Protocol Analysis for Triggered RIP
RFC 2092
Network Working Group S. Sherry
Request for Comments: 2092 Xyplex
Category: Informational G. Meyer
Shiva
January 1997
Protocol Analysis for Triggered RIP
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
As required by Routing Protocol Criteria [1], this report documents
the key features of Triggered Extensions to RIP to Support Demand
Circuits [2] and the current implementation experience.
As a result of the improved characteristics of Triggered RIP, it is
proposed that Demand RIP [5] be obsoleted.
Acknowledgements
The authors wish to thank Johanna Kruger and Jim Pearl of Xyplex for
many comments and suggestions which improved this effort.
1. Protocol Documents
"Triggered Extensions to RIP to Support Demand Circuits" [2] suggests
an enhancement to the "Routing Internet Protocol" (RIP) [3] and
"RIP-2" [4] to allow them to run more cost-effectively on Wide Area
Networks (WANs).
2. Applicability
Triggered RIP requires that there is an underlying mechanism for
determining unreachability in a finite predictable period.
The triggered extensions to RIP are particularly appropriate for WANs
where the cost - either financial or packet overhead - would make
periodic transmission of routing (or service advertising) updates
unacceptable:
o Connection oriented Public Data Networks - for example X.25 packet
switched networks or ISDN.
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RFC 2092 Triggered RIP Protocol Analysis January 1997
o Point-to-point links supporting PPP link quality monitoring or
echo request to determine link failure.
A triggered RIP implementation runs standard RIP on Local Area
Networks (LANs) allowing them to interoperate transparently with
implementations adhering to the original specifications.
3. Key Features
The proposal shares the same basic algorithms as RIP or RIP-2 when
running on LANs; Packet formats, broadcast frequency, triggered
update operation and database timeouts are all unmodified.
The new features operate on WANs which use switched circuits on
demand to achieve intermittent connectivity; Or on permanent WAN
connections where there is a desire to keep routing packet overhead
to a minimum. Instead of using periodic 'broadcasts', information is
only sent as triggered updates. The proposal makes use of features
of the underlying connection oriented service to provide feedback on
connectivity.
3.1 Triggered Updates
Updates are only sent on the WAN when an event changes the routing
database. Each update is retransmitted until acknowledged.
Information received in an update is not timed out.
The packet format of a RIP response is modified (with a different
unique command field) to include sequence number information. An
acknowledgement packet is also defined.
3.2 Circuit Manager
The circuit manager running below the IP network layer is responsible
for establishing a circuit to the next hop router whenever there is
data (or a routing update) to transfer. After a period of inactivity
the circuit will be closed by the circuit manager.
If the circuit manager fails to make a connection a circuit down
indication is sent to the routing application. The circuit manager
will then attempt at (increasing) intervals to establish a
connection. When successful a circuit up indication is sent to the
routing application.
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RFC 2092 Triggered RIP Protocol Analysis January 1997
3.3 Technology Restrictions
There is a small but nontrivial possiblility of an incorrectly
configured or poorly operating link causing severe data loss,
resulting in a 'black hole' in routing. This is often unidirectional
- the link that route updates cross works properly, but not the
return path.
Triggered RIP will NOT fuction properly (and should NOT be used) if a
routing information will be retained/advertised for an arbitrarily
long period of time (until an update in the opposite direction fails
to obtain a response).
To detect black holes in technologies which use PPP encapsulation,
either Echo Request/Response or Link Quality Monitoring should be
used. When a black hole is detected a circuit down indication must
be sent to the routing application.
Current (and future) technologies which do not use PPP, need to use
an equivalent 'are-you-there' mechanism - or should NOT be used with
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