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rfc:rfc791

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

                                                              [Page i]
                                                        September 1981

Internet Protocol

[Page ii]

September 1981

                                                     Internet Protocol
                              PREFACE

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

                                                         Jon Postel
                                                         Editor
                                                            [Page iii]
                                                        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
                                                              [Page 1]
                                                        September 1981

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.

[Page 2]

September 1981

                                                     Internet Protocol
                                                          Introduction
The Options provide for control functions needed or useful in some
situations but unnecessary for the most common communications.  The
options include provisions for timestamps, security, and special
routing.
The Header Checksum provides a verification that the information used
in processing internet datagram has been transmitted correctly.  The
data may contain errors.  If the header checksum fails, the internet
datagram is discarded at once by the entity which detects the error.
The internet protocol does not provide a reliable communication
facility.  There are no acknowledgments either end-to-end or
hop-by-hop.  There is no error control for data, only a header
checksum.  There are no retransmissions.  There is no flow control.
Errors detected may be reported via the Internet Control Message
Protocol (ICMP) [3] which is implemented in the internet protocol
module.
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                                                        September 1981

Internet Protocol

[Page 4]

September 1981

                                                     Internet Protocol
                            2.  OVERVIEW

2.1. Relation to Other Protocols

The following diagram illustrates the place of the internet protocol
in the protocol hierarchy:
                                  
               +------+ +-----+ +-----+     +-----+  
               |Telnet| | FTP | | TFTP| ... | ... |  
               +------+ +-----+ +-----+     +-----+  
                     |   |         |           |     
                    +-----+     +-----+     +-----+  
                    | TCP |     | UDP | ... | ... |  
                    +-----+     +-----+     +-----+  
                       |           |           |     
                    +--------------------------+----+
                    |    Internet Protocol & ICMP   |
                    +--------------------------+----+
                                   |                 
                      +---------------------------+  
                      |   Local Network Protocol  |  
                      +---------------------------+  
                       Protocol Relationships
                             Figure 1.
Internet protocol interfaces on one side to the higher level
host-to-host protocols and on the other side to the local network
protocol.  In this context a "local network" may be a small network in
a building or a large network such as the ARPANET.

2.2. Model of Operation

The  model of operation for transmitting a datagram from one
application program to another is illustrated by the following
scenario:
  We suppose that this transmission will involve one intermediate
  gateway.
  The sending application program prepares its data and calls on its
  local internet module to send that data as a datagram and passes the
  destination address and other parameters as arguments of the call.
  The internet module prepares a datagram header and attaches the data
  to it.  The internet module determines a local network address for
  this internet address, in this case it is the address of a gateway.
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                                                        September 1981

Internet Protocol Overview

  It sends this datagram and the local network address to the local
  network interface.
  The local network interface creates a local network header, and
  attaches the datagram to it, then sends the result via the local
  network.
  The datagram arrives at a gateway host wrapped in the local network
  header, the local network interface strips off this header, and
  turns the datagram over to the internet module.  The internet module
  determines from the internet address that the datagram is to be
  forwarded to another host in a second network.  The internet module
  determines a local net address for the destination host.  It calls
  on the local network interface for that network to send the
  datagram.
  This local network interface creates a local network header and
  attaches the datagram sending the result to the destination host.
  At this destination host the datagram is stripped of the local net
  header by the local network interface and handed to the internet
  module.
  The internet module determines that the datagram is for an
  application program in this host.  It passes the data to the
  application program in response to a system call, passing the source
  address and other parameters as results of the call.
                                  
 Application                                           Application
 Program                                                   Program
       \                                                   /      
     Internet Module      Internet Module      Internet Module    
           \                 /       \                /           
           LNI-1          LNI-1      LNI-2         LNI-2          
              \           /             \          /              
             Local Network 1           Local Network 2            
                          Transmission Path
                              Figure 2

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September 1981

                                                     Internet Protocol
                                                              Overview

