Detecting Inactive Neighbors over OSPF Demand Circuits (DC)
RFC 3883

Document Type RFC - Proposed Standard (October 2004; No errata)
Updates RFC 1793
Last updated 2013-03-02
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IESG IESG state RFC 3883 (Proposed Standard)
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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]
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