MPLS WG                                                      K. Kompella
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                            L. Contreras
Expires: May 4, 2016                                      Telefonica I+D
                                                        November 1, 2015

                          Resilient MPLS Rings


   This document describes the use of the MPLS control and data planes
   on ring topologies.  It describes the special nature of rings, and
   proceeds to show how MPLS can be effectively used in such topologies.
   It describes how MPLS rings are configured, auto-discovered and
   signaled, as well as how the data plane works.  Companion documents
   describe the details of discovery and signaling for specific

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on May 4, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Provisioning  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Ring Nodes  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Ring Links and Directions . . . . . . . . . . . . . . . .   6
       3.3.1.  Bypass Links  . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Ring LSPs . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Installing Primary LFIB Entries . . . . . . . . . . . . .   7
     3.6.  Installing FRR LFIB Entries . . . . . . . . . . . . . . .   7
     3.7.  Protection  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Autodiscovery . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Ring Announcement Phase . . . . . . . . . . . . . . . . .  10
     4.3.  Mastership Phase  . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Ring Identification Phase . . . . . . . . . . . . . . . .  11
     4.5.  Ring Changes  . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Ring Signaling  . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Ring OAM  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Rings are a very common topology in transport networks.  A ring is
   the simplest topology offering link and node resilience.  Rings are
   nearly ubiquitous in access and aggregation networks.  As MPLS
   increases its presence in such networks, and takes on a greater role
   in transport, it is imperative that MPLS handles rings well; this is
   not the case today.

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   This document describes the special nature of rings, and the special
   needs of MPLS on rings.  It then shows how these needs can be met in
   several ways, some of which involve extensions to protocols such as
   IS-IS [RFC5305], OSPF[RFC3630], RSVP-TE [RFC3209] and LDP [RFC5036].

   The intent of this document is to handle rings that "occur
   naturally".  Many access and aggregation networks in metros have
   their start as a simple ring.  They may then grow into more complex
   topologies, for example, by adding parallel links to the ring, or by
   adding "bypass" links.  The goal here is to discover these rings
   (with some guidance), and run MPLS over them efficiently.  The intent
   is not to construct rings in a mesh network, and use those for

1.1.  Definitions

   A (directed) graph G = (V, E) consists of a set of vertices (or
   nodes) V and a set of edges (or links) E.  An edge is an ordered pair
   of nodes (a, b), where a and b are in V.  (In this document, the
   terms node and link will be used instead of vertex and edge.)

   A ring is a subgraph of G.  A ring consists of a subset of n nodes
   {R_i, 0 <= i < n} of V.  The directed edges {(R_i, R_i+1) and (R_i+1,
   R_i), 0 <= i < n-1} must be a subset of E (note that index arithmetic
   is done modulo n).  We define the direction from node R_i to R_i+1 as
   "clockwise" (CW) and the reverse direction as "anticlockwise" (AC).
   As there may be several rings in a graph, we number each ring with a
   distinct ring ID RID.

                                R0 . . . R1
                              .             .
                           R7                 R2
              Anti-     |  .        Ring       .  |
              Clockwise |  .                   .  | Clockwise
                        v  .      RID = 17     .  v
                           R6                 R3
                              .             .
                                R5 . . . R4

                        Figure 1: Ring with 8 nodes

   The following terminology is used for ring LSPs:

   Ring ID (RID):  A non-zero number that identifies a ring; this is
      unique in some scope of a Service Provider's network.  A node may
      belong to multiple rings.

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   Ring node:  A member of a ring.  Note that a device may belong to
      several rings.

   Node index:  A logical numbering of nodes in a ring, from zero upto
      one less than the ring size.  Used purely for exposition in this

   Ring master:  The ring master initiates the ring identification
      process.  Mastership is indicated in the IGP by a two-bit field.

   Ring neighbors:  Nodes whose indices differ by one (modulo ring

   Ring links:  Links that connnect ring neighbors.

