RFC2092 - Protocol Analysis for Triggered RIP

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　　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.

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.

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
Triggered RIP.

3.4 Presumption of Reachability

In a stable network there is no requirement to propagate routing
information on a circuit, so if no routing information is (being)
received on a circuit it is assumed that:

o The most recently received information is accurate.

o The intervening path is operational (although there may be no
current connection).

If the circuit manager determines that the intervening path is NOT
operational routing information previously received on that circuit
is timed out. It is worth stressing that it can be ANY routed
datagram which triggers the event.

When the circuit manager re-establishes a connection, the application
exchanges full routing information with its peer.

3.5 Routing Information Flow Control

If the circuit manager reports a circuit as down, the routing
application is flow controlled from sending further information on
the circuit.

4. Relationship to Demand RIP

The Triggered RIP proposal [2] has a number of efficiency advantages
over the Demand RIP proposal [5]:

o When routing information changes Demand RIP sends the full
database to its partner.

Once a router has exchanged all routing information with its
partner, Triggered RIP sends only the changed information to the
partner. This can dramatically decrease the quantity of
information requiring propagation when a route change occurs.

o Demand RIP requires a full routing update to be stored by the
receiver, before applying changes to the routing database.

A Triggered RIP receiver is able to apply all changes immediately
upon receiving each packet from its partner. Systems therefore
need to use less memory than Demand RIP.

o Demand RIP has an upper limit of 255 fragments in an update. This
sets an upper limit on the sizes of routing and service
advertising databases which can be propagated.

This restriction is lifted in Triggered RIP (which does not use
fragmentation).

In all other respects Demand RIP and Triggered RIP perform the same
function.

5. Obsoleting Demand RIP

While it is possible that systems could be able to support Demand RIP
and Triggered RIP, the internal maintenance of structures is likely
to differ significantly. The method of propagating the information
also differs significantly. In practice it is unlikely that systems
would support Demand RIP and Triggered RIP.

As a result of the improved characteristics of Triggered RIP, it is
proposed that Demand RIP [5] be obsoleted.

6. Implementations

At this stage there are believed to be two completed implementation.

The Xyplex implementation supports all the features outlined for IP
RIP-1, IP RIP-2, IPX RIP, and IPX SAP. However, it only supports one
outstanding acknowledgement per partner. The implementation has been
tested against itself on X.25, ISDN, Frame Relay, V42bis CSU/DSUs,
and asynchronous modems. It has also been tested in operation with
various router and host implementations on Ethernet LANs.

The DEC implementation supports IP RIP-1 over ISDN, Frame Relay,
leased lines and V.25bis. The Xyplex and DEC IP RIP-1
implementations have been checked for interoperability over ISDN
without problems.

7. Restrictions

Demand RIP relies on the ability to place a call in either direction.
Some dialup services - for example DTR dialing - allow calls to be
made in one direction only.

Demand RIP can not operate with third-party advertisement of routes
on the WAN. The next hop IP address in RIP-2 should always be
0.0.0.0 for any routes advertised on the WAN.

8. Security Considerations

Security is provided through authentication of the logical and
physical address of the sender of the routing update. Incoming call
requests are matched by the circuit manager against a list of
physical addresses (used to make outgoing calls). The routing
application makes a further check against the logical address of an
incoming update.

Additional security can be provided by RIP-2 authentication [2] where
appropriate.

References

[1] Hinden, R., "Internet Engineering Task Force Internet Routing
Protocol Standardization Criteria", RFC1264, Bolt Beranek and
Newman, Inc, October 1991.

[2] Meyer. G.M. and Sherry, S., "Triggered Extensions to RIP to
Support Demand Circuits", RFC2091, Shiva and Xyplex, Aug 1995.

[3] Hedrick. C., "Routing Information Protocol", RFC1058, Rutgers
University, June 1988.

[4] Malkin. G., "RIP Version 2 - Carrying Additional Information",
RFC1723, Xylogics, November 1994.

[5] Meyer. G., "Extensions to RIP to Support Demand Circuits",
Spider Systems, February 1994.

Authors' Address:

Steve Sherry
Xyplex
295 Foster St.
Littleton, MA 01460

Phone: (US) 508 952 4745
Fax: (US) 508 952 4887
Email: shs@xyplex.com

Gerry Meyer
Shiva Europe Ltd
Stanwell Street
Edinburgh EH6 5NG
Scotland, UK

Phone: (UK) 131 561 4223
Fax: (UK) 131 561 4083
Email: gerry@europe.shiva.com

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