Network Working Group M. Jork Internet Draft NextPoint Networks Category: Informational Alia Atlas Expires:
May 2008June 17, 2009 British Telecom L. Fang Cisco Systems, Inc. November 3,December 17, 2008 LDP IGP Synchronization draft-ietf-mpls-ldp-igp-sync-03.txtdraft-ietf-mpls-ldp-igp-sync-04.txt Status of thisThis Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 ofBCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txthttp://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 18, 2009. Copyright and License Notice Copyright (C) The(c) 2008 IETF Trust (2008).and the persons identified as the document authors. All rights reserved. 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 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract In certain networks there is a dependency on edge-to-edge LSPsLabel Switched Paths (LSPs) setup by LDP, e.g.Label Distribution Protocol (LDP), e.g., networks that are used for MPLS VPNMultiProtocol Label Switching (MPLS) Virtual Private Network (VPN) applications. For such applications it is not possible to rely on IPInternet Protocol (IP) forwarding if the MPLS LSP is not operating appropriately. Blackholing of labeled traffic can occur in situations where the IGPInterior 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 LDP IGP Synchronization November 2008forwarding, for example, MPLS VPN; BGPBorder Gateway Protocol (BGP) route free core; or IP address carried in the packet is out of the RFC1918RFC 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..................................................2Introduction..................................................3 2. Proposed Solution.............................................3 3. Applicability.................................................4Applicability.................................................5 4. Interaction Withwith TE Tunnels...................................5 5. Security Considerations.......................................5Considerations.......................................6 6. IANA Considerations...........................................5Considerations...........................................6 7. Normative References..........................................6 8. Informational References......................................6 9. Author's Addresses............................................6Authors' Addresses............................................7 10. Acknowledgements.............................................8Acknowledgments..............................................7 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]. LDP IGP Synchronization December 2008 1. Introduction LDP [RFC5036][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 OSPF [RFC2328]Open Shortest Path First (OSPF) [RFC 2328] or IS-ISIntermediate system to intermediate system (IS-IS) [ISO.10589.1992], i.e.i.e., all links in the domain MAYhave 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 L2layer 2 (L2) or L3layer 3 (L3) VPN scenario for example, a given PEProvider-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 LDP IGP Synchronization November 2008or 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 can notcannot re-direct traffic to an alternate IGP path. 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. LDP IGP Synchronization December 2008 This has some similarity to the mechanism specified in [RFC3137][RFC 3137] which allows an OSPF router to advertise that it should not be used as a transit router. One difference is that [RFC3137][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 [RFC3137].[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 [RFC3784])[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 LDP IGP Synchronization November 2008condition can notcannot 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 LIBLabel 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 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].[LDP Failure Recovery]. 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. LDP IGP Synchronization November 2008On 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 Withwith 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. 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 is to preventprevents the use of the link to be usedwhen 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 triggedtriggered by operational error or implementation error. They are considered as general Security issues and should follow the current best security practice.practice [MPLS-GMPLS-Security]. 6. IANA Considerations LDP IGP Synchronization November 2008This document has no actions for IANA. 7. Normative References [RFC5036][RFC 5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B. Thomas, "LDP Specification", RFC 5036, October 2007. [RFC2328][RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. 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 [RFC3137][RFC 3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. [RFC3784] Smit, H.,[RFC 5305] Li, T., Smit, H., Intermediate System to Intermediate System (IS-IS) Extension for Traffic Engineering, June 2004.October 2008. 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-Fail][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. Author'sAuthors' Addresses Markus Jork NextPoint Networks 3 Fedral St. Billerica, MA 01821 USA Email: email@example.com Alia Atlas British Telecom Email: firstname.lastname@example.org LDP IGP Synchronization November 2008Luyuan Fang Cisco Systems, Inc. 300 Beaver Brook Road Boxborough, MA 01719 USA Email: email@example.com Full Copyright Statement Copyright (C) The IETF Trust (2008). 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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- firstname.lastname@example.org. LDP IGP Synchronization November 2008Phone: 1 (978) 936-1633 10. AcknowledgementsAcknowledgments 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, thankBruno Decraene for his in depth discussion, comments and helpful suggestions,discussion and comments; thank Dave Ward for his ADhelpful review and input; and thank Loa Andersson, Ross Callon, Amanda Baber, Francis Dupont, Donald Eastlake, Russ Housley, Pasi Eronen, Dan Romascanu, LDP IGP Synchronization December 2008 Bin Mo, Lan Zheng, Bob Thomas, and Dave Ojemann for their review commentsand suggestions.comments.