   Bypass links:  Links that connnect non-neighboring ring nodes.

   Ring direction:  A two-bit field in the IGP indicating the direction
      of a link.  The choices are:

      UN: 00  undefined link

      CW: 01  clockwise ring link

      AC: 10  anticlockwise ring link

      BY: 11  bypass link

   Ring Identification:  The process of discovering ring nodes, ring
      links, link directions, and bypass links.

   The following notation is used for ring LSPs:

   R_k:  A ring node with index k.  R_k has AC neighbor R_(k-1) and CW
      neighbor R_(k+1).

   RL_k:  A (unicast) Ring LSP anchored on node R_k.

   CL_jk (AL_jk):  A label allocated by R_j for RL_k in the CW (AC)

   P_jk (Q_jk):  A Path (Resv) message sent by R_j for RL_k.

2.  Motivation

   A ring is the simplest topology that offers resilience.  This is
   perhaps the main reason to lay out fiber in a ring.  Thus, effective
   mechanisms for fast failover on rings are needed.  Furthermore, there

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   are large numbers of rings.  Thus, configuration of rings needs to be
   as simple as possible.  Finally, bandwidth management on access rings
   is very important, as bandwidth is generally quite constrained here.

   The goals of this document are to present mechanisms for improved
   MPLS-based resilience in ring networks (using ideas that are
   reminiscent of Bidirectional Line Switched Rings), for automatic
   bring-up of LSPs, better bandwidth management and for auto-hierarchy.
   These goals can be achieved using extensions to existing IGP and MPLS
   signaling protocols, using central provisioning, or in other ways.

3.  Theory of Operation

   Say a ring has ring ID RID.  The ring is provisioned by choosing one
   or more ring masters for the ring and assigning them the RID.  Other
   nodes in the ring may also be assigned this RID, or may be configured
   as "promiscuous".  Ring discovery then kicks in.  When each ring node
   knows its CW and AC ring neighbors and its ring links, and all bypass
   links have been identified, ring identification is complete.

   Once ring identification is complete, each node signals one or more
   ring LSPs RL_i.  RL_i, anchored on node R_i, consists of two counter-
   rotating unicast LSPs that start and end at R_i.  A ring LSP is
   "multipoint": any node R_j can use RL_i to send traffic to R_i; this
   can be in either the CW or AC directions, or both (i.e., load
   balanced).  Both of these counter-rotating LSPs are "active"; the
   choice of direction to send traffic to R_i is determined by policy at
   the node where traffic is injected into the ring.  The default is to
   send traffic along the shortest path.  Bidirectional connectivity
   between nodes R_i and R_j is achieved by using two different ring
   LSPs: R_i uses RL_j to reach R_j, and R_j uses RL_i to reach R_i.

3.1.  Provisioning

   The goal here is to provision rings with the absolute minimum
   configuration.  The exposition below aims to achieve that using auto-
   discovery via a link-state IGP (see Section 4).  Of course, auto-
   discovery can be overriden by configuration.  For example, a link
   that would otherwise be classified by auto-discovery as a ring link
   might be configured not to be used for ring LSPs.

3.2.  Ring Nodes

   Ring nodes have a loopback address, and run a link-state IGP and an
   MPLS signaling protocol.  To provision a node as a ring node for ring
   RID, the node is simply assigned that RID.  A node may be part of
   several rings, and thus may be assigned several ring IDs.

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   To simplify ring provisioning even further, a node N may be made
   "promiscuous" by being assigned an RID of 0.  A promiscuous node
   listens to RIDs in its IGP neighbors' link-state updates.  If N hears
   a non-zero RID from a neighbor, it joins that ring by taking on that
   RID.  However, if N hears more than one non-zero RID from its
   neighbors, N remains in promiscuous mode.  In many situations, the
   use of promiscuous mode means that only one or two nodes in the ring
   needs to be provisioned; everything else is auto-discovered.

