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Network Working Group                                Sira Panduranga Rao
Internet Draft                                                       UTA
Expiration Date: March 2003                                   Alex Zinin
File name: draft-ietf-ospf-dc-04.txt                             Alcatel
                                                               Abhay Roy
                                                           Cisco Systems

                                                          September 2002

         Detecting Inactive Neighbors over OSPF Demand Circuits
                       draft-ietf-ospf-dc-04.txt


Status of this Memo

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

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Abstract
   OSPF [RFC2328] 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 addresses the above problem by
   the neighbor probing mechanism.

1. Motivation

   In some situations, when operating over demand circuits, the remote



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   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.

   The 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, and the routers may probe each other. The routers
   may also periodically probe each other any time the link is up (could
   be implemented as a configurable option) with the caution that OSPF
   packets sent as part of neighbor probing are not 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).

   The case when one or more of the router's links are oversubscribed



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   (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 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 effect further flooding until the
   LSA's sequence number is incremented.

   Again, the implementation should insure (through internal mechanisms)



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   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 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. References


[RFC2328]
     J.Moy, OSPF Version 2. Technical Report RFC2328 Internet Engineer-
     ing Task Force, 1998 ftp://ftp.isi.edu/in-notes/rfc2328.txt

[RFC1793]
     J.Moy, Extending OSPF to support Demand Circuits.  Technical Report
     RFC1793 Internet Engineering Task Force, 1995 ftp://ftp.isi.edu/in-
     notes/rfc1793.txt

8. Authors' addresses



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   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
   USA
   E-mail: akr@cisco.com







































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