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[Note that this file is a concatenation of more than one RFC.]

RFC:  791
                                    
                                    
                                    
                                    
                                    
                                    
                                    
                           INTERNET PROTOCOL
                                    
                                    
                         DARPA INTERNET PROGRAM
                                    
                         PROTOCOL SPECIFICATION
                                    
                                    
                                    
                             September 1981













                              prepared for

               Defense Advanced Research Projects Agency
                Information Processing Techniques Office
                         1400 Wilson Boulevard
                       Arlington, Virginia  22209







                                   by

                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291





September 1981                                                          
                                                       Internet Protocol



                           TABLE OF CONTENTS

    PREFACE ........................................................ iii

1.  INTRODUCTION ..................................................... 1

  1.1  Motivation .................................................... 1
  1.2  Scope ......................................................... 1
  1.3  Interfaces .................................................... 1
  1.4  Operation ..................................................... 2

2.  OVERVIEW ......................................................... 5

  2.1  Relation to Other Protocols ................................... 9
  2.2  Model of Operation ............................................ 5
  2.3  Function Description .......................................... 7
  2.4  Gateways ...................................................... 9

3.  SPECIFICATION ................................................... 11

  3.1  Internet Header Format ....................................... 11
  3.2  Discussion ................................................... 23
  3.3  Interfaces ................................................... 31

APPENDIX A:  Examples & Scenarios ................................... 34
APPENDIX B:  Data Transmission Order ................................ 39

GLOSSARY ............................................................ 41

REFERENCES .......................................................... 45





















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                                                          September 1981
Internet Protocol






















































[Page ii]                                                               



September 1981                                                          
                                                       Internet Protocol



                                PREFACE



This document specifies the DoD Standard Internet Protocol.  This
document is based on six earlier editions of the ARPA Internet Protocol
Specification, and the present text draws heavily from them.  There have
been many contributors to this work both in terms of concepts and in
terms of text.  This edition revises aspects of addressing, error
handling, option codes, and the security, precedence, compartments, and
handling restriction features of the internet protocol.

                                                           Jon Postel

                                                           Editor




































                                                              [Page iii]





                                                          September 1981


RFC:  791
Replaces:  RFC 760
IENs 128, 123, 111,
80, 54, 44, 41, 28, 26

                           INTERNET PROTOCOL

                         DARPA INTERNET PROGRAM
                         PROTOCOL SPECIFICATION



                            1.  INTRODUCTION

1.1.  Motivation

  The Internet Protocol is designed for use in interconnected systems of
  packet-switched computer communication networks.  Such a system has
  been called a "catenet" [1].  The internet protocol provides for
  transmitting blocks of data called datagrams from sources to
  destinations, where sources and destinations are hosts identified by
  fixed length addresses.  The internet protocol also provides for
  fragmentation and reassembly of long datagrams, if necessary, for
  transmission through "small packet" networks.

1.2.  Scope

  The internet protocol is specifically limited in scope to provide the
  functions necessary to deliver a package of bits (an internet
  datagram) from a source to a destination over an interconnected system
  of networks.  There are no mechanisms to augment end-to-end data
  reliability, flow control, sequencing, or other services commonly
  found in host-to-host protocols.  The internet protocol can capitalize
  on the services of its supporting networks to provide various types
  and qualities of service.

1.3.  Interfaces

  This protocol is called on by host-to-host protocols in an internet
  environment.  This protocol calls on local network protocols to carry
  the internet datagram to the next gateway or destination host.

  For example, a TCP module would call on the internet module to take a
  TCP segment (including the TCP header and user data) as the data
  portion of an internet datagram.  The TCP module would provide the
  addresses and other parameters in the internet header to the internet
  module as arguments of the call.  The internet module would then
  create an internet datagram and call on the local network interface to
  transmit the internet datagram.

  In the ARPANET case, for example, the internet module would call on a


                                                                [Page 1]
 




                                                          September 1981
Internet Protocol
Introduction



  local net module which would add the 1822 leader [2] to the internet
  datagram creating an ARPANET message to transmit to the IMP.  The
  ARPANET address would be derived from the internet address by the
  local network interface and would be the address of some host in the
  ARPANET, that host might be a gateway to other networks.

1.4.  Operation

  The internet protocol implements two basic functions:  addressing and
  fragmentation.

  The internet modules use the addresses carried in the internet header
  to transmit internet datagrams toward their destinations.  The
  selection of a path for transmission is called routing.

  The internet modules use fields in the internet header to fragment and
  reassemble internet datagrams when necessary for transmission through
  "small packet" networks.

  The model of operation is that an internet module resides in each host
  engaged in internet communication and in each gateway that
  interconnects networks.  These modules share common rules for
  interpreting address fields and for fragmenting and assembling
  internet datagrams.  In addition, these modules (especially in
  gateways) have procedures for making routing decisions and other
  functions.

  The internet protocol treats each internet datagram as an independent
  entity unrelated to any other internet datagram.  There are no
  connections or logical circuits (virtual or otherwise).

  The internet protocol uses four key mechanisms in providing its
  service:  Type of Service, Time to Live, Options, and Header Checksum.

  The Type of Service is used to indicate the quality of the service
  desired.  The type of service is an abstract or generalized set of
  parameters which characterize the service choices provided in the
  networks that make up the internet.  This type of service indication
  is to be used by gateways to select the actual transmission parameters
  for a particular network, the network to be used for the next hop, or
  the next gateway when routing an internet datagram.

  The Time to Live is an indication of an upper bound on the lifetime of
  an internet datagram.  It is set by the sender of the datagram and
  reduced at the points along the route where it is processed.  If the
  time to live reaches zero before the internet datagram reaches its
  destination, the internet datagram is destroyed.  The time to live can
  be thought of as a self destruct time limit.


[Page 2]
 
                                                                



September 1981                                                          
                                                       Internet Protocol
                                                            Introduction



  The Options provide for control functions needed or useful in some
  situations but unnecessary for the most common communications.  The
  options include provisions for timestamps, security, and special
  routing.

  The Header Checksum provides a verification that the information used
  in processing internet datagram has been transmitted correctly.  The
  data may contain errors.  If the header checksum fails, the internet
  datagram is discarded at once by the entity which detects the error.

  The internet protocol does not provide a reliable communication
  facility.  There are no acknowledgments either end-to-end or
  hop-by-hop.  There is no error control for data, only a header
  checksum.  There are no retransmissions.  There is no flow control.

  Errors detected may be reported via the Internet Control Message
  Protocol (ICMP) [3] which is implemented in the internet protocol
  module.

  






























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                                                          September 1981
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[Page 4]
 
                                                                



September 1981                                                          
                                                       Internet Protocol



                              2.  OVERVIEW

2.1.  Relation to Other Protocols

  The following diagram illustrates the place of the internet protocol
  in the protocol hierarchy:

                                    
                 +------+ +-----+ +-----+     +-----+  
                 |Telnet| | FTP | | TFTP| ... | ... |  
                 +------+ +-----+ +-----+     +-----+  
                       |   |         |           |     
                      +-----+     +-----+     +-----+  
                      | TCP |     | UDP | ... | ... |  
                      +-----+     +-----+     +-----+  
                         |           |           |     
                      +--------------------------+----+
                      |    Internet Protocol & ICMP   |
                      +--------------------------+----+
                                     |                 
                        +---------------------------+  
                        |   Local Network Protocol  |  
                        +---------------------------+  

                         Protocol Relationships

                               Figure 1.

  Internet protocol interfaces on one side to the higher level
  host-to-host protocols and on the other side to the local network
  protocol.  In this context a "local network" may be a small network in
  a building or a large network such as the ARPANET.

2.2.  Model of Operation

  The  model of operation for transmitting a datagram from one
  application program to another is illustrated by the following
  scenario:

    We suppose that this transmission will involve one intermediate
    gateway.

    The sending application program prepares its data and calls on its
    local internet module to send that data as a datagram and passes the
    destination address and other parameters as arguments of the call.

    The internet module prepares a datagram header and attaches the data
    to it.  The internet module determines a local network address for
    this internet address, in this case it is the address of a gateway.


                                                                [Page 5]
 




                                                          September 1981
Internet Protocol
Overview



    It sends this datagram and the local network address to the local
    network interface.

    The local network interface creates a local network header, and
    attaches the datagram to it, then sends the result via the local
    network.

    The datagram arrives at a gateway host wrapped in the local network
    header, the local network interface strips off this header, and
    turns the datagram over to the internet module.  The internet module
    determines from the internet address that the datagram is to be
    forwarded to another host in a second network.  The internet module
    determines a local net address for the destination host.  It calls
    on the local network interface for that network to send the
    datagram.

    This local network interface creates a local network header and
    attaches the datagram sending the result to the destination host.

    At this destination host the datagram is stripped of the local net
    header by the local network interface and handed to the internet
    module.

    The internet module determines that the datagram is for an
    application program in this host.  It passes the data to the
    application program in response to a system call, passing the source
    address and other parameters as results of the call.

                                    
   Application                                           Application
   Program                                                   Program
         \                                                   /      
       Internet Module      Internet Module      Internet Module    
             \                 /       \                /           
             LNI-1          LNI-1      LNI-2         LNI-2          
                \           /             \          /              
               Local Network 1           Local Network 2            



                            Transmission Path

                                Figure 2







[Page 6]
 
                                                                



September 1981                                                          
                                                       Internet Protocol
                                                                Overview



2.3.  Function Description

  The function or purpose of Internet Protocol is to move datagrams
  through an interconnected set of networks.  This is done by passing
  the datagrams from one internet module to another until the
  destination is reached.  The internet modules reside in hosts and
  gateways in the internet system.  The datagrams are routed from one
  internet module to another through individual networks based on the
  interpretation of an internet address.  Thus, one important mechanism
  of the internet protocol is the internet address.

  In the routing of messages from one internet module to another,
  datagrams may need to traverse a network whose maximum packet size is
  smaller than the size of the datagram.  To overcome this difficulty, a
  fragmentation mechanism is provided in the internet protocol.

  Addressing

    A distinction is made between names, addresses, and routes [4].   A
    name indicates what we seek.  An address indicates where it is.  A
    route indicates how to get there.  The internet protocol deals
    primarily with addresses.  It is the task of higher level (i.e.,
    host-to-host or application) protocols to make the mapping from
    names to addresses.   The internet module maps internet addresses to
    local net addresses.  It is the task of lower level (i.e., local net
    or gateways) procedures to make the mapping from local net addresses
    to routes.

    Addresses are fixed length of four octets (32 bits).  An address
    begins with a network number, followed by local address (called the
    "rest" field).  There are three formats or classes of internet
    addresses:  in class a, the high order bit is zero, the next 7 bits
    are the network, and the last 24 bits are the local address; in
    class b, the high order two bits are one-zero, the next 14 bits are
    the network and the last 16 bits are the local address; in class c,
    the high order three bits are one-one-zero, the next 21 bits are the
    network and the last 8 bits are the local address.

    Care must be taken in mapping internet addresses to local net
    addresses; a single physical host must be able to act as if it were
    several distinct hosts to the extent of using several distinct
    internet addresses.  Some hosts will also have several physical
    interfaces (multi-homing).

    That is, provision must be made for a host to have several physical
    interfaces to the network with each having several logical internet
    addresses.



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                                                          September 1981
Internet Protocol
Overview



    Examples of address mappings may be found in "Address Mappings" [5].

  Fragmentation

    Fragmentation of an internet datagram is necessary when it
    originates in a local net that allows a large packet size and must
    traverse a local net that limits packets to a smaller size to reach
    its destination.

    An internet datagram can be marked "don't fragment."  Any internet
    datagram so marked is not to be internet fragmented under any
    circumstances.  If internet datagram marked don't fragment cannot be
    delivered to its destination without fragmenting it, it is to be
    discarded instead.

    Fragmentation, transmission and reassembly across a local network
    which is invisible to the internet protocol module is called
    intranet fragmentation and may be used [6].

    The internet fragmentation and reassembly procedure needs to be able
    to break a datagram into an almost arbitrary number of pieces that
    can be later reassembled.  The receiver of the fragments uses the
    identification field to ensure that fragments of different datagrams
    are not mixed.  The fragment offset field tells the receiver the
    position of a fragment in the original datagram.  The fragment
    offset and length determine the portion of the original datagram
    covered by this fragment.  The more-fragments flag indicates (by
    being reset) the last fragment.  These fields provide sufficient
    information to reassemble datagrams.

    The identification field is used to distinguish the fragments of one
    datagram from those of another.  The originating protocol module of
    an internet datagram sets the identification field to a value that
    must be unique for that source-destination pair and protocol for the
    time the datagram will be active in the internet system.  The
    originating protocol module of a complete datagram sets the
    more-fragments flag to zero and the fragment offset to zero.

    To fragment a long internet datagram, an internet protocol module
    (for example, in a gateway), creates two new internet datagrams and
    copies the contents of the internet header fields from the long
    datagram into both new internet headers.  The data of the long
    datagram is divided into two portions on a 8 octet (64 bit) boundary
    (the second portion might not be an integral multiple of 8 octets,
    but the first must be).  Call the number of 8 octet blocks in the
    first portion NFB (for Number of Fragment Blocks).  The first
    portion of the data is placed in the first new internet datagram,
    and the total length field is set to the length of the first


[Page 8]
 
 



September 1981 
                                                       Internet Protocol
                                                                Overview



    datagram.  The more-fragments flag is set to one.  The second
    portion of the data is placed in the second new internet datagram,
    and the total length field is set to the length of the second
    datagram.  The more-fragments flag carries the same value as the
    long datagram.  The fragment offset field of the second new internet
    datagram is set to the value of that field in the long datagram plus
    NFB.

    This procedure can be generalized for an n-way split, rather than
    the two-way split described.

    To assemble the fragments of an internet datagram, an internet
    protocol module (for example at a destination host) combines
    internet datagrams that all have the same value for the four fields:
    identification, source, destination, and protocol.  The combination
    is done by placing the data portion of each fragment in the relative
    position indicated by the fragment offset in that fragment's
    internet header.  The first fragment will have the fragment offset
    zero, and the last fragment will have the more-fragments flag reset
    to zero.

2.4.  Gateways

  Gateways implement internet protocol to forward datagrams between
  networks.  Gateways also implement the Gateway to Gateway Protocol
  (GGP) [7] to coordinate routing and other internet control
  information.

  In a gateway the higher level protocols need not be implemented and
  the GGP functions are added to the IP module.

                                    
                   +-------------------------------+   
                   | Internet Protocol & ICMP & GGP|   
                   +-------------------------------+   
                           |                 |         
                 +---------------+   +---------------+ 
                 |   Local Net   |   |   Local Net   | 
                 +---------------+   +---------------+ 

                           Gateway Protocols

                               Figure 3.

  





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September 1981                                                          
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                           3.  SPECIFICATION

3.1.  Internet Header Format

  A summary of the contents of the internet header follows:

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  IHL  |Type of Service|          Total Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Identification        |Flags|      Fragment Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Time to Live |    Protocol   |         Header Checksum       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Example Internet Datagram Header

                               Figure 4.

  Note that each tick mark represents one bit position.

  Version:  4 bits

    The Version field indicates the format of the internet header.  This
    document describes version 4.

  IHL:  4 bits

    Internet Header Length is the length of the internet header in 32
    bit words, and thus points to the beginning of the data.  Note that
    the minimum value for a correct header is 5.












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                                                          September 1981
Internet Protocol
Specification



  Type of Service:  8 bits

    The Type of Service provides an indication of the abstract
    parameters of the quality of service desired.  These parameters are
    to be used to guide the selection of the actual service parameters
    when transmitting a datagram through a particular network.  Several
    networks offer service precedence, which somehow treats high
    precedence traffic as more important than other traffic (generally
    by accepting only traffic above a certain precedence at time of high
    load).  The major choice is a three way tradeoff between low-delay,
    high-reliability, and high-throughput.

      Bits 0-2:  Precedence.
      Bit    3:  0 = Normal Delay,      1 = Low Delay.
      Bits   4:  0 = Normal Throughput, 1 = High Throughput.
      Bits   5:  0 = Normal Relibility, 1 = High Relibility.
      Bit  6-7:  Reserved for Future Use.

         0     1     2     3     4     5     6     7
      +-----+-----+-----+-----+-----+-----+-----+-----+
      |                 |     |     |     |     |     |
      |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
      |                 |     |     |     |     |     |
      +-----+-----+-----+-----+-----+-----+-----+-----+

        Precedence

          111 - Network Control
          110 - Internetwork Control
          101 - CRITIC/ECP
          100 - Flash Override
          011 - Flash
          010 - Immediate
          001 - Priority
          000 - Routine

    The use of the Delay, Throughput, and Reliability indications may
    increase the cost (in some sense) of the service.  In many networks
    better performance for one of these parameters is coupled with worse
    performance on another.  Except for very unusual cases at most two
    of these three indications should be set.

    The type of service is used to specify the treatment of the datagram
    during its transmission through the internet system.  Example
    mappings of the internet type of service to the actual service
    provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
    is given in "Service Mappings" [8].



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September 1981                                                          
                                                       Internet Protocol
                                                           Specification



    The Network Control precedence designation is intended to be used
    within a network only.  The actual use and control of that
    designation is up to each network. The Internetwork Control
    designation is intended for use by gateway control originators only.
    If the actual use of these precedence designations is of concern to
    a particular network, it is the responsibility of that network to
    control the access to, and use of, those precedence designations.

  Total Length:  16 bits

    Total Length is the length of the datagram, measured in octets,
    including internet header and data.  This field allows the length of
    a datagram to be up to 65,535 octets.  Such long datagrams are
    impractical for most hosts and networks.  All hosts must be prepared
    to accept datagrams of up to 576 octets (whether they arrive whole
    or in fragments).  It is recommended that hosts only send datagrams
    larger than 576 octets if they have assurance that the destination
    is prepared to accept the larger datagrams.

    The number 576 is selected to allow a reasonable sized data block to
    be transmitted in addition to the required header information.  For
    example, this size allows a data block of 512 octets plus 64 header
    octets to fit in a datagram.  The maximal internet header is 60
    octets, and a typical internet header is 20 octets, allowing a
    margin for headers of higher level protocols.

  Identification:  16 bits

    An identifying value assigned by the sender to aid in assembling the
    fragments of a datagram.

  Flags:  3 bits

    Various Control Flags.

      Bit 0: reserved, must be zero
      Bit 1: (DF) 0 = May Fragment,  1 = Don't Fragment.
      Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.

          0   1   2
        +---+---+---+
        |   | D | M |
        | 0 | F | F |
        +---+---+---+

  Fragment Offset:  13 bits

    This field indicates where in the datagram this fragment belongs.


