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rfc:ien:ien111
                                                           August 1979

IEN: 111

                                  
                                  
                                  
                                  
                                  
                                  
                                  
                         INTERNET PROTOCOL
                                  
                                  
                                  
                            August 1979
                            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

August 1979

                                                     Internet Protocol
                         TABLE OF CONTENTS
  PREFACE ........................................................ iii

1. INTRODUCTION …………………………………………….. 1

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

2. OVERVIEW ………………………………………………… 5

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

3. SPECIFICATION …………………………………………… 11

3.1  Internet Header Format ....................................... 11
3.2  Discussion ................................................... 22
3.3  Examples & Scenarios ......................................... 30
3.4  Interfaces ................................................... 34

GLOSSARY …………………………………………………… 37

REFERENCES …………………………………………………. 41

                                                              [Page i]
                                                           August 1979

Internet Protocol

[Page ii]

August 1979

                                                     Internet Protocol
                              PREFACE

This document describes the Internet Protocol. There have been three previous editions of this specification, and the present text draws heavily from them. There have been many contributors to this document both in terms of concepts and in terms of text.

                                                         Jon Postel
                                                         Editor
                                                            [Page iii]

August 1979 IEN: 111 Replaces: IENs 80, 54, 44, 41, 28, 26

                  Internet 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 promote data reliability,
flow control, sequencing, or other services commonly found in
host-to-host protocols.

1.3. Interfaces

This protocol is called on by host-to-host protocols in an internet
environment.  This protocol calls on local network protocols to carry
the internet datagram to the next gateway or destination host.
For example, a TCP module would call on the internet module to take a
TCP segment (including the TCP header and user data) as the data
portion of an internet datagram.  The TCP module would provide the
addresses and other parameters in the internet header to the internet
module as arguments of the call.  The internet module would then
create an internet datagram and call on the local network interface to
transmit the internet datagram.
In the ARPANET case, for example, the internet module would call on a
local net module which would add the 1822 leader [2] to the internet
datagram creating an ARPANET message to transmit to the IMP.  The
ARPANET address would be derived from the internet address by the
local network interface and would be the address of some host in the
ARPANET, that host might be a gateway to other networks.
                                                              [Page 1]
                                                           August 1979

Internet Protocol Introduction

1.4. Operation

The internet protocol implements two basic functions:  addressing and
fragmentation.
The internet modules use the addresses carried in the internet header
to transmit the internet datagram toward their destinations.  The
selection of a path for transmission is called routing.
The internet modules use fields in the internet header to fragment and
reassemble internet datagrams when necessary for transmission through
"small packet" networks.
The model of operation is that an internet module resides in each host
engaged in internet communication and in each gateway that
interconnects networks.  These modules share common rules for
interpreting address fields and for fragmenting and assembling
internet datagrams.  In addition, these modules (especially in
gateways) may have procedures for making routing decisions and other
functions.
The internet protocol treats each internet datagram as an independent
entity unrelated to any other internet datagram.  There are no
connections or logical circuits (virtual or otherwise).
The internet protocol uses four key mechanisms in providing its
service:  Type of Service, Time to Live, Options, and Header Checksum.
The Type of Service is used to indicate the quality of the service
desired; this may be thought of as selecting among Interactive, Bulk,
or Real Time, for example.  The type of service is an abstract or
generalized set of parameters which characterize the service choices
provided in the networks that make up the internet.  This type of
service indication is to be used by gateways to select the actual
transmission parameters for a particular network, the network to be
used for the next hop, or the next gateway when routing an internet
datagram.
The Time to Live is an indication of the lifetime of an internet
datagram.  It is set by the sender of the datagram and reduced at the
points along the route where it is processed.  If the time to live
reaches zero before the internet datagram reaches its destination, the
internet datagram is destroyed.  The time to live can be thought of as
a self destruct time limit.
The Options provide for control functions needed or useful in some
situations but unnecessary for the most common communications.  The

[Page 2]

August 1979

                                                     Internet Protocol
                                                          Introduction
options include provisions for timestamps, error reports, and special
routing.
The Header Checksum provides a verification that the information used
in processing internet datagram has been transmitted correctly.  The
data may contain errors.  If the header checksum fails, the internet
datagram is discarded at once by the entity which detects the error.
The internet protocol does not provide a reliable communication
facility.  There are no acknowledgments either end-to-end or
hop-by-hop.  There is no error control for data, only a header
checksum.  There are no retransmissions.  There is no flow control.
                                                              [Page 3]
                                                           August 1979

Internet Protocol

[Page 4]

August 1979

                                                     Internet Protocol
                            2.  OVERVIEW

2.1. Relation to Other Protocols

The following diagram illustrates the place of the internet protocol
in the protocol hierarchy:
                                  
               +------+ +-----+ +-----+       +-----+
               |Telnet| | FTP | |Voice|  ...  |     |
               +------+ +-----+ +-----+       +-----+
                     |   |         |             |   
                    +-----+     +-----+       +-----+
                    | TCP |     | RTP |  ...  |     |
                    +-----+     +-----+       +-----+
                       |           |             |   
                    +-------------------------------+
                    |       Internet Protocol       |
                    +-------------------------------+
                                   |                 
                      +---------------------------+  
                      |   Local Network Protocol  |  
                      +---------------------------+  
                                   |                 
                       Protocol Relationships
                             Figure 1.
Internet protocol interfaces on one side to the higher level
host-to-host protocols and on the other side to the local network
protocol.

