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[Text version]

This RFC is updated by: RFC 6864

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

Internet Protocol

                                                       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
                                                          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

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.

                                                       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.

Internet Protocol

                                                       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.

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
                                                       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.

Internet Protocol Overview

    Examples of address mappings may be found in "Address Mappings" [5].
  Fragmentation
    Fragmentation of an internet datagram is necessary when it
    originates in a local net that allows a large packet size and must
    traverse a local net that limits packets to a smaller size to reach
    its destination.
    An internet datagram can be marked "don't fragment."  Any internet
    datagram so marked is not to be internet fragmented under any
    circumstances.  If internet datagram marked don't fragment cannot be
    delivered to its destination without fragmenting it, it is to be
    discarded instead.
    Fragmentation, transmission and reassembly across a local network
    which is invisible to the internet protocol module is called
    intranet fragmentation and may be used [6].
    The internet fragmentation and reassembly procedure needs to be able
    to break a datagram into an almost arbitrary number of pieces that
    can be later reassembled.  The receiver of the fragments uses the
    identification field to ensure that fragments of different datagrams
    are not mixed.  The fragment offset field tells the receiver the
    position of a fragment in the original datagram.  The fragment
    offset and length determine the portion of the original datagram
    covered by this fragment.  The more-fragments flag indicates (by
    being reset) the last fragment.  These fields provide sufficient
    information to reassemble datagrams.
    The identification field is used to distinguish the fragments of one
    datagram from those of another.  The originating protocol module of
    an internet datagram sets the identification field to a value that
    must be unique for that source-destination pair and protocol for the
    time the datagram will be active in the internet system.  The
    originating protocol module of a complete datagram sets the
    more-fragments flag to zero and the fragment offset to zero.
    To fragment a long internet datagram, an internet protocol module
    (for example, in a gateway), creates two new internet datagrams and
    copies the contents of the internet header fields from the long
    datagram into both new internet headers.  The data of the long
    datagram is divided into two portions on a 8 octet (64 bit) boundary
    (the second portion might not be an integral multiple of 8 octets,
    but the first must be).  Call the number of 8 octet blocks in the
    first portion NFB (for Number of Fragment Blocks).  The first
    portion of the data is placed in the first new internet datagram,
    and the total length field is set to the length of the first
                                                       Internet Protocol
                                                                Overview
    datagram.  The more-fragments flag is set to one.  The second
    portion of the data is placed in the second new internet datagram,
    and the total length field is set to the length of the second
    datagram.  The more-fragments flag carries the same value as the
    long datagram.  The fragment offset field of the second new internet
    datagram is set to the value of that field in the long datagram plus
    NFB.
    This procedure can be generalized for an n-way split, rather than
    the two-way split described.
    To assemble the fragments of an internet datagram, an internet
    protocol module (for example at a destination host) combines
    internet datagrams that all have the same value for the four fields:
    identification, source, destination, and protocol.  The combination
    is done by placing the data portion of each fragment in the relative
    position indicated by the fragment offset in that fragment's
    internet header.  The first fragment will have the fragment offset
    zero, and the last fragment will have the more-fragments flag reset
    to zero.

2.4. Gateways

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

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

                   +-------------------------------+   
                   | Internet Protocol & ICMP & GGP|   
                   +-------------------------------+   
                           |                 |         
                 +---------------+   +---------------+ 
                 |   Local Net   |   |   Local Net   | 
                 +---------------+   +---------------+ 
                           Gateway Protocols
                               Figure 3.

Internet Protocol

                                                       Internet Protocol
                           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.

