Network Working Group
S. Bryant Internet DraftM. Shand Expiration Date: Jan 2008Internet-Draft S. Bryant Intended status: Standards Track S. Previdi Expires: August 28, 2008 Cisco Systems February 25, 2008 IP Fast Reroute Using Not-via Addresses <draft-ietf-rtgwg-ipfrr-notvia-addresses-01.txt>draft-ietf-rtgwg-ipfrr-notvia-addresses-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 other groups may also distribute working documents as Internet-Drafts.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".progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.htmlhttp://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.htmlhttp://www.ietf.org/shadow.html. This Internet-Draft will expire on August 28, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). All rights reserved.(2008). Abstract This draft describes a mechanism that provides fast reroute in an IP network through encapsulation to "not-via" addresses. A single level of encapsulation is used. The mechanism protects unicast, multicast and LDP traffic against link, router and shared risk group failure, regardless of network topology and metrics. Table of Contents 1. 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 RFC 2119 [RFC2119]. Table of Contents 1.. . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview of Not-via Repairs.........................................3 1.1Repairs . . . . . . . . . . . . . . . . . 3 3.1. Use of Equal Cost Multi-Path.....................................5 1.2Multi-Path . . . . . . . . . . . . . . . 5 3.2. Use of LFA repairs...............................................5 2.repairs . . . . . . . . . . . . . . . . . . . . 5 4. Not-via Repair Path Computation.....................................5 3.Computation . . . . . . . . . . . . . . . 5 4.1. Computing not-via repairs in routing vector protocols . . 6 5. Operation of Repairs................................................6 3.1Repairs . . . . . . . . . . . . . . . . . . . . . 7 5.1. Node Failure.....................................................7 3.2 Link Failure.....................................................7 3.3 Multi-homed Prefix...............................................8 3.4 Installation of Repair Paths.....................................9 4. Compound failures..................................................10 4.1 Shared RiskFailure . . . . . . . . . . . . . . . . . . . . . . . 7 5.2. Link Groups.........................................10 4.1.1 Use of LFAs with SRLGs......................................14 4.2 Local Area Networks.............................................14 4.2.1 Simple LAN Repair...........................................15 4.2.2 LAN Component Repair........................................16 4.2.3 LAN RepairFailure . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 8 5.4. Installation of Repair Paths . . . . . . . . . . . . . . . 9 6. Compound Failures . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Shared Risk Link Groups . . . . . . . . . . . . . . . . . 11 6.1.1. Use of LFAs with SRLGs . . . . . . . . . . . . . . . . 14 6.2. Local Area Networks . . . . . . . . . . . . . . . . . . . 15 6.2.1. Simple LAN Repair . . . . . . . . . . . . . . . . . . 15 6.2.2. LAN Component Repair . . . . . . . . . . . . . . . . . 16 6.2.3. LAN Repair Using Diagnostics................................17 5.Diagnostics . . . . . . . . . . . . . 17 7. Multiple Simultaneous Failures.....................................17 6.Failures . . . . . . . . . . . . . . . . 17 8. Optimizing not-via computations using LFAs.........................18 7. Multicast..........................................................18 8. Fast Reroute inLFAs . . . . . . . . . . 17 9. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 18 10. Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 19 11. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 19 12. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 19 13. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 20 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 15. Security Considerations . . . . . . . . . . . . . . . . . . . 20 16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 17.1. Normative References . . . . . . . . . . . . . . . . . . . 21 17.2. Informative References . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 Intellectual Property and Copyright Statements . . . . . . . . . . 23 1. 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 RFC 2119 [RFC2119]. 2. Introduction When a link or a router fails, only the neighbors of the failure are initially aware that the failure has occurred. In a network operating IP fast reroute [I-D.ietf-rtgwg-ipfrr-framework], the routers that are the neighbors of the failure repair the failure. These repairing routers have to steer packets to their destinations despite the fact that most other routers in the network are unaware of the nature and location of the failure. A common limitation in most IPFRR mechanisms is an inability to indicate the identity of the failure and to explicitly steer the repaired packet round the failure. The extent to which this limitation affects the repair coverage is topology dependent. The mechanism proposed here is to encapsulate the packet to an MPLS LDP Network................................19 9. Encapsulation......................................................19 10. Routing Extensions................................................19 11. Incremental Deployment............................................20 12. IANA considerations...............................................20 13. Security Considerations...........................................20 Introductionaddress that explicitly identifies the network component that the repair must avoid. This produces a repair mechanism, which, provided the network is not partitioned by the failure, will always achieve a repair. 