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                                                          September 1981
Internet Protocol
Specification



    The fragment offset is measured in units of 8 octets (64 bits).  The
    first fragment has offset zero.

  Time to Live:  8 bits

    This field indicates the maximum time the datagram is allowed to
    remain in the internet system.  If this field contains the value
    zero, then the datagram must be destroyed.  This field is modified
    in internet header processing.  The time is measured in units of
    seconds, but since every module that processes a datagram must
    decrease the TTL by at least one even if it process the datagram in
    less than a second, the TTL must be thought of only as an upper
    bound on the time a datagram may exist.  The intention is to cause
    undeliverable datagrams to be discarded, and to bound the maximum
    datagram lifetime.

  Protocol:  8 bits

    This field indicates the next level protocol used in the data
    portion of the internet datagram.  The values for various protocols
    are specified in "Assigned Numbers" [9].

  Header Checksum:  16 bits

    A checksum on the header only.  Since some header fields change
    (e.g., time to live), this is recomputed and verified at each point
    that the internet header is processed.

    The checksum algorithm is:

      The checksum field is the 16 bit one's complement of the one's
      complement sum of all 16 bit words in the header.  For purposes of
      computing the checksum, the value of the checksum field is zero.

    This is a simple to compute checksum and experimental evidence
    indicates it is adequate, but it is provisional and may be replaced
    by a CRC procedure, depending on further experience.

  Source Address:  32 bits

    The source address.  See section 3.2.

  Destination Address:  32 bits

    The destination address.  See section 3.2.





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September 1981                                                          
                                                       Internet Protocol
                                                           Specification



  Options:  variable

    The options may appear or not in datagrams.  They must be
    implemented by all IP modules (host and gateways).  What is optional
    is their transmission in any particular datagram, not their
    implementation.

    In some environments the security option may be required in all
    datagrams.

    The option field is variable in length.  There may be zero or more
    options.  There are two cases for the format of an option:

      Case 1:  A single octet of option-type.

      Case 2:  An option-type octet, an option-length octet, and the
               actual option-data octets.

    The option-length octet counts the option-type octet and the
    option-length octet as well as the option-data octets.

    The option-type octet is viewed as having 3 fields:

      1 bit   copied flag,
      2 bits  option class,
      5 bits  option number.

    The copied flag indicates that this option is copied into all
    fragments on fragmentation.

      0 = not copied
      1 = copied

    The option classes are:

      0 = control
      1 = reserved for future use
      2 = debugging and measurement
      3 = reserved for future use











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Internet Protocol
Specification



    The following internet options are defined:

      CLASS NUMBER LENGTH DESCRIPTION
      ----- ------ ------ -----------
        0     0      -    End of Option list.  This option occupies only
                          1 octet; it has no length octet.
        0     1      -    No Operation.  This option occupies only 1
                          octet; it has no length octet.
        0     2     11    Security.  Used to carry Security,
                          Compartmentation, User Group (TCC), and
                          Handling Restriction Codes compatible with DOD
                          requirements.
        0     3     var.  Loose Source Routing.  Used to route the
                          internet datagram based on information
                          supplied by the source.
        0     9     var.  Strict Source Routing.  Used to route the
                          internet datagram based on information
                          supplied by the source.
        0     7     var.  Record Route.  Used to trace the route an
                          internet datagram takes.
        0     8      4    Stream ID.  Used to carry the stream
                          identifier.
        2     4     var.  Internet Timestamp.

      

    Specific Option Definitions

      End of Option List

        +--------+
        |00000000|
        +--------+
          Type=0

        This option indicates the end of the option list.  This might
        not coincide with the end of the internet header according to
        the internet header length.  This is used at the end of all
        options, not the end of each option, and need only be used if
        the end of the options would not otherwise coincide with the end
        of the internet header.

        May be copied, introduced, or deleted on fragmentation, or for
        any other reason.






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                                                           Specification



      No Operation

        +--------+
        |00000001|
        +--------+
          Type=1

        This option may be used between options, for example, to align
        the beginning of a subsequent option on a 32 bit boundary.

        May be copied, introduced, or deleted on fragmentation, or for
        any other reason.

      Security

        This option provides a way for hosts to send security,
        compartmentation, handling restrictions, and TCC (closed user
        group) parameters.  The format for this option is as follows:

          +--------+--------+---//---+---//---+---//---+---//---+
          |10000010|00001011|SSS  SSS|CCC  CCC|HHH  HHH|  TCC   |
          +--------+--------+---//---+---//---+---//---+---//---+
           Type=130 Length=11

        Security (S field):  16 bits

          Specifies one of 16 levels of security (eight of which are
          reserved for future use).

            00000000 00000000 - Unclassified
            11110001 00110101 - Confidential
            01111000 10011010 - EFTO
            10111100 01001101 - MMMM
            01011110 00100110 - PROG
            10101111 00010011 - Restricted
            11010111 10001000 - Secret
            01101011 11000101 - Top Secret
            00110101 11100010 - (Reserved for future use)
            10011010 11110001 - (Reserved for future use)
            01001101 01111000 - (Reserved for future use)
            00100100 10111101 - (Reserved for future use)
            00010011 01011110 - (Reserved for future use)
            10001001 10101111 - (Reserved for future use)
            11000100 11010110 - (Reserved for future use)
            11100010 01101011 - (Reserved for future use)





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        Compartments (C field):  16 bits

          An all zero value is used when the information transmitted is
          not compartmented.  Other values for the compartments field
          may be obtained from the Defense Intelligence Agency.

        Handling Restrictions (H field):  16 bits

          The values for the control and release markings are
          alphanumeric digraphs and are defined in the Defense
          Intelligence Agency Manual DIAM 65-19, "Standard Security
          Markings".

        Transmission Control Code (TCC field):  24 bits

          Provides a means to segregate traffic and define controlled
          communities of interest among subscribers. The TCC values are
          trigraphs, and are available from HQ DCA Code 530.

        Must be copied on fragmentation.  This option appears at most
        once in a datagram.

      Loose Source and Record Route

        +--------+--------+--------+---------//--------+
        |10000011| length | pointer|     route data    |
        +--------+--------+--------+---------//--------+
         Type=131

        The loose source and record route (LSRR) option provides a means
        for the source of an internet datagram to supply routing
        information to be used by the gateways in forwarding the
        datagram to the destination, and to record the route
        information.

        The option begins with the option type code.  The second octet
        is the option length which includes the option type code and the
        length octet, the pointer octet, and length-3 octets of route
        data.  The third octet is the pointer into the route data
        indicating the octet which begins the next source address to be
        processed.  The pointer is relative to this option, and the
        smallest legal value for the pointer is 4.

        A route data is composed of a series of internet addresses.
        Each internet address is 32 bits or 4 octets.  If the pointer is
        greater than the length, the source route is empty (and the
        recorded route full) and the routing is to be based on the
        destination address field.


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        If the address in destination address field has been reached and
        the pointer is not greater than the length, the next address in
        the source route replaces the address in the destination address
        field, and the recorded route address replaces the source
        address just used, and pointer is increased by four.

        The recorded route address is the internet module's own internet
        address as known in the environment into which this datagram is
        being forwarded.

        This procedure of replacing the source route with the recorded
        route (though it is in the reverse of the order it must be in to
        be used as a source route) means the option (and the IP header
        as a whole) remains a constant length as the datagram progresses
        through the internet.

        This option is a loose source route because the gateway or host
        IP is allowed to use any route of any number of other
        intermediate gateways to reach the next address in the route.

        Must be copied on fragmentation.  Appears at most once in a
        datagram.

      Strict Source and Record Route

        +--------+--------+--------+---------//--------+
        |10001001| length | pointer|     route data    |
        +--------+--------+--------+---------//--------+
         Type=137

        The strict source and record route (SSRR) option provides a
        means for the source of an internet datagram to supply routing
        information to be used by the gateways in forwarding the
        datagram to the destination, and to record the route
        information.

        The option begins with the option type code.  The second octet
        is the option length which includes the option type code and the
        length octet, the pointer octet, and length-3 octets of route
        data.  The third octet is the pointer into the route data
        indicating the octet which begins the next source address to be
        processed.  The pointer is relative to this option, and the
        smallest legal value for the pointer is 4.

        A route data is composed of a series of internet addresses.
        Each internet address is 32 bits or 4 octets.  If the pointer is
        greater than the length, the source route is empty (and the



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        recorded route full) and the routing is to be based on the
        destination address field.

        If the address in destination address field has been reached and
        the pointer is not greater than the length, the next address in
        the source route replaces the address in the destination address
        field, and the recorded route address replaces the source
        address just used, and pointer is increased by four.

        The recorded route address is the internet module's own internet
        address as known in the environment into which this datagram is
        being forwarded.

        This procedure of replacing the source route with the recorded
        route (though it is in the reverse of the order it must be in to
        be used as a source route) means the option (and the IP header
        as a whole) remains a constant length as the datagram progresses
        through the internet.

        This option is a strict source route because the gateway or host
        IP must send the datagram directly to the next address in the
        source route through only the directly connected network
        indicated in the next address to reach the next gateway or host
        specified in the route.

        Must be copied on fragmentation.  Appears at most once in a
        datagram.

      Record Route

        +--------+--------+--------+---------//--------+
        |00000111| length | pointer|     route data    |
        +--------+--------+--------+---------//--------+
          Type=7

        The record route option provides a means to record the route of
        an internet datagram.

        The option begins with the option type code.  The second octet
        is the option length which includes the option type code and the
        length octet, the pointer octet, and length-3 octets of route
        data.  The third octet is the pointer into the route data
        indicating the octet which begins the next area to store a route
        address.  The pointer is relative to this option, and the
        smallest legal value for the pointer is 4.

        A recorded route is composed of a series of internet addresses.
        Each internet address is 32 bits or 4 octets.  If the pointer is


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        greater than the length, the recorded route data area is full.
        The originating host must compose this option with a large
        enough route data area to hold all the address expected.  The
        size of the option does not change due to adding addresses.  The
        intitial contents of the route data area must be zero.

        When an internet module routes a datagram it checks to see if
        the record route option is present.  If it is, it inserts its
        own internet address as known in the environment into which this
        datagram is being forwarded into the recorded route begining at
        the octet indicated by the pointer, and increments the pointer
        by four.

        If the route data area is already full (the pointer exceeds the
        length) the datagram is forwarded without inserting the address
        into the recorded route.  If there is some room but not enough
        room for a full address to be inserted, the original datagram is
        considered to be in error and is discarded.  In either case an
        ICMP parameter problem message may be sent to the source
        host [3].

        Not copied on fragmentation, goes in first fragment only.
        Appears at most once in a datagram.

      Stream Identifier

        +--------+--------+--------+--------+
        |10001000|00000010|    Stream ID    |
        +--------+--------+--------+--------+
         Type=136 Length=4

        This option provides a way for the 16-bit SATNET stream
        identifier to be carried through networks that do not support
        the stream concept.

        Must be copied on fragmentation.  Appears at most once in a
        datagram.













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      Internet Timestamp

        +--------+--------+--------+--------+
        |01000100| length | pointer|oflw|flg|
        +--------+--------+--------+--------+
        |         internet address          |
        +--------+--------+--------+--------+
        |             timestamp             |
        +--------+--------+--------+--------+
        |                 .                 |
                          .
                          .
        Type = 68

        The Option Length is the number of octets in the option counting
        the type, length, pointer, and overflow/flag octets (maximum
        length 40).

        The Pointer is the number of octets from the beginning of this
        option to the end of timestamps plus one (i.e., it points to the
        octet beginning the space for next timestamp).  The smallest
        legal value is 5.  The timestamp area is full when the pointer
        is greater than the length.

        The Overflow (oflw) [4 bits] is the number of IP modules that
        cannot register timestamps due to lack of space.

        The Flag (flg) [4 bits] values are

          0 -- time stamps only, stored in consecutive 32-bit words,

          1 -- each timestamp is preceded with internet address of the
               registering entity,

          3 -- the internet address fields are prespecified.  An IP
               module only registers its timestamp if it matches its own
               address with the next specified internet address.

        The Timestamp is a right-justified, 32-bit timestamp in
        milliseconds since midnight UT.  If the time is not available in
        milliseconds or cannot be provided with respect to midnight UT
        then any time may be inserted as a timestamp provided the high
        order bit of the timestamp field is set to one to indicate the
        use of a non-standard value.

        The originating host must compose this option with a large
        enough timestamp data area to hold all the timestamp information
        expected.  The size of the option does not change due to adding


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        timestamps.  The intitial contents of the timestamp data area
        must be zero or internet address/zero pairs.

        If the timestamp data area is already full (the pointer exceeds
        the length) the datagram is forwarded without inserting the
        timestamp, but the overflow count is incremented by one.

        If there is some room but not enough room for a full timestamp
        to be inserted, or the overflow count itself overflows, the
        original datagram is considered to be in error and is discarded.
        In either case an ICMP parameter problem message may be sent to
        the source host [3].

        The timestamp option is not copied upon fragmentation.  It is
        carried in the first fragment.  Appears at most once in a
        datagram.

  Padding:  variable

    The internet header padding is used to ensure that the internet
    header ends on a 32 bit boundary.  The padding is zero.

3.2.  Discussion

  The implementation of a protocol must be robust.  Each implementation
  must expect to interoperate with others created by different
  individuals.  While the goal of this specification is to be explicit
  about the protocol there is the possibility of differing
  interpretations.  In general, an implementation must be conservative
  in its sending behavior, and liberal in its receiving behavior.  That
  is, it must be careful to send well-formed datagrams, but must accept
  any datagram that it can interpret (e.g., not object to technical
  errors where the meaning is still clear).

  The basic internet service is datagram oriented and provides for the
  fragmentation of datagrams at gateways, with reassembly taking place
  at the destination internet protocol module in the destination host.
  Of course, fragmentation and reassembly of datagrams within a network
  or by private agreement between the gateways of a network is also
  allowed since this is transparent to the internet protocols and the
  higher-level protocols.  This transparent type of fragmentation and
  reassembly is termed "network-dependent" (or intranet) fragmentation
  and is not discussed further here.

  Internet addresses distinguish sources and destinations to the host
  level and provide a protocol field as well.  It is assumed that each
  protocol will provide for whatever multiplexing is necessary within a
  host.


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  Addressing

    To provide for flexibility in assigning address to networks and
    allow for the  large number of small to intermediate sized networks
    the interpretation of the address field is coded to specify a small
    number of networks with a large number of host, a moderate number of
    networks with a moderate number of hosts, and a large number of
    networks with a small number of hosts.  In addition there is an
    escape code for extended addressing mode.

    Address Formats:

      High Order Bits   Format                           Class
      ---------------   -------------------------------  -----
            0            7 bits of net, 24 bits of host    a
            10          14 bits of net, 16 bits of host    b
            110         21 bits of net,  8 bits of host    c
            111         escape to extended addressing mode

      A value of zero in the network field means this network.  This is
      only used in certain ICMP messages.  The extended addressing mode
      is undefined.  Both of these features are reserved for future use.

    The actual values assigned for network addresses is given in
    "Assigned Numbers" [9].

    The local address, assigned by the local network, must allow for a
    single physical host to act as several distinct internet hosts.
    That is, there must be a mapping between internet host addresses and
    network/host interfaces that allows several internet addresses to
    correspond to one interface.  It must also be allowed for a host to
    have several physical interfaces and to treat the datagrams from
    several of them as if they were all addressed to a single host.

    Address mappings between internet addresses and addresses for
    ARPANET, SATNET, PRNET, and other networks are described in "Address
    Mappings" [5].

  Fragmentation and Reassembly.

    The internet identification field (ID) is used together with the
    source and destination address, and the protocol fields, to identify
    datagram fragments for reassembly.

    The More Fragments flag bit (MF) is set if the datagram is not the
    last fragment.  The Fragment Offset field identifies the fragment
    location, relative to the beginning of the original unfragmented
    datagram.  Fragments are counted in units of 8 octets.  The


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    fragmentation strategy is designed so than an unfragmented datagram
    has all zero fragmentation information (MF = 0, fragment offset =
    0).  If an internet datagram is fragmented, its data portion must be
    broken on 8 octet boundaries.

    This format allows 2**13 = 8192 fragments of 8 octets each for a
    total of 65,536 octets.  Note that this is consistent with the the
    datagram total length field (of course, the header is counted in the
    total length and not in the fragments).

    When fragmentation occurs, some options are copied, but others
    remain with the first fragment only.

    Every internet module must be able to forward a datagram of 68
    octets without further fragmentation.  This is because an internet
    header may be up to 60 octets, and the minimum fragment is 8 octets.

    Every internet destination must be able to receive a datagram of 576
    octets either in one piece or in fragments to be reassembled.

    The fields which may be affected by fragmentation include:

      (1) options field
      (2) more fragments flag
      (3) fragment offset
      (4) internet header length field
      (5) total length field
      (6) header checksum

    If the Don't Fragment flag (DF) bit is set, then internet
    fragmentation of this datagram is NOT permitted, although it may be
    discarded.  This can be used to prohibit fragmentation in cases
    where the receiving host does not have sufficient resources to
    reassemble internet fragments.

    One example of use of the Don't Fragment feature is to down line
    load a small host.  A small host could have a boot strap program
    that accepts a datagram stores it in memory and then executes it.

    The fragmentation and reassembly procedures are most easily
    described by examples.  The following procedures are example
    implementations.

    General notation in the following pseudo programs: "=<" means "less
    than or equal", "#" means "not equal", "=" means "equal", "<-" means
    "is set to".  Also, "x to y" includes x and excludes y; for example,
    "4 to 7" would include 4, 5, and 6 (but not 7).



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    An Example Fragmentation Procedure

      The maximum sized datagram that can be transmitted through the
      next network is called the maximum transmission unit (MTU).

      If the total length is less than or equal the maximum transmission
      unit then submit this datagram to the next step in datagram
      processing; otherwise cut the datagram into two fragments, the
      first fragment being the maximum size, and the second fragment
      being the rest of the datagram.  The first fragment is submitted
      to the next step in datagram processing, while the second fragment
      is submitted to this procedure in case it is still too large.