2.3. Function Description

The function or purpose of Internet Protocol is to move datagrams
through an interconnected set of networks.  This is done by passing
the datagrams from one internet module to another until the
destination is reached.  The internet modules reside in hosts and
gateways in the internet system.  The datagrams are routed from one
internet module to another through individual networks based on the
interpretation of an internet address.  Thus, one important mechanism
of the internet protocol is the internet address.
In the routing of messages from one internet module to another,
datagrams may need to traverse a network whose maximum packet size is
smaller than the size of the datagram.  To overcome this difficulty, a
fragmentation mechanism is provided in the internet protocol.
Addressing
  A distinction is made between names, addresses, and routes [4].   A
  name indicates what we seek.  An address indicates where it is.  A
  route indicates how to get there.  The internet protocol deals
  primarily with addresses.  It is the task of higher level (i.e.,
  host-to-host or application) protocols to make the mapping from
  names to addresses.   The internet module maps internet addresses to
  local net addresses.  It is the task of lower level (i.e., local net
  or gateways) procedures to make the mapping from local net addresses
  to routes.
  Addresses are fixed length of four octets (32 bits).  An address
  begins with a network number, followed by local address (called the
  "rest" field).  There are three formats or classes of internet
  addresses:  in class a, the high order bit is zero, the next 7 bits
  are the network, and the last 24 bits are the local address; in
  class b, the high order two bits are one-zero, the next 14 bits are
  the network and the last 16 bits are the local address; in class c,
  the high order three bits are one-one-zero, the next 21 bits are the
  network and the last 8 bits are the local address.
  Care must be taken in mapping internet addresses to local net
  addresses; a single physical host must be able to act as if it were
  several distinct hosts to the extent of using several distinct
  internet addresses.  Some hosts will also have several physical
  interfaces (multi-homing).
  That is, provision must be made for a host to have several physical
  interfaces to the network with each having several logical internet
  addresses.
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Internet Protocol Overview

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

[Page 8]

September 1981

                                                     Internet Protocol
                                                              Overview
  datagram.  The more-fragments flag is set to one.  The second
  portion of the data is placed in the second new internet datagram,
  and the total length field is set to the length of the second
  datagram.  The more-fragments flag carries the same value as the
  long datagram.  The fragment offset field of the second new internet
  datagram is set to the value of that field in the long datagram plus
  NFB.
  This procedure can be generalized for an n-way split, rather than
  the two-way split described.
  To assemble the fragments of an internet datagram, an internet
  protocol module (for example at a destination host) combines
  internet datagrams that all have the same value for the four fields:
  identification, source, destination, and protocol.  The combination
  is done by placing the data portion of each fragment in the relative
  position indicated by the fragment offset in that fragment's
  internet header.  The first fragment will have the fragment offset
  zero, and the last fragment will have the more-fragments flag reset
  to zero.

2.4. Gateways

Gateways implement internet protocol to forward datagrams between
networks.  Gateways also implement the Gateway to Gateway Protocol
(GGP) [7] to coordinate routing and other internet control
information.
In a gateway the higher level protocols need not be implemented and
the GGP functions are added to the IP module.
                                  
                 +-------------------------------+   
                 | Internet Protocol & ICMP & GGP|   
                 +-------------------------------+   
                         |                 |         
               +---------------+   +---------------+ 
               |   Local Net   |   |   Local Net   | 
               +---------------+   +---------------+ 
                         Gateway Protocols
                             Figure 3.
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Internet Protocol

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September 1981

                                                     Internet Protocol
                         3.  SPECIFICATION

3.1. Internet Header Format

A summary of the contents of the internet header follows:
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Version|  IHL  |Type of Service|          Total Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Identification        |Flags|      Fragment Offset    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Time to Live |    Protocol   |         Header Checksum       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Source Address                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Destination Address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Options                    |    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Example Internet Datagram Header
                             Figure 4.
Note that each tick mark represents one bit position.
Version:  4 bits
  The Version field indicates the format of the internet header.  This
  document describes version 4.
IHL:  4 bits
  Internet Header Length is the length of the internet header in 32
  bit words, and thus points to the beginning of the data.  Note that
  the minimum value for a correct header is 5.
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Internet Protocol Specification