3. Overview of Not-via Repairs When a link or a router fails, only the neighbors of the failure are initially aware that the failure has occurred. In a network operating IP fast reroute [IPFRR],[I-D.ietf-rtgwg-ipfrr-framework], the routers that are the neighbors of the failure repair the failure. These repairing routers have to steer packets to their destinations despite the fact that most other routers in the network are unaware of the nature and location of the failure. A common limitation in most IPFRR mechanisms is an inability to indicate the identity of the failure and to explicitly steer the repaired packet round the failure. The extent to which this limitation affects the repair coverage is topology dependent. The mechanism proposed here is to encapsulate the packet to an address that explicitly identifies the network component that the repair must avoid. This produces a repair mechanism, which, provided the network is not partitioned by the failure, will always achieve a repair. 1. Overview of Not-via Repairs The purpose of a repair is to deliver packets to their destination without traversing a known failure in the network, i.e. to deliver the packet not via the failure. A special address is assigned to each protected component. This address is called the not-via address. The semantics of a not-via address are that a packet addressed to a not-via address must be delivered to the router advertising that address, not via (i.e. without traversing or attempting to traverse) the protected component (link, node, SRLG etc.) with which that address is associated. A simple example would be node repair in which an additional address is assigned to each interface in the network. To repair a failure, the repairing router encapsulates the packet to the not-via address of the router interface on the far side of the failure. The routers on the repair path then know to which router they must deliver the packet, and which network component they must avoid. The network fragment shown in Figure 1 illustrates a not-via repair for the case of a router failure.A | Bp is the address to use to get | a packet to B not-via P | S----------P----------B. . . . . . . . . .D \ | Bp^ \ | | \ | | \ C | \ | ----------------+ Repair to Bp Figure 1: Not-via repair of router failure Assume that S has a packet for some destination D that it would normally send via P and B, and that S suspects that P has failed. S encapsulates the packet to Bp. The path from S to Bp is the shortest path from S to B not going via P. If the network contains a path from S to B that does not transit router P, i.e. the network is not partitioned by the failure of P, then the packet will be successfully delivered to B. When the packet addressed to Bp arrives at B, B removes the encapsulation and forwards the repaired packet towards its final destination. Note that if the path from B to the final destination includes one or more nodes that are included in the repair path, a packet may back track after the encapsulation is removed. However, because the decapsulating router is always closer to the packet destination than the encapsulating router, the packet will not loop. For complete protection, all of P's neighbors will require a not-via address that allows traffic to be directed to them without traversing P. This is shown in Figure 2. A |Ap | Sp Pa|Pb S----------P----------B Ps|Pc Bp | Cp| C Figure 2: The set of Not-via P Addresses 1.13.1. Use of Equal Cost Multi-Path A router can use an equal cost multi-path (ECMP) repair in place of a not-via repair. A router computing a not-via repair path MAY subject the repair to ECMP. 1.23.2. Use of LFA repairs The not-via approach provides complete repair coverage and therefore may be used as the sole repair mechanism. There are, however, advantages in using not-via in combination with loop free alternates (LFA) and or downstream paths as documented in [LFA].[I-D.ietf-rtgwg-ipfrr-spec-base]. LFAs are computed on a per destination basis and in general, only a subset of the destinations requiring repair will have a suitable LFA repair. In this case, those destinations which are repairable by LFAs are so repaired and the remainder of the destinations are repaired using the not-via encapsulation. This has the advantage of reducing the volume of traffic that requires encapsulation. On the other hand, the path taken by an LFA repair may be less optimal than that of the equivalent not-via repair for traffic destined to nodes close to the far end of the failure, but may be more optimal for some other traffic. The description in this document assumes that LFAs will be used where available, but the distribution of repairs between the two mechanisms is a local implementation choice. 2.4. Not-via Repair Path Computation The not-via repair mechanism requires that all routers on the path from S to B (Figure 1) have a route to Bp. They can calculate this by failing node P, running an SPF, and finding the shortest route to B. A router has no simple way of knowing whether it is on the shortest path for any particular repair. It is therefore necessary for every router to calculate the path it would use in the event of any possible router failure. Each router therefore "fails" every router in the network, one at a time, and calculates its own best route to each of the neighbors of that router. In other words, with reference to Figure 2,1, some router X will consider each router in turn to be P, fail P, and then calculate its own route to each of the not-via P addresses advertised by the neighbors of P. i.e. X calculates its route to Sp, Ap, Bp, and Cp, in each case, not via P. To calculate the repair paths a router has to calculate n-1 SPFs where n is the number of routers in the network. This is expensive to compute. However, the problem is amenable to a solution in which each router (X) proceeds as follows. X first calculates the base topology with all routers functional and determines its normal path to all not-via addresses. This can be performed as part of the normal SPF computation. For each router P in the topology, X then performs the following actions:- 1. Removes router P from the topology. 