Network Working Group S. Rao
Request for Comments: 3883 UTA
Updates: 1793 A. Zinin
Category: Standards Track Alcatel
October 2004 Detecting Inactive Neighbors over OSPF Demand Circuits (DC)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004).
OSPF is a link-state intra-domain routing protocol used in IP
networks. OSPF behavior over demand circuits (DC) is optimized in
RFC 1793 to minimize the amount of overhead traffic. A part of the
OSPF demand circuit extensions is the Hello suppression mechanism.
This technique allows a demand circuit to go down when no interesting
traffic is going through the link. However, it also introduces a
problem, where it becomes impossible to detect an OSPF-inactive
neighbor over such a link. This memo introduces a new mechanism
called "neighbor probing" to address the above problem.
In some situations, when operating over demand circuits, the remote
neighbor may be unable to run OSPF [RFC2328], and, as a possible
result, unable to route application traffic. Possible scenarios
o The OSPF process might have died on the remote neighbor.
o Oversubscription (Section 7 of [RFC1793]) may cause a continuous
drop of application data at the link level.
The problem here is that the local router cannot identify problems
such as this, since the Hello exchange is suppressed on demand
circuits. If the topology of the network is such that other routers
cannot communicate their knowledge about the remote neighbor via
flooding, the local router and all the routers behind it will never
know about the problem, so application traffic may continue being
forwarded to the OSPF-incapable router.
This memo describes a backward-compatible neighbor probing mechanism
based on the details of the standard flooding procedure followed by
2. Proposed Solution
The solution this document proposes uses the link-state update
packets to detect whether the OSPF process is operational on the
remote neighbor. We call this process "Neighbor probing". The idea
behind this technique is to allow either of the two neighbors
connected over a demand circuit to test the remote neighbor at any
time (see Section 2.1).
The routers across the demand circuit can be connected by either a
point-to-point link, a virtual link, or a point-to-multipoint
interface. The case of routers connected by broadcast networks or
Non-Broadcast Multi-Access (NBMA) links is not considered, since
Hello suppression is not used in these cases (Section 3.2 [RFC1793]).
The neighbor probing mechanism is used as follows. After a router
has synchronized the Link State Database (LSDB) with its neighbor
over the demand circuit, the demand circuit may be torn down if there
is no more application traffic. When application traffic starts
going over the link, the link is brought up. If ospfIfDemandNbrProbe
is enabled, the routers SHOULD probe each other. While the link is
up, the routers may also periodically probe each other every
ospfIfDemandNbrProbeInterval. Neighbor probing should not be
considered as interesting traffic and should not cause the demand
circuit to remain up (relevant details of implementation are outside
of the scope of this document).
The case when one or more of the router's links are oversubscribed
(see section 7 of [RFC1793]) should be considered by the
implementations. In such a situation, even if the link status is up
and application data is being sent on the link, only a limited number
of neighbors are really reachable. To make sure temporarily
unreachable neighbors are not mistakenly declared down, Neighbor
probing should be restricted to those neighbors that are actually
reachable (i.e., there is a circuit established with the neighbor at
the moment the probing procedure needs to be initiated). This check
itself is also considered an implementation detail.
2.1. Neighbor Probing
The neighbor probing method described in this section is completely
compatible with standard OSPF implementations, because it is based on
standard behavior that must be followed by OSPF implementations in
order to keep their LSDBs synchronized.
When a router needs to verify the OSPF capability of a neighbor
reachable through a demand circuit, it should flood to the neighbor
any LSA in its LSDB that would normally be sent to the neighbor
during the initial LSDB synchronization process (in most cases, such
an LSA must have already been flooded to the neighbor by the time the
probing procedure starts). For example, the router may flood its own
router-LSA (without originating a new version), or the neighbor's own
router-LSA. If the neighbor is still alive and OSPF-capable, it
replies with a link state acknowledgement or a link state update (an
implied acknowledgement), and the LSA is removed from the neighbor's
retransmission list. The implementations should limit the number of
times an LSA can be retransmitted to ospfIfDemandNbrProbeRetxLimit,
when used for neighbor probing. If no acknowledgement (explicit or
implicit) is received for a predefined period of time, the probing
router should treat this as evidence of the neighbor's unreachability
(proving wrong the assumption of reachability used in [RFC1793]) and
should bring the adjacency down.
Note that when the neighbor being probed receives such a link state
update packet, the received LSA has the same contents as the LSA in
the neighbor's LSDB, and hence should normally not cause any
additional flooding. However, since LSA refreshes are not flooded
over demand circuits, the received LSA may have a higher Sequence
Number. This will result in the first probe LSA being flooded
further by the neighbor. Note that if the current version of the
probe LSA has already been flooded to the neighbor, it will not be
propagated any further by the neighbor. Also note that in any case,
subsequent (non-first) probe LSAs will not cause further flooding
until the LSA's sequence number is incremented.
Again, the implementation should insure (through internal mechanisms)
that OSPF link state update packets sent over the demand circuit for
the purpose of neighbor probing do not prevent that circuit from
being torn down.
3. Support of Virtual Links and Point-to-multipoint Interfaces
Virtual links can be treated analogously to point-to-point links, so
the techniques described in this memo are applicable to virtual links
as well. The case of point-to-multipoint interface running as a
demand circuit (section 3.5 [RFC1793]) can be treated as individual
point-to-point links, for which the solution has been described in
4. Compatibility Issues
All mechanisms described in this document are backward-compatible
with standard OSPF implementations.
5. Deployment Considerations
In addition to the lost functionality mentioned in Section 6 of
[RFC1793], there is additional overhead in terms of the amount of
data (link state updates and acknowledgements) being transmitted due
to neighbor probing whenever the link is up, thereby increasing the
The original idea of limiting the number of LSA retransmissions on
demand circuits (used as part of the solution described in this
document) and its implementation belong to Padma Pillay-Esnault and
The authors would like to thank John Moy, Vijayapal Reddy Patil, SVR
Anand, and Peter Psenak for their comments on this work.
A significant portion of Sira's work was carried out as part of the
HFCL-IISc Research Project (HIRP), Bangalore, India. He would like
to thank the team for their insightful discussions.
7. Security Considerations
The mechanism described in this document does not modify security
aspects of the OSPF routing protocol.
8. Normative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC1793] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.
Appendix A. Configurable Parameters
This memo defines the following additional configuration parameters
for OSPF interfaces.
Indicates whether or not neighbor probing is enabled to
determine whether the neighbor is inactive. Neighbor probing
is disabled by default.
The number of consecutive LSA retransmissions before the
neighbor is deemed inactive and the neighbor adjacency is
brought down. Sample value is 10 consecutive LSA
Defines how often the neighbor will be probed. The sample
value is 2 minutes.
Sira Panduranga Rao
The University of Texas at Arlington
416 Yates Street, 300 Nedderman Hall
Arlington, TX 76019
701 E Middlefield Rd
Mountain View, CA 94043
170 W. Tasman Dr.
San Jose,CA 95134
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