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Network Working Group                                            N. ShenRequest for Comments: 3906                              Redback NetworksCategory: Informational                                          H. Smit                                                            October 2004Calculating Interior Gateway Protocol (IGP) RoutesOver Traffic Engineering TunnelsStatus 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) The Internet Society (2004).Abstract   This document describes how conventional hop-by-hop link-state   routing protocols interact with new Traffic Engineering capabilities   to create Interior Gateway Protocol (IGP) shortcuts.  In particular,   this document describes how Dijkstra's Shortest Path First (SPF)   algorithm can be adapted so that link-state IGPs will calculate IP   routes to forward traffic over tunnels that are set up by Traffic   Engineering.1.  Introduction   Link-state protocols like integrated Intermediate System to   Intermediate System (IS-IS) [1] and OSPF [2] use Dijkstra's SPF   algorithm to compute a shortest path tree to all nodes in the   network.  Routing tables are derived from this shortest path tree.   The routing tables contain tuples of destination and first-hop   information.  If a router does normal hop-by-hop routing, the first-   hop will be a physical interface attached to the router.  New traffic   engineering algorithms calculate explicit routes to one or more nodes   in the network.  At the router that originates explicit routes, such   routes can be viewed as logical interfaces which supply Label   Switched Paths through the network.  In the context of this document,   we refer to these Label Switched Paths as Traffic Engineering tunnels   (TE-tunnels).  Such capabilities are specified in [3] and [4].   The existence of TE-tunnels in the network and how the traffic in the   network is switched over those tunnels are orthogonal issues.  A node   may define static routes pointing to the TE-tunnels, it may match theShen & Smit                  Informational                      [Page 1]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 2004   recursive route next-hop with the TE-tunnel end-point address, or it   may define local policy such as affinity based tunnel selection for   switching certain traffic.  This document describes a mechanism   utilizing link-state IGPs to dynamically install IGP routes over   those TE-tunnels.   The tunnels under consideration are tunnels created explicitly by the   node performing the calculation, and with an end-point address known   to this node.  For use in algorithms such as the one described in   this document, it does not matter whether the tunnel itself is   strictly or loosely routed.  A simple constraint can ensure that the   mechanism be loop free.  When a router chooses to inject a packet   addressed to a destination D, the router may inject the packet into a   tunnel where the end-point is closer (according to link-state IGP   topology) to the destination D than is the injecting router.  In   other words, the tail-end of the tunnel has to be a downstream IGP   node for the destination D.  The algorithms that follow are one way   that a router may obey this rule and dynamically make intelligent   choices about when to use TE-tunnels for traffic.  This algorithm may   be used in conjunction with other mechanisms such as statically   defined routes over TE-tunnels or traffic flow and QoS based TE-   tunnel selection.   This IGP shortcut mechanism assumes the TE-tunnels have already been   setup.  The TE-tunnels in the network may be used for QoS, bandwidth,   redundancy, or fastreroute reasons.  When an IGP shortcut mechanism   is applied on those tunnels, or other mechanisms are used in   conjunction with an IGP shortcut, the physical traffic switching   through those tunnels may not match the initial traffic engineering   setup goal.  Also the traffic pattern in the network may change with   time.  Some forwarding plane measurement and feedback into the   adjustment of TE-tunnel attributes need to be there to ensure that   the network is being traffic engineered efficiently [6].2.  Enhancement to the Shortest Path First Computation   During each step of the SPF computation, a router discovers the path   to one node in the network.  If that node is directly connected to   the calculating router, the first-hop information is derived from the   adjacency database.  If a node is not directly connected to the   calculating router, it inherits the first-hop information from the   parent(s) of that node.  Each node has one or more parents.  Each   node is the parent of zero or more down-stream nodes.   For traffic engineering purposes, each router maintains a list of all   TE-tunnels that originate at this router.  For each of those TE-   tunnels, the router at the tail-end is known.Shen & Smit                  Informational                      [Page 2]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 2004   During SPF, when a router finds the path to a new node (in other   words, this new node is moved from the TENTative list to the PATHS   list), the router must determine the first-hop information.  There   are three possible ways to do this:      -  Examine the list of tail-end routers directly reachable via a         TE-tunnel.  