Type of Service:  8 bits
  The Type of Service provides an indication of the abstract
  parameters of the quality of service desired.  These parameters are
  to be used to guide the selection of the actual service parameters
  when transmitting a datagram through a particular network.  Several
  networks offer service precedence, which somehow treats high
  precedence traffic as more important than other traffic (generally
  by accepting only traffic above a certain precedence at time of high
  load).  The major choice is a three way tradeoff between low-delay,
  high-reliability, and high-throughput.
    Bits 0-2:  Precedence.
    Bit    3:  0 = Normal Delay,      1 = Low Delay.
    Bits   4:  0 = Normal Throughput, 1 = High Throughput.
    Bits   5:  0 = Normal Relibility, 1 = High Relibility.
    Bit  6-7:  Reserved for Future Use.
       0     1     2     3     4     5     6     7
    +-----+-----+-----+-----+-----+-----+-----+-----+
    |                 |     |     |     |     |     |
    |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
    |                 |     |     |     |     |     |
    +-----+-----+-----+-----+-----+-----+-----+-----+
      Precedence
        111 - Network Control
        110 - Internetwork Control
        101 - CRITIC/ECP
        100 - Flash Override
        011 - Flash
        010 - Immediate
        001 - Priority
        000 - Routine
  The use of the Delay, Throughput, and Reliability indications may
  increase the cost (in some sense) of the service.  In many networks
  better performance for one of these parameters is coupled with worse
  performance on another.  Except for very unusual cases at most two
  of these three indications should be set.
  The type of service is used to specify the treatment of the datagram
  during its transmission through the internet system.  Example
  mappings of the internet type of service to the actual service
  provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
  is given in "Service Mappings" [8].

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September 1981

                                                     Internet Protocol
                                                         Specification
  The Network Control precedence designation is intended to be used
  within a network only.  The actual use and control of that
  designation is up to each network. The Internetwork Control
  designation is intended for use by gateway control originators only.
  If the actual use of these precedence designations is of concern to
  a particular network, it is the responsibility of that network to
  control the access to, and use of, those precedence designations.
Total Length:  16 bits
  Total Length is the length of the datagram, measured in octets,
  including internet header and data.  This field allows the length of
  a datagram to be up to 65,535 octets.  Such long datagrams are
  impractical for most hosts and networks.  All hosts must be prepared
  to accept datagrams of up to 576 octets (whether they arrive whole
  or in fragments).  It is recommended that hosts only send datagrams
  larger than 576 octets if they have assurance that the destination
  is prepared to accept the larger datagrams.
  The number 576 is selected to allow a reasonable sized data block to
  be transmitted in addition to the required header information.  For
  example, this size allows a data block of 512 octets plus 64 header
  octets to fit in a datagram.  The maximal internet header is 60
  octets, and a typical internet header is 20 octets, allowing a
  margin for headers of higher level protocols.
Identification:  16 bits
  An identifying value assigned by the sender to aid in assembling the
  fragments of a datagram.
Flags:  3 bits
  Various Control Flags.
    Bit 0: reserved, must be zero
    Bit 1: (DF) 0 = May Fragment,  1 = Don't Fragment.
    Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
        0   1   2
      +---+---+---+
      |   | D | M |
      | 0 | F | F |
      +---+---+---+
Fragment Offset:  13 bits
  This field indicates where in the datagram this fragment belongs.
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Internet Protocol Specification

  The fragment offset is measured in units of 8 octets (64 bits).  The
  first fragment has offset zero.
Time to Live:  8 bits
  This field indicates the maximum time the datagram is allowed to
  remain in the internet system.  If this field contains the value
  zero, then the datagram must be destroyed.  This field is modified
  in internet header processing.  The time is measured in units of
  seconds, but since every module that processes a datagram must
  decrease the TTL by at least one even if it process the datagram in
  less than a second, the TTL must be thought of only as an upper
  bound on the time a datagram may exist.  The intention is to cause
  undeliverable datagrams to be discarded, and to bound the maximum
  datagram lifetime.
Protocol:  8 bits
  This field indicates the next level protocol used in the data
  portion of the internet datagram.  The values for various protocols
  are specified in "Assigned Numbers" [9].
Header Checksum:  16 bits
  A checksum on the header only.  Since some header fields change
  (e.g., time to live), this is recomputed and verified at each point
  that the internet header is processed.
  The checksum algorithm is:
    The checksum field is the 16 bit one's complement of the one's
    complement sum of all 16 bit words in the header.  For purposes of
    computing the checksum, the value of the checksum field is zero.
  This is a simple to compute checksum and experimental evidence
  indicates it is adequate, but it is provisional and may be replaced
  by a CRC procedure, depending on further experience.
Source Address:  32 bits
  The source address.  See section 3.2.
Destination Address:  32 bits
  The destination address.  See section 3.2.

