💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc888.txt captured on 2023-01-29 at 07:24:56.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
RFC 888
"STUB" EXTERIOR GATEWAY PROTOCOL
Linda J. Seamonson
Eric C. Rosen
BBN Communications
January 1984
This note describes the Exterior Gateway Protocol used to connect Stub
Gateways to an Autonomous System of core Gateways. This document specifies
the working protocol, and defines an ARPA official protocol. All
implementers of Gateways should carefully review this document.
RFC 888 JANUARY 1984
Table of Contents
1 INTRODUCTION.......................................... 1
2 DEFINITIONS AND OVERVIEW.............................. 4
3 NEIGHBOR ACQUISITION.................................. 7
4 NEIGHBOR REACHABILITY PROTOCOL....................... 10
5 NETWORK REACHABILITY (NR) MESSAGE.................... 15
6 POLLING FOR NR MESSAGES.............................. 22
7 SENDING NR MESSAGES.................................. 24
8 INDIRECT NEIGHBORS................................... 26
9 LIMITATIONS.......................................... 27
A APPENDIX A - EGP MESSAGE FORMATS..................... 28
A.1 NEIGHBOR ACQUISITION MESSAGE....................... 28
A.2 NEIGHBOR HELLO/I HEARD YOU MESSAGE................. 30
A.3 NR POLL MESSAGE.................................... 32
A.4 NETWORK REACHABILITY MESSAGE....................... 34
A.5 EGP ERROR MESSAGE.................................. 37
- i -
RFC 888 JANUARY 1984
1 INTRODUCTION
The DARPA Catenet is expected to be a continuously expanding
system, with more and more hosts on more and more networks
participating in it. Of course, this will require more and more
gateways. In the past, such expansion has taken place in a
relatively unstructured manner. New gateways, often containing
radically different software than the existing gateways, would be
added and would immediately begin participating in the common
routing algorithm via the GGP protocol. However, as the internet
grows larger and larger, this simple method of expansion becomes
less and less feasible. There are a number of reasons for this:
- the overhead of the routing algorithm becomes excessively
large;
- the proliferation of radically different gateways
participating in a single common routing algorithm makes
maintenance and fault isolation nearly impossible, since
it becomes impossible to regard the internet as an
integrated communications system;
- the gateway software and algorithms, especially the
routing algorithm, become too rigid and inflexible, since
- 1 -
RFC 888 JANUARY 1984
any proposed change must be made in too many different
places and by too many different people.
In the future, the internet is expected to evolve into a set
of separate sections or "autonomous systems", each of which
consists of a set of one or more relatively homogeneous gateways.
The protocols, and in particular the routing algorithm which
these gateways use among themselves, will be a private matter,
and need never be implemented in gateways outside the particular
sections or system.
In the simplest case, an autonomous system might consist of
just a single gateway connecting, for example, a local network to
the ARPANET. Such a gateway might be called a "stub gateway",
since its only purpose is to interface the local network to the
rest of the internet, and it is not intended to be used for
handling any traffic which neither originated in nor is destined
for that particular local network. In the near-term future, we
will begin to think of the internet as a set of autonomous
systems, one of which consists of the DARPA gateways on ARPANET
and SATNET, and the others of which are stub gateways to local
networks. The former system, which we shall call the "core"
- 2 -
RFC 888 JANUARY 1984
system, will be used as a transport or "long-haul" system by the
latter systems.
Ultimately, the internet may consist of a number of co-equal
autonomous systems, any of which may be used as a transport
medium for traffic originating in any system and destined for any
system. This more general case is still the subject of research.
This paper describes only how stub gateways connect to the core
system using the Exterior Gateway Protocol (EGP).
- 3 -
RFC 888 JANUARY 1984
2 DEFINITIONS AND OVERVIEW
For the purposes of this paper, a "stub gateway" is defined
as follows:
- it is not a core gateway
- it shares a network with at least one core gateway (has an
interface on the same network as some core gateway)
- it has interfaces to one or more networks which have no
core gateways
- all other nets which are reachable from the core system
via the stub have no other path to the core system except
via the stub
The stub gateway is expected to fully execute the Internet
Control Message Protocol (ICMP), as well as the EGP protocol. In
particular, it must respond to ICMP echo requests, and must send
ICMP destination dead messages as appropriate. It is also
required to send ICMP Redirect messages as appropriate.
