[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
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Internet Protocol
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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
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RFC: 791
Replaces: RFC 760
IENs 128, 123, 111,
80, 54, 44, 41, 28, 26
INTERNET PROTOCOL
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION
1. INTRODUCTION
1.1. Motivation
The Internet Protocol is designed for use in interconnected systems of
packet-switched computer communication networks. Such a system has
been called a "catenet" [1]. The internet protocol provides for
transmitting blocks of data called datagrams from sources to
destinations, where sources and destinations are hosts identified by
fixed length addresses. The internet protocol also provides for
fragmentation and reassembly of long datagrams, if necessary, for
transmission through "small packet" networks.
1.2. Scope
The internet protocol is specifically limited in scope to provide the
functions necessary to deliver a package of bits (an internet
datagram) from a source to a destination over an interconnected system
of networks. There are no mechanisms to augment end-to-end data
reliability, flow control, sequencing, or other services commonly
found in host-to-host protocols. The internet protocol can capitalize
on the services of its supporting networks to provide various types
and qualities of service.
1.3. Interfaces
This protocol is called on by host-to-host protocols in an internet
environment. This protocol calls on local network protocols to carry
the internet datagram to the next gateway or destination host.
For example, a TCP module would call on the internet module to take a
TCP segment (including the TCP header and user data) as the data
portion of an internet datagram. The TCP module would provide the
addresses and other parameters in the internet header to the internet
module as arguments of the call. The internet module would then
create an internet datagram and call on the local network interface to
transmit the internet datagram.
In the ARPANET case, for example, the internet module would call on a
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Internet Protocol
Introduction
local net module which would add the 1822 leader [2] to the internet
datagram creating an ARPANET message to transmit to the IMP. The
ARPANET address would be derived from the internet address by the
local network interface and would be the address of some host in the
ARPANET, that host might be a gateway to other networks.
1.4. Operation
The internet protocol implements two basic functions: addressing and
fragmentation.
The internet modules use the addresses carried in the internet header
to transmit internet datagrams toward their destinations. The
selection of a path for transmission is called routing.
The internet modules use fields in the internet header to fragment and
reassemble internet datagrams when necessary for transmission through
"small packet" networks.
The model of operation is that an internet module resides in each host
engaged in internet communication and in each gateway that
interconnects networks. These modules share common rules for
interpreting address fields and for fragmenting and assembling
internet datagrams. In addition, these modules (especially in
gateways) have procedures for making routing decisions and other
functions.
The internet protocol treats each internet datagram as an independent
entity unrelated to any other internet datagram. There are no
connections or logical circuits (virtual or otherwise).
The internet protocol uses four key mechanisms in providing its
service: Type of Service, Time to Live, Options, and Header Checksum.
The Type of Service is used to indicate the quality of the service
desired. The type of service is an abstract or generalized set of
parameters which characterize the service choices provided in the
networks that make up the internet. This type of service indication
is to be used by gateways to select the actual transmission parameters
for a particular network, the network to be used for the next hop, or
the next gateway when routing an internet datagram.
The Time to Live is an indication of an upper bound on the lifetime of
an internet datagram. It is set by the sender of the datagram and
reduced at the points along the route where it is processed. If the
time to live reaches zero before the internet datagram reaches its
destination, the internet datagram is destroyed. The time to live can
be thought of as a self destruct time limit.
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Introduction
The Options provide for control functions needed or useful in some
situations but unnecessary for the most common communications. The
options include provisions for timestamps, security, and special
routing.
The Header Checksum provides a verification that the information used
in processing internet datagram has been transmitted correctly. The
data may contain errors. If the header checksum fails, the internet
datagram is discarded at once by the entity which detects the error.
The internet protocol does not provide a reliable communication
facility. There are no acknowledgments either end-to-end or
hop-by-hop. There is no error control for data, only a header
checksum. There are no retransmissions. There is no flow control.