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                                                     Internet Protocol
                                                         Specification
Options:  variable
  The options may appear or not in datagrams.  They must be
  implemented by all IP modules (host and gateways).  What is optional
  is their transmission in any particular datagram, not their
  implementation.
  In some environments the security option may be required in all
  datagrams.
  The option field is variable in length.  There may be zero or more
  options.  There are two cases for the format of an option:
    Case 1:  A single octet of option-type.
    Case 2:  An option-type octet, an option-length octet, and the
             actual option-data octets.
  The option-length octet counts the option-type octet and the
  option-length octet as well as the option-data octets.
  The option-type octet is viewed as having 3 fields:
    1 bit   copied flag,
    2 bits  option class,
    5 bits  option number.
  The copied flag indicates that this option is copied into all
  fragments on fragmentation.
    0 = not copied
    1 = copied
  The option classes are:
    0 = control
    1 = reserved for future use
    2 = debugging and measurement
    3 = reserved for future use
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Internet Protocol Specification

  The following internet options are defined:
    CLASS NUMBER LENGTH DESCRIPTION
    ----- ------ ------ -----------
      0     0      -    End of Option list.  This option occupies only
                        1 octet; it has no length octet.
      0     1      -    No Operation.  This option occupies only 1
                        octet; it has no length octet.
      0     2     11    Security.  Used to carry Security,
                        Compartmentation, User Group (TCC), and
                        Handling Restriction Codes compatible with DOD
                        requirements.
      0     3     var.  Loose Source Routing.  Used to route the
                        internet datagram based on information
                        supplied by the source.
      0     9     var.  Strict Source Routing.  Used to route the
                        internet datagram based on information
                        supplied by the source.
      0     7     var.  Record Route.  Used to trace the route an
                        internet datagram takes.
      0     8      4    Stream ID.  Used to carry the stream
                        identifier.
      2     4     var.  Internet Timestamp.
  Specific Option Definitions
    End of Option List
      +--------+
      |00000000|
      +--------+
        Type=0
      This option indicates the end of the option list.  This might
      not coincide with the end of the internet header according to
      the internet header length.  This is used at the end of all
      options, not the end of each option, and need only be used if
      the end of the options would not otherwise coincide with the end
      of the internet header.
      May be copied, introduced, or deleted on fragmentation, or for
      any other reason.

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                                                     Internet Protocol
                                                         Specification
    No Operation
      +--------+
      |00000001|
      +--------+
        Type=1
      This option may be used between options, for example, to align
      the beginning of a subsequent option on a 32 bit boundary.
      May be copied, introduced, or deleted on fragmentation, or for
      any other reason.
    Security
      This option provides a way for hosts to send security,
      compartmentation, handling restrictions, and TCC (closed user
      group) parameters.  The format for this option is as follows:
        +--------+--------+---//---+---//---+---//---+---//---+
        |10000010|00001011|SSS  SSS|CCC  CCC|HHH  HHH|  TCC   |
        +--------+--------+---//---+---//---+---//---+---//---+
         Type=130 Length=11
      Security (S field):  16 bits
        Specifies one of 16 levels of security (eight of which are
        reserved for future use).
          00000000 00000000 - Unclassified
          11110001 00110101 - Confidential
          01111000 10011010 - EFTO
          10111100 01001101 - MMMM
          01011110 00100110 - PROG
          10101111 00010011 - Restricted
          11010111 10001000 - Secret
          01101011 11000101 - Top Secret
          00110101 11100010 - (Reserved for future use)
          10011010 11110001 - (Reserved for future use)
          01001101 01111000 - (Reserved for future use)
          00100100 10111101 - (Reserved for future use)
          00010011 01011110 - (Reserved for future use)
          10001001 10101111 - (Reserved for future use)
          11000100 11010110 - (Reserved for future use)
          11100010 01101011 - (Reserved for future use)
                                                             [Page 17]
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Internet Protocol Specification

      Compartments (C field):  16 bits
        An all zero value is used when the information transmitted is
        not compartmented.  Other values for the compartments field
        may be obtained from the Defense Intelligence Agency.
      Handling Restrictions (H field):  16 bits
        The values for the control and release markings are
        alphanumeric digraphs and are defined in the Defense
        Intelligence Agency Manual DIAM 65-19, "Standard Security
        Markings".
      Transmission Control Code (TCC field):  24 bits
        Provides a means to segregate traffic and define controlled
        communities of interest among subscribers. The TCC values are
        trigraphs, and are available from HQ DCA Code 530.
      Must be copied on fragmentation.  This option appears at most
      once in a datagram.
    Loose Source and Record Route
      +--------+--------+--------+---------//--------+
      |10000011| length | pointer|     route data    |
      +--------+--------+--------+---------//--------+
       Type=131
      The loose source and record route (LSRR) option provides a means
      for the source of an internet datagram to supply routing
      information to be used by the gateways in forwarding the
      datagram to the destination, and to record the route
      information.
      The option begins with the option type code.  The second octet
      is the option length which includes the option type code and the
      length octet, the pointer octet, and length-3 octets of route
      data.  The third octet is the pointer into the route data
      indicating the octet which begins the next source address to be
      processed.  The pointer is relative to this option, and the
      smallest legal value for the pointer is 4.
      A route data is composed of a series of internet addresses.
      Each internet address is 32 bits or 4 octets.  If the pointer is
      greater than the length, the source route is empty (and the
      recorded route full) and the routing is to be based on the
      destination address field.