2. Performs an incremental SPF [ISPF] on the modified topology. The iSPF process involves detaching the sub-tree affected by the removal of router P, and then re-attaching the detached nodes. However, it is not necessary to run the iSPF to completion. It is sufficient to run the iSPF up to the point where all of the nodes advertising not-via P addresses have been re-attached to the SPT, and then terminate it. 3. Reverts to the base topology. This algorithm is significantly less expensive than a set of full SPFs. Thus, although a router has to calculate the repair paths for n-1 failures, the computational effort is much less than n-1 SPFs. Experiments on a selection of real world network topologies with between 40 and 400 nodes suggest that the worst-case computational complexity using the above optimizations is equivalent to performing between 5 and 13 full SPFs. Further optimizations are described in section 6. 2.14.1. Computing not-via repairs in routing vector protocols While this document focuses on link state routing protocols, it is equally possible to compute not-via repairs in distance vector (e.g. RIP) or path vector (e.g. BGP) routing protocols. This can be achieved with very little protocol modification by advertising the not-via address in the normal way, but ensuring that the information about a not-via address Ps is not propagated through the node S. In the case of link protection this simply means that the advertisement from P to S is suppressed, with the result that S and all other nodes compute a route to Ps which doesn't traverse S, as required. In the case of node protection, where P is the protected node, and N is some neighbor, the advertisement of Np must be suppressed not only across the link N->P, but also across any link to P. The simplest way of achieving this is for P itself to perform the suppression of any address of the form Xp. 3.5. Operation of Repairs This section explains the basic operation of the not-via repair of node and link failure. 3.15.1. Node Failure When router P fails (Figure 2) S encapsulates any packet that it would send to B via P to Bp, and then sends the encapsulated packet on the shortest path to Bp. S follows the same procedure for routers A and C in Figure 2. The packet is decapsulated at the repair target (A, B or C) and then forwarded normally to its destination. The repair target can be determined as part of the normal SPF by recording the "next-next-hop" for each destination in addition to the normal next-hop. Notice that with this technique only one level of encapsulation is needed, and that it is possible to repair ANY failure regardless of link metrics and any asymmetry that may be present in the network. The only exception to this is where the failure was a single point of failure that partitioned the network, in which case ANY repair is clearly impossible. 3.25.2. Link Failure The normal mode of operation of the network would be to assume router failure. However, where some destinations are only reachable through the failed router, it is desirable that an attempt be made to repair to those destinations by assuming that only a link failure has occurred. To perform a link repair, S encapsulates to Ps (i.e. it instructs the network to deliver the packet to P not-via S). All of the neighbors of S will have calculated a path to Ps in case S itself had failed. S could therefore give the packet to any of its neighbors (except, of course, P). However, S should preferably send the encapsulated packet on the shortest available path to P. This path is calculated by running an SPF with the link SP failed. Note that this may again be an incremental calculation, which can terminate when address Ps has been reattached. It is necessary to consider the behavior of IPFRR solutions when a link repair is attempted in the presence of node failure. In its simplest form the not-via IPFRR solution prevents the formation of loops forming as a result of mutual repair, by never providing a repair path for a not-via address. Referring to Figure 2, if A was the neighbor of P that was on the link repair path from S to P, and P itself had failed, the repaired packet from S would arrive at A encapsulated to Ps. A would have detected that the AP link had failed and would normally attempt to repair the packet. However, no repair path is provided for any not-via address, and so A would be forced to drop the packet, thus preventing the formation of loop. 3.35.3. Multi-homed PrefixPrefixes A multi-homed Prefix (MHP) is a prefix that is reachable via more than one router in the network. Some of these may be repairable using LFAs as described in [LFA].