      Notation:

        FO    -  Fragment Offset
        IHL   -  Internet Header Length
        DF    -  Don't Fragment flag
        MF    -  More Fragments flag
        TL    -  Total Length
        OFO   -  Old Fragment Offset
        OIHL  -  Old Internet Header Length
        OMF   -  Old More Fragments flag
        OTL   -  Old Total Length
        NFB   -  Number of Fragment Blocks
        MTU   -  Maximum Transmission Unit

      Procedure:

        IF TL =< MTU THEN Submit this datagram to the next step
             in datagram processing ELSE IF DF = 1 THEN discard the
        datagram ELSE
        To produce the first fragment:
        (1)  Copy the original internet header;
        (2)  OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
        (3)  NFB <- (MTU-IHL*4)/8;
        (4)  Attach the first NFB*8 data octets;
        (5)  Correct the header:
             MF <- 1;  TL <- (IHL*4)+(NFB*8);
             Recompute Checksum;
        (6)  Submit this fragment to the next step in
             datagram processing;
        To produce the second fragment:
        (7)  Selectively copy the internet header (some options
             are not copied, see option definitions);
        (8)  Append the remaining data;
        (9)  Correct the header:
             IHL <- (((OIHL*4)-(length of options not copied))+3)/4;


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             TL <- OTL - NFB*8 - (OIHL-IHL)*4);
             FO <- OFO + NFB;  MF <- OMF;  Recompute Checksum;
        (10) Submit this fragment to the fragmentation test; DONE.

      In the above procedure each fragment (except the last) was made
      the maximum allowable size.  An alternative might produce less
      than the maximum size datagrams.  For example, one could implement
      a fragmentation procedure that repeatly divided large datagrams in
      half until the resulting fragments were less than the maximum
      transmission unit size.

    An Example Reassembly Procedure

      For each datagram the buffer identifier is computed as the
      concatenation of the source, destination, protocol, and
      identification fields.  If this is a whole datagram (that is both
      the fragment offset and the more fragments  fields are zero), then
      any reassembly resources associated with this buffer identifier
      are released and the datagram is forwarded to the next step in
      datagram processing.

      If no other fragment with this buffer identifier is on hand then
      reassembly resources are allocated.  The reassembly resources
      consist of a data buffer, a header buffer, a fragment block bit
      table, a total data length field, and a timer.  The data from the
      fragment is placed in the data buffer according to its fragment
      offset and length, and bits are set in the fragment block bit
      table corresponding to the fragment blocks received.

      If this is the first fragment (that is the fragment offset is
      zero)  this header is placed in the header buffer.  If this is the
      last fragment ( that is the more fragments field is zero) the
      total data length is computed.  If this fragment completes the
      datagram (tested by checking the bits set in the fragment block
      table), then the datagram is sent to the next step in datagram
      processing; otherwise the timer is set to the maximum of the
      current timer value and the value of the time to live field from
      this fragment; and the reassembly routine gives up control.

      If the timer runs out, the all reassembly resources for this
      buffer identifier are released.  The initial setting of the timer
      is a lower bound on the reassembly waiting time.  This is because
      the waiting time will be increased if the Time to Live in the
      arriving fragment is greater than the current timer value but will
      not be decreased if it is less.  The maximum this timer value
      could reach is the maximum time to live (approximately 4.25
      minutes).  The current recommendation for the initial timer
      setting is 15 seconds.  This may be changed as experience with


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      this protocol accumulates.  Note that the choice of this parameter
      value is related to the buffer capacity available and the data
      rate of the transmission medium; that is, data rate times timer
      value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).

      Notation:

        FO    -  Fragment Offset
        IHL   -  Internet Header Length
        MF    -  More Fragments flag
        TTL   -  Time To Live
        NFB   -  Number of Fragment Blocks
        TL    -  Total Length
        TDL   -  Total Data Length
        BUFID -  Buffer Identifier
        RCVBT -  Fragment Received Bit Table
        TLB   -  Timer Lower Bound

      Procedure:

        (1)  BUFID <- source|destination|protocol|identification;
        (2)  IF FO = 0 AND MF = 0
        (3)     THEN IF buffer with BUFID is allocated
        (4)             THEN flush all reassembly for this BUFID;
        (5)          Submit datagram to next step; DONE.
        (6)     ELSE IF no buffer with BUFID is allocated
        (7)             THEN allocate reassembly resources
                             with BUFID;
                             TIMER <- TLB; TDL <- 0;
        (8)          put data from fragment into data buffer with
                     BUFID from octet FO*8 to
                                         octet (TL-(IHL*4))+FO*8;
        (9)          set RCVBT bits from FO
                                        to FO+((TL-(IHL*4)+7)/8);
        (10)         IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
        (11)         IF FO = 0 THEN put header in header buffer
        (12)         IF TDL # 0
        (13)          AND all RCVBT bits from 0
                                             to (TDL+7)/8 are set
        (14)            THEN TL <- TDL+(IHL*4)
        (15)                 Submit datagram to next step;
        (16)                 free all reassembly resources
                             for this BUFID; DONE.
        (17)         TIMER <- MAX(TIMER,TTL);
        (18)         give up until next fragment or timer expires;
        (19) timer expires: flush all reassembly with this BUFID; DONE.

      In the case that two or more fragments contain the same data


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      either identically or through a partial overlap, this procedure
      will use the more recently arrived copy in the data buffer and
      datagram delivered.

  Identification

    The choice of the Identifier for a datagram is based on the need to
    provide a way to uniquely identify the fragments of a particular
    datagram.  The protocol module assembling fragments judges fragments
    to belong to the same datagram if they have the same source,
    destination, protocol, and Identifier.  Thus, the sender must choose
    the Identifier to be unique for this source, destination pair and
    protocol for the time the datagram (or any fragment of it) could be
    alive in the internet.

    It seems then that a sending protocol module needs to keep a table
    of Identifiers, one entry for each destination it has communicated
    with in the last maximum packet lifetime for the internet.

    However, since the Identifier field allows 65,536 different values,
    some host may be able to simply use unique identifiers independent
    of destination.

    It is appropriate for some higher level protocols to choose the
    identifier. For example, TCP protocol modules may retransmit an
    identical TCP segment, and the probability for correct reception
    would be enhanced if the retransmission carried the same identifier
    as the original transmission since fragments of either datagram
    could be used to construct a correct TCP segment.

  Type of Service

    The type of service (TOS) is for internet service quality selection.
    The type of service is specified along the abstract parameters
    precedence, delay, throughput, and reliability.  These abstract
    parameters are to be mapped into the actual service parameters of
    the particular networks the datagram traverses.

    Precedence.  An independent measure of the importance of this
    datagram.

    Delay.  Prompt delivery is important for datagrams with this
    indication.

    Throughput.  High data rate is important for datagrams with this
    indication.




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    Reliability.  A higher level of effort to ensure delivery is
    important for datagrams with this indication.

    For example, the ARPANET has a priority bit, and a choice between
    "standard" messages (type 0) and "uncontrolled" messages (type 3),
    (the choice between single packet and multipacket messages can also
    be considered a service parameter). The uncontrolled messages tend
    to be less reliably delivered and suffer less delay.  Suppose an
    internet datagram is to be sent through the ARPANET.  Let the
    internet type of service be given as:

      Precedence:    5
      Delay:         0
      Throughput:    1
      Reliability:   1

    In this example, the mapping of these parameters to those available
    for the ARPANET would be  to set the ARPANET priority bit on since
    the Internet precedence is in the upper half of its range, to select
    standard messages since the throughput and reliability requirements
    are indicated and delay is not.  More details are given on service
    mappings in "Service Mappings" [8].

  Time to Live

    The time to live is set by the sender to the maximum time the
    datagram is allowed to be in the internet system.  If the datagram
    is in the internet system longer than the time to live, then the
    datagram must be destroyed.

    This field must be decreased at each point that the internet header
    is processed to reflect the time spent processing the datagram.
    Even if no local information is available on the time actually
    spent, the field must be decremented by 1.  The time is measured in
    units of seconds (i.e. the value 1 means one second).  Thus, the
    maximum time to live is 255 seconds or 4.25 minutes.  Since every
    module that processes a datagram must decrease the TTL by at least
    one even if it process the datagram in less than a second, the TTL
    must be thought of only as an upper bound on the time a datagram may
    exist.  The intention is to cause undeliverable datagrams to be
    discarded, and to bound the maximum datagram lifetime.

    Some higher level reliable connection protocols are based on
    assumptions that old duplicate datagrams will not arrive after a
    certain time elapses.  The TTL is a way for such protocols to have
    an assurance that their assumption is met.




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  Options

    The options are optional in each datagram, but required in
    implementations.  That is, the presence or absence of an option is
    the choice of the sender, but each internet module must be able to
    parse every option.  There can be several options present in the
    option field.

    The options might not end on a 32-bit boundary.  The internet header
    must be filled out with octets of zeros.  The first of these would
    be interpreted as the end-of-options option, and the remainder as
    internet header padding.

    Every internet module must be able to act on every option.  The
    Security Option is required if classified, restricted, or
    compartmented traffic is to be passed.

  Checksum

    The internet header checksum is recomputed if the internet header is
    changed.  For example, a reduction of the time to live, additions or
    changes to internet options, or due to fragmentation.  This checksum
    at the internet level is intended to protect the internet header
    fields from transmission errors.

    There are some applications where a few data bit errors are
    acceptable while retransmission delays are not.  If the internet
    protocol enforced data correctness such applications could not be
    supported.

  Errors

    Internet protocol errors may be reported via the ICMP messages [3].

3.3.  Interfaces

  The functional description of user interfaces to the IP is, at best,
  fictional, since every operating system will have different
  facilities.  Consequently, we must warn readers that different IP
  implementations may have different user interfaces.  However, all IPs
  must provide a certain minimum  set of services to guarantee that all
  IP implementations can support the same protocol hierarchy.  This
  section specifies the functional interfaces required of all IP
  implementations.

  Internet protocol interfaces on one side to the local network and on
  the other side to either a higher level protocol or an application
  program.  In the following, the higher level protocol or application


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Internet Protocol
Specification



  program (or even a gateway program) will be called the "user" since it
  is using the internet module.  Since internet protocol is a datagram
  protocol, there is minimal memory or state maintained between datagram
  transmissions, and each call on the internet protocol module by the
  user supplies all information necessary for the IP to perform the
  service requested.

  An Example Upper Level Interface

  The following two example calls satisfy the requirements for the user
  to internet protocol module communication ("=>" means returns):

  SEND (src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt => result)

    where:

      src = source address
      dst = destination address
      prot = protocol
      TOS = type of service
      TTL = time to live
      BufPTR = buffer pointer
      len = length of buffer
      Id  = Identifier
      DF = Don't Fragment
      opt = option data
      result = response
        OK = datagram sent ok
        Error = error in arguments or local network error

    Note that the precedence is included in the TOS and the
    security/compartment is passed as an option.

  RECV (BufPTR, prot, => result, src, dst, TOS, len, opt)

    where:

      BufPTR = buffer pointer
      prot = protocol
      result = response
        OK = datagram received ok
        Error = error in arguments
      len = length of buffer
      src = source address
      dst = destination address
      TOS = type of service
      opt = option data



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                                                       Internet Protocol
                                                           Specification



  When the user sends a datagram, it executes the SEND call supplying
  all the arguments.  The internet protocol module, on receiving this
  call, checks the arguments and prepares and sends the message.  If the
  arguments are good and the datagram is accepted by the local network,
  the call returns successfully.  If either the arguments are bad, or
  the datagram is not accepted by the local network, the call returns
  unsuccessfully.  On unsuccessful returns, a reasonable report must be
  made as to the cause of the problem, but the details of such reports
  are up to individual implementations.

  When a datagram arrives at the internet protocol module from the local
  network, either there is a pending RECV call from the user addressed
  or there is not.  In the first case, the pending call is satisfied by
  passing the information from the datagram to the user.  In the second
  case, the user addressed is notified of a pending datagram.  If the
  user addressed does not exist, an ICMP error message is returned to
  the sender, and the data is discarded.

  The notification of a user may be via a pseudo interrupt or similar
  mechanism, as appropriate in the particular operating system
  environment of the implementation.

  A user's RECV call may then either be immediately satisfied by a
  pending datagram, or the call may be pending until a datagram arrives.

  The source address is included in the send call in case the sending
  host has several addresses (multiple physical connections or logical
  addresses).  The internet module must check to see that the source
  address is one of the legal address for this host.

  An implementation may also allow or require a call to the internet
  module to indicate interest in or reserve exclusive use of a class of
  datagrams (e.g., all those with a certain value in the protocol
  field).

  This section functionally characterizes a USER/IP interface.  The
  notation used is similar to most procedure of function calls in high
  level languages, but this usage is not meant to rule out trap type
  service calls (e.g., SVCs, UUOs, EMTs), or any other form of
  interprocess communication.

  








                                                               [Page 33]
 




                                                          September 1981
Internet Protocol



APPENDIX A:  Examples & Scenarios

Example 1:

  This is an example of the minimal data carrying internet datagram:

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver= 4 |IHL= 5 |Type of Service|        Total Length = 21      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Identification = 111     |Flg=0|   Fragment Offset = 0   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Time = 123  |  Protocol = 1 |        header checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         source address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      destination address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     data      |                                                
   +-+-+-+-+-+-+-+-+                                                

                       Example Internet Datagram

                               Figure 5.

  Note that each tick mark represents one bit position.

  This is a internet datagram in version 4 of internet protocol; the
  internet header consists of five 32 bit words, and the total length of
  the datagram is 21 octets.  This datagram is a complete datagram (not
  a fragment).


















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September 1981                                                          
                                                       Internet Protocol



Example 2:

  In this example, we show first a moderate size internet datagram (452
  data octets), then two internet fragments that might result from the
  fragmentation of this datagram if the maximum sized transmission
  allowed were 280 octets.

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 472      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Identification = 111      |Flg=0|     Fragment Offset = 0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Time = 123  | Protocol = 6  |        header checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         source address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      destination address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   \                                                               \
   \                                                               \
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             data              |                                
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                

                       Example Internet Datagram

                               Figure 6.

















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                                                          September 1981
Internet Protocol



  Now the first fragment that results from splitting the datagram after
  256 data octets.

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 276      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Identification = 111      |Flg=1|     Fragment Offset = 0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Time = 119  | Protocol = 6  |        Header Checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         source address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      destination address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   \                                                               \
   \                                                               \
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Example Internet Fragment

                               Figure 7.





















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September 1981                                                          
                                                       Internet Protocol



  And the second fragment.

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 216      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Identification = 111      |Flg=0|  Fragment Offset  =  32 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Time = 119  | Protocol = 6  |        Header Checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         source address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      destination address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   \                                                               \
   \                                                               \
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            data               |                                
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                

                       Example Internet Fragment

                               Figure 8.






















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                                                          September 1981
Internet Protocol



Example 3:

  Here, we show an example of a datagram containing options:

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver= 4 |IHL= 8 |Type of Service|       Total Length = 576      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Identification = 111    |Flg=0|     Fragment Offset = 0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Time = 123  |  Protocol = 6 |       Header Checksum         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        source address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      destination address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opt. Code = x | Opt.  Len.= 3 | option value  | Opt. Code = x |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opt. Len. = 4 |           option value        | Opt. Code = 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Opt. Code = y | Opt. Len. = 3 |  option value | Opt. Code = 0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   \                                                               \
   \                                                               \
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Example Internet Datagram

                               Figure 9.
















[Page 38]
 
                                                               



September 1981                                                          
                                                       Internet Protocol



APPENDIX B:  Data Transmission Order

The order of transmission of the header and data described in this
document is resolved to the octet level.  Whenever a diagram shows a
group of octets, the order of transmission of those octets is the normal
order in which they are read in English.  For example, in the following
diagram the octets are transmitted in the order they are numbered.

                                    
    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |       2       |       3       |       4       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |       6       |       7       |       8       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       9       |      10       |      11       |      12       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Transmission Order of Bytes

                               Figure 10.

Whenever an octet represents a numeric quantity the left most bit in the
diagram is the high order or most significant bit.  That is, the bit
labeled 0 is the most significant bit.  For example, the following
diagram represents the value 170 (decimal).

                                    
                            0 1 2 3 4 5 6 7 
                           +-+-+-+-+-+-+-+-+
                           |1 0 1 0 1 0 1 0|
                           +-+-+-+-+-+-+-+-+

                          Significance of Bits

                               Figure 11.

Similarly, whenever a multi-octet field represents a numeric quantity
the left most bit of the whole field is the most significant bit.  When
a multi-octet quantity is transmitted the most significant octet is
transmitted first.









                                                               [Page 39]
 




                                                          September 1981
Internet Protocol






















































[Page 40]
 
                                                               



September 1981                                                          
                                                       Internet Protocol



                                GLOSSARY



1822
          BBN Report 1822, "The Specification of the Interconnection of
          a Host and an IMP".  The specification of interface between a
          host and the ARPANET.

ARPANET leader
          The control information on an ARPANET message at the host-IMP
          interface.

ARPANET message
          The unit of transmission between a host and an IMP in the
          ARPANET.  The maximum size is about 1012 octets (8096 bits).

ARPANET packet
          A unit of transmission used internally in the ARPANET between
          IMPs. The maximum size is about 126 octets (1008 bits).

Destination
          The destination address, an internet header field.

DF
          The Don't Fragment bit carried in the flags field.

Flags
          An internet header field carrying various control flags.

Fragment Offset
          This internet header field indicates where in the internet
          datagram a fragment belongs.

GGP
          Gateway to Gateway Protocol, the protocol used primarily
          between gateways to control routing and other gateway
          functions.

header
          Control information at the beginning of a message, segment,
          datagram, packet or block of data.

ICMP
          Internet Control Message Protocol, implemented in the internet
          module, the ICMP is used from gateways to hosts and between
          hosts to report errors and make routing suggestions.




                                                               [Page 41]
 




                                                          September 1981
Internet Protocol
Glossary



Identification
          An internet header field carrying the identifying value
          assigned by the sender to aid in assembling the fragments of a
          datagram.

IHL
          The internet header field Internet Header Length is the length
          of the internet header measured in 32 bit words.

IMP
          The Interface Message Processor, the packet switch of the
          ARPANET.

Internet Address
          A four octet (32 bit) source or destination address consisting
          of a Network field and a Local Address field.

internet datagram
          The unit of data exchanged between a pair of internet modules
          (includes the internet header).

internet fragment
          A portion of the data of an internet datagram with an internet
          header.

Local Address
          The address of a host within a network.  The actual mapping of
          an internet local address on to the host addresses in a
          network is quite general, allowing for many to one mappings.

MF
          The More-Fragments Flag carried in the internet header flags
          field.

module
          An implementation, usually in software, of a protocol or other
          procedure.

more-fragments flag
          A flag indicating whether or not this internet datagram
          contains the end of an internet datagram, carried in the
          internet header Flags field.

NFB
          The Number of Fragment Blocks in a the data portion of an
          internet fragment.  That is, the length of a portion of data
          measured in 8 octet units.



[Page 42]
 
                                                               



September 1981                                                          
                                                       Internet Protocol
                                                                Glossary



octet
          An eight bit byte.

Options
          The internet header Options field may contain several options,
          and each option may be several octets in length.

