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Table of Contents

[Note that this file is a concatenation of more than one RFC.]

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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Internet Protocol

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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.
                                                              [Page 7]
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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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                                                     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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                                                     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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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]

Network Working Group J. Mogul (Stanford) Request for Comments: 950 J. Postel (ISI)

                                                           August 1985
               Internet Standard Subnetting Procedure

Status Of This Memo

 This RFC specifies a protocol for the ARPA-Internet community.  If
 subnetting is implemented it is strongly recommended that these
 procedures be followed.  Distribution of this memo is unlimited.

Overview

 This memo discusses the utility of "subnets" of Internet networks,
 which are logically visible sub-sections of a single Internet
 network.  For administrative or technical reasons, many organizations
 have chosen to divide one Internet network into several subnets,
 instead of acquiring a set of Internet network numbers.  This memo
 specifies procedures for the use of subnets.  These procedures are
 for hosts (e.g., workstations).  The procedures used in and between
 subnet gateways are not fully described.  Important motivation and
 background information for a subnetting standard is provided in
 RFC-940 [7].

Acknowledgment

 This memo is based on RFC-917 [1].  Many people contributed to the
 development of the concepts described here.  J. Noel Chiappa, Chris
 Kent, and Tim Mann, in particular, provided important suggestions.
 Additional contributions in shaping this memo were made by Zaw-Sing
 Su, Mike Karels, and the Gateway Algorithms and Data Structures Task
 Force (GADS).

Mogul & Postel [Page 1]

RFC 950 August 1985 Internet Standard Subnetting Procedure

1. Motivation

 The original view of the Internet universe was a two-level hierarchy:
 the top level the Internet as a whole, and the level below it
 individual networks, each with its own network number.  The Internet
 does not have a hierarchical topology, rather the interpretation of
 addresses is hierarchical.  In this two-level model, each host sees
 its network as a single entity; that is, the network may be treated
 as a "black box" to which a set of hosts is connected.
 While this view has proved simple and powerful, a number of
 organizations have found it inadequate, and have added a third level
 to the interpretation of Internet addresses.  In this view, a given
 Internet network is divided into a collection of subnets.
 The three-level model is useful in networks belonging to moderately
 large organizations (e.g., Universities or companies with more than
 one building), where it is often necessary to use more than one LAN
 cable to cover a "local area".  Each LAN may then be treated as a
 subnet.
 There are several reasons why an organization might use more than one
 cable to cover a campus:
  1. Different technologies: Especially in a research environment,

there may be more than one kind of LAN in use; e.g., an

      organization may have some equipment that supports Ethernet, and
      some that supports a ring network.
  1. Limits of technologies: Most LAN technologies impose limits,

based on electrical parameters, on the number of hosts

      connected, and on the total length of the cable.  It is easy to
      exceed these limits, especially those on cable length.
  1. Network congestion: It is possible for a small subset of the

hosts on a LAN to monopolize most of the bandwidth. A common

      solution to this problem is to divide the hosts into cliques of
      high mutual communication, and put these cliques on separate
      cables.
  1. Point-to-Point links: Sometimes a "local area", such as a

university campus, is split into two locations too far apart to

      connect using the preferred LAN technology.  In this case,
      high-speed point-to-point links might connect several LANs.
 An organization that has been forced to use more than one LAN has
 three choices for assigning Internet addresses:

Mogul & Postel [Page 2]

RFC 950 August 1985 Internet Standard Subnetting Procedure

    1. Acquire a distinct Internet network number for each cable;
       subnets are not used at all.
    2. Use a single network number for the entire organization, but
       assign host numbers without regard to which LAN a host is on
       ("transparent subnets").
    3. Use a single network number, and partition the host address
       space by assigning subnet numbers to the LANs ("explicit
       subnets").
 Each of these approaches has disadvantages.  The first, although not
 requiring any new or modified protocols, results in an explosion in
 the size of Internet routing tables.  Information about the internal
 details of local connectivity is propagated everywhere, although it
 is of little or no use outside the local organization.  Especially as
 some current gateway implementations do not have much space for
 routing tables, it would be good to avoid this problem.
 The second approach requires some convention or protocol that makes
 the collection of LANs appear to be a single Internet network.  For
 example, this can be done on LANs where each Internet address is
 translated to a hardware address using an Address Resolution Protocol
 (ARP), by having the bridges between the LANs intercept ARP requests
 for non-local targets, see RFC-925 [2].  However, it is not possible
 to do this for all LAN technologies, especially those where ARP
 protocols are not currently used, or if the LAN does not support
 broadcasts.  A more fundamental problem is that bridges must discover
 which LAN a host is on, perhaps by using a broadcast algorithm.  As
 the number of LANs grows, the cost of broadcasting grows as well;
 also, the size of translation caches required in the bridges grows
 with the total number of hosts in the network.
 The third approach is to explicitly support subnets.  This does have
 a disadvantage, in that it is a modification of the Internet
 Protocol, and thus requires changes to IP implementations already in
 use (if these implementations are to be used on a subnetted network).
 However, these changes are relatively minor, and once made, yield a
 simple and efficient solution to the problem.  Also, the approach
 avoids any changes that would be incompatible with existing hosts on
 non-subnetted networks.
 Further, when appropriate design choices are made, it is possible for
 hosts which believe they are on a non-subnetted network to be used on
 a subnetted one, as explained in RFC-917 [1].  This is useful when it
 is not possible to modify some of the hosts to support subnets
 explicitly, or when a gradual transition is preferred.

Mogul & Postel [Page 3]

RFC 950 August 1985 Internet Standard Subnetting Procedure

2. Standards for Subnet Addressing

 This section first describes a proposal for interpretation of
 Internet addresses to support subnets.  Next it discusses changes to
 host software to support subnets.  Finally, it presents a procedures
 for discovering what address interpretation is in use on a given
 network (i.e., what address mask is in use).
 2.1. Interpretation of Internet Addresses
    Suppose that an organization has been assigned an Internet network
    number, has further divided that network into a set of subnets,
    and wants to assign host addresses: how should this be done?
    Since there are minimal restrictions on the assignment of the
    "local address" part of the Internet address, several approaches
    have been proposed for representing the subnet number:
       1. Variable-width field:  Any number of the bits of the local
          address part are used for the subnet number; the size of
          this field, although constant for a given network, varies
          from network to network.  If the field width is zero, then
          subnets are not in use.
       2. Fixed-width field:  A specific number of bits (e.g., eight)
          is used for the subnet number, if subnets are in use.
       3. Self-encoding variable-width field:  Just as the width
          (i.e., class) of the network number field is encoded by its
          high-order bits, the width of the subnet field is similarly
          encoded.
       4. Self-encoding fixed-width field:  A specific number of bits
          is used for the subnet number.
       5. Masked bits:  Use a bit mask ("address mask") to identify
          which bits of the local address field indicate the subnet
          number.
    What criteria can be used to choose one of these five schemes?
    First, should we use a self-encoding scheme?  And, should it be
    possible to tell from examining an Internet address if it refers
    to a subnetted network, without reference to any other
    information?
       An interesting feature of self-encoding is that it allows the

Mogul & Postel [Page 4]

RFC 950 August 1985 Internet Standard Subnetting Procedure

       address space of a network to be divided into subnets of
       different sizes, typically one subnet of half the address space
       and a set of small subnets.
          For example, consider a class C network that uses a
          self-encoding scheme with one bit to indicate if it is the
          large subnet or not and an additional three bits to identify
          the small subnet.  If the first bit is zero then this is the
          large subnet, if the first bit is one then the following
          bits (3 in this example) give the subnet number.  There is
          one subnet with 128 host addresses, and eight subnets with
          16 hosts each.
       To establish a subnetting standard the parameters and
       interpretation of the self-encoding scheme must be fixed and
       consistent throughout the Internet.
       It could be assumed that all networks are subnetted.  This
       would allow addresses to be interpreted without reference to
       any other information.
          This is a significant advantage, that given the Internet
          address no additional information is needed for an
          implementation to determine if two addresses are on the same
          subnet.  However, this can also be viewed as a disadvantage:
          it may cause problems for networks which have existing host
          numbers that use arbitrary bits in the local address part.
          In other words, it is useful to be able to control whether a
          network is subnetted independently from the assignment of
          host addresses.
       The alternative is to have the fact that a network is subnetted
       kept separate from the address.  If one finds, somehow, that
       the network is subnetted then the standard self-encoded
       subnetted network address rules are followed, otherwise the
       non-subnetted network addressing rules are followed.
    If a self-encoding scheme is not used, there is no reason to use a
    fixed-width field scheme: since there must in any case be some
    per-network "flag" to indicate if subnets are in use, the
    additional cost of using an integer (a subnet field width or
    address mask) instead of a boolean is negligible.  The advantage
    of using the address mask scheme is that it allows each
    organization to choose the best way to allocate relatively scarce
    bits of local address to subnet and host numbers.  Therefore, we
    choose the address-mask scheme: it is the most flexible scheme,
    yet costs no more to implement than any other.

Mogul & Postel [Page 5]

RFC 950 August 1985 Internet Standard Subnetting Procedure

    For example, the Internet address might be interpreted as:
       <network-number><subnet-number><host-number>
    where the <network-number> field is as defined by IP [3], the
    <host-number> field is at least 1-bit wide, and the width of the
    <subnet-number> field is constant for a given network.  No further
    structure is required for the <subnet-number> or <host-number>
    fields.  If the width of the <subnet-number> field is zero, then
    the network is not subnetted (i.e., the interpretation of [3] is
    used).
    For example, on a Class B network with a 6-bit wide subnet field,
    an address would be broken down like this:
                         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 0|        NETWORK            |  SUBNET   |    Host Number    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Since the bits that identify the subnet are specified by a
    bitmask, they need not be adjacent in the address.  However, we
    recommend that the subnet bits be contiguous and located as the
    most significant bits of the local address.
    Special Addresses:
       From the Assigned Numbers memo [9]:
          "In certain contexts, it is useful to have fixed addresses
          with functional significance rather than as identifiers of
          specific hosts.  When such usage is called for, the address
          zero is to be interpreted as meaning "this", as in "this
          network".  The address of all ones are to be interpreted as
          meaning "all", as in "all hosts".  For example, the address
          128.9.255.255 could be interpreted as meaning all hosts on
          the network 128.9.  Or, the address 0.0.0.37 could be
          interpreted as meaning host 37 on this network."
       It is useful to preserve and extend the interpretation of these
       special addresses in subnetted networks.  This means the values
       of all zeros and all ones in the subnet field should not be
       assigned to actual (physical) subnets.
          In the example above, the 6-bit wide subnet field may have
          any value except 0 and 63.