Autonomous systems will be assigned 16-bit identification
numbers (in much the same ways as network and protocol numbers
are now assigned), and every EGP message header contains a field
- 4 -
RFC 888 JANUARY 1984
for this number. Zero will not be assigned to any autonomous
system; the use of zero as an autonomous system number is
reserved for future use.
We call two gateways "neighbors" if there is a network to
which each has an interface. If two neighbors are part of the
same autonomous system, we call them INTERIOR NEIGHBORS; for
example, any two core gateways on the same network are interior
neighbors of each other. If two neighbors are not part of the
same autonomous system, we call them EXTERIOR NEIGHBORS; for
example, a stub gateway and any core gateway that share a network
are exterior neighbors of each other. In order for one system to
use another as a transport medium, gateways which are exterior
neighbors of each other must be able to find out which networks
can be reached through the other. The Exterior Gateway Protocol
enables this information to be passed between exterior neighbors.
Since it is a polling protocol, it also enables each gateway to
control the rate at which it sends and receives network
reachability information, allowing each system to control its own
overhead. It also enables each system to have an independent
routing algorithm whose operation cannot be disrupted by failures
of other systems.
- 5 -
RFC 888 JANUARY 1984
The Exterior Gateway Protocol has three parts: (a) Neighbor
Acquisition Protocol, (b) Neighbor Reachability Protocol, and (c)
Network Reachability determination. Note that all messages
defined by EGP are intended to travel only a single "hop". That
is, they originate at one gateway and are sent to a neighboring
gateway without the mediation of any intervening gateway.
Therefore, the time-to-live field should be set to a very small
value. Gateways which encounter EGP messages in their message
streams which are not addressed to them may discard them.
Each EGP message contains a sequence number. The gateway
should maintain one sequence number per neighbor.
- 6 -
RFC 888 JANUARY 1984
3 NEIGHBOR ACQUISITION
Before it is possible to obtain routing information from an
exterior gateway, it is necessary to acquire that gateway as a
direct neighbor. (The distinction between direct and indirect
neighbors will be made in a later section.) In order for two
gateways to become direct neighbors, they must be neighbors, in
the sense defined above, and they must execute the NEIGHBOR
ACQUISITION PROTOCOL, which is simply a standard two-way
handshake.
A gateway that wishes to initiate neighbor acquisition with
another sends it a Neighbor Acquisition Request. This message
should be repeatedly transmitted (at a reasonable rate, perhaps
once every 30 seconds or so) until a Neighbor Acquisition Reply
or a Neighbor Acquisition Refusal is received. The Request will
contain an identification number which is copied into the reply
so that request and reply can be matched up.
A gateway receiving a Neighbor Acquisition Request must
determine whether it wishes to become a direct neighbor of the
source of the Request. If not, it may, at its option, respond
with a Neighbor Acquisition Refusal message, optionally
specifying the reason for refusal. Otherwise, it should send a
- 7 -
RFC 888 JANUARY 1984
Neighbor Acquisition Reply message.
The gateway that sent the Request should consider the
Neighbor Acquisition complete when it has received the neighbor's
Reply. The gateway that sent the Reply should consider the
acquisition complete when it has sent the Reply.
Unmatched Replies or Refusals should be discarded after a
reasonable period of time. However, information about any such
unmatched messages may be useful for diagnostic purposes.
A Neighbor Acquisition Request from a gateway which is
already a direct neighbor should be responded to with a Reply.
A Neighbor Acquisition Request or Reply from gateway G to
gateway G' carries the minimum interval in seconds with which G
is willing to answer Neighbor Reachability Hello Messages from G'
and the minimum interval in seconds with which G is willing to be
polled for NR messages (see below).
If a gateway wishes to cease being a neighbor of a
particular exterior gateway, it sends a Neighbor Cease message.