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                                                         Specification
      If the address in destination address field has been reached and
      the pointer is not greater than the length, the next address in
      the source route replaces the address in the destination address
      field, and the recorded route address replaces the source
      address just used, and pointer is increased by four.
      The recorded route address is the internet module's own internet
      address as known in the environment into which this datagram is
      being forwarded.
      This procedure of replacing the source route with the recorded
      route (though it is in the reverse of the order it must be in to
      be used as a source route) means the option (and the IP header
      as a whole) remains a constant length as the datagram progresses
      through the internet.
      This option is a loose source route because the gateway or host
      IP is allowed to use any route of any number of other
      intermediate gateways to reach the next address in the route.
      Must be copied on fragmentation.  Appears at most once in a
      datagram.
    Strict Source and Record Route
      +--------+--------+--------+---------//--------+
      |10001001| length | pointer|     route data    |
      +--------+--------+--------+---------//--------+
       Type=137
      The strict source and record route (SSRR) option provides a
      means for the source of an internet datagram to supply routing
      information to be used by the gateways in forwarding the
      datagram to the destination, and to record the route
      information.
      The option begins with the option type code.  The second octet
      is the option length which includes the option type code and the
      length octet, the pointer octet, and length-3 octets of route
      data.  The third octet is the pointer into the route data
      indicating the octet which begins the next source address to be
      processed.  The pointer is relative to this option, and the
      smallest legal value for the pointer is 4.
      A route data is composed of a series of internet addresses.
      Each internet address is 32 bits or 4 octets.  If the pointer is
      greater than the length, the source route is empty (and the
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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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                                                         Specification
      greater than the length, the recorded route data area is full.
      The originating host must compose this option with a large
      enough route data area to hold all the address expected.  The
      size of the option does not change due to adding addresses.  The
      intitial contents of the route data area must be zero.
      When an internet module routes a datagram it checks to see if
      the record route option is present.  If it is, it inserts its
      own internet address as known in the environment into which this
      datagram is being forwarded into the recorded route begining at
      the octet indicated by the pointer, and increments the pointer
      by four.
      If the route data area is already full (the pointer exceeds the
      length) the datagram is forwarded without inserting the address
      into the recorded route.  If there is some room but not enough
      room for a full address to be inserted, the original datagram is
      considered to be in error and is discarded.  In either case an
      ICMP parameter problem message may be sent to the source
      host [3].
      Not copied on fragmentation, goes in first fragment only.
      Appears at most once in a datagram.
    Stream Identifier
      +--------+--------+--------+--------+
      |10001000|00000010|    Stream ID    |
      +--------+--------+--------+--------+
       Type=136 Length=4
      This option provides a way for the 16-bit SATNET stream
      identifier to be carried through networks that do not support
      the stream concept.
      Must be copied on fragmentation.  Appears at most once in a
      datagram.
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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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                                                         Specification
      timestamps.  The intitial contents of the timestamp data area
      must be zero or internet address/zero pairs.
      If the timestamp data area is already full (the pointer exceeds
      the length) the datagram is forwarded without inserting the
      timestamp, but the overflow count is incremented by one.
      If there is some room but not enough room for a full timestamp
      to be inserted, or the overflow count itself overflows, the
      original datagram is considered to be in error and is discarded.
      In either case an ICMP parameter problem message may be sent to
      the source host [3].
      The timestamp option is not copied upon fragmentation.  It is
      carried in the first fragment.  Appears at most once in a
      datagram.
Padding:  variable
  The internet header padding is used to ensure that the internet
  header ends on a 32 bit boundary.  The padding is zero.