If there is a TE-tunnel to this node, we use the         TE-tunnel as the first-hop.      -  If there is no TE-tunnel, and the node is directly connected,         we use the first-hop information from the adjacency database.      -  If the node is not directly connected, and is not directly         reachable via a TE-tunnel, we copy the first-hop information         from the parent node(s) to the new node.   The result of this algorithm is that traffic to nodes that are the   tail-end of TE-tunnels, will flow over those TE-tunnels.  Traffic to   nodes that are downstream of the tail-end nodes will also flow over   those TE-tunnels.  If there are multiple TE-tunnels to different   intermediate nodes on the path to destination node X, traffic will   flow over the TE-tunnel whose tail-end node is closest to node X.  In   certain applications, there is a need to carry both the native   adjacency and the TE-tunnel next-hop information for the TE-tunnel   tail-end and its downstream nodes.  The head-end node may   conditionally switch the data traffic onto TE-tunnels based on user   defined criteria or events; the head-end node may also split flow of   traffic towards either types of the next-hops; the head-end node may   install the routes with two different types of next-hops into two   separate RIBs.  Multicast protocols running over physical links may   have to perform RPF checks using the native adjacency next-hops   rather than the TE-tunnel next-hops.3.  Special Cases and Exceptions   The Shortest Path First algorithm will find equal-cost parallel paths   to destinations.  The enhancement described in this document does not   change this.  Traffic can be forwarded over one or more native IP   paths, over one or more TE-tunnels, or over a combination of native   IP paths and TE-tunnels.   A special situation occurs in the following topology:      rtrA -- rtrB -- rtrC               |       |              rtrD -- rtrEShen & Smit                  Informational                      [Page 3]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 2004   Assume all links have the same cost.  Assume a TE-tunnel is set up   from rtrA to rtrD.  When the SPF calculation puts rtrC on the   TENTative list, it will realize that rtrC is not directly connected,   and thus it will use the first-hop information from the parent, which   is rtrB.  When the SPF calculation on rtrA moves rtrD from the   TENTative list to the PATHS list, it realizes that rtrD is the tail-   end of a TE-tunnel.  Thus rtrA will install a route to rtrD via the   TE-tunnel, and not via rtrB.   When rtrA puts rtrE on the TENTative list, it realizes that rtrE is   not directly connected, and that rtrE is not the tail-end of a TE-   tunnel.  Therefore, rtrA will copy the first-hop information from the   parents (rtrC and rtrD) to the first-hop information of rtrE.   Traffic to rtrE will now load-balance over the native IP path via   rtrA->rtrB->rtrC, and the TE-tunnel rtrA->rtrD.   In the case where both parallel native IP paths and paths over TE-   tunnels are available, implementations can allow the network   administrator to force traffic to flow over only TE-tunnels (or only   over native IP paths) or both to be used for load sharing.4.  Metric Adjustment of IP Routes over TE-tunnels   When an IGP route is installed in the routing table with a TE-tunnel   as the next hop, an interesting question is what should be the cost   or metric of this route?  The most obvious answer is to assign a   metric that is the same as the IGP metric of the native IP path as if   the TE-tunnels did not exist.  For example, rtrA can reach rtrC over   a path with a cost of 20.  X is an IP prefix advertised by rtrC.  We   install the route to X in rtrA's routing table with a cost of 20.   When a TE-tunnel from rtrA to rtrC comes up, by default the route is   still installed with metric of 20, only the next-hop information for   X is changed.   While this scheme works well, in some networks it might be useful to   change the cost of the path over a TE-tunnel, to make the route over   the TE-tunnel less or more preferred than other routes.   For instance, when equal cost paths exist over a TE-tunnel and over a   native IP path, by adjusting the cost of the path over the TE-tunnel,   we can force traffic to prefer the path via the TE-tunnel, to prefer   the native IP path, or to load-balance among them.  Another example   is when multiple TE-tunnels go to the same or different destinations.   Adjusting TE-tunnel metrics can force the traffic to prefer some TE-   tunnels over others regardless of underlining IGP cost to those   destinations.Shen & Smit                  Informational                      [Page 4]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 2004   Setting a manual metric on a TE-tunnel does not impact the SPF   algorithm itself.  It only affects the comparison of the new route   with existing routes in the routing table.  Existing routes can be   either IP routes to another router that advertises the same IP   prefix, or it can be a path to the same router, but via a different   outgoing interface or different TE-tunnel.  All routes to IP prefixes   advertised by the tail-end router will be affected by the TE-tunnel   metric.  Also, the metrics of paths to routers that are downstream of   the tail-end router will be influenced by the manual TE-tunnel   metric.   