A gateway receiving a Neighbor Cease message should always
respond with a Neighbor Cease Acknowledgment. It should cease to
treat the sender of the message as a neighbor in any way. Since
- 8 -
RFC 888 JANUARY 1984
there is a significant amount of protocol run between direct
neighbors (see below), if some gateway no longer needs to be a
direct neighbor of some other, it is "polite" to indicate this
fact with a Neighbor Cease Message. The Neighbor Cease Message
should be retransmitted (up to some number of times) until an
acknowledgment for it is received.
Once a Neighbor Cease message has been received, the
Neighbor Reachability Protocol (below) should cease to be
executed.
A stub should have tables configured in with the addresses
of a small number of the core gateways (no more than two or
three) with which it has a common network. It will be the
responsibility of the stub to initiate neighbor acquisition with
these gateways. If the direct neighbors of a stub should all
fail, it will be the responsibility of the stub to acquire at
least one new direct neighbor. It can do so by choosing one of
the core gateways which it has had as an indirect neighbor (see
below), and executing the neighbor acquisition protocol with it.
(It is possible that no more than one core gateway will ever
agree to become a direct neighbor with any given stub gateway at
any one time.)
- 9 -
RFC 888 JANUARY 1984
4 NEIGHBOR REACHABILITY PROTOCOL
It is important for a gateway to keep real-time information
as to the reachability of its neighbors. If a gateway concludes
that a particular neighbor cannot be reached, it should cease
forwarding traffic to that gateway. To make that determination,
a NEIGHBOR REACHABILITY protocol is needed. The EGP protocol
provides two messages types for this purpose -- a "Hello" message
and an "I Heard You" message.
When a "Hello" message is received from a direct neighbor,
an "I Heard You" must be returned to that neighbor "immediately".
The delay between receiving a "Hello" and returning an "I Heard
You" should never be more than a few seconds.
Core gateways will use the following algorithm for
determining reachablility of an exterior neighbor:
A reachable neighbor shall be declared unreachable if,
during the time in which the core gateway sent its last n
"Hello"s, it received fewer than k "I Heard You"s in return. An
unreachable neighbor shall be declared reachable if, during the
time in which the core gateway sent its last m "Hello"s, it
received at least j "I Heard You"s in return.
- 10 -
RFC 888 JANUARY 1984
Stub gateways may also send "Hello"s to their direct
neighbors and receive "I Heard You"s in return. The algorithm
for determining reachability may be similar to the algorithm
described above. However, it is not necessary for stubs to send
"Hello"s. The "Hello" and "I Heard You" messages have a status
field which the sending gateway uses to indicate whether it
thinks the receiving gateway is reachable or not. This
information can be useful for diagnostic purposes. It also
allows a stub gateway to make its reachability determination
parasitic on its core neighbor: only the core gateway actually
needs to send "Hello" messages, and the stub can declare it up or
down based on the status field in the "Hello". That is, the stub
gateway (which sends only "I Heard You"s) declares the core
gateway (which sends only "Hello"s) to be reachable when the
"Hello"s from the core indicate that it has declared the stub to
be reachable.
The frequency with which the "Hello"s are sent, and the
values of the parameters k, n, j, and m cannot be specified here.
For best results, this will depend on the characteristics of the
neighbor and of the network which the neighbors have in common.
THIS IMPLIES THAT THE PROPER PARAMETERS MAY NEED TO BE DETERMINED
JOINTLY BY THE DESIGNERS AND IMPLEMENTERS OF THE TWO NEIGHBORING
- 11 -
RFC 888 JANUARY 1984
GATEWAYS; choosing algorithms and parameters in isolation,
without considering the characteristics of the neighbor and the
connecting network, would not be expected to result in optimum
reachability determinations.
However, the Neighbor Acquisition Request and Reply messages
provide neighbors with a way to inform each other of the minimum
frequency at which they are willing to answer Hellos. When
gateway G sends a Neighbor Acquisition Request to gateway G', it
states that it does not wish to answer Hellos from G' more
frequently than once every X seconds. G' in its Neighbor
Acquisition Reply states that it does not wish to answer Hellos
from G more frequently than once every Y seconds. The two
frequencies do not have to be the same, but each neighbor must
conform to the interval requested by the other. A gateway may
send Hellos less frequently than requested, but not more.