3.2. Discussion

The implementation of a protocol must be robust.  Each implementation
must expect to interoperate with others created by different
individuals.  While the goal of this specification is to be explicit
about the protocol there is the possibility of differing
interpretations.  In general, an implementation must be conservative
in its sending behavior, and liberal in its receiving behavior.  That
is, it must be careful to send well-formed datagrams, but must accept
any datagram that it can interpret (e.g., not object to technical
errors where the meaning is still clear).
The basic internet service is datagram oriented and provides for the
fragmentation of datagrams at gateways, with reassembly taking place
at the destination internet protocol module in the destination host.
Of course, fragmentation and reassembly of datagrams within a network
or by private agreement between the gateways of a network is also
allowed since this is transparent to the internet protocols and the
higher-level protocols.  This transparent type of fragmentation and
reassembly is termed "network-dependent" (or intranet) fragmentation
and is not discussed further here.
Internet addresses distinguish sources and destinations to the host
level and provide a protocol field as well.  It is assumed that each
protocol will provide for whatever multiplexing is necessary within a
host.
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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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                                                     Internet Protocol
                                                         Specification
  fragmentation strategy is designed so than an unfragmented datagram
  has all zero fragmentation information (MF = 0, fragment offset =
  0).  If an internet datagram is fragmented, its data portion must be
  broken on 8 octet boundaries.
  This format allows 2**13 = 8192 fragments of 8 octets each for a
  total of 65,536 octets.  Note that this is consistent with the the
  datagram total length field (of course, the header is counted in the
  total length and not in the fragments).
  When fragmentation occurs, some options are copied, but others
  remain with the first fragment only.
  Every internet module must be able to forward a datagram of 68
  octets without further fragmentation.  This is because an internet
  header may be up to 60 octets, and the minimum fragment is 8 octets.
  Every internet destination must be able to receive a datagram of 576
  octets either in one piece or in fragments to be reassembled.
  The fields which may be affected by fragmentation include:
    (1) options field
    (2) more fragments flag
    (3) fragment offset
    (4) internet header length field
    (5) total length field
    (6) header checksum
  If the Don't Fragment flag (DF) bit is set, then internet
  fragmentation of this datagram is NOT permitted, although it may be
  discarded.  This can be used to prohibit fragmentation in cases
  where the receiving host does not have sufficient resources to
  reassemble internet fragments.
  One example of use of the Don't Fragment feature is to down line
  load a small host.  A small host could have a boot strap program
  that accepts a datagram stores it in memory and then executes it.
  The fragmentation and reassembly procedures are most easily
  described by examples.  The following procedures are example
  implementations.
  General notation in the following pseudo programs: "=<" means "less
  than or equal", "#" means "not equal", "=" means "equal", "<-" means
  "is set to".  Also, "x to y" includes x and excludes y; for example,
  "4 to 7" would include 4, 5, and 6 (but not 7).
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  An Example Fragmentation Procedure
    The maximum sized datagram that can be transmitted through the
    next network is called the maximum transmission unit (MTU).
    If the total length is less than or equal the maximum transmission
    unit then submit this datagram to the next step in datagram
    processing; otherwise cut the datagram into two fragments, the
    first fragment being the maximum size, and the second fragment
    being the rest of the datagram.  The first fragment is submitted
    to the next step in datagram processing, while the second fragment
    is submitted to this procedure in case it is still too large.
    Notation:
      FO    -  Fragment Offset
      IHL   -  Internet Header Length
      DF    -  Don't Fragment flag
      MF    -  More Fragments flag
      TL    -  Total Length
      OFO   -  Old Fragment Offset
      OIHL  -  Old Internet Header Length
      OMF   -  Old More Fragments flag
      OTL   -  Old Total Length
      NFB   -  Number of Fragment Blocks
      MTU   -  Maximum Transmission Unit
    Procedure:
      IF TL =< MTU THEN Submit this datagram to the next step
           in datagram processing ELSE IF DF = 1 THEN discard the
      datagram ELSE
      To produce the first fragment:
      (1)  Copy the original internet header;
      (2)  OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
      (3)  NFB <- (MTU-IHL*4)/8;
      (4)  Attach the first NFB*8 data octets;
      (5)  Correct the header:
           MF <- 1;  TL <- (IHL*4)+(NFB*8);
           Recompute Checksum;
      (6)  Submit this fragment to the next step in
           datagram processing;
      To produce the second fragment:
      (7)  Selectively copy the internet header (some options
           are not copied, see option definitions);
      (8)  Append the remaining data;
      (9)  Correct the header:
           IHL <- (((OIHL*4)-(length of options not copied))+3)/4;

