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Versions: (draft-ietf-mpls-igp-sync) 00 01 02 03 04 RFC 5443

   Network Working Group                                     M. Jork
   Internet Draft                                 NextPoint Networks
   Category: Informational                                Alia Atlas
   Expires: August 2008                              British Telecom
                                                             L. Fang
                                                 Cisco Systems, Inc.



                                                       February 2008


                          LDP IGP Synchronization
                      draft-ietf-mpls-ldp-igp-sync-01.txt

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Copyright Notice
   Copyright (C) The IETF Trust (2008).


Abstract

   In certain networks there is a dependency on edge-to-edge LSPs setup
   by LDP, e.g. networks that are used for MPLS VPN applications. For
   such applications it is not possible to rely on IP forwarding if the
   MPLS LSP is not operating appropriately. Blackholing of labeled

   M. Jork, A. Atlas, and L. Fang
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   LDP IGP Synchronization                             February 2008

   traffic can occur in situations where the 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 traffic with
   packets carrying a label stack of more than one label or when the IP
   address carried in the packet is out of the RFC1918 space. This
   document describes a mechanism to avoid traffic loss due to this
   condition without introducing any protocol changes.



Table of Contents

   1. Introduction..................................................2
   2. Proposed Solution.............................................3
   3. Applicability.................................................4
   4. Interaction With TE Tunnels...................................5
   5. Security Considerations.......................................5
   6. IANA Considerations...........................................5
   7. Normative References..........................................6
   8. Informational References......................................6
   9. Author's Addresses............................................6
   10.  Acknowledgements............................................8


Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC2119 [RFC
   2119].


1. Introduction

   LDP [RFC5036] 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.

   M. Jork, Alia Atlas, and L. Fang                                2

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   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.

   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.


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
   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.

   M. Jork, Alia Atlas, and L. Fang                                3

   LDP IGP Synchronization                             February 2008


   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].


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

   M. Jork, Alia Atlas, and L. Fang                                4

   LDP IGP Synchronization                             February 2008

   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.

5. Security Considerations

   A DoS attack 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 is to prevent the link to be used 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 trigged by
   operational error or implementation error. They are considered as
   general Security issues and should follow the current best security
   practice.


6. IANA Considerations

   This document has no actions for IANA.



   M. Jork, Alia Atlas, and L. Fang                                5

   LDP IGP Synchronization                             February 2008

7. Normative References

   [RFC5036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
   and B. Thomas, "LDP Specification", RFC 5036, October 2007.

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


8. Informational References

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

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

   [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-Fail] 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.


9. Author's Addresses

   Markus Jork
   NextPoint Networks
   3 Fedral St.
   Billerica, MA 01821
   USA
   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
   USA
   Email: lufang@cisco.com


Intellectual Property

   M. Jork, Alia Atlas, and L. Fang                                6

   LDP IGP Synchronization                             February 2008


   The IETF takes no position regarding the validity or scope of any
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   Information on the procedures with respect to rights in RFC
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   of such proprietary rights by implementers or users of this
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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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   This document and the information contained herein are provided on
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Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that

   M. Jork, Alia Atlas, and L. Fang                                7

   LDP IGP Synchronization                             February 2008

   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.



10.     Acknowledgements

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

   The authors would like to thank Loa Andersson for his review and
   comments.
























   M. Jork, Alia Atlas, and L. Fang                                8


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