Padding
          The internet header Padding field is used to ensure that the
          data begins on 32 bit word boundary.  The padding is zero.

Protocol
          In this document, the next higher level protocol identifier,
          an internet header field.

Rest
          The local address portion of an Internet Address.

Source
          The source address, an internet header field.

TCP
          Transmission Control Protocol:  A host-to-host protocol for
          reliable communication in internet environments.

TCP Segment
          The unit of data exchanged between TCP modules (including the
          TCP header).

TFTP
          Trivial File Transfer Protocol:  A simple file transfer
          protocol built on UDP.

Time to Live
          An internet header field which indicates the upper bound on
          how long this internet datagram may exist.

TOS
          Type of Service

Total Length
          The internet header field Total Length is the length of the
          datagram in octets including internet header and data.

TTL
          Time to Live




                                                               [Page 43]
 




                                                          September 1981
Internet Protocol
Glossary



Type of Service
          An internet header field which indicates the type (or quality)
          of service for this internet datagram.

UDP
          User Datagram Protocol:  A user level protocol for transaction
          oriented applications.

User
          The user of the internet protocol.  This may be a higher level
          protocol module, an application program, or a gateway program.

Version
          The Version field indicates the format of the internet header.




































[Page 44]
 
                                                               



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                                                       Internet Protocol



                               REFERENCES



[1]  Cerf, V., "The Catenet Model for Internetworking," Information
     Processing Techniques Office, Defense Advanced Research Projects
     Agency, IEN 48, July 1978.

[2]  Bolt Beranek and Newman, "Specification for the Interconnection of
     a Host and an IMP," BBN Technical Report 1822, Revised May 1978.

[3]  Postel, J., "Internet Control Message Protocol - DARPA Internet
     Program Protocol Specification," RFC 792, USC/Information Sciences
     Institute, September 1981.

[4]  Shoch, J., "Inter-Network Naming, Addressing, and Routing,"
     COMPCON, IEEE Computer Society, Fall 1978.

[5]  Postel, J., "Address Mappings," RFC 796, USC/Information Sciences
     Institute, September 1981.

[6]  Shoch, J., "Packet Fragmentation in Inter-Network Protocols,"
     Computer Networks, v. 3, n. 1, February 1979.

[7]  Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and
     Newman, August 1979.

[8]  Postel, J., "Service Mappings," RFC 795, USC/Information Sciences
     Institute, September 1981.

[9]  Postel, J., "Assigned Numbers," RFC 790, USC/Information Sciences
     Institute, September 1981.



















                                                               [Page 45]
 




========================================================================

Network Working Group                                J. Mogul (Stanford)
Request for Comments: 950                                J. Postel (ISI)
                                                             August 1985

                 Internet Standard Subnetting Procedure


Status Of This Memo

   This RFC specifies a protocol for the ARPA-Internet community.  If
   subnetting is implemented it is strongly recommended that these
   procedures be followed.  Distribution of this memo is unlimited.

Overview

   This memo discusses the utility of "subnets" of Internet networks,
   which are logically visible sub-sections of a single Internet
   network.  For administrative or technical reasons, many organizations
   have chosen to divide one Internet network into several subnets,
   instead of acquiring a set of Internet network numbers.  This memo
   specifies procedures for the use of subnets.  These procedures are
   for hosts (e.g., workstations).  The procedures used in and between
   subnet gateways are not fully described.  Important motivation and
   background information for a subnetting standard is provided in
   RFC-940 [7].

Acknowledgment

   This memo is based on RFC-917 [1].  Many people contributed to the
   development of the concepts described here.  J. Noel Chiappa, Chris
   Kent, and Tim Mann, in particular, provided important suggestions.
   Additional contributions in shaping this memo were made by Zaw-Sing
   Su, Mike Karels, and the Gateway Algorithms and Data Structures Task
   Force (GADS).



















Mogul & Postel                                                  [Page 1]
 





RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


1.  Motivation

   The original view of the Internet universe was a two-level hierarchy:
   the top level the Internet as a whole, and the level below it
   individual networks, each with its own network number.  The Internet
   does not have a hierarchical topology, rather the interpretation of
   addresses is hierarchical.  In this two-level model, each host sees
   its network as a single entity; that is, the network may be treated
   as a "black box" to which a set of hosts is connected.

   While this view has proved simple and powerful, a number of
   organizations have found it inadequate, and have added a third level
   to the interpretation of Internet addresses.  In this view, a given
   Internet network is divided into a collection of subnets.

   The three-level model is useful in networks belonging to moderately
   large organizations (e.g., Universities or companies with more than
   one building), where it is often necessary to use more than one LAN
   cable to cover a "local area".  Each LAN may then be treated as a
   subnet.

   There are several reasons why an organization might use more than one
   cable to cover a campus:

      - Different technologies:  Especially in a research environment,
        there may be more than one kind of LAN in use; e.g., an
        organization may have some equipment that supports Ethernet, and
        some that supports a ring network.

      - Limits of technologies:  Most LAN technologies impose limits,
        based on electrical parameters, on the number of hosts
        connected, and on the total length of the cable.  It is easy to
        exceed these limits, especially those on cable length.

      - Network congestion:  It is possible for a small subset of the
        hosts on a LAN to monopolize most of the bandwidth.  A common
        solution to this problem is to divide the hosts into cliques of
        high mutual communication, and put these cliques on separate
        cables.

      - Point-to-Point links:  Sometimes a "local area", such as a
        university campus, is split into two locations too far apart to
        connect using the preferred LAN technology.  In this case,
        high-speed point-to-point links might connect several LANs.

   An organization that has been forced to use more than one LAN has
   three choices for assigning Internet addresses:


Mogul & Postel                                                  [Page 2]
 





RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


      1. Acquire a distinct Internet network number for each cable;
         subnets are not used at all.

      2. Use a single network number for the entire organization, but
         assign host numbers without regard to which LAN a host is on
         ("transparent subnets").

      3. Use a single network number, and partition the host address
         space by assigning subnet numbers to the LANs ("explicit
         subnets").

   Each of these approaches has disadvantages.  The first, although not
   requiring any new or modified protocols, results in an explosion in
   the size of Internet routing tables.  Information about the internal
   details of local connectivity is propagated everywhere, although it
   is of little or no use outside the local organization.  Especially as
   some current gateway implementations do not have much space for
   routing tables, it would be good to avoid this problem.

   The second approach requires some convention or protocol that makes
   the collection of LANs appear to be a single Internet network.  For
   example, this can be done on LANs where each Internet address is
   translated to a hardware address using an Address Resolution Protocol
   (ARP), by having the bridges between the LANs intercept ARP requests
   for non-local targets, see RFC-925 [2].  However, it is not possible
   to do this for all LAN technologies, especially those where ARP
   protocols are not currently used, or if the LAN does not support
   broadcasts.  A more fundamental problem is that bridges must discover
   which LAN a host is on, perhaps by using a broadcast algorithm.  As
   the number of LANs grows, the cost of broadcasting grows as well;
   also, the size of translation caches required in the bridges grows
   with the total number of hosts in the network.

   The third approach is to explicitly support subnets.  This does have
   a disadvantage, in that it is a modification of the Internet
   Protocol, and thus requires changes to IP implementations already in
   use (if these implementations are to be used on a subnetted network).
   However, these changes are relatively minor, and once made, yield a
   simple and efficient solution to the problem.  Also, the approach
   avoids any changes that would be incompatible with existing hosts on
   non-subnetted networks.

   Further, when appropriate design choices are made, it is possible for
   hosts which believe they are on a non-subnetted network to be used on
   a subnetted one, as explained in RFC-917 [1].  This is useful when it
   is not possible to modify some of the hosts to support subnets
   explicitly, or when a gradual transition is preferred.


Mogul & Postel                                                  [Page 3]
 





RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


2.  Standards for Subnet Addressing

   This section first describes a proposal for interpretation of
   Internet addresses to support subnets.  Next it discusses changes to
   host software to support subnets.  Finally, it presents a procedures
   for discovering what address interpretation is in use on a given
   network (i.e., what address mask is in use).

   2.1. Interpretation of Internet Addresses

      Suppose that an organization has been assigned an Internet network
      number, has further divided that network into a set of subnets,
      and wants to assign host addresses: how should this be done?
      Since there are minimal restrictions on the assignment of the
      "local address" part of the Internet address, several approaches
      have been proposed for representing the subnet number:

         1. Variable-width field:  Any number of the bits of the local
            address part are used for the subnet number; the size of
            this field, although constant for a given network, varies
            from network to network.  If the field width is zero, then
            subnets are not in use.

         2. Fixed-width field:  A specific number of bits (e.g., eight)
            is used for the subnet number, if subnets are in use.

         3. Self-encoding variable-width field:  Just as the width
            (i.e., class) of the network number field is encoded by its
            high-order bits, the width of the subnet field is similarly
            encoded.

         4. Self-encoding fixed-width field:  A specific number of bits
            is used for the subnet number.

         5. Masked bits:  Use a bit mask ("address mask") to identify
            which bits of the local address field indicate the subnet
            number.

      What criteria can be used to choose one of these five schemes?
      First, should we use a self-encoding scheme?  And, should it be
      possible to tell from examining an Internet address if it refers
      to a subnetted network, without reference to any other
      information?

         An interesting feature of self-encoding is that it allows the




Mogul & Postel                                                  [Page 4]
 





RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         address space of a network to be divided into subnets of
         different sizes, typically one subnet of half the address space
         and a set of small subnets.

            For example, consider a class C network that uses a
            self-encoding scheme with one bit to indicate if it is the
            large subnet or not and an additional three bits to identify
            the small subnet.  If the first bit is zero then this is the
            large subnet, if the first bit is one then the following
            bits (3 in this example) give the subnet number.  There is
            one subnet with 128 host addresses, and eight subnets with
            16 hosts each.

         To establish a subnetting standard the parameters and
         interpretation of the self-encoding scheme must be fixed and
         consistent throughout the Internet.

         It could be assumed that all networks are subnetted.  This
         would allow addresses to be interpreted without reference to
         any other information.

            This is a significant advantage, that given the Internet
            address no additional information is needed for an
            implementation to determine if two addresses are on the same
            subnet.  However, this can also be viewed as a disadvantage:
            it may cause problems for networks which have existing host
            numbers that use arbitrary bits in the local address part.
            In other words, it is useful to be able to control whether a
            network is subnetted independently from the assignment of
            host addresses.

         The alternative is to have the fact that a network is subnetted
         kept separate from the address.  If one finds, somehow, that
         the network is subnetted then the standard self-encoded
         subnetted network address rules are followed, otherwise the
         non-subnetted network addressing rules are followed.

      If a self-encoding scheme is not used, there is no reason to use a
      fixed-width field scheme: since there must in any case be some
      per-network "flag" to indicate if subnets are in use, the
      additional cost of using an integer (a subnet field width or
      address mask) instead of a boolean is negligible.  The advantage
      of using the address mask scheme is that it allows each
      organization to choose the best way to allocate relatively scarce
      bits of local address to subnet and host numbers.  Therefore, we
      choose the address-mask scheme: it is the most flexible scheme,
      yet costs no more to implement than any other.


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


      For example, the Internet address might be interpreted as:

         <network-number><subnet-number><host-number>

      where the <network-number> field is as defined by IP [3], the
      <host-number> field is at least 1-bit wide, and the width of the
      <subnet-number> field is constant for a given network.  No further
      structure is required for the <subnet-number> or <host-number>
      fields.  If the width of the <subnet-number> field is zero, then
      the network is not subnetted (i.e., the interpretation of [3] is
      used).

      For example, on a Class B network with a 6-bit wide subnet field,
      an address would be broken down like this:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 0|        NETWORK            |  SUBNET   |    Host Number    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Since the bits that identify the subnet are specified by a
      bitmask, they need not be adjacent in the address.  However, we
      recommend that the subnet bits be contiguous and located as the
      most significant bits of the local address.

      Special Addresses:

         From the Assigned Numbers memo [9]:

            "In certain contexts, it is useful to have fixed addresses
            with functional significance rather than as identifiers of
            specific hosts.  When such usage is called for, the address
            zero is to be interpreted as meaning "this", as in "this
            network".  The address of all ones are to be interpreted as
            meaning "all", as in "all hosts".  For example, the address
            128.9.255.255 could be interpreted as meaning all hosts on
            the network 128.9.  Or, the address 0.0.0.37 could be
            interpreted as meaning host 37 on this network."

         It is useful to preserve and extend the interpretation of these
         special addresses in subnetted networks.  This means the values
         of all zeros and all ones in the subnet field should not be
         assigned to actual (physical) subnets.

            In the example above, the 6-bit wide subnet field may have
            any value except 0 and 63.


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         Please note that there is no effect or new restriction on the
         addresses of hosts on non-subnetted networks.

   2.2. Changes to Host Software to Support Subnets

      In most implementations of IP, there is code in the module that
      handles outgoing datagrams to decide if a datagram can be sent
      directly to the destination on the local network or if it must be
      sent to a gateway.

      Generally the code is something like this:

         IF ip_net_number(dg.ip_dest) = ip_net_number(my_ip_addr)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                                  gateway_to(ip_net_number(dg.ip_dest)))

      (If the code supports multiply-connected networks, it will be more
      complicated, but this is irrelevant to the current discussion.)

      To support subnets, it is necessary to store one more 32-bit
      quantity, called my_ip_mask.  This is a bit-mask with bits set in
      the fields corresponding to the IP network number, and additional
      bits set corresponding to the subnet number field.

      The code then becomes:

         IF bitwise_and(dg.ip_dest, my_ip_mask)
                                   = bitwise_and(my_ip_addr, my_ip_mask)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                        gateway_to(bitwise_and(dg.ip_dest, my_ip_mask)))

      Of course, part of the expression in the conditional can be
      pre-computed.

      It may or may not be necessary to modify the "gateway_to"
      function, so that it too takes the subnet field bits into account
      when performing comparisons.

      To support multiply-connected hosts, the code can be changed to




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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


      keep  the "my_ip_addr" and "my_ip_mask" quantities on a
      per-interface basis; the expression in the conditional must then
      be evaluated for each interface.

   2.3. Finding the Address Mask

      How can a host determine what address mask is in use on a subnet
      to which it is connected?  The problem is analogous to several
      other "bootstrapping" problems for Internet hosts: how a host
      determines its own address, and how it locates a gateway on its
      local network.  In all three cases, there are two basic solutions:
      "hardwired" information, and broadcast-based protocols.

      Hardwired information is that available to a host in isolation
      from a network.  It may be compiled-in, or (preferably) stored in
      a disk file.  However, for the increasingly common case of a
      diskless workstation that is bootloaded over a LAN, neither
      hardwired solution is satisfactory.

      Instead, since most LAN technology supports broadcasting, a better
      method is for the newly-booted host to broadcast a request for the
      necessary information.  For example, for the purpose of
      determining its Internet address, a host may use the "Reverse
      Address Resolution Protocol" (RARP) [4].

      However, since a newly-booted host usually needs to gather several
      facts (e.g., its IP address, the hardware address of a gateway,
      the IP address of a domain name server, the subnet address mask),
      it would be better to acquire all this information in one request
      if possible, rather than doing numerous broadcasts on the network.
      The mechanisms designed to boot diskless workstations can also
      load per-host specific configuration files that contain the
      required information (e.g., see RFC-951 [8]).  It is possible, and
      desirable, to obtain all the facts necessary to operate a host
      from a boot server using only one broadcast message.

      In the case where it is necessary for a host to find the address
      mask as a separate operation the following mechanism is provided:

         To provide the address mask information the ICMP protocol [5]
         is extended by adding a new pair of ICMP message types,
         "Address Mask Request" and "Address Mask Reply", analogous to
         the "Information Request" and "Information Reply" ICMP
         messages.  These are described in detail in Appendix I.

         The intended use of these new ICMP messages is that a host,
         when booting, broadcast an "Address Mask Request" message.  A


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         gateway (or a host acting in lieu of a gateway) that receives
         this message responds with an "Address Mask Reply".  If there
         is no indication in the request which host sent it (i.e., the
         IP Source Address is zero), the reply is broadcast as well.
         The requesting host will hear the response, and from it
         determine the address mask.

         Since there is only one possible value that can be sent in an
         "Address Mask Reply" on any given LAN, there is no need for the
         requesting host to match the responses it hears against the
         request it sent; similarly, there is no problem if more than
         one gateway responds.  We assume that hosts reboot
         infrequently, so the broadcast load on a network from use of
         this protocol should be small.

      If a host is connected to more than one LAN, it might have to find
      the address mask for each.

      One potential problem is what a host should do if it can not find
      out the address mask, even after a reasonable number of tries.
      Three interpretations can be placed on the situation:

         1. The local net exists in (permanent) isolation from all other
            nets.

         2. Subnets are not in use, and no host can supply the address
            mask.

         3. All gateways on the local net are (temporarily) down.

      The first and second situations imply that the address mask is
      identical with the Internet network number mask.  In the third
      situation, there is no way to determine what the proper value is;
      the safest choice is thus a mask identical with the Internet
      network number mask.  Although this might later turn out to be
      wrong, it will not prevent transmissions that would otherwise
      succeed.  It is possible for a host to recover from a wrong
      choice: when a gateway comes up, it should broadcast an "Address
      Mask Reply"; when a host receives such a message that disagrees
      with its guess, it should change its mask to conform to the
      received value.  No host or gateway should send an "Address Mask
      Reply" based on a "guessed" value.

      Finally, note that no host is required to use this ICMP protocol
      to discover the address mask; it is perfectly reasonable for a
      host with non-volatile storage to use stored information
      (including a configuration file from a boot server).


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


Appendix I.  Address Mask ICMP

   Address Mask Request or Address Mask Reply

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |      Code     |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Identifier          |       Sequence Number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Address Mask                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      IP Fields:

         Addresses

            The address of the source in an address mask request message
            will be the destination of the address mask reply message.
            To form an address mask reply message, the source address of
            the request becomes the destination address of the reply,
            the source address of the reply is set to the replier's
            address, the type code changed to AM2, the address mask
            value inserted into the Address Mask field, and the checksum
            recomputed.  However, if the source address in the request
            message is zero, then the destination address for the reply
            message should denote a broadcast.

      ICMP Fields:

         Type

            AM1 for address mask request message

            AM2 for address mask reply message

         Code

            0 for address mask request message

            0 for address mask reply message

         Checksum

            The checksum is the 16-bit one's complement of the one's



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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


            complement sum of the ICMP message starting with the ICMP
            Type.  For computing the checksum, the checksum field should
            be zero.  This checksum may be replaced in the future.

         Identifier

            An identifier to aid in matching requests and replies, may
            be zero.