A direct neighbor gateway should also be declared
unreachable if the network connecting it supplies lower level
protocol information from which this can be deduced. Thus, for
example, if a gateway receives an 1822 Destination Dead message
from the ARPANET which indicates that a direct neighbor is dead,
it should declare that neighbor unreachable. The neighbor should
- 12 -
RFC 888 JANUARY 1984
not be declared reachable again until the requisite number of
Hello/I-Heard-You packets have been exchanged.
A direct neighbor which has become unreachable does not
thereby cease to be a direct neighbor. The neighbor can be
declared reachable again without any need to go through the
neighbor acquisition protocol again. However, if the neighbor
remains unreachable for an extremely long period of time, such as
an hour, the gateway should cease to treat it as a neighbor,
i.e., should cease sending Hello messages to it. The neighbor
acquisition protocol would then need to be repeated before it
could become a direct neighbor again.
"Hello" messages from sources other than direct neighbors
should simply be ignored. However, logging the presence of any
such messages might provide useful diagnostic information.
A gateway which is going down, or whose interface to the
network which connects it to a particular neighbor is going down,
should send a Neighbor Cease message to all direct neighbors
which will no longer be able to reach it. The Cease message
should use the info field to specify the reason as "going down".
It should retransmit that message (up to some number of times)
until it receives a Neighbor Cease Acknowledgment. This provides
- 13 -
RFC 888 JANUARY 1984
the neighbors with an advance warning of an outage, and enables
them to prepare for it in a way which will minimize disruption to
existing traffic.
- 14 -
RFC 888 JANUARY 1984
5 NETWORK REACHABILITY (NR) MESSAGE
Terminology: Let gateway G have an interface to network N.
We say that G is AN APPROPRIATE FIRST HOP to network M relative
to network N (where M and N are distinct networks) if and only if
the following condition holds:
Traffic which is destined for network M, and which arrives
at gateway G over its network N interface, will be forwarded
to M by G over a path which does not include any other
gateway with an interface to network N.
In short, G is an appropriate first hop for network M
relative to network N just in case there is no better gateway on
network N through which to route traffic which is destined for
network M. For optimal routing, traffic in network N which is
destined for network M ought always to be forwarded to a gateway
which is an appropriate first hop.
In order for exterior neighbors G and G' (which are
neighbors over network N) to be able to use each other as packet
switches for forwarding traffic to remote networks, each needs to
know the list of networks for which the other is an appropriate
first hop. The Exterior Gateway Protocol defines a message,
- 15 -
RFC 888 JANUARY 1984
called the Network Reachability Message (or NR message), for
transferring this information.
Let G be a gateway on network N. Then the NR message which
G sends about network N must contain the following information:
A list of all the networks for which G is an appropriate
first hop relative to network N.
If G' can obtain this information from exterior neighbor G, then
it knows that no traffic destined for networks which are NOT in
that list should be forwarded to G. (It cannot simply conclude,
however, that all traffic for any networks in that list ought to
be forwarded via G, since G' may also have other neighbors which
are also appropriate first hops to network N. For example, G and
G'' might each be neighbors of G', but might be "equidistant"
from some network M. Then each could be an appropriate first
hop.)
For each network in the list, the NR message also specifies
the "distance" (according to some metric whose definition is left
to the designers of the autonomous system of which gateway G is a
member) from G to that network. Core gateways will report
distances less than 128 for networks that can be reached without
- 16 -
RFC 888 JANUARY 1984
leaving the core system, and greater than or equal to 128
otherwise. A stub gateway should report distances less than 128
for all networks listed in its NR messages.
The maximum value of distance (255.) shall be taken to mean
that the network is UNREACHABLE. ALL OTHER VALUES WILL BE TAKEN
TO MEAN THAT THE NETWORK IS REACHABLE.
If an NR message from some gateway G fails to mention some
network N which was mentioned in the previous NR message from G,
it is possible that N has become unreachable from G. If several
successive NR messages from G omit mention of N, it should be
taken to mean that N is no longer reachable from G. This
procedure is necessary to ensure that networks which can no
longer be reached, but which are never explicitly declared
unreachable, are timed out and removed from the list of reachable
networks.
