Advertising LSPs with Segment Routing

describes the segment routing architecture. In segment routing, LDP-like forwarding behavior along shortest paths is achieved using globally-known node labels. Globally-known node labels can be distributed in one of two ways. Each router can directly advertise a globally unique node label in the IGP. Or each router can advertise a globally unique node index value as well a locally assigned label block, allowing any router in the IGP area to determine the mapping of locally-assigned label to globally unique node index for any other router in the area. In order to forward traffic along strict explicit paths, segment routing uses stacks of local adjacency labels. Each router uses the IGP to advertise locally significant adjacency labels corresponding to each of the router's outgoing interfaces. This allows any router in the IGP area to construct an arbitrary forwarding path by imposing a stack of adjacency labels on a packet. Forwarding is accomplished at each router by reading the top label on the stack to determine next-hop interface (based on its adjacency label to interface mapping), popping that top label, and forwarding the packet out the next-hop interface. The above is only a short description of the use of node and adjacency labels in segment routing. See for more detail on node and adjacency label semantics as well as combining node and adjacency labels in a label stack. In addition to node and adjacency label advertisements, which advertise label bindings corresponding to nodes and interfaces, it is useful to advertise more abstract label bindings in the IGP, using a Binding advertisement. This draft describes how label bindings corresponding to RSVP, LDP, BGP labeled-unicast, and static LSPs are advertised in segment routing and how these labels can be combined with other segment routing labels to create forwarding paths. This draft also describes how context labels for egress node protection are advertised using a Binding advertisement.

An LSP and its associated label is advertised in the IGP using the Binding advertisement extensions defined in and . The router which is the ingress for the LSP advertises the label as well as the Forwarding Equivalency Class(FEC) associated with the LSP. The advertisement may include other information that describes the LSP. An Explicit Route Object(ERO) may be included to describe the path taken by the LSP. An ERO metric value may be included to indicate the cumulative IGP or TE metric associated with the LSP along the path described by the ERO.

Consider the network shown in . A router R4 advertising a label L1 and a FEC F using a Binding advertisement to advertise an LSP is indicating the following forwarding behavior. (See for a description of the segment routing label operation and forwarding terminology.) Assume that a particular packet P arrives at R4 with a segment list/label stack whose incoming active segment is label L1. R4 performs a NEXT operation on the segment list (equivalent to a label POP operation). Assume that this results in a label stack of depth m-1, with the m-1 label being the new active segment with respect to segment routing forwarding actions. R4 forwards P to the router associated with FEC F (call it R7) by means of a Label Switched Path. The precise set of MPLS label operations that get P from R4 to R7 is not specified. However, the label operations MUST satisfy the properties of a "Label Switched Path of level m" described in Section 3.15, where R4 plays the role of LSP Ingress and R7 plays the role of LSP Egress. In particular, if R4 (acting as the LSP Ingress) pushes a level m label onto P's label stack, then the forwarding decision on each router between R4 and R7 cannot make use of label information below the level m label in the label stack. Instead, the forwarding decision at R7 (acting as the LSP Egress) does make use of label information (or packet header information) below the level m label. Note that the level m LSP from R4 to R7 is not exclusively reserved for carrying traffic that enters at R4 via the segment routing mechanism described above. In general, it will also carry labeled or unlabeled packets that enter the LSP via other mechanisms. Also, the packets may have entered the LSP at a router other than R4 in the case of label merging. For example, in situations where R7 needs to make use of the value of the level m label for the processing of other traffic, R7 may distribute a non-null label to the penultimate hop router in the level m LSP from R4 to R7. R7 needs to be able to properly and consistently process the packet P that originated at R4 via the segment routing mechanism described above, regardless of the details of how R7 is performing the LSP egress role for other traffic. If m=1, then R7 should forward P based on the received packet header information. If m>1, then the m-1 label of P at R7 MUST correspond to a label distributed by R7. If the m-1 label corresponds to a segment routing label, then R7 MUST treat the m-1 label as the incoming active segment and perform the corresponding incoming label operations and forwarding action. R7 MUST the perform any necessary additional label operations to ensure that the next SR-capable router that receives the packet can correctly determine the incoming active segment. For example, if R7 distributed a non-null or explicit null label to the penultimate hop router in the level m LSP from R4 to R7, then R7 MUST POP that label before performing the segment routing label operation required by the incoming active segment, the m-1 label. Continuing with the example above, a segment routing capable router R1 uses the information describing the LSP contained in the Binding advertisement (FEC, ERO, metric, etc.) to determine if it wants to use that LSP as part of a longer forwarding path. If so, R1 uses the label L1 advertised by R4 for the LSP in the construction of a label stack. R1 can use any combination of segment routing labels stacked above L1 to define a path to reach R1. The only requirement is that label L1 be the incoming active segment when the packet reaches R4. This will ensure that R4 forwards the packet to R7. If R1 has determined that R7 supports segment routing, then R1 can also include additional segment routing labels below L1 in the label stack to specify the forwarding path beyond R7. The description above assumes that R1 is responsible for computing the forwarding path and the associated label stack. However, the same forwarding behavior can be achieved if a centralized controller is used to compute the path and communicate the associated label stack to R1 via PCEP with the appropriate segment routing extensions (see ). The same is true of the examples for specific label distribution protocols provided below.

