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Versions: 00 01 02                                                      
Network Working Group                                      Eric C. Rosen
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: March 2005

                                                          September 2004

  Detecting and Reacting to Failures of the Full Mesh in IPLS and VPLS


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   Certain L2VPN architectures [IPLS, VPLS] rely on there being a full
   mesh of pseudowires [PWE3-ARCH] among a set of entities.  This mesh
   is used to provide a "LAN-like" service among the entities.  If one
   or more of these pseudowires is absent, so that there is not really a
   full mesh, various higher layers (from routing to bridge control
   protocols) that expect a LAN-like service may fail to work as
   expected.  Therefore it is desirable to have procedures that enable
   the pseudowire endpoints to determine automatically whether there is
   really a full mesh or not.  It is also desirable to have procedures
   that cause the L2VPNs to adapt to pseudowire failures.  This document
   proposes a set of procedures to meet these goals.  Detailed protocol
   encodings are not present, but will be added in future versions.

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    1        Introduction  .........................................   2
    2        Detection of Partially Connected EEs  .................   4
    3        Actions Taken Upon Detection  .........................   5
    4        Normative References  .................................   7
    5        Informative References  ...............................   7
    6        Author's Information  .................................   7
    7        Intellectual Property Statement  ......................   7
    8        Full Copyright Statement  .............................   8

1. Introduction

   IPLS [IPLS] interconnects a set of CEs.  With respect to a particular
   IPLS instance and a particular PE supporting that IPLS instance, the
   set of CEs can be divided into the PE's "local CEs" and the PE's
   "remote CEs".  The local CEs are directly attached to the PE.
   ("Directly attached" means attached via an "Attachment Circuit" in
   the sense of [L2VPN-Framework].)  The PE must ensure that each of its
   local CEs is bound, by a Pseudowire (PW), to each of the remote CEs.
   When this condition holds for all the PEs supporting a given IPLS
   instance, we say that the IPLS instance is fully meshed.

   VPLS [VPLS} interconnects a set of "VPLS Forwarders" [L2VPN-
   FRAMEWORK], which are virtual entities inside PEs; for a given VPLS
   instance, there is one VPLS Forwarder in a given PE.  Some of these
   are considered "spokes", and some are considered "hubs".  In a given
   VPLS instance, there must be a PW binding every hub VPLS Forwarder to
   every other hub VPLS Forwarder; this means that every hub PE in the
   VPLS instance must have a PW to every other hub PE in the VPLS
   instance.  When this condition holds, we say that the VPLS instance
   is fully meshed.

   We will use the term "LS" to mean "IPLS or VPLS".

   In each LS instance, there is a set of "endpoint entities" (EEs).  In
   VPLS, the EEs are hub VPLS Forwarders inside the PEs, in IPLS the EEs
   are CEs.  In either case, we say say that the LS instance is "fully
   meshed" if every pair of EEs which are not local to the same PE are
   bound together by a PW.

   (For present purposes, it does not matter whether two EEs are bound

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   by a single bidirectional point-to-point PW or by a pair of
   unidirectional point-to-multipoint PWs.)

   It is possible that a given LS instance may fail to be fully meshed.
   This may happen for the following reasons:

     - Configuration errors.

     - Failure of the auto-discovery process.

     - Failure of the control plane to properly establish all the
       necessary PWs.  This in turn may be due to bugs, or to resource
       shortages at the PEs.

     - Failure of the data plane to carry traffic correctly on all the
       established PWs.  This can occur if there are bugs in the
       encapsulation/decapsulation procedures at the PEs, or bugs in the
       forwarding procedures at intermediate nodes (especially in
       technologies where the data and control planes are decoupled.

   When an LS instance is not fully meshed, we will say that one or more
   of its EEs are "partially connected".  An EE is regarded as
   "partially connected" at a particular time if one of the following
   conditions holds:

     - PW not established: at that time, some PW binding that EE to
       another EE has not been properly established, as determined by
       the PW control plane.