Errors detected may be reported via the Internet Control Message
Protocol (ICMP) [3] which is implemented in the internet protocol
module.
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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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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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Internet Protocol
Overview
2.3. Function Description
The function or purpose of Internet Protocol is to move datagrams
through an interconnected set of networks. This is done by passing
the datagrams from one internet module to another until the
destination is reached. The internet modules reside in hosts and
gateways in the internet system. The datagrams are routed from one
internet module to another through individual networks based on the
interpretation of an internet address. Thus, one important mechanism
of the internet protocol is the internet address.
In the routing of messages from one internet module to another,
datagrams may need to traverse a network whose maximum packet size is
smaller than the size of the datagram. To overcome this difficulty, a
fragmentation mechanism is provided in the internet protocol.
Addressing
A distinction is made between names, addresses, and routes [4]. A
name indicates what we seek. An address indicates where it is. A
route indicates how to get there. The internet protocol deals
primarily with addresses. It is the task of higher level (i.e.,
host-to-host or application) protocols to make the mapping from
names to addresses. The internet module maps internet addresses to
local net addresses. It is the task of lower level (i.e., local net
or gateways) procedures to make the mapping from local net addresses
to routes.
Addresses are fixed length of four octets (32 bits). An address
begins with a network number, followed by local address (called the
"rest" field). There are three formats or classes of internet
addresses: in class a, the high order bit is zero, the next 7 bits
are the network, and the last 24 bits are the local address; in
class b, the high order two bits are one-zero, the next 14 bits are
the network and the last 16 bits are the local address; in class c,
the high order three bits are one-one-zero, the next 21 bits are the
network and the last 8 bits are the local address.
Care must be taken in mapping internet addresses to local net
addresses; a single physical host must be able to act as if it were
several distinct hosts to the extent of using several distinct
internet addresses. Some hosts will also have several physical
interfaces (multi-homing).
That is, provision must be made for a host to have several physical
interfaces to the network with each having several logical internet
addresses.
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Internet Protocol
Overview
Examples of address mappings may be found in "Address Mappings" [5].
Fragmentation
Fragmentation of an internet datagram is necessary when it
originates in a local net that allows a large packet size and must
traverse a local net that limits packets to a smaller size to reach
its destination.
An internet datagram can be marked "don't fragment." Any internet
datagram so marked is not to be internet fragmented under any
circumstances. If internet datagram marked don't fragment cannot be
delivered to its destination without fragmenting it, it is to be
discarded instead.
Fragmentation, transmission and reassembly across a local network
which is invisible to the internet protocol module is called
intranet fragmentation and may be used [6].
The internet fragmentation and reassembly procedure needs to be able
to break a datagram into an almost arbitrary number of pieces that
can be later reassembled. The receiver of the fragments uses the
identification field to ensure that fragments of different datagrams
are not mixed. The fragment offset field tells the receiver the
position of a fragment in the original datagram. The fragment
offset and length determine the portion of the original datagram
covered by this fragment. The more-fragments flag indicates (by
being reset) the last fragment. These fields provide sufficient
information to reassemble datagrams.
The identification field is used to distinguish the fragments of one
datagram from those of another. The originating protocol module of
an internet datagram sets the identification field to a value that
must be unique for that source-destination pair and protocol for the
time the datagram will be active in the internet system. The
originating protocol module of a complete datagram sets the
more-fragments flag to zero and the fragment offset to zero.
To fragment a long internet datagram, an internet protocol module
(for example, in a gateway), creates two new internet datagrams and
copies the contents of the internet header fields from the long
datagram into both new internet headers. The data of the long
datagram is divided into two portions on a 8 octet (64 bit) boundary
(the second portion might not be an integral multiple of 8 octets,
but the first must be). Call the number of 8 octet blocks in the
first portion NFB (for Number of Fragment Blocks). The first
portion of the data is placed in the first new internet datagram,
and the total length field is set to the length of the first
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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
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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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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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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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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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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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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No Operation
+--------+
|00000001|
+--------+
Type=1
This option may be used between options, for example, to align
the beginning of a subsequent option on a 32 bit boundary.
