MPLS Working Group S. Kini (Ed) Internet Draft W. Lu (Ed) Updates: 5443 (if approved) Ericsson Intended Status: Informational October 25, 2010 Expires: April 2011 LDP IGP Synchronization for broadcast networks draft-ietf-mpls-ldp-igp-sync-bcast-05.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 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." 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Kini & Lu Expires April 28, 2011 [Page 1] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 Abstract RFC 5443 describes a mechanism to achieve LDP IGP Synchronization to prevent black-holing traffic (e.g. VPN) when an interior gateway protocol (IGP) is operational on a link but Label Distribution Protocol (LDP) is not. If this mechanism is applied to broadcast links that have more than one LDP/IGP peer, the metric increase procedure can only be applied to the link as a whole but not an individual peer. When a new LDP peer comes up on a broadcast network, this can result in loss of traffic through other established peers on that network. This document describes a mechanism to address that use-case without dropping traffic. The mechanism does not introduce any protocol message changes. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions used in this document . . . . . . . . . . . . . . . 3 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3 4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 10.2. Informative References . . . . . . . . . . . . . . . . . 7 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Computation of 'cut-edge' . . . . . . . . . . . . . . 9 Appendix B. Sync without support at one end . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Kini & Lu Expires April 28, 2011 [Page 2] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 1. Introduction In RFC 5443 ([LDP-IGP-SYNC]), when [LDP] is not fully operational on a link, the IGP advertises the link with maximum cost to avoid any transit traffic on the link if possible. When LDP becomes operational i.e., all the label bindings have been exchanged, the link is advertised with its correct cost. This tries to ensure that all along the IGP shortest path, the LDP LSP is available. The mechanisms in [LDP-IGP-SYNC] have limitations when applied to a broadcast link. These are described in section 3. A solution is defined in section 4. 2. 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]. 3. Problem Statement On broadcast networks, a router's link-state advertisement (LSA) contains a single cost to the broadcast network, rather than a separate cost to each peer on the broadcast network. The operation of the mechanism in [LDP-IGP-SYNC] is analyzed using the sample topology of Figure 1 below where routers A, B, C and E are attached to a common broadcast network. Say all links in that topology have a cost of 1 except the link A-PE3 that has a cost of 10. The use-case when router B's link to the broadcast network comes up is analyzed. Before that link comes up, traffic between PE1 and PE2 flows along the bi- directional path PE1-A-C-D-PE2 and traffic between PE1 and PE3 flows along the bi-directional path PE1-A-E-PE3. Kini & Lu Expires April 28, 2011 [Page 3] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 | +---+ +---+ |----| B |-----------|PE2| | +---+ +---+ +---+ +---+ | | |PE1|----| A |----| | +---+ +---+ | | | | +---+ +---+ | | |----| C |----| D |----+ | | +---+ +---+ | | | | | | | | +---+ | |----| E |-------------+ | | +---+ | | | | | | | +---+ +---------------------------|PE3| +---+ Figure 1 LDP IGP Sync on a broadcast network In one interpretation of the applicability of [LDP-IGP-SYNC] to broadcast networks, when a new router is discovered on a broadcast network, that network should avoid transit traffic till LDP becomes operational between all routers on that network. This can be achieved by having all the attached routers advertise maximum cost to that network. This should result in traffic that is being sent via that broadcast network to be diverted. However, traffic might be inadvertently diverted to the link that just came up. Till LDP becomes operational, that traffic will be black-holed. An additional problem is route churn in the entire network that results in traffic that should be unaffected taking sub-optimal paths until the high cost metric is reverted to the normal cost. In Figure 1, when B's link to the broadcast network comes up and it is discovered by routers A, C and E, then A, B, C and E can all start advertising maximum cost to the broadcast network. A will have B as next-hop to PE2 and will not have a LDP LSP path to PE2 resulting in VPN traffic from PE1 to PE2 to be black-holed at A. The route churn at A also results in traffic between PE1 and PE3 to be unnecessarily diverted to the sub-optimal path PE1-A-PE3 until the maximum cost advertisement is reverted