    RFC:  791

                               INTERNET PROTOCOL

                             DARPA INTERNET PROGRAM

                             PROTOCOL SPECIFICATION

                                 September 1981

                                  prepared for

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

                                       by

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

    September 1981
                                                           Internet Protocol

                               TABLE OF CONTENTS

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

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

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

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

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

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

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

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

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

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

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                                                           Internet Protocol

                                    PREFACE

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

                                                               Jon Postel

                                                               Editor

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                                                              September 1981

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

                               INTERNET PROTOCOL

                             DARPA INTERNET PROGRAM
                             PROTOCOL SPECIFICATION

                                1.  INTRODUCTION

    1.1.  Motivation

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

    1.2.  Scope

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

    1.3.  Interfaces

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

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

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

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    Internet Protocol
    Introduction

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

    1.4.  Operation

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

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

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

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

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

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

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

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

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                                                           Internet Protocol
                                                                Introduction

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

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

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

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

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                                                           Internet Protocol

                                  2.  OVERVIEW

    2.1.  Relation to Other Protocols

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

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

                             Protocol Relationships

                                   Figure 1.

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

    2.2.  Model of Operation

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

        We suppose that this transmission will involve one intermediate
        gateway.

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

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

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    Internet Protocol
    Overview

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

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

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

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

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

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

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

                                Transmission Path

                                    Figure 2

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

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

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

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

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

      Fragmentation

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

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

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

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

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

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

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                                                                    Overview

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

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

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

    2.4.  Gateways

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

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

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

                               Gateway Protocols

                                   Figure 3.

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                               3.  SPECIFICATION

    3.1.  Internet Header Format

      A summary of the contents of the internet header follows:

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

                        Example Internet Datagram Header

                                   Figure 4.

      Note that each tick mark represents one bit position.

      Version:  4 bits

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

      IHL:  4 bits

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

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      Type of Service:  8 bits

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

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

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

            Precedence

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

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

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

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                                                               Specification

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

      Total Length:  16 bits

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

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

      Identification:  16 bits

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

      Flags:  3 bits

        Various Control Flags.

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

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

      Fragment Offset:  13 bits

        This field indicates where in the datagram this fragment belongs.

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

      Time to Live:  8 bits

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

      Protocol:  8 bits

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

      Header Checksum:  16 bits

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

        The checksum algorithm is:

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

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

      Source Address:  32 bits

        The source address.  See section 3.2.

      Destination Address:  32 bits

        The destination address.  See section 3.2.

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      Options:  variable

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

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

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

          Case 1:  A single octet of option-type.

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

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

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

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

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

          0 = not copied
          1 = copied

        The option classes are:

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

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        The following internet options are defined:

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

        Specific Option Definitions

          End of Option List

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

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

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

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          No Operation

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

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

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

          Security

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

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

            Security (S field):  16 bits

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

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

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    Internet Protocol
    Specification

            Compartments (C field):  16 bits

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

            Handling Restrictions (H field):  16 bits

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

            Transmission Control Code (TCC field):  24 bits

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

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

          Loose Source and Record Route

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

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

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

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

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

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

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

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

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

          Strict Source and Record Route

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

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

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

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

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    Specification

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

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

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

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

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

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

          Record Route

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

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

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

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

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

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

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

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

          Stream Identifier

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

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

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

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    Internet Protocol
    Specification

          Internet Timestamp

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

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

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

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

            The Flag (flg) [4 bits] values are

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

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

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

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

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

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                                                               Specification

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

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

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

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

      Padding:  variable

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

    3.2.  Discussion

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

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

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

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    Internet Protocol
    Specification

      Addressing

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

        Address Formats:

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

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

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

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

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

      Fragmentation and Reassembly.

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

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

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

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

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

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

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

        The fields which may be affected by fragmentation include:

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

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

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

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

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

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        An Example Fragmentation Procedure

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

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

          Notation:

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

          Procedure:

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

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                                                               Specification

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

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

        An Example Reassembly Procedure

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

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

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

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

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

          Notation:

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

          Procedure:

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

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

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                                                               Specification

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

      Identification

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

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

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

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

      Type of Service

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

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

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

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

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

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

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

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

      Time to Live

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

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

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

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      Options

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

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

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

      Checksum

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

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

      Errors

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

    3.3.  Interfaces

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

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

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    Specification

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

      An Example Upper Level Interface

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

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

        where:

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

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

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

        where:

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

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    September 1981
                                                           Internet Protocol
                                                               Specification

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

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

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

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

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

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

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

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    Internet Protocol

    APPENDIX A:  Examples & Scenarios

    Example 1:

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

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

                           Example Internet Datagram

                                   Figure 5.

      Note that each tick mark represents one bit position.

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

    [Page 34]

    September 1981
                                                           Internet Protocol

    Example 2:

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

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

                           Example Internet Datagram

                                   Figure 6.

                                                                   [Page 35]

                                                              September 1981
    Internet Protocol

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

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

                           Example Internet Fragment

                                   Figure 7.

    [Page 36]

    September 1981
                                                           Internet Protocol

      And the second fragment.

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

                           Example Internet Fragment

                                   Figure 8.

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                                                              September 1981
    Internet Protocol

    Example 3:

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

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

                           Example Internet Datagram

                                   Figure 9.

    [Page 38]

    September 1981
                                                           Internet Protocol

    APPENDIX B:  Data Transmission Order

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

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

                          Transmission Order of Bytes

                                   Figure 10.

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

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

                              Significance of Bits

                                   Figure 11.

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

                                                                   [Page 39]

                                                              September 1981
    Internet Protocol

    [Page 40]

    September 1981
                                                           Internet Protocol

                                    GLOSSARY

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

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

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

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

    Destination
              The destination address, an internet header field.

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

    Flags
              An internet header field carrying various control flags.

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

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

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

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

                                                                   [Page 41]

                                                              September 1981
    Internet Protocol
    Glossary

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

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

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

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

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

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

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

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

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

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

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

    [Page 42]

    September 1981
                                                           Internet Protocol
                                                                    Glossary

    octet
              An eight bit byte.

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

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

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

    Rest
              The local address portion of an Internet Address.

    Source
              The source address, an internet header field.

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

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

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

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

    TOS
              Type of Service

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

    TTL
              Time to Live

                                                                   [Page 43]

                                                              September 1981
    Internet Protocol
    Glossary

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

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

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

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

    [Page 44]

    September 1981
                                                           Internet Protocol

                                   REFERENCES

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

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

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

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

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

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

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

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

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

                                                                   [Page 45]