May be copied, introduced, or deleted on fragmentation, or for
any other reason.
Security
This option provides a way for hosts to send security,
compartmentation, handling restrictions, and TCC (closed user
group) parameters. The format for this option is as follows:
+--------+--------+---//---+---//---+---//---+---//---+
|10000010|00001011|SSS SSS|CCC CCC|HHH HHH| TCC |
+--------+--------+---//---+---//---+---//---+---//---+
Type=130 Length=11
Security (S field): 16 bits
Specifies one of 16 levels of security (eight of which are
reserved for future use).
00000000 00000000 - Unclassified
11110001 00110101 - Confidential
01111000 10011010 - EFTO
10111100 01001101 - MMMM
01011110 00100110 - PROG
10101111 00010011 - Restricted
11010111 10001000 - Secret
01101011 11000101 - Top Secret
00110101 11100010 - (Reserved for future use)
10011010 11110001 - (Reserved for future use)
01001101 01111000 - (Reserved for future use)
00100100 10111101 - (Reserved for future use)
00010011 01011110 - (Reserved for future use)
10001001 10101111 - (Reserved for future use)
11000100 11010110 - (Reserved for future use)
11100010 01101011 - (Reserved for future use)
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Compartments (C field): 16 bits
An all zero value is used when the information transmitted is
not compartmented. Other values for the compartments field
may be obtained from the Defense Intelligence Agency.
Handling Restrictions (H field): 16 bits
The values for the control and release markings are
alphanumeric digraphs and are defined in the Defense
Intelligence Agency Manual DIAM 65-19, "Standard Security
Markings".
Transmission Control Code (TCC field): 24 bits
Provides a means to segregate traffic and define controlled
communities of interest among subscribers. The TCC values are
trigraphs, and are available from HQ DCA Code 530.
Must be copied on fragmentation. This option appears at most
once in a datagram.
Loose Source and Record Route
+--------+--------+--------+---------//--------+
|10000011| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=131
The loose source and record route (LSRR) option provides a means
for the source of an internet datagram to supply routing
information to be used by the gateways in forwarding the
datagram to the destination, and to record the route
information.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, the pointer octet, and length-3 octets of route
data. The third octet is the pointer into the route data
indicating the octet which begins the next source address to be
processed. The pointer is relative to this option, and the
smallest legal value for the pointer is 4.
A route data is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. If the pointer is
greater than the length, the source route is empty (and the
recorded route full) and the routing is to be based on the
destination address field.
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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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Internet Protocol
Specification
recorded route full) and the routing is to be based on the
destination address field.
If the address in destination address field has been reached and
the pointer is not greater than the length, the next address in
the source route replaces the address in the destination address
field, and the recorded route address replaces the source
address just used, and pointer is increased by four.
The recorded route address is the internet module's own internet
address as known in the environment into which this datagram is
being forwarded.
This procedure of replacing the source route with the recorded
route (though it is in the reverse of the order it must be in to
be used as a source route) means the option (and the IP header
as a whole) remains a constant length as the datagram progresses
through the internet.
This option is a strict source route because the gateway or host
IP must send the datagram directly to the next address in the
source route through only the directly connected network
indicated in the next address to reach the next gateway or host
specified in the route.
Must be copied on fragmentation. Appears at most once in a
datagram.
Record Route
+--------+--------+--------+---------//--------+
|00000111| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=7
The record route option provides a means to record the route of
an internet datagram.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, the pointer octet, and length-3 octets of route
data. The third octet is the pointer into the route data
indicating the octet which begins the next area to store a route
address. The pointer is relative to this option, and the
smallest legal value for the pointer is 4.
A recorded route is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. If the pointer is
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greater than the length, the recorded route data area is full.
