Internet Draft

Netwok Working Group                                     Danny McPherson
Internet Draft                                            Amber Networks
                                                         Tony Przygienda
                                                                 Redback


draft-mcpherson-ospf-transient-00.txt                          July 2000




                   OSPF Transient Blackhole Avoidance


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

   This document describes a simple, interoperable mechanism that can be
   employed in OSPF networks in order to decrease data loss associated
   with deterministic blackholing of packets during transient network
   conditions.  The mechanism proposed here requires no OSPF protocol
   changes and is completely interoperable with the existing OSPF
   specification.


Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].







Table of Contents

  1. Overview
  2. Discussion
  3. Deployment Considerations
  4. Security Considerations
  5. Acknowledgements
  6. References
  7. Authors' Address


1. Overview

   When an OSPF router that was previously a transit router becomes
   unavailable as a result of some transient condition such as a reboot,
   other routers within the routing domain must select an alternative
   path to reach destinations which had previously transited the failed
   router.  Presumably, the newly selected router(s) comprising the path
   have been available for some time and, as a result, have complete
   forwarding information bases (FIBs) which contain a full set of
   reachibility information for both internal and external (e.g. BGP)
   destinations.

   When the previously failed router becomes available again, in only a
   few seconds paths that had previously transited the router are again
   selected as the optimal path by the IGP.  As a result, forwarding
   tables are updated and packets are once again forwarded along the
   path.  Unfortunately, external destination reachibility information
   (e.g. learned via BGP) is not yet available to the router, and as a
   result, packets bound for destinations not learned via the IGP are
   unnecessarily discarded.

   A mechanism to alleviate the offshoot associated with this
   deterministic behavior is discussed below.


2. Discussion

   This document describes a simple, interoperable mechanism that can be
   employed in OSPF [RFC2328] networks in order to avoid transition to a
   newly available path until other associated routing protocols such as
   BGP have had sufficient time to converge.

   The benefits of such a mechanism can realized when considering the
   following scenario.


                    D.1
                     |
                 +-------+
                 | RtrD  |
                 +-------+
                 /       \  
                /         \ 
           +-------+    +-------+
           | RtrB  |    | RtrC  |
           +-------+    +-------+
                 \        /
                  \      /
                  +-------+
                  | RtrA  |
                  +-------+
                      |
                     S.1

   Host S.1 is transmitting data to destination D.1 via a primary path
   of RtrA->RtrB->RtrD.  Routers A, B and C learn of reachibility to
   destination D.1 via BGP from RtrD.  RtrA's primary path to D.1 is
   selected because when calculatng the path to BGP NEXT_HOP of RtrD the
   sum of the OSPF link costs on the RtrA-RtrB-RtrD path is less than
   the sum of the costs of the RtrA-RtrC-RtrD path.

   Assume RtrB becomes unavailable and as a result the RtrC path to RtrD
   is used.  Once RtrA's FIB is updated and it begins forwarding packets
   to RtrC everything should behave properly as RtrC has existing
   forwarding information regarding destination D.1's availability via
   BGP NEXT_HOP RtrD.

   Assume now that RtrB comes back online.  In only a few seconds OSPF
   neighbor state is been established with RtrA and RtrD and database
   synchronization has occurred.  RtrA now realizes that the best path
   to destination D.1 is via RtrB, and therefore updates it FIB
   appropriately.  RtrA begins to forward packets destined to D.1 to
   RtrB.  Though, because RtrB has yet to establish and synchronization
   it's BGP neighbor relationship and routing information with RtrD,
   RtrB has no knowledge regarding reachibililty of destination D.1, and
   therefore discards the packets received from RtrA.

   If RtrB were to temporarily set it's link costs to 0xFFFF while
   synchronizing with BGP tables with it's neighbors, RtrA would
   continue to use the working RtrA->RtrC->RtrD path.  Upon intial
   synchronization of BGP tables with neighboring router, RtrB would
   generate a new LSA describing the actual link costs associated with
   each connection, and RtrA could again begin using the optimal path
   via RtrB.

   However, if no alternative path were available to destination D.1
   RtrA would still be able to use the path via RtrB, and manipulation
   of the link costs would result in no adverse effect.


3. Deployment Considerations

   Such a mechanism increases overall network availablity and allows
   network operators to alleviate the deterministic blackholing behavoir
   introduced in this scenario.  The IS-IS overload bit has been
   employed in IS-IS routing domains to achieve similar behavior.

   Triggers for setting the link costs as described are left to the
   implementor.  Some potential triggers could include N seconds after
   booting, or N number of BGP prefixes in the BGP Loc-RIB.

   Also, understand that this mechanism assumes actual deployments
   assign substantially lower values for link costs (and the sum of
   subsequent path costs), and that a value of 0xFFFF for an individual
   link within a path would be sufficiently large enough to discourage
   transit traffic from entering the router.





4. Security Considerations

   Security issues are not discussed in this memo.


5. Acknowledgements

   The authors would like to acknowledge John Moy of Sycamore
   Networks for his valuable input.


6. References

      [RFC2328]   Moy, J., "OSPF Version 2", RFC 2328, April 1998.


7. Authors' Address

   Danny McPherson
   Amber Networks
   2465 Augustine Drive
   Santa Clara, CA  95054
   Phone: +1 408.486.6336
   Email: danny@ambernetworks.com
   John Moy
   Sycamore Networks

   Tony Przygienda
   Redback
   350 Holger Way
   San Jose, CA 95134
   Email: prz@redback.com