Network Working Group Hannes Gredler Internet Draft Juniper Networks Intended status: Standards Track Expires: April 2014 Yakov Rekhter Juniper Networks Sriganesh Kini Ericsson October 20 2013 Supporting Source/Explicitly Routed Tunnels via Stacked LSPs draft-gredler-spring-mpls-01.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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Gredler [Page 1] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Abstract This document describes how source/explicitly routed tunnels could be realized using stacked Label Switched Paths (LSPs). This document also describes how use of IS-IS/OSPF as a label distribution protocol fits into the MPLS architecture. Gredler [Page 2] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 Table of Contents 1 Specification of Requirements ......................... 3 2 Terminology ........................................... 4 3 Introduction .......................................... 4 4 Constructing Explicitly Routed Tunnels by using Stacked LSPs 5 4.1 Examples of Constructing Explicitly Routed Tunnels by Stacked LSPs 8 4.1.1 Explicitly Routed Tunnel with Single Hops ............. 8 4.1.2 Explicitly Routed Tunnel with Multi-Hops .............. 11 5 IS-IS or OSPF as Label Distribution Protocol .......... 13 5.1 Example of IS-IS/OSPF as Label Distribution Protocols . 14 6 IANA Considerations ................................... 15 7 Security Considerations ............................... 15 8 Acknowledgements ...................................... 15 9 Normative References .................................. 15 10 Informative References ................................ 16 11 Authors' Addresses .................................... 17 1. Specification of Requirements 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]. Gredler [Page 3] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 2. Terminology We use the term "explicitly routed tunnels" as a synonym for such terms as "source routed tunnels" and "source-initiated routed tunnels". Note that the term "source routed tunnel", or "source-initiated routed tunnel" does not imply that intermediate nodes of such a tunnel forward packets traversing the tunnel based upon source addresses of these packets. In the context of "source routed tunnels" and "source-initiated routed tunnels" the term "source" refers to the tunnels' ingress. This document assumes that a reader is familiar with the MPLS architecture [RFC3031] terminology. For a given Label Switched Path (LSP) of level m, as defined in section 3.15 of [RFC3031]: + the ingress node/router is the LSR that pushes the level m label, + intermediate nodes/routers are the LSRs making their forwarding decision on a level m label 3. Introduction MPLS architecture [RFC3031] defines the concept of explicitly routed tunnel as follows: If a Tunneled Packet travels from Ru to Rd over a path other than the Hop-by-hop path, we say that it is in an "Explicitly Routed Tunnel" where Ru and Rd are Label Switch Routers (LSRs). To realize explicitly routed tunnels [RFC3031] proposes to use explicitly routed Label Switched Paths (LSPs): An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an Explicitly Routed LSP Up until now there have been two possible protocols to instantiate/signal such explicitly routed LSPs - RSVP-TE ([RFC3209]) and CR-LDP ([RFC3212]). Gredler [Page 4] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 MPLS architecture ([RFC3031]) defines the notion of LSP hierarchy, as LSP tunnels within LSPs. Use of MPLS label stack mechanism allows LSP hierarchy to nest to any depth. In this document we specify the procedures to realize explicitly routed point-to-point tunnels by using LSP hierarchy, thus defining yet another possible mechanism to realize such tunnels. (Note though that the idea of using LSP hierarchy to realize explicitly routed tunnels is not new - e.g., Remote LFA [R-LFA] uses explicitly routed tunnels constructed by LSP hierarchy.) An essential part of MPLS is the notion of label distribution protocol. On the subject of whether it should be one, or more then one label distribution protocol, MPLS architecture ([RFC3031]) said the following: THE ARCHITECTURE DOES NOT ASSUME THAT THERE IS ONLY A SINGLE LABEL DISTRIBUTION PROTOCOL. In fact, a number of different label distribution protocols are being standardized. Up until now IETF standardized the following label distribution protocols for unicast: LDP ([RFC5036]), CR-LDP ([RFC3212]), RSVP-TE [RFC3209] and BGP ([RFC3107], [RFC4364], [RFC4761]). Recently there have been proposals ([gredler-isis], [gredler-ospf], [previdi-isis], [psenak-ospf]) to extend IS-IS [RFC1142] and OSPF [RFC1583] to make them yet another label distribution protocols. This document describes how use of IS-IS or OSPF as label distribution protocols fits into the MPLS