2.2. Model of Operation

The  model of operation for transmitting a datagram from one
application program to another is illustrated by the following
scenario:
  We suppose that this transmission will involve one intermediate
  gateway.
  The sending application program prepares its data and calls on its
  local internet module to send that data as a datagram and passes the
  destination address and other parameters as arguments of the call.
  The internet module prepares a datagram header and attaches the data
                                                              [Page 5]
                                                           August 1979

Internet Protocol Overview

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

[Page 6]

August 1979

                                                     Internet Protocol
                                                              Overview

2.3. Function Description

The function or purpose of Internet Protocol is to move datagrams
through an interconnected set of networks.  This is done by passing
the datagrams from one internet module to another until the
destination is reached.  The internet modules reside in hosts and
gateways in the internet system.  The datagrams are routed from one
internet module to another through individual networks based on the
interpretation of an internet address.  Thus, one important mechanism
of the internet protocol is the internet address.
In the routing of messages from one internet module to another,
datagrams may need to traverse a network whose maximum packet size is
smaller than the size of the datagram.  To overcome this difficulty, a
fragmentation mechanism is provided in the internet protocol.
Addressing
  A distinction is made between names, addresses, and routes [3].   A
  name indicates what we seek.  An address indicates where it is.  A
  route indicates how to get there.  The internet protocol deals
  primarily with addresses.  It is the task of higher level (i.e.,
  host-to-host or application) protocols to make the mapping from
  names to addresses.   The internet module maps internet addresses to
  local net addresses.  It is the task of lower level (i.e., local net
  or gateways) procedures to make the mapping from local net addresses
  addresses to routes.
  Addresses are fixed length of four octets (32 bits).  An address
  begins with a one octet network number, followed by a three octet
  local address.  This three octet field is called the "rest" field.
  Care must be taken in mapping internet addresses to local net
  addresses; a single physical host must be able to act as if it were
  several distinct hosts to the extent of using several distinct
  internet addresses.  A host should also be able to have several
  physical interfaces (multi-homing).
  That is, a host should be allowed several physical interfaces to the
  network with each having several logical internet addresses.
  Examples of address mappings may be found in reference [4].
Fragmentation
  Fragmentation of an internet datagram may be necessary when it
  originates in a local net that allows a large packet size and must
                                                              [Page 7]
                                                           August 1979

Internet Protocol Overview

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

[Page 8]

August 1979

                                                     Internet Protocol
                                                              Overview
  datagram is set to the value of that field in the long datagram plus
  NFB.
  This procedure can be generalized for an n-way split, rather than
  the two-way split described.
  To assemble the fragments of an internet datagram, an internet
  protocol module (for example at a destination host) combines
  internet datagram that all have the same value for the four fields:
  identification, source, destination, and protocol.  The combination
  is done by placing the data portion of each fragment in the relative
  position indicated by the fragment offset in that fragment's
  internet header.  The first fragment will have the fragment offset
  zero, and the last fragment will have the more-fragments flag reset
  to zero.
                                                              [Page 9]
                                                           August 1979

Internet Protocol

[Page 10]

August 1979

                                                     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 3.
Note that each tick mark represents one bit position.
Version:  4 bits
  The Version field indicates the format of the internet header.  This
  document describes version 4.
IHL:  4 bits
  Internet Header Length is the length of the internet header in 32
  bit words, and thus points to the beginning of the data.  Note that
  the minimum value for a correct header is 5.
                                                             [Page 11]
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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 a priority service, which somehow treats high
  priority traffic as more important than other traffic.  A few
  networks offer a Stream service, whereby one can achieve a smoother
  service at some cost.  Typically this involves the reservation of
  resources within the network.  Another choice involves a low-delay
  vs. high-reliability trade off.  Typically networks invoke more
  complex (and delay producing) mechanisms as the need for reliability
  increases.
    Bits 0-1:  Priority.
    Bit    2:  Stream or Datagram.
    Bits 3-4:  Reliability.
    Bit    5:  Speed over Reliability.
    Bits 6-7:  Speed.
       0     1     2     3     4     5     6     7
    +-----+-----+-----+-----+-----+-----+-----+-----+
    |           |     |           |     |           |
    |  PRIORITY |STRM |RELIABILITY| S/R |   SPEED   |
    |           |     |           |     |           |
    +-----+-----+-----+-----+-----+-----+-----+-----+
    PRIORITY    STRM      RELIABILITY  S/R     SPEED
    11-highest  1-STREAM  11-highest   1-speed 11-highest
    10-higher   0-DTGRM   10-higher    0-rlblt 10-higher
    01-lower              01-lower             01-lower
    00-lowest             00-lowest            00-lowest
  The type of service is used to specify the treatment of the datagram
  during its transmission through the internet system.  In the
  discussion (section 3.2) below, a chart shows the relationship of
  the internet type of service to the actual service provided on the
  ARPANET, the SATNET, and the PRNET.
Total Length:  16 bits
  Total Length is the length of the datagram, measured in octets,
  including internet header and data.  This field allows the length of
  a datagram to be up to 65,535 octets.  Such long datagrams are
  impractical for most hosts and networks.  All hosts must be prepared
  to accept datagrams of up to 576 octets (whether they arrive whole
  in fragments).  It is recommended that hosts only send datagrams