         Sequence Number

            A sequence number to aid in matching requests and replies,
            may be zero.

         Address Mask

            A 32-bit mask.

      Description

         A gateway receiving an address mask request should return it
         with the address mask field set to the 32-bit mask of the bits
         identifying the subnet and network, for the subnet on which the
         request was received.

         If the requesting host does not know its own IP address, it may
         leave the source field zero; the reply should then be
         broadcast.  However, this approach should be avoided if at all
         possible, since it increases the superfluous broadcast load on
         the network.  Even when the replies are broadcast, since there
         is only one possible address mask for a subnet, there is no
         need to match requests with replies.  The "Identifier" and
         "Sequence Number" fields can be ignored.

            Type AM1 may be received from a gateway or a host.

            Type AM2 may be received from a gateway, or a host acting in
            lieu of a gateway.











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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


Appendix II.  Examples

   These examples show how a host can find out the address mask using
   the ICMP Address Mask Request and Address Mask Reply messages.  For
   the following examples, assume that address 255.255.255.255 denotes
   "broadcast to this physical medium" [6].

   1.  A Class A Network Case

      For this case, assume that the requesting host is on class A
      network 36.0.0.0, has address 36.40.0.123, that there is a gateway
      at 36.40.0.62, and that a 8-bit wide subnet field is in use, that
      is, the address mask is 255.255.0.0.

      The most efficient method, and the one we recommend, is for a host
      to first discover its own address (perhaps using "RARP" [4]), and
      then to send the ICMP request to 255.255.255.255:

         Source address:          36.40.0.123
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      The gateway can then respond directly to the requesting host.

         Source address:          36.40.0.62
         Destination address:     36.40.0.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

      Suppose that 36.40.0.123 is a diskless workstation, and does not
      know even its own host number.  It could send the following
      datagram:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      36.40.0.62 will hear the datagram, and should respond with this
      datagram:


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         Source address:          36.40.0.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

      Note that the gateway uses the narrowest possible broadcast to
      reply.  Even so, the over use of broadcasts presents an
      unnecessary load to all hosts on the subnet, and so the use of the
      "anonymous" (0.0.0.0) source address must be kept to a minimum.

      If broadcasting is not allowed, we assume that hosts have wired-in
      information about neighbor gateways; thus, 36.40.0.123 might send
      this datagram:

         Source address:          36.40.0.123
         Destination address:     36.40.0.62
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      36.40.0.62 should respond exactly as in the previous case.

         Source address:          36.40.0.62
         Destination address:     36.40.0.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

   2.  A Class B Network Case

      For this case, assume that the requesting host is on class B
      network 128.99.0.0, has address 128.99.4.123, that there is a
      gateway at 128.99.4.62, and that a 6-bit wide subnet field is in
      use, that is, the address mask is 255.255.252.0.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          128.99.4.123
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


      The gateway can then respond directly to the requesting host.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      128.99.4.62 will hear the datagram, and should respond with this
      datagram:

         Source address:          128.99.4.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      If broadcasting is not allowed 128.99.4.123 sends:

         Source address:          128.99.4.123
         Destination address:     128.99.4.62
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      128.99.4.62 should respond exactly as in the previous case.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0




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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


   3.  A Class C Network Case (illustrating non-contiguous subnet bits)

      For this case, assume that the requesting host is on class C
      network 192.1.127.0, has address 192.1.127.19, that there is a
      gateway at 192.1.127.50, and that on network an 3-bit subnet field
      is in use (01011000), that is, the address mask is 255.255.255.88.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          192.1.127.19
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      The gateway can then respond directly to the requesting host.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      192.1.127.50 will hear the datagram, and should respond with this
      datagram:

         Source address:          192.1.127.50
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      If broadcasting is not allowed 192.1.127.19 sends:




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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         Source address:          192.1.127.19
         Destination address:     192.1.127.50
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      192.1.127.50 should respond exactly as in the previous case.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88

Appendix III.  Glossary

   Bridge

      A node connected to two or more administratively indistinguishable
      but physically distinct subnets, that automatically forwards
      datagrams when necessary, but whose existence is not known to
      other hosts.  Also called a "software repeater".

   Gateway

      A node connected to two or more administratively distinct networks
      and/or subnets, to which hosts send datagrams to be forwarded.

   Host Field

      The bit field in an Internet address used for denoting a specific
      host.

   Internet

      The collection of connected networks using the IP protocol.

   Local Address

      The rest field of the Internet address (as defined in [3]).

   Network

      A single Internet network (which may or may not be divided into
      subnets).


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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


   Network Number

      The network field of the Internet address.

   Subnet

      One or more physical networks forming a subset of an Internet
      network.  A subnet is explicitly identified in the Internet
      address.

   Subnet Field

      The bit field in an Internet address denoting the subnet number.
      The bits making up this field are not necessarily contiguous in
      the address.

   Subnet Number

      A number identifying a subnet within a network.

Appendix IV.  Assigned Numbers

   The following assignments are made for protocol parameters used in
   the support of subnets.  The only assignments needed are for the
   Internet Control Message Protocol (ICMP) [5].

   ICMP Message Types

      AM1 = 17

      AM2 = 18


















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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


References

   [1]  Mogul, J., "Internet Subnets", RFC-917, Stanford University,
        October 1984.

   [2]  Postel, J., "Multi-LAN Address Resolution", RFC-925,
        USC/Information Sciences Institute, October 1984.

   [3]  Postel, J., "Internet Protocol", RFC-791, USC/Information
        Sciences Institute, September 1981.

   [4]  Finlayson, R., T. Mann, J. Mogul, M. Theimer, "A Reverse Address
        Resolution Protocol", RFC-903, Stanford University, June 1984.

   [5]  Postel, J., "Internet Control Message Protocol", RFC-792,
        USC/Information Sciences Institute, September 1981.

   [6]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
        University, October 1984.

   [7]  GADS, "Towards an Internet Standard Scheme for Subnetting",
        RFC-940, Network Information Center, SRI International,
        April 1985.

   [8]  Croft, B., and J. Gilmore, "BOOTP -- UDP Bootstrap Protocol",
        RFC-951, Stanford University, August 1985.

   [9]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-943,
        USC/Information Sciences Institute, April 1985.

   


















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========================================================================

Network Working Group                                      Jeffrey Mogul
Request for Comments: 919                    Computer Science Department
                                                     Stanford University
                                                            October 1984

                     BROADCASTING INTERNET DATAGRAMS


Status of this Memo

   We propose simple rules for broadcasting Internet datagrams on local
   networks that support broadcast, for addressing broadcasts, and for
   how gateways should handle them.

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

Acknowledgement

   This proposal is the result of discussion with several other people,
   especially J. Noel Chiappa and Christopher A. Kent, both of whom both
   pointed me at important references.

1. Introduction

   The use of broadcasts, especially on high-speed local area networks,
   is a good base for many applications.  Since broadcasting is not
   covered in the basic IP specification [13], there is no agreed-upon
   way to do it, and so protocol designers have not made use of it. (The
   issue has been touched upon before, e.g. [6], but has not been the
   subject of a standard.)

   We consider here only the case of unreliable, unsequenced, possibly
   duplicated datagram broadcasts (for a discussion of TCP broadcasting,
   see [11].) Even though unreliable and limited in length, datagram
   broadcasts are quite useful [1].

   We assume that the data link layer of the local network supports
   efficient broadcasting.  Most common local area networks do support
   broadcast; for example, Ethernet [7, 5], ChaosNet [10], token ring
   networks [2], etc.

   We do not assume, however, that broadcasts are reliably delivered.
   (One might consider providing a reliable broadcast protocol as a
   layer above IP.) It is quite expensive to guarantee delivery of
   broadcasts; instead, what we assume is that a host will receive most
   of the broadcasts that are sent.  This is important to avoid
   excessive use of broadcasts; since every host on the network devotes
   at least some effort to every broadcast, they are costly.



Mogul                                                           [Page 1]
 





RFC 919                                                     October 1984
Broadcasting Internet Datagrams


   When a datagram is broadcast, it imposes a cost on every host that
   hears it.  Therefore, broadcasting should not be used
   indiscriminately, but rather only when it is the best solution to a
   problem.

   Note: some organizations have divided their IP networks into subnets,
   for which a standard [8] has been proposed.  This RFC does not cover
   the numerous complications arising from the interactions between
   subnets and broadcasting; see [9] for a complete discussion.

2. Terminology

   Because broadcasting depends on the specific data link layer in use
   on a local network, we must discuss it with reference to both
   physical networks and logical networks.

   The terms we will use in referring to physical networks are, from the
   point of view of the host sending or forwarding a broadcast:

   Local Hardware Network

      The physical link to which the host is attached.

   Remote Hardware Network

      A physical network which is separated from the host by at least
      one gateway.

   Collection of Hardware Networks

      A set of hardware networks (transitively) connected by gateways.

   The IP world includes several kinds of logical network.  To avoid
   ambiguity, we will use the following terms:

   Internet

      The DARPA Internet collection of IP networks.

   IP Network

      One or a collection of several hardware networks that have one
      specific IP network number.






Mogul                                                           [Page 2]
 





RFC 919                                                     October 1984
Broadcasting Internet Datagrams


3. Why Broadcast?

   Broadcasts are useful when a host needs to find information without
   knowing exactly what other host can supply it, or when a host wants
   to provide information to a large set of hosts in a timely manner.

   When a host needs information that one or more of its neighbors might
   have, it could have a list of neighbors to ask, or it could poll all
   of its possible neighbors until one responds.  Use of a wired-in list
   creates obvious network management problems (early binding is
   inflexible).  On the other hand, asking all of one's neighbors is
   slow if one must generate plausible host addresses, and try them
   until one works.  On the ARPANET, for example, there are roughly 65
   thousand plausible host numbers.  Most IP implementations have used
   wired-in lists (for example, addresses of "Prime" gateways.)
   Fortunately, broadcasting provides a fast and simple way for a host
   to reach all of its neighbors.

   A host might also use a broadcast to provide all of its neighbors
   with some information; for example, a gateway might announce its
   presence to other gateways.

   One way to view broadcasting is as an imperfect substitute for
   multicasting, the sending of messages to a subset of the hosts on a
   network.  In practice, broadcasts are usually used where multicasts
   are what is wanted; packets are broadcast at the hardware level, but
   filtering software in the receiving hosts gives the effect of
   multicasting.

   For more examples of broadcast applications, see [1, 3].

4. Broadcast Classes

   There are several classes of IP broadcasting:

      - Single-destination datagram broadcast on the local IP net: A
        datagrams is destined for a specific IP host, but the sending
        host broadcasts it at the data link layer, perhaps to avoid
        having to do routing.  Since this is not an IP broadcast, the IP
        layer is not involved, except that a host should discard
        datagrams not meant for it without becoming flustered (i.e.,
        printing an error message).

      - Broadcast to all hosts on the local IP net: A distinguished
        value for the host-number part of the IP address denotes
        broadcast instead of a specific host.  The receiving IP layer
        must be able to recognize this address as well as its own.


Mogul                                                           [Page 3]
 





RFC 919                                                     October 1984
Broadcasting Internet Datagrams


        However, it might still be useful to distinguish at higher
        levels between broadcasts and non-broadcasts, especially in
        gateways. This is the most useful case of broadcast; it allows a
        host to discover gateways without wired-in tables, it is the
        basis for address resolution protocols, and it is also useful
        for accessing such utilities as name servers, time servers,
        etc., without requiring wired-in addresses.

      - Broadcast to all hosts on a remote IP network: It is
        occasionally useful to send a broadcast to all hosts on a
        non-local network; for example, to find the latest version of a
        hostname database, to bootload a host on an IP network without a
        bootserver, or to monitor the timeservers on the IP network.
        This case is the same as local-network broadcasts; the datagram
        is routed by normal mechanisms until it reaches a gateway
        attached to the destination IP network, at which point it is
        broadcast. This class of broadcasting is also known as "directed
        broadcasting", or quaintly as sending a "letter bomb" [1].

      - Broadcast to the entire Internet: This is probably not useful,
        and almost certainly not desirable.

   For reasons of performance or security, a gateway may choose not to
   forward broadcasts; especially, it may be a good idea to ban
   broadcasts into or out of an autonomous group of networks.

5. Broadcast Methods

   A host's IP receiving layer must be modified to support broadcasting.
   In the absence of broadcasting, a host determines if it is the
   recipient of a datagram by matching the destination address against
   all of its IP addresses.  With broadcasting, a host must compare the
   destination address not only against the host's addresses, but also
   against the possible broadcast addresses for that host.

   The problem of how best to send a broadcast has been extensively
   discussed [1, 3, 4, 14, 15].  Since we assume that the problem has
   already been solved at the data link layer, an IP host wishing to
   send either a local broadcast or a directed broadcast need only
   specify the appropriate destination address and send the datagram as
   usual.  Any sophisticated algorithms need only reside in gateways.








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RFC 919                                                     October 1984
Broadcasting Internet Datagrams


6. Gateways and Broadcasts

   Most of the complexity in supporting broadcasts lies in gateways.  If
   a gateway receives a directed broadcast for a network to which it is
   not connected, it simply forwards it using the usual mechanism.
   Otherwise, it must do some additional work.

   When a gateway receives a local broadcast datagram, there are several
   things it might have to do with it.  The situation is unambiguous,
   but without due care it is possible to create infinite loops.

   The appropriate action to take on receipt of a broadcast datagram
   depends on several things: the subnet it was received on, the
   destination network, and the addresses of the gateway.

      - The primary rule for avoiding loops is "never broadcast a
        datagram on the hardware network it was received on". It is not
        sufficient simply to avoid repeating datagrams that a gateway
        has heard from itself; this still allows loops if there are
        several gateways on a hardware network.

      - If the datagram is received on the hardware network to which it
        is addressed, then it should not be forwarded.  However, the
        gateway should consider itself to be a destination of the
        datagram (for example, it might be a routing table update.)

      - Otherwise, if the datagram is addressed to a hardware network to
        which the gateway is connected, it should be sent as a (data
        link layer) broadcast on that network.  Again, the gateway
        should consider itself a destination of the datagram.

      - Otherwise, the gateway should use its normal routing procedure
        to choose a subsequent gateway, and send the datagram along to
        it.

7. Broadcast IP Addressing - Proposed Standards

   If different IP implementations are to be compatible, there must be a
   distinguished number to denote "all hosts".

   Since the local network layer can always map an IP address into data
   link layer address, the choice of an IP "broadcast host number" is
   somewhat arbitrary.  For simplicity, it should be one not likely to
   be assigned to a real host.  The number whose bits are all ones has
   this property; this assignment was first proposed in [6].  In the few
   cases where a host has been assigned an address with a host-number
   part of all ones, it does not seem onerous to require renumbering.


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RFC 919                                                     October 1984
Broadcasting Internet Datagrams


   The address 255.255.255.255 denotes a broadcast on a local hardware
   network, which must not be forwarded.  This address may be used, for
   example, by hosts that do not know their network number and are
   asking some server for it.

   Thus, a host on net 36, for example, may:

      - broadcast to all of its immediate neighbors by using
        255.255.255.255

      - broadcast to all of net 36 by using 36.255.255.255

   (Note that unless the network has been broken up into subnets, these
   two methods have identical effects.)

   If the use of "all ones" in a field of an IP address means
   "broadcast", using "all zeros" could be viewed as meaning
   "unspecified".  There is probably no reason for such addresses to
   appear anywhere but as the source address of an ICMP Information
   Request datagram.  However, as a notational convention, we refer to
   networks (as opposed to hosts) by using addresses with zero fields.
   For example, 36.0.0.0 means "network number 36" while 36.255.255.255
   means "all hosts on network number 36".

   7.1. ARP Servers and Broadcasts

      The Address Resolution Protocol (ARP) described in [12] can, if
      incorrectly implemented, cause problems when broadcasts are used
      on a network where not all hosts share an understanding of what a
      broadcast address is.  The temptation exists to modify the ARP
      server so that it provides the mapping between an IP broadcast
      address and the hardware broadcast address.

      This temptation must be resisted.  An ARP server should never
      respond to a request whose target is a broadcast address.  Such a
      request can only come from a host that does not recognize the
      broadcast address as such, and so honoring it would almost
      certainly lead to a forwarding loop.  If there are N such hosts on
      the physical network that do not recognize this address as a
      broadcast, then a datagram sent with a Time-To-Live of T could
      potentially give rise to T**N spurious re-broadcasts.








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RFC 919                                                     October 1984
Broadcasting Internet Datagrams


8. References

   1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
        University, January 1982.

   2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
        Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516, 1978.

   3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
        Computer Networks.  Ph.D. Th., Stanford University, April 1977.

   4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
        of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048, December
        1978.

   5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
        Layer Specifications.  Version 1.0, Digital Equipment
        Corporation, Intel, Xerox, September 1980.

   6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
        Addressing Issues.  IEN-212, Bolt Beranek and Newman, September
        1982.

   7.    R.M. Metcalfe and D.R. Boggs. "Ethernet: Distributed Packet
        Switching for Local Computer Networks".  Comm. ACM 19, 7,
        pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
        Center, reprinted in CSL-80-2.

   8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
        October 1984.

   9.   Jeffrey Mogul.  Broadcasting Internet Packets in the Presence of
        Subnets.  RFC-922, Stanford University, October 1984.

   10.  David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
        Institute of Technology Artificial Intelligence Laboratory, June
        1981.

   11.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, Bolt
        Beranek and Newman, March 1977.

   12.  David Plummer.  An Ethernet Address Resolution Protocol.
        RFC-826, Symbolics, September 1982.

   13.  Jon Postel.  Internet Protocol.  RFC 791, ISI, September 1981.




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RFC 919                                                     October 1984
Broadcasting Internet Datagrams


   14.  David W. Wall.  Mechanisms for Broadcast and Selective
        Broadcast.  Ph.D. Th., Stanford University, June 1980.

   15.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
        Computer Systems Lab Technical Report TR189, Stanford
        University, June 1980.











































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========================================================================

Network Working Group                                      Jeffrey Mogul
Request for Comments: 922                    Computer Science Department
                                                     Stanford University
                                                            October 1984

       BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS


Status of this Memo

   We propose simple rules for broadcasting Internet datagrams on local
   networks that support broadcast, for addressing broadcasts, and for
   how gateways should handle them.

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

Acknowledgement

   This proposal here is the result of discussion with several other
   people, especially J. Noel Chiappa and Christopher A. Kent, both of
   whom both pointed me at important references.

 1. Introduction

   The use of broadcasts, especially on high-speed local area networks,
   is a good base for many applications.  Since broadcasting is not
   covered in the basic IP specification [12], there is no agreed-upon
   way to do it, and so protocol designers have not made use of it. (The
   issue has been touched upon before, e.g. [6], but has not been the
   subject of a standard.)