It will often be the case that where a core gateway G and a
stub gateway G' are direct neighbors on network N, G knows of
many more gateway neighbors on network N, and knows for which
networks those gateway neighbors are the appropriate first hop.
Since the stub G' may not know about all these other neighbors,
it is convenient and often more efficient for it to be able to
- 17 -
RFC 888 JANUARY 1984
obtain this information from G. Therefore, the EGP NR message
also contains fields which allow the core gateway G to specify
the following information:
a) A list of all neighbors (both interior and exterior) of G
(on network N) which G has reliably determined to be
reachable. G may also include indirect neighbors in this
list (see below.)
b) For each of those neighbors, the list of networks for
which that neighbor is an appropriate first hop (relative
to network N).
c) For each such <neighbor, network> pair, the "distance"
from that neighbor to that network.
Thus the NR message provides a means of allowing a gateway
to "discover" new neighbors by seeing whether a neighbor that it
already knows of has any additional neighbors on the same
network. This information also makes possible the implementation
of the INDIRECT NEIGHBOR strategy defined below.
A more precise description of the NR message is the
following.
- 18 -
RFC 888 JANUARY 1984
The data portion of the message will consist largely of
blocks of data. Each block will be headed by a gateway address,
which will be the address either of the gateway sending the
message or of one of that gateway's neighbors. Each gateway
address will be followed by a list of the networks for which that
gateway is an appropriate first hop. All networks at the same
distance from the gateway will be grouped together in this list,
preceded by the distance itself and the number of networks at
that distance. The whole list is preceded by a count of the
distance-groups in the list.
Preceding the list of data blocks is:
a) The count (one byte) of the number of interior neighbors
of G for which this message contains data blocks. By
convention, this count will include the data block for G
itself, which should be the first one to appear.
b) The count (one byte) of the number of exterior neighbors
of G for which this message contains data blocks.
c) The address of the network which this message is about.
If G and G' are neighbors on network N, then in the NR
message going from G to G', this is the address of
- 19 -
RFC 888 JANUARY 1984
network N. For convenience, four bytes have been
allocated for this address -- the trailing one, two, or
three bytes should be zero.
Then follow the data blocks themselves, first the block for
G itself, then the blocks for all the interior neighbors of G (if
any), then the blocks for the exterior neighbors. Since all
gateways mentioned are on the same network, whose address has
already been given, the gateway addresses are given with the
network address part (one, two, or three bytes) omitted, to save
space.
In the list of networks, each network address is either one,
two, or three bytes, depending on whether it is a class A, class
B, or class C network. No trailing bytes are used.
The NR message sent by a stub should be the simplest
allowable. That is, it should have only a single data block,
headed by its own address (on the network it has in common with
the neighboring core gateway), listing just the networks to which
it is an appropriate first hop. These will be just the networks
that can be reached no other way, in general.
- 20 -
RFC 888 JANUARY 1984
The core gateways will send complete NR messages, containing
information about all other gateways on the common network, both
core gateways (which shall be listed as interior neighbors) and
other gateways (which shall be listed as exterior neighbors, and
may include the stub itself). This information will enable the
stub to become an indirect neighbor (see below) of all these
other gateways. That is, the stub shall forward traffic directly
to these other gateways as appropriate, but shall not become
direct neighbors with them.
The stub should NEVER forward to any (directly or
indirectly) neighboring core gateway any traffic for which that
gateway is not an appropriate first hop, as indicated in an NR
message. Of course, this does not apply to datagrams which are
using the source route option; any such datagrams should always
be forwarded as indicated in the source route option field, even
if that requires forwarding to a gateway which is not an
appropriate first hop.
- 21 -
RFC 888 JANUARY 1984
6 POLLING FOR NR MESSAGES
No gateway is required to send NR messages to any other
gateway, except as a response to an NR Poll from a direct
neighbor. However, a gateway is required to respond to an NR
Poll from a direct neighbor within several seconds (subject to
the qualification two paragraphs hence), even if the gateway
believes that neighbor to be down.