[Page 12]

August 1979

                                                     Internet Protocol
                                                         Specification
  larger than 576 octets if they have assurance that the destination
  is prepared to accept the larger datagrams.
  The number 576 is selected to allow a reasonable sized data block to
  be transmitted in addition to the required header information.  For
  example, this size allows a data block of 512 octets plus 64 header
  octets to fit in a datagram.  The maximal internet header is 60
  octets, and a typical internet header is 20 octets, allowing a
  margin for headers of higher level protocols.
Identification:  16 bits
  An identifying value assigned by the sender to aid in assembling the
  fragments of a datagram.
Flags:  3 bits
  Various Control Flags.
    Bit 0: reserved, must be zero
    Bit 1: Don't Fragment This Datagram (DF).
    Bit 2: More Fragments Flag (MF).
        0   1   2
      +---+---+---+
      |   | D | M |
      | 0 | F | F |
      +---+---+---+
Fragment Offset:  13 bits
  This field indicates where in the datagram this fragment belongs.
  The fragment offset is measured in units of 8 octets (64 bits).  The
  first fragment has offset zero.
Time to Live:  8 bits
  This field indicates the maximum time the datagram is allowed to
  remain the internet system.  If this field contains the value zero,
  then the datagram should be destroyed.  This field is modified in
  internet header processing.  The time is measured in units of
  seconds.  The intention is to cause undeliverable datagrams to be
  discarded.
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Internet Protocol Specification

Protocol:  8 bits
  This field indicates the next level protocol used in the data
  portion of the internet datagram.  The values for various protocols
  are specified in reference [6].
Header Checksum:  16 bits
  A checksum on the header only.  Since some header fields may change
  (e.g., time to live), this is recomputed and verified at each point
  that the internet header is processed.
  The checksum algorithm is:
    The checksum field is the 16 bit one's complement of the one's
    complement sum of all 16 bit words in the header.  For purposes of
    computing the checksum, the value of the checksum field is zero.
  This is a simple to compute checksum and experimental evidence
  indicates it is adequate, but it is provisional and may be replaced
  by a CRC procedure, depending on further experience.
Source Address:  32 bits
  The source address.  The first octet is the Source Network, and the
  following three octets are the Source Local Address.
Destination Address:  32 bits
  The destination address.  The first octet is the Destination
  Network, and the following three octets are the Destination Local
  Address.

[Page 14]

August 1979

                                                     Internet Protocol
                                                         Specification
Options:  variable
  The option field is variable in length.  There may be zero or more
  options.  There are two case for the format of an option:
    Case 1:  A single octet of option-type.
    Case 2:  An option-type octet, an option-length octet, and the
             actual option-data octets.
  The option-length octet counts the option-type octet and the
  option-length octet as well as the option-data octets.
  The option-type octet is viewed as having 3 fields:
    1 bit   reserved, must be zero
    2 bits  option class,
    5 bits  option number.
  The option classes are:
    0 = control
    1 = internet error
    2 = experimental debugging and measurement
    3 = reserved for future use
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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      4    S/P/T.  Used to carry Security, Precedence,
                        and user group (TCC) information compatible
                        with AUTODIN II requirements.
      0     3     var.  Source Routing.  Used to route the internet
                        datagram based on information supplied by the
                        source.
      0     4     var.  BCR Open.
      0     5     var.  BCR Close.
      0     6     var.  BCR other.
      0     7     var.  Return Route.  Used to record the route an
                        internet datagram takes.
      0     8      4    Stream ID.  Used to carry the stream
                        identifier.
      1     1     var.  General Error Report.  Used to report errors
                        in internet datagram processing.
      2     4      6    Internet Timestamp.  Used to accumulate
                        timestamping information during internet
                        transit.  The length field is variable and may
                        change as the internet datagram traverses the
                        networks and gateways of the internet system.
      2     5      6    Satellite Timestamp.  Used as above for
                        special satellite network testing.
  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.