   We consider here only the case of unreliable, unsequenced, possibly
   duplicated datagram broadcasts (for a discussion of TCP broadcasting,
   see [10].) Even though unreliable and limited in length, datagram
   broadcasts are quite useful [1].

   We assume that the data link layer of the local network supports
   efficient broadcasting.  Most common local area networks do support
   broadcast; for example, Ethernet [7, 5], ChaosNet [9], token ring
   networks [2], etc.

   We do not assume, however, that broadcasts are reliably delivered.
   (One might consider providing a reliable datagram broadcast protocol
   as a layer above IP.) It is quite expensive to guarantee delivery of
   broadcasts; instead, what we assume is that a host will receive most
   of the broadcasts that are sent.  This is important to avoid
   excessive use of broadcasts; since every host on the network devotes
   at least some effort to every broadcast, they are costly.



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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


   When a datagram is broadcast, it imposes a cost on every host that
   hears it.  Therefore, broadcasting should not be used
   indiscriminately, but rather only when it is the best solution to a
   problem.

 2. Terminology

   Because broadcasting depends on the specific data link layer in use
   on a local network, we must discuss it with reference to both
   physical networks and logical networks.

   The terms we will use in referring to physical networks are, from the
   point of view of the host sending or forwarding a broadcast:

   Local Hardware Network

      The physical link to which the host is attached.

   Remote Hardware Network

      A physical network which is separated from the host by at least
      one gateway.

   Collection of Hardware Networks

      A set of hardware networks (transitively) connected by gateways.

   The IP world includes several kinds of logical network.  To avoid
   ambiguity, we will use the following terms:

   Internet

      The DARPA Internet collection of IP networks.

   IP Network

      One or a collection of several hardware networks that have one
      specific IP network number.

   Subnet

      A single member of the collection of hardware networks that
      compose an IP network.  Host addresses on a given subnet share an
      IP network number with hosts on all other subnets of that IP
      network, but the local-address part is divided into subnet-number




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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


      and host-number fields to indicate which subnet a host is on.  We
      do not assume a particular division of the local-address part;
      this could vary from network to network.

   The introduction of a subnet level in the addressing hierarchy is at
   variance with the IP specification [12], but as the use of
   addressable subnets proliferates it is obvious that a broadcasting
   scheme should support subnetting.  For more on subnets, see [8].

   In this paper, the term "host address" refers to the host-on-subnet
   address field of a subnetted IP network, or the host-part field
   otherwise.

   An IP network may consist of a single hardware network or a
   collection of subnets; from the point of view of a host on another IP
   network, it should not matter.

 3. Why Broadcast?

   Broadcasts are useful when a host needs to find information without
   knowing exactly what other host can supply it, or when a host wants
   to provide information to a large set of hosts in a timely manner.

   When a host needs information that one or more of its neighbors might
   have, it could have a list of neighbors to ask, or it could poll all
   of its possible neighbors until one responds.  Use of a wired-in list
   creates obvious network management problems (early binding is
   inflexible).  On the other hand, asking all of one's neighbors is
   slow if one must generate plausible host addresses, and try them
   until one works.  On the ARPANET, for example, there are roughly 65
   thousand plausible host numbers.  Most IP implementations have used
   wired-in lists (for example, addresses of "Prime" gateways.)
   Fortunately, broadcasting provides a fast and simple way for a host
   to reach all of its neighbors.

   A host might also use a broadcast to provide all of its neighbors
   with some information; for example, a gateway might announce its
   presence to other gateways.

   One way to view broadcasting is as an imperfect substitute for
   multicasting, the sending of messages to a subset of the hosts on a
   network.  In practice, broadcasts are usually used where multicasts
   are what is wanted; datagrams are broadcast at the hardware level,
   but filtering software in the receiving hosts gives the effect of
   multicasting.

   For more examples of broadcast applications, see [1, 3].


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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


 4. Broadcast Classes

   There are several classes of IP broadcasting:

      - Single-destination datagrams broadcast on the local hardware
        net: A datagram is destined for a specific IP host, but the
        sending host broadcasts it at the data link layer, perhaps to
        avoid having to do routing.  Since this is not an IP broadcast,
        the IP layer is not involved, except that a host should discard
        datagram not meant for it without becoming flustered (i.e.,
        printing an error message).

      - Broadcast to all hosts on the local hardware net: A
        distinguished value for the host-number part of the IP address
        denotes broadcast instead of a specific host.  The receiving IP
        layer must be able to recognize this address as well as its own.
        However, it might still be useful to distinguish at higher
        levels between broadcasts and non-broadcasts, especially in
        gateways.  This is the most useful case of broadcast; it allows
        a host to discover gateways without wired-in tables, it is the
        basis for address resolution protocols, and it is also useful
        for accessing such utilities as name servers, time servers,
        etc., without requiring wired-in addresses.

      - Broadcast to all hosts on a remote hardware network: It is
        occasionally useful to send a broadcast to all hosts on a
        non-local network; for example, to find the latest version of a
        hostname database, to bootload a host on a subnet without a
        bootserver, or to monitor the timeservers on the subnet.  This
        case is the same as local-network broadcasts; the datagram is
        routed by normal mechanisms until it reaches a gateway attached
        to the destination hardware network, at which point it is
        broadcast.  This class of broadcasting is also known as
        "directed broadcasting", or quaintly as sending a "letter bomb"
        [1].

      - Broadcast to all hosts on a subnetted IP network (Multi-subnet
        broadcasts): A distinguished value for the subnet-number part of
        the IP address is used to denote "all subnets".  Broadcasts to
        all hosts of a remote subnetted IP network are done just as
        directed broadcasts to a single subnet.

      - Broadcast to the entire Internet: This is probably not useful,
        and almost certainly not desirable.





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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


   For reasons of performance or security, a gateway may choose not to
   forward broadcasts; especially, it may be a good idea to ban
   broadcasts into or out of an autonomous group of networks.

 5. Broadcast Methods

   A host's IP receiving layer must be modified to support broadcasting.
   In the absence of broadcasting, a host determines if it is the
   recipient of a datagram by matching the destination address against
   all of its IP addresses.  With broadcasting, a host must compare the
   destination address not only against the host's addresses, but also
   against the possible broadcast addresses for that host.

   The problem of how best to send a broadcast has been extensively
   discussed [1, 3, 4, 13, 14].  Since we assume that the problem has
   already been solved at the data link layer, an IP host wishing to
   send either a local broadcast or a directed broadcast need only
   specify the appropriate destination address and send the datagram as
   usual.  Any sophisticated algorithms need only reside in gateways.

   The problem of broadcasting to all hosts on a subnetted IP network is
   apparently somewhat harder.  However, even in this case it turns out
   that the best known algorithms require no additional complexity in
   non-gateway hosts.  A good broadcast method will meet these
   additional criteria:

      - No modification of the IP datagram format.

      - Reasonable efficiency in terms of the number of excess copies
        generated and the cost of paths chosen.

      - Minimization of gateway modification, in both code and data
        space.

      - High likelihood of delivery.

   The algorithm that appears best is the Reverse Path Forwarding (RPF)
   method [4].  While RPF is suboptimal in cost and reliability, it is
   quite good, and is extremely simple to implement, requiring no
   additional data space in a gateway.









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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


 6. Gateways and Broadcasts

   Most of the complexity in supporting broadcasts lies in gateways.  If
   a gateway receives a directed broadcast for a network to which it is
   not connected, it simply forwards it using the usual mechanism.
   Otherwise, it must do some additional work.

    6.1. Local Broadcasts

      When a gateway receives a local broadcast datagram, there are
      several things it might have to do with it.  The situation is
      unambiguous, but without due care it is possible to create
      infinite loops.

      The appropriate action to take on receipt of a broadcast datagram
      depends on several things: the subnet it was received on, the
      destination network, and the addresses of the gateway.

         - The primary rule for avoiding loops is "never broadcast a
           datagram on the hardware network it was received on". It is
           not sufficient simply to avoid repeating datagram that a
           gateway has heard from itself; this still allows loops if
           there are several gateways on a hardware network.

         - If the datagram is received on the hardware network to which
           it is addressed, then it should not be forwarded.  However,
           the gateway should consider itself to be a destination of the
           datagram (for example, it might be a routing table update.)

         - Otherwise, if the datagram is addressed to a hardware network
           to which the gateway is connected, it should be sent as a
           (data link layer) broadcast on that network.  Again, the
           gateway should consider itself a destination of the datagram.

         - Otherwise, the gateway should use its normal routing
           procedure to choose a subsequent gateway, and send the
           datagram along to it.












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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


    6.2. Multi-subnet broadcasts

      When a gateway receives a broadcast meant for all subnets of an IP
      network, it must use the Reverse Path Forwarding algorithm to
      decide what to do.  The method is simple: the gateway should
      forward copies of the datagram along all connected links, if and
      only if the datagram arrived on the link which is part of the best
      route between the gateway and the source of the datagram.
      Otherwise, the datagram should be discarded.

      This algorithm may be improved if some or all of the gateways
      exchange among themselves additional information; this can be done
      transparently from the point of view of other hosts and even other
      gateways.  See [4, 3] for details.

    6.3. Pseudo-Algol Routing Algorithm

      This is a pseudo-Algol description of the routing algorithm a
      gateway should use.  The algorithm is shown in figure 1.  Some
      definitions are:

      RouteLink(host)

         A function taking a host address as a parameter and returning
         the first-hop link from the gateway to the host.

      RouteHost(host)

         As above but returns the first-hop host address.

      ResolveAddress(host)

         Returns the hardware address for an IP host.

      IncomingLink

         The link on which the packet arrived.

      OutgoingLinkSet

         The set of links on which the packet should be sent.

      OutgoingHardwareHost

         The hardware host address to send the packet to.




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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


      Destination.host

         The host-part of the destination address.

      Destination.subnet

         The subnet-part of the destination address.

      Destination.ipnet

         The IP-network-part of the destination address.






































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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets

BEGIN
   IF Destination.ipnet IN AllLinks THEN
      BEGIN
         IF IsSubnetted(Destination.ipnet) THEN
            BEGIN
               IF Destination.subnet = BroadcastSubnet THEN
                  BEGIN      /* use Reverse Path Forwarding algorithm */
                     IF IncomingLink = RouteLink(Source) THEN
                        BEGIN IF Destination.host = BroadcastHost THEN
                              OutgoingLinkSet <- AllLinks -
                           IncomingLink;
                           OutgoingHost <- BroadcastHost;
                           Examine packet for possible internal use;
                        END
                     ELSE  /* duplicate from another gateway, discard */
                        Discard;
                  END
               ELSE
                  IF Destination.subnet = IncomingLink.subnet THEN
                     BEGIN           /* forwarding would cause a loop */
                        IF Destination.host = BroadcastHost THEN
                           Examine packet for possible internal use;
                        Discard;
                     END
                  ELSE BEGIN    /* forward to (possibly local) subnet */
                        OutgoingLinkSet <- RouteLink(Destination);
                        OutgoingHost <- RouteHost(Destination);
                     END
            END
         ELSE BEGIN         /* destined for one of our local networks */
               IF Destination.ipnet = IncomingLink.ipnet THEN
                  BEGIN              /* forwarding would cause a loop */
                     IF Destination.host = BroadcastHost THEN
                        Examine packet for possible internal use;
                     Discard;
                  END
               ELSE BEGIN                     /* might be a broadcast */
                     OutgoingLinkSet <- RouteLink(Destination);
                     OutgoingHost <- RouteHost(Destination);
                  END
            END
      END
   ELSE BEGIN                    /* forward to a non-local IP network */
         OutgoingLinkSet <- RouteLink(Destination);
         OutgoingHost <- RouteHost(Destination);
      END
   OutgoingHardwareHost <- ResolveAddress(OutgoingHost);
END

Figure 1: Pseudo-Algol algorithm for routing broadcasts by gateways

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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


 7. Broadcast IP Addressing - Conventions

   If different IP implementations are to be compatible, there must be
   convention distinguished number to denote "all hosts" and "all
   subnets".

   Since the local network layer can always map an IP address into data
   link layer address, the choice of an IP "broadcast host number" is
   somewhat arbitrary.  For simplicity, it should be one not likely to
   be assigned to a real host.  The number whose bits are all ones has
   this property; this assignment was first proposed in [6].  In the few
   cases where a host has been assigned an address with a host-number
   part of all ones, it does not seem onerous to require renumbering.

   The "all subnets" number is also all ones; this means that a host
   wishing to broadcast to all hosts on a remote IP network need not
   know how the destination address is divided up into subnet and host
   fields, or if it is even divided at all.  For example, 36.255.255.255
   may denote all the hosts on a single hardware network, or all the
   hosts on a subnetted IP network with 1 byte of subnet field and 2
   bytes of host field, or any other possible division.

   The address 255.255.255.255 denotes a broadcast on a local hardware
   network that must not be forwarded.  This address may be used, for
   example, by hosts that do not know their network number and are
   asking some server for it.

   Thus, a host on net 36, for example, may:

      - broadcast to all of its immediate neighbors by using
        255.255.255.255

      - broadcast to all of net 36 by using 36.255.255.255

   without knowing if the net is subnetted; if it is not, then both
   addresses have the same effect. A robust application might try the
   former address, and if no response is received, then try the latter.
   See [1] for a discussion of such "expanding ring search" techniques.

   If the use of "all ones" in a field of an IP address means
   "broadcast", using "all zeros" could be viewed as meaning
   "unspecified".  There is probably no reason for such addresses to
   appear anywhere but as the source address of an ICMP Information
   Request datagram.  However, as a notational convention, we refer to
   networks (as opposed to hosts) by using addresses with zero fields.
   For example, 36.0.0.0 means "network number 36" while 36.255.255.255
   means "all hosts on network number 36".


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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


    7.1. ARP Servers and Broadcasts

      The Address Resolution Protocol (ARP) described in [11] can, if
      incorrectly implemented, cause problems when broadcasts are used
      on a network where not all hosts share an understanding of what a
      broadcast address is.  The temptation exists to modify the ARP
      server so that it provides the mapping between an IP broadcast
      address and the hardware broadcast address.

      This temptation must be resisted.  An ARP server should never
      respond to a request whose target is a broadcast address.  Such a
      request can only come from a host that does not recognize the
      broadcast address as such, and so honoring it would almost
      certainly lead to a forwarding loop.  If there are N such hosts on
      the physical network that do not recognize this address as a
      broadcast, then a datagram sent with a Time-To-Live of T could
      potentially give rise to T**N spurious re-broadcasts.

 8. References

    1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
        University, January 1982.

    2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
        Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516,
        November 1978.

    3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
        Computer Networks.  Ph.D. Th., Stanford University, April 1977.

    4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
        of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048,
        December 1978.

    5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
        Layer Specifications.  Version 1.0, Digital Equipment
        Corporation, Intel, Xerox, September 1980.

    6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
        Addressing Issues.  IEN-212, BBN, September 1982.

    7.   R.M. Metcalfe and D.R. Boggs.  "Ethernet: Distributed Packet
        Switching for Local Computer Networks".  Comm. ACM 19, 7,
        pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
        Center, reprinted in CSL-80-2.




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RFC 922                                                     October 1984
Broadcasting Internet Datagrams in the Presence of Subnets


    8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
        October 1984.

    9.   David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
        Institute of Technology Artificial Intelligence Laboratory,
        June 1981.

    10.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, BBN,
        March 1977.

    11.  David Plummer.  An Ethernet Address Resolution Protocol.
        RFC-826, Symbolics, September 1982.

    12.  Jon Postel.  Internet Protocol.  RFC-791, ISI, September 1981.

    13.  David W. Wall.  Mechanisms for Broadcast and Selective
        Broadcast.  Ph.D. Th., Stanford University, June 1980.

    14.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
        Computer Systems Lab Technical Report TR189, Stanford
        University, June 1980.




























Mogul                                                          [Page 12]
 



========================================================================

Network Working Group                                          J. Postel
Request for Comments:  792                                           ISI
                                                          September 1981
Updates:  RFCs 777, 760
Updates:  IENs 109, 128

                   INTERNET CONTROL MESSAGE PROTOCOL

                         DARPA INTERNET PROGRAM
                         PROTOCOL SPECIFICATION



Introduction

   The Internet Protocol (IP) [1] is used for host-to-host datagram
   service in a system of interconnected networks called the
   Catenet [2].  The network connecting devices are called Gateways.
   These gateways communicate between themselves for control purposes
   via a Gateway to Gateway Protocol (GGP) [3,4].  Occasionally a
   gateway or destination host will communicate with a source host, for
   example, to report an error in datagram processing.  For such
   purposes this protocol, the Internet Control Message Protocol (ICMP),
   is used.  ICMP, uses the basic support of IP as if it were a higher
   level protocol, however, ICMP is actually an integral part of IP, and
   must be implemented by every IP module.

   ICMP messages are sent in several situations:  for example, when a
   datagram cannot reach its destination, when the gateway does not have
   the buffering capacity to forward a datagram, and when the gateway
   can direct the host to send traffic on a shorter route.

   The Internet Protocol is not designed to be absolutely reliable.  The
   purpose of these control messages is to provide feedback about
   problems in the communication environment, not to make IP reliable.
   There are still no guarantees that a datagram will be delivered or a
   control message will be returned.  Some datagrams may still be
   undelivered without any report of their loss.  The higher level
   protocols that use IP must implement their own reliability procedures
   if reliable communication is required.

   The ICMP messages typically report errors in the processing of
   datagrams.  To avoid the infinite regress of messages about messages
   etc., no ICMP messages are sent about ICMP messages.  Also ICMP
   messages are only sent about errors in handling fragment zero of
   fragemented datagrams.  (Fragment zero has the fragment offeset equal
   zero).







                                                                [Page 1]
 




                                                          September 1981
RFC 792



Message Formats

   ICMP messages are sent using the basic IP header.  The first octet of
   the data portion of the datagram is a ICMP type field; the value of
   this field determines the format of the remaining data.  Any field
   labeled "unused" is reserved for later extensions and must be zero
   when sent, but receivers should not use these fields (except to
   include them in the checksum).  Unless otherwise noted under the
   individual format descriptions, the values of the internet header
   fields are as follows:

   Version

      4

   IHL

      Internet header length in 32-bit words.

   Type of Service

      0

   Total Length

      Length of internet header and data in octets.

   Identification, Flags, Fragment Offset

      Used in fragmentation, see [1].

   Time to Live

      Time to live in seconds; as this field is decremented at each
      machine in which the datagram is processed, the value in this
      field should be at least as great as the number of gateways which
      this datagram will traverse.