The EGP NR Poll message is defined for this purpose. No
gateway may poll another for an NR message more often than once
per minute. A gateway receiving more than one poll per minute
may simply ignore the excess polls, or may return an error
message.
The minimum interval which gateway G will accept as the
polling interval from gateway G' and the minimum interval which
G' will accept as the polling interval from G are specified at
the time that G and G' become direct neighbors. Both the
Neighbor Acquisition Request and the Neighbor Acquisition Reply
allow the sender to specify, in seconds, its desired minimum
polling interval. If G specifies to G' that its minimum polling
interval is X, G' should not poll G more frequently than once
every X seconds. G will not guarantee to answer more frequent
- 22 -
RFC 888 JANUARY 1984
polls.
Polls must only be sent to direct neighbors which are
declared reachable by the neighbor reachability protocol.
An NR Poll message contains a sequence number chosen by the
polling gateway. The polled gateway will return this number in
the NR message it sends in response to the poll, to enable the
polling gateway to match up received NR messages with polls.
In general, a poll should be retransmitted some number of
times (with a reasonable interval between retransmissions) until
an NR message is received. IF NO NR MESSAGE IS RECEIVED AFTER
THE MAXIMUM NUMBER OF RETRANSMISSIONS, THE POLLING GATEWAY SHOULD
ASSUME THAT THE POLLED GATEWAY IS NOT AN APPROPRIATE FIRST HOP
FOR ANY NETWORK WHATSOEVER. The optimum parameters for the
polling/retransmission algorithm will be dependent on the
characteristics of the two neighbors and of the network
connecting them.
Received NR messages whose identification numbers do not
match the identification number of the most recently sent poll
shall be ignored. There is no provision for multiple outstanding
polls to the same neighbor.
- 23 -
RFC 888 JANUARY 1984
7 SENDING NR MESSAGES
In general, NR messages are to be sent only in response to a
poll. However, between two successive polls from an exterior
neighbor, a gateway may send one and only one unsolicited NR
message to that neighbor. This gives it limited ability to
quickly announce network reachability changes that may have
occurred in the interval since the last poll. Excess unsolicited
NR messages may be ignored, or an error message may be returned.
An NR message should be sent within several seconds after
receipt of a poll. Failure to respond in a timely manner to an
NR poll may result in the polling gateway's deciding that the
polled gateway is not an appropriate first hop to any network.
NR messages sent in response to polls carry the sequence
number of the poll message in their "sequence number" fields.
Unsolicited NR messages carry the identification number of the
last poll received, and have the "unsolicited" bit set. (Note
that this allows for only a single unsolicited NR message per
polling period.)
Polls from non-neighbors, from neighbors which are not
declared reachable, or with bad IP source network fields, should
- 24 -
RFC 888 JANUARY 1984
be responded to with an EGP error message with the appropriate
"reason" field. If G sends an NR poll to G' with IP source
network N, and G' is not a neighbor of G on its interface to
network N (or G' does not have an interface to network N), then
the source network field is considered "bad".
A gateway is normally not required to send more than one NR
message within the minimum interval specified at the time of the
neighbor acquisition. An exception to this must be made for
duplicate polls (successive polls with the same sequence number),
which occur when an NR message is lost in transit. A gateway
should send an NR message containing its most recent information
in response to a duplicate poll.
- 25 -
RFC 888 JANUARY 1984
8 INDIRECT NEIGHBORS
Becoming a "direct neighbor" of an exterior gateway requires
three steps: (a) neighbor acquisition, (b) running a neighbor
reachability protocol, and (c) polling the neighbor periodically
for NR messages. Suppose, however, that gateway G receives an NR
message from G', in which G' indicates the presence of other
neighbors G1, ..., Gn, each of which is an appropriate first hop
for some set of networks to which G' itself is not an appropriate
first hop. Then G should be allowed to forward traffic for those
networks directly to the appropriate one of G1, ..., Gn, without
having to send it to G' first. In this case, G may be considered
an INDIRECT NEIGHBOR of G1, ..., Gn, since it is a neighbor of
these other gateways for the purpose of forwarding traffic, but
does not perform neighbor acquisition, neighbor reachability, or
exchange of NR messages with them. Neighbor and network
reachability information is obtained indirectly via G', hence the
designation "indirect neighbor". We say that G is an indirect
neighbor of G1, ..., Gn VIA G'.