architecture. This document also describes the benefits of using IS-IS/OSPF as label distribution protocols for the purpose of constructing explicitly routed tunnels with stacked LSPs. 4. Constructing Explicitly Routed Tunnels by using Stacked LSPs Instead of explicitly routed LSPs, one can use LSP hierarchy (stack of LSPs) to construct explicitly routed point-to-point tunnels as follows. Consider an explicitly routed point-to-point tunnel with an explicit route , where R(0) is the ingress of the tunnel and R(n) is the egress of the tunnel. Denote the LSPs needed to realize such a tunnel via an LSP stack as , where LSP(1) is the topmost and LSP(n) is the bottommost LSP in the stack. These LSPs are constructed as follows: Gredler [Page 5] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 + All the LSPs in the stack are constructed with the same ingress - R(0). (See further down on why this is needed.) + LSP(i) is constructed with R(i) as its egress (e.g., LSP(1) is constructed with R(1) as its egress, LSP(2) with R(2) as its egress, etc... LSP(n) with R(n) as its egress). + For every 0 < i < n, the first intermediate router of LSP(i+1). is constructed to be the same as the egress router of LSP(i). + The first intermediate router of LSP(1) is constructed to be one hop away from R(0). If R(1) is one hop away from R(0), then this intermediate router is also the egress of LSP(1) (in which case LSP(1) is a one-hop LSP). If R(1) is more than one hop away from R(0), then this intermediate router is some router other than R(1), and R(1) is still the egress of that LSP. + The first intermediate router of any LSP in the stack, could be either single or multi-hop away from the egress of that LSP. + All the LSPs in the stack are constructed with the penultimate hop popping. That is, for each LSP in the stack the penultimate router of that LSP pops the label corresponding to that LSP off the label stack before sending data to the egress router of that LSP. When R(i) and R(i+1) are single hop away from each other, the first intermediate router of LSP(i+1) is one hop away from the egress of that LSP. When R(i) and R(i+1) are multi-hop away from each other, the first intermediate router of LSP(i+1) is multi-hop away from the egress of that LSP. Following the above procedures, the LSPs in the stack satisfy the following properties: + All LSPs in the stack have the same ingress. + The egress of a given LSP in the stack is the first intermediate router of the next LSP in the stack. + The first intermediate router of the LSP at the top of the stack is one hop away from the ingress. + The first intermediate router of any LSP in the stack could be either single or multi-hop away from the egress of that LSP. (Thus the egress of a given LSP in the stack could be either single or multi-hop away from the egress of the next LSP in the Gredler [Page 6] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 stack.) Such stack of LSPs provides the functionality to forward a packet through a sequence of egresses of the LSPs on the stack - the sequence of these egresses represents the explicit route of the explicitly routed point-to-point tunnel constructed by using these stacked LSPs. The ingress of all these LSPs is the ingress of the tunnel. When the first intermediate router of a given LSP in the stack is multi-hop away from the egress of that LSP, the existing label distribution protocols (LDP, RSVP-TE, etc. ) can be used to establish a multi-hop LSP fragment for this LSP. When IS-IS or OSPF, in addition to being a routing protocol, is also used as a label distribution protocol (see section "IS-IS or OSPF as Label Distribution Protocol"), it can also be used to establish such multi- hop LSP fragment. To construct the label stack associated with the stack of LSPs the ingress of all these LSPs, R0, uses the following procedures: + For (n > i > 0) R(0) obtains from R(i) label binding for LSP(i+1) and places the label onto the stack, starting from the bottommost label (the label that corresponds to LSP(n). In section "IS-IS or OSPF as Label Distribution Protocol" we describe how IS-IS or OSPF with appropriate extensions could be used as a label distribution protocol to obtain such label bindings. If the first intermediate router of LSP(i+1) is either (a) a single hop away from the egress of that LSP, or (b) multi-hop away, and LDP is used as a label distribution protocol to establish a multi-hop LSP fragment between the first intermediate router and the egress of that LSP, then R(0) can use targeted LDP session with R(i) to obtain such label bindings. + For LSP(1) if R(1) is one hop away from R(0), then no label is needed (as LSP(1), just like all other LSPs in the stack, is constructed with the penultimate hop popping), and the label stack construction terminates with the topmost label that R(0) obtains from