RFC 827
Eric C. Rosen
Bolt Beranek and Newman Inc.
October 1982
Eric C. Rosen
Table of Contents
1 INTRODUCTION
2 NEIGHBOR ACQUISITION
3 NEIGHBOR REACHABILITY PROTOCOL
4 NETWORK REACHABILITY (NR) MESSAGE
5 POLLING FOR NR MESSAGES
6 SENDING NR MESSAGES
7 INDIRECT NEIGHBORS
8 HOW TO BE A STUB GATEWAY
9 LIMITATIONS
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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
any proposed change must be made in too many different
places and by too many different people.
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In the future, the internet is expected to evolve into a set
of separate domains 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
domain 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"
system, will be used as a transport or "long-haul" system by the
latter systems.
Ultimately, however, the internet may consist of a number of
co-equal autonomous systems, any of which may be used (with
certain restrictions which will be discussed later) as a
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transport medium for traffic originating in any system and
destined for any system. When this more complex configuration
comes into being, it will be inappropriate to regard any one
autonomous system as a "core" system. For the sake of
concreteness, however, and because the initial implementations of
the Exterior Gateway Protocol are expected to focus on the the
case of connecting "stub gateways" to the DARPA gateways on
ARPANET and SATNET, we will often use the term "core" gateways in
our examples and discussion.
The purpose of the Exterior Gateway Protocol (EGP) is to
enable one or more autonomous systems to be used as transport
media for traffic originating in some other autonomous system and
destined for yet another, while allowing the end-user to see the
composite of all the autonomous systems as a single internet,
with a flat, uniform address space. The route which a datagram
takes through the internet, and the number of autonomous systems
which it traverses, are to be transparent to the end-user
(unless, of course, the end-user makes use of the IP "source
route" option).
In describing the Exterior Gateway Protocol, we have
deliberately left a great deal of latitude to the designers and
implementers of particular autonomous systems, particularly with
regard to timer values. We have done this because we expect that
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different gateway implementations and different internet
environments may just have different requirements and goals, so
that no single strict implementation specification could apply to
all. However, this does NOT mean that ANY implementation which
conforms to the specification will work well, or that the areas
in which we have left latitude are not crucial to performance.
The fact that some time-out value, for example, is not specified
here does not mean that everything will work no matter what value
is assigned.
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 one word
for this number. Zero will not be assigned to any autonomous
system; rather, the presence of a zero in this field will
indicate that no number is present.
We need to introduce the concept of one gateway being a
NEIGHBOR of another. In the simplest and most common case, we
call two gateways "neighbors" if there is a network to which each
has an interface. However, we will need a somewhat more general
notion of "neighbor" to allow the following two cases:
a) Two gateways may be regarded as neighbors if they are
directly connected not by a network (in the usual sense
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of the term), but by a simple wire, or HDLC line, or some
similar means of "direct connection".
b) Two gateways may be regarded as neighbors if they are
connected by an "internet" which is transparent to them.
That is, we would like to be able to say that two
gateways are neighbors even if they are connected by an
internet, as long as the gateways utilize no knowledge of
the internal structure of that internet in their own
packet-forwarding algorithms.
In order to handle all these cases, let us say that two gateways
are NEIGHBORS if they are connected by some communications medium
whose internal structure is transparent to them. (See IEN 184
for a more general discussion of this notion of neighbor.)
If two neighbors are part of the same autonomous system, we
call them INTERIOR NEIGHBORS; if two neighbors are not part of
the same autonomous system, we call them EXTERIOR NEIGHBORS. 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
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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.
It must be clearly understood that any autonomous system in
which routing needs to be performed among gateways within that
system must implement its own routing algorithm. (A routing
algorithm is not generally necessary for a simple autonomous
system which consists of a single stub gateway.) The Exterior
Gateway Protocol is NOT a routing algorithm. It enables exterior
neighbors to exchange information which is likely to be needed by
any routing algorithm, but it does NOT specify what the gateways
are to do with this information. The "routing updates" of some
autonomous system's interior routing algorithm may or may not be
similar in format to the messages of the exterior gateway
protocol. The gateways in the DARPA "core" system will initially
use the GGP protocol (the old Gateway-Gateway protocol) as their
routing algorithm, but this will be subject to change. Gateways
in other autonomous systems may use their own Interior Gateway
Protocols (IGPs), which may or may not be similar to the IGP of
any other autonomous system. They may, of course, use GGP, but
will not be permitted to exchange GGP messages with gateways in
other autonomous systems.