The originating host must compose this option with a large
enough route data area to hold all the address expected. The
size of the option does not change due to adding addresses. The
intitial contents of the route data area must be zero.
When an internet module routes a datagram it checks to see if
the record route option is present. If it is, it inserts its
own internet address as known in the environment into which this
datagram is being forwarded into the recorded route begining at
the octet indicated by the pointer, and increments the pointer
by four.
If the route data area is already full (the pointer exceeds the
length) the datagram is forwarded without inserting the address
into the recorded route. If there is some room but not enough
room for a full address to be inserted, the original datagram is
considered to be in error and is discarded. In either case an
ICMP parameter problem message may be sent to the source
host [3].
Not copied on fragmentation, goes in first fragment only.
Appears at most once in a datagram.
Stream Identifier
+--------+--------+--------+--------+
|10001000|00000010| Stream ID |
+--------+--------+--------+--------+
Type=136 Length=4
This option provides a way for the 16-bit SATNET stream
identifier to be carried through networks that do not support
the stream concept.
Must be copied on fragmentation. Appears at most once in a
datagram.
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Internet Timestamp
+--------+--------+--------+--------+
|01000100| length | pointer|oflw|flg|
+--------+--------+--------+--------+
| internet address |
+--------+--------+--------+--------+
| timestamp |
+--------+--------+--------+--------+
| . |
.
.
Type = 68
The Option Length is the number of octets in the option counting
the type, length, pointer, and overflow/flag octets (maximum
length 40).
The Pointer is the number of octets from the beginning of this
option to the end of timestamps plus one (i.e., it points to the
octet beginning the space for next timestamp). The smallest
legal value is 5. The timestamp area is full when the pointer
is greater than the length.
The Overflow (oflw) [4 bits] is the number of IP modules that
cannot register timestamps due to lack of space.
The Flag (flg) [4 bits] values are
0 -- time stamps only, stored in consecutive 32-bit words,
1 -- each timestamp is preceded with internet address of the
registering entity,
3 -- the internet address fields are prespecified. An IP
module only registers its timestamp if it matches its own
address with the next specified internet address.
The Timestamp is a right-justified, 32-bit timestamp in
milliseconds since midnight UT. If the time is not available in
milliseconds or cannot be provided with respect to midnight UT
then any time may be inserted as a timestamp provided the high
order bit of the timestamp field is set to one to indicate the
use of a non-standard value.
The originating host must compose this option with a large
enough timestamp data area to hold all the timestamp information
expected. The size of the option does not change due to adding
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timestamps. The intitial contents of the timestamp data area
must be zero or internet address/zero pairs.
If the timestamp data area is already full (the pointer exceeds
the length) the datagram is forwarded without inserting the
timestamp, but the overflow count is incremented by one.
If there is some room but not enough room for a full timestamp
to be inserted, or the overflow count itself overflows, the
original datagram is considered to be in error and is discarded.
In either case an ICMP parameter problem message may be sent to
the source host [3].
The timestamp option is not copied upon fragmentation. It is
carried in the first fragment. Appears at most once in a
datagram.
Padding: variable
The internet header padding is used to ensure that the internet
header ends on a 32 bit boundary. The padding is zero.
3.2. Discussion
The implementation of a protocol must be robust. Each implementation
must expect to interoperate with others created by different
individuals. While the goal of this specification is to be explicit
about the protocol there is the possibility of differing
interpretations. In general, an implementation must be conservative
in its sending behavior, and liberal in its receiving behavior. That
is, it must be careful to send well-formed datagrams, but must accept
any datagram that it can interpret (e.g., not object to technical
errors where the meaning is still clear).
The basic internet service is datagram oriented and provides for the
fragmentation of datagrams at gateways, with reassembly taking place
at the destination internet protocol module in the destination host.
Of course, fragmentation and reassembly of datagrams within a network
or by private agreement between the gateways of a network is also
allowed since this is transparent to the internet protocols and the
higher-level protocols. This transparent type of fragmentation and
reassembly is termed "network-dependent" (or intranet) fragmentation
and is not discussed further here.