R(1) for LSP(2). + Otherwise, if R(1) is more than one hop away from R(0), then R(0) obtains label binding for LSP(1) from the first intermediate router of LSP(1), and places this label at the top of the stack. Gredler [Page 7] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 Note that the above procedures require all the LSPs in the stack to have the same ingress - R(0). This requirement comes from the observation that (a) R(0) is the router that constructs the whole label stack needed to realize the explicitly routed tunnel, and (b) according to MPLS architecture [RFC3031] when a router wants to create a label stack, the router has to be the head-end of all the LSPs corresponding to the labels in the stack. Since the MPLS label stack mechanism allows stack of LSPs to nest to any depth, use of LSP hierarchy for explicitly routed tunnels does not place any protocol restrictions on the number of entries in the explicit route of an explicitly routed tunnel. Note though that there may be some other restrictions (e.g., due to MTU, or hardware) that would place an upper bound on the depth of the label stack, and thus on the number of entries in the explicit route. Also, the depth of the label stack may have implications on ECMP, and specifically on the use of the Entropy label (see [kini] for more). 4.1. Examples of Constructing Explicitly Routed Tunnels by Stacked LSPs In this section we illustrate how to construct an explicitly routed tunnel by using stacked LSPs. The first example illustrates this for an explicitly routed tunnel where consecutive hops that define the tunnel are one hop away from each other. The second example illustrates this when these hops are more than one hop away from each other. 4.1.1. Explicitly Routed Tunnel with Single Hops Consider a network topology shown below: R0-----R4 | /| | / | | / | | / | | / | R1 R3 \ / \ / R2 Assume that R0 wants to construct an explicitly routed tunnel with Gredler [Page 8] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 (R0, R1, R2, R3, R4). The consecutive hops that define the tunnel are one hop away from each other. That is, R0 is one hop away from R1, R2 is one hop away from R1, R3 is one hop away from R2, and R4 is one hop away from R3. R0 constructs this tunnel using the following stack of LSPs: LSP1: (R0, R1) - top of the stack LSP2: (R0, R1, R2) LSP3: (R0, R2, R3) LSP4: (R0, R3, R4) - bottom of the stack Note that this stack of LSPs meets the requirements specified in section "Constructing Explicitly Routed Tunnels by using Stacked LSPs". Specifically, + All four LSPs in the stack have the same ingress - R0, which is also the ingress of the explicitly routed tunnel. + The egress of LSP1, R1, is the first intermediate router of the next LSP in the stack, LSP2. The egress of LSP2, R2, is the first intermediate router of the next LSP in the stack, LSP3. Likewise, the egress of LSP3, R3, is the first intermediate router of the next (and the last) LSP in the stack, LSP4. + The LSP at the top of the stack, LSP1, has its first intermediate router, R1, one hop away from its ingress, R0. Because of that, this intermediate router is also the egress of that LSP, and that LSP is a one-hop LSP. + In that particular example the first intermediate router of every LSP in the stack is one hop away from the egress of that LSP. That is, the first intermediate router of LSP2, R1, is one hop away from the egress of that LSP, R2; the first intermediate router of LSP3, R2, is one hop away from the egress of that LSP, R3; and the first intermediate router of LSP4, R3, is one hop away from the egress of that LSP, R4. As a result, the egress of a given LSP in the stack is one hop away from the egress of the next LSP in the stack. The first intermediate router of each of these LSPs creates label bindings for these LSPs as follows. R3 creates label binding for LSP4 by binding a particular label, L1, to the address of R4, creating a Next Hop Label Forwarding Entry (NHLFE) whose next hop is the link from R3 to R4, and setting the Incoming Label Map (ILM) so that L1 maps to that NHLFE. Likewise, R2 creates label binding for LSP3 by binding a particular label, L2, to the address of R3, creating an Gredler [Page 9] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 NHLFE whose next hop is the link from R2 to R3, and setting the ILM so that L2 maps to that NHLFE. Finally, R1 creates label binding for LSP2 by binding a particular label, L3, to the address of R2, creating an NHLFE whose next hop is the link from R1 to R2, and setting the ILM so that L3 maps to that NHLFE. To get from the first hop of LSP4, R0, to the second hop of LSP4, R3, the packet has to go through the LSP tunnel provided by LSP3. To get from the first hop of LSP3, R0, to the second hop of LSP3, R2, a packet has to go through the LSP tunnel provided by LSP2. To get from the