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Network Working Group                                            M. JorkRequest for Comments: 5443                                       GENBANDCategory: Informational                                         A. Atlas                                                         British Telecom                                                                 L. Fang                                                     Cisco Systems, Inc.                                                              March 2009LDP IGP SynchronizationStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (c) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008. The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Jork, et al.                 Informational                      [Page 1]

RFC 5443                LDP IGP Synchronization               March 2009Abstract   In certain networks, there is dependency on the edge-to-edge Label   Switched Paths (LSPs) setup by the 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 on which LDP is not.  While the link   could still be used for IP forwarding, it is not useful for MPLS   forwarding, for example, MPLS VPN applications or Border Gateway   Protocol (BGP) route-free cores.  This document describes a mechanism   to avoid traffic loss due to this condition without introducing any   protocol changes.Table of Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. Proposed Solution ...............................................33. Applicability ...................................................44. Interaction with TE Tunnels .....................................55. Security Considerations .........................................56. References ......................................................66.1. Normative References .......................................66.2. Informative References .....................................67. Acknowledgments .................................................61.  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 Open Shortest Path   First (OSPF) [RFC2328] 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 all the links along the IP shortest   path from one PE router to the other 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 isJork, et al.                 Informational                      [Page 2]

RFC 5443                LDP IGP Synchronization               March 2009   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 this 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 has not yet     distributed all the label bindings.   The 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 path.1.1.  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].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   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.  In the case of OSPF, this cost is   LSInfinity (16-bit value 0xFFFF), as proposed in [RFC3137].  In the   case of ISIS, the maximum metric value is 2^24-2 (0xFFFFFE).  Indeed,   if a link is configured with 2^24-1 (the maximum link metric per   [RFC5305]), then this link is not advertised in the topology.  It is   important to keep the link in the topology to allow IP traffic to use   the link as a last resort in case of massive failure.Jork, et al.                 Informational                      [Page 3]

RFC 5443                LDP IGP Synchronization               March 2009   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   estimation 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 concerns of the large   variations of the Label Information Base (LIB) table size and   equipment capabilities.  In addition, there is a current work in   progress on LDP End-of-LIB as specified in [End-of-LIB], which   enables the LDP speaker to signal the completion of its initial   advertisement following session establishment.  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   document.   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-Fail-Rec].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 are 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.Jork, et al.                 Informational                      [Page 4]

RFC 5443                LDP IGP Synchronization               March 2009   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 the 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 to 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 that 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   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 a link into the IGP, this becomes a local issue of the   tunnel headend router.5.  Security Considerations   A Denial-of-Service (DoS) attack that brings down LDP service on a   link or prevents it from becoming operational on a link could be one   possible cause of 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 where LDP is not   operational while IGP is.  Assigning the IGP cost to maximum on such   a link should not introduce new security threats.  The operation is   internal to the router to allow LDP and IGP to communicate and react.   Making many LDP links unavailable, however, is a security threat that   can cause dropped traffic due to limited available network capacity.   This may be triggered by operational error or implementation error.   These errors are considered general security issues and implementors   should follow the current best security practice [MPLS-GMPLS-Sec].Jork, et al.                 Informational                      [Page 5]

RFC 5443                LDP IGP Synchronization               March 20096.  References6.1.  Normative References   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate                    Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2328]        Moy, J., "OSPF Version 2", STD 54,RFC 2328, April                    1998.   [RFC5036]        Andersson, L., Ed., Minei, I., Ed., and B. Thomas,                    Ed., "LDP Specification",RFC 5036, October 2007.6.2.  Informative References   [End-of-LIB]     Asati, R., LDP End-of-LIB, Work in Progress, January                    2009.   [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-Rec]   Fang, L., Atlas, A., Chiussi, F., Kompella, K., and                    G. Swallow, "LDP Failure Detection and Recovery",                    IEEE Communications Magazine, Vol.42, No.10, October                    2004.   [MPLS-GMPLS-Sec] Fang. L., Ed., "Security Framework for MPLS and                    GMPLS Networks", Work in Progress, November 2008.   [RFC3137]        Retana, A., Nguyen, L., White, R., Zinin, A., and D.                    McPherson, "OSPF Stub Router Advertisement",RFC3137, June 2001.   [RFC5305]        Li, T. and H. Smit, "IS-IS Extensions for Traffic                    Engineering",RFC 5305, October 2008.7.  Acknowledgments   The authors would like to thank Bruno Decraene for his in-depth   discussion and comments, Dave Ward for his helpful review and input,   and Loa Andersson, Ross Callon, Amanda Baber, Francis Dupont, Donald   Eastlake, Russ Housley, Pasi Eronen, Dan Romascanu, Bin Mo, Lan   Zheng, Bob Thomas, and Dave Ojemann for their reviews and comments.Jork, et al.                 Informational                      [Page 6]

RFC 5443                LDP IGP Synchronization               March 2009Authors' Addresses   Markus Jork   GENBAND   3 Federal St.   Billerica, MA 01821   USA   EMail: Markus.Jork@genband.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   Phone: 1 (978) 936-1633Jork, et al.                 Informational                      [Page 7]