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It must also be clearly understood that the Exterior Gateway
Protocol is NOT intended to provide information which could be
used as input to a completely general area or hierarchical
routing algorithm. It is intended for a set of autonomous
systems which are connected in a tree, with no cycles. It does
not enable the passing of sufficient information to prevent
routing loops if cycles in the topology do exist.
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.
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2 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 three-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
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
Neighbor Acquisition Reply message. It must also send a Neighbor
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Acquisition Request message, unless it has done so already.
Two gateways become direct neighbors when each has sent a
Neighbor Acquisition Message to, and received the corresponding
Neighbor Acquisition Reply from, the other.
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 Message from a gateway which is
already a direct neighbor should be responded to with a Reply and
a Neighbor Acquisition Message.
If a Neighbor Acquisition Reply is received from a
prospective neighbor, but a period of time passes during which no
Neighbor Acquisition Message is received from that prospective
neighbor, the neighbor acquisition protocol shall be deemed
incomplete. A Neighbor Cease message (see below) should then be
sent. If one gateway still desires to acquire the other as a
neighbor, the protocol must be repeated from the beginning.
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
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treat the sender of the message as a neighbor in any way. Since
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.
NOTE THAT WE HAVE NOT SPECIFIED THE WAY IN WHICH ONE GATEWAY
INITIALLY DECIDES THAT IT WANTS TO BECOME A NEIGHBOR OF ANOTHER.
While this is hardly a trivial problem, it is not part of the
External Gateway Protocol.
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3 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.
At the current time, the reachability determination
algorithm is left to the designers of a particular gateway. We
have in mind algorithms like the following:
A reachable neighbor shall be declared unreachable if,
during the time in which we sent our last n "Hello"s, we received
fewer than k "I Heard You"s in return. An unreachable neighbor
shall be declared reachable if, during the time in which we sent
our last m "Hello"s, we received at least j "I Heard You"s in
return.
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However, 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 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.
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 one gateway to
make its reachability determination parasitic on the other: only
one gateway actually needs to send "Hello" messages, and the
other can declare it up or down based on the status field in the
"Hello". That is, the "passive" gateway (which sends only "I
Heard You"s) declares the "active" one (which sends only
"Hello"s) to be reachable when the "Hello"s from the active one
indicate that it has declared the passive one to be reachable.
Of course, this can only work if there is prior agreement as to
which neighbor is to be the active one. (Ways of coming to this
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"prior agreement" are not part of the Exterior Gateway Protocol.)
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
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,
acquisition protocol would then need to be repeated before it
could become a direct neighbor again.
"Hello" and "I Heard You" messages from gateway G to gateway
G' also carry the identification number of the NR poll message
(see below) which G has most recently received from G'.
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"Hello" and "I Heard You" messages from gateway G to gateway
G' also carry the minimum interval in minutes with which G is
willing to be polled by G' for NR messages (see below).
"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 Gateway Going Down message to all direct neighbors
which will no longer be able to reach it. It should retransmit
that message (up to some number of times) until it receives a
Gateway Going Down Acknowledgment. This provides 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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4 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,
called the Network Reachability Message (or NR message), for
transferring this information.
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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 contains a
byte which 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.
This information might (or might not) be useful in the interior
routing algorithm of gateway G', or for diagnostic purposes.
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.
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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 shall be assumed that N is still reachable from G. HOWEVER,
IF N IS NOT MENTIONED IN TWO SUCCESSIVE NR MESSAGES FROM G, THAT
SHALL 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 may often be the case that where G and G' are exterior
neighbors on network N, G knows of many more gateway neighbors on
network N, and knows for which networks those other neighbors are
the appropriate first hop. Since G' may not know about all these
other neighbors, it is convenient and often more efficient for it
to be able to obtain this information from G. Therefore, the EGP
NR message also contains fields which allow 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. Gateways should be included in this list only
if G is actively running its neighbor reachability
protocol with them.
