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Versions: 00 01 02 03 04 05 06 07 RFC 3883

Network Working Group                                Sira Panduranga Rao
Internet Draft                                                       UTA
Expiration Date: July 2001                                    Alex Zinin
File name: draft-ietf-ospf-dc-00.txt                       Cisco Systems

                                                           November 2000

         Detecting Inactive Neighbors over OSPF Demand Circuits
                       draft-ietf-ospf-dc-00.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
   introducing three mechanisms---Hello probes, limitation of the number
   of LSA retransmits and flushing of self-originated LSAs.

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



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INTERNET DRAFT    OSPF DC Inactive Neighbor Detection       October 2000


   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 prob-
   lems such as this, since Hello exchange is suppressed on demand cir-
   cuits.  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 for-
   warded to the OSPF-incapable router.

   This memo describes two techniques that solve the described problem.
   First, a neighbor probing mechanism using Hellos is introduced, and
   second, the number of LSA retransmit attempts on demand circuits is
   limited.  We also encourage flushing of self-originated LSAs when the
   OSPF process is going down.

2. Proposed Solution

   The first part of the solution this document proposes makes use of
   Hellos to detect whether the OSPF process is operational on the
   remote neighbor.  We call this process "Hello probing". The idea
   behind this technique is to allow either of the two neighbors con-
   nected over a demand circuit to test the remote neighbor at any time
   (see Section 2.1.2). The routers across the demand circuit can be
   connected by either a point-to-point link, or 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]).  Since Hellos are
   suppressed on demand circuit interfaces, the local router must make
   sure the remote router supports Hello probing before testing it. Oth-
   erwise the remote router may be mistakenly declared inoperational. To
   accomplish this, we introduce a new capability bit that is exchanged
   in DBD packets (see Section 2.1.1).

   The Hello 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 probe each other any time the link is up (could be imple-
   mented as a configurable option) with the caution that OSPF Hello
   packets are not considered as interesting traffic and do not cause



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   the demand circuit to remain up.

   The case when one or more of the router's links are oversubscribed
   (see section 7 of [RFC1793]) should be considered by the implementa-
   tions. In such a situation even if the link status is up and applica-
   tion data 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, Hello probing should be restricted
   to those neighbors that are actually reachable (i.e., there is a cir-
   cuit established with the neighbor at the moment the probing pro-
   cedure needs to be initiated). This check itself is considered an
   implementation detail.

   The second part of the solution is limiting the number of times LSAs
   can be retransmitted over a demand circuit. See Section 2.2 for more
   details.

   The third part of the solution is flushing of self-originated LSAs
   whenever the OSPF process on a router is going down.

   Hello probing and LSA retransmission limit may be used together or
   alone.  This memo does not dictate which one and how many of them
   must be implemented, but only provides mechanisms to solve the
   described problem. This memo, however, recommends to flush some
   locally originated LSAs when possible when OSPF process is going
   down.


 2.1 Hello Probing

   The Hello probing mechanism allows routers connected over a demand
   circuit to test each other's OSPF capabilities. In order to do so,
   both routers need to support this functionality, otherwise opera-
   tional routers may mistakenly be declared unreachable. We insure this
   by introducing a new capability bit in the Extended Options TLV
   announced in the link-local signaling (LLS) data block of DBD packets
   (see [LLS] for more information on LLS).

   We also use the same bit in Hello packets as a Hello reply request
   (RR) flag.  This helps avoid racing conditions when a Hello sent in
   reply causes another reply to be sent, and so on. When a router needs
   to probe its neighbor, it sends a Hello with the RR bit set. The
   receiving side sends a Hello packet in reply with RR bit clear.

  2.1.1 Extended Options TLV

   The Extended Options TLV is a part of LLS specification (see [LLS])
   and is announced in both Hello and DBD packets.



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   A new bit is introduced in the Value field of this TLV as shown in
   Figure 1. The value of the bit is 0x00000004.