first hop of LSP2, R0, to the second hop of LSP2, R1, a packet has to go through the LSP tunnel provided by LSP1. In order to accomplish this R0 constructs the label stack for the explicitly routed tunnel as follows: + Step 1: R0 obtains label binding L1 created by R3 for LSP4 (R0, R3, R4), and starts building the label stack by pushing L1 onto the label stack. + Step 2: R0 obtains label binding L2 created by R2 for LSP3 (R0, R2, R3), and pushes L2 into the stack. At this point the stack contains (L2, L1). + Step 3: R0 obtains label binding L3 created by R1 for LSP2 (R0, R1, R2), and pushes L3 into the stack. At this point the stack contains (L3, L2, L1). + Step 4: Since R0 and R1 are one hop away from each other the label stack construction is completed (R0 does not need a label for one-hop LSP1, as all the LSPs use penultimate hop popping). So far we did not say anything about how R0 obtains from R3 label binding for LSP4, from R2 label binding for LSP3, and from R1 label binding for LSP2. At least in principle, these label bindings could be obtain by such already defined label distribution protocols as LDP (to be more precise, targeted LDP if the two routers are more than one hop away from each other). E.g., if one uses targeted LDP, then R0 would need to dynamically establish and maintain a targeted LDP session with R3 and another targeted LDP session with R2 (R0 would maintain a "vanilla" LDP session with R1). Using these LDP sessions R0 would obtain from R3 label binding for LSP4, from R2 label binding for LSP3, and from R1 label binding for LSP2. Note that obtaining such labels bindings with targeted LDP may also require defining a new FEC to be used by targeted LDP. Gredler [Page 10] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 In section "IS-IS or OSPF as Label Distribution Protocol" we describe how IS-IS or OSPF with appropriate extensions could be used as a label distribution protocol to obtain such label bindings. When R0 wants to forward a packet along the explicitly constructed tunnel (R0, R1, R2, R3, R4), R0 pushes (L3, L2, L1) onto the label stack of the packet, and forwards the packet to R1. R1 performs the lookup on the topmost label, L3, and based on this lookup forwards the packet to R2. Prior to forwarding the packet to R2, R1 (acting as a penultimate hop for LSP2) pops the topmost label, L3. When R2 receives the packet, R2 performs the lookup on the topmost label, L2, and based on this lookup forwards the packet to R3. Prior to forwarding the packet to R3, R2 (acting as a penultimate hop for LSP3) pops the topmost label, L2. When R3 receives the packet, R3 performs the lookup on the topmost label, L1, and based on this lookup forwards the packet to R4. Prior to forwarding the packet to R4, R3 (acting as a penultimate hop for LSP4) pops the topmost label, L1. 4.1.2. Explicitly Routed Tunnel with Multi-Hops Consider a network topology shown below: R0---R1 / \ R2 R5---R6 \ / R3---R4 Assume that R0 wants to construct an explicitly routed tunnel with (R0, R4, R6) as hops. Note that R4 is multi-hop away from R0, and R6 is multi-hop away from R4. R0 constructs this tunnel using the following stack of LSPs: LSP1: (R0, R2, R3, R4) - top of the stack LSP2: (R0, R4, R5, R6) - bottom of the stack Note that this stack of LSPs meets the requirements specified in section "Constructing Explicitly Routed Tunnels by using Stacked LSPs". Specifically, + Both LSPs in the stack have the same ingress - R0, which is also the ingress of the explicitly routed tunnel. Gredler [Page 11] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 + The egress of LSP1, R4, is the first intermediate router of the next LSP in the stack, LSP2. + The LSP at the top of the stack, LSP1, has its first intermediate router, R2, one hop away from its ingress, R0. However, this intermediate router is not the egress of that LSP, and therefore this LSP is a multi-hop LSP. + In that particular example the first intermediate router of every LSP in the stack is multi-hop away from the egress of that LSP. That is, the first intermediate router of LSP1, R2, is multi-hop away from the egress of that LSP, R4; the first intermediate router of LSP2, R4, is multi-hop away from the egress of that LSP, R6. As a result, the egress of a given LSP in the stack is multi-hop away from the egress of the next LSP in the stack. In this example we assume that LDP is used as a label distribution protocol for both LSP1 and LSP2. Since R0 and R4 are not IGP neighbors, they are remote label distribution peers. Thus R0 and R4 use targeted LDP for label distribution. All other routers use "vanilla" LDP procedures. To get from the first hop of LSP2, R0, to its second hop, R4, the packet has to go through the LSP tunnel provided by LSP1. In order to accomplish this R0 constructs the label stack for the explicitly routed