Internet Protocol Specification

  Type of Service:  8 bits
    The Type of Service provides an indication of the abstract
    parameters of the quality of service desired.  These parameters are
    to be used to guide the selection of the actual service parameters
    when transmitting a datagram through a particular network.  Several
    networks offer service precedence, which somehow treats high
    precedence traffic as more important than other traffic (generally
    by accepting only traffic above a certain precedence at time of high
    load).  The major choice is a three way tradeoff between low-delay,
    high-reliability, and high-throughput.
      Bits 0-2:  Precedence.
      Bit    3:  0 = Normal Delay,      1 = Low Delay.
      Bits   4:  0 = Normal Throughput, 1 = High Throughput.
      Bits   5:  0 = Normal Relibility, 1 = High Relibility.
      Bit  6-7:  Reserved for Future Use.
         0     1     2     3     4     5     6     7
      +-----+-----+-----+-----+-----+-----+-----+-----+
      |                 |     |     |     |     |     |
      |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
      |                 |     |     |     |     |     |
      +-----+-----+-----+-----+-----+-----+-----+-----+
        Precedence
          111 - Network Control
          110 - Internetwork Control
          101 - CRITIC/ECP
          100 - Flash Override
          011 - Flash
          010 - Immediate
          001 - Priority
          000 - Routine
    The use of the Delay, Throughput, and Reliability indications may
    increase the cost (in some sense) of the service.  In many networks
    better performance for one of these parameters is coupled with worse
    performance on another.  Except for very unusual cases at most two
    of these three indications should be set.
    The type of service is used to specify the treatment of the datagram
    during its transmission through the internet system.  Example
    mappings of the internet type of service to the actual service
    provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
    is given in "Service Mappings" [8].
                                                       Internet Protocol
                                                           Specification
    The Network Control precedence designation is intended to be used
    within a network only.  The actual use and control of that
    designation is up to each network. The Internetwork Control
    designation is intended for use by gateway control originators only.
    If the actual use of these precedence designations is of concern to
    a particular network, it is the responsibility of that network to
    control the access to, and use of, those precedence designations.
  Total Length:  16 bits
    Total Length is the length of the datagram, measured in octets,
    including internet header and data.  This field allows the length of
    a datagram to be up to 65,535 octets.  Such long datagrams are
    impractical for most hosts and networks.  All hosts must be prepared
    to accept datagrams of up to 576 octets (whether they arrive whole
    or in fragments).  It is recommended that hosts only send datagrams
    larger than 576 octets if they have assurance that the destination
    is prepared to accept the larger datagrams.
    The number 576 is selected to allow a reasonable sized data block to
    be transmitted in addition to the required header information.  For
    example, this size allows a data block of 512 octets plus 64 header
    octets to fit in a datagram.  The maximal internet header is 60
    octets, and a typical internet header is 20 octets, allowing a
    margin for headers of higher level protocols.
  Identification:  16 bits
    An identifying value assigned by the sender to aid in assembling the
    fragments of a datagram.
  Flags:  3 bits
    Various Control Flags.
      Bit 0: reserved, must be zero
      Bit 1: (DF) 0 = May Fragment,  1 = Don't Fragment.
      Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
          0   1   2
        +---+---+---+
        |   | D | M |
        | 0 | F | F |
        +---+---+---+
  Fragment Offset:  13 bits
    This field indicates where in the datagram this fragment belongs.

Internet Protocol Specification

    The fragment offset is measured in units of 8 octets (64 bits).  The
    first fragment has offset zero.
  Time to Live:  8 bits
    This field indicates the maximum time the datagram is allowed to
    remain in the internet system.  If this field contains the value
    zero, then the datagram must be destroyed.  This field is modified
    in internet header processing.  The time is measured in units of
    seconds, but since every module that processes a datagram must
    decrease the TTL by at least one even if it process the datagram in
    less than a second, the TTL must be thought of only as an upper
    bound on the time a datagram may exist.  The intention is to cause
    undeliverable datagrams to be discarded, and to bound the maximum
    datagram lifetime.
  Protocol:  8 bits
    This field indicates the next level protocol used in the data
    portion of the internet datagram.  The values for various protocols
    are specified in "Assigned Numbers" [9].
  Header Checksum:  16 bits
    A checksum on the header only.  Since some header fields change
    (e.g., time to live), this is recomputed and verified at each point
    that the internet header is processed.
    The checksum algorithm is:
      The checksum field is the 16 bit one's complement of the one's
      complement sum of all 16 bit words in the header.  For purposes of
      computing the checksum, the value of the checksum field is zero.
    This is a simple to compute checksum and experimental evidence
    indicates it is adequate, but it is provisional and may be replaced
    by a CRC procedure, depending on further experience.
  Source Address:  32 bits
    The source address.  See section 3.2.
  Destination Address:  32 bits
    The destination address.  See section 3.2.
                                                       Internet Protocol
                                                           Specification
  Options:  variable
    The options may appear or not in datagrams.  They must be
    implemented by all IP modules (host and gateways).  What is optional
    is their transmission in any particular datagram, not their
    implementation.
    In some environments the security option may be required in all
    datagrams.
    The option field is variable in length.  There may be zero or more
    options.  There are two cases for the format of an option:
      Case 1:  A single octet of option-type.
      Case 2:  An option-type octet, an option-length octet, and the
               actual option-data octets.
    The option-length octet counts the option-type octet and the
    option-length octet as well as the option-data octets.
    The option-type octet is viewed as having 3 fields:
      1 bit   copied flag,
      2 bits  option class,
      5 bits  option number.
    The copied flag indicates that this option is copied into all
    fragments on fragmentation.
      0 = not copied
      1 = copied
    The option classes are:
      0 = control
      1 = reserved for future use
      2 = debugging and measurement
      3 = reserved for future use