Mogul & Postel [Page 6]

RFC 950 August 1985 Internet Standard Subnetting Procedure

       Please note that there is no effect or new restriction on the
       addresses of hosts on non-subnetted networks.
 2.2. Changes to Host Software to Support Subnets
    In most implementations of IP, there is code in the module that
    handles outgoing datagrams to decide if a datagram can be sent
    directly to the destination on the local network or if it must be
    sent to a gateway.
    Generally the code is something like this:
       IF ip_net_number(dg.ip_dest) = ip_net_number(my_ip_addr)
           THEN
               send_dg_locally(dg, dg.ip_dest)
           ELSE
               send_dg_locally(dg,
                                gateway_to(ip_net_number(dg.ip_dest)))
    (If the code supports multiply-connected networks, it will be more
    complicated, but this is irrelevant to the current discussion.)
    To support subnets, it is necessary to store one more 32-bit
    quantity, called my_ip_mask.  This is a bit-mask with bits set in
    the fields corresponding to the IP network number, and additional
    bits set corresponding to the subnet number field.
    The code then becomes:
       IF bitwise_and(dg.ip_dest, my_ip_mask)
                                 = bitwise_and(my_ip_addr, my_ip_mask)
           THEN
               send_dg_locally(dg, dg.ip_dest)
           ELSE
               send_dg_locally(dg,
                      gateway_to(bitwise_and(dg.ip_dest, my_ip_mask)))
    Of course, part of the expression in the conditional can be
    pre-computed.
    It may or may not be necessary to modify the "gateway_to"
    function, so that it too takes the subnet field bits into account
    when performing comparisons.
    To support multiply-connected hosts, the code can be changed to

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RFC 950 August 1985 Internet Standard Subnetting Procedure

    keep  the "my_ip_addr" and "my_ip_mask" quantities on a
    per-interface basis; the expression in the conditional must then
    be evaluated for each interface.
 2.3. Finding the Address Mask
    How can a host determine what address mask is in use on a subnet
    to which it is connected?  The problem is analogous to several
    other "bootstrapping" problems for Internet hosts: how a host
    determines its own address, and how it locates a gateway on its
    local network.  In all three cases, there are two basic solutions:
    "hardwired" information, and broadcast-based protocols.
    Hardwired information is that available to a host in isolation
    from a network.  It may be compiled-in, or (preferably) stored in
    a disk file.  However, for the increasingly common case of a
    diskless workstation that is bootloaded over a LAN, neither
    hardwired solution is satisfactory.
    Instead, since most LAN technology supports broadcasting, a better
    method is for the newly-booted host to broadcast a request for the
    necessary information.  For example, for the purpose of
    determining its Internet address, a host may use the "Reverse
    Address Resolution Protocol" (RARP) [4].
    However, since a newly-booted host usually needs to gather several
    facts (e.g., its IP address, the hardware address of a gateway,
    the IP address of a domain name server, the subnet address mask),
    it would be better to acquire all this information in one request
    if possible, rather than doing numerous broadcasts on the network.
    The mechanisms designed to boot diskless workstations can also
    load per-host specific configuration files that contain the
    required information (e.g., see RFC-951 [8]).  It is possible, and
    desirable, to obtain all the facts necessary to operate a host
    from a boot server using only one broadcast message.
    In the case where it is necessary for a host to find the address
    mask as a separate operation the following mechanism is provided:
       To provide the address mask information the ICMP protocol [5]
       is extended by adding a new pair of ICMP message types,
       "Address Mask Request" and "Address Mask Reply", analogous to
       the "Information Request" and "Information Reply" ICMP
       messages.  These are described in detail in Appendix I.
       The intended use of these new ICMP messages is that a host,
       when booting, broadcast an "Address Mask Request" message.  A

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RFC 950 August 1985 Internet Standard Subnetting Procedure

       gateway (or a host acting in lieu of a gateway) that receives
       this message responds with an "Address Mask Reply".  If there
       is no indication in the request which host sent it (i.e., the
       IP Source Address is zero), the reply is broadcast as well.
       The requesting host will hear the response, and from it
       determine the address mask.
       Since there is only one possible value that can be sent in an
       "Address Mask Reply" on any given LAN, there is no need for the
       requesting host to match the responses it hears against the
       request it sent; similarly, there is no problem if more than
       one gateway responds.  We assume that hosts reboot
       infrequently, so the broadcast load on a network from use of
       this protocol should be small.
    If a host is connected to more than one LAN, it might have to find
    the address mask for each.
    One potential problem is what a host should do if it can not find
    out the address mask, even after a reasonable number of tries.
    Three interpretations can be placed on the situation:
       1. The local net exists in (permanent) isolation from all other
          nets.
       2. Subnets are not in use, and no host can supply the address
          mask.
       3. All gateways on the local net are (temporarily) down.
    The first and second situations imply that the address mask is
    identical with the Internet network number mask.  In the third
    situation, there is no way to determine what the proper value is;
    the safest choice is thus a mask identical with the Internet
    network number mask.  Although this might later turn out to be
    wrong, it will not prevent transmissions that would otherwise
    succeed.  It is possible for a host to recover from a wrong
    choice: when a gateway comes up, it should broadcast an "Address
    Mask Reply"; when a host receives such a message that disagrees
    with its guess, it should change its mask to conform to the
    received value.  No host or gateway should send an "Address Mask
    Reply" based on a "guessed" value.
    Finally, note that no host is required to use this ICMP protocol
    to discover the address mask; it is perfectly reasonable for a
    host with non-volatile storage to use stored information
    (including a configuration file from a boot server).

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RFC 950 August 1985 Internet Standard Subnetting Procedure

Appendix I. Address Mask ICMP

 Address Mask Request or Address Mask Reply
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |      Code     |          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Identifier          |       Sequence Number         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Address Mask                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    IP Fields:
       Addresses
          The address of the source in an address mask request message
          will be the destination of the address mask reply message.
          To form an address mask reply message, the source address of
          the request becomes the destination address of the reply,
          the source address of the reply is set to the replier's
          address, the type code changed to AM2, the address mask
          value inserted into the Address Mask field, and the checksum
          recomputed.  However, if the source address in the request
          message is zero, then the destination address for the reply
          message should denote a broadcast.
    ICMP Fields:
       Type
          AM1 for address mask request message
          AM2 for address mask reply message
       Code
          0 for address mask request message
          0 for address mask reply message
       Checksum
          The checksum is the 16-bit one's complement of the one's

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RFC 950 August 1985 Internet Standard Subnetting Procedure

          complement sum of the ICMP message starting with the ICMP
          Type.  For computing the checksum, the checksum field should
          be zero.  This checksum may be replaced in the future.
       Identifier
          An identifier to aid in matching requests and replies, may
          be zero.
       Sequence Number
          A sequence number to aid in matching requests and replies,
          may be zero.
       Address Mask
          A 32-bit mask.
    Description
       A gateway receiving an address mask request should return it
       with the address mask field set to the 32-bit mask of the bits
       identifying the subnet and network, for the subnet on which the
       request was received.
       If the requesting host does not know its own IP address, it may
       leave the source field zero; the reply should then be
       broadcast.  However, this approach should be avoided if at all
       possible, since it increases the superfluous broadcast load on
       the network.  Even when the replies are broadcast, since there
       is only one possible address mask for a subnet, there is no
       need to match requests with replies.  The "Identifier" and
       "Sequence Number" fields can be ignored.
          Type AM1 may be received from a gateway or a host.
          Type AM2 may be received from a gateway, or a host acting in
          lieu of a gateway.

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RFC 950 August 1985 Internet Standard Subnetting Procedure

Appendix II. Examples

 These examples show how a host can find out the address mask using
 the ICMP Address Mask Request and Address Mask Reply messages.  For
 the following examples, assume that address 255.255.255.255 denotes
 "broadcast to this physical medium" [6].
 1.  A Class A Network Case
    For this case, assume that the requesting host is on class A
    network 36.0.0.0, has address 36.40.0.123, that there is a gateway
    at 36.40.0.62, and that a 8-bit wide subnet field is in use, that
    is, the address mask is 255.255.0.0.
    The most efficient method, and the one we recommend, is for a host
    to first discover its own address (perhaps using "RARP" [4]), and
    then to send the ICMP request to 255.255.255.255:
       Source address:          36.40.0.123
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    The gateway can then respond directly to the requesting host.
       Source address:          36.40.0.62
       Destination address:     36.40.0.123
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.0.0
    Suppose that 36.40.0.123 is a diskless workstation, and does not
    know even its own host number.  It could send the following
    datagram:
       Source address:          0.0.0.0
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    36.40.0.62 will hear the datagram, and should respond with this
    datagram:

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RFC 950 August 1985 Internet Standard Subnetting Procedure

       Source address:          36.40.0.62
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.0.0
    Note that the gateway uses the narrowest possible broadcast to
    reply.  Even so, the over use of broadcasts presents an
    unnecessary load to all hosts on the subnet, and so the use of the
    "anonymous" (0.0.0.0) source address must be kept to a minimum.
    If broadcasting is not allowed, we assume that hosts have wired-in
    information about neighbor gateways; thus, 36.40.0.123 might send
    this datagram:
       Source address:          36.40.0.123
       Destination address:     36.40.0.62
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    36.40.0.62 should respond exactly as in the previous case.
       Source address:          36.40.0.62
       Destination address:     36.40.0.123
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.0.0
 2.  A Class B Network Case
    For this case, assume that the requesting host is on class B
    network 128.99.0.0, has address 128.99.4.123, that there is a
    gateway at 128.99.4.62, and that a 6-bit wide subnet field is in
    use, that is, the address mask is 255.255.252.0.
    The host sends the ICMP request to 255.255.255.255:
       Source address:          128.99.4.123
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0

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RFC 950 August 1985 Internet Standard Subnetting Procedure

    The gateway can then respond directly to the requesting host.
       Source address:          128.99.4.62
       Destination address:     128.99.4.123
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.252.0
    In the diskless workstation case the host sends:
       Source address:          0.0.0.0
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    128.99.4.62 will hear the datagram, and should respond with this
    datagram:
       Source address:          128.99.4.62
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.252.0
    If broadcasting is not allowed 128.99.4.123 sends:
       Source address:          128.99.4.123
       Destination address:     128.99.4.62
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    128.99.4.62 should respond exactly as in the previous case.
       Source address:          128.99.4.62
       Destination address:     128.99.4.123
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.252.0

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RFC 950 August 1985 Internet Standard Subnetting Procedure

 3.  A Class C Network Case (illustrating non-contiguous subnet bits)
    For this case, assume that the requesting host is on class C
    network 192.1.127.0, has address 192.1.127.19, that there is a
    gateway at 192.1.127.50, and that on network an 3-bit subnet field
    is in use (01011000), that is, the address mask is 255.255.255.88.
    The host sends the ICMP request to 255.255.255.255:
       Source address:          192.1.127.19
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    The gateway can then respond directly to the requesting host.
       Source address:          192.1.127.50
       Destination address:     192.1.127.19
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.255.88.
    In the diskless workstation case the host sends:
       Source address:          0.0.0.0
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    192.1.127.50 will hear the datagram, and should respond with this
    datagram:
       Source address:          192.1.127.50
       Destination address:     255.255.255.255
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.255.88.
    If broadcasting is not allowed 192.1.127.19 sends:

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RFC 950 August 1985 Internet Standard Subnetting Procedure

       Source address:          192.1.127.19
       Destination address:     192.1.127.50
       Protocol:                ICMP = 1
       Type:                    Address Mask Request = AM1
       Code:                    0
       Mask:                    0
    192.1.127.50 should respond exactly as in the previous case.
       Source address:          192.1.127.50
       Destination address:     192.1.127.19
       Protocol:                ICMP = 1
       Type:                    Address Mask Reply = AM2
       Code:                    0
       Mask:                    255.255.255.88

Appendix III. Glossary

 Bridge
    A node connected to two or more administratively indistinguishable
    but physically distinct subnets, that automatically forwards
    datagrams when necessary, but whose existence is not known to
    other hosts.  Also called a "software repeater".
 Gateway
    A node connected to two or more administratively distinct networks
    and/or subnets, to which hosts send datagrams to be forwarded.
 Host Field
    The bit field in an Internet address used for denoting a specific
    host.
 Internet
    The collection of connected networks using the IP protocol.
 Local Address
    The rest field of the Internet address (as defined in [3]).
 Network
    A single Internet network (which may or may not be divided into
    subnets).

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RFC 950 August 1985 Internet Standard Subnetting Procedure

 Network Number
    The network field of the Internet address.
 Subnet
    One or more physical networks forming a subset of an Internet
    network.  A subnet is explicitly identified in the Internet
    address.
 Subnet Field
    The bit field in an Internet address denoting the subnet number.
    The bits making up this field are not necessarily contiguous in
    the address.
 Subnet Number
    A number identifying a subnet within a network.