tunnel as follows: + Step 1: R0 (using targeted LDP) obtains label binding L1 created by R4 for LSP2 (R0, R4, R5, R6), and starts building the label stack by pushing L1 onto the label stack. + Step 2: R0 (using "vanilla" LDP procedures) obtains label binding L2 created by R2 for LSP1 (R0, R3, R4), and pushes L2 into the stack. At this point the stack contains (L2, L1). + Step 3: Since R0 and R1 are one hop away from each other the label stack construction is completed (R0 does not need a label for one-hop LSP1). A reader familiar with Remote LFA FRR [R-LFA] should be able to notice that the example described in this section is nothing more than an instance of Remote LFA FRR, where Remote LFA FRR provides fast reroute to the traffic going from R0 to R6 in the presence of the (R0, R2) link failure, with R0 being the Point of Local Repair (PLR), R4 being the PQ-node, and R6 being the ultimate destination. Gredler [Page 12] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 The explicitly routed tunnel (R0, R4, R6) consists of the PLR as the head-node, the PQ-node as the next hop, and the ultimate destination as yet another hop. 5. IS-IS or OSPF as Label Distribution Protocol When OSPF or IS-IS, in addition to being a routing protocol, is also used as a label distribution protocol (as proposed in [gredler-isis], [gredler-ospf], [previdi-isis], [psenak-ospf), the OSPF/IS-IS Link State Advertisements originated by a router carry label bindings for LSPs that either transit or originated by the router. Doing this allows to extend such LSPs. The criteria for selecting among all these LSPs a subset for which the router would originate label binding advertisements in IS-IS/OSPF are purely local to the router. The router could be either single or multi-hop away from the egresses of the LSPs in the subset. Existing label distribution protocols (LDP, RSVP-TE, etc.) can be used to establish multi-hop LSP fragments if the router is multi-hop away from the egress of a particular LSP in the subset. When IS-IS or OSPF, in addition to being a routing protocol, is also used as a label distribution protocol, it can also be used to establish such multi-hop LSP fragments. MPLS architecture [RFC3031] defines the notion of local/remote label distribution peers as follows: When two LSRs are IGP neighbors, we will refer to them as "local label distribution peers". When two LSRs may be label distribution peers, but are not IGP neighbors, we will refer to them as "remote label distribution peers." Following OSPF/IS-IS procedures each router passes Link State Advertisements originated by other routers unmodified. When these advertisements carry label binding information, this information is also passed unmodified. Therefore, the router that originates label bindings advertisements in IS-IS/OSPF can be either single or multi- hop away from the routers that receive and use these bindings. In the former case the IGP neighbors of the router that originates the advertisements will be the local label distribution peers of the router. In the latter case other routers in the same IGP domain will be the remote label distribution peers of the router. Use of OSPF or IS-IS as a label distribution protocol provides scalable support for remote label distribution peering in terms of the number of label distribution peers a given router has to maintain. This is because label distribution protocol messages (Link State Advertisements) are exchanged only between IGP neighbors, without requiring control plane peering between a router that Gredler [Page 13] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 originates Link State Advertisements and each of its remote label distribution peers. It is important to note that the existing MPLS control plane already has mechanisms/protocols to support remote label distribution peering (using BGP or targeted LDP [RFC5036]). Thus the practical relevance of the ability to provide scalable support for remote label distribution peering with IS-IS or OSPF as a label distribution protocol depends on a particular use case. If for a given subset of routers within an MPLS network each router within the subset is assigned a distinct index, then one could compress announcements of labels bound to the LSPs whose FECs are the IP addresses of these routers by (a) advertising these indices in IS- IS/OSPF, and (b) making each router advertise a label block in IS- IS/OSPF as well. A router R1 that advertises a given label block algorithmically binds a FEC associated with an IP address of some other router R2 to the label from that block that is identified by the index that R2 advertises in IGP. A router R1 that receives label block originated by some other router R2 can determine the label bound to a FEC associated with an IP address of some other router R3 by using the index advertised by R3 as an