rfc823.txt
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if the source gateway requests a routing
update from the destination gateway, and
set to 0 if not.
n-distances An 8-bit field. The number of
distance-groups reported in this update.
Each distance-group consists of a
distance value and a number of nets,
followed by the actual net numbers which
are reachable at that distance. Not all
distances need be reported.
distance 1 hop count (or other distance measure)
which applies to this distance-group.
n1-dist number of nets which are reported in
this distance-group.
net11 1, 2, or 3 bytes for the first net at
distance "distance 1".
net12 second net
...
net1n1 etc.
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RFC 823
ACKNOWLEDGMENT or NEGATIVE ACKNOWLEDGMENT
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gateway Type | Unused | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Gateway Type Acknowledgments are type 2. Negative
acknowledgments are type 10.
Sequence Number The 16-bit sequence number that the
gateway is acknowledging or negatively
acknowledging.
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GGP ECHO and ECHO REPLY
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gateway Type | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Gateway Type 8 for echo message; 0 for echo reply.
Source Address In an echo message, this is the address
of the gateway on the same network as
the neighbor to which it is sending the
echo message. In an echo reply message,
the source and destination addresses are
simply reversed, and the remainder is
returned unchanged.
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NETWORK INTERFACE STATUS
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Gateway Type ! unused !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Gateway Type 9
Source Address
Destination Address The address of the gateway's network
interface. The gateway can send Net
Interface Status messages to itself to
determine if it is able to send and
receive traffic on its network
interface.
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RFC 823
APPENDIX B. Information Maintained by Gateways
In order to implement the shortest-path routing algorithm,
gateways must maintain information about their connectivity to
networks and other gateways. This section explains the
information maintained by each gateway; this information can be
organized into the following tables and variables.
o Number of Networks
The number of networks for which the gateway maintains
routing information and to which it can forward traffic.
o Number of Neighbors
The number of neighbor gateways with which the gateway
exchanges routing information.
o Gateway Addresses
The addresses of the gateway's network interfaces.
o Neighbor Gateway Addresses
The address of each neighbor gateway's network interface
that is on the same network as this gateway.
o Neighbor Connectivity Vector
A vector of the connectivity between this gateway and each
of its neighbors.
o Distance Matrix
A matrix of the routing updates received from the neighbor
gateways.
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RFC 823
o Minimum Distance Vector
A vector containing the minimum distance to each network.
o Routing Updates from Non-Routing Gateways
The routing updates that would have been received from each
non-routing neighbor gateway which does not participate in
this routing strategy.
o Routing Table
A table containing, for each network, a list of the neighbor
gateways on a minimum-distance route to the network.
o Send Sequence Number
The sequence number that will be used to send the next
routing update.
o Receive Sequence Numbers
The sequence numbers that the gateway received in the last
routing update from each of its neighbors.
o Received Acknowledgment Vector
A vector indicating whether or not each neighbor has
acknowledged the sequence number in the most recent routing
update sent.
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DARPA Internet Gateway September 1982
RFC 823
APPENDIX C. GGP Events and Responses
The following list shows the GGP events that occur at a
gateway and the gateway's responses. The variables and tables
referred to are listed above.
o Connectivity to an attached network changes.
a. Update the Minimum Distance Vector.
b. Recompute the Routing Updates.
c. Recompute the Routing Table.
d. If any routing update has changed, send the new routing
updates to the neighbors.
o Connectivity to a neighbor gateway changes.
a. Update the Neighbor Connectivity Vector.
b. Recompute the Minimum Distance Vector.
c. Recompute the Routing Updates.
d. Recompute the Routing Table.
e. If any routing update has changed, send the new routing
updates to the neighbors.
o A Routing Update message is received.
a. Compare the Internet source address of the Routing Update
message to the Neighbor Addresses. If the address is not
on the list, add it to the list of Neighbor Addresses,
increment the Number of Neighbors, and set the Receive
Sequence Number for this neighbor to the sequence number
in the Routing Update message.
b. Compare the Receive Sequence Number for this neighbor to
the sequence number in the Routing Update message to
determine whether or not to accept this message. If the
message is rejected, send a Negative Acknowledgment
message. If the message is accepted, send an
Acknowledgment message and proceed with the following
steps.
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RFC 823
c. Compare the networks reported in the Routing Update
message to the Number of Networks. If new networks are
reported, enter them in the network vectors, increase the
number of networks, and expand the Distance Matrix to
account for the new networks.
d. Copy the routing update received into the appropriate row
of the Distance Matrix.
e. Recompute the Minimum Distance Vector.
f. Recompute the Routing Updates.
g. Recompute the Routing Table.
h. If any routing update has changed, send the new routing
updates to the neighbors.
o An Acknowledgment message is received.
Compare the sequence number in the message to the Send
Sequence Number. If the Send Sequence Number is
acknowledged, update the entry in the Received
Acknowledgment Vector for the neighbor that sent the
acknowledgment.
o A Negative Acknowledgment message is received.
Compare the sequence number in the message to the Send
Sequence Number. If necessary, replace the Send Sequence
Number, and retransmit the routing updates.
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DARPA Internet Gateway September 1982
RFC 823
REFERENCES
[1] Postel, J. (ed.), "Internet Protocol - DARPA Internet
Program Protocol Specification," RFC 791, USC/Information
Sciences Institute, September 1981.
[2] Strazisar, V., "Gateway Routing: An Implementation
Specification," IEN-30, Bolt Beranek and Newman Inc., August
1979.
[3] Strazisar, V., "How to Build a Gateway," IEN-109, Bolt
Beranek and Newman Inc., August 1979.
[4] Postel, J., "Internet Control Message Protocol - DARPA
Internet Program Protocol Specification," RFC 792,
USC/Information Sciences Institute, September 1981.
[5] Postel, J., "Assigned Numbers," RFC 790, USC/Information
Sciences Institute, September 1981.
[6] Littauer, B., Huang, A., Hinden, R., "A Host Monitoring
Protocol," IEN-197, Bolt Beranek and Newman Inc., September
1981.
[7] Santos, P., Chalstrom, H., Linn, J., Herman, J.,
"Architecture of a Network Monitoring, Control and
Management System," Proc. of the 5th Int. Conference on
Computer Communication, October 1980.
[8] Haverty, J., "XNET Formats for Internet Protocol Version 4,"
IEN-158, Bolt Beranek and Newman Inc., October 1980.
[9] Mathis, J., Klemba, K., Poggio, "TIU Notebook- Volume 2,
Software Documentation," SRI, May 1979.
[10] Rosen, E., "Exterior Gateway Protocol," IEN-209, Bolt
Beranek and Newman Inc., August 1982.
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