Appendix IV. Assigned Numbers

 The following assignments are made for protocol parameters used in
 the support of subnets.  The only assignments needed are for the
 Internet Control Message Protocol (ICMP) [5].
 ICMP Message Types
    AM1 = 17
    AM2 = 18

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RFC 950 August 1985 Internet Standard Subnetting Procedure

References

 [1]  Mogul, J., "Internet Subnets", RFC-917, Stanford University,
      October 1984.
 [2]  Postel, J., "Multi-LAN Address Resolution", RFC-925,
      USC/Information Sciences Institute, October 1984.
 [3]  Postel, J., "Internet Protocol", RFC-791, USC/Information
      Sciences Institute, September 1981.
 [4]  Finlayson, R., T. Mann, J. Mogul, M. Theimer, "A Reverse Address
      Resolution Protocol", RFC-903, Stanford University, June 1984.
 [5]  Postel, J., "Internet Control Message Protocol", RFC-792,
      USC/Information Sciences Institute, September 1981.
 [6]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
      University, October 1984.
 [7]  GADS, "Towards an Internet Standard Scheme for Subnetting",
      RFC-940, Network Information Center, SRI International,
      April 1985.
 [8]  Croft, B., and J. Gilmore, "BOOTP -- UDP Bootstrap Protocol",
      RFC-951, Stanford University, August 1985.
 [9]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-943,
      USC/Information Sciences Institute, April 1985.

Mogul & Postel [Page 18]

Network Working Group Jeffrey Mogul Request for Comments: 919 Computer Science Department

                                                   Stanford University
                                                          October 1984
                   BROADCASTING INTERNET DATAGRAMS

Status of this Memo

 We propose simple rules for broadcasting Internet datagrams on local
 networks that support broadcast, for addressing broadcasts, and for
 how gateways should handle them.
 This RFC suggests a proposed protocol for the ARPA-Internet
 community, and requests discussion and suggestions for improvements.
 Distribution of this memo is unlimited.

Acknowledgement

 This proposal is the result of discussion with several other people,
 especially J. Noel Chiappa and Christopher A. Kent, both of whom both
 pointed me at important references.

1. Introduction

 The use of broadcasts, especially on high-speed local area networks,
 is a good base for many applications.  Since broadcasting is not
 covered in the basic IP specification [13], there is no agreed-upon
 way to do it, and so protocol designers have not made use of it. (The
 issue has been touched upon before, e.g. [6], but has not been the
 subject of a standard.)
 We consider here only the case of unreliable, unsequenced, possibly
 duplicated datagram broadcasts (for a discussion of TCP broadcasting,
 see [11].) Even though unreliable and limited in length, datagram
 broadcasts are quite useful [1].
 We assume that the data link layer of the local network supports
 efficient broadcasting.  Most common local area networks do support
 broadcast; for example, Ethernet [7, 5], ChaosNet [10], token ring
 networks [2], etc.
 We do not assume, however, that broadcasts are reliably delivered.
 (One might consider providing a reliable broadcast protocol as a
 layer above IP.) It is quite expensive to guarantee delivery of
 broadcasts; instead, what we assume is that a host will receive most
 of the broadcasts that are sent.  This is important to avoid
 excessive use of broadcasts; since every host on the network devotes
 at least some effort to every broadcast, they are costly.

Mogul [Page 1]

RFC 919 October 1984 Broadcasting Internet Datagrams

 When a datagram is broadcast, it imposes a cost on every host that
 hears it.  Therefore, broadcasting should not be used
 indiscriminately, but rather only when it is the best solution to a
 problem.
 Note: some organizations have divided their IP networks into subnets,
 for which a standard [8] has been proposed.  This RFC does not cover
 the numerous complications arising from the interactions between
 subnets and broadcasting; see [9] for a complete discussion.

2. Terminology

 Because broadcasting depends on the specific data link layer in use
 on a local network, we must discuss it with reference to both
 physical networks and logical networks.
 The terms we will use in referring to physical networks are, from the
 point of view of the host sending or forwarding a broadcast:
 Local Hardware Network
    The physical link to which the host is attached.
 Remote Hardware Network
    A physical network which is separated from the host by at least
    one gateway.
 Collection of Hardware Networks
    A set of hardware networks (transitively) connected by gateways.
 The IP world includes several kinds of logical network.  To avoid
 ambiguity, we will use the following terms:
 Internet
    The DARPA Internet collection of IP networks.
 IP Network
    One or a collection of several hardware networks that have one
    specific IP network number.

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RFC 919 October 1984 Broadcasting Internet Datagrams

3. Why Broadcast?

 Broadcasts are useful when a host needs to find information without
 knowing exactly what other host can supply it, or when a host wants
 to provide information to a large set of hosts in a timely manner.
 When a host needs information that one or more of its neighbors might
 have, it could have a list of neighbors to ask, or it could poll all
 of its possible neighbors until one responds.  Use of a wired-in list
 creates obvious network management problems (early binding is
 inflexible).  On the other hand, asking all of one's neighbors is
 slow if one must generate plausible host addresses, and try them
 until one works.  On the ARPANET, for example, there are roughly 65
 thousand plausible host numbers.  Most IP implementations have used
 wired-in lists (for example, addresses of "Prime" gateways.)
 Fortunately, broadcasting provides a fast and simple way for a host
 to reach all of its neighbors.
 A host might also use a broadcast to provide all of its neighbors
 with some information; for example, a gateway might announce its
 presence to other gateways.
 One way to view broadcasting is as an imperfect substitute for
 multicasting, the sending of messages to a subset of the hosts on a
 network.  In practice, broadcasts are usually used where multicasts
 are what is wanted; packets are broadcast at the hardware level, but
 filtering software in the receiving hosts gives the effect of
 multicasting.
 For more examples of broadcast applications, see [1, 3].

4. Broadcast Classes

 There are several classes of IP broadcasting:
  1. Single-destination datagram broadcast on the local IP net: A

datagrams is destined for a specific IP host, but the sending

      host broadcasts it at the data link layer, perhaps to avoid
      having to do routing.  Since this is not an IP broadcast, the IP
      layer is not involved, except that a host should discard
      datagrams not meant for it without becoming flustered (i.e.,
      printing an error message).
  1. Broadcast to all hosts on the local IP net: A distinguished

value for the host-number part of the IP address denotes

      broadcast instead of a specific host.  The receiving IP layer
      must be able to recognize this address as well as its own.

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RFC 919 October 1984 Broadcasting Internet Datagrams

      However, it might still be useful to distinguish at higher
      levels between broadcasts and non-broadcasts, especially in
      gateways. This is the most useful case of broadcast; it allows a
      host to discover gateways without wired-in tables, it is the
      basis for address resolution protocols, and it is also useful
      for accessing such utilities as name servers, time servers,
      etc., without requiring wired-in addresses.
  1. Broadcast to all hosts on a remote IP network: It is

occasionally useful to send a broadcast to all hosts on a

      non-local network; for example, to find the latest version of a
      hostname database, to bootload a host on an IP network without a
      bootserver, or to monitor the timeservers on the IP network.
      This case is the same as local-network broadcasts; the datagram
      is routed by normal mechanisms until it reaches a gateway
      attached to the destination IP network, at which point it is
      broadcast. This class of broadcasting is also known as "directed
      broadcasting", or quaintly as sending a "letter bomb" [1].
  1. Broadcast to the entire Internet: This is probably not useful,

and almost certainly not desirable.

 For reasons of performance or security, a gateway may choose not to
 forward broadcasts; especially, it may be a good idea to ban
 broadcasts into or out of an autonomous group of networks.

5. Broadcast Methods

 A host's IP receiving layer must be modified to support broadcasting.
 In the absence of broadcasting, a host determines if it is the
 recipient of a datagram by matching the destination address against
 all of its IP addresses.  With broadcasting, a host must compare the
 destination address not only against the host's addresses, but also
 against the possible broadcast addresses for that host.
 The problem of how best to send a broadcast has been extensively
 discussed [1, 3, 4, 14, 15].  Since we assume that the problem has
 already been solved at the data link layer, an IP host wishing to
 send either a local broadcast or a directed broadcast need only
 specify the appropriate destination address and send the datagram as
 usual.  Any sophisticated algorithms need only reside in gateways.

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RFC 919 October 1984 Broadcasting Internet Datagrams

6. Gateways and Broadcasts

 Most of the complexity in supporting broadcasts lies in gateways.  If
 a gateway receives a directed broadcast for a network to which it is
 not connected, it simply forwards it using the usual mechanism.
 Otherwise, it must do some additional work.
 When a gateway receives a local broadcast datagram, there are several
 things it might have to do with it.  The situation is unambiguous,
 but without due care it is possible to create infinite loops.
 The appropriate action to take on receipt of a broadcast datagram
 depends on several things: the subnet it was received on, the
 destination network, and the addresses of the gateway.
  1. The primary rule for avoiding loops is "never broadcast a

datagram on the hardware network it was received on". It is not

      sufficient simply to avoid repeating datagrams that a gateway
      has heard from itself; this still allows loops if there are
      several gateways on a hardware network.
  1. If the datagram is received on the hardware network to which it

is addressed, then it should not be forwarded. However, the

      gateway should consider itself to be a destination of the
      datagram (for example, it might be a routing table update.)
  1. Otherwise, if the datagram is addressed to a hardware network to

which the gateway is connected, it should be sent as a (data

      link layer) broadcast on that network.  Again, the gateway
      should consider itself a destination of the datagram.
  1. Otherwise, the gateway should use its normal routing procedure

to choose a subsequent gateway, and send the datagram along to

      it.

7. Broadcast IP Addressing - Proposed Standards

 If different IP implementations are to be compatible, there must be a
 distinguished number to denote "all hosts".
 Since the local network layer can always map an IP address into data
 link layer address, the choice of an IP "broadcast host number" is
 somewhat arbitrary.  For simplicity, it should be one not likely to
 be assigned to a real host.  The number whose bits are all ones has
 this property; this assignment was first proposed in [6].  In the few
 cases where a host has been assigned an address with a host-number
 part of all ones, it does not seem onerous to require renumbering.

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RFC 919 October 1984 Broadcasting Internet Datagrams

 The address 255.255.255.255 denotes a broadcast on a local hardware
 network, which must not be forwarded.  This address may be used, for
 example, by hosts that do not know their network number and are
 asking some server for it.
 Thus, a host on net 36, for example, may:
  1. broadcast to all of its immediate neighbors by using

255.255.255.255

  1. broadcast to all of net 36 by using 36.255.255.255
 (Note that unless the network has been broken up into subnets, these
 two methods have identical effects.)
 If the use of "all ones" in a field of an IP address means
 "broadcast", using "all zeros" could be viewed as meaning
 "unspecified".  There is probably no reason for such addresses to
 appear anywhere but as the source address of an ICMP Information
 Request datagram.  However, as a notational convention, we refer to
 networks (as opposed to hosts) by using addresses with zero fields.
 For example, 36.0.0.0 means "network number 36" while 36.255.255.255
 means "all hosts on network number 36".
 7.1. ARP Servers and Broadcasts
    The Address Resolution Protocol (ARP) described in [12] can, if
    incorrectly implemented, cause problems when broadcasts are used
    on a network where not all hosts share an understanding of what a
    broadcast address is.  The temptation exists to modify the ARP
    server so that it provides the mapping between an IP broadcast
    address and the hardware broadcast address.
    This temptation must be resisted.  An ARP server should never
    respond to a request whose target is a broadcast address.  Such a
    request can only come from a host that does not recognize the
    broadcast address as such, and so honoring it would almost
    certainly lead to a forwarding loop.  If there are N such hosts on
    the physical network that do not recognize this address as a
    broadcast, then a datagram sent with a Time-To-Live of T could
    potentially give rise to T**N spurious re-broadcasts.