[I-D.ietf-rtgwg-ipfrr-spec-base]. Only those without such a repair need be considered here. When IPFRR router S (Figure 3) discovers that P has failed, it needs to send packets addressed to the MHP X, which is normally reachable through P, to an alternate router, which is still able to reach X. X X X | | | | | | | Sp |Pb | Z...............S----------P----------B...............Y Ps|Pc Bp | Cp| C Figure 3: Multi-homed Prefixes S should choose the closest router that can reach X during the failure as the alternate router. S determines which router to use as the alternate while running the SPF with P failed. This is accomplished by the normal process of re-attaching a leaf node to the core topology (this is sometimes known as a "partial SPF"). First, consider the case where the shortest alternate path to X is via Z. S can reach Z without using the failed router P. However, S cannot just send the packet towards Z, because the other routers in the network will not be aware of the failure of P, and may loop the packet back to S. S therefore encapsulates the packet to Z (using a normal address for Z). When Z receives the encapsulated packet it removes the encapsulation and forwards the packet to X. Now consider the case where the shortest alternate path to X is via Y, which S reaches via P and B. To reach Y, S must first repair the packet to B using the normal not-via repair mechanism. To do this S encapsulates the packet for X to Bp. When B receives the packet it removes the encapsulation and discovers that the packet is intended for MHP X. The situation now reverts to the previous case, in which the shortest alternate path does not require traversal of the failure. B therefore follows the algorithm above and encapsulates the packet to Y (using a normal address for Y). Y removes the encapsulation and forwards the packet to X. It may be that the cost of reaching X using local delivery from the alternate router (i.e. Z or Y) is greater than the cost of reaching X via P. Under those circumstances, the alternate router would normally forward to X via P, which would cause the IPFRR repair to loop. To prevent the repair from looping the alternate router must locally deliver a packet received via a repair encapsulation. This may be specified by using a special address with the above semantics. Note that only one such address is required per node. Notice that using the not-via approach, only one level of encapsulation was needed to repair MHPs to the alternate router. 3.45.4. Installation of Repair Paths The following algorithm is used by node S (Figure 3) to pre- calculate and install repair paths in the FIB, ready for immediate use in the event of a failure. It is assumed that the not-via repair paths have already been calculated as described above. For each neighbor P, consider all destinations which are reachable via P in the current topology:- 1. For all destinations with an ECMP or LFA repair (as described in [LFA] )[I-D.ietf-rtgwg-ipfrr-spec-base]) install that repair. 2. For each destination (DR) that remains, identify in the current topology the next-next-hop (H) (i.e. the neighbor of P that P will use to send the packet to DR). This can be determined during the normal SPF run by recording the additional information. If S has a path to the not-via address Hp (H not via P), install a not-via repair to Hp for the destination DR. 3. Identify all remaining destinations (M) which can still be reached when node P fails. These will be multi-homed prefixes that are not repairable by LFA, for which the normal attachment node is P, or a router for which P is a single point of failure, and have an alternative attachment point that is reachable after P has failed. One way of determining these destinations would be to run an SPF rooted at S with node P removed, but an implementation may record alternative attachment points during the normal SPF run. In either case, the next best point of attachment can also be determined for use in step (4) below. 4. For each multi-homed prefix (M) identified in step (3):- a.A. Identify the new attachment node (as shown in Figure 3). This may be:- i.a. Y, where the next hop towards Y is P, or ii.b. Z, where the next hop towards Z is not P. b.If the attachment node is Z, install the repair for M as a tunnel to Z' (where Z' is the address of Z that is used to force local forwarding). c.B. For the subset of prefixes (M) that remain (having attachment point Y), install the repair path previously installed for destination Y. 5.For each destination (DS) that remains, install a not-via repair to Ps (P not via S). Note, these are destinations for which node P is a single point of failure, and can only be repaired by assuming that the apparent failure of node P was simply a failure of the S-P link. Note that, if available, a downstream path to P may be used for such a repair. This cannot generate a persistent loop in the event of the failure of node P, but if one neighbor of P uses a not-via repair and another uses a downstream path, it is possible for a packet sent on the downstream path to be returned to the sending node inside a not-via encapsulation. Since packets destined to not-via addresses are not repaired, the packet will be dropped after executing a single turn loop. 4.6. Compound Failures The following types of failures 4.1involve more tha one component. 6.1. Shared Risk Link Groups A Shared Risk Link Group (SRLG) is a set of links whose failure can be caused by a single action such as a conduit cut or line card failure. When repairing the failure of a link that is a member of an SRLG, it must be assumed that all the other links that are also members of the SRLG have also failed. Consequently, any repair path must be computed to avoid not just the adjacent link, but also all the links which are members of the same SRLG. In Figure 4 below, the links S-P and A-B are both members of SRLG "a". The semantics of the not-via address Ps changes from simply "P not-via the link S-P" to be "P not-via the link S-P or any other link with which S-P shares an SRLG" In Figure 4 this is the links that are members of SRLG "a". I.e. links S-P and A-B. Since the information about SRLG membership of all links is available in the Link State Database, all nodes computing routes to the not-via address Ps can infer these