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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.
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, and the distance from that
gateway to each network.
Preceding the list of data blocks is:
a) The address of the network which this message is about.
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If G and G' are neighbors on network N, then in the NR
message going from G to G', this is the address of
network N. For convenience, four bytes have been
allocated for this address -- the trailing one, two, or
three bytes should be zero.
b) 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.
c) The count (one byte) of the number of exterior neighbors
of G for which this message contains data blocks.
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.
Each block includes a one-byte count of the number of
networks for which that gateway is the appropriate first hop. 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
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class C network. No trailing bytes are used.
It may sometimes be necessary to fragment the NR message.
The NR message contains a byte indicating the number of this
fragment (fragments will be numbered from zero), and a byte
containing the number of the last fragment (NOT the number of
fragments). If fragmentation is not used, these bytes must both
be zero. EACH FRAGMENT MUST BE A FULLY SELF-CONTAINED NR
MESSAGE. That is, each fragment will begin with a count of
interior and exterior neighbors, and will have some integral
number of gateway data blocks. The number of data blocks in each
fragment must correspond to the neighbor counts at the beginning
of that fragment. However, only the first fragment should begin
with a data block describing the sending gateway.
This scheme enables each fragment to be processed
independently, and requires no complex reassembly mechanisms. It
also enables processing of a message all of whose fragments have
not been received. If, after some amount of time and some number
of retransmissions of a poll, not all fragments have been
received, the fragments which are present shall be processed as
if they constituted the complete NR message. (This means that
networks mentioned only in the missing fragment will retain the
"distance" values they had in the previous NR message from that
gateway. However, if no new value for a particular network is
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received in the next NR message from that gateway, the network
will be declared unreachable.)
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5 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 Hello and I Heard You messages which gateway G
sends to gateway G' indicate the minimum interval which G will
accept as the polling interval from G'. That is, G' will not
guarantee to respond to polls from G that arrive less than that
interval apart.
Polls must only be sent to direct neighbors which are
declared reachable by the neighbor reachability protocol.
An NR Poll message contains an identification 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
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polls. It will be the responsibility of the polling gateway to
choose an identification number which is sufficiently "unique" to
allow detection of out-of-date NR messages which may still be
floating around the network. Since polls are relatively
infrequent, this is not expected to be much of a problem.
However, to aid in choosing an identification number, the Hello
and I Heard You messages carry the identification number of the
last NR poll received from the neighbor to which they are being
sent.
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.
If only some fragments of an NR message are received after
the maximum number of retransmissions, the fragments that are
present shall be treated as constituting the whole of the NR
message.
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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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6 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
identification number of the poll message in their
"identification 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.)
To facilitate the sending of unsolicited NR messages, the NR
poll message has a byte indicating the polling interval in
minutes.
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Polls from non-neighbors, from neighbors which are not
declared reachable, or with bad IP source network fields, should
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".
Duplicated polls (successive polls with the same
identification number) should be responded to with duplicates of
the same NR message. If that message is fragmented, the same
fragments shall be sent each time. Note that there is no
provision for handling multiple outstanding polls from a single
neighbor. NOTE THAT IF THE SAME FRAGMENTS ARE NOT SENT IN
RESPONSE TO DUPLICATED POLLS, INCORRECT REASSEMBLY WILL BE THE
PROBABLE RESULT. If fragmentation is not being used, however,
then no harm should result from responding to a duplicate poll
with a different (presumably more recent) NR message.
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7 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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8 HOW TO BE A STUB GATEWAY
The most common application of EGP will probably be its use
to enable a stub gateway to communicate with one of the DARPA
core gateways, so as to enable data flow between networks
accessible only via the stub and networks accessible only via the
system of core gateways. As discussed previously, a stub gateway
can be considered to be a one-gateway internet system with no
interior neighbors. It is probably used to interface a local
network or networks to a long range transport network (such as
ARPANET or SATNET) on which there is a core gateway. In this
case, the stub will not want the core gateways to forward it any
traffic other than traffic which is destined for the network or
networks which can be reached only via the stub. In general, the
stub will not want to perform any services for the internet
transport system which are not needed in order to be able to pass
traffic to and from the networks that cannot be otherwise
reached.
