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Network Working Group S. Rao Request for Comments: 3883 UTA Updates: 1793 A. Zinin Category: Standards Track Alcatel

                                                                A. Roy
                                                         Cisco Systems
                                                          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 Notice

 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.

1. Motivation

 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.

Rao, et al. Standards Track [Page 1] RFC 3883 OSPF DC Inactive Neighbor Detection October 2004

 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
 OSPF routers.

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

Rao, et al. Standards Track [Page 2] RFC 3883 OSPF DC Inactive Neighbor Detection October 2004

 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.

Rao, et al. Standards Track [Page 3] RFC 3883 OSPF DC Inactive Neighbor Detection October 2004

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
 section 2.

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
 overall cost.

6. Acknowledgements

 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
 Derek Yeung.
 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.

Rao, et al. Standards Track [Page 4] RFC 3883 OSPF DC Inactive Neighbor Detection October 2004

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.

Authors' Addresses

 Sira Panduranga Rao
 The University of Texas at Arlington
 416 Yates Street, 300 Nedderman Hall
 Arlington, TX 76019
 Alex Zinin
 701 E Middlefield Rd
 Mountain View, CA 94043
 Abhay Roy
 Cisco Systems
 170 W. Tasman Dr.
 San Jose,CA 95134

Rao, et al. Standards Track [Page 5] RFC 3883 OSPF DC Inactive Neighbor Detection October 2004

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Rao, et al. Standards Track [Page 6]

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