RFC 760
This document is obsolete. Please refer to RFC 791.

RFC: 760
IEN: 128

                              DOD STANDARD
                           INTERNET PROTOCOL
                              January 1980

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


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

January 1980                                                            
                                                       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 ................................... 5
  2.2  Model of Operation ............................................ 5
  2.3  Function Description .......................................... 7

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

  3.1  Internet Header Format ....................................... 11
  3.2  Discussion ................................................... 21
  3.3  Examples & Scenarios ......................................... 30
  3.4  Interfaces ................................................... 34

GLOSSARY ............................................................ 37

REFERENCES .......................................................... 41

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This document specifies the DoD Standard Internet Protocol.  This
document is based on five 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 the details security,
compartmentation, and precedence features of the internet protocol.

                                                           Jon Postel


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

                              DOD STANDARD

                           INTERNET PROTOCOL

                            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 promote data reliability,
  flow control, sequencing, or other services commonly found in
  host-to-host protocols.

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
  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.

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1.4.  Operation

  The internet protocol implements two basic functions:  addressing and

  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) may have procedures for making routing decisions and other

  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; this may be thought of as selecting among Interactive, Bulk,
  or Real Time, for example.  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

  The Time to Live is an indication of 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.

  The Options provide for control functions needed or useful in some
  situations but unnecessary for the most common communications.  The

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  options include provisions for timestamps, error reports, and special

  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.


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                              2.  OVERVIEW

2.1.  Relation to Other Protocols

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

                 +------+ +-----+ +-----+       +-----+
                 |Telnet| | FTP | |Voice|  ...  |     |
                 +------+ +-----+ +-----+       +-----+
                       |   |         |             |  
                      +-----+     +-----+       +-----+
                      | TCP |     | RTP |  ...  |     |
                      +-----+     +-----+       +-----+
                         |           |             |  
                      |       Internet Protocol       |
                        |   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

2.2.  Model of Operation

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

    We suppose that this transmission will involve one intermediate

    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

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    to it.  The internet module determines a local network address for
    this internet address, in this case it is the address of a gateway.
    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

    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 should 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

    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

    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

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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.


    A distinction is made between names, addresses, and routes [3].   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 one octet network number, followed by a three octet
    local address.  This three octet field is called the "rest" field.

    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.  A host should also be able to have several
    physical interfaces (multi-homing).

    That is, a host should be allowed several physical interfaces to the
    network with each having several logical internet addresses.

    Examples of address mappings may be found in reference [4].


    Fragmentation of an internet datagram may be necessary when it
    originates in a local net that allows a large packet size and must

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    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 [5].

    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
    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

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    datagram is set to the value of that field in the long datagram plus

    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 datagram 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.


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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 3.

  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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  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.  A few
    networks offer a Stream service, whereby one can achieve a smoother
    service at some cost.  Typically this involves the reservation of
    resources within the network.  Another choice involves a low-delay
    vs. high-reliability trade off.  Typically networks invoke more
    complex (and delay producing) mechanisms as the need for reliability

      Bits 0-2:  Precedence.
      Bit    3:  Stream or Datagram.
      Bits 4-5:  Reliability.
      Bit    6:  Speed over Reliability.
      Bits   7:  Speed.

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

      111-Flash Override  1-STREAM  11-highest   1-speed  1-high
      110-Flash           0-DTGRM   10-higher    0-rlblt  0-low
      11X-Immediate                 01-lower
      01X-Priority                  00-lowest

    The type of service is used to specify the treatment of the datagram
    during its transmission through the internet system.  In the
    discussion (section 3.2) below, a chart shows the relationship of
    the internet type of service to the actual service provided on the
    ARPANET, the SATNET, and the PRNET.

  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

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    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: Don't Fragment This Datagram (DF).
      Bit 2: More Fragments Flag (MF).

          0   1   2
        |   | D | M |
        | 0 | F | F |

  Fragment Offset:  13 bits

    This field indicates where in the datagram this fragment belongs.
    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 the internet system.  If this field contains the value zero,
    then the datagram should be destroyed.  This field is modified in
    internet header processing.  The time is measured in units of
    seconds.  The intention is to cause undeliverable datagrams to be

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  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 reference [6].

  Header Checksum:  16 bits

    A checksum on the header only.  Since some header fields may 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.  The first octet is the Source Network, and the
    following three octets are the Source Local Address.

  Destination Address:  32 bits

    The destination address.  The first octet is the Destination
    Network, and the following three octets are the Destination Local

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  Options:  variable

    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   reserved, must be zero
      2 bits  option class,
      5 bits  option number.