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                                                     Internet Protocol
                                                         Specification
      May be copied, introduced, or deleted on fragmentation.
    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.
    S/P/T
      This option provides a way for AUTODIN II hosts to send
      security, precedence, and TCC (closed user groups) parameters
      through networks whose transport leader does not contain fields
      for this information.  The format for this option is as follows:
        +--------+--------+---------+--------+
        |00000010|00000100|Prec|Sec |  TCC   |
        +--------+--------+---------+--------+
          Type=2  Length=4
      Precedence:  4 bits
        Specifies one of 16 levels of precedence
      Security:  4 bits
        Specifies one of 16 levels of security
      Transmission Control Code:  8 bits
        Provides a means to compartmentalize traffic and define
        controlled communities of interest among subscribers.
      This option might be used between hosts on the AUTODIN II
      network and other networks, such as the EDN at DCEC.
      Must be copied on fragmentation.
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Internet Protocol Specification

    Source Route
      +--------+--------+--------+---------//--------+
      |00000011| length |        source route        |
      +--------+--------+--------+---------//--------+
        Type=3
      The source route option provides a means for the source of an
      internet datagram to supply routing information to be used by
      the gateways in forwarding the datagram to the destination.
      The option begins with the option type code.  The second octet
      is the option length which includes the option type code and the
      length octet, as well as length-2 octets of source route data.
      A source route is composed of a series of internet addresses.
      Each internet address is 32 bits or 4 octets.  The length
      defaults to two, which indicates the source route is empty and
      the remaining routing is to be based on the destination address
      field.
      If the address in destination address field has been reached and
      this option's length is not two, the next address in the source
      route replaces the address in the destination address field, and
      is deleted from the source route and this option's length is
      reduced by four.  (The Internet Header Length Field must be
      changed also.)
      Must be copied on fragmentation.
    Return Route
      +--------+--------+--------+---------//--------+
      |00000111| length |        return route        |
      +--------+--------+--------+---------//--------+
        Type=7
      The return route option provides a means to record the route of
      an internet datagram.
      The option begins with the option type code.  The second octet
      is the option length which includes the option type code and the
      length octet, as well as length-2 octets of return route data.
      A return route is composed of a series of internet addresses.
      The length defaults to two, which indicates the return route is
      empty.

[Page 18]

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                                                     Internet Protocol
                                                         Specification
      When an internet module routes a datagram it checks to see if
      the return route option is present.  If it is, it inserts its
      own internet address as known in the environment into which this
      datagram is being forwarded into the return route at the front
      of the address string and increments the length by four.
      Not copied on fragmentation, goes in first fragment only.
    BCR Options
      BCR OPEN
      +--------+--------+------
      |00000100| length | data 
      +--------+--------+------
        Type=4
      BCR CLOSE
      +--------+--------+------
      |00000101| length | data 
      +--------+--------+------
        Type=5
      BCR OTHER
      +--------+--------+------
      |00000110| length | data 
      +--------+--------+------
        Type=6
      These options are used with the BCR.
      These options are not copied on fragmentation, they are carried
      only in the first fragment.
    Stream Identifier
      +--------+--------+---------+--------+
      |00001000|00000010|     Stream ID    |
      +--------+--------+---------+--------+
        Type=8  Length=4
      This option provides a way for the 16-bit SATNET stream
      identifier to be carried through networks that do not support
      the stream concept.
      Must be copied on fragmentation.
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Internet Protocol Specification

    General Error Report
      +--------+--------+--------+--------+--------+----//----+
      |00100001| length |err code|        id       |          |
      +--------+--------+--------+--------+--------+----//----+
       Type=33
      The general error report is used to report an error detected in
      processing an internet datagram to the source internet module of
      that datagram.  The "err code" indicates the type of error
      detected, and the "id" is copied from the identification field
      of the datagram in error, additional octets of error information
      may be present depending on the err code.
      If an internet datagram containing the general error report
      option is found to be in error or must be discarded, no error
      report is sent.
      ERR CODE:
        0 - Undetermined Error, used when no information is available
        about the type of error or the error does not fit any defined
        class.  Following the id should be as much of the datagram
        (starting with the internet header) as fits in the option
        space.
        1 - Datagram Discarded, used when specific information is
        available about the reason for discarding the datagram can be
        reported.  Following the id should be the original (4-octets)
        destination address, and the (1-octet) reason.
          Reason   Description
          ------   -----------
             0     No Reason
             1     No One Wants It - No higher level protocol or
                   application program at destination wants this
                   datagram.
             2     Fragmentation Needed & DF - Cannot deliver with out
                   fragmenting and has don't fragment bit set.
             3     Reassembly Problem - Destination could not
                   reassemble due to missing fragments when time to
                   live expired.
             4     Gateway Congestion - Gateway discarded datagram due
                   to congestion.
      The error report is placed in a datagram with the following
      values in the internet header fields:

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                                                     Internet Protocol
                                                         Specification
        Version:  Same as the datagram in error.
        IHL:  As computed.
        Type of Service:  Zero.
        Total Length:  As computed.
        Identification:  A new identification is selected.
        Flags:  Zero.
        Fragment Offset:  Zero.
        Time to Live:  Sixty.
        Protocol:  Same as the datagram in error.
        Header Checksum:  As computed.
        Source Address:  Address of the error reporting module.
        Destination Address:  Source address of the datagram in error.
        Options:  The General Error Report Option.
        Padding:  As needed.
      Not copied on fragmentation, goes with first fragment.
    Internet Timestamp
      +--------+--------+--------+--------+--------+--------+
      |01000100|00000100|        time in milliseconds       |
      +--------+--------+--------+--------+--------+--------+
       Type=68  Length=6
      The data of the timestamp is a 32 bit time measured in
      milliseconds.
      Not copied on fragmentation, goes with first fragment
    Satellite Timestamp
      +--------+--------+--------+--------+--------+--------+
      |01000101|00000100|        time in milliseconds       |
      +--------+--------+--------+--------+--------+--------+
       Type=69  Length=6
      The data of the timestamp is a 32 bit time measured in
      milliseconds.
      Not copied on fragmentation, goes with first fragment
Padding:  variable
  The internet header padding is used to ensure that the internet
  header ends on a 32 bit boundary.  The padding is zero.
                                                             [Page 21]
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Internet Protocol Specification

3.2. Discussion

The implementation of a protocol must be robust.  Each implementation
must expect to interoperate with others created by different
individuals.  While the goal of this specification is to be explicit
about the protocol there is the possibility of differing
interpretations.  In general, an implementation should be conservative
in its sending behavior, and liberal in its receiving behavior.  That
is, it should be careful to send well-formed datagrams, but should
accept any datagram that it can interpret (e.g., not object to
technical errors where the meaning is still clear).
The basic internet service is datagram oriented and provides for the
fragmentation of datagrams at gateways, with reassembly taking place
at the destination internet protocol module in the destination host.
Of course, fragmentation and reassembly of datagrams within a network
or by private agreement between the gateways of a network is also
allowed since this is transparent to the internet protocols and the
higher-level protocols.  This transparent type of fragmentation and
reassembly is termed "network-dependent" (or intranet) fragmentation
and is not discussed further here.
Internet addresses distinguish sources and destinations to the host
level and provide a protocol field as well.  It is assumed that each
protocol will provide for whatever multiplexing is necessary within a
host.
Addressing
  The 8 bit network number, which is the first octet of the variable
  length address, has a value as specified in reference [6].
  The 24 bit local address, assigned by the local network, should
  allow for a single physical host to act as several distinct internet
  hosts.  That is, there should be mapping between internet host
  addresses and network/host interfaces that allows several internet
  addresses to correspond to one interface.  It should also be allowed
  for a host to have several physical interfaces and to treat the
  datagrams from several of them as if they were all addressed to a
  single host.  Address mappings between internet addresses and
  addresses for ARPANET, SATNET, PRNET, and other networks are
  described in reference [4].
Fragmentation and Reassembly.
  The internet identification field (ID) is used together with the
  source and destination address, and the protocol fields, to identify
  datagrams fragments for reassembly.

[Page 22]

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                                                     Internet Protocol
                                                         Specification
  The More Fragments flag bit (MF) is set if the datagram is not the
  last fragment.  The Fragment Offset field identifies the fragment
  location, relative to the beginning of the original unfragmented
  datagram.  Fragments are counted in units of 8 octets.  The
  fragmentation strategy is designed so than an unfragmented datagram
  has all zero fragmentation information (MF = 0, fragment offset =
  0).  If an internet datagram is fragmented, its data portion must be
  broken on 8 octet boundaries.
  This format allows 2**13 = 8192 fragments of 8 octets each for a
  total of 65,536 octets.  Note that this is consistent with the the
  datagram total length field.
  When fragmentation occurs, options some are not 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.
  General notation in the following pseudo programs: "=<" means "less
  than or equal", "#" means "not equal", "=" means "equal", "<-" means
  "is set to".  Also, "x to y" includes x and excludes y; for example,
  "4 to 7" would include 4, 5, and 6 (but not 7).
  Fragmentation Procedure
    The maximum sized message that can be transmitted through the next
    network is called the maximum transmission unit (MTU).
                                                             [Page 23]
                                                           August 1979