semantics, and perform the computation by failing all the links in the SRLG when running the iSPF. Note that it is not necessary for S to consider repairs to any other nodes attached to members of the SRLG (such as B). It is sufficient for S to repair to the other end of the adjacent link (P in this case). a Ps S----------P---------D | | | a | A----------B | | | | C----------E Figure 4: Shared Risk Link Group In some cases, it may be that the links comprising the SRLG occur in series on the path from S to the destination D, as shown in Figure 5. In this case, multiple consecutive repairs may be necessary. S will first repair to Ps, then P will repair to Dp. In both cases, because the links concerned are members of SRLG "a" the paths are computed to avoid all members of SRLG "a". a Ps a Dp S----------P---------D | | | | a | | A----------B | | | | | | | C----------E---------F Figure 5: Shared Risk Link Group members in series While the use of multiple repairs in series introduces some additional overhead, these semantics avoid the potential combinatorial explosion of not-via addresses that could otherwise occur. Note that although multiple repairs are used, only a single level of encapsulation is required. This is because the first repair packet is de-capsulated before the packet is re-encapsulated using the not- via address corresponding to the far side of the next link which is a member of the same SRLG. In some cases the de-capsulation and re- encapsulation takes place (at least notionally) at a single node, while in other cases, these functions may be performed by different nodes. This scenario is illustrated in Figure 6 below. a Ps a Dg S----------P---------G--------D | | | | | a | | | A----------B | | | | | | | | | | C----------E---------F--------H Figure 6: Shared Risk Link Group members in series In this case, S first encapsulates to Ps, and node P decapsulates the packet and forwards it "native" to G using its normal FIB entry for destination D. G then repairs the packet to Dg. It can be shown that such multiple repairs can never form a loop because each repair causes the packet to move closer to its destination. It is often the case that a single link may be a member of multiple SRLGs, and those SRLG may not be isomorphic. This is illustrated in Figure 7 below. ab Ps a Dg S----------P---------G--------D | | | | | a | | | A----------B | | | | | | | b | | b | C----------E---------F--------H | | | | J----------K Figure 7: Multiple Shared Risk Link Groups The link SP is a member of SRLGs "a" and "b". When a failure of the link SP is detected, it must be assumed that BOTH SRLGs have failed. Therefore the not-via path to Ps must be computed by failing all links which are members of SRLG "a" or SRLG "b". I.e. the semantics of Ps is now "P not-via any links which are members of any of the SRLGs of which link SP is a member". This is illustrated in Figure 8 below. ab Ps a Dg S----/-----P---------G---/----D | | | | | a | | | A----/-----B | | | | | | | b | | b | C----/-----E---------F---/----H | | | | J----------K Figure 8: Topology used for repair computation for link S-P In this case, the repair path to Ps will be S-A-C-J-K-E-B-P. It may appear that there is no path to D because GD is a member of SRLG "a" and FH is a member of SRLG "b". This is true if BOTH SRLGs "a" and "b" have in fact failed. But that would be an instance of multiple uncorrelated failures which are out of scope for this design. In practice it is likely that there is only a single failure, i.e. either SRLG "a" or SRLG "b" has failed, but not both. These two possibilities are indistinguishable from the point of view of the repairing router S and so it must repair on the assumption that both are unavailable. However, each link repair is considered independently. The repair to Ps delivers the packet to P which then forwards the packet to G. When the packet arrives at G, if SRLG "a" has failed it will be repaired around the path G-F-H-D. This is illustrated in Figure 9 below. If, on the other hand, SRLG "b" has failed, link GD will still be available. In this case the packet will be delivered as normal across the link GD. ab Ps a Dg S----/-----P---------G---/----D | | | | | a | | | A----/-----B | | | | | | | b | | b | C----------E---------F--------H | | | | J----------K Figure 9: Topology used for repair computation for link G-D A repair strategy that assumes the worst-case failure for each link can often result in longer repair paths than necessary. In cases where only a single link fails, rather than the full SRLG, this strategy may occasionally fail to identify a repair even though a viable repair path exists in the network. The use of sub-optimal repair paths is an inevitable consequence of this compromise approach. The failure to identify any repair is a serious deficiency, but is a rare occurrence in a robustly designed network. This problem can be addressed by:- 1. Reporting that the link in question is irreparable, so that the network designer can take appropriate action. 2. Modifying the design of the network to avoid this possibility. 3. Using some form of SRLG diagnostic (for example, by running BFD over alternate repair paths) to determine which SRLG member(s) has actually failed and using this information to select an appropriate pre-computed repair path. However, aside from the complexity of performing the diagnostics, this requires multiple not-via addresses per interface, which has poor scaling properties. 22.214.171.124.1. Use of LFAs with SRLGs Section 4.1 above describes the repair of links which are members of one or more SRLGs. LFAs can be used for the repair of such links provided that any other link with which S-P shares an SRLG is avoided when computing the LFA. This is described for the simple case of "local-SRLGs" in [LFA]. 