The 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. When a stub and a core gateway become direct
neighbors, the core gateway will begin sending Hello messages.
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Eric C. Rosen
When the stub declares the core gateways which are direct
neighbors to be reachable, it should poll those gateways for NR
messages at a rate not to exceed once per minute (or as specified
in the Hello messages from the core gateways). The core gateways
will also poll the stub for NR messages.
The NR message sent by the 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.
The core gateways will send complete NR messages, containing
information about all other gateways on the common networks, 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 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 core gateways will report distances less than 128 if the
network can be reached without leaving the core system (i.e.,
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Eric C. Rosen
without traversing any gateway other than a core gateway), and
greater than or equal to 128 otherwise.
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.
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, 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.)
If the stub gateway does not respond in a timely manner to
Hello messages from the core gateway, it may be declared
unreachable. If it does not respond to NR poll messages in a
timely manner, its networks may be declared unreachable. In both
these cases, the core gateways may discard traffic destined for
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Eric C. Rosen
those networks, returning ICMP "destination network unreachable"
to the source hosts.
The stub gateway is expected to fully execute the ICMP
protocol, 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.
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Eric C. Rosen
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 of the set of autonomous systems is not tree-structured
(i.e., if it has cycles), 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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Eric C. Rosen
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 # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Identification # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description:
The Neighbor Acquisition messages are used by interior and exterior gateways to become neighbors of each other.
EGP Version #
1
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
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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Eric C. Rosen
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.
Identification Number
An identification number to aid in matching requests and replies.
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Eric C. Rosen
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 # !Min Poll Intvl ! Zero !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Last Poll Id # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 #
1
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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Eric C. Rosen
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
Last Poll Id Number
The identification number of the most recently received NR poll message from the neighbor to which this message is being sent.
Minimum Polling Interval
This gateway should not be polled for NR messages more often than once in this number of minutes.
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Eric C. Rosen
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 # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IP Source Network ! Interval !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Identification # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 #
1
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
containing the gateway which is the source of this message.
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Eric C. Rosen
Identification Number
An identification 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.
Interval
The polling interval in minutes.
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Eric C. Rosen
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 # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Fragment # !# of last frg. ! Identification # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IP Source Network !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! # of Int Gwys ! # of Ext Gwys !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! # of Nets ! ; # of nets for
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Gateway 1
! Gateway 1 IP address (without network #) ! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! distance !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! distance !
+-+-+-+-+-+-+-+-+
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net 1,m !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; m nets reachable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; via Gateway 1
.
.
+-+-+-+-+-+-+-+-+
! # of nets ! ;number of nets for Gateway n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Gateway n IP address (without network #) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,1 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! distance !
+-+-+-+-+-+-+-+-+
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Eric C. Rosen
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,2 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! distance ! .
+-+-+-+-+-+-+-+-+ .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! net n,m !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; m nets reachable
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ; via Gateway n
! distance !
+-+-+-+-+-+-+-+-+
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Eric C. Rosen
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 #
1
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.
Identification Number
The identification 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.
Fragment Number
Which Fragment this is in the NR Message. Zero, if fragmentation is not used.
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Eric C. Rosen
Number of Last Fragment
Number of the last fragment in the NR Message. Zero, if fragmentation is not used.
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.
# of Networks
The number of networks for which the gateway whose IP address immediately follows is the appropriate first hop.
Gateway IP address
1, 2 or 3 bytes of Gateway IP address (without network #).
Network address
1, 2, or 3 bytes of network address of network which can be reached via the preceding gateway.
Distance
1 byte of distance in # of hops.
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Eric C. Rosen
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 # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Error Type ! Error Code ! Id. # of Erroneous Msg. !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sequence # !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 #
1
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 #
This 16-bit number identifies the autonomous system
containing the gateway which is the source of this message.
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Eric C. Rosen
Sequence Number
A sequence number assigned by the gateway sending the error message.
Error Type
The Type of the EGP message that was in error.
Error Code
The Code of the EGP message that was in error.
Identification number of erroneous message
The Sequence number of the EGP message that was in error.
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
9 - Not all fragments of NR message received
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