to the normal cost. This interpretation has the additional complexity of requiring the maximum cost advertisement to be reverted by all routers after LDP peering between all the routers on the broadcast network is Kini & Lu Expires April 28, 2011 [Page 4] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 operational. This is non-trivial and needs co-ordination between all the routers. In another alternative interpretation of the applicability of [LDP- IGP-SYNC] to broadcast networks, only the router whose link to the broadcast network comes up, advertises maximum cost for that link but other routers continue to advertise the normal cost. In Figure 1 when B's link to the broadcast network comes up, it advertises a high cost to the broadcast network. After the IGP has converged but the LDP peering A-B is not yet operational, A will have B as the next-hop for PE2 and will not have a LDP LSP path to PE2. Since A's cost to reach B is not high, A-B-PE2 becomes the shortest path. VPN traffic from PE1 to PE2 will be dropped at A. 4. Solution The problem described above exists because the link-state database (LSDB) of the IGP does not describe a link coming up on a broadcast network with a high bi-directional cost to all other routers on that broadcast network. A broadcast network is advertised as a pseudo-node containing a list of routers that the broadcast network is connected to and the cost of all these links from the pseudo-node to each router is zero when computing SPF (Shortest Path First). The solution proposed below removes the link that is coming up from the LSDB unless absolutely necessary. Only the router whose link is coming up plays a role in ensuring this. The other routers on the broadcast network are not involved. The following text describes this in more detail. During the intra-area SPF algorithm execution, an additional computation is made to detect an alternate path to a directly connected network that does not have any IGP adjacencies. If a router has a directly connected network that does not have an alternate path to reach it, then the interface to that network is a 'cut-edge' in the topology for that router. When a 'cut-edge' goes down, the network is partitioned into two disjoint sub-graphs. This property of whether or not an interface is a 'cut-edge' is used when an IGP adjacency comes up on that interface. The method to determine whether an interface is a 'cut-edge' is described in Appendix A. During IGP procedures when the router's first adjacency to the broadcast network is coming up and the LSA is about to be updated with a link to the pseudo-node of the broadcast interface, a check is made whether that interface is a 'cut-edge'. If it is not a 'cut- edge' then the updating of the LSA with that link to the pseudo-node is postponed until LDP is operational with all the LDP peers on that Kini & Lu Expires April 28, 2011 [Page 5] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 broadcast interface. After LDP is operational, the LSA is updated with that link to the pseudo-node (and the LSA is flooded). If the interface is a 'cut-edge' then the updating of the LSA MUST NOT be delayed by LDP's operational state. Note that the IGP and LDP adjacency bring-up procedures are unchanged. The conditional check whether the interface is a 'cut-edge' must be done just before the adjacency is about to be reflected in the LSA. If the IGP is [OSPF], the Router-LSA is not updated with a 'Link Type 2' (link to transit network) for that subnet, until LDP is operational with all neighboring routers on that subnet. Similarly, if the IGP is [ISIS], the 'Link State PDU' is updated with an 'IS Reachability TLV' (or an 'Extended IS Reachability TLV') to the pseudo-node after LDP is operational with all neighboring routers on that subnet. Note that this solution can be introduced in a gradual manner in a network without any backward compatibility issues. 5. Scope This document is agnostic to the method that detects LDP to be operational with a neighbor. It does not define any new method to detect that LDP is operational. At the time of publishing this document [LDP-EOL] seems to be the preferred method. Issues arising out of LDP not being configured on some routers or on some interfaces are not specific to the method described in this document and are considered outside the scope of this solution. 6. Applicability The method described in this document can be easily extended to point-to-point (p2p) links. However, an implementation may continue to apply the method described in [LDP-IGP-SYNC] to p2p links but apply the method described in this document to broadcast networks. Both methods can co-exist in a network. The techniques used in this document's solution enable LDP IGP synchronization in many scenarios where one end of the IGP adjacency does not support any LDP IGP sync method. This is an optional benefit and is for further study. Some ways to apply this technique to achieve that benefit are discussed in Appendix B. 7. Security Considerations Kini & Lu Expires April 28, 2011 [Page 6] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 This document does not introduce any new security considerations beyond those already described in [LDP-IGP-SYNC]. 