If G is an indirect neighbor of G' via G'', and then G
receives an NR message from G'' which does not mention G', G
should treat G' as having become unreachable.
- 26 -
RFC 888 JANUARY 1984
9 LIMITATIONS
It must be clearly understood that the Exterior Gateway
Protocol does not in itself constitute a network routing
algorithm. In addition, it does not provide all the information
needed to implement a general area routing algorithm. If the
topology does not obey the rules given for stubs above, the
Exterior Gateway Protocol does not provide enough topological
information to prevent loops.
If any gateway sends an NR message with false information,
claiming to be an appropriate first hop to a network which it in
fact cannot even reach, traffic destined to that network may
never be delivered. Implementers must bear this in mind.
- 27 -
RFC 888 JANUARY 1984
A APPENDIX A - EGP MESSAGE FORMATS
The Exterior Gateway Protocol runs under Internet Protocol as
protocol number 8 (decimal).
A.1 NEIGHBOR ACQUISITION 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EGP Version # ! Type ! Code ! Info !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Checksum ! Autonomous System # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # ! NR Hello interval !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! NR poll interval !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description:
The Neighbor Acquisition messages are used by interior and
exterior gateways to become neighbors of each other.
EGP Version #
2
Type
3
Code
Code = 0 Neighbor Acquisition Request
Code = 1 Neighbor Acquisition Reply
Code = 2 Neighbor Acquisition Refusal (see Info field)
Code = 3 Neighbor Cease Message (see Info field)
Code = 4 Neighbor Cease Acknowledgment
Checksum
- 28 -
RFC 888 JANUARY 1984
The EGP checksum is the 16-bit one's complement of the one's
complement sum of the EGP message starting with the EGP
version number field. For computing the checksum, the
checksum field should be zero.
Autonomous System #
This 16-bit number identifies the autonomous system
containing the gateway which is the source of this message.
Info
For Refusal message, gives reason for refusal:
0 Unspecified
1 Out of table space
2 Administrative prohibition
For Cease message, gives reason for ceasing to be neighbor:
0 Unspecified
1 Going down
2 No longer needed
Otherwise, this field MUST be zero.
Sequence Number
A sequence number to aid in matching requests and
replies.
NR Hello Interval
Minimum Hello polling interval(seconds).
NR Poll Interval
Minumum NR polling interval(seconds).
- 29 -
RFC 888 JANUARY 1984
A.2 NEIGHBOR HELLO/I HEARD YOU 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EGP Version # ! Type ! Code ! Status !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Checksum ! Autonomous System # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description:
Exterior neighbors use EGP Neighbor Hello and I Heard You
Messages to determine neighbor connectivity. When a gateway
receives an EGP Neighbor Hello message from a neighbor it
should respond with an EGP I Heard You message.
EGP Version #
2
Type
5
Code
Code = 0 for Hello
Code = 1 for I Heard you
Checksum
The EGP checksum is the 16-bit one's complement of the one's
complement sum of the EGP message starting with the EGP
version number field. For computing the checksum, the
checksum field should be zero.
Autonomous System #
This 16-bit number identifies the autonomous system
containing the gateway which is the source of this message.
- 30 -
RFC 888 JANUARY 1984
Sequence Number
A sequence number to aid in matching requests and replies.
Status
0 No status given
1 You appear reachable to me
2 You appear unreachable to me due to neighbor
reachability protocol
3 You appear unreachable to me due to network
reachability information (such as 1822 "destination
dead" messages from ARPANET)
4 You appear unreachable to me due to problems
with my network interface
- 31 -
RFC 888 JANUARY 1984
A.3 NR POLL 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EGP Version # ! Type ! Code ! Unused !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Checksum ! Autonomous System # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # ! Unused !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IP Source Network !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description:
A gateway that wants to receive an NR message from an
Exterior Gateway will send an NR Poll message. Each gateway
mentioned in the NR message will have an interface on the
network that is in the IP source network field.