offset into the label block advertised by R2. Note that to avoid wasting labels this scheme requires a fairly dense assignment of indices. Also note that to expand the number of labels that a router advertises using label blocks, the router may advertise more than one label block. Note though, that the benefits of scaling improvements in terms of label distribution peering come at a cost, as every router in the domain ends up keeping all the labels assigned/bounded by every other router in the domain, whether it really needs to know them or not. Whether this cost is of practical significance depends on (a) the number of label bindings being advertised, and (b) the encoding of label bindings (e.g., use of label blocks vs enumerating each label binding). 5.1. Example of IS-IS/OSPF as Label Distribution Protocols In this section we illustrate how IS-IS/OSPF with extensions, as defined in [gredler-isis], [gredler-ospf], [previdi-isis], [psenak- ospf] could be used as a label distribution protocol to support explicitly routed tunnels realized by stacked LSPs. For the purpose of this illustration we assume the scenario described in section "Example of Constructing Explicitly Routed Tunnels by Stacked LSPs". In that example one of the key issues is the ability of R0 to obtain from R3 label binding for LSP4, from R2 label binding for LSP3, and from R1 label binding for LSP2. Gredler [Page 14] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 To obtains such label bindings, the Link State Advertisement originated by R3 carries label L1 (this is the label that R3 binds to LSP4). Using IS-IS/OSPF procedures this Link State Advertisement is propagated by R2 and R1 (as well as by R4) to R0. This is how R0 obtains from R3 label binding for LSP4. In a similar fashion, the Link State Advertisement originated by R2 carries label L2 (which is the label that R2 binds to LSP3). Using IS-IS/OSPF procedures this Link State Advertisement is propagated by R1 (as well as by R3 and R4) to R0. This is how R0 obtains from R2 label binding for LSP3. Likewise, the Link State Advertisement originated by R1 carries label L3 (which is the label that R1 binds to LSP2). Using IS-IS/OSPF procedures this Link State Advertisement is delivered to R0. This is how R0 obtains from R1 label binding for LSP2. Note that while from R0's perspective both R2 and R3 are remote label distribution peers, R0 does not maintain any control plane peering (e.g., targeted LDP) with either R2 or R3. 6. IANA Considerations This document introduces no new IANA Considerations. 7. Security Considerations TBD 8. Acknowledgements We would like to thank John Drake (Juniper Networks) and John Scudder (Juniper Networks) for their review and comments. We would also like to thank Bruno Decraene (Orange) for his review and comments. 9. Normative References [RFC3031] Rosen, E., et. al., "Multiprotocol Label Switching Architecture", RFC3031, January 2001 Gredler [Page 15] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 10. Informative References [kini] Kini, S., et. al., "Entropy labels for source routed stacked tunnels", draft-kini-mpls-entropy-label-src-stacked-tunnels, work in progress [gredler] Gredler, H., et. al., "Advertising MPLS labels in IGPs" draft-gredler-rtgwg-igp-label-advertisement, work in progress [gredler-isis] Gredler, H., et. al., "Advertising MPLS labels in IS- IS" draft-gredler-isis-label-advertisement, work in progress [gredler-ospf] Gredler, H., et. al., "Advertising MPLS labels in OSPF" draft-gredler-ospf-label-advertisement, work in progress [previdi-isis] Previdi, S., et. al., "IS-IS Extensions for Segment Routing" draft-previdi-isis-segment-routing-extensions, work in progress [psenak-ospf] Psenak, P., et. al., "OSPF Extensions for Segment Routing" draft-psenak-ospf-segment-routing-extensions, work in progress [R-LFA] Bryant, S., et. al., "Remote LFA FRR", draft-ietf-rtgwg- remote-lfa, work in progress [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC2119, March 1997 [RFC3107] Rekhter, Y., et. al., "Carrying Label Information in BGP-4", RFC3107, May 2001 [RFC3209] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC3209, December 2001 [RFC3212] Jamoussi, B., et. al., "Constraint-Based LSP Setup using LDP", RFC3212, January 2002 [RFC4364] Rosen E., et. al., "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC4364, February 2006 [RFC4761] Kompella, K., et. al., "Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling", RFC4761, January 2007 [RFC5036] L. Andersson, et. al., "LDP Specification", RFC5036, October 2007 Gredler [Page 16] Internet Draft draft-gredler-spring-mpls-01.txt October 2013 11. Authors' Addresses Hannes Gredler Juniper Networks e-mail: hannes@juniper.net Yakov Rekhter Juniper Networks e-mail: yakov@juniper.net Sriganesh Kini Ericsson Email: sriganesh.kini@ericsson.com Gredler [Page 17]