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RFC Number : 888

Title : 'STUB' Exterior Gateway Protocol.
















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

















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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



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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'




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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).































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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




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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.





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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.

























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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



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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



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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.)




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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.




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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



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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




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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



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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.










































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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,




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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




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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



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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 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.





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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



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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.







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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.













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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



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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.




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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




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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.























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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.




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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.






















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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



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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).









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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.




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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
































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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



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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.































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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 !



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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.





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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.




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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 #




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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

























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