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Scenix Lib IO OSI3 Tcpip Documentation Rfcs RFC791.TXT

 






       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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                                       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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                                                                   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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                                     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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           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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       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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           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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           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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               Compartments (C field):  16 bits

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

               Handling Restrictions (H field):  16 bits

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

               Transmission Control Code (TCC field):  24 bits

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

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

             Loose Source and Record Route

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

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

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

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


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

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

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

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

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

             Strict Source and Record Route

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

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

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

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



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

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

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

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

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

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

             Record Route

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

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

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

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


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

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

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

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

             Stream Identifier

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

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

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













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             Internet Timestamp

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

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

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

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

               The Flag (flg) [4 bits] values are

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

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

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

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

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


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

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

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

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

         Padding:  variable

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

       3.2.  Discussion

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

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

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


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         Addressing

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

           Address Formats:

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

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

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

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

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

         Fragmentation and Reassembly.

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

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


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

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

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

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

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

           The fields which may be affected by fragmentation include:

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

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

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

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

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



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           An Example Fragmentation Procedure

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

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

             Notation:

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

             Procedure:

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


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

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

           An Example Reassembly Procedure

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

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

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

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


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

             Notation:

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

             Procedure:

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

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


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

         Identification

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

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

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

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

         Type of Service

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

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

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

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




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

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

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

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

         Time to Live

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

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

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




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         Options

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

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

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

         Checksum

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

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

         Errors

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

       3.3.  Interfaces

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

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


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         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



       [Page 32]







       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.










                                                                      [Page 33]







                                                                 September 1981
       Internet Protocol



       APPENDIX A:  Examples & Scenarios

       Example 1:

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


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

                              Example Internet Datagram

                                      Figure 5.

         Note that each tick mark represents one bit position.

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


















       [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.






















                                                                      [Page 37]







                                                                 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]



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