      +---+---+---+---+---+---+---+- -+---+---+---+---+-----+---+---+
      | * | * | * | * | * | * | * |...| * | * | * | * |HP/RR| RS| OR|
      +---+---+---+---+---+---+---+- -+---+---+---+---+-----+---+---+

                  Figure 1. Bits in Extended Options TLV

   When used in DBD packets, the new bit indicates router's Hello Prob-
   ing capability and is called the HP-bit.  When used in Hello packets,
   the new bit means that a Hello must be sent in reply and is called
   the Reply Request (RR) bit.

   Routers supporting Hello probing must always set the HP bit in their
   DBD packets.

   For description of RS and OR bits, see [HELLO] and [OOB] correspond-
   ingly.

  2.1.2 Hello Probing Procedure

   OSPF routers are allowed to perform Hello probing at any time. How-
   ever, it is not recommended to do so when the link is down, because,
   in its one extreme, it will keep the demand circuit up or bouncing,
   or, in its other extreme, it may cause a neighbor to be mistakenly
   declared unreachable.

   It is recommended that both sides perform Hello probing whenever the
   demand circuit goes up, and periodically if the circuit stays in the
   active state. Note however that care must be taken not to let OSPF
   Hello probes keep the circuit in the active state without any appli-
   cation traffic going through it.

   When a router needs to probe a neighbor, it should start its Hello
   and Dead timers and send Hello packets with the RR-bit set. If asso-
   ciated interface is point-to-multipoint, it is recommended to account
   for neighbor-specific timers and send Hello probes as IP unicasts.
   On the receiving side, when a packet with the RR-bit set is received,
   the router should immediately reply with a unicast Hello packet
   without setting the RR-bit. Unicast Hello limits the scope of Hello
   probing.

   The described procedure makes it possible for the sides to probe
   their corresponding neighbors asynchronously and without coordina-
   tion.





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 2.2 Limiting Number of LSA Retransmissions

   An alternative method (that can be used together with Hello probing)
   to identify OSPF-incapable neighbors is to limit the amount of LSA
   retransmits over a demand circuit. The router should count the number
   of retransmit attempts for each neighbor. When an LSA is acknowledged
   by the neighbor, the router should zero the counter. When the counter
   reaches a predefined (or configured) value, a KillNbr event should be
   generated for the neighbor experiencing the problem.

   Note that this method does not require cooperation of the routers on
   both sides of a demand circuit and can be used with already installed
   OSPF routers without requiring them to be upgraded with new software.

 2.3 OSPF Process Shutdown and Flushing of LSAs

   It is recommended for an OSPF process to flush its self-originated
   LSAs when the OSPF process is going down. This way the router informs
   all other routers in the area that they should not consider it tran-
   sit any more and should look for alternative routes.

   Care must be taken not to introduce instability in the network by
   flushing all LSAs. It is acceptable to flush only the self-originated
   router-LSA in the appropriate area and let other LSAs age out.

   Note that there can happen situations where the router cannot reli-
   ably flush its LSAs within reasonable time frame. This could be due
   to the loss of the packets, the demand circuit being down or the
   delay in establishing a path to the neighbor. A situation highlight-
   ing this problem is when the router is oversubscribed (see Section 7
   of [RFC1793]) and thus cannot communicate the news to its neighbors.

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

   Backward compatibility of the Hello probing mechanism is insured by
   introducing the HP bit in the Extended Options TLV.

   Limiting the number of LSA retransmission is a backward-compatible
   technique by its nature.



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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
   (hello packets) being transmitted due to Hello 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.

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

[LLS] Zinin, Friedman, Roy, Nguyen, Yeung, "OSPF Link-local Signaling",
     draft-ietf-ospf-lls-00.txt, Work in progress.

[HELLO]
      Zinin, Roy, Nguyen, "OSPF Restart Signaling", draft-ietf-ospf-
     restart-00.txt, Work in progress.

[OOB] Zinin, Roy, Nguyen, "OSPF Out-of-band LSDB resynchronization",
     draft-ietf-ospf-oob-resync-00.txt, Work in progress.

8. Authors' addresses

   Sira Panduranga Rao
   The University of Texas at Arlington
   Arlington, TX 76013
   Email: siraprao@hotmail.com

   Alex Zinin
   Cisco Systems
   150 West Tasman Dr.
   San Jose, CA 95134
   Email: azinin@cisco.com





Rao, Zinin                                                      [Page 6]


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