    The option classes are:

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

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    The following internet options are defined:

      ----- ------ ------ -----------
        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      4    Security.  Used to carry Security, and user
                          group (TCC) information compatible with DOD
        0     3     var.  Source Routing.  Used to route the internet
                          datagram based on information supplied by the
        0     7     var.  Return Route.  Used to record the route an
                          internet datagram takes.
        0     8      4    Stream ID.  Used to carry the stream
        1     1     var.  General Error Report.  Used to report errors
                          in internet datagram processing.
        2     4      6    Internet Timestamp.
        2     5      6    Satellite Timestamp.


    Specific Option Definitions

      End of Option List


        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.

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      No Operation


        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.


        This option provides a way for DOD hosts to send security and
        TCC (closed user groups) parameters through networks whose
        transport leader does not contain fields for this information.
        The format for this option is as follows:

          |00000010|00000100|000000SS |  TCC   |
            Type=2  Length=4

        Security:  2 bits

          Specifies one of 4 levels of security

            11-top secret

        Transmission Control Code:  8 bits

          Provides a means to compartmentalize traffic and define
          controlled communities of interest among subscribers.

        Note that this option does not require processing by the
        internet module but does require that this information be passed
        to higher level protocol modules.  The security and TCC
        information might be used to supply class level and compartment
        information for transmitting datagrams into or through
        AUTODIN II.

        Must be copied on fragmentation.

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      Source Route

        |00000011| length |        source route        |

        The source route 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.

        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, as well as length-2 octets of source route data.

        A source route is composed of a series of internet addresses.
        Each internet address is 32 bits or 4 octets.  The length
        defaults to two, which indicates the source route is empty and
        the remaining routing is to be based on the destination address

        If the address in destination address field has been reached and
        this option's length is not two, the next address in the source
        route replaces the address in the destination address field, and
        is deleted from the source route and this option's length is
        reduced by four.  (The Internet Header Length Field must be
        changed also.)

        Must be copied on fragmentation.

      Return Route

        |00000111| length |        return route        |

        The return 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, as well as length-2 octets of return route data.

        A return route is composed of a series of internet addresses.
        The length defaults to two, which indicates the return route is

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        When an internet module routes a datagram it checks to see if
        the return 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 return route at the front
        of the address string and increments the length by four.

        Not copied on fragmentation, goes in first fragment only.

      Stream Identifier

        |00001000|00000010|     Stream ID    |
          Type=8  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.

      General Error Report

        |00100001| length |err code|        id       |          |

        The general error report is used to report an error detected in
        processing an internet datagram to the source internet module of
        that datagram.  The "err code" indicates the type of error
        detected, and the "id" is copied from the identification field
        of the datagram in error, additional octets of error information
        may be present depending on the err code.

        If an internet datagram containing the general error report
        option is found to be in error or must be discarded, no error
        report is sent.

        ERR CODE:

          0 - Undetermined Error, used when no information is available
          about the type of error or the error does not fit any defined
          class.  Following the id should be as much of the datagram
          (starting with the internet header) as fits in the option

          1 - Datagram Discarded, used when specific information is

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          available about the reason for discarding the datagram can be
          reported.  Following the id should be the original (4-octets)
          destination address, and the (1-octet) reason.

            Reason   Description
            ------   -----------
               0     No Reason
               1     No One Wants It - No higher level protocol or
                     application program at destination wants this
               2     Fragmentation Needed & DF - Cannot deliver with out
                     fragmenting and has don't fragment bit set.
               3     Reassembly Problem - Destination could not
                     reassemble due to missing fragments when time to
                     live expired.
               4     Gateway Congestion - Gateway discarded datagram due
                     to congestion.

        The error report is placed in a datagram with the following
        values in the internet header fields:

          Version:  Same as the datagram in error.
          IHL:  As computed.
          Type of Service:  Zero.
          Total Length:  As computed.
          Identification:  A new identification is selected.
          Flags:  Zero.
          Fragment Offset:  Zero.
          Time to Live:  Sixty.
          Protocol:  Same as the datagram in error.
          Header Checksum:  As computed.
          Source Address:  Address of the error reporting module.
          Destination Address:  Source address of the datagram in error.
          Options:  The General Error Report Option.
          Padding:  As needed.

        Not copied on fragmentation, goes with first fragment.

      Internet Timestamp

        |01000100|00000100|        time in milliseconds       |
         Type=68  Length=6

        The data of the timestamp is a 32 bit time measured in

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        Not copied on fragmentation, goes with first fragment

      Satellite Timestamp

        |01000101|00000100|        time in milliseconds       |
         Type=69  Length=6

        The data of the timestamp is a 32 bit time measured in

        Not copied on fragmentation, goes with first fragment

  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 should be conservative
  in its sending behavior, and liberal in its receiving behavior.  That
  is, it should be careful to send well-formed datagrams, but should
  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

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    The 8 bit network number, which is the first octet of the address,
    has a value as specified in reference [6].

    The 24 bit local address, assigned by the local network, should
    allow for a single physical host to act as several distinct internet
    hosts.  That is, there should be mapping between internet host
    addresses and network/host interfaces that allows several internet
    addresses to correspond to one interface.  It should 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 reference [4].

