Internet DRAFT - draft-ietf-mpls-ldp-igp-sync


   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  
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   Copyright (c) 2008 IETF Trust and the persons identified as the 
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   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 
   ( in effect on the date of 
   publication of this document. Please review these documents 
   carefully, as they describe your rights and restrictions with 
   respect to this document. 
   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]. 
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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 
   Alia Atlas 
   British Telecom 
   Luyuan Fang 
   Cisco Systems, Inc. 
   300 Beaver Brook Road 
   Boxborough, MA 01719 
   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]