Internet Protocol Specification

    The following internet options are defined:
      CLASS NUMBER LENGTH DESCRIPTION
      ----- ------ ------ -----------
        0     0      -    End of Option list.  This option occupies only
                          1 octet; it has no length octet.
        0     1      -    No Operation.  This option occupies only 1
                          octet; it has no length octet.
        0     2     11    Security.  Used to carry Security,
                          Compartmentation, User Group (TCC), and
                          Handling Restriction Codes compatible with DOD
                          requirements.
        0     3     var.  Loose Source Routing.  Used to route the
                          internet datagram based on information
                          supplied by the source.
        0     9     var.  Strict Source Routing.  Used to route the
                          internet datagram based on information
                          supplied by the source.
        0     7     var.  Record Route.  Used to trace the route an
                          internet datagram takes.
        0     8      4    Stream ID.  Used to carry the stream
                          identifier.
        2     4     var.  Internet Timestamp.
    Specific Option Definitions
      End of Option List
        +--------+
        |00000000|
        +--------+
          Type=0
        This option indicates the end of the option list.  This might
        not coincide with the end of the internet header according to
        the internet header length.  This is used at the end of all
        options, not the end of each option, and need only be used if
        the end of the options would not otherwise coincide with the end
        of the internet header.
        May be copied, introduced, or deleted on fragmentation, or for
        any other reason.
                                                       Internet Protocol
                                                           Specification
      No Operation
        +--------+
        |00000001|
        +--------+
          Type=1
        This option may be used between options, for example, to align
        the beginning of a subsequent option on a 32 bit boundary.
        May be copied, introduced, or deleted on fragmentation, or for
        any other reason.
      Security
        This option provides a way for hosts to send security,
        compartmentation, handling restrictions, and TCC (closed user
        group) parameters.  The format for this option is as follows:
          +--------+--------+---//---+---//---+---//---+---//---+
          |10000010|00001011|SSS  SSS|CCC  CCC|HHH  HHH|  TCC   |
          +--------+--------+---//---+---//---+---//---+---//---+
           Type=130 Length=11
        Security (S field):  16 bits
          Specifies one of 16 levels of security (eight of which are
          reserved for future use).
            00000000 00000000 - Unclassified
            11110001 00110101 - Confidential
            01111000 10011010 - EFTO
            10111100 01001101 - MMMM
            01011110 00100110 - PROG
            10101111 00010011 - Restricted
            11010111 10001000 - Secret
            01101011 11000101 - Top Secret
            00110101 11100010 - (Reserved for future use)
            10011010 11110001 - (Reserved for future use)
            01001101 01111000 - (Reserved for future use)
            00100100 10111101 - (Reserved for future use)
            00010011 01011110 - (Reserved for future use)
            10001001 10101111 - (Reserved for future use)
            11000100 11010110 - (Reserved for future use)
            11100010 01101011 - (Reserved for future use)

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.
                                                       Internet Protocol
                                                           Specification
        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

Internet Protocol 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
                                                       Internet Protocol
                                                           Specification
        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.

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
                                                       Internet Protocol
                                                           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.

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
                                                       Internet Protocol
                                                           Specification
    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).

Internet Protocol Specification

    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;
                                                       Internet Protocol
                                                           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

Internet Protocol Specification

      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
                                                       Internet Protocol
                                                           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.

Internet Protocol Specification

    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.
                                                       Internet Protocol
                                                           Specification
  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

Internet Protocol Specification

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

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

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

    where:
      src = source address
      dst = destination address
      prot = protocol
      TOS = type of service
      TTL = time to live
      BufPTR = buffer pointer
      len = length of buffer
      Id  = Identifier
      DF = Don't Fragment
      opt = option data
      result = response
        OK = datagram sent ok
        Error = error in arguments or local network error
    Note that the precedence is included in the TOS and the
    security/compartment is passed as an option.

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

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

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).

                                                       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.

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.
                                                       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.

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.
                                                       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.

Internet Protocol

                                                       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.

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.
                                                       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

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.
                                                       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.