Mogul [Page 6]

RFC 919 October 1984 Broadcasting Internet Datagrams

8. References

 1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
      University, January 1982.
 2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
      Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516, 1978.
 3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
      Computer Networks.  Ph.D. Th., Stanford University, April 1977.
 4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
      of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048, December
      1978.
 5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
      Layer Specifications.  Version 1.0, Digital Equipment
      Corporation, Intel, Xerox, September 1980.
 6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
      Addressing Issues.  IEN-212, Bolt Beranek and Newman, September
      1982.
 7.    R.M. Metcalfe and D.R. Boggs. "Ethernet: Distributed Packet
      Switching for Local Computer Networks".  Comm. ACM 19, 7,
      pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
      Center, reprinted in CSL-80-2.
 8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
      October 1984.
 9.   Jeffrey Mogul.  Broadcasting Internet Packets in the Presence of
      Subnets.  RFC-922, Stanford University, October 1984.
 10.  David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
      Institute of Technology Artificial Intelligence Laboratory, June
      1981.
 11.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, Bolt
      Beranek and Newman, March 1977.
 12.  David Plummer.  An Ethernet Address Resolution Protocol.
      RFC-826, Symbolics, September 1982.
 13.  Jon Postel.  Internet Protocol.  RFC 791, ISI, September 1981.

Mogul [Page 7]

RFC 919 October 1984 Broadcasting Internet Datagrams

 14.  David W. Wall.  Mechanisms for Broadcast and Selective
      Broadcast.  Ph.D. Th., Stanford University, June 1980.
 15.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
      Computer Systems Lab Technical Report TR189, Stanford
      University, June 1980.

Mogul [Page 8]

Network Working Group Jeffrey Mogul Request for Comments: 922 Computer Science Department

                                                   Stanford University
                                                          October 1984
     BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS

Status of this Memo

 We propose simple rules for broadcasting Internet datagrams on local
 networks that support broadcast, for addressing broadcasts, and for
 how gateways should handle them.
 This RFC suggests a proposed protocol for the ARPA-Internet
 community, and requests discussion and suggestions for improvements.
 Distribution of this memo is unlimited.

Acknowledgement

 This proposal here is the result of discussion with several other
 people, especially J. Noel Chiappa and Christopher A. Kent, both of
 whom both pointed me at important references.

1. Introduction

 The use of broadcasts, especially on high-speed local area networks,
 is a good base for many applications.  Since broadcasting is not
 covered in the basic IP specification [12], there is no agreed-upon
 way to do it, and so protocol designers have not made use of it. (The
 issue has been touched upon before, e.g. [6], but has not been the
 subject of a standard.)
 We consider here only the case of unreliable, unsequenced, possibly
 duplicated datagram broadcasts (for a discussion of TCP broadcasting,
 see [10].) Even though unreliable and limited in length, datagram
 broadcasts are quite useful [1].
 We assume that the data link layer of the local network supports
 efficient broadcasting.  Most common local area networks do support
 broadcast; for example, Ethernet [7, 5], ChaosNet [9], token ring
 networks [2], etc.
 We do not assume, however, that broadcasts are reliably delivered.
 (One might consider providing a reliable datagram broadcast protocol
 as a layer above IP.) It is quite expensive to guarantee delivery of
 broadcasts; instead, what we assume is that a host will receive most
 of the broadcasts that are sent.  This is important to avoid
 excessive use of broadcasts; since every host on the network devotes
 at least some effort to every broadcast, they are costly.

Mogul [Page 1]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

 When a datagram is broadcast, it imposes a cost on every host that
 hears it.  Therefore, broadcasting should not be used
 indiscriminately, but rather only when it is the best solution to a
 problem.

2. Terminology

 Because broadcasting depends on the specific data link layer in use
 on a local network, we must discuss it with reference to both
 physical networks and logical networks.
 The terms we will use in referring to physical networks are, from the
 point of view of the host sending or forwarding a broadcast:
 Local Hardware Network
    The physical link to which the host is attached.
 Remote Hardware Network
    A physical network which is separated from the host by at least
    one gateway.
 Collection of Hardware Networks
    A set of hardware networks (transitively) connected by gateways.
 The IP world includes several kinds of logical network.  To avoid
 ambiguity, we will use the following terms:
 Internet
    The DARPA Internet collection of IP networks.
 IP Network
    One or a collection of several hardware networks that have one
    specific IP network number.
 Subnet
    A single member of the collection of hardware networks that
    compose an IP network.  Host addresses on a given subnet share an
    IP network number with hosts on all other subnets of that IP
    network, but the local-address part is divided into subnet-number

Mogul [Page 2]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

    and host-number fields to indicate which subnet a host is on.  We
    do not assume a particular division of the local-address part;
    this could vary from network to network.
 The introduction of a subnet level in the addressing hierarchy is at
 variance with the IP specification [12], but as the use of
 addressable subnets proliferates it is obvious that a broadcasting
 scheme should support subnetting.  For more on subnets, see [8].
 In this paper, the term "host address" refers to the host-on-subnet
 address field of a subnetted IP network, or the host-part field
 otherwise.
 An IP network may consist of a single hardware network or a
 collection of subnets; from the point of view of a host on another IP
 network, it should not matter.

3. Why Broadcast?

 Broadcasts are useful when a host needs to find information without
 knowing exactly what other host can supply it, or when a host wants
 to provide information to a large set of hosts in a timely manner.
 When a host needs information that one or more of its neighbors might
 have, it could have a list of neighbors to ask, or it could poll all
 of its possible neighbors until one responds.  Use of a wired-in list
 creates obvious network management problems (early binding is
 inflexible).  On the other hand, asking all of one's neighbors is
 slow if one must generate plausible host addresses, and try them
 until one works.  On the ARPANET, for example, there are roughly 65
 thousand plausible host numbers.  Most IP implementations have used
 wired-in lists (for example, addresses of "Prime" gateways.)
 Fortunately, broadcasting provides a fast and simple way for a host
 to reach all of its neighbors.
 A host might also use a broadcast to provide all of its neighbors
 with some information; for example, a gateway might announce its
 presence to other gateways.
 One way to view broadcasting is as an imperfect substitute for
 multicasting, the sending of messages to a subset of the hosts on a
 network.  In practice, broadcasts are usually used where multicasts
 are what is wanted; datagrams are broadcast at the hardware level,
 but filtering software in the receiving hosts gives the effect of
 multicasting.
 For more examples of broadcast applications, see [1, 3].

Mogul [Page 3]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

4. Broadcast Classes

 There are several classes of IP broadcasting:
  1. Single-destination datagrams broadcast on the local hardware

net: A datagram is destined for a specific IP host, but the

      sending host broadcasts it at the data link layer, perhaps to
      avoid having to do routing.  Since this is not an IP broadcast,
      the IP layer is not involved, except that a host should discard
      datagram not meant for it without becoming flustered (i.e.,
      printing an error message).
  1. Broadcast to all hosts on the local hardware net: A

distinguished value for the host-number part of the IP address

      denotes broadcast instead of a specific host.  The receiving IP
      layer must be able to recognize this address as well as its own.
      However, it might still be useful to distinguish at higher
      levels between broadcasts and non-broadcasts, especially in
      gateways.  This is the most useful case of broadcast; it allows
      a host to discover gateways without wired-in tables, it is the
      basis for address resolution protocols, and it is also useful
      for accessing such utilities as name servers, time servers,
      etc., without requiring wired-in addresses.
  1. Broadcast to all hosts on a remote hardware network: It is

occasionally useful to send a broadcast to all hosts on a

      non-local network; for example, to find the latest version of a
      hostname database, to bootload a host on a subnet without a
      bootserver, or to monitor the timeservers on the subnet.  This
      case is the same as local-network broadcasts; the datagram is
      routed by normal mechanisms until it reaches a gateway attached
      to the destination hardware network, at which point it is
      broadcast.  This class of broadcasting is also known as
      "directed broadcasting", or quaintly as sending a "letter bomb"
      [1].
  1. Broadcast to all hosts on a subnetted IP network (Multi-subnet

broadcasts): A distinguished value for the subnet-number part of

      the IP address is used to denote "all subnets".  Broadcasts to
      all hosts of a remote subnetted IP network are done just as
      directed broadcasts to a single subnet.
  1. Broadcast to the entire Internet: This is probably not useful,

and almost certainly not desirable.

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RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

 For reasons of performance or security, a gateway may choose not to
 forward broadcasts; especially, it may be a good idea to ban
 broadcasts into or out of an autonomous group of networks.

5. Broadcast Methods

 A host's IP receiving layer must be modified to support broadcasting.
 In the absence of broadcasting, a host determines if it is the
 recipient of a datagram by matching the destination address against
 all of its IP addresses.  With broadcasting, a host must compare the
 destination address not only against the host's addresses, but also
 against the possible broadcast addresses for that host.
 The problem of how best to send a broadcast has been extensively
 discussed [1, 3, 4, 13, 14].  Since we assume that the problem has
 already been solved at the data link layer, an IP host wishing to
 send either a local broadcast or a directed broadcast need only
 specify the appropriate destination address and send the datagram as
 usual.  Any sophisticated algorithms need only reside in gateways.
 The problem of broadcasting to all hosts on a subnetted IP network is
 apparently somewhat harder.  However, even in this case it turns out
 that the best known algorithms require no additional complexity in
 non-gateway hosts.  A good broadcast method will meet these
 additional criteria:
  1. No modification of the IP datagram format.
  1. Reasonable efficiency in terms of the number of excess copies

generated and the cost of paths chosen.

  1. Minimization of gateway modification, in both code and data

space.

  1. High likelihood of delivery.
 The algorithm that appears best is the Reverse Path Forwarding (RPF)
 method [4].  While RPF is suboptimal in cost and reliability, it is
 quite good, and is extremely simple to implement, requiring no
 additional data space in a gateway.

Mogul [Page 5]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

6. Gateways and Broadcasts

 Most of the complexity in supporting broadcasts lies in gateways.  If
 a gateway receives a directed broadcast for a network to which it is
 not connected, it simply forwards it using the usual mechanism.
 Otherwise, it must do some additional work.
 6.1. Local Broadcasts
    When a gateway receives a local broadcast datagram, there are
    several things it might have to do with it.  The situation is
    unambiguous, but without due care it is possible to create
    infinite loops.
    The appropriate action to take on receipt of a broadcast datagram
    depends on several things: the subnet it was received on, the
    destination network, and the addresses of the gateway.
  1. The primary rule for avoiding loops is "never broadcast a

datagram on the hardware network it was received on". It is

         not sufficient simply to avoid repeating datagram that a
         gateway has heard from itself; this still allows loops if
         there are several gateways on a hardware network.
  1. If the datagram is received on the hardware network to which

it is addressed, then it should not be forwarded. However,

         the gateway should consider itself to be a destination of the
         datagram (for example, it might be a routing table update.)
  1. Otherwise, if the datagram is addressed to a hardware network

to which the gateway is connected, it should be sent as a

         (data link layer) broadcast on that network.  Again, the
         gateway should consider itself a destination of the datagram.
  1. Otherwise, the gateway should use its normal routing

procedure to choose a subsequent gateway, and send the

         datagram along to it.

Mogul [Page 6]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

 6.2. Multi-subnet broadcasts
    When a gateway receives a broadcast meant for all subnets of an IP
    network, it must use the Reverse Path Forwarding algorithm to
    decide what to do.  The method is simple: the gateway should
    forward copies of the datagram along all connected links, if and
    only if the datagram arrived on the link which is part of the best
    route between the gateway and the source of the datagram.
    Otherwise, the datagram should be discarded.
    This algorithm may be improved if some or all of the gateways
    exchange among themselves additional information; this can be done
    transparently from the point of view of other hosts and even other
    gateways.  See [4, 3] for details.
 6.3. Pseudo-Algol Routing Algorithm
    This is a pseudo-Algol description of the routing algorithm a
    gateway should use.  The algorithm is shown in figure 1.  Some
    definitions are:
    RouteLink(host)
       A function taking a host address as a parameter and returning
       the first-hop link from the gateway to the host.
    RouteHost(host)
       As above but returns the first-hop host address.
    ResolveAddress(host)
       Returns the hardware address for an IP host.
    IncomingLink
       The link on which the packet arrived.
    OutgoingLinkSet
       The set of links on which the packet should be sent.
    OutgoingHardwareHost
       The hardware host address to send the packet to.