4.2[I-D.ietf-rtgwg-ipfrr-spec-base]. 6.2. Local Area Networks LANs are a special type of SRLG and are solved using the SRLG mechanisms outlined above. With all SRLGs there is a trade-off between the sophistication of the fault detection and the size of the SRLG. Protecting against link failure of the LAN link(s) is relatively straightforward, but as with all fast reroute mechanisms, the problem becomes more complex when it is desired to protect against the possibility of failure of the nodes attached to the LAN as well as the LAN itself. +--------------Q------C | | | A--------S-------(N)-------------P------B | | | +--------------R------D Figure 10: Local Area Networks Consider the LAN shown in Figure 10. For connectivity purposes, we consider that the LAN is represented by the pseudonode (N). To provide IPFRR protection, S must run a connectivity check to each of its protected LAN adjacencies P, Q, and R, using, for example BFD [BFD].[I-D.ietf-bfd-base]. When S discovers that it has lost connectivity to P, it is unsure whether the failure is: .o its own interface to the LAN, .o the LAN itself, .o the LAN interface of P, .o the node P. 126.96.36.199.1. Simple LAN Repair A simple approach to LAN repair is to consider the LAN and all of its connected routers as a single SRLG. Thus, the address P not via the LAN (Pl) would require P to be reached not-via any router connected to the LAN. This is shown in Figure 11. Ql Cl +-------------Q--------C | Qc | As Sl | Pl Bl A--------S-------(N)------------P--------B Sa | Pb | | Rl Dl +-------------R--------D Rd Figure 11: Local Area Networks - LAN SRLG In this case, when S detected that P had failed it would send traffic reached via P and B to B not-via the LAN or any router attached to the LAN (i.e. to Bl). Any destination only reachable through P would be addressed to P not-via the LAN or any router attached to the LAN (except of course P). Whilst this approach is simple, it assumes that a large portion of the network adjacent to the failure has also failed. This will result in the use of sub-optimal repair paths and in some cases the inability to identify a viable repair. 188.8.131.52.2. LAN Component Repair In this approach, possible failures are considered at a finer granularity, but without the use of diagnostics to identify the specific component that has failed. Because S is unable to diagnose the failure it must repair traffic sent through P and B, to B not- via P,N (i.e. not via P and not via N), on the conservative assumption that both the entire LAN and P have failed. Destinations for which P is a single point of failure must as usual be sent to P using an address that avoids the interface by which P is reached from S, i.e. to P not-via N. Similarly for routers Q and R. Notice that each router that is connected to a LAN must, as usual, advertise one not-via address for each neighbor. In addition, each router on the LAN must advertise an extra address not via the pseudonode (N). Notice also that each neighbor of a router connected to a LAN must advertise two not-via addresses, the usual one not via the neighbor and an additional one, not via either the neighbor or the pseudonode. The required set of LAN address assignments is shown in Figure 12 below. Each router on the LAN, and each of its neighbors, is advertising exactly one address more than it would otherwise have advertised if this degree of connectivity had been achieved using point-to-point links. Qs Qp Qc Cqn +--------------Q---------C | Qr Qn Cq | Asn Sa Sp Sq | Ps Pq Pb Bpn A--------S-------(N)-------------P---------B As Sr Sn | Pr Pn Bp | | Rs Rp Pd Drn +--------------R---------D Rq Rn Dr Figure 12: Local Area Networks 184.108.40.206.3. LAN Repair Using Diagnostics A more specific LAN repair can be undertaken by using diagnostics. In order to explicitly diagnose the failed network component, S correlates the connectivity reports from P and one or more of the other routers on the LAN, in this case, Q and R. If it lost connectivity to P alone, it could deduce that the LAN was still functioning and that the fault lay with either P, or the interface connecting P to the LAN. It would then repair to B not via P (and P not-via N for destinations for which P is a single point of failure) in the usual way. If S lost connectivity to more than one router on the LAN, it could conclude that the fault lay only with the LAN, and could repair to P, Q and R not-via N, again in the usual way. 5.7. Multiple Simultaneous Failures The failure of a node or an SRLG can result in multiple correlated failures, which may be repaired using the mechanisms described in this design. This design will not correctly repair a set of unanticipated multiple failures. Such failures are out of scope of this design and are for further study. It is important that the routers in the network are able to discriminate between these two classes of failure, and take appropriate action. 