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Versions: (draft-ietf-mpls-igp-sync) 00 01 02 03           Informational
          04 rfc5443                                                    
   Network Working Group                                     M. Jork
   Internet Draft                                 NextPoint Networks
   Category: Informational                                Alia Atlas
   Expires: June 17, 2009                            British Telecom
                                                             L. Fang
                                                 Cisco Systems, Inc.

                                                   December 17, 2008

                          LDP IGP Synchronization

   Status of This Memo

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   Copyright and License Notice

   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   M. Jork, A. Atlas, and L. Fang                             [Page 1]

   LDP IGP Synchronization                             December 2008

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
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   In certain networks there is a dependency on edge-to-edge Label
   Switched Paths (LSPs) setup by Label Distribution Protocol (LDP),
   e.g., networks that are used for MultiProtocol Label Switching
   (MPLS) Virtual Private Network (VPN) applications. For such
   applications it is not possible to rely on Internet Protocol (IP)
   forwarding if the MPLS LSP is not operating appropriately.
   Blackholing of labeled traffic can occur in situations where the
   Interior Gateway Protocol (IGP) is operational on a link but LDP is
   not operational on that link. While the link could still be used for
   IP forwarding, it is not useful for MPLS forwarding, for example,
   MPLS VPN; Border Gateway Protocol (BGP) route free core; or IP
   address carried in the packet is out of the RFC 1918 [RFC 1918]
   space. This document describes a mechanism to avoid traffic loss due
   to this condition without introducing any protocol changes.

Table of Contents

   1. Introduction..................................................3
   2. Proposed Solution.............................................3
   3. Applicability.................................................5
   4. Interaction with TE Tunnels...................................5
   5. Security Considerations.......................................6
   6. IANA Considerations...........................................6
   7. Normative References..........................................6
   8. Informational References......................................6
   9. Authors' Addresses............................................7
   10. Acknowledgments..............................................7

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC2119 [RFC

   M. Jork, Alia Atlas, and L. Fang                           [Page 2]

   LDP IGP Synchronization                             December 2008

1. Introduction

   LDP [RFC 5036] 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 Open Shortest
   Path First (OSPF) [RFC 2328] or Intermediate system to intermediate
   system (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 layer 2 (L2) or layer 3 (L3) VPN
   scenario for example, a given Provider-Edge(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

     - A configuration error

     - An implementation bug

     - 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.

   LDP protocol has currently no way to correct the issue, LDP is not
   a routing protocol; it cannot re-direct traffic to an alternate IGP

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 discourage a link from
   being used for IP forwarding as long as LDP is not fully

   M. Jork, Alia Atlas, and L. Fang                           [Page 3]

   LDP IGP Synchronization                             December 2008

   This has some similarity to the mechanism specified in [RFC 3137]
   which allows an OSPF router to advertise that it should not be used
   as a transit router.  One difference is that [RFC 3137] 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
   [RFC 3137]. In the case of ISIS, the max metric value is 2^24-2
   (0xFFFFFE). Indeed, if a link is configured with 2^24-1 (the
   maximum link metric per [RFC 5305]) then this link is not
   advertised in the topology. It is important to keep the link in the
   topology to allow for IP traffic to use the link as a last resort
   in case of massive failure.

   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. At the present time, the latter
   condition cannot generally be verified by a router and some
   estimated may have to be used. A simple implementation strategy is
   to use a configurable hold down timer to allow LDP session
   establishment before declaring LDP fully operational. The default
   timer is not defined in this document due to the concerns of the
   large variations of the Label Information Base (LIB) table size and
   the equipment capabilities. In addition, this is a current work in
   progress on LDP End-of-LIB as specified in [LDP End-of-LIB], it
   enables the LDP speaker to signal the completion of its initial
   advertisement following session establish. When LDP End-of-LIB is
   implemented, the configurable hold down timer is no longer needed.
   The neighbor LDP session is considered fully operational when the
   End-of-LIB notification message is received.

   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.

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

   M. Jork, Alia Atlas, and L. Fang                           [Page 4]

   LDP IGP Synchronization                             December 2008

3.  Applicability

   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.

   Example network scenarios that benefit from the mechanism described
   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.

   The usefulness of this mechanism also depends on the availability
   of alternate paths with sufficient bandwidth in the network should
   one link be assigned to the maximum cost 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.

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.

   M. Jork, Alia Atlas, and L. Fang                           [Page 5]

   LDP IGP Synchronization                             December 2008

5. Security Considerations

   A DoS attack that brings down LDP service on a link or prevents it
   from becoming operational on a link could be one of the
   possibilities that causes LDP related traffic blackholing. This
   document does not address how to prevent LDP session failure. The
   mechanism described here prevents the use of the link when LDP is
   not operational while IGP is. Assigning the IGP cost to maximum on
   the link where LDP is failed and IGP is not should not introduce
   new security threats. The operation is internal in the router to
   allow LDP and IGP to communicate and react. Making many LDP links
   unavailable, however, is a security threat which can cause traffic
   being dropped due to limited available network capacity. This may
   be triggered by operational error or implementation error. They are
   considered as general Security issues and should follow the current
   best security practice [MPLS-GMPLS-Security].

6. IANA Considerations

   This document has no actions for IANA.

7. Normative References

   [RFC 5036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
   and B. Thomas, "LDP Specification", RFC 5036, October 2007.

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

8. Informational References

   [RFC 1918] Rekhter, Y., "Address Allocation for Private Internets",
   BCP: 5, RFC 1918, February 1996.

   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997

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

   [RFC 5305] Li, T., Smit, H., Intermediate System to Intermediate
   System (IS-IS) Extension for Traffic Engineering, October 2008.

   M. Jork, Alia Atlas, and L. Fang                           [Page 6]

   LDP IGP Synchronization                             December 2008

   [ISO.10589.1992]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.

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

   [LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-ietf-mpls-ldp-
   end-of-lib-01.txt, work in progress, September 2008.

   [MPLS-GMPLS-Security] Fang. L., Ed., "Security Framework for MPLS
   and GMPLS Networks", draft-ietf-mpls-mpls-and-gmpls-security-
   framework-04.txt, work in progress, November 2008.

9. Authors' Addresses

   Markus Jork
   NextPoint Networks
   3 Fedral St.
   Billerica, MA 01821
   Email: mjork@nextpointnetworks.com

   Alia Atlas
   British Telecom
   Email: alia.atlas@bt.com

   Luyuan Fang
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA 01719
   Email: lufang@cisco.com
   Phone: 1 (978) 936-1633

10.     Acknowledgments

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

   The authors would like to thank Bruno Decraene for his in depth
   discussion and comments; thank Dave Ward for his helpful review and
   input; and thank Loa Andersson, Ross Callon, Amanda Baber, Francis
   Dupont, Donald Eastlake, Russ Housley, Pasi Eronen, Dan Romascanu,

   M. Jork, Alia Atlas, and L. Fang                           [Page 7]

   LDP IGP Synchronization                             December 2008

   Bin Mo, Lan Zheng, Bob Thomas, and Dave Ojemann for their review
   and comments.

   M. Jork, Alia Atlas, and L. Fang                           [Page 8]