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RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

    Destination.host
       The host-part of the destination address.
    Destination.subnet
       The subnet-part of the destination address.
    Destination.ipnet
       The IP-network-part of the destination address.

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RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

BEGIN

 IF Destination.ipnet IN AllLinks THEN
    BEGIN
       IF IsSubnetted(Destination.ipnet) THEN
          BEGIN
             IF Destination.subnet = BroadcastSubnet THEN
                BEGIN      /* use Reverse Path Forwarding algorithm */
                   IF IncomingLink = RouteLink(Source) THEN
                      BEGIN IF Destination.host = BroadcastHost THEN
                            OutgoingLinkSet <- AllLinks -
                         IncomingLink;
                         OutgoingHost <- BroadcastHost;
                         Examine packet for possible internal use;
                      END
                   ELSE  /* duplicate from another gateway, discard */
                      Discard;
                END
             ELSE
                IF Destination.subnet = IncomingLink.subnet THEN
                   BEGIN           /* forwarding would cause a loop */
                      IF Destination.host = BroadcastHost THEN
                         Examine packet for possible internal use;
                      Discard;
                   END
                ELSE BEGIN    /* forward to (possibly local) subnet */
                      OutgoingLinkSet <- RouteLink(Destination);
                      OutgoingHost <- RouteHost(Destination);
                   END
          END
       ELSE BEGIN         /* destined for one of our local networks */
             IF Destination.ipnet = IncomingLink.ipnet THEN
                BEGIN              /* forwarding would cause a loop */
                   IF Destination.host = BroadcastHost THEN
                      Examine packet for possible internal use;
                   Discard;
                END
             ELSE BEGIN                     /* might be a broadcast */
                   OutgoingLinkSet <- RouteLink(Destination);
                   OutgoingHost <- RouteHost(Destination);
                END
          END
    END
 ELSE BEGIN                    /* forward to a non-local IP network */
       OutgoingLinkSet <- RouteLink(Destination);
       OutgoingHost <- RouteHost(Destination);
    END
 OutgoingHardwareHost <- ResolveAddress(OutgoingHost);

END

Figure 1: Pseudo-Algol algorithm for routing broadcasts by gateways

Mogul [Page 9]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

7. Broadcast IP Addressing - Conventions

 If different IP implementations are to be compatible, there must be
 convention distinguished number to denote "all hosts" and "all
 subnets".
 Since the local network layer can always map an IP address into data
 link layer address, the choice of an IP "broadcast host number" is
 somewhat arbitrary.  For simplicity, it should be one not likely to
 be assigned to a real host.  The number whose bits are all ones has
 this property; this assignment was first proposed in [6].  In the few
 cases where a host has been assigned an address with a host-number
 part of all ones, it does not seem onerous to require renumbering.
 The "all subnets" number is also all ones; this means that a host
 wishing to broadcast to all hosts on a remote IP network need not
 know how the destination address is divided up into subnet and host
 fields, or if it is even divided at all.  For example, 36.255.255.255
 may denote all the hosts on a single hardware network, or all the
 hosts on a subnetted IP network with 1 byte of subnet field and 2
 bytes of host field, or any other possible division.
 The address 255.255.255.255 denotes a broadcast on a local hardware
 network that must not be forwarded.  This address may be used, for
 example, by hosts that do not know their network number and are
 asking some server for it.
 Thus, a host on net 36, for example, may:
  1. broadcast to all of its immediate neighbors by using

255.255.255.255

  1. broadcast to all of net 36 by using 36.255.255.255
 without knowing if the net is subnetted; if it is not, then both
 addresses have the same effect. A robust application might try the
 former address, and if no response is received, then try the latter.
 See [1] for a discussion of such "expanding ring search" techniques.
 If the use of "all ones" in a field of an IP address means
 "broadcast", using "all zeros" could be viewed as meaning
 "unspecified".  There is probably no reason for such addresses to
 appear anywhere but as the source address of an ICMP Information
 Request datagram.  However, as a notational convention, we refer to
 networks (as opposed to hosts) by using addresses with zero fields.
 For example, 36.0.0.0 means "network number 36" while 36.255.255.255
 means "all hosts on network number 36".

Mogul [Page 10]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

 7.1. ARP Servers and Broadcasts
    The Address Resolution Protocol (ARP) described in [11] can, if
    incorrectly implemented, cause problems when broadcasts are used
    on a network where not all hosts share an understanding of what a
    broadcast address is.  The temptation exists to modify the ARP
    server so that it provides the mapping between an IP broadcast
    address and the hardware broadcast address.
    This temptation must be resisted.  An ARP server should never
    respond to a request whose target is a broadcast address.  Such a
    request can only come from a host that does not recognize the
    broadcast address as such, and so honoring it would almost
    certainly lead to a forwarding loop.  If there are N such hosts on
    the physical network that do not recognize this address as a
    broadcast, then a datagram sent with a Time-To-Live of T could
    potentially give rise to T**N spurious re-broadcasts.

8. References

 1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
      University, January 1982.
 2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
      Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516,
      November 1978.
 3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
      Computer Networks.  Ph.D. Th., Stanford University, April 1977.
 4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
      of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048,
      December 1978.
 5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
      Layer Specifications.  Version 1.0, Digital Equipment
      Corporation, Intel, Xerox, September 1980.
 6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
      Addressing Issues.  IEN-212, BBN, September 1982.
 7.   R.M. Metcalfe and D.R. Boggs.  "Ethernet: Distributed Packet
      Switching for Local Computer Networks".  Comm. ACM 19, 7,
      pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
      Center, reprinted in CSL-80-2.

Mogul [Page 11]

RFC 922 October 1984 Broadcasting Internet Datagrams in the Presence of Subnets

 8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
      October 1984.
 9.   David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
      Institute of Technology Artificial Intelligence Laboratory,
      June 1981.
 10.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, BBN,
      March 1977.
 11.  David Plummer.  An Ethernet Address Resolution Protocol.
      RFC-826, Symbolics, September 1982.
 12.  Jon Postel.  Internet Protocol.  RFC-791, ISI, September 1981.
 13.  David W. Wall.  Mechanisms for Broadcast and Selective
      Broadcast.  Ph.D. Th., Stanford University, June 1980.
 14.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
      Computer Systems Lab Technical Report TR189, Stanford
      University, June 1980.

Mogul [Page 12]

Network Working Group J. Postel Request for Comments: 792 ISI

                                                        September 1981

Updates: RFCs 777, 760 Updates: IENs 109, 128

                 INTERNET CONTROL MESSAGE PROTOCOL
                       DARPA INTERNET PROGRAM
                       PROTOCOL SPECIFICATION

Introduction

 The Internet Protocol (IP) [1] is used for host-to-host datagram
 service in a system of interconnected networks called the
 Catenet [2].  The network connecting devices are called Gateways.
 These gateways communicate between themselves for control purposes
 via a Gateway to Gateway Protocol (GGP) [3,4].  Occasionally a
 gateway or destination host will communicate with a source host, for
 example, to report an error in datagram processing.  For such
 purposes this protocol, the Internet Control Message Protocol (ICMP),
 is used.  ICMP, uses the basic support of IP as if it were a higher
 level protocol, however, ICMP is actually an integral part of IP, and
 must be implemented by every IP module.
 ICMP messages are sent in several situations:  for example, when a
 datagram cannot reach its destination, when the gateway does not have
 the buffering capacity to forward a datagram, and when the gateway
 can direct the host to send traffic on a shorter route.
 The Internet Protocol is not designed to be absolutely reliable.  The
 purpose of these control messages is to provide feedback about
 problems in the communication environment, not to make IP reliable.
 There are still no guarantees that a datagram will be delivered or a
 control message will be returned.  Some datagrams may still be
 undelivered without any report of their loss.  The higher level
 protocols that use IP must implement their own reliability procedures
 if reliable communication is required.
 The ICMP messages typically report errors in the processing of
 datagrams.  To avoid the infinite regress of messages about messages
 etc., no ICMP messages are sent about ICMP messages.  Also ICMP
 messages are only sent about errors in handling fragment zero of
 fragemented datagrams.  (Fragment zero has the fragment offeset equal
 zero).
                                                              [Page 1]
                                                        September 1981

RFC 792

Message Formats

 ICMP messages are sent using the basic IP header.  The first octet of
 the data portion of the datagram is a ICMP type field; the value of
 this field determines the format of the remaining data.  Any field
 labeled "unused" is reserved for later extensions and must be zero
 when sent, but receivers should not use these fields (except to
 include them in the checksum).  Unless otherwise noted under the
 individual format descriptions, the values of the internet header
 fields are as follows:
 Version
    4
 IHL
    Internet header length in 32-bit words.
 Type of Service
 Total Length
    Length of internet header and data in octets.
 Identification, Flags, Fragment Offset
    Used in fragmentation, see [1].
 Time to Live
    Time to live in seconds; as this field is decremented at each
    machine in which the datagram is processed, the value in this
    field should be at least as great as the number of gateways which
    this datagram will traverse.
 Protocol
    ICMP = 1
 Header Checksum
    The 16 bit one's complement of the one's complement sum of all 16
    bit words in the header.  For computing the checksum, the checksum
    field should be zero.  This checksum may be replaced in the
    future.

[Page 2]

September 1981 RFC 792

 Source Address
    The address of the gateway or host that composes the ICMP message.
    Unless otherwise noted, this can be any of a gateway's addresses.
 Destination Address
    The address of the gateway or host to which the message should be
    sent.
                                                              [Page 3]
                                                        September 1981

RFC 792

Destination Unreachable Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             unused                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Internet Header + 64 bits of Original Data Datagram      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Destination Address
    The source network and address from the original datagram's data.
 ICMP Fields:
 Type
    3
 Code
    0 = net unreachable;
    1 = host unreachable;
    2 = protocol unreachable;
    3 = port unreachable;
    4 = fragmentation needed and DF set;
    5 = source route failed.
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Internet Header + 64 bits of Data Datagram
    The internet header plus the first 64 bits of the original

[Page 4]

September 1981 RFC 792

    datagram's data.  This data is used by the host to match the
    message to the appropriate process.  If a higher level protocol
    uses port numbers, they are assumed to be in the first 64 data
    bits of the original datagram's data.
 Description
    If, according to the information in the gateway's routing tables,
    the network specified in the internet destination field of a
    datagram is unreachable, e.g., the distance to the network is
    infinity, the gateway may send a destination unreachable message
    to the internet source host of the datagram.  In addition, in some
    networks, the gateway may be able to determine if the internet
    destination host is unreachable.  Gateways in these networks may
    send destination unreachable messages to the source host when the
    destination host is unreachable.
    If, in the destination host, the IP module cannot deliver the
    datagram  because the indicated protocol module or process port is
    not active, the destination host may send a destination
    unreachable message to the source host.
    Another case is when a datagram must be fragmented to be forwarded
    by a gateway yet the Don't Fragment flag is on.  In this case the
    gateway must discard the datagram and may return a destination
    unreachable message.
    Codes 0, 1, 4, and 5 may be received from a gateway.  Codes 2 and
    3 may be received from a host.
                                                              [Page 5]
                                                        September 1981

RFC 792

Time Exceeded Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             unused                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Internet Header + 64 bits of Original Data Datagram      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Destination Address
    The source network and address from the original datagram's data.
 ICMP Fields:
 Type
    11
 Code
    0 = time to live exceeded in transit;
    1 = fragment reassembly time exceeded.
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Internet Header + 64 bits of Data Datagram
    The internet header plus the first 64 bits of the original
    datagram's data.  This data is used by the host to match the
    message to the appropriate process.  If a higher level protocol
    uses port numbers, they are assumed to be in the first 64 data
    bits of the original datagram's data.
 Description
    If the gateway processing a datagram finds the time to live field

[Page 6]

September 1981 RFC 792

    is zero it must discard the datagram.  The gateway may also notify
    the source host via the time exceeded message.
    If a host reassembling a fragmented datagram cannot complete the
    reassembly due to missing fragments within its time limit it
    discards the datagram, and it may send a time exceeded message.
    If fragment zero is not available then no time exceeded need be
    sent at all.
    Code 0 may be received from a gateway.  Code 1 may be received
    from a host.
                                                              [Page 7]
                                                        September 1981

RFC 792

Parameter Problem Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Pointer    |                   unused                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Internet Header + 64 bits of Original Data Datagram      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Destination Address
    The source network and address from the original datagram's data.
 ICMP Fields:
 Type
    12
 Code
    0 = pointer indicates the error.
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Pointer
    If code = 0, identifies the octet where an error was detected.
 Internet Header + 64 bits of Data Datagram
    The internet header plus the first 64 bits of the original
    datagram's data.  This data is used by the host to match the
    message to the appropriate process.  If a higher level protocol
    uses port numbers, they are assumed to be in the first 64 data
    bits of the original datagram's data.