6.8. Optimizing not-via computations using LFAs If repairing node S has an LFA to the repair endpoint it is not necessary for any router to perform the incremental SPF with the link SP removed in order to compute the route to the not-via address Ps. This is because the correct routes will already have been computed as a result of the SPF on the base topology. Node S can signal this condition to all other routers by including a bit in its LSP or LSA associated with each LFA protected link. Routers computing not-via routes can then omit the running of the iSPF for links with this bit set. When running the iSPF for a particular link AB, the calculating router first checks whether the link AB is present in the existing SPT. If the link is not present in the SPT, no further work is required. This check is a normal part of the iSPF computation. If the link is present in the SPT, this optimization introduces a further check to determine whether the link is marked as protected by an LFA in the direction in which the link appears in the SPT. If so the iSPF need not be performed. For example, if the link appears in the SPT in the direction A->B and A has indicated that the link AB is protected by an LFA no further action is required for this link. If the receipt of this information is delayed, the correct operation of the protocol is not compromised provided that the necessity to perform a not-via computation is re-evaluated whenever new information arrives. This optimization is not particularly beneficial to nodes close to the repair since, as has been observed above, the computation for nodes on the LFA path is trivial. However, for nodes upstream of the link SP for which S-P is in the path to P, there is a significant reduction in the computation required. 7.9. Multicast Multicast traffic can be repaired in a similar way to unicast. The multicast forwarder is able to use the not-via address to which the multicast packet was addressed as an indication of the expected receive interface and hence to correctly run the required RPF check. In some cases, all the destinations, including the repair endpoint, are repairable by an LFA. In this case, all unicast traffic may be repaired without encapsulation. Multicast traffic still requires encapsulation, but for the nodes on the LFA repair path the computation of the not-via forwarding entry is unnecessary since, by definition, their normal path to the repair endpoint is not via the failure. A more complete description of multicast operation will be provided in a future version of this draft. 8.10. Fast Reroute in an MPLS LDP Network. Not-via addresses are IP addresses and LDP [LDP][RFC5036] will distribute labels for them in the usual way. The not-via repair mechanism may therefore be used to provide fast re-route in an MPLS network by first pushing the label which the repair endpoint uses to forward the packet, and then pushing the label corresponding to the not-via address needed to effect the repair. Referring once again to Figure 1, if S has a packet destined for D that it must reach via P and B, S first pushes B's label for D. S then pushes the label that its next hop to Bp needs to reach Bp. Note that in an MPLS LDP network it is necessary for S to have the repair endpoint's label for the destination. When S is effecting a link repair it already has this. In the case of a node repair, S either needs to set up a directed LDP session with each of its neighbor's neighbors, or it needs to use the next-next hop label distribution mechanism proposed in [NNHL]. 9.[I-D.shen-mpls-ldp-nnhop-label]. 11. Encapsulation Any IETF specified IP in IP encapsulation may be used to carry a not-vianot- via repair. IP in IP [IPIP],[RFC2003], GRE [GRE][RFC1701] and L2TPv3 [L2TPv3],[RFC3931], all have the necessary and sufficient properties. The requirement is that both the encapsulating router and the router to which the encapsulated packet is addressed have a common ability to process the chosen encapsulation type. When an MPLS LDP network is being protected, the encapsulation would normally be an additional MPLS label. In an MPLS enabled IP network an MPLS label may be used in place of an IP in IP encapsulation in the case above. 10.12. Routing Extensions IPFRR requires IGP extensions. Each IPFRR router that is directly connected to a protected network component must advertise a not-via address for that component. This must be advertised in such a way that the association between the protected component (link, router or SRLG) and the not-via address can be determined by the other routers in the network. It is necessary that not-via capable routers advertise in the IGP that they will calculate not-via routes. It is necessary for routers to advertise the type of encapsulation that they support (MPLS, GRE [GRE],GRE, L2TPv3 etc). However, the deployment of mixed IP encapsulation types within a network is discouraged. 11.13. Incremental Deployment Incremental deployment is supported by excluding routers that are not calculating not-via routes (as indicated by their capability information flooded with their link state information) from the base topology used for the computation of repair paths. In that way repairs may be steered around islands of routers that are not IPFRR capable. Routers that are protecting a network component need to have the capability to encapsulate and de-capsulate packets. However, routers that are on the repair path only need to be capable of calculating not-via paths and including the not-via addresses in their FIB i.e. these routers do not need any changes to their forwarding mechanism. 12.14. IANA considerationsConsiderations There are no IANA considerations that arise from this draft. 13.15. Security Considerations The repair endpoints present vulnerability in that they might be used as a method of disguising the delivery of a packet to a point in the network. The primary method of protection should be through the use of a private address space for the not-via addresses. These addresses MUST NOT be advertised outside the area, and SHOULD be filtered at the network entry points. In addition, a mechanism might be developed that allowed the use of the mild security available through the use of a key [GRE] [L2TPv3].[RFC1701] [RFC3931]. With the deployment of such mechanisms, the repair endpoints would not increase the security risk beyond that of existing IP tunnel mechanisms. An attacker may attempt to overload a router by addressing an excessive traffic load to the de-capsulation endpoint. Typically, routers take a 50% performance penalty in decapsulating a packet. The attacker could not be certain that the router would be impacted, and the extremely high volume of traffic needed, would easily be detected as an anomaly. If an attacker were able to influence the availability of a link, they could cause the network to invoke the not-via repair mechanism. A network protected by not-via IPFRR is less vulnerable to such an attack than a network that undertook a full convergence in response to a link up/down event. Intellectual Property Statement16. Acknowledgements The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertainauthors would like to the implementation oracknowledge contributions made by Alia Atlas and John Harper. 17. References 17.1. Normative References [RFC2119] Bradner, S., "Key words for use of the technology describedin this document or the extentRFCs to which any license under such rights might or might not be available; nor does it represent that it has made any independent effortIndicate Requirement Levels", BCP 14, RFC 2119, March 1997. 17.2. Informative References [I-D.ietf-bfd-base] Katz, D. and D. Ward, "Bidirectional Forwarding Detection", draft-ietf-bfd-base-07 (work in progress), January 2008. [I-D.ietf-rtgwg-ipfrr-framework] Shand, M. and S. Bryant, "IP Fast Reroute Framework", draft-ietf-rtgwg-ipfrr-framework-07 (work in progress), July 2007. [I-D.ietf-rtgwg-ipfrr-spec-base] Atlas, A., Zinin, A., Torvi, R., Choudhury, G., Martin, C., Imhoff, B., and D. Fedyk, "Basic Specification for IP Fast-Reroute: Loop-free Alternates", draft-ietf-rtgwg-ipfrr-spec-base-10 (work in progress), November 2007. [I-D.shen-mpls-ldp-nnhop-label] Shen, N., "Discovering LDP Next-Nexthop Labels", draft-shen-mpls-ldp-nnhop-label-02 (work in progress), May 2005. [ISPF] McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing Algorithm Improvements"", BBN Technical Report 3803, 1978. [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, October 1994. [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP Specification", RFC 5036, October 2007. Authors' Addresses Mike Shand Cisco Systems 250, Longwater Avenue. Reading, Berks RG2 6GB UK Email: firstname.lastname@example.org Stewart Bryant Cisco Systems 250, Longwater Avenue. Reading, Berks RG2 6GB UK Email: email@example.com Stefano Previdi Cisco Systems Via Del Serafico, 200 00142 Rome, Italy Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to identify any such rights. Information onthe procedures with respect to rights in RFC documents can be found in BCP 78rights, licenses and restrictions contained in BCP 79. Copies of IPR disclosures made to the IETF Secretariat78, 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. 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 toexcept as set forth therein, the IETF at ietf- email@example.com. Disclaimer of Validityauthors 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, THE IETF TRUST 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. Copyright statement Copyright (C)Intellectual Property The IETF Trust (2007). This document is subjecttakes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the rights, licenses and restrictions containedimplementation or use of the technology described in BCP 78, and except as set forth therein,this document or the authors retain all theirextent 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. Normative References There are no normative references. Informative References Internet-drafts are works in progress available from <http://www.ietf.org/internet-drafts/> [BFD] Katz, D., Ward, D., "Bidirectional Forwarding Detection", < draft-ietf-bfd-base-06.txt>, March 2007, (workInformation on the procedures with respect to rights in progress). [GRE] S. Hanks, T. Li, D. Farinacci, P. Traina. "Generic Routing Encapsulation (GRE)",RFC 1701,. October 1994. [IPFRR] Shand, M., Bryant, S., "IP Fast-reroute Framework", <draft-ietf-rtgwg-ipfrr-framework-07.txt>, June 2007, (workdocuments can be found in progress). [IPIP] Perkins, C., "IP encapsulation within IP", RFC 2003, October 1996 [ISPF] McQuillan, J., I. RicherBCP 78 and E. Rosen, "ARPANET Routing Algorithm Improvements", BBN Technical Report 3803, April 1978. [L2TPv3] J. Lau, Ed., et al., "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. [LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A.BCP 79. Copies of IPR disclosures made to the IETF Secretariat and B. Thomas, "LDP Specification", RFC 3036, January 2001. [LFA] A. Atlas, Ed, A. Zinin, Ed, "Basic Specification for IP Fast-Reroute: Loop-free Alternates", <draft-ietf-rtgwg-ipfrr-spec-base-06.txt>, March 2007, (work in progress). [NNHL] Shen, N., et al "Discovering LDP Next-Nexthop Labels", <draft-shen-mpls-ldp-nnhop-label-02.txt>, May 2005, (work in progress) [RFC2119] Bradner, S., "Key words for use in RFCsany assurances of licenses to Indicate Requirement Levels", RFC 2119 Acknowledgements The authors acknowledgebe made available, or the contributionsresult of an attempt made to obtain a general license or permission for the following peopleuse 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. 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 work:- Alia Atlas, John Harper. Authors' Addresses Stewart Bryant Cisco Systems, 250, Longwater Avenue, Green Park, Reading, RG2 6GB, United Kingdom. Email: firstname.lastname@example.org Stefano Previdi Cisco Systems Via Del Serafico, 200 00142 Rome, Italy Email: email@example.com Mike Shand Cisco Systems, 250, Longwater Avenue, Green Park, Reading, RG2 6GB, United Kingdom. Email: firstname.lastname@example.org. Please address the information to the IETF at email@example.com. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).