   Protocol

      ICMP = 1

   Header Checksum

      The 16 bit one's complement of the one's complement sum of all 16
      bit words in the header.  For computing the checksum, the checksum
      field should be zero.  This checksum may be replaced in the
      future.


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   Source Address

      The address of the gateway or host that composes the ICMP message.
      Unless otherwise noted, this can be any of a gateway's addresses.

   Destination Address

      The address of the gateway or host to which the message should be
      sent.









































                                                                [Page 3]
 




                                                          September 1981
RFC 792



Destination Unreachable Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      3

   Code

       0 = net unreachable;

       1 = host unreachable;

       2 = protocol unreachable;

       3 = port unreachable;

       4 = fragmentation needed and DF set;

       5 = source route failed.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original


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      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      If, according to the information in the gateway's routing tables,
      the network specified in the internet destination field of a
      datagram is unreachable, e.g., the distance to the network is
      infinity, the gateway may send a destination unreachable message
      to the internet source host of the datagram.  In addition, in some
      networks, the gateway may be able to determine if the internet
      destination host is unreachable.  Gateways in these networks may
      send destination unreachable messages to the source host when the
      destination host is unreachable.

      If, in the destination host, the IP module cannot deliver the
      datagram  because the indicated protocol module or process port is
      not active, the destination host may send a destination
      unreachable message to the source host.

      Another case is when a datagram must be fragmented to be forwarded
      by a gateway yet the Don't Fragment flag is on.  In this case the
      gateway must discard the datagram and may return a destination
      unreachable message.

      Codes 0, 1, 4, and 5 may be received from a gateway.  Codes 2 and
       3 may be received from a host.





















                                                                [Page 5]
 




                                                          September 1981
RFC 792



Time Exceeded Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      11

   Code

       0 = time to live exceeded in transit;

       1 = fragment reassembly time exceeded.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      If the gateway processing a datagram finds the time to live field


[Page 6]
 
                                                                



September 1981                                                          
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      is zero it must discard the datagram.  The gateway may also notify
      the source host via the time exceeded message.

      If a host reassembling a fragmented datagram cannot complete the
      reassembly due to missing fragments within its time limit it
      discards the datagram, and it may send a time exceeded message.

      If fragment zero is not available then no time exceeded need be
      sent at all.

      Code 0 may be received from a gateway.  Code 1 may be received
      from a host.






































                                                                [Page 7]
 




                                                          September 1981
RFC 792



Parameter Problem Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Pointer    |                   unused                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      12

   Code

       0 = pointer indicates the error.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Pointer

      If code = 0, identifies the octet where an error was detected.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.




[Page 8]
 
                                                                



September 1981                                                          
RFC 792



   Description

      If the gateway or host processing a datagram finds a problem with
      the header parameters such that it cannot complete processing the
      datagram it must discard the datagram.  One potential source of
      such a problem is with incorrect arguments in an option.  The
      gateway or host may also notify the source host via the parameter
      problem message.  This message is only sent if the error caused
      the datagram to be discarded.

      The pointer identifies the octet of the original datagram's header
      where the error was detected (it may be in the middle of an
      option).  For example, 1 indicates something is wrong with the
      Type of Service, and (if there are options present) 20 indicates
      something is wrong with the type code of the first option.

      Code 0 may be received from a gateway or a host.

































                                                                [Page 9]
 




                                                          September 1981
RFC 792



Source Quench Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address of the original datagram's data.

   ICMP Fields:

   Type

      4

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      A gateway may discard internet datagrams if it does not have the
      buffer space needed to queue the datagrams for output to the next
      network on the route to the destination network.  If a gateway


[Page 10]
 
                                                               



September 1981                                                          
RFC 792



      discards a datagram, it may send a source quench message to the
      internet source host of the datagram.  A destination host may also
      send a source quench message if datagrams arrive too fast to be
      processed.  The source quench message is a request to the host to
      cut back the rate at which it is sending traffic to the internet
      destination.  The gateway may send a source quench message for
      every message that it discards.  On receipt of a source quench
      message, the source host should cut back the rate at which it is
      sending traffic to the specified destination until it no longer
      receives source quench messages from the gateway.  The source host
      can then gradually increase the rate at which it sends traffic to
      the destination until it again receives source quench messages.

      The gateway or host may send the source quench message when it
      approaches its capacity limit rather than waiting until the
      capacity is exceeded.  This means that the data datagram which
      triggered the source quench message may be delivered.

      Code 0 may be received from a gateway or a host.































                                                               [Page 11]
 




                                                          September 1981
RFC 792



Redirect Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Gateway Internet Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address of the original datagram's data.

   ICMP Fields:

   Type

      5

   Code

       0 = Redirect datagrams for the Network.

       1 = Redirect datagrams for the Host.

       2 = Redirect datagrams for the Type of Service and Network.

       3 = Redirect datagrams for the Type of Service and Host.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Gateway Internet Address

      Address of the gateway to which traffic for the network specified
      in the internet destination network field of the original
      datagram's data should be sent.




[Page 12]
 
                                                               



September 1981                                                          
RFC 792



   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      The gateway sends a redirect message to a host in the following
      situation.  A gateway, G1, receives an internet datagram from a
      host on a network to which the gateway is attached.  The gateway,
      G1, checks its routing table and obtains the address of the next
      gateway, G2, on the route to the datagram's internet destination
      network, X.  If G2 and the host identified by the internet source
      address of the datagram are on the same network, a redirect
      message is sent to the host.  The redirect message advises the
      host to send its traffic for network X directly to gateway G2 as
      this is a shorter path to the destination.  The gateway forwards
      the original datagram's data to its internet destination.

      For datagrams with the IP source route options and the gateway
      address in the destination address field, a redirect message is
      not sent even if there is a better route to the ultimate
      destination than the next address in the source route.

      Codes 0, 1, 2, and 3 may be received from a gateway.






















                                                               [Page 13]
 




                                                          September 1981
RFC 792



Echo or Echo Reply Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Data ...
   +-+-+-+-+-

   IP Fields:

   Addresses

      The address of the source in an echo message will be the
      destination of the echo reply message.  To form an echo reply
      message, the source and destination addresses are simply reversed,
      the type code changed to 0, and the checksum recomputed.

   IP Fields:

   Type

       8 for echo message;

       0 for echo reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      If the total length is odd, the received data is padded with one
      octet of zeros for computing the checksum.  This checksum may be
      replaced in the future.

   Identifier

      If code = 0, an identifier to aid in matching echos and replies,
      may be zero.

   Sequence Number


[Page 14]
 
                                                               



September 1981                                                          
RFC 792



      If code = 0, a sequence number to aid in matching echos and
      replies, may be zero.

   Description

      The data received in the echo message must be returned in the echo
      reply message.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the echo requests.  For
      example, the identifier might be used like a port in TCP or UDP to
      identify a session, and the sequence number might be incremented
      on each echo request sent.  The echoer returns these same values
      in the echo reply.

      Code 0 may be received from a gateway or a host.


































                                                               [Page 15]
 




                                                          September 1981
RFC 792



Timestamp or Timestamp Reply Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |      Code     |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Originate Timestamp                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Receive Timestamp                                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Transmit Timestamp                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Addresses

      The address of the source in a timestamp message will be the
      destination of the timestamp reply message.  To form a timestamp
      reply message, the source and destination addresses are simply
      reversed, the type code changed to 14, and the checksum
      recomputed.

   IP Fields:

   Type

       13 for timestamp message;

       14 for timestamp reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Identifier




[Page 16]
 
                                                               



September 1981                                                          
RFC 792



      If code = 0, an identifier to aid in matching timestamp and
      replies, may be zero.

   Sequence Number

      If code = 0, a sequence number to aid in matching timestamp and
      replies, may be zero.

   Description

      The data received (a timestamp) in the message is returned in the
      reply together with an additional timestamp.  The timestamp is 32
      bits of milliseconds since midnight UT.  One use of these
      timestamps is described by Mills [5].

      The Originate Timestamp is the time the sender last touched the
      message before sending it, the Receive Timestamp is the time the
      echoer first touched it on receipt, and the Transmit Timestamp is
      the time the echoer last touched the message on sending it.

      If the time is not available in miliseconds or cannot be provided
      with respect to midnight UT then any time can be inserted in a
      timestamp provided the high order bit of the timestamp is also set
      to indicate this non-standard value.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the requests.  For example,
      the identifier might be used like a port in TCP or UDP to identify
      a session, and the sequence number might be incremented on each
      request sent.  The destination returns these same values in the
      reply.

      Code 0 may be received from a gateway or a host.

















                                                               [Page 17]
 




                                                          September 1981
RFC 792



Information Request or Information Reply Message

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |      Code     |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Addresses

      The address of the source in a information request message will be
      the destination of the information reply message.  To form a
      information reply message, the source and destination addresses
      are simply reversed, the type code changed to 16, and the checksum
      recomputed.

   IP Fields:

   Type

       15 for information request message;

       16 for information reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Identifier

      If code = 0, an identifier to aid in matching request and replies,
      may be zero.

   Sequence Number

      If code = 0, a sequence number to aid in matching request and
      replies, may be zero.


[Page 18]
 
                                                               



September 1981                                                          
RFC 792



   Description

      This message may be sent with the source network in the IP header
      source and destination address fields zero (which means "this"
      network).  The replying IP module should send the reply with the
      addresses fully specified.  This message is a way for a host to
      find out the number of the network it is on.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the requests.  For example,
      the identifier might be used like a port in TCP or UDP to identify
      a session, and the sequence number might be incremented on each
      request sent.  The destination returns these same values in the
      reply.

      Code 0 may be received from a gateway or a host.


































                                                               [Page 19]
 




                                                          September 1981
RFC 792



Summary of Message Types

     0  Echo Reply

     3  Destination Unreachable

     4  Source Quench

     5  Redirect

     8  Echo

    11  Time Exceeded

    12  Parameter Problem

    13  Timestamp

    14  Timestamp Reply

    15  Information Request

    16  Information Reply



























[Page 20]
 
                                                               



September 1981                                                          
RFC 792



References

   [1]  Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
         Protocol Specification," RFC 791, USC/Information Sciences
         Institute, September 1981.

   [2]   Cerf, V., "The Catenet Model for Internetworking," IEN 48,
         Information Processing Techniques Office, Defense Advanced
         Research Projects Agency, July 1978.

   [3]   Strazisar, V., "Gateway Routing:  An Implementation
         Specification", IEN 30, Bolt Beranek and Newman, April 1979.

   [4]   Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek
         and Newman, August 1979.

   [5]   Mills, D., "DCNET Internet Clock Service," RFC 778, COMSAT
         Laboratories, April 1981.

   






























                                                               [Page 21]
 



========================================================================




Network Working Group                                        S. Deering
Request for Comments: 1112                          Stanford University
Obsoletes: RFCs 988, 1054                                   August 1989


                  Host Extensions for IP Multicasting

 1. STATUS OF THIS MEMO

   This memo specifies the extensions required of a host implementation
   of the Internet Protocol (IP) to support multicasting.  It is the
   recommended standard for IP multicasting in the Internet.
   Distribution of this memo is unlimited.

 2. INTRODUCTION

   IP multicasting is the transmission of an IP datagram to a "host
   group", a set of zero or more hosts identified by a single IP
   destination address.  A multicast datagram is delivered to all
   members of its destination host group with the same "best-efforts"
   reliability as regular unicast IP datagrams, i.e., the datagram is
   not guaranteed to arrive intact at all members of the destination
   group or in the same order relative to other datagrams.

   The membership of a host group is dynamic; that is, hosts may join
   and leave groups at any time.  There is no restriction on the
   location or number of members in a host group.  A host may be a
   member of more than one group at a time.  A host need not be a member
   of a group to send datagrams to it.

   A host group may be permanent or transient.  A permanent group has a
   well-known, administratively assigned IP address.  It is the address,
   not the membership of the group, that is permanent; at any time a
   permanent group may have any number of members, even zero.  Those IP
   multicast addresses that are not reserved for permanent groups are
   available for dynamic assignment to transient groups which exist only
   as long as they have members.

   Internetwork forwarding of IP multicast datagrams is handled by
   "multicast routers" which may be co-resident with, or separate from,
   internet gateways.  A host transmits an IP multicast datagram as a
   local network multicast which reaches all immediately-neighboring
   members of the destination host group.  If the datagram has an IP
   time-to-live greater than 1, the multicast router(s) attached to the
   local network take responsibility for forwarding it towards all other
   networks that have members of the destination group.  On those other
   member networks that are reachable within the IP time-to-live, an
   attached multicast router completes delivery by transmitting the



Deering                                                         [Page 1]
 



RFC 1112          Host Extensions for IP Multicasting        August 1989


   datagram as a local multicast.

   This memo specifies the extensions required of a host IP
   implementation to support IP multicasting, where a "host" is any
   internet host or gateway other than those acting as multicast
   routers.  The algorithms and protocols used within and between
   multicast routers are transparent to hosts and will be specified in
   separate documents.  This memo also does not specify how local
   network multicasting is accomplished for all types of network,
   although it does specify the required service interface to an
   arbitrary local network and gives an Ethernet specification as an
   example.  Specifications for other types of network will be the
   subject of future memos.

 3. LEVELS OF CONFORMANCE

   There are three levels of conformance to this specification:

      Level 0: no support for IP multicasting.

   There is, at this time, no requirement that all IP implementations
   support IP multicasting.  Level 0 hosts will, in general, be
   unaffected by multicast activity.  The only exception arises on some
   types of local network, where the presence of level 1 or 2 hosts may
   cause misdelivery of multicast IP datagrams to level 0 hosts.  Such
   datagrams can easily be identified by the presence of a class D IP
   address in their destination address field; they should be quietly
   discarded by hosts that do not support IP multicasting.  Class D
   addresses are described in section 4 of this memo.

      Level 1: support for sending but not receiving multicast IP
      datagrams.

   Level 1 allows a host to partake of some multicast-based services,
   such as resource location or status reporting, but it does not allow
   a host to join any host groups.  An IP implementation may be upgraded
   from level 0 to level 1 very easily and with little new code.  Only
   sections 4, 5, and 6 of this memo are applicable to level 1
   implementations.

      Level 2: full support for IP multicasting.

   Level 2 allows a host to join and leave host groups, as well as send
   IP datagrams to host groups.  It requires implementation of the
   Internet Group Management Protocol (IGMP) and extension of the IP and
   local network service interfaces within the host.  All of the
   following sections of this memo are applicable to level 2
   implementations.



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 4. HOST GROUP ADDRESSES

   Host groups are identified by class D IP addresses, i.e., those with
   "1110" as their high-order four bits.  Class E IP addresses, i.e.,
   those with "1111" as their high-order four bits, are reserved for
   future addressing modes.

   In Internet standard "dotted decimal" notation, host group addresses
   range from 224.0.0.0 to 239.255.255.255.  The address 224.0.0.0 is
   guaranteed not to be assigned to any group, and 224.0.0.1 is assigned
   to the permanent group of all IP hosts (including gateways).  This is
   used to address all multicast hosts on the directly connected
   network.  There is no multicast address (or any other IP address) for
   all hosts on the total Internet.  The addresses of other well-known,
   permanent groups are to be published in "Assigned Numbers".

   Appendix II contains some background discussion of several issues
   related to host group addresses.

































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 5. MODEL OF A HOST IP IMPLEMENTATION

   The multicast extensions to a host IP implementation are specified in
   terms of the layered model illustrated below.  In this model, ICMP
   and (for level 2 hosts) IGMP are considered to be implemented within
   the IP module, and the mapping of IP addresses to local network
   addresses is considered to be the responsibility of local network
   modules.  This model is for expository purposes only, and should not
   be construed as constraining an actual implementation.

         |                                                          |
         |              Upper-Layer Protocol Modules                |
         |__________________________________________________________|

      --------------------- IP Service Interface -----------------------
          __________________________________________________________
         |                            |              |              |
         |                            |     ICMP     |     IGMP     |
         |             IP             |______________|______________|
         |           Module                                         |
         |                                                          |
         |__________________________________________________________|

      ---------------- Local Network Service Interface -----------------
          __________________________________________________________
         |                            |                             |
         |           Local            | IP-to-local address mapping |
         |          Network           |         (e.g., ARP)         |
         |          Modules           |_____________________________|
         |      (e.g., Ethernet)                                    |
         |                                                          |

   To provide level 1 multicasting, a host IP implementation must
   support the transmission of multicast IP datagrams.  To provide level
    2 multicasting, a host must also support the reception of multicast
   IP datagrams.  Each of these two new services is described in a
   separate section, below.  For each service, extensions are specified
   for the IP service interface, the IP module, the local network
   service interface, and an Ethernet local network module.  Extensions
   to local network modules other than Ethernet are mentioned briefly,
   but are not specified in detail.










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 6. SENDING MULTICAST IP DATAGRAMS

 6.1. Extensions to the IP Service Interface

   Multicast IP datagrams are sent using the same "Send IP" operation
   used to send unicast IP datagrams; an upper-layer protocol module
   merely specifies an IP host group address, rather than an individual
   IP address, as the destination.  However, a number of extensions may
   be necessary or desirable.

   First, the service interface should provide a way for the upper-layer
   protocol to specify the IP time-to-live of an outgoing multicast
   datagram, if such a capability does not already exist.  If the
   upper-layer protocol chooses not to specify a time-to-live, it should
   default to 1 for all multicast IP datagrams, so that an explicit
   choice is required to multicast beyond a single network.

   Second, for hosts that may be attached to more than one network, the
   service interface should provide a way for the upper-layer protocol
   to identify which network interface is be used for the multicast
   transmission.  Only one interface is used for the initial
   transmission; multicast routers are responsible for forwarding to any
   other networks, if necessary.  If the upper-layer protocol chooses
   not to identify an outgoing interface, a default interface should be
   used, preferably under the control of system management.

   Third (level 2 implementations only), for the case in which the host
   is itself a member of a group to which a datagram is being sent, the
   service interface should provide a way for the upper-layer protocol
   to inhibit local delivery of the datagram; by default, a copy of the
   datagram is looped back.  This is a performance optimization for
   upper-layer protocols that restrict the membership of a group to one
   process per host (such as a routing protocol), or that handle
   loopback of group communication at a higher layer (such as a
   multicast transport protocol).

 6.2. Extensions to the IP Module

   To support the sending of multicast IP datagrams, the IP module must
   be extended to recognize IP host group addresses when routing
   outgoing datagrams.  Most IP implementations include the following
   logic:

        if IP-destination is on the same local network,
           send datagram locally to IP-destination
        else
           send datagram locally to GatewayTo( IP-destination )




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   To allow multicast transmissions, the routing logic must be changed
   to:

        if IP-destination is on the same local network
        or IP-destination is a host group,
           send datagram locally to IP-destination
        else
           send datagram locally to GatewayTo( IP-destination )


   If the sending host is itself a member of the destination group on
   the outgoing interface, a copy of the outgoing datagram must be
   looped-back for local delivery, unless inhibited by the sender.
   (Level 2 implementations only.)