     - PW not operational:  at that time, although the control plane
       indicates that all the PWs binding other EEs to the given EE are
       properly established, one or more of those PW is incapable of
       passing data to the given EE for some reason.  Note that
       "operational" status is a unidirectional attribute.

   If an LS instance is not fully meshed, then it will not be able to
   provide the "LAN-like" service on which its users are depending.  For
   instance, if a link state routing algorithm is using its LAN
   procedures over an LS instance which is not fully meshed, the
   selected set of routes may have "black holes".

   It is desirable therefore to have procedures which will automatically
   identify any partially connected EEs.  This document proposes a set
   of procedures to meet these goals.  Detailed protocol encodings are
   not present, but will be added in future versions if the WG has
   interest in proceeding in this direction.

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2. Detection of Partially Connected EEs

   Each PE in a particular LS instance must have some sort of control
   plane relationship with each of the other PEs in the same LS
   instance.  (For the time being we ignore the situation in which PWs
   are spliced together; this concepts discussed here are readily
   extended to that case.)

   There must be a status message, which we call the "Mesh Status"
   message, which a PE sends to each of the other PEs in the same LS
   instance.  The Mesh Status message identifies the LS instance (by its
   globally unique VPN identifier, for example), and lists the set of EE
   pairs for which the originating PE has operational PWs.  This message
   would need to be resent whenever the list changes.  As long as the
   control protocol can reliably transport control messages, this
   message would not have to be sent unless there is a change; in fact,
   only changes would need to be sent.  (However, this would require two
   variants of the Mesh Status message: an "Add" and a "Remove".)  A
   PE's Mesh Status messages should also indicate which of the EEs are
   locally attached to that PE.

   Thus every PE in an LS instance maintains the Mesh Status of every
   other PE supporting that same LS instance.

   When the control connection to a particular remote PE is lost, the
   Mesh Status of the remote PE is flushed, and no longer considered for
   the purposes of Partially Connected EE Detection.

   By including a pair of EEs in its Mesh Status messages, a PE is
   stating that there is an OPERATIONAL PW binding the two EEs together,
   not merely an established PW.  Each PE is responsible for determining
   whether each of its local PWs is operational in the outgoing
   direction.  This may require the use of some sort of per-PW test of
   the data plane. It is advisable to construct the test for operational
   status so as to avoid the possibility of flapping, perhaps by not
   allowing a non-operational PW to return to operational status in less
   than a specified time period.  The test for operational status should
   also ensure that a PW is not declared non-operational due to ordinary
   network conditions, such as occasional packet loss, and that a PW is
   not declared non-operational due to routing transients.

   It is understood that it is much easier to lay down such requirements
   than it is to devise procedures to meet them.  The specification of
   such procedures however is outside the scope of the current document.

   When a PE in a particular LS instance has received a Mesh Status
   message from every other PE (that it knows about) in that instance,
   it can compute the set {EE} of all the EEs in the LS instance.  This

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   is the union of the set of EEs mentioned in all the Mesh Status

   The IPLS or VPLS instance is fully meshed if and only if the
   following condition holds:

       For every PE p and every EE e, either e is one of p's local EEs,
       or p reports an operational PW from each of its local EEs to e.

   If this condition doesn't hold, there are one or more Partially
   Connected PWs .  The set of Partially Connected EEs is defined as

       An EE e is "Partially Connected" if and only if there is some PE
       p such that e is not locally attached to p, and p has a locally
       attached EE e' such that there is either no operational PW from e
       to e' or there is no operational PW from e' to e.

   If the configuration and/or auto-discovery procedures identify a set
   of EEs whose local PE just happens to be down (or otherwise
   unreachable), no PEs will have operational PWs for any of those EEs,
   and the above procedures will not result in the determination that
   there are any Partially Connected EEs. However, misconfigurations or
   auto-discovery problems which cause different PEs to learn about
   different sets of EEs will result in the detection of Partially
   Connected EEs.