To simplify the diagrams and descriptions of the examples in this draft, we assume that all routers advertise a router-id of the form 192.0.2.XX, where XX is the router number of the form RXX. For example, in R16 advertises router-id=192.0.2.16 as a loopback address. Unless otherwise stated, links between routers all have the same IGP metric of 10. The node-SID index value for a router with name RXX will be XX. For example, R23 in has an index value of 23. We assume that all routers are advertising the same SR Global Block of 1000-1099. For example, for R11 to send a packet to R23 on the shortest path, R11 sends the packet to R13 with the top label equal to 1023.

| |<--- SR --->| |<------RSVP------>| ]]> shows a network that uses both segment routing and RSVP. Segment routing is enabled on R11, R13, R21, R22, R23, R16, R17, R18, R26, R27, and R28. RSVP is enabled on R13, R14, R15, R16, R23, R24, R25, R26, and R35. Note that both segment routing and RSVP are enabled on R13, R23, R16, and R26. R23 uses RSVP to signal an LSP from R23 to R16, following the path R23->R24->R25->R26->R16. R23 uses a Binding advertisement to advertise the following values: label value = 2099 FEC = 192.0.2.16/32 ERO = (192.0.2.24[strict], 192.0.2.25[strict], 192.0.2.26[strict], 192.0.2.16[strict]) R11 receives the Binding advertisement and decides (based on some policy) to forward traffic from R11 to R18 along a path that consists of the following partial paths: the shortest path from R11 to R23 the path from R23 to R16 following the explicit path R23->R24->R25->R26->R16 the shortest path from R16 to R18 In order to accomplish this, R11 sends packets to R13 with the label stack = <1023,2099,1018>. Label 1023 corresponds to the node-SID label for R23, and thus results in forwarding along the shortest path from R11 to R23. The packets will arrive at R23 with label stack = <2099,1018>. The top label value of 2099 at R23 will result in forwarding of packets along the path R23->R24->R25->R26->R16 using the label SWAP operations signalled by RSVP for this LSP. With penultimate hop popping, the packets will arrive at R16 with label stack = <1018>. Label 1018 corresponds to the node-SID label for R18, and thus results in forwarding along the shortest path from R16 to R18. R11 may use the information about the primary path in this Binding advertisement to decide whether or not to construct SR label stacks that use this RSVP LSP. For example, R11 may have a requirement to avoid forwarding traffic over primary paths that include R35. The ERO advertised in this Binding advertisement satisfies this requirement. Note that the scenario described in this example is very similar to the commonly deployed LDP-over-RSVP architecture, with shortest path routing with LDP at the edges and explicit routing with RSVP in the core. However, it is difficult to achieve fine-grained forwarding behavior described in this example using LDP-over-RSVP. In an LDP-over-RSVP network, the only way to influence which LDP/edge traffic gets tunnelled over which RSVP LSPs is to advertise the RSVP LSPs as forwarding adjacencies (FAs) and tune the IGP metrics of the FAs. It may be difficult or impossible to achieve the desired mapping of LDP/edge traffic over RSVP LSPs by tuning the metrics of FAs. Instead, with the SR-and-RSVP architecture described above, it is possible to achieve an arbitrary mapping of edge traffic to core RSVP LSPs using a maximum label stack depth of 3, assuming shortest path forwarding between edge and core nodes via node-SIDs. One could also build label stacks using adjacency labels advertised by SR-capable routers in the edge networks in order to forward traffic along non-shortest paths in the edge networks. More explicit control of forwarding paths in the edge networks would come at the expense of deeper label stacks.

