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Network Working Group                                            M. Jork
Internet-Draft                                                  A. Atlas
Expires: April 16, 2005                                            Avici
                                                                 L. Fang
                                                        October 16, 2004

                        LDP IGP Synchronization

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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Copyright Notice

   Copyright (C) The Internet Society (2004).


   In networks depending on edge-to-edge establishment of MPLS
   forwarding paths via LDP, blackholing of traffic can occur in
   situations where the IGP is operational on a link and thus the link
   is used for IP forwarding but LDP is not operational on that link for
   whatever reason.  This document describes a mechanism to avoid

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   traffic loss due to this condition without introducing any protocol

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1.  Motivation

   LDP [RFC3036] establishes MPLS LSPs along the shortest path to a
   destination as determined by IP forwarding.  In a common network
   design, LDP is used to provide label switched paths throughout the
   complete network domain covered by an IGP such as OSPF [RFC2328] or
   IS-IS [ISO.10589.1992], i.e.  all links in the domain have IGP as
   well as LDP adjacencies.

   A variety of services a network provider may want to deploy over an
   LDP enabled network depend on the availability of edge to edge label
   switched paths.  In a L2 or L3 VPN scenario for example, a given PE
   router relies on the availability of a complete MPLS forwarding path
   to the other PE routers for the VPNs it serves.  This means that
   along the IP shortest path from one PE router to the other, all the
   links need to have operational LDP sessions and the necessary label
   binding must have been exchanged over those sessions.  If only one
   link along the IP shortest path is not covered by an LDP session, a
   blackhole exists and services depending on MPLS forwarding will fail.
   This might be a transient or a persistent error condition.  Some of
   the reasons for it could be
   o  a configuration error,
   o  an implementation bug,
   o  the link has just come up and has an IGP adjacency but LDP has
      either not yet established an adjacency or session or distributed
      all the label bindings.

   The LDP protocol itself has currently no means to indicate to a
   service depending on it whether there is an uninterrupted label
   switched path available to the desired destination or not.

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2.  Proposed Solution

   The problem described above exists because LDP is tied to IP
   forwarding decisions but no coupling between the IGP and LDP
   operational state on a given link exists.  If IGP is operational on a
   link but LDP is not, a potential network problem exists.  So the
   solution described by this document is to prevent a link from being
   used for IP forwarding as long as LDP is not fully operational.  This
   has some similarity to the mechanism specified in [RFC3137] which
   allows an OSPF router to advertise that it should not be used as a
   transit router.  One difference is that [RFC3137] raises the link
   costs on all (stub) router links, while the mechanism described in
   here applies on a per-link basis.

   In detail: when LDP is not "fully operational" (see below) on a given
   link, the IGP will advertise the link with maximum cost to avoid any
   transit traffic over it if possible.  In the case of OSPF this cost
   is LSInfinity (16-bit value 0xFFFF) as proposed in [RFC3137].  Note
   that the link is not just simply removed from the topology because
   LDP depends on the IP reachability to establish its adjacency and
   session.  Also, if there is no other link in the network to reach a
   particular destination, no additional harm is done by making this
   link available for IP forwarding at maximum cost.

   LDP is considered fully operational on a link when an LDP hello
   adjacency exists on it, an LDP session is established for it and all
   label bindings have been exchanged over the session.  The latter
   condition can not generally be verified by a router and some
   heuristics may have to be used.  A simple implementation strategy is
   to wait some time after LDP session establishment before declaring
   LDP fully operational in order to allow for the exchange of label
   bindings.  This is typically sufficient to deal with the link when it
   is being brought up.  LDP protocol extensions to indicate the
   complete transmission of all currently available label bindings after
   a session has come up are conceivable but not addressed in this

   The mechanism described in this document does not entail any protocol
   changes and is a local implementation issue.  However, it is
   recommended that both sides of a link implement this mechanism to be
   effective and to avoid asymmetric link costs which could cause
   problems with IP multicast forwarding.

   The problem space and solution specified in this document have also
   been discussed in an IEEE Communications Magazine paper [LDP-Fail].

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3.  Applicability

   Example network scenarios that benefit from the mechanism described
   in here are MPLS VPNs and BGP-free core network designs where traffic
   can only be forwarded through the core when LDP forwarding state is
   available throughout.

   In general, the proposed procedure is applicable in networks where
   the availability of LDP signaled MPLS LSPs and avoidance of
   blackholes for MPLS traffic is more important than always choosing an
   optimal path for IP forwarded traffic.  Note however that non-optimal
   IP forwarding only occurs for a short time after a link comes up or
   when there is a genuine problem on a link.  In the latter case an
   implementation should issue network management alerts to report the
   error condition and enable the operator to address it.

   The usefulness of this mechanism also depends on the availability of
   alternate paths with sufficient bandwidth in the network should one
   link get costed out due to unavailability of LDP service over it.

   On broadcast links with more than one IGP/LDP peer, the cost-out
   procedure can only be applied to the link as a whole and not an
   individual peer.  So a policy decision has to be made whether the
   unavailability of LDP service to one peer should result in the
   traffic being diverted away from all the peers on the link.

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4.  Security Considerations

   A DoS attack that brings down LDP service on a link or prevents it
   from becoming operational on a link will now additionally cause
   non-optimal IP forwarding within the network.  However, as discussed
   above this is considered beneficial as it prevents MPLS traffic from
   being dropped.

5  References

   [RFC3036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
              B. Thomas, "LDP Specification", RFC 3036, January 2001.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A. and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 3137,
              June 2001.

              International Organization for Standardization,
              "Intermediate system to intermediate system
              intra-domain-routing routine information exchange protocol
              for use in conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO
              Standard 10589, 1992.

              Fang, L., Atlas, A., Chiussi, F., Kompella, K. and G.
              Swallow, "LDP Failure Detection and Recovery", IEEE
              Communications Vol.42 No.10, October 2004.

Authors' Addresses

   Markus Jork
   Avici Systems, Inc.
   101 Billerica Ave
   North Billerica, MA  01862

   Phone: +1 978 964 2142
   EMail: mjork@avici.com

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   Alia Atlas
   Avici Systems, Inc.
   101 Billerica Ave
   North Billerica, MA  01862

   Phone: +1 978 964 2070
   EMail: aatlas@avici.com

   Luyuan Fang
   Room C2-3B35
   200 Laurel Avenue
   Middletown, NJ  07748

   Phone: +1 732 420 1921
   EMail: luyuanfang@att.com

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