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Versions: 00 01 02 03 04 05 06 07 rfc3883                Standards Track
Network Working Group                                Sira Panduranga Rao
Internet Draft                                                       UTA
Expiration Date: July 2004                                    Alex Zinin
File name: draft-ietf-ospf-dc-07.txt                             Alcatel
                                                               Abhay Roy
                                                           Cisco Systems

                                                            January 2004

         Detecting Inactive Neighbors over OSPF Demand Circuits

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet Drafts are working documents of the Internet Engineering
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   OSPF is a link-state intra-domain routing protocol used in IP
   networks. OSPF behavior over demand circuits is optimized in RFC1793
   to minimize the amount of overhead traffic. A part of 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 a OSPF-inactive neighbor over such a
   link. This memo introduces a new mechanism called "neighbor probing"
   to address the above problem.

Rao, Zinin, Roy                                                 [Page 1]

Internet Draft     OSPF DC Inactive Neighbor Detection      January 2004

1. Motivation

   In some situations, when operating over demand circuits, the remote
   neighbor may be unable to run OSPF, and, as a possible result, unable
   to route application traffic. Possible scenarios include:

       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.

   Problem here is that the local router cannot identify the problems
   such as this, since 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 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
   NBMA 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 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
   do not cause the demand circuit to remain up (relevant details of
   implementation are outside of the scope of this document).

Rao, Zinin, Roy                                                 [Page 2]

Internet Draft     OSPF DC Inactive Neighbor Detection      January 2004

   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 is 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 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 (it 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

Rao, Zinin, Roy                                                 [Page 3]

Internet Draft     OSPF DC Inactive Neighbor Detection      January 2004

   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 analogous to point-to-point links and so
   the techniques described in this memo are applicable to virtual links
   as well.  The case of point-to-multipoint interface running as 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. Considerations

   In addition to the lost functionality mentioned in Section 6 of
   [RFC1793], there is an added overhead in terms of the amount of data
   (link state updates and acknowledgements) being transmitted due to
   neighbor probing whenever the link is up and 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.

Rao, Zinin, Roy                                                 [Page 4]

Internet Draft     OSPF DC Inactive Neighbor Detection      January 2004

8. Normative References

     Moy, J., "OSPF Version 2", RFC 2328, April 1998.

     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 or not 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. Sample value is
         2 minutes.

Authors' addresses

   Sira Panduranga Rao                       Alex Zinin
   The University of Texas at Arlington      Alcatel
   Arlington, TX 76013                       Sunnyvale, CA
   Email: siraprao@hotmail.com               E-mail: zinin@psg.com

   Abhay Roy
   Cisco Systems
   170 W. Tasman Dr.
   San Jose,CA 95134
   E-mail: akr@cisco.com

Rao, Zinin, Roy                                                 [Page 5]