Continuing with the example of , it may also be desirable for R23 to provide information about backup paths that may be used in the event of a failure affecting the primary path. For example, assume that R23 has signalled a primary LSP along the path R23->R24->R25->R26->R16 and a backup LSP along the path R23->R13->R14->R35->R16. R23 uses a Binding advertisement to advertise the following values: label value = 2099 FEC = 192.0.2.16/32 ERO = (192.0.2.24[strict], 192.0.2.25[strict], 192.0.2.26[strict], 192.0.2.16[strict]) backup ERO = (192.0.2.13[strict], 192.0.2.14[strict], 192.0.2.35[strict], 192.0.2.16[strict]) R11 may use the information about the backup path in this Binding advertisement to decide whether or not to construct SR label stacks that use this RSVP LSP. For example, R11 may have a requirement to avoid forwarding traffic over primary or backup paths that include R35.

| |<--- LDP --->| ]]> shows a network that uses both segment routing and LDP. Segment routing is enabled on R31-34, while LDP is enabled on R34-36. Note that both segment routing and LDP are enabled on R34. Also note that LDP is NOT enabled on R31, R32 and R33. R34 has received a label mapping for FEC=192.0.2.36/32 from R35 using LDP, corresponding to an LDP LSP from R34 to R36 along the shortest path. R34 uses a Binding advertisement to advertise the following values: label value = 2088 FEC = 192.0.2.36/32 After receiving this Binding advertisement, R31 can forward traffic to R36 by sending packets to next-hop R32 with label stack = <1034,2088>. The packets will arrive at R34 with label stack = <2088>. The top label value of 2088 at R34 will result in forwarding of packets along the shortest path to R36 using the label SWAP operations for FEC = 192.0.2.36/32 signalled by LDP. Now we look at what is needed to forward labeled traffic from R36 in the LDP-only domain to R31 in the SR-only domain. R34 can get a packet to R31 using R31's node-SID label (1031). In order for R34 to apply label=1031 to packets in FEC 192.0.2.31, packets need to arrive at R34 with a label corresponding to FEC 192.0.2.31. Therefore R34 should not use penultimate hop popping when it distributes a label to the LDP-only domain. The routers in the LDP-only domain (R34, R35, and R36) advertise label mappings for FEC 192.0.2.31/32 using LDP. This corresponds to normal LDP behavior based on since R34, R35, and R36 each has an entry in its routing table for 192.0.2.31/32, and in the absence of segment routing, R34 is an egress LSR with respect to FEC 192.0.2.31/32. This will allow packets to travel from R36 to R34 using LDP labels. Via LDP, R34 advertises the label value 3154 for FEC 192.0.2.31/32 (instead of implicit or explicit null). Packets from R36 arrive at R34 with label=3154. R34 recognizes that label=3154 corresponds to FEC 192.0.2.31/32, so it swaps the label with outgoing label=1031 (the node-SID for R31), and forwards the packet to next-hop R33. The packet will ultimately be delivered to R31 over the shortest path in the SR-only network using the node-SID. Note that describes a different method (utilizing a Segment Routing Mapping Server) to allow SR-enabled nodes to send traffic to LDP nodes that do not support SR.