Internet addresses distinguish sources and destinations to the host
level and provide a protocol field as well. It is assumed that each
protocol will provide for whatever multiplexing is necessary within a
host.
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Addressing
To provide for flexibility in assigning address to networks and
allow for the large number of small to intermediate sized networks
the interpretation of the address field is coded to specify a small
number of networks with a large number of host, a moderate number of
networks with a moderate number of hosts, and a large number of
networks with a small number of hosts. In addition there is an
escape code for extended addressing mode.
Address Formats:
High Order Bits Format Class
--------------- ------------------------------- -----
0 7 bits of net, 24 bits of host a
10 14 bits of net, 16 bits of host b
110 21 bits of net, 8 bits of host c
111 escape to extended addressing mode
A value of zero in the network field means this network. This is
only used in certain ICMP messages. The extended addressing mode
is undefined. Both of these features are reserved for future use.
The actual values assigned for network addresses is given in
"Assigned Numbers" [9].
The local address, assigned by the local network, must allow for a
single physical host to act as several distinct internet hosts.
That is, there must be a mapping between internet host addresses and
network/host interfaces that allows several internet addresses to
correspond to one interface. It must also be allowed for a host to
have several physical interfaces and to treat the datagrams from
several of them as if they were all addressed to a single host.
Address mappings between internet addresses and addresses for
ARPANET, SATNET, PRNET, and other networks are described in "Address
Mappings" [5].
Fragmentation and Reassembly.
The internet identification field (ID) is used together with the
source and destination address, and the protocol fields, to identify
datagram fragments for reassembly.
The More Fragments flag bit (MF) is set if the datagram is not the
last fragment. The Fragment Offset field identifies the fragment
location, relative to the beginning of the original unfragmented
datagram. Fragments are counted in units of 8 octets. The
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fragmentation strategy is designed so than an unfragmented datagram
has all zero fragmentation information (MF = 0, fragment offset =
0). If an internet datagram is fragmented, its data portion must be
broken on 8 octet boundaries.
This format allows 2**13 = 8192 fragments of 8 octets each for a
total of 65,536 octets. Note that this is consistent with the the
datagram total length field (of course, the header is counted in the
total length and not in the fragments).
When fragmentation occurs, some options are copied, but others
remain with the first fragment only.
Every internet module must be able to forward a datagram of 68
octets without further fragmentation. This is because an internet
header may be up to 60 octets, and the minimum fragment is 8 octets.
Every internet destination must be able to receive a datagram of 576
octets either in one piece or in fragments to be reassembled.
The fields which may be affected by fragmentation include:
(1) options field
(2) more fragments flag
(3) fragment offset
(4) internet header length field
(5) total length field
(6) header checksum
If the Don't Fragment flag (DF) bit is set, then internet
fragmentation of this datagram is NOT permitted, although it may be
discarded. This can be used to prohibit fragmentation in cases
where the receiving host does not have sufficient resources to
reassemble internet fragments.
One example of use of the Don't Fragment feature is to down line
load a small host. A small host could have a boot strap program
that accepts a datagram stores it in memory and then executes it.
The fragmentation and reassembly procedures are most easily
described by examples. The following procedures are example
implementations.
General notation in the following pseudo programs: "=<" means "less
than or equal", "#" means "not equal", "=" means "equal", "<-" means
"is set to". Also, "x to y" includes x and excludes y; for example,
"4 to 7" would include 4, 5, and 6 (but not 7).
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An Example Fragmentation Procedure
The maximum sized datagram that can be transmitted through the
next network is called the maximum transmission unit (MTU).
If the total length is less than or equal the maximum transmission
unit then submit this datagram to the next step in datagram
processing; otherwise cut the datagram into two fragments, the
first fragment being the maximum size, and the second fragment
being the rest of the datagram. The first fragment is submitted
to the next step in datagram processing, while the second fragment
is submitted to this procedure in case it is still too large.