   A ring node indicates in its IGP updates the ring LSP signaling
   protocols it supports.  This can be LDP and/or RSVP-TE.  Ideally,
   each node should support both.

3.3.  Ring Links and Directions

   Ring links must be MPLS-capable.  They are by default unnumbered,
   point-to-point (from the IGP point of view) and "auto-bundled".  The
   last attribute means that parallel links between ring neighbors are
   considered as a single link, without the need for explicit
   configuration for bundling (such as a Link Aggregation Group).  Note
   that each component may be advertised separately in the IGP; however,
   signaling messages and labels across one component link apply to all
   components.  Parallel links between a pair of ring nodes is often the
   result of having multiple lambdas or fibers between those nodes.  RMR
   is primarily intended for operation at the packet layer; however,
   parallel links at the lambda or fiber layer result in parallel links
   at the packet layer.

   A ring link is not provisioned as belonging to the ring; it is
   discovered to belong to ring RID if both its adjacent nodes belong to
   RID.  A ring link's direction (CW or AC) is also discovered; this
   process is initiated by the ring's ring master.  Note that the above
   two attributes can be overridden by provisioning if needed; it is
   then up to the provisioning system to maintain consistency across the

3.3.1.  Bypass Links

   Bypass links are discovered once ring nodes, ring links and
   directions have been established.  As defined earlier, bypass links
   are links joining non-neighboring ring nodes; often, this may be the
   result of optically bypassing ring nodes.  The use of bypass links
   will be described in a future version of this document.

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3.4.  Ring LSPs

   Ring LSPs are not provisioned.  Once a ring node R_i knows its RID,
   its ring links and directions, it kicks off ring LSP signaling
   automatically.  R_i allocates CW and AC labels for each ring LSP
   RL_k.  R_i also initiates the creation of RL_i.  As the signaling
   propagates around the ring, CW and AC labels are exchanged.  When R_i
   receives CW and AC labels for RL_k from its ring neighbors, primary
   and fast reroute (FRR) paths for RL_k are installed at R_i.  More
   details are given in Section 5.

   For RSVP-TE LSPs, bandwidths may be signaled in both directions.
   However, these are not provisioned either; rather, one does "reverse
   call admission control".  When a service needs to use an LSP, the
   ring node where the traffic enters the ring attempts to increase the
   bandwidth on the LSP to the egress.  If successful, the service is
   admitted to the ring.

3.5.  Installing Primary LFIB Entries

   In setting up RL_k, a node R_j sends out two labels: CL_jk to R_j-1
   and AL_jk to R_j+1.  R_j also receives two labels: CL_j+1,k from
   R_j+1, and AL_j-1,k from R_j-1.  R_j can now set up the forwarding
   entries for RL_k.  In the CW direction, R_j swaps incoming label
   CL_jk with CL_j+1,k with next hop R_j+1; these allow R_j to act as
   LSR for RL_k.  R_j also installs an LFIB entry to push CL_j+1,k with
   next hop R_j+1 to act as ingress for RL_k.  Similarly, in the AC
   direction, R_j swaps incoming label AL_jk with AL_j-1,k with next hop
   R_j-1 (as LSR), and an entry to push AL_j-1,k with next hop R_j-1 (as

   Clearly, R_k does not act as ingress for its own LSPs.  However, if
   these LSPs use UHP, then R_k installs LFIB entries to pop CL_k,k for
   packets received from R_k-1 and to pop AL_k,k for packets received
   from R_k+1.

3.6.  Installing FRR LFIB Entries

   At the same time that R_j sets up its primary CW and AC LFIB entries,
   it can also set up the protection forwarding entries for RL_k.  In
   the CW direction, R_j sets up an FRR LFIB entry to swap incoming
   label CL_jk with AL_j-1,k with next hop R_j-1.  In the AC direction,
   R_j sets up an FRR LFIB entry to swap incoming label AL_jk with
   CL_j+1,k with next hop R_j+1.  Again, R_k does not install FRR LFIB
   entries in this manner.