|<----- LDP ----->|<---- LDP --->| ^ ^ ^ ^ ^ ^ ^ ^ \___/ \_____/ \___/ \_____/ BGP-LU BGP-LU BGP-LU BGP-LU ]]> shows a network that uses segment routing together with BGP labeled-unicast (BGP-LU) . In this example, segment routing is enabled on R71-75 (region 1). LDP is enabled on R81-85 (region 2) and R91-95 (region 3). In addition, BGP-LU sessions exist between R75 and R83, R83 and R85, R85 and R93, and R93 and R95. Via LDP, R93 learns a label value of 3009 from R94 corresponding to FEC 192.0.2.95/32. Via its BGP-LU session with R85, R93 advertises a label value of 4021 corresponding to prefix 192.0.2.95/32, with a next-hop of 192.0.2.93. Via its BGP-LU session with R83, R85 advertises a label value of 4031 corresponding to prefix 192.0.2.95/32, with a next-hop of 192.0.2.85. Via its BGP-LU session with R75, R83 advertises a label value of 4041 corresponding to prefix 192.0.2.95/32, with a next-hop of 192.0.2.83. R75 uses a segment routing Binding advertisement to advertise the following values: label value = 2077 FEC = 192.0.2.95/32 ERO = (192.0.2.83[strict]) In this example, R75 has included an ERO list with a single element corresponding to the directly connected next-hop from R75 to R83. Its inclusion by R75 is optional, but the information may be useful to routers that receive the Binding advertisement. R75 could also optionally include a loose hop for R93 in the ERO since it knows that information from the BGP Originator_ID attribute of the BGP-LU advertisement. R71 receives the Binding advertisement via the IGP. In order to send a labeled packet to R95, R71 needs to construct a label stack that causes the packet to arrive at R75 with top label=2077. If R71 wants the packet to take the shortest path from R71 to R75, then it sends the packet to R72 with label stack = <1075,2077>. (Label 1075 is the node-SID for R75 based on the conventions in .) The packet arrives at R75 with label stack = <2077>. R75 swaps label 2077 with label 4041 and forwards the packet to next-hop R83. R83 swaps label 4031 with label 4021, pushes the LDP label distributed by R84 for FEC=192.0.2.85, and forwards the packet to next-hop R84. The packet arrives at R85 with label 4031 exposed, so R85 swaps label 4031 with label 4021, and forwards the packet to next-hop R93. Finally R93 swaps label 4021 with label 3009 and forwards the packet to next-hop R94, allowing the packet to be delivered to R95 via LDP label operations. If instead R71 wants the packet to take the path R71->R73->R74->R75, then it would send the packet to R73 with two adjacency labels corresponding to the links between R73 and R74 and between R74 and R75, followed by the label 2077. The above description accounts for sending labeled packets from a source in a segment routing region to a destination in another region using paths established by BGP-LU. Since the example is not symmetric with respect to source and destination, for completeness, we illustrate how traffic to send traffic from another region to a destination in a segment routing region using LSPs established by BGP-LU, which in this example corresponds to sending a packet from R95 to R71. R75 knows that it can send a packet to R71 by imposing a node-SID label of 1071. Via its BGP-LU session with R83, R75 advertises a label value of 4052 for prefix 192.0.2.71/32, with a next-hop of 192.0.2.75. The string of BGP-LU sessions from R83 to R85 to R93 to R95 advertise label bindings for prefix 192.0.2.71/32 such that R95 can send a packet to R93 with the appropriate BGP-LU label that the packet will arrive at R75 with label 4052 exposed as the top label. When R75 receives this packet, it swaps label 4052 with label 1071 and forwards the packet to next-hop R72, resulting in the packet being delivered to R71.

