RFC1582 - Extensions to RIP to Support Demand Circuits(13)

the entry is advertised as unreachable on other interfaces. On
expiry of the hold down timer the entry is deleted.
For IP RIP the hold down timer should always run for 120 seconds, to
be consistent with RIP usage on broadcast networks. The database
entry timer should by default run for 180 seconds. The network can
be made more responsive by reducing the database entry timer value.
However, making this timer too short can lead to network
instabilities. The duration of the database entry timer allows a
period of grace in which contention for network resources can be
resolved by the circuit manager.
7.2 Retransmission Timer
The routing task runs a retransmission timer:
o When a triggered request is sent it will be retransmitted
periodically while a triggered response packet is not received.
o When a triggered response is sent a note of the sequence number
and fragment number(s) of the routing update is kept.
Fragments will be retransmitted at periodic intervals while a
triggered acknowledgement packet is not received for the
appropriate fragment.
With call set up time on the WAN being of the order of a second, a
value of 5 seconds for the retransmission timer is appropriate.
If no response is received after 10 retransmissions, routes via the
next hop router are marked as unreachable, the hold down timer MUST
be started and the entry is advertised as unreachable on other
interfaces. On expiry of the hold down timer the entry is deleted.
The next hop router is then polled using a triggered request packet
at 60 second intervals. If a response is received the routers should
exchange routing information using triggered response packets.
It may not be desirable to poll indefinitely, since a lack of
response (when a circuit is up) is most likely caused by incorrect
configuration of the next hop router. An administrator definable
number of polls (5 or greater) should be provided.
If the circuit manager indicates that the next hop router is
unreachable, the retransmission is suppressed until the circuit
manager indicates that the next hop router is reachable once more.
Counting of the number of retransmissions continues from where it
left off prior to the circuit down indication.
7.3 Reassembly Timer
When a router receives a triggered response update it MUST
acknowledge each fragment. If the routing update is fragmented over
more than one packet, the receiving router MUST store the fragments
until ALL fragments are received.
On receiving the first fragment a timer should be started. If all
fragments of the routing update are not received within that period
they are discarded - and a triggered request is sent back to the
originator (with retransmissions if necessary). The originator MUST
then resend ALL triggered response fragments.
The reassembly timer should be set to four times the value of the
retransmission timer. With a suggested retransmission timer value of
5 seconds, the suggested reassembly timer value SHOULD be 20 seconds.
Implementations MAY allow the reassembly timer and retransmission
timer to be configurable (in the 1:4 ratio), but interoperability
will be compromised on WANs where all participating routers DO NOT
support the same values for these timers.
Fragments MUST also be discarded if a new fragment with a different
sequence number is received. A triggered request MUST not be sent in
this instance.
8. Implementation Considerations
In the implementation described in this memo, it is assumed that
there is a close binding between the circuit manager and the routing
applications - that they are in some way the same "program". This is
not necessarily true of all products which are routers.