Notation:
FO - Fragment Offset
IHL - Internet Header Length
DF - Don't Fragment flag
MF - More Fragments flag
TL - Total Length
OFO - Old Fragment Offset
OIHL - Old Internet Header Length
OMF - Old More Fragments flag
OTL - Old Total Length
NFB - Number of Fragment Blocks
MTU - Maximum Transmission Unit
Procedure:
IF TL =< MTU THEN Submit this datagram to the next step
in datagram processing ELSE IF DF = 1 THEN discard the
datagram ELSE
To produce the first fragment:
(1) Copy the original internet header;
(2) OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
(3) NFB <- (MTU-IHL*4)/8;
(4) Attach the first NFB*8 data octets;
(5) Correct the header:
MF <- 1; TL <- (IHL*4)+(NFB*8);
Recompute Checksum;
(6) Submit this fragment to the next step in
datagram processing;
To produce the second fragment:
(7) Selectively copy the internet header (some options
are not copied, see option definitions);
(8) Append the remaining data;
(9) Correct the header:
IHL <- (((OIHL*4)-(length of options not copied))+3)/4;
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TL <- OTL - NFB*8 - (OIHL-IHL)*4);
FO <- OFO + NFB; MF <- OMF; Recompute Checksum;
(10) Submit this fragment to the fragmentation test; DONE.
In the above procedure each fragment (except the last) was made
the maximum allowable size. An alternative might produce less
than the maximum size datagrams. For example, one could implement
a fragmentation procedure that repeatly divided large datagrams in
half until the resulting fragments were less than the maximum
transmission unit size.
An Example Reassembly Procedure
For each datagram the buffer identifier is computed as the
concatenation of the source, destination, protocol, and
identification fields. If this is a whole datagram (that is both
the fragment offset and the more fragments fields are zero), then
any reassembly resources associated with this buffer identifier
are released and the datagram is forwarded to the next step in
datagram processing.
If no other fragment with this buffer identifier is on hand then
reassembly resources are allocated. The reassembly resources
consist of a data buffer, a header buffer, a fragment block bit
table, a total data length field, and a timer. The data from the
fragment is placed in the data buffer according to its fragment
offset and length, and bits are set in the fragment block bit
table corresponding to the fragment blocks received.
If this is the first fragment (that is the fragment offset is
zero) this header is placed in the header buffer. If this is the
last fragment ( that is the more fragments field is zero) the
total data length is computed. If this fragment completes the
datagram (tested by checking the bits set in the fragment block
table), then the datagram is sent to the next step in datagram
processing; otherwise the timer is set to the maximum of the
current timer value and the value of the time to live field from
this fragment; and the reassembly routine gives up control.
If the timer runs out, the all reassembly resources for this
buffer identifier are released. The initial setting of the timer
is a lower bound on the reassembly waiting time. This is because
the waiting time will be increased if the Time to Live in the
arriving fragment is greater than the current timer value but will
not be decreased if it is less. The maximum this timer value
could reach is the maximum time to live (approximately 4.25
minutes). The current recommendation for the initial timer
setting is 15 seconds. This may be changed as experience with
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Specification
this protocol accumulates. Note that the choice of this parameter
value is related to the buffer capacity available and the data
rate of the transmission medium; that is, data rate times timer
value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).
Notation:
FO - Fragment Offset
IHL - Internet Header Length
MF - More Fragments flag
TTL - Time To Live
NFB - Number of Fragment Blocks
TL - Total Length
TDL - Total Data Length
BUFID - Buffer Identifier
RCVBT - Fragment Received Bit Table
TLB - Timer Lower Bound
Procedure:
(1) BUFID <- source|destination|protocol|identification;
(2) IF FO = 0 AND MF = 0
(3) THEN IF buffer with BUFID is allocated
(4) THEN flush all reassembly for this BUFID;
(5) Submit datagram to next step; DONE.