[Page 8]

September 1981 RFC 792

 Description
    If the gateway or host processing a datagram finds a problem with
    the header parameters such that it cannot complete processing the
    datagram it must discard the datagram.  One potential source of
    such a problem is with incorrect arguments in an option.  The
    gateway or host may also notify the source host via the parameter
    problem message.  This message is only sent if the error caused
    the datagram to be discarded.
    The pointer identifies the octet of the original datagram's header
    where the error was detected (it may be in the middle of an
    option).  For example, 1 indicates something is wrong with the
    Type of Service, and (if there are options present) 20 indicates
    something is wrong with the type code of the first option.
    Code 0 may be received from a gateway or a host.
                                                              [Page 9]
                                                        September 1981

RFC 792

Source Quench Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             unused                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Internet Header + 64 bits of Original Data Datagram      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Destination Address
    The source network and address of the original datagram's data.
 ICMP Fields:
 Type
    4
 Code
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Internet Header + 64 bits of Data Datagram
    The internet header plus the first 64 bits of the original
    datagram's data.  This data is used by the host to match the
    message to the appropriate process.  If a higher level protocol
    uses port numbers, they are assumed to be in the first 64 data
    bits of the original datagram's data.
 Description
    A gateway may discard internet datagrams if it does not have the
    buffer space needed to queue the datagrams for output to the next
    network on the route to the destination network.  If a gateway

[Page 10]

September 1981 RFC 792

    discards a datagram, it may send a source quench message to the
    internet source host of the datagram.  A destination host may also
    send a source quench message if datagrams arrive too fast to be
    processed.  The source quench message is a request to the host to
    cut back the rate at which it is sending traffic to the internet
    destination.  The gateway may send a source quench message for
    every message that it discards.  On receipt of a source quench
    message, the source host should cut back the rate at which it is
    sending traffic to the specified destination until it no longer
    receives source quench messages from the gateway.  The source host
    can then gradually increase the rate at which it sends traffic to
    the destination until it again receives source quench messages.
    The gateway or host may send the source quench message when it
    approaches its capacity limit rather than waiting until the
    capacity is exceeded.  This means that the data datagram which
    triggered the source quench message may be delivered.
    Code 0 may be received from a gateway or a host.
                                                             [Page 11]
                                                        September 1981

RFC 792

Redirect Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Gateway Internet Address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Internet Header + 64 bits of Original Data Datagram      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Destination Address
    The source network and address of the original datagram's data.
 ICMP Fields:
 Type
    5
 Code
    0 = Redirect datagrams for the Network.
    1 = Redirect datagrams for the Host.
    2 = Redirect datagrams for the Type of Service and Network.
    3 = Redirect datagrams for the Type of Service and Host.
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Gateway Internet Address
    Address of the gateway to which traffic for the network specified
    in the internet destination network field of the original
    datagram's data should be sent.

[Page 12]

September 1981 RFC 792

 Internet Header + 64 bits of Data Datagram
    The internet header plus the first 64 bits of the original
    datagram's data.  This data is used by the host to match the
    message to the appropriate process.  If a higher level protocol
    uses port numbers, they are assumed to be in the first 64 data
    bits of the original datagram's data.
 Description
    The gateway sends a redirect message to a host in the following
    situation.  A gateway, G1, receives an internet datagram from a
    host on a network to which the gateway is attached.  The gateway,
    G1, checks its routing table and obtains the address of the next
    gateway, G2, on the route to the datagram's internet destination
    network, X.  If G2 and the host identified by the internet source
    address of the datagram are on the same network, a redirect
    message is sent to the host.  The redirect message advises the
    host to send its traffic for network X directly to gateway G2 as
    this is a shorter path to the destination.  The gateway forwards
    the original datagram's data to its internet destination.
    For datagrams with the IP source route options and the gateway
    address in the destination address field, a redirect message is
    not sent even if there is a better route to the ultimate
    destination than the next address in the source route.
    Codes 0, 1, 2, and 3 may be received from a gateway.
                                                             [Page 13]
                                                        September 1981

RFC 792

Echo or Echo Reply Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Code      |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Identifier          |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Data ...
 +-+-+-+-+-
 IP Fields:
 Addresses
    The address of the source in an echo message will be the
    destination of the echo reply message.  To form an echo reply
    message, the source and destination addresses are simply reversed,
    the type code changed to 0, and the checksum recomputed.
 IP Fields:
 Type
    8 for echo message;
    0 for echo reply message.
 Code
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    If the total length is odd, the received data is padded with one
    octet of zeros for computing the checksum.  This checksum may be
    replaced in the future.
 Identifier
    If code = 0, an identifier to aid in matching echos and replies,
    may be zero.
 Sequence Number

[Page 14]

September 1981 RFC 792

    If code = 0, a sequence number to aid in matching echos and
    replies, may be zero.
 Description
    The data received in the echo message must be returned in the echo
    reply message.
    The identifier and sequence number may be used by the echo sender
    to aid in matching the replies with the echo requests.  For
    example, the identifier might be used like a port in TCP or UDP to
    identify a session, and the sequence number might be incremented
    on each echo request sent.  The echoer returns these same values
    in the echo reply.
    Code 0 may be received from a gateway or a host.
                                                             [Page 15]
                                                        September 1981

RFC 792

Timestamp or Timestamp Reply Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |      Code     |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Identifier          |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Originate Timestamp                                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Receive Timestamp                                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Transmit Timestamp                                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Addresses
    The address of the source in a timestamp message will be the
    destination of the timestamp reply message.  To form a timestamp
    reply message, the source and destination addresses are simply
    reversed, the type code changed to 14, and the checksum
    recomputed.
 IP Fields:
 Type
    13 for timestamp message;
    14 for timestamp reply message.
 Code
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Identifier

[Page 16]

September 1981 RFC 792

    If code = 0, an identifier to aid in matching timestamp and
    replies, may be zero.
 Sequence Number
    If code = 0, a sequence number to aid in matching timestamp and
    replies, may be zero.
 Description
    The data received (a timestamp) in the message is returned in the
    reply together with an additional timestamp.  The timestamp is 32
    bits of milliseconds since midnight UT.  One use of these
    timestamps is described by Mills [5].
    The Originate Timestamp is the time the sender last touched the
    message before sending it, the Receive Timestamp is the time the
    echoer first touched it on receipt, and the Transmit Timestamp is
    the time the echoer last touched the message on sending it.
    If the time is not available in miliseconds or cannot be provided
    with respect to midnight UT then any time can be inserted in a
    timestamp provided the high order bit of the timestamp is also set
    to indicate this non-standard value.
    The identifier and sequence number may be used by the echo sender
    to aid in matching the replies with the requests.  For example,
    the identifier might be used like a port in TCP or UDP to identify
    a session, and the sequence number might be incremented on each
    request sent.  The destination returns these same values in the
    reply.
    Code 0 may be received from a gateway or a host.
                                                             [Page 17]
                                                        September 1981

RFC 792

Information Request or Information Reply Message

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |      Code     |          Checksum             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Identifier          |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IP Fields:
 Addresses
    The address of the source in a information request message will be
    the destination of the information reply message.  To form a
    information reply message, the source and destination addresses
    are simply reversed, the type code changed to 16, and the checksum
    recomputed.
 IP Fields:
 Type
    15 for information request message;
    16 for information reply message.
 Code
 Checksum
    The checksum is the 16-bit ones's complement of the one's
    complement sum of the ICMP message starting with the ICMP Type.
    For computing the checksum , the checksum field should be zero.
    This checksum may be replaced in the future.
 Identifier
    If code = 0, an identifier to aid in matching request and replies,
    may be zero.
 Sequence Number
    If code = 0, a sequence number to aid in matching request and
    replies, may be zero.

[Page 18]

September 1981 RFC 792

 Description
    This message may be sent with the source network in the IP header
    source and destination address fields zero (which means "this"
    network).  The replying IP module should send the reply with the
    addresses fully specified.  This message is a way for a host to
    find out the number of the network it is on.
    The identifier and sequence number may be used by the echo sender
    to aid in matching the replies with the requests.  For example,
    the identifier might be used like a port in TCP or UDP to identify
    a session, and the sequence number might be incremented on each
    request sent.  The destination returns these same values in the
    reply.
    Code 0 may be received from a gateway or a host.
                                                             [Page 19]
                                                        September 1981

RFC 792

Summary of Message Types

  0  Echo Reply
  3  Destination Unreachable
  4  Source Quench
  5  Redirect
  8  Echo
 11  Time Exceeded
 12  Parameter Problem
 13  Timestamp
 14  Timestamp Reply
 15  Information Request
 16  Information Reply

[Page 20]

September 1981 RFC 792

References

 [1]  Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
       Protocol Specification," RFC 791, USC/Information Sciences
       Institute, September 1981.
 [2]   Cerf, V., "The Catenet Model for Internetworking," IEN 48,
       Information Processing Techniques Office, Defense Advanced
       Research Projects Agency, July 1978.
 [3]   Strazisar, V., "Gateway Routing:  An Implementation
       Specification", IEN 30, Bolt Beranek and Newman, April 1979.
 [4]   Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek
       and Newman, August 1979.
 [5]   Mills, D., "DCNET Internet Clock Service," RFC 778, COMSAT
       Laboratories, April 1981.
                                                             [Page 21]

Network Working Group S. Deering Request for Comments: 1112 Stanford University Obsoletes: RFCs 988, 1054 August 1989

                Host Extensions for IP Multicasting

1. STATUS OF THIS MEMO

 This memo specifies the extensions required of a host implementation
 of the Internet Protocol (IP) to support multicasting.  It is the
 recommended standard for IP multicasting in the Internet.
 Distribution of this memo is unlimited.

2. INTRODUCTION

 IP multicasting is the transmission of an IP datagram to a "host
 group", a set of zero or more hosts identified by a single IP
 destination address.  A multicast datagram is delivered to all
 members of its destination host group with the same "best-efforts"
 reliability as regular unicast IP datagrams, i.e., the datagram is
 not guaranteed to arrive intact at all members of the destination
 group or in the same order relative to other datagrams.
 The membership of a host group is dynamic; that is, hosts may join
 and leave groups at any time.  There is no restriction on the
 location or number of members in a host group.  A host may be a
 member of more than one group at a time.  A host need not be a member
 of a group to send datagrams to it.
 A host group may be permanent or transient.  A permanent group has a
 well-known, administratively assigned IP address.  It is the address,
 not the membership of the group, that is permanent; at any time a
 permanent group may have any number of members, even zero.  Those IP
 multicast addresses that are not reserved for permanent groups are
 available for dynamic assignment to transient groups which exist only
 as long as they have members.
 Internetwork forwarding of IP multicast datagrams is handled by
 "multicast routers" which may be co-resident with, or separate from,
 internet gateways.  A host transmits an IP multicast datagram as a
 local network multicast which reaches all immediately-neighboring
 members of the destination host group.  If the datagram has an IP
 time-to-live greater than 1, the multicast router(s) attached to the
 local network take responsibility for forwarding it towards all other
 networks that have members of the destination group.  On those other
 member networks that are reachable within the IP time-to-live, an
 attached multicast router completes delivery by transmitting the

Deering [Page 1] RFC 1112 Host Extensions for IP Multicasting August 1989

 datagram as a local multicast.
 This memo specifies the extensions required of a host IP
 implementation to support IP multicasting, where a "host" is any
 internet host or gateway other than those acting as multicast
 routers.  The algorithms and protocols used within and between
 multicast routers are transparent to hosts and will be specified in
 separate documents.  This memo also does not specify how local
 network multicasting is accomplished for all types of network,
 although it does specify the required service interface to an
 arbitrary local network and gives an Ethernet specification as an
 example.  Specifications for other types of network will be the
 subject of future memos.