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                                                         Specification
           TL <- OTL - NFB*8 - (OIHL-IHL)*4);
           FO <- OFO + NFB;  MF <- OMF;  Recompute Checksum;
      (10) Submit this fragment to the fragmentation test; DONE.
    In the above procedure each fragment (except the last) was made
    the maximum allowable size.  An alternative might produce less
    than the maximum size datagrams.  For example, one could implement
    a fragmentation procedure that repeatly divided large datagrams in
    half until the resulting fragments were less than the maximum
    transmission unit size.
  An Example Reassembly Procedure
    For each datagram the buffer identifier is computed as the
    concatenation of the source, destination, protocol, and
    identification fields.  If this is a whole datagram (that is both
    the fragment offset and the more fragments  fields are zero), then
    any reassembly resources associated with this buffer identifier
    are released and the datagram is forwarded to the next step in
    datagram processing.
    If no other fragment with this buffer identifier is on hand then
    reassembly resources are allocated.  The reassembly resources
    consist of a data buffer, a header buffer, a fragment block bit
    table, a total data length field, and a timer.  The data from the
    fragment is placed in the data buffer according to its fragment
    offset and length, and bits are set in the fragment block bit
    table corresponding to the fragment blocks received.
    If this is the first fragment (that is the fragment offset is
    zero)  this header is placed in the header buffer.  If this is the
    last fragment ( that is the more fragments field is zero) the
    total data length is computed.  If this fragment completes the
    datagram (tested by checking the bits set in the fragment block
    table), then the datagram is sent to the next step in datagram
    processing; otherwise the timer is set to the maximum of the
    current timer value and the value of the time to live field from
    this fragment; and the reassembly routine gives up control.
    If the timer runs out, the all reassembly resources for this
    buffer identifier are released.  The initial setting of the timer
    is a lower bound on the reassembly waiting time.  This is because
    the waiting time will be increased if the Time to Live in the
    arriving fragment is greater than the current timer value but will
    not be decreased if it is less.  The maximum this timer value
    could reach is the maximum time to live (approximately 4.25
    minutes).  The current recommendation for the initial timer
    setting is 15 seconds.  This may be changed as experience with
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    this protocol accumulates.  Note that the choice of this parameter
    value is related to the buffer capacity available and the data
    rate of the transmission medium; that is, data rate times timer
    value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).
    Notation:
      FO    -  Fragment Offset
      IHL   -  Internet Header Length
      MF    -  More Fragments flag
      TTL   -  Time To Live
      NFB   -  Number of Fragment Blocks
      TL    -  Total Length
      TDL   -  Total Data Length
      BUFID -  Buffer Identifier
      RCVBT -  Fragment Received Bit Table
      TLB   -  Timer Lower Bound
    Procedure:
      (1)  BUFID <- source|destination|protocol|identification;
      (2)  IF FO = 0 AND MF = 0
      (3)     THEN IF buffer with BUFID is allocated
      (4)             THEN flush all reassembly for this BUFID;
      (5)          Submit datagram to next step; DONE.
      (6)     ELSE IF no buffer with BUFID is allocated
      (7)             THEN allocate reassembly resources
                           with BUFID;
                           TIMER <- TLB; TDL <- 0;
      (8)          put data from fragment into data buffer with
                   BUFID from octet FO*8 to
                                       octet (TL-(IHL*4))+FO*8;
      (9)          set RCVBT bits from FO
                                      to FO+((TL-(IHL*4)+7)/8);
      (10)         IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
      (11)         IF FO = 0 THEN put header in header buffer
      (12)         IF TDL # 0
      (13)          AND all RCVBT bits from 0
                                           to (TDL+7)/8 are set
      (14)            THEN TL <- TDL+(IHL*4)
      (15)                 Submit datagram to next step;
      (16)                 free all reassembly resources
                           for this BUFID; DONE.
      (17)         TIMER <- MAX(TIMER,TTL);
      (18)         give up until next fragment or timer expires;
      (19) timer expires: flush all reassembly with this BUFID; DONE.
    In the case that two or more fragments contain the same data