This mechanism is loop free since the TE-tunnels are source-routed   and the tunnel egress is a downstream node to reach the computed   destinations.  The end result of TE-tunnel metric adjustment is more   control over traffic loadsharing.  If there is only one way to reach   a particular IP prefix through a single TE-tunnel, then no matter   what metric is assigned, the traffic has only one path to go.   The routing table described in this section can be viewed as the   private RIB for the IGP.  The metric is an important attribute to the   routes in the routing table.  A path or paths with lower metric will   be selected over other paths for the same route in the routing table.4.1.  Absolute and Relative Metrics   It is possible to represent the TE-tunnel metric in two different   ways: an absolute (or fixed) metric or a relative metric, which is   merely an adjustment of the dynamic IGP metric as calculated by the   SPF computation.  When using an absolute metric on a TE-tunnel, the   cost of the IP routes in the routing table does not depend on the   topology of the network.  Note that this fixed metric is not only   used to compute the cost of IP routes advertised by the router that   is the tail-end of the TE-tunnel, but also for all the routes that   are downstream of this tail-end router.  For example, if we have TE-   tunnels to two core routers in a remote POP, and one of them is   assigned with an absolute metric of 1, then all the traffic going to   that POP will traverse this low-metric TE-tunnel.   By setting a relative metric, the cost of IP routes in the routing   table is based on the IGP metric as calculated by the SPF   computation.  This relative metric can be a positive or a negative   number.  Not configuring a metric on a TE-tunnel is a special case of   the relative metric scheme.  No metric is the same as a relative   metric of 0.  The relative metric is bounded by minimum and maximum   allowed metric values while the positive metric disables the TE-   tunnel in the SPF calculation.Shen & Smit                  Informational                      [Page 5]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 20044.2.  Examples of Metric Adjustment   Assume the following topology.  X, Y, and Z are IP prefixes   advertised by rtrC, rtrD, and rtrE respectively.  T1 is a TE-tunnel   from rtrA to rtrC.  Each link in the network has an IGP metric of 10.        ===== T1 =====>      rtrA -- rtrB -- rtrC -- rtrD -- rtrE           10      10  |   10  |   10  |                       X       Y       Z   Without TE-tunnel T1, rtrA will install IP routes X, Y, and Z in the   routing table with metrics 20, 30, and 40 respectively.  When rtrA   has brought up TE-tunnel T1 to rtrC, and if rtrA is configured with   the relative metric of -5 on tunnel T1, then the routes X, Y, and Z   will be installed in the routing table with metrics 15, 25, and 35.   If an absolute metric of 5 is configured on tunnel T1, then rtrA will   install routes X, Y, and Z all with metrics 5, 15, and 25   respectively.5.  Security Considerations   This document does not change the security aspects of IS-IS or OSPF.   Security considerations specific to each protocol still apply.  For   more information see [5] and [2].6.  Acknowledgments   The authors would like to thank Joel Halpern and Christian Hopps for   their comments on this document.7.  Informative References   [1] ISO.  Information Technology - Telecommunications and Information       Exchange between Systems - Intermediate System to Intermediate       System Routing Exchange Protocol for Use in Conjunction with the       Protocol for Providing the Connectionless-Mode Network Service.       ISO, 1990.   [2] Moy, J., "OSPF Version 2",RFC 2328, April 1998.   [3] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.       McManus, "Requirements for Traffic Engineering Over MPLS",RFC2702, September 1999.   [4] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G.       Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels",RFC 3209,       December 2001.Shen & Smit                  Informational                      [Page 6]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 2004   [5] Li, T. and R. Atkinson, "Intermediate System to Intermediate       System (IS-IS) Cryptographic Authentication",RFC 3567, July       2003.   [6] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X. Xiao,       "Overview and Principles of Internet Traffic Engineering",RFC3272, May 2002.8.  Authors' Addresses   Naiming Shen   Redback Networks, Inc.   300 Holger Way   San Jose, CA 95134   EMail: naiming@redback.com   Henk Smit   EMail: hhwsmit@xs4all.nlShen & Smit                  Informational                      [Page 7]

RFC 3906              IGP ShortCut Over MPLS LSPs           October 20049.  Full Copyright Statement   Copyright (C) The Internet Society (2004).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE 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 it has   made any independent effort to identify any such rights.  Information   on the IETF's procedures with respect to rights in IETF Documents can   be found inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   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-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Shen & Smit                  Informational                      [Page 8]