3. LEVELS OF CONFORMANCE

 There are three levels of conformance to this specification:
    Level 0: no support for IP multicasting.
 There is, at this time, no requirement that all IP implementations
 support IP multicasting.  Level 0 hosts will, in general, be
 unaffected by multicast activity.  The only exception arises on some
 types of local network, where the presence of level 1 or 2 hosts may
 cause misdelivery of multicast IP datagrams to level 0 hosts.  Such
 datagrams can easily be identified by the presence of a class D IP
 address in their destination address field; they should be quietly
 discarded by hosts that do not support IP multicasting.  Class D
 addresses are described in section 4 of this memo.
    Level 1: support for sending but not receiving multicast IP
    datagrams.
 Level 1 allows a host to partake of some multicast-based services,
 such as resource location or status reporting, but it does not allow
 a host to join any host groups.  An IP implementation may be upgraded
 from level 0 to level 1 very easily and with little new code.  Only
 sections 4, 5, and 6 of this memo are applicable to level 1
 implementations.
    Level 2: full support for IP multicasting.
 Level 2 allows a host to join and leave host groups, as well as send
 IP datagrams to host groups.  It requires implementation of the
 Internet Group Management Protocol (IGMP) and extension of the IP and
 local network service interfaces within the host.  All of the
 following sections of this memo are applicable to level 2
 implementations.

Deering [Page 2] RFC 1112 Host Extensions for IP Multicasting August 1989

4. HOST GROUP ADDRESSES

 Host groups are identified by class D IP addresses, i.e., those with
 "1110" as their high-order four bits.  Class E IP addresses, i.e.,
 those with "1111" as their high-order four bits, are reserved for
 future addressing modes.
 In Internet standard "dotted decimal" notation, host group addresses
 range from 224.0.0.0 to 239.255.255.255.  The address 224.0.0.0 is
 guaranteed not to be assigned to any group, and 224.0.0.1 is assigned
 to the permanent group of all IP hosts (including gateways).  This is
 used to address all multicast hosts on the directly connected
 network.  There is no multicast address (or any other IP address) for
 all hosts on the total Internet.  The addresses of other well-known,
 permanent groups are to be published in "Assigned Numbers".
 Appendix II contains some background discussion of several issues
 related to host group addresses.

Deering [Page 3] RFC 1112 Host Extensions for IP Multicasting August 1989

5. MODEL OF A HOST IP IMPLEMENTATION

 The multicast extensions to a host IP implementation are specified in
 terms of the layered model illustrated below.  In this model, ICMP
 and (for level 2 hosts) IGMP are considered to be implemented within
 the IP module, and the mapping of IP addresses to local network
 addresses is considered to be the responsibility of local network
 modules.  This model is for expository purposes only, and should not
 be construed as constraining an actual implementation.
       |                                                          |
       |              Upper-Layer Protocol Modules                |
       |__________________________________________________________|
  1. ——————– IP Service Interface ———————–