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                                                         Specification
    either identically or through a partial overlap, this procedure
    will use the more recently arrived copy in the data buffer and
    datagram delivered.
Identification
  The choice of the Identifier for a datagram is based on the need to
  provide a way to uniquely identify the fragments of a particular
  datagram.  The protocol module assembling fragments judges fragments
  to belong to the same datagram if they have the same source,
  destination, protocol, and Identifier.  Thus, the sender must choose
  the Identifier to be unique for this source, destination pair and
  protocol for the time the datagram (or any fragment of it) could be
  alive in the internet.
  It seems then that a sending protocol module needs to keep a table
  of Identifiers, one entry for each destination it has communicated
  with in the last maximum packet lifetime for the internet.
  However, since the Identifier field allows 65,536 different values,
  some host may be able to simply use unique identifiers independent
  of destination.
  It is appropriate for some higher level protocols to choose the
  identifier. For example, TCP protocol modules may retransmit an
  identical TCP segment, and the probability for correct reception
  would be enhanced if the retransmission carried the same identifier
  as the original transmission since fragments of either datagram
  could be used to construct a correct TCP segment.
Type of Service
  The type of service (TOS) is for internet service quality selection.
  The type of service is specified along the abstract parameters
  precedence, delay, throughput, and reliability.  These abstract
  parameters are to be mapped into the actual service parameters of
  the particular networks the datagram traverses.
  Precedence.  An independent measure of the importance of this
  datagram.
  Delay.  Prompt delivery is important for datagrams with this
  indication.
  Throughput.  High data rate is important for datagrams with this
  indication.
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  Reliability.  A higher level of effort to ensure delivery is
  important for datagrams with this indication.
  For example, the ARPANET has a priority bit, and a choice between
  "standard" messages (type 0) and "uncontrolled" messages (type 3),
  (the choice between single packet and multipacket messages can also
  be considered a service parameter). The uncontrolled messages tend
  to be less reliably delivered and suffer less delay.  Suppose an
  internet datagram is to be sent through the ARPANET.  Let the
  internet type of service be given as:
    Precedence:    5
    Delay:         0
    Throughput:    1
    Reliability:   1
  In this example, the mapping of these parameters to those available
  for the ARPANET would be  to set the ARPANET priority bit on since
  the Internet precedence is in the upper half of its range, to select
  standard messages since the throughput and reliability requirements
  are indicated and delay is not.  More details are given on service
  mappings in "Service Mappings" [8].
Time to Live
  The time to live is set by the sender to the maximum time the
  datagram is allowed to be in the internet system.  If the datagram
  is in the internet system longer than the time to live, then the
  datagram must be destroyed.
  This field must be decreased at each point that the internet header
  is processed to reflect the time spent processing the datagram.
  Even if no local information is available on the time actually
  spent, the field must be decremented by 1.  The time is measured in
  units of seconds (i.e. the value 1 means one second).  Thus, the
  maximum time to live is 255 seconds or 4.25 minutes.  Since every
  module that processes a datagram must decrease the TTL by at least
  one even if it process the datagram in less than a second, the TTL
  must be thought of only as an upper bound on the time a datagram may
  exist.  The intention is to cause undeliverable datagrams to be
  discarded, and to bound the maximum datagram lifetime.
  Some higher level reliable connection protocols are based on
  assumptions that old duplicate datagrams will not arrive after a
  certain time elapses.  The TTL is a way for such protocols to have
  an assurance that their assumption is met.

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                                                         Specification
Options
  The options are optional in each datagram, but required in
  implementations.  That is, the presence or absence of an option is
  the choice of the sender, but each internet module must be able to
  parse every option.  There can be several options present in the
  option field.
  The options might not end on a 32-bit boundary.  The internet header
  must be filled out with octets of zeros.  The first of these would
  be interpreted as the end-of-options option, and the remainder as
  internet header padding.
  Every internet module must be able to act on every option.  The
  Security Option is required if classified, restricted, or
  compartmented traffic is to be passed.
Checksum
  The internet header checksum is recomputed if the internet header is
  changed.  For example, a reduction of the time to live, additions or
  changes to internet options, or due to fragmentation.  This checksum
  at the internet level is intended to protect the internet header
  fields from transmission errors.
  There are some applications where a few data bit errors are
  acceptable while retransmission delays are not.  If the internet
  protocol enforced data correctness such applications could not be
  supported.
Errors
  Internet protocol errors may be reported via the ICMP messages [3].

3.3. Interfaces

The functional description of user interfaces to the IP is, at best,
fictional, since every operating system will have different
facilities.  Consequently, we must warn readers that different IP
implementations may have different user interfaces.  However, all IPs
must provide a certain minimum  set of services to guarantee that all
IP implementations can support the same protocol hierarchy.  This
section specifies the functional interfaces required of all IP
implementations.
Internet protocol interfaces on one side to the local network and on
the other side to either a higher level protocol or an application
program.  In the following, the higher level protocol or application
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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

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                                                     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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