EGP Version #
2
Type
2
Code
0
Checksum
The EGP checksum is the 16-bit one's complement of the one's
complement sum of the EGP message starting with the EGP
version number field. For computing the checksum, the
checksum field should be zero.
Autonomous System #
This 16-bit number identifies the autonomous system
- 32 -
RFC 888 JANUARY 1984
containing the gateway which is the source of this message.
Sequence Number
A sequence number to aid in matching requests and
replies.
IP Source Network
Each gateway mentioned in the NR message will have an
interface on the network that is in the IP source network
field. The IP source network is coded as one byte of
network number followed by two bytes of zero for class A
networks, two bytes of network number followed by one byte
of zero for class B networks, and three bytes of network
number for class C networks.
- 33 -
RFC 888 JANUARY 1984
A.4 NETWORK REACHABILITY 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EGP Version # ! Type ! Code !U! Zeroes !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Checksum ! Autonomous System # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # ! # of Int Gwys ! # of Ext Gwys !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IP Source Network !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Gateway 1 IP address (without network #) ! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! # Distances !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Distance 1 ! # Nets !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,1,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,1,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Distance 2 ! # Nets !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,2,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,2,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Gateway n IP address (without network #) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! # Distances !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Distance 1 ! # Nets !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,1,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,1,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Distance 2 ! # Nets !
- 34 -
RFC 888 JANUARY 1984
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,2,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,2,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
Description:
The Network Reachability message (NR) is used to discover
which networks may be reached through Exterior Gateways. The NR
message is sent in response to an NR Poll message.
EGP Version #
2
Type
1
Code
0
Checksum
The EGP checksum is the 16-bit one's complement of the one's
complement sum of the EGP message starting with the EGP
version number field. For computing the checksum, the
checksum field should be zero.
Autonomous System #
This 16-bit number identifies the autonomous system
containing the gateway which is the source of this message.
U (Unsolicited) bit
This bit is set if the NR message is being sent unsolicited.
- 35 -
RFC 888 JANUARY 1984
Sequence Number
The sequence number of the last NR poll message
received from the neighbor to whom this NR message is being
sent. This number is used to aid in matching polls and
replies.
IP Source Network
Each gateway mentioned in the NR message will have an
interface on the network that is in the IP source network
field.
# of Interior Gateways
The number of interior gateways that are mentioned in this
message.
# of Exterior Gateways
The number of exterior gateways that are mentioned in this
message.
Gateway IP address
1, 2 or 3 bytes of Gateway IP address (without network #).
# of Distances
The number of distances in the gateway block.
Distance
The distance.
# of Nets
The number of nets at this distance.
Network address
1, 2, or 3 bytes of network address of network which can be
reached via the preceding gateway.
- 36 -
RFC 888 JANUARY 1984
A.5 EGP ERROR 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EGP Version # ! Type ! Code ! Unused !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Checksum ! Autonomous System # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # ! Reason !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
! Error Message Header !
! (first three 32-bit words of EGP header) !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description:
An EGP Error Message is sent in response to an EGP Message
that has a bad checksum or has an incorrect value in one of
its fields.
EGP Version #
2
Type
8
Code
0
Checksum
The EGP checksum is the 16-bit one's complement of the one's
complement sum of the EGP message starting with the EGP
version number field. For computing the checksum, the
checksum field should be zero.
Autonomous System #
- 37 -
RFC 888 JANUARY 1984
This 16-bit number identifies the autonomous system
containing the gateway which is the source of this message.
Sequence Number
A sequence number assigned by the gateway sending the error
message.
Reason
The reason that the EGP message was in error. The following
reasons are defined:
0 - unspecified
1 - Bad EGP checksum
2 - Bad IP Source address in NR Poll or Response
3 - Undefined EGP Type or Code
4 - Received poll from non-neighbor
5 - Received excess unsolicted NR message
6 - Received excess poll
7 - Erroneous counts in received NR message
8 - No response received to NR poll
- 38 -