Internet Protocol Specification

    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 to fragments, the
    first fragment being the maximum size (modulo the fragment block
    size), and the second fragment being the rest of the datagram.
    The first fragment is submitted to the next step in datagram
    processing, while the second fragment is submitted to this
    procedure in case it still too large.
    Notation:
      FO    -  Fragment Offset
      IHL   -  Internet Header Length
      MF    -  More Fragments flag
      TL    -  Total Length
      OFO   -  Old Fragment Offset
      OIHL  -  Old Internet Header Length
      OMF   -  Old More Fragments flag
      OTL   -  Old Total Length
      NFB   -  Number of Fragment Blocks
      MTU   -  Maximum Transmission Unit
    Procedure:
      IF TL =< MTU THEN Submit this datagram to the next step
           in datagram processing ELSE
      To produce the first fragment:
      (1)  Copy the original internet header;
      (2)  OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
      (3)  NFB <- (MTU-IHL*4)/8;
      (4)  Attach the first NFB*8 data octets;
      (5)  Correct the header:
           MF <- 1;  TL <- (IHL*4)+(NFB*8);
           Recompute Checksum;
      (6)  Submit this fragment to the next step in
           datagram processing;
      To produce the second fragment:
      (7)  Selectively copy the internet header (some options
           are not copied, see option definitions);
      (8)  Append the remaining data;
      (9)  Correct the header:
           IHL <- OIHL - ((length of options not copied)+3)/4;
           TL <- OTL - NFB*8 - (OIHL-IHL)*4);
           FO <- OFO + NFB;  MF <- OMF;  Recompute Checksum;
      (10) Submit this fragment to the fragmentation test; DONE.

[Page 24]

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                                                     Internet Protocol
                                                         Specification
  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.
    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
      RECBT -  Fragment Received Bit Table
                                                             [Page 25]
                                                           August 1979

Internet Protocol Specification

    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 <- 2 minutes; 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 runout;
      (19) timer runout: flush all reassembly with this BUFID; DONE.
    In the case that two or more fragments contain the same data
    either identically or through a partial overlap, this procedure
    will use the more recently arrived copy in the data buffer and
    datagram delivered.
Identification
  The choice of the Identifier for a datagram is based on the need to
  provide a way to uniquely identify the fragments of a particular
  datagram.  The protocol module assembling fragments judges fragments
  to belong to the same datagram if they have the same source,
  destination, protocol, and Identifier.  Thus, the sender must choose
  the Identifier to be unique for this source, destination pair and
  protocol for the time the datagram (or any fragment of it) could be
  alive in the internet.
  It seems then that a sending protocol module needs to keep a table
  of Identifiers, one entry for each destination it has communicated
  with in the last maximum packet lifetime for the internet.

[Page 26]

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                                                     Internet Protocol
                                                         Specification
  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
  priority, reliability, and speed.  A further concern is the
  possibility of efficient handling of streams of datagrams.  These
  abstract parameters are to be mapped into the actual service
  parameters of the particular networks the datagram traverses.
  Priority.  An independent measure of the importance of this
  datagram.
  Stream or Datagram.  Indicates if there will be other datagrams from
  this source to this destination at regular frequent intervals
  justifying the maintenance of stream processing information.
  Reliability.  A measure of the level of effort desired to ensure
  delivery of this datagram.
  Speed over Reliability.  Indicates the relative importance of speed
  and reliability when a conflict arises in meeting the pair of
  requests.
  Speed.  A measure of the importance of prompt delivery of this
  datagram.
  For example, the ARPANET has a priority bit, and a choice between
  "standard" messages (type 0) and "uncontrolled" messages (type 3),
  (the choice between single packet and multipacket messages can also
  be considered a service parameter). The uncontrolled messages tend
  to be less reliably delivered and suffer less delay.  Suppose an
  internet datagram is to be sent through the ARPANET.  Let the
  internet type of service be given as:
                                                             [Page 27]
                                                           August 1979

Internet Protocol Specification

    Priority:    2
    Stream:      0
    Reliability: 1
    S/R:         1
    Speed:       1
  The mapping of these parameters to those available for the ARPANET
  would be  to set the ARPANET priority bit on since the Internet
  priority is in the upper have of its range, to select uncontrolled
  messages since the speed and reliability requirements are equal and
  speed is prefered.
  The following chart presents the recommended mappings from the
  internet protocol type of service into the service parameters
  actually available on the ARPANET, the PRNET, and the SATNET:
    +------------+----------+----------+----------+----------+
    |Application | INTERNET | ARPANET  | PRNET    | SATNET   |
    +------------+----------+----------+----------+----------+
    |TELNET      |S/D:stream| T: 3     | R: ptp   | T: block |
    |  on        |  R:normal| S: S     | A: no    | D: min   |
    |   TCP      |S/R:speed |          |          | H: inf   |
    |            |  S:fast  |          |          | R: no    |
    +------------+----------+----------+----------+----------+
    |FTP         |S/D:stream| T: 0     | R: ptp   | T: block |
    |  on        |  R:normal| S: M     | A: no    | D: normal|
    |   TCP      |S/R:rlblt |          |          | H: inf   |
    |            |  S:normal|          |          | R: no    |
    +------------+----------+----------+----------+----------+
    |interactive |S/D:strm* | T: 3     | R: ptp   | T: stream|
    |narrow band |  R:least | S: S     | A: no    | D: min   |
    |  speech    |  P:speed |          |          | H: short |
    |            |  S:asap  |          |          | R: no    |
    +------------+----------+----------+----------+----------+
    |datagram    |S/D:dtgrm | T: 3 or 0| R:station| T: block |
    |            |  R:normal| S: S or M| A: no    | D: min   |
    |            |S/R:speed |          |          | H: short |
    |            |  S:fast  |          |          | R: no    |
    +------------+----------+----------+----------+----------+
     key:    S/D=strm/dtgrm   T=type     R=route  T=type
             R=reliability    S=size     A=ack    D=delay
             S/R=speed/rlblt                      H=holding time
             S=speed                              R=reliability
             *=requires stream set up