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3.7.  Protection

   In this scheme, there are no protection LSPs as such -- no node or
   link bypasses, no standby LSPs, no detours, and no LFA-type
   protection.  Protection is via the "other" direction around the ring,
   which is why ring LSPs are in counter-rotating pairs.  Protection
   works in the same way for link, node and ring LSP failures.

   If a node R_j detects a failure from R_j+1 -- either all links to
   R_j+1 fail, or R_j+1 itself fails, R_j switches traffic on all CW
   ring LSPs to the AC direction using the FRR LFIB entries.  If the
   failure is specific to a single ring LSP, R_j switches traffic just
   for that LSP.  In either case, this switchover can be very fast, as
   the FRR LFIB entries can be preprogrammed.  Fast detection and fast
   switchover lead to minimal traffic loss.

   R_j then sends an indication to R_j-1 that the CW direction is not
   working, so that R_j-1 can similarly switch traffic to the AC
   direction.  For RSVP-TE, this indication can be a PathErr or a
   Notify; other signaling protocols have similar indications.  These
   indications propagate AC until each traffic source on the ring AC of
   the failure uses the AC direction.  Thus, within a short period,
   traffic will be flowing in the optimal path, given that there is a
   failure on the ring.  This contrasts with (say) bypass protection,
   where until the ingress recomputes a new path, traffic will be

   Note that the failure of a node or a link will not necessarily affect
   all ring LSPs.  Thus, it is important to identify the affected LSPs
   (and switch them), but to leave the rest alone.

   One point to note is that when a ring node, say R_j, fails, RL_j is
   clearly unusable.  However, the above protection scheme will cause a
   traffic loop: R_j-1 detects a failure CW, and protects by sending CW
   traffic on RL_j back all the way to R_j+1, which in turn sends
   traffic to R_j-1, etc.  There are three proposals to avoid this:

   1.  Each ring node acting as ingress sends traffic with a TTL of at
       most 2*n, where n is the number of nodes in the ring.

   2.  A ring node sends protected traffic (i.e., traffic switched from
       CW to AC or vice versa) with TTL just large enough to reach the

   3.  A ring node sends protected traffic with a special purpose label
       below the ring LSP label.  A protecting node first checks for the
       presence of this label; if present, it means that the traffic is
       looping and MUST be dropped.

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   It is recommended that (2) be implemented.  The other methods are

4.  Autodiscovery

4.1.  Overview

   Auto-discovery proceeds in three phases.  The first phase is the
   announcement phase.  The second phase is the mastership phase.  The
   third phase is the ring identification phase.

                 /   \
                |     R0 . . . R1          R0 has MV = 11
                |  .    \        .         R1 has MV = 10
                R7       \________ R2      All other nodes have MV = 00
   Anti-     |  .                   .  |
   clockwise |  .        Ring       .  | Clockwise
             v  .      RID = 17     .  v
                R6                 R3
                   .             .
                     R5 . . . R4
                       \      /
                        \    /

               Figure 2: Ring with non-ring nodes and links

   In what follows, we refer to a ring Type-Length-Value (TLV).  This is
   a new TLV that contains an RID and associated flags.  A ring link TLV
   is a ring TLV that appears as a sub-TLV of a traffic engineering TLV
   (TE TLV) of each link that is identified as a ring link or a bypass
   link.  For IS-IS, the TE TLV is the extended reachability TLV; for
   OSPF, it is the Link TLV in the opaque TE LSA.  A ring node TLV is a
   ring TLV that appears as a sub-TLV of a "node TLV" once for each ring
   this node is participating in.  In IS-IS, the node TLV is the Router
   ID TLV; in OSPF, it is a new top-level TLV of the TE LSA.  The ring
   direction field is ignored in ring node TLVs.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Type (TBD)  |   Length = 8  |     Ring ID (4 octets) ...    |
     |      ... (RID continued)      |     Ring Flags (2 octets)     |

                           IS-IS Ring TLV Format

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |           Type (TBD)          |          Length = 12          |
     |                       Ring ID (4 octets)                      |
     |     Ring Flags (2 octets)     |         Pad (2 octets)        |
     Pad is set to zero when sending and ignored on receipt.