| | | | | | ICMP | IGMP | | IP ||| | Module | | | ||

  1. ————— Local Network Service Interface —————–

| | | | Local | IP-to-local address mapping | | Network | (e.g., ARP) | | Modules |_| | (e.g., Ethernet) | | | To provide level 1 multicasting, a host IP implementation must support the transmission of multicast IP datagrams. To provide level 2 multicasting, a host must also support the reception of multicast IP datagrams. Each of these two new services is described in a separate section, below. For each service, extensions are specified for the IP service interface, the IP module, the local network service interface, and an Ethernet local network module. Extensions to local network modules other than Ethernet are mentioned briefly, but are not specified in detail. Deering [Page 4] RFC 1112 Host Extensions for IP Multicasting August 1989 6. SENDING MULTICAST IP DATAGRAMS 6.1. Extensions to the IP Service Interface Multicast IP datagrams are sent using the same "Send IP" operation used to send unicast IP datagrams; an upper-layer protocol module merely specifies an IP host group address, rather than an individual IP address, as the destination. However, a number of extensions may be necessary or desirable. First, the service interface should provide a way for the upper-layer protocol to specify the IP time-to-live of an outgoing multicast datagram, if such a capability does not already exist. If the upper-layer protocol chooses not to specify a time-to-live, it should default to 1 for all multicast IP datagrams, so that an explicit choice is required to multicast beyond a single network. Second, for hosts that may be attached to more than one network, the service interface should provide a way for the upper-layer protocol to identify which network interface is be used for the multicast transmission. Only one interface is used for the initial transmission; multicast routers are responsible for forwarding to any other networks, if necessary. If the upper-layer protocol chooses not to identify an outgoing interface, a default interface should be used, preferably under the control of system management. Third (level 2 implementations only), for the case in which the host is itself a member of a group to which a datagram is being sent, the service interface should provide a way for the upper-layer protocol to inhibit local delivery of the datagram; by default, a copy of the datagram is looped back. This is a performance optimization for upper-layer protocols that restrict the membership of a group to one process per host (such as a routing protocol), or that handle loopback of group communication at a higher layer (such as a multicast transport protocol). 6.2. Extensions to the IP Module To support the sending of multicast IP datagrams, the IP module must be extended to recognize IP host group addresses when routing outgoing datagrams. Most IP implementations include the following logic: if IP-destination is on the same local network, send datagram locally to IP-destination else send datagram locally to GatewayTo( IP-destination ) Deering [Page 5] RFC 1112 Host Extensions for IP Multicasting August 1989 To allow multicast transmissions, the routing logic must be changed to: if IP-destination is on the same local network or IP-destination is a host group, send datagram locally to IP-destination else send datagram locally to GatewayTo( IP-destination ) If the sending host is itself a member of the destination group on the outgoing interface, a copy of the outgoing datagram must be looped-back for local delivery, unless inhibited by the sender. (Level 2 implementations only.) The IP source address of the outgoing datagram must be one of the individual addresses corresponding to the outgoing interface. A host group address must never be placed in the source address field or anywhere in a source route or record route option of an outgoing IP datagram. 6.3. Extensions to the Local Network Service Interface No change to the local network service interface is required to support the sending of multicast IP datagrams. The IP module merely specifies an IP host group destination, rather than an individual IP destination, when it invokes the existing "Send Local" operation. 6.4. Extensions to an Ethernet Local Network Module The Ethernet directly supports the sending of local multicast packets by allowing multicast addresses in the destination field of Ethernet packets. All that is needed to support the sending of multicast IP datagrams is a procedure for mapping IP host group addresses to Ethernet multicast addresses. An IP host group address is mapped to an Ethernet multicast address by placing the low-order 23-bits of the IP address into the low-order 23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex). Because there are 28 significant bits in an IP host group address, more than one host group address may map to the same Ethernet multicast address. 6.5. Extensions to Local Network Modules other than Ethernet Other networks that directly support multicasting, such as rings or buses conforming to the IEEE 802.2 standard, may be handled the same Deering [Page 6] RFC 1112 Host Extensions for IP Multicasting August 1989 way as Ethernet for the purpose of sending multicast IP datagrams. For a network that supports broadcast but not multicast, such as the Experimental Ethernet, all IP host group addresses may be mapped to a single local broadcast address (at the cost of increased overhead on all local hosts). For a point-to-point link joining two hosts (or a host and a multicast router), multicasts should be transmitted exactly like unicasts. For a store-and-forward network like the ARPANET or a public X.25 network, all IP host group addresses might be mapped to the well-known local address of an IP multicast router; a router on such a network would take responsibility for completing multicast delivery within the network as well as among networks. 7. RECEIVING MULTICAST IP DATAGRAMS 7.1. Extensions to the IP Service Interface Incoming multicast IP datagrams are received by upper-layer protocol modules using the same "Receive IP" operation as normal, unicast datagrams. Selection of a destination upper-layer protocol is based on the protocol field in the IP header, regardless of the destination IP address. However, before any datagrams destined to a particular group can be received, an upper-layer protocol must ask the IP module to join that group. Thus, the IP service interface must be extended to provide two new operations: JoinHostGroup ( group-address, interface ) LeaveHostGroup ( group-address, interface ) The JoinHostGroup operation requests that this host become a member of the host group identified by "group-address" on the given network interface. The LeaveGroup operation requests that this host give up its membership in the host group identified by "group-address" on the given network interface. The interface argument may be omitted on hosts that support only one interface. For hosts that may be attached to more than one network, the upper-layer protocol may choose to leave the interface unspecified, in which case the request will apply to the default interface for sending multicast datagrams (see section 6.1). It is permissible to join the same group on more than one interface, in which case duplicate multicast datagrams may be received. It is also permissible for more than one upper-layer protocol to request membership in the same group. Both operations should return immediately (i.e., they are non- blocking operations), indicating success or failure. Either operation may fail due to an invalid group address or interface Deering [Page 7] RFC 1112 Host Extensions for IP Multicasting August 1989 identifier. JoinHostGroup may fail due to lack of local resources. LeaveHostGroup may fail because the host does not belong to the given group on the given interface. LeaveHostGroup may succeed, but the membership persist, if more than one upper-layer protocol has requested membership in the same group. 7.2. Extensions to the IP Module To support the reception of multicast IP datagrams, the IP module must be extended to maintain a list of host group memberships associated with each network interface. An incoming datagram destined to one of those groups is processed exactly the same way as datagrams destined to one of the host's individual addresses. Incoming datagrams destined to groups to which the host does not belong are discarded without generating any error report or log entry. On hosts with more than one network interface, if a datagram arrives via one interface, destined for a group to which the host belongs only on a different interface, the datagram is quietly discarded. (These cases should occur only as a result of inadequate multicast address filtering in a local network module.) An incoming datagram is not rejected for having an IP time-to-live of 1 (i.e., the time-to-live should not automatically be decremented on arriving datagrams that are not being forwarded). An incoming datagram with an IP host group address in its source address field is quietly discarded. An ICMP error message (Destination Unreachable, Time Exceeded, Parameter Problem, Source Quench, or Redirect) is never generated in response to a datagram destined to an IP host group. The list of host group memberships is updated in response to JoinHostGroup and LeaveHostGroup requests from upper-layer protocols. Each membership should have an associated reference count or similar mechanism to handle multiple requests to join and leave the same group. On the first request to join and the last request to leave a group on a given interface, the local network module for that interface is notified, so that it may update its multicast reception filter (see section 7.3). The IP module must also be extended to implement the IGMP protocol, specified in Appendix I. IGMP is used to keep neighboring multicast routers informed of the host group memberships present on a particular local network. To support IGMP, every level 2 host must join the "all-hosts" group (address 224.0.0.1) on each network interface at initialization time and must remain a member for as long as the host is active. Deering [Page 8] RFC 1112 Host Extensions for IP Multicasting August 1989 (Datagrams addressed to the all-hosts group are recognized as a special case by the multicast routers and are never forwarded beyond a single network, regardless of their time-to-live. Thus, the all- hosts address may not be used as an internet-wide broadcast address. For the purpose of IGMP, membership in the all-hosts group is really necessary only while the host belongs to at least one other group. However, it is specified that the host shall remain a member of the all-hosts group at all times because (1) it is simpler, (2) the frequency of reception of unnecessary IGMP queries should be low enough that overhead is negligible, and (3) the all-hosts address may serve other routing-oriented purposes, such as advertising the presence of gateways or resolving local addresses.) 7.3. Extensions to the Local Network Service Interface Incoming local network multicast packets are delivered to the IP module using the same "Receive Local" operation as local network unicast packets. To allow the IP module to tell the local network module which multicast packets to accept, the local network service interface is extended to provide two new operations: JoinLocalGroup ( group-address ) LeaveLocalGroup ( group-address ) where "group-address" is an IP host group address. The JoinLocalGroup operation requests the local network module to accept and deliver up subsequently arriving packets destined to the given IP host group address. The LeaveLocalGroup operation requests the local network module to stop delivering up packets destined to the given IP host group address. The local network module is expected to map the IP host group addresses to local network addresses as required to update its multicast reception filter. Any local network module is free to ignore LeaveLocalGroup requests, and may deliver up packets destined to more addresses than just those specified in JoinLocalGroup requests, if it is unable to filter incoming packets adequately. The local network module must not deliver up any multicast packets that were transmitted from that module; loopback of multicasts is handled at the IP layer or higher. 7.4. Extensions to an Ethernet Local Network Module To support the reception of multicast IP datagrams, an Ethernet module must be able to receive packets addressed to the Ethernet multicast addresses that correspond to the host's IP host group addresses. It is highly desirable to take advantage of any address Deering [Page 9] RFC 1112 Host Extensions for IP Multicasting August 1989 filtering capabilities that the Ethernet hardware interface may have, so that the host receives only those packets that are destined to it. Unfortunately, many current Ethernet interfaces have a small limit on the number of addresses that the hardware can be configured to recognize. Nevertheless, an implementation must be capable of listening on an arbitrary number of Ethernet multicast addresses, which may mean "opening up" the address filter to accept all multicast packets during those periods when the number of addresses exceeds the limit of the filter. For interfaces with inadequate hardware address filtering, it may be desirable (for performance reasons) to perform Ethernet address filtering within the software of the Ethernet module. This is not mandatory, however, because the IP module performs its own filtering based on IP destination addresses. 7.5. Extensions to Local Network Modules other than Ethernet Other multicast networks, such as IEEE 802.2 networks, can be handled the same way as Ethernet for the purpose of receiving multicast IP datagrams. For pure broadcast networks, such as the Experimental Ethernet, all incoming broadcast packets can be accepted and passed to the IP module for IP-level filtering. On point-to-point or store-and-forward networks, multicast IP datagrams will arrive as local network unicasts, so no change to the local network module should be necessary. Deering [Page 10] RFC 1112 Host Extensions for IP Multicasting August 1989 APPENDIX I. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP) The Internet Group Management Protocol (IGMP) is used by IP hosts to report their host group memberships to any immediately-neighboring multicast routers. IGMP is an asymmetric protocol and is specified here from the point of view of a host, rather than a multicast router. (IGMP may also be used, symmetrically or asymmetrically, between multicast routers. Such use is not specified here.) Like ICMP, IGMP is a integral part of IP. It is required to be implemented by all hosts conforming to level 2 of the IP multicasting specification. IGMP messages are encapsulated in IP datagrams, with an IP protocol number of 2. All IGMP messages of concern to hosts have the following format: 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| Type | Unused | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version This memo specifies version 1 of IGMP. Version 0 is specified in RFC-988 and is now obsolete. Type There are two types of IGMP message of concern to hosts: 1 = Host Membership Query 2 = Host Membership Report Unused Unused field, zeroed when sent, ignored when received. Checksum The checksum is the 16-bit one's complement of the one's complement sum of the 8-octet IGMP message. For computing the checksum, the checksum field is zeroed. Group Address In a Host Membership Query message, the group address field Deering [Page 11] RFC 1112 Host Extensions for IP Multicasting August 1989 is zeroed when sent, ignored when received. In a Host Membership Report message, the group address field holds the IP host group address of the group being reported. Informal Protocol Description Multicast routers send Host Membership Query messages (hereinafter called Queries) to discover which host groups have members on their attached local networks. Queries are addressed to the all-hosts group (address 224.0.0.1), and carry an IP time-to-live of 1. Hosts respond to a Query by generating Host Membership Reports (hereinafter called Reports), reporting each host group to which they belong on the network interface from which the Query was received. In order to avoid an "implosion" of concurrent Reports and to reduce the total number of Reports transmitted, two techniques are used: 1. When a host receives a Query, rather than sending Reports immediately, it starts a report delay timer for each of its group memberships on the network interface of the incoming Query. Each timer is set to a different, randomly-chosen value between zero and D seconds. When a timer expires, a Report is generated for the corresponding host group. Thus, Reports are spread out over a D second interval instead of all occurring at once. 2. A Report is sent with an IP destination address equal to the host group address being reported, and with an IP time-to-live of 1, so that other members of the same group on the same network can overhear the Report. If a host hears a Report for a group to which it belongs on that network, the host stops its own timer for that group and does not generate a Report for that group. Thus, in the normal case, only one Report will be generated for each group present on the network, by the member host whose delay timer expires first. Note that the multicast routers receive all IP multicast datagrams, and therefore need not be addressed explicitly. Further note that the routers need not know which hosts belong to a group, only that at least one host belongs to a group on a particular network. There are two exceptions to the behavior described above. First, if a report delay timer is already running for a group membership when a Query is received, that timer is not reset to a new random value, but rather allowed to continue running with its current value. Second, a report delay timer is never set for a host's membership in the all- hosts group (224.0.0.1), and that membership is never reported. Deering [Page 12] RFC 1112 Host Extensions for IP Multicasting August 1989 If a host uses a pseudo-random number generator to compute the reporting delays, one of the host's own individual IP address should be used as part of the seed for the generator, to reduce the chance of multiple hosts generating the same sequence of delays. A host should confirm that a received Report has the same IP host group address in its IP destination field and its IGMP group address field, to ensure that the host's own Report is not cancelled by an erroneous received Report. A host should quietly discard any IGMP message of type other than Host Membership Query or Host Membership Report. Multicast routers send Queries periodically to refresh their knowledge of memberships present on a particular network. If no Reports are received for a particular group after some number of Queries, the routers assume that that group has no local members and that they need not forward remotely-originated multicasts for that group onto the local network. Queries are normally sent infrequently (no more than once a minute) so as to keep the IGMP overhead on hosts and networks very low. However, when a multicast router starts up, it may issue several closely-spaced Queries in order to build up its knowledge of local memberships quickly. When a host joins a new group, it should immediately transmit a Report for that group, rather than waiting for a Query, in case it is the first member of that group on the network. To cover the possibility of the initial Report being lost or damaged, it is recommended that it be repeated once or twice after short delays. (A simple way to accomplish this is to act as if a Query had been received for that group only, setting the group's random report delay timer. The state transition diagram below illustrates this approach.) Note that, on a network with no multicast routers present, the only IGMP traffic is the one or more Reports sent whenever a host joins a new group. State Transition Diagram IGMP behavior is more formally specified by the state transition diagram below. A host may be in one of three possible states, with respect to any single IP host group on any single network interface: - Non-Member state, when the host does not belong to the group on the interface. This is the initial state for all memberships on all network interfaces; it requires no storage in the host. Deering [Page 13] RFC 1112 Host Extensions for IP Multicasting August 1989 - Delaying Member state, when the host belongs to the group on the interface and has a report delay timer running for that membership. - Idle Member state, when the host belongs to the group on the interface and does not have a report delay timer running for that membership. There are five significant events that can cause IGMP state transitions: - "join group" occurs when the host decides to join the group on the interface. It may occur only in the Non-Member state. - "leave group" occurs when the host decides to leave the group on the interface. It may occur only in the Delaying Member and Idle Member states. - "query received" occurs when the host receives a valid IGMP Host Membership Query message. To be valid, the Query message must be at least 8 octets long, have a correct IGMP checksum and have an IP destination address of 224.0.0.1. A single Query applies to all memberships on the interface from which the Query is received. It is ignored for memberships in the Non-Member or Delaying Member state. - "report received" occurs when the host receives a valid IGMP Host Membership Report message. To be valid, the Report message must be at least 8 octets long, have a correct IGMP checksum, and contain the same IP host group address in its IP destination field and its IGMP group address field. A Report applies only to the membership in the group identified by the Report, on the interface from which the Report is received. It is ignored for memberships in the Non-Member or Idle Member state. - "timer expired" occurs when the report delay timer for the group on the interface expires. It may occur only in the Delaying Member state. All other events, such as receiving invalid IGMP messages, or IGMP messages other than Query or Report, are ignored in all states. There are three possible actions that may be taken in response to the above events: - "send report" for the group on the interface. Deering [Page 14] RFC 1112 Host Extensions for IP Multicasting August 1989 - "start timer" for the group on the interface, using a random delay value between 0 and D seconds. - "stop timer" for the group on the interface. In the following diagram, each state transition arc is labelled with the event that causes the transition, and, in parentheses, any actions taken during the transition. | | | | | | | | ———>| Non-Member |←——– | | | | | | | | | | | | | || | | | | | leave group | join group | leave group | (stop timer) |(send report, | | | start timer) | | | | | |←——– | | | | | | | |←——————| | | | query received | | | Delaying Member | (start timer) | Idle Member | | |——————→| | | | report received | | | | (stop timer) | | |_|——————→|___|

                              timer expired
                              (send report)
 The all-hosts group (address 224.0.0.1) is handled as a special case.
 The host starts in Idle Member state for that group on every
 interface, never transitions to another state, and never sends a
 report for that group.

Protocol Parameters

 The maximum report delay, D, is 10 seconds.

Deering [Page 15] RFC 1112 Host Extensions for IP Multicasting August 1989

APPENDIX II. HOST GROUP ADDRESS ISSUES

 This appendix is not part of the IP multicasting specification, but
 provides background discussion of several issues related to IP host
 group addresses.

Group Address Binding

 The binding of IP host group addresses to physical hosts may be
 considered a generalization of the binding of IP unicast addresses.
 An IP unicast address is statically bound to a single local network
 interface on a single IP network.  An IP host group address is
 dynamically bound to a set of local network interfaces on a set of IP
 networks.
 It is important to understand that an IP host group address is NOT
 bound to a set of IP unicast addresses.  The multicast routers do not
 need to maintain a list of individual members of each host group.
 For example, a multicast router attached to an Ethernet need
 associate only a single Ethernet multicast address with each host
 group having local members, rather than a list of the members'
 individual IP or Ethernet addresses.

Allocation of Transient Host Group Addresses

 This memo does not specify how transient group address are allocated.
 It is anticipated that different portions of the IP transient host
 group address space will be allocated using different techniques.
 For example, there may be a number of servers that can be contacted
 to acquire a new transient group address.  Some higher-level
 protocols (such as VMTP, specified in RFC-1045) may generate higher-
 level transient "process group" or "entity group" addresses which are
 then algorithmically mapped to a subset of the IP transient host
 group addresses, similarly to the way that IP host group addresses
 are mapped to Ethernet multicast addresses.  A portion of the IP
 group address space may be set aside for random allocation by
 applications that can tolerate occasional collisions with other
 multicast users, perhaps generating new addresses until a suitably
 "quiet" one is found.
 In general, a host cannot assume that datagrams sent to any host
 group address will reach only the intended hosts, or that datagrams
 received as a member of a transient host group are intended for the
 recipient.  Misdelivery must be detected at a level above IP, using
 higher-level identifiers or authentication tokens.  Information
 transmitted to a host group address should be encrypted or governed
 by administrative routing controls if the sender is concerned about
 unwanted listeners.

Deering [Page 16] RFC 1112 Host Extensions for IP Multicasting August 1989

Author's Address

 Steve Deering
 Stanford University
 Computer Science Department
 Stanford, CA 94305-2140
 Phone: (415) 723-9427
 EMail: deering@PESCADERO.STANFORD.EDU

Deering [Page 17]

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