--- 1/draft-ietf-mpls-ldp-igp-sync-01.txt 2008-07-01 02:12:15.000000000 +0200 +++ 2/draft-ietf-mpls-ldp-igp-sync-02.txt 2008-07-01 02:12:15.000000000 +0200 @@ -1,22 +1,22 @@ Network Working Group M. Jork Internet Draft NextPoint Networks Category: Informational Alia Atlas - Expires: August 2008 British Telecom + Expires: December 2008 British Telecom L. Fang Cisco Systems, Inc. - February 2008 + June 28, 2008 LDP IGP Synchronization - draft-ietf-mpls-ldp-igp-sync-01.txt + draft-ietf-mpls-ldp-igp-sync-02.txt Status of this Memo 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 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that @@ -36,29 +36,24 @@ 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 - 1 - 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 + still be used for IP forwarding, it is not useful for MPLS + forwarding, for example, MPLS VPN; BGP route free core; or 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 @@ -83,94 +78,96 @@ 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 - LDP IGP Synchronization February 2008 - 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. + LDP protocol has currently no way to correct the issue, LDP is not + a routing protocol; it can not 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. 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 + 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]. In the case of ISIS, the max matrix value is 0xFFFFFE + per [RFC 3784]. 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. 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. + exchanged over the session. At the present time, the latter + condition can not 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 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. 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. + 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]. 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- @@ -185,24 +182,20 @@ 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 @@ -230,49 +223,51 @@ 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 [RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. + [RFC 3784] Smit, H., Li, T., Intermediate System to Intermediate + System (IS-IS) Extension for Traffic Engineering, June 2004. + [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. + [LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-asati-mpls-ldp- + end-of-lib-01.txt, November 2007. + 9. Author's Addresses Markus Jork NextPoint Networks 3 Fedral St. Billerica, MA 01821 USA Email: mjork@nextpointnetworks.com Alia Atlas @@ -271,33 +266,29 @@ 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 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 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 @@ -330,24 +321,20 @@ ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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. @@ -357,13 +344,12 @@ 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 + comments, and thank Bruno Decraene for his in depth discussion, + comments and helpful suggestions.