                           OSPF Ring TLV Format

              0                   1
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             |MV |RD |SP |OP |M|     MBZ     |
             MV: Mastership Value
             RD: Ring Direction
             SP: Signaling Protocols (10 = RSVP-TE; 01 = LDP)
             OP: OAM Protocols (10 = BFD; 01 = EFM)
             M : Elected Master (0 = no, 1 = yes)

                             Ring Flags Format

4.2.  Ring Announcement Phase

   Each node participating in an MPLS ring is assigned an RID; in the
   example, RID = 17.  A node is also provisioned with a mastership
   value.  Each node advertises a ring node TLV for each ring it is
   participating in, along with the associated flags.  It then starts
   timer T1.

   A node in promiscuous mode doesn't advertise any ring node TLVs.  If
   it hears exactly one non-zero RID from its IGP neighbors, it joins
   that ring, and sends one ring node TLV with that RID.  If it hears
   more than one RID from its IGP neighbors, it doesn't join any rings,
   and withdraws any ring node TLVs it may have advertised.

   The announcement phase allows a ring node to discover other ring
   nodes in the same ring so that a ring master can be elected and ring
   links be identified.

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4.3.  Mastership Phase

   When timer T1 fires, a node enters the mastership phase.  In this
   phase, each ring node N starts timer T2 and checks if it is master.
   If it is the node with the lowest loopback address of all nodes with
   the highest mastership values, N declares itself master by
   readvertising its ring node TLV with the M bit set.

   When timer T2 fires, each node examines the ring node TLVs from all
   other nodes in the ring to identify the ring master.  There should be
   exaclty one; if not, each node restarts timer T2 and tries again.
   The nodes that set their M bit should be extra careful in advertising
   their M bit in subsequent tries.

4.4.  Ring Identification Phase

   When there is exactly one ring master M, M enters the Ring
   Identification Phase.  M indicates that it has successfully completed
   this phase by advertising ring link TLVs.  This is the trigger for
   M's CW neighbor to enter the Ring Identification Phase.  This phase
   passes CW until all ring nodes have completed ring identification.

   In the Ring Identification Phase, a node X that has two or more IGP
   neighbors that belong to the ring picks one of them to be its CW ring
   neighbor.  If X is the ring master, it also picks a node as its AC
   ring neighbor.  If there are exactly two such nodes, this step is
   trivial.  If not, X computes a ring that includes all nodes that have
   completed the Ring Identification Phase (as seen by their ring link
   TLVs) and further contains the maximal number of nodes that belong to
   the ring.  Based on that, X picks a CW neighbor and inserts ring link
   TLVs with ring direction CW for each link to its CW neighbor; X also
   inserts a ring link TLV with direction AC for each link to its AC
   neighbor.  Then, X determines its bypass links.  These are links
   connected to ring nodes that are not ring neighbors.  X advertises
   ring link TLVs for bypass links by setting the link direction to
   "bypass link".

4.5.  Ring Changes

   The main changes to a ring are:

      ring link addition;

      ring link deletion;

      ring node addition; and

      ring node deletion.

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   The main goal of handling ring changes is (as much as possible) not
   to perturb existing ring operation.  Thus, if the ring master hasn't
   changed, all of the above changes should be local to the point of
   change.  Link adds just update the IGP; signaling should take
   advantage of the new capacity as soon as it learns.  Link deletions
   in the case of parallel links also show up as a change in capacity
   (until the last link in the bundle is removed.)

   The removal of the last ring link between two nodes, or the removal
   of a ring node is an event that triggers protection switching.  In a
   simple ring, the result is a broken ring.  However, if a ring has