  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
    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.

    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.

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    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.

    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).

    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 still too large.


        FO    -  Fragment Offset
        IHL   -  Internet Header Length
        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

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        IF TL =< MTU THEN Submit this datagram to the next step
             in datagram processing 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;
             TL <- OTL - NFB*8 - (OIHL-IHL)*4);
             FO <- OFO + NFB;  MF <- OMF;  Recompute Checksum;
        (10) Submit this fragment to the fragmentation test; DONE.

    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

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      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
      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).


        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

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


    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.

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    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, reliability, and speed.  A further concern is the
    possibility of efficient handling of streams of datagrams.  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

    Stream or Datagram.  Indicates if there will be other datagrams from
    this source to this destination at regular frequent intervals
    justifying the maintenance of stream processing information.

    Reliability.  A measure of the level of effort desired to ensure
    delivery of this datagram.

    Speed over Reliability.  Indicates the relative importance of speed
    and reliability when a conflict arises in meeting the pair of

    Speed.  A measure of the importance of prompt delivery of this

    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:

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      Precedence:    5
      Stream:        0
      Reliability:   1
      S/R:           1
      Speed:         1

    The mapping of these parameters to those available for the ARPANET
    would be  to set the ARPANET priority bit on since the Internet
    priority is in the upper half of its range, to select uncontrolled
    messages since the speed and reliability requirements are equal and
    speed is preferred.

    The following chart presents the recommended mappings from the
    internet protocol type of service into the service parameters
    actually available on the ARPANET, the PRNET, and the SATNET:

      |Application | INTERNET | ARPANET  | PRNET    | SATNET   |
      |TELNET      |S/D:stream| T: 3     | R: ptp   | T: block |
      |  on        |  R:normal| S: S     | A: no    | D: min   |
      |   TCP      |S/R:speed |          |          | H: inf   |
      |            |  S:fast  |          |          | R: no    |
      |FTP         |S/D:stream| T: 0     | R: ptp   | T: block |
      |  on        |  R:normal| S: M     | A: no    | D: normal|
      |   TCP      |S/R:rlblt |          |          | H: inf   |
      |            |  S:normal|          |          | R: no    |
      |interactive |S/D:strm* | T: 3     | R: ptp   | T: stream|
      |narrow band |  R:least | S: S     | A: no    | D: min   |
      |  speech    |  P:speed |          |          | H: short |
      |            |  S:asap  |          |          | R: no    |
      |datagram    |S/D:dtgrm | T: 3 or 0| R:station| T: block |
      |            |  R:normal| S: S or M| A: no    | D: min   |
      |            |S/R:speed |          |          | H: short |
      |            |  S:fast  |          |          | R: no    |
       key:    S/D=strm/dtgrm   T=type     R=route  T=type
               R=reliability    S=size     A=ack    D=delay
               S/R=speed/rlblt                      H=holding time
               S=speed                              R=reliability
               *=requires stream set up

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  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 should be destroyed.  This field should 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 should 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.


    The options are just that, optional.  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
    should 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 the following options:
    End of Option List (0), No Operation (1), Source Route (3), Return
    Route (7), General Error Report (33), and Internet Timestamp (68).
    The Security Option (2) is required only if classified or
    compartmented traffic is to be passed.


    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.

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3.3.  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 4.

    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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  Example 2:

    In this example, we show first a moderate size internet datagram
    (552 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 5.

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    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 6.

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    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 7.

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  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 8.

3.4.  Interfaces

  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
  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 the necessary information.

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

    SEND (dest, TOS, TTL, BufPTR, len, Id, DF, options => result)


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

    RECV (BufPTR => result, source, dest, prot, TOS, len)


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

  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 should
  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

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  user addressed does not exist, an error datagram 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.

  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


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          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 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).

          The destination address, an internet header field.

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

          An internet header field carrying various control flags.

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

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

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

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

          The Interface Message Processor, the packet switch of the

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

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

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

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

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.

          The More-Fragments Flag carried in the internet header flags

          An implementation, usually in software, of a protocol or other

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.

          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.

          An eight bit byte.

          The internet header Options field may contain several options,
          and each option may be several octets in length.  The options
          are used primarily in testing situations, for example to carry

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

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

          The 3 octet (24 bit) local address portion of an Internet

          Real Time Protocol:  A host-to-host protocol for communication
          of time critical information.

          The source address, an internet header field.

          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).

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

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

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

          The Version field indicates the format of the internet header.

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[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, May 1978 (Revised).

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

[4]  Postel, J., "Address Mappings," IEN 115, USC/Information Sciences
     Institute, August 1979.

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

[6]  Postel, J., "Assigned Numbers," RFC 762, IEN 127, USC/Information
     Sciences Institute, January 1980.

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