    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

                                       by

                         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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                                                           Internet Protocol

                                    PREFACE

    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

                                                               Editor

                                                                  [Page iii]

    January 1980
    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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    Introduction

    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) may 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; 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
      datagram.

      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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                                                           Internet Protocol
                                                                Introduction

      options include provisions for timestamps, error reports, 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.

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                                                           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 | |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
      protocol.

    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

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    Internet Protocol
    Overview

        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
        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 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
        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

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                                                           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 [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

        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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    Internet Protocol
    Overview

        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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                                                                    Overview

        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 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
        increases.

          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
          +-----+-----+-----+-----+-----+-----+-----+-----+
          |                 |     |           |     |     |
          |   PRECEDENCE    | STRM|RELIABILITY| S/R |SPEED|
          |                 |     |           |     |     |
          +-----+-----+-----+-----+-----+-----+-----+-----+

          PRECEDENCE          STRM      RELIABILITY  S/R      SPEED
          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
          00X-Routine

        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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                                                           Internet Protocol
                                                               Specification

        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
        discarded.

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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
        Address.

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                                                               Specification

      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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    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      4    Security.  Used to carry Security, and user
                              group (TCC) information compatible with DOD
                              requirements.
            0     3     var.  Source Routing.  Used to route the internet
                              datagram based on information supplied by the
                              source.
            0     7     var.  Return Route.  Used to record the route an
                              internet datagram takes.
            0     8      4    Stream ID.  Used to carry the stream
                              identifier.
            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

            +--------+
            |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.

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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.

          Security

            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
                10-secret
                01-confidential
                00-unclassified

            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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    Internet Protocol
    Specification

          Source Route

            +--------+--------+--------+---------//--------+
            |00000011| length |        source route        |
            +--------+--------+--------+---------//--------+
              Type=3

            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
            field.

            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        |
            +--------+--------+--------+---------//--------+
              Type=7

            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
            empty.

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                                                           Internet Protocol
                                                               Specification

            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       |          |
            +--------+--------+--------+--------+--------+----//----+
             Type=33

            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
              space.

              1 - Datagram Discarded, used when specific information is

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    Internet Protocol
    Specification

              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
                         datagram.
                   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
            milliseconds.

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                                                           Internet Protocol
                                                               Specification

            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
            milliseconds.

            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
      host.

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      Addressing

        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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                                                           Internet Protocol
                                                               Specification

        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.

          Notation:

            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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          Procedure:

            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

    [Page 24]

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                                                           Internet Protocol
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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).

          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

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          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
          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.

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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
        datagram.

        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
        requests.

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

        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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    Specification

          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.

      Options

        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.

      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.

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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.

    [Page 32]

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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)

          where:

            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)

          where:

            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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    Internet Protocol
    Specification

      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
      field).

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                                                           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 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.

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

    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.

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                                                                January 1980
    Internet Protocol
    Glossary

    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
              header.

    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
              interface.

    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.

    octet
              An eight bit byte.

    Options
              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
              timestamps.

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    January 1980
                                                           Internet Protocol
                                                                    Glossary

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

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

    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).

    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.

    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.

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    Internet Protocol

    [Page 40]

    January 1980
                                                           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, 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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    Internet Protocol

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