   The IP source address of the outgoing datagram must be one of the
   individual addresses corresponding to the outgoing interface.

   A host group address must never be placed in the source address field
   or anywhere in a source route or record route option of an outgoing
   IP datagram.

 6.3. Extensions to the Local Network Service Interface

   No change to the local network service interface is required to
   support the sending of multicast IP datagrams.  The IP module merely
   specifies an IP host group destination, rather than an individual IP
   destination, when it invokes the existing "Send Local" operation.

 6.4. Extensions to an Ethernet Local Network Module

   The Ethernet directly supports the sending of local multicast packets
   by allowing multicast addresses in the destination field of Ethernet
   packets.  All that is needed to support the sending of multicast IP
   datagrams is a procedure for mapping IP host group addresses to
   Ethernet multicast addresses.

   An IP host group address is mapped to an Ethernet multicast address
   by placing the low-order 23-bits of the IP address into the low-order
    23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
   Because there are 28 significant bits in an IP host group address,
   more than one host group address may map to the same Ethernet
   multicast address.

 6.5. Extensions to Local Network Modules other than Ethernet

   Other networks that directly support multicasting, such as rings or
   buses conforming to the IEEE 802.2 standard, may be handled the same



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   way as Ethernet for the purpose of sending multicast IP datagrams.
   For a network that supports broadcast but not multicast, such as the
   Experimental Ethernet, all IP host group addresses may be mapped to a
   single local broadcast address (at the cost of increased overhead on
   all local hosts).  For a point-to-point link joining two hosts (or a
   host and a multicast router), multicasts should be transmitted
   exactly like unicasts.  For a store-and-forward network like the
   ARPANET or a public X.25 network, all IP host group addresses might
   be mapped to the well-known local address of an IP multicast router;
   a router on such a network would take responsibility for completing
   multicast delivery within the network as well as among networks.

 7. RECEIVING MULTICAST IP DATAGRAMS

 7.1. Extensions to the IP Service Interface

   Incoming multicast IP datagrams are received by upper-layer protocol
   modules using the same "Receive IP" operation as normal, unicast
   datagrams.  Selection of a destination upper-layer protocol is based
   on the protocol field in the IP header, regardless of the destination
   IP address.  However, before any datagrams destined to a particular
   group can be received, an upper-layer protocol must ask the IP module
   to join that group.  Thus, the IP service interface must be extended
   to provide two new operations:

                 JoinHostGroup  ( group-address, interface )

                 LeaveHostGroup ( group-address, interface )

   The JoinHostGroup operation requests that this host become a member
   of the host group identified by "group-address" on the given network
   interface.  The LeaveGroup operation requests that this host give up
   its membership in the host group identified by "group-address" on the
   given network interface.  The interface argument may be omitted on
   hosts that support only one interface.  For hosts that may be
   attached to more than one network, the upper-layer protocol may
   choose to leave the interface unspecified, in which case the request
   will apply to the default interface for sending multicast datagrams
   (see section 6.1).

   It is permissible to join the same group on more than one interface,
   in which case duplicate multicast datagrams may be received.  It is
   also permissible for more than one upper-layer protocol to request
   membership in the same group.

   Both operations should return immediately (i.e., they are non-
   blocking operations), indicating success or failure.  Either
   operation may fail due to an invalid group address or interface



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   identifier.  JoinHostGroup may fail due to lack of local resources.
   LeaveHostGroup may fail because the host does not belong to the given
   group on the given interface.  LeaveHostGroup may succeed, but the
   membership persist, if more than one upper-layer protocol has
   requested membership in the same group.

 7.2. Extensions to the IP Module

   To support the reception of multicast IP datagrams, the IP module
   must be extended to maintain a list of host group memberships
   associated with each network interface.  An incoming datagram
   destined to one of those groups is processed exactly the same way as
   datagrams destined to one of the host's individual addresses.

   Incoming datagrams destined to groups to which the host does not
   belong are discarded without generating any error report or log
   entry.  On hosts with more than one network interface, if a datagram
   arrives via one interface, destined for a group to which the host
   belongs only on a different interface, the datagram is quietly
   discarded.  (These cases should occur only as a result of inadequate
   multicast address filtering in a local network module.)

   An incoming datagram is not rejected for having an IP time-to-live of
    1 (i.e., the time-to-live should not automatically be decremented on
   arriving datagrams that are not being forwarded).  An incoming
   datagram with an IP host group address in its source address field is
   quietly discarded.  An ICMP error message (Destination Unreachable,
   Time Exceeded, Parameter Problem, Source Quench, or Redirect) is
   never generated in response to a datagram destined to an IP host
   group.

   The list of host group memberships is updated in response to
   JoinHostGroup and LeaveHostGroup requests from upper-layer protocols.
   Each membership should have an associated reference count or similar
   mechanism to handle multiple requests to join and leave the same
   group.  On the first request to join and the last request to leave a
   group on a given interface, the local network module for that
   interface is notified, so that it may update its multicast reception
   filter (see section 7.3).

   The IP module must also be extended to implement the IGMP protocol,
   specified in Appendix I. IGMP is used to keep neighboring multicast
   routers informed of the host group memberships present on a
   particular local network.  To support IGMP, every level 2 host must
   join the "all-hosts" group (address 224.0.0.1) on each network
   interface at initialization time and must remain a member for as long
   as the host is active.




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   (Datagrams addressed to the all-hosts group are recognized as a
   special case by the multicast routers and are never forwarded beyond
   a single network, regardless of their time-to-live.  Thus, the all-
   hosts address may not be used as an internet-wide broadcast address.
   For the purpose of IGMP, membership in the all-hosts group is really
   necessary only while the host belongs to at least one other group.
   However, it is specified that the host shall remain a member of the
   all-hosts group at all times because (1) it is simpler, (2) the
   frequency of reception of unnecessary IGMP queries should be low
   enough that overhead is negligible, and (3) the all-hosts address may
   serve other routing-oriented purposes, such as advertising the
   presence of gateways or resolving local addresses.)

 7.3. Extensions to the Local Network Service Interface

   Incoming local network multicast packets are delivered to the IP
   module using the same "Receive Local" operation as local network
   unicast packets.  To allow the IP module to tell the local network
   module which multicast packets to accept, the local network service
   interface is extended to provide two new operations:

                      JoinLocalGroup  ( group-address )

                      LeaveLocalGroup ( group-address )

   where "group-address" is an IP host group address.  The
   JoinLocalGroup operation requests the local network module to accept
   and deliver up subsequently arriving packets destined to the given IP
   host group address.  The LeaveLocalGroup operation requests the local
   network module to stop delivering up packets destined to the given IP
   host group address.  The local network module is expected to map the
   IP host group addresses to local network addresses as required to
   update its multicast reception filter.  Any local network module is
   free to ignore LeaveLocalGroup requests, and may deliver up packets
   destined to more addresses than just those specified in
   JoinLocalGroup requests, if it is unable to filter incoming packets
   adequately.

   The local network module must not deliver up any multicast packets
   that were transmitted from that module; loopback of multicasts is
   handled at the IP layer or higher.

 7.4. Extensions to an Ethernet Local Network Module

   To support the reception of multicast IP datagrams, an Ethernet
   module must be able to receive packets addressed to the Ethernet
   multicast addresses that correspond to the host's IP host group
   addresses.  It is highly desirable to take advantage of any address



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   filtering capabilities that the Ethernet hardware interface may have,
   so that the host receives only those packets that are destined to it.

   Unfortunately, many current Ethernet interfaces have a small limit on
   the number of addresses that the hardware can be configured to
   recognize.  Nevertheless, an implementation must be capable of
   listening on an arbitrary number of Ethernet multicast addresses,
   which may mean "opening up" the address filter to accept all
   multicast packets during those periods when the number of addresses
   exceeds the limit of the filter.

   For interfaces with inadequate hardware address filtering, it may be
   desirable (for performance reasons) to perform Ethernet address
   filtering within the software of the Ethernet module.  This is not
   mandatory, however, because the IP module performs its own filtering
   based on IP destination addresses.

 7.5. Extensions to Local Network Modules other than Ethernet

   Other multicast networks, such as IEEE 802.2 networks, can be handled
   the same way as Ethernet for the purpose of receiving multicast IP
   datagrams.  For pure broadcast networks, such as the Experimental
   Ethernet, all incoming broadcast packets can be accepted and passed
   to the IP module for IP-level filtering.  On point-to-point or
   store-and-forward networks, multicast IP datagrams will arrive as
   local network unicasts, so no change to the local network module
   should be necessary.
























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APPENDIX I. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)

   The Internet Group Management Protocol (IGMP) is used by IP hosts to
   report their host group memberships to any immediately-neighboring
   multicast routers.  IGMP is an asymmetric protocol and is specified
   here from the point of view of a host, rather than a multicast
   router.  (IGMP may also be used, symmetrically or asymmetrically,
   between multicast routers.  Such use is not specified here.)

   Like ICMP, IGMP is a integral part of IP.  It is required to be
   implemented by all hosts conforming to level 2 of the IP multicasting
   specification.  IGMP messages are encapsulated in IP datagrams, with
   an IP protocol number of 2.  All IGMP messages of concern to hosts
   have the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version| Type  |    Unused     |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Group Address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Version

         This memo specifies version 1 of IGMP.  Version 0 is specified
         in RFC-988 and is now obsolete.

      Type

         There are two types of IGMP message of concern to hosts:

             1 = Host Membership Query
             2 = Host Membership Report

      Unused

         Unused field, zeroed when sent, ignored when received.

      Checksum

         The checksum is the 16-bit one's complement of the one's
         complement sum of the 8-octet IGMP message.  For computing
         the checksum, the checksum field is zeroed.

      Group Address

         In a Host Membership Query message, the group address field



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         is zeroed when sent, ignored when received.

         In a Host Membership Report message, the group address field
         holds the IP host group address of the group being reported.

Informal Protocol Description

   Multicast routers send Host Membership Query messages (hereinafter
   called Queries) to discover which host groups have members on their
   attached local networks.  Queries are addressed to the all-hosts
   group (address 224.0.0.1), and carry an IP time-to-live of 1.

   Hosts respond to a Query by generating Host Membership Reports
   (hereinafter called Reports), reporting each host group to which they
   belong on the network interface from which the Query was received.
   In order to avoid an "implosion" of concurrent Reports and to reduce
   the total number of Reports transmitted, two techniques are used:

       1. When a host receives a Query, rather than sending Reports
         immediately, it starts a report delay timer for each of its
         group memberships on the network interface of the incoming
         Query.  Each timer is set to a different, randomly-chosen
         value between zero and D seconds.  When a timer expires, a
         Report is generated for the corresponding host group.  Thus,
         Reports are spread out over a D second interval instead of
         all occurring at once.

       2. A Report is sent with an IP destination address equal to the
         host group address being reported, and with an IP
         time-to-live of 1, so that other members of the same group on
         the same network can overhear the Report.  If a host hears a
         Report for a group to which it belongs on that network, the
         host stops its own timer for that group and does not generate
         a Report for that group.  Thus, in the normal case, only one
         Report will be generated for each group present on the
         network, by the member host whose delay timer expires first.
         Note that the multicast routers receive all IP multicast
         datagrams, and therefore need not be addressed explicitly.
         Further note that the routers need not know which hosts
         belong to a group, only that at least one host belongs to a
         group on a particular network.

   There are two exceptions to the behavior described above.  First, if
   a report delay timer is already running for a group membership when a
   Query is received, that timer is not reset to a new random value, but
   rather allowed to continue running with its current value.  Second, a
   report delay timer is never set for a host's membership in the all-
   hosts group (224.0.0.1), and that membership is never reported.



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   If a host uses a pseudo-random number generator to compute the
   reporting delays, one of the host's own individual IP address should
   be used as part of the seed for the generator, to reduce the chance
   of multiple hosts generating the same sequence of delays.

   A host should confirm that a received Report has the same IP host
   group address in its IP destination field and its IGMP group address
   field, to ensure that the host's own Report is not cancelled by an
   erroneous received Report.  A host should quietly discard any IGMP
   message of type other than Host Membership Query or Host Membership
   Report.

   Multicast routers send Queries periodically to refresh their
   knowledge of memberships present on a particular network.  If no
   Reports are received for a particular group after some number of
   Queries, the routers assume that that group has no local members and
   that they need not forward remotely-originated multicasts for that
   group onto the local network.  Queries are normally sent infrequently
   (no more than once a minute) so as to keep the IGMP overhead on hosts
   and networks very low.  However, when a multicast router starts up,
   it may issue several closely-spaced Queries in order to build up its
   knowledge of local memberships quickly.

   When a host joins a new group, it should immediately transmit a
   Report for that group, rather than waiting for a Query, in case it is
   the first member of that group on the network.  To cover the
   possibility of the initial Report being lost or damaged, it is
   recommended that it be repeated once or twice after short delays.  (A
   simple way to accomplish this is to act as if a Query had been
   received for that group only, setting the group's random report delay
   timer.  The state transition diagram below illustrates this
   approach.)

   Note that, on a network with no multicast routers present, the only
   IGMP traffic is the one or more Reports sent whenever a host joins a
   new group.

State Transition Diagram

   IGMP behavior is more formally specified by the state transition
   diagram below.  A host may be in one of three possible states, with
   respect to any single IP host group on any single network interface:

      - Non-Member state, when the host does not belong to the group
        on the interface.  This is the initial state for all
        memberships on all network interfaces; it requires no storage
        in the host.




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      - Delaying Member state, when the host belongs to the group on
        the interface and has a report delay timer running for that
        membership.

      - Idle Member state, when the host belongs to the group on the
        interface and does not have a report delay timer running for
        that membership.

   There are five significant events that can cause IGMP state
   transitions:

      - "join group" occurs when the host decides to join the group on
        the interface.  It may occur only in the Non-Member state.

      - "leave group" occurs when the host decides to leave the group
        on the interface.  It may occur only in the Delaying Member
        and Idle Member states.

      - "query received" occurs when the host receives a valid IGMP
        Host Membership Query message.  To be valid, the Query message
        must be at least 8 octets long, have a correct IGMP
        checksum and have an IP destination address of 224.0.0.1.
        A single Query applies to all memberships on the
        interface from which the Query is received.  It is ignored for
        memberships in the Non-Member or Delaying Member state.

      - "report received" occurs when the host receives a valid IGMP
        Host Membership Report message.  To be valid, the Report
        message must be at least 8 octets long, have a correct IGMP
        checksum, and contain the same IP host group address in its IP
        destination field and its IGMP group address field.  A Report
        applies only to the membership in the group identified by the
        Report, on the interface from which the Report is received.
        It is ignored for memberships in the Non-Member or Idle Member
        state.

      - "timer expired" occurs when the report delay timer for the
        group on the interface expires.  It may occur only in the
        Delaying Member state.

   All other events, such as receiving invalid IGMP messages, or IGMP
   messages other than Query or Report, are ignored in all states.

   There are three possible actions that may be taken in response to the
   above events:

      - "send report" for the group on the interface.




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      - "start timer" for the group on the interface, using a random
        delay value between 0 and D seconds.

      - "stop timer" for the group on the interface.

   In the following diagram, each state transition arc is labelled with
   the event that causes the transition, and, in parentheses, any
   actions taken during the transition.

                              ________________
                             |                |
                             |                |
                             |                |
                             |                |
                   --------->|   Non-Member   |<---------
                  |          |                |          |
                  |          |                |          |
                  |          |                |          |
                  |          |________________|          |
                  |                   |                  |
                  | leave group       | join group       | leave group
                  | (stop timer)      |(send report,     |
                  |                   | start timer)     |
          ________|________           |          ________|________
         |                 |<---------          |                 |
         |                 |                    |                 |
         |                 |<-------------------|                 |
         |                 |   query received   |                 |
         | Delaying Member |    (start timer)   |   Idle Member   |
         |                 |------------------->|                 |
         |                 |   report received  |                 |
         |                 |    (stop timer)    |                 |
         |_________________|------------------->|_________________|
                                timer expired
                                (send report)

   The all-hosts group (address 224.0.0.1) is handled as a special case.
   The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

Protocol Parameters

   The maximum report delay, D, is 10 seconds.







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RFC 1112          Host Extensions for IP Multicasting        August 1989


APPENDIX II. HOST GROUP ADDRESS ISSUES

   This appendix is not part of the IP multicasting specification, but
   provides background discussion of several issues related to IP host
   group addresses.

Group Address Binding

   The binding of IP host group addresses to physical hosts may be
   considered a generalization of the binding of IP unicast addresses.
   An IP unicast address is statically bound to a single local network
   interface on a single IP network.  An IP host group address is
   dynamically bound to a set of local network interfaces on a set of IP
   networks.

   It is important to understand that an IP host group address is NOT
   bound to a set of IP unicast addresses.  The multicast routers do not
   need to maintain a list of individual members of each host group.
   For example, a multicast router attached to an Ethernet need
   associate only a single Ethernet multicast address with each host
   group having local members, rather than a list of the members'
   individual IP or Ethernet addresses.

Allocation of Transient Host Group Addresses

   This memo does not specify how transient group address are allocated.
   It is anticipated that different portions of the IP transient host
   group address space will be allocated using different techniques.
   For example, there may be a number of servers that can be contacted
   to acquire a new transient group address.  Some higher-level
   protocols (such as VMTP, specified in RFC-1045) may generate higher-
   level transient "process group" or "entity group" addresses which are
   then algorithmically mapped to a subset of the IP transient host
   group addresses, similarly to the way that IP host group addresses
   are mapped to Ethernet multicast addresses.  A portion of the IP
   group address space may be set aside for random allocation by
   applications that can tolerate occasional collisions with other
   multicast users, perhaps generating new addresses until a suitably
   "quiet" one is found.

   In general, a host cannot assume that datagrams sent to any host
   group address will reach only the intended hosts, or that datagrams
   received as a member of a transient host group are intended for the
   recipient.  Misdelivery must be detected at a level above IP, using
   higher-level identifiers or authentication tokens.  Information
   transmitted to a host group address should be encrypted or governed
   by administrative routing controls if the sender is concerned about
   unwanted listeners.



Deering                                                        [Page 16]
 



RFC 1112          Host Extensions for IP Multicasting        August 1989


Author's Address

   Steve Deering
   Stanford University
   Computer Science Department
   Stanford, CA 94305-2140

   Phone: (415) 723-9427

   EMail: deering@PESCADERO.STANFORD.EDU









































Deering                                                        [Page 17]