3. Actions Taken Upon Detection

   Upon identification of a Partially Connected EE, an alarm should be
   raised so that the network operators are aware of the situation.

   In general, the LS service will not function properly if there are
   Partially Connected EEs.  It can however be made to function properly
   if the Partially Connected EEs are removed from service entirely,
   until such time as they becomes fully connected.  In effect, once the
   problematic EEs are removed from the mesh entirely, the LS service is
   once again fully meshed, though with fewer EEs.  Any users who
   connect via the removed EEs will of experience degraded service, if
   not complete loss of service, but other users may continue to receive

   If a PE determines that one of its locally attached EEs is Partially
   Connected, it should remove that EE from service.  In the case of
   VPLS, this means that an Emulated LAN interface [L2VPN-Framework] is
   brought down.  In the case of IPLS, this means that the Attachment
   Circuit to a particular set of CEs is brought down.  PWs which are

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   bound to the Emulated LAN interface or Attachment Circuit should NOT
   be disestablished and the testing of the data plane of such PWs
   should continue.

   If a PE determines that a remote EE is Partially Connected, the PE
   will cease to send or receive data to or from that EE.  The
   corresponding PWs should NOT be disestablished, and the testing of
   the data plane of such PWs should continue.

   There may be methods of returning the LS service to a full mesh which
   do not require removing a Partially Connected EE from service
   entirely.  For example, in VPLS it may be possible to change a
   Partially Connected EE from a hub to a spoke, thereby removing it
   from the mesh without bringing it out of service.  [HUB-TO-SPOKE]

   If, at some later time, an EE ceases to be Partially Connected,
   normal operations can resume.

   It must be understood that when an EE first becomes known, there will
   be a period of time during which PEs are trying to bring up PWs to
   it.  From the time the first PW to/from it becomes operational to the
   time the last PW to/from it becomes operational, the EE will be
   detected as Partially Connected. As this is a normal transient, there
   should be a specified period of time during which a newly discovered
   EE may be Partially Connected before any action is taken.
   Determination that a previously known EE has become Partially
   Connected should cause immediate actions, however.

   If a PE detects that one of its PWs has ceased to be operational, the
   remote EE does not necessarily get treated immediately as being
   Partially Connected.  Before declaring the EE to be Partially
   Connected, the PE should wait a period of time to see if that EE
   disappears from the Mesh Status messages generated by all the other
   PEs.  After all, a very likely cause for a PW to become non-
   operational is for the remote PE to fail or to become unreachable.
   As this will no result in a partial mesh, no special action needs to
   be take.

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4. Normative References

   [IPLS] "IP-only LAN Service (IPLS)", H. Shah, K. Arvind, E. Rosen, F.
   Le Faucheur, G. Heron, V. Radoaca, draft-ietf-l2vpn-ipls-00.txt,
   November 2003

   [L2VPN-FRAMEWORK] "L2VPN Framework", L. Andersson, E. Rosen, editors,
   draft-ietf-l2vpn-l2-framework-05.txt, June 2004

   [PWE3-ARCH] "PWE3 Architecture", S. Bryant, P.Pate, editors, draft-
   ietf-pwe3-arch-07.txt, March 2004

   [VPLS] "Virtual Private LAN Services over MPLS", M. Lasserre, V.
   Kompella, et. al., draft-ietf-l2vpn-vpls-ldp-05.txt, September 2004

5. Informative References

   [HUB-TO-SPOKE] as suggested by Vach Kompella on the L2VPN mailing

6. Author's Information

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA, 01719

   E-mail: erosen@cisco.com

7. Intellectual Property Statement

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   Copies of IPR disclosures made to the IETF Secretariat and any

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   assurances of licenses to be made available, or the result of an
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8. Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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