[Page 28]

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                                                     Internet Protocol
                                                         Specification
Time to Live
  The time to live is set by the sender to the maximum time the
  datagram is allowed to be in the internet system.  If the datagram
  is in the internet system longer than the time to live, then the
  datagram should be destroyed.  This field should be decreased at
  each point that the internet header is processed to reflect the time
  spent processing the datagram.  Even if no local information is
  available on the time actually spent, the field should be
  decremented by 1.  The time is measured in units of seconds (i.e.
  the value 1 means one second).  Thus, the maximum time to live is
  255 seconds or 4.25 minutes.
Options
  The options are just that, optional.  That is, the presence or
  absence of an option is the choice of the sender, but each internet
  module must implement every option.
  There can be several options present in the option field.  Every
  internet module must be able to parse and act on every option.
  The options might not end on a 32-bit boundary.  The internet header
  should be filled out with octets of zeros.  The first of these would
  be interpreted as the end-of-options option, and the remainder as
  internet header padding.
Checksum
  The internet header checksum is recomputed if the internet header is
  changed.  For example, a reduction of the time to live, additions or
  changes to internet options, or due to fragmentation.  This checksum
  at the internet level is intended to protect the internet header
  fields from transmission errors.
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Internet Protocol Specification

3.3. Examples & Scenarios

Example 1:
  This is an example of the minimal data carrying internet datagram:
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver= 4 |IHL= 5 |Type of Service|        Total Length = 21      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Identification = 111     |Flg=0|   Fragment Offset = 0   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Time = 123  |  Protocol = 1 |        header checksum        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         source address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      destination address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     data      |                                                
 +-+-+-+-+-+-+-+-+                                                
                     Example Internet Datagram
                             Figure 4.
  Note that each tick mark represents one bit position.
  This is a internet datagram in version 4 of internet protocol; the
  internet header consists of five 32 bit words, and the total length
  of the datagram is 21 octets.  This datagram is a complete datagram
  (not a fragment).

[Page 30]

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                                                     Internet Protocol
                                                         Specification
Example 2:
  In this example, we show first a moderate size internet datagram
  (552 data octets), then two internet fragments that might result
  from the fragmentation of this datagram if the maximum sized
  transmission allowed were 280 octets.
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 472      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Identification = 111      |Flg=0|     Fragment Offset = 0 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Time = 123  | Protocol = 6  |        header checksum        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         source address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      destination address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 \                                                               \
 \                                                               \
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             data              |                                
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                
                     Example Internet Datagram
                             Figure 5.
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                                                           August 1979

Internet Protocol Specification

  Now the first fragment that results from splitting the datagram
  after 256 data octets.
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 276      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Identification = 111      |Flg=1|     Fragment Offset = 0 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Time = 119  | Protocol = 6  |        Header Checksum        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         source address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      destination address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 \                                                               \
 \                                                               \
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Example Internet Fragment
                             Figure 6.

[Page 32]

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                                                     Internet Protocol
                                                         Specification
  And the second fragment.
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver= 4 |IHL= 5 |Type of Service|       Total Length = 216      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Identification = 111      |Flg=0|  Fragment Offset  =  32 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Time = 119  | Protocol = 6  |        Header Checksum        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         source address                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      destination address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 \                                                               \
 \                                                               \
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            data               |                                
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                
                     Example Internet Fragment
                             Figure 7.
                                                             [Page 33]
                                                           August 1979

Internet Protocol Specification

Example 3:
  Here, we show an example of a datagram containing options:
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver= 4 |IHL= 8 |Type of Service|       Total Length = 576      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Identification = 111    |Flg=0|     Fragment Offset = 0 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Time = 123  |  Protocol = 6 |       Header Checksum         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        source address                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      destination address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Opt. Code = x | Opt.  Len.= 3 | option value  | Opt. Code = x |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Opt. Len. = 4 |           option value        | Opt. Code = 1 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Opt. Code = y | Opt. Len. = 3 |  option value | Opt. Code = 0 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 \                                                               \
 \                                                               \
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Example Internet Datagram
                             Figure 8.