8. IANA Considerations This document has no actions for IANA. 9. Conclusions This document complements [LDP-IGP-SYNC] by providing a solution to achieve LDP IGP synchronization for broadcast networks. It can also co-exist with that solution in a network that has a combination of p2p links and broadcast networks. It can also be introduced into a network without backward compatibility issues. The solution in this document can also be used exclusively to achieve LDP IGP synchronization since this solution applies to both p2p links as well as broadcast networks. This solution also has useful properties that can be optionally used to achieve LDP IGP synchronization when only one end of the IGP adjacency supports this solution but the other end supports neither this solution nor the one in [LDP-IGP-SYNC]. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [LDP-IGP-SYNC] Jork, M., et al, "LDP IGP Synchronization", RFC 5443, March 2009. [LDP] Andersson, L., et al, "LDP Specification", RFC 5036, October 2007. [OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [ISIS] 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. 10.2. Informative References [LDP-EOL] Asati, R., et al, "Signaling LDP Label Advertisement Kini & Lu Expires April 28, 2011 [Page 7] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 Completion", RFC 5919, June 2010. 11. Acknowledgements The authors would like to thank Luyuan Fang, Mikael Abrahamsson, Ben Niven-Jenkins, Bruno Decraene, Jeff Tantsura and Acee Lindem for their review and useful comments. Kini & Lu Expires April 28, 2011 [Page 8] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 Appendix A. Computation of 'cut-edge' A 'cut-edge' can be computed during an intra-area SPF run or by using results of the previous SPF run. If a SPF run was scheduled but is pending execution, that SPF MUST be executed immediately before any procedure checks whether an interface is a 'cut-edge'. An interface is considered a 'cut-edge' if during intra-area SPF (using Dijkstra's algorithm - see [OSPF] sec 16) there is no alternate path for the directly connected network. Alternately, lack of connectivity to the router-id of a directly connected peer via an alternate path as detected by the last run of SPF can be used. The router-id can be known during the adjacency bring-up process. A 'cut-edge' computation should not require any extra SPF runs. It should not increase the algorithmic complexity of SPF. Kini & Lu Expires April 28, 2011 [Page 9] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 Appendix B. Sync without support at one end A useful property of the solution described in this document is that LDP IGP synchronization is achievable in many scenarios where one end of the IGP adjacency does not support any LDP IGP sync method. For p2p links (or broadcast links on which the IGP operates in p2p mode) the applicability is straightforward. An IGP can establish a p2p adjacency on a p2p link or a broadcast link with the IGP in p2p mode. When a p2p adjacency comes up, the end of the adjacency that supports the solution in this document would not advertise the link to the other router in its LSA unless the edge is a 'cut-edge' or until LDP becomes operational. Hence neither of the two routers will have IGP next-hop as the other router unless the link is a 'cut- edge'. Consider Figure 1 modified such that the broadcast network is replaced by p2p links between each of A, B, C and E. Say link A-B is coming up but only A has implemented the solution in this document whereas B has implemented neither the solution in this document nor the solution in [LDP-IGP-SYNC]. Since A's LSA does not advertise a link to B until LDP is operational, B does not have A as next-hop. After LDP is operational, A advertises the link to B in its LSA. Hence there is no traffic loss due to LDP LSP not being present. For broadcast networks the applicability is not straightforward and should be considered a topic for future study. One way is for the DR to stop advertising the link in the pseudo-node to the router whose link is coming up until LDP is operational. Kini & Lu Expires April 28, 2011 [Page 10] Internet Draft LDP IGP Sync for broadcast networks Oct 4, 2010 Authors' Addresses Sriganesh Kini Ericsson 300 Holger Way, San Jose, CA 95134 EMail: sriganesh.kini@ericsson.com Wenhu Lu Ericsson 300 Holger Way, San Jose, CA 95134 EMail: wenhu.lu@ericsson.com Kini & Lu Expires April 28, 2011 [Page 11]