Network Working Group                                            M. Jork
Internet-Draft                                                Reef Point
Expires: March 9, 2008                                          A. Atlas
                                                                 L. Fang
                                                     Cisco Systems, Inc.
                                                       September 6, 2007

                        LDP IGP Synchronization

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   Copyright (C) The IETF Trust (2007).

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   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
   traffic loss due to this condition without introducing any protocol

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1.  Introduction

   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, a suitable associated LDP session (matching
   the LDP Identifier of the hello adjacency) is established to the peer
   at the other end of the link 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 document.

   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.  Interaction With TE Tunnels

   In some networks, LDP is used in conjunction with RSVP-TE which sets
   up traffic-engineered tunnels.  The path computation for the TE
   tunnels is based on the TE link cost which is flooded by the IGP in
   addition to the regular IP link cost.  The mechanism described in
   this document should only be applied to the IP link cost to prevent
   any unnecessary TE tunnel reroutes.

   In order to establish LDP LSPs across a TE tunnel, a targeted LDP
   session between the tunnel endpoints needs to exist.  This presents a
   problem very similar to the case of a regular LDP session over a link
   (the case discussed so far): when the TE tunnel is used for IP
   forwarding, the targeted LDP session needs to be operational to avoid
   LDP connectivity problems.  Again, raising the IP cost of the tunnel
   while there is no operational LDP session will solve the problem.
   When there is no IGP adjacency over the tunnel and the tunnel is not
   advertised as link into the IGP, this becomes a local issue of the
   tunnel headend router.

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5.  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.

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6.  IANA Considerations

   This document has no actions for IANA.

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7.  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.

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Authors' Addresses

   Markus Jork
   Reef Point Systems
   8 New England Executive Park
   Burlington, MA  01803

   Phone: +1 781 359 5071

   Alia Atlas
   Google, Inc.
   One Broadway, 7th Floor
   Cambridge, MA  02142


   Luyuan Fang
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA  01719


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