3.4. Interfaces

Internet protocol interfaces on one side to the local network and on
the other side to either a higher level protocol or an application
program.  In the following, the higher level protocol or application
program (or even a gateway program) will be called the "user" since it
is using the internet module.  Since internet protocol is a datagram
protocol, there is minimal memory or state maintained between datagram
transmissions, and each call on the internet protocol module by the
user supplies all the necessary information.

[Page 34]

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                                                     Internet Protocol
                                                         Specification
For example, the following two calls satisfy the requirements for the
user to internet protocol module communication ("=>" means returns):
  SEND (dest, TOS, TTL, BufPTR, len, Id, DF, options => result)
    where:
      dest = destination address
      TOS = type of service
      TTL = time to live
      BufPTR = buffer pointer
      len = length of buffer
      Id  = Identifier
      DF = Don't Fragment
      options = option data
      result = response
        OK = datagram sent ok
        Error = error in arguments or local network error
  RECV (BufPTR => result, source, dest, prot, TOS, len)
    where:
      BufPTR = buffer pointer
      result = response
        OK = datagram received ok
        Error = error in arguments
      source = source address
      dest = destination address
      prot = protocol
      TOS = type of service
      len = length of buffer
When the user sends a datagram, it executes the SEND call supplying
all the arguments.  The internet protocol module, on receiving this
call, checks the arguments and prepares and sends the message.  If the
arguments are good and the datagram is accepted by the local network,
the call returns successfully.  If either the arguments are bad, or
the datagram is not accepted by the local network, the call returns
unsuccessfully.  On unsuccessful returns, a reasonable report should
be made as to the cause of the problem, but the details of such
reports are up to individual implementations.
When a datagram arrives at the internet protocol module from the local
network, either there is a pending RECV call from the user addressed
or there is not.  In the first case, the pending call is satisfied by
passing the information from the datagram to the user.  In the second
case, the user addressed is notified of a pending datagram.  If the
                                                             [Page 35]
                                                           August 1979

Internet Protocol Specification

user addressed does not exist, an error datagram is returned to the
sender, and the data is discarded.
The notification of a user may be via a pseudo interrupt or similar
mechanism, as appropriate in the particular operating system
environment of the implementation.
A user's RECV call may then either be immediately satisfied by a
pending datagram, or the call may be pending until a datagram arrives.
An implementation may also allow or require a call to the internet
module to indicate interest in or reserve exclusive use of a class of
datagrams (e.g., all those with a certain value in the protocol
field).

[Page 36]

August 1979

                                                     Internet Protocol
                              GLOSSARY

1822

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

ARPANET message

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

ARPANET packet

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

Destination

        The destination address, an internet header field.

DF

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

Flags

        An internet header field carrying various control flags.

Fragment Offset

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

header

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

Identification

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

IHL

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

IMP

        The Interface Message Processor, the packet switch of the
        ARPANET.
                                                             [Page 37]
                                                           August 1979

Internet Protocol Glossary

Internet Address

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

internet fragment

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

internet datagram

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

ARPANET leader

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

Local Address

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

MF

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

module

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

more-fragments flag

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

NFB

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

octet

        An eight bit byte.

Options

        The internet header Options field may contain several options,
        and each option may be several octets in length.  The options
        are used primarily in testing situations, for example to carry
        timestamps.

[Page 38]

August 1979

                                                     Internet Protocol
                                                              Glossary

Padding

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

Protocol

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

Rest

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

RTP

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

Source

        The source address, an internet header field.

TCP

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

TCP Segment

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

Total Length

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

Type of Service

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

User

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

Version

        The Version field indicates the format of the internet header.
                                                             [Page 39]
                                                           August 1979

Internet Protocol

[Page 40]

August 1979

                                                     Internet Protocol
                             REFERENCES

[1] Cerf, V., "The Catenet Model for Internetworking," Information

   Processing Techniques Office, Defense Advanced Research Projects
   Agency, IEN 48, July 1978.

[2] Bolt Beranek and Newman, "Specification for the Interconnection of

   a Host and an IMP," BBN Technical Report 1822, May 1978 (Revised).

[3] Shoch, J., "Inter-Network Naming, Addressing, and Routing,"

   COMPCON, IEEE Computer Society, Fall 1978.

[4] Postel, J., "Address Mappings," IEN 115, USC/Information Sciences

   Institute, August 1979.

[5] Shoch, J., "Packet Fragmentation in Inter-Network Protocols,"

   Computer Networks, v. 3, n. 1, February 1979.

[6] Postel, J., "Assigned Numbers," RFC 758, IEN 117, USC/Information

   Sciences Institute, August 1979.
                                                             [Page 41]
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