(6) ELSE IF no buffer with BUFID is allocated
(7) THEN allocate reassembly resources
with BUFID;
TIMER <- TLB; TDL <- 0;
(8) put data from fragment into data buffer with
BUFID from octet FO*8 to
octet (TL-(IHL*4))+FO*8;
(9) set RCVBT bits from FO
to FO+((TL-(IHL*4)+7)/8);
(10) IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
(11) IF FO = 0 THEN put header in header buffer
(12) IF TDL # 0
(13) AND all RCVBT bits from 0
to (TDL+7)/8 are set
(14) THEN TL <- TDL+(IHL*4)
(15) Submit datagram to next step;
(16) free all reassembly resources
for this BUFID; DONE.
(17) TIMER <- MAX(TIMER,TTL);
(18) give up until next fragment or timer expires;
(19) timer expires: flush all reassembly with this BUFID; DONE.
In the case that two or more fragments contain the same data
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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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Internet Protocol
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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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Internet Protocol
Specification
program (or even a gateway program) will be called the "user" since it
is using the internet module. Since internet protocol is a datagram
protocol, there is minimal memory or state maintained between datagram
transmissions, and each call on the internet protocol module by the
user supplies all information necessary for the IP to perform the
service requested.
An Example Upper Level Interface
The following two example calls satisfy the requirements for the user
to internet protocol module communication ("=>" means returns):
SEND (src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt => result)
where:
src = source address
dst = destination address
prot = protocol
TOS = type of service
TTL = time to live
BufPTR = buffer pointer
len = length of buffer
Id = Identifier
DF = Don't Fragment
opt = option data
result = response
OK = datagram sent ok
Error = error in arguments or local network error
Note that the precedence is included in the TOS and the
security/compartment is passed as an option.
RECV (BufPTR, prot, => result, src, dst, TOS, len, opt)
where:
BufPTR = buffer pointer
prot = protocol
result = response
OK = datagram received ok
Error = error in arguments
len = length of buffer
src = source address
dst = destination address
TOS = type of service
opt = option data
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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.
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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).
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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.
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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.
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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:
- 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.
- 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.
- 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.
- 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
Mogul & Postel [Page 7]
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
Mogul & Postel [Page 8]
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).
Mogul & Postel [Page 9]
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
Mogul & Postel [Page 10]
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.
Mogul & Postel [Page 11]
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:
Mogul & Postel [Page 12]
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
Mogul & Postel [Page 13]
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
Mogul & Postel [Page 14]
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:
Mogul & Postel [Page 15]
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).
Mogul & Postel [Page 16]
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
Mogul & Postel [Page 17]
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.
Mogul [Page 2]
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:
- 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).
- 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.
Mogul [Page 3]
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.
- 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].
- 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.
Mogul [Page 4]
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.
- 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.
- 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.)
- 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.
- 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.
Mogul [Page 5]
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:
- broadcast to all of its immediate neighbors by using
255.255.255.255
- 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:
- 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).
- 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.
- 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].
- 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.
- Broadcast to the entire Internet: This is probably not useful,
and almost certainly not desirable.
Mogul [Page 4]
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:
- No modification of the IP datagram format.
- Reasonable efficiency in terms of the number of excess copies
generated and the cost of paths chosen.
- Minimization of gateway modification, in both code and data
space.
- 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.
- 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.
- 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.)
- 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.
- 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.
Mogul [Page 7]
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.
Mogul [Page 8]
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:
- broadcast to all of its immediate neighbors by using
255.255.255.255
- 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
0
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
0
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.
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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]
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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
0
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
0
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
0
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.
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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 |
|__________________________________________________________|
--------------------- IP Service Interface -----------------------
__________________________________________________________
| | | |
| | ICMP | IGMP |
| IP |______________|______________|
| Module |
| |
|__________________________________________________________|
---------------- 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.
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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.
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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.
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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.
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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:
[email protected]
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