| ]]> In , each grouping of routers (R41-46, R51-54, R61-66, and R71-74) represents a different autonomous system (AS 4,5,6, and 7 respectively). Segment routing is enabled on R41-46. R43 has two interfaces that connect to routers outside of its AS. One egress interface connects to R51 while another connects to R61. It is desirable for routers in AS4 to be able to send traffic to R43 with a label stack that indicates the interface that R43 should use to send the traffic. This can be accomplished by having R43 advertise label bindings for one-hop static LSPs corresponding to each of its egress interfaces. In order to advertise the egress interface connected to R51, R43 uses a segment routing Binding advertisement to advertise the following values: label value = 2033 FEC = 192.0.2.51/32 ERO = (192.0.2.51[strict]) In order to advertise the egress interface connected to R61, R43 uses a segment routing Binding advertisement to advertise the following values: label value = 2034 FEC = 192.0.2.61/32 ERO = (192.0.2.61[strict]) R41 receives the Binding advertisements via the IGP. In order to send a packet out R43's interface to R51, R41 constructs a packet with label stack = <1043,2033> and sends it to R42. (Label 1043 is the node-SID for R43 based on the conventions in .) The packet arrives at R43 with label stack = <2033>. R43 will POP label 2033 and send the unlabeled packet out the interface to R51. Similarly in order to send a packet out R43's interface to R56, R41 constructs a packet with label stack = <1043,2034> and sends it to R42.

| ]]> describes a mechanism to provide fast protection of RFC 2547/4364 based VPN traffic against egress PE failure. In the example in , CE1 and CE2 are each dual-homed to two different PEs and belong to the VPN-A. In the absence of a failure, traffic travels from CE1 to CE2 over the path CE1->PE3->P7->P8->PE5->CE2. Using the mechanism described in , upon the failure of PE5, the point of local repair (P8) can use a loop-free alternate (LFA) to divert the traffic destined for protected PE (PE5) to a Protector function co-located with an alternate egress PE for CE2 (PE6). Before forwarding the traffic to PE6, P8 pushes a context label associated with PE5 (the protected PE). This context label allows PE6 to interpret the VPN label in the context of PE5 VPN label advertisements, since the VPN label on these packets was originally imposed by the ingress PE (PE3) based on the assumption that they would be delivered to PE5. We assume in this example that there is a single context-identifier corresponding to the protected PE (PE5) with a value of 203.0.113.5. The prefix 203.0.113.5/32 is advertised in the IGP and in LDP by PE5. PE5 advertises VPN-IP prefixes via BGP with next-hop = 203.0.113.5. This causes VPN traffic from PE3 to PE5 will take an LDP transport tunnel corresponding to FEC 203.0.113.5/32. PE6 advertises a context label for context-identifier 203.0.113.5 using a segment routing Binding advertisement with the following values: label value = 2066 FEC = 203.0.113.5/32 Mirror context = TRUE When P8 receives this Binding advertisement via the IGP, it creates a forwarding table entry for LDP traffic in FEC 203.0.113.5 that will be activated immediately when the link to PE5 fails. This behavior is triggered by setting Mirror context = TRUE in the advertisement. This backup forwarding table entry uses a loop-free alternate (LFA) or remote LFA to send traffic to PE6 (using FEC 192.0.2.6 which is the router-id of PE6) along a path that avoids passing through PE5. Importantly, the backup forwarding table entry pushes label 2066 into the packet before applying any labels associated with the repair path to PE6. When the link to PE5 fails and P8 activates the backup forwarding table entry for LDP traffic in FEC 203.0.113.5, that traffic will be diverted to PE6. The packets will arrive at PE6 with top label = 2066, followed by the VPN label advertised by PE5. PE6 pops label 2066, and interprets the next label as a VPN label advertised by PE5. PE6 has been listening in on PE5s BGP advertisements, so it knows the mapping between a given VPN label advertised by PE5 and the actual VPN.

This document introduces no new IANA Considerations.

TBD

The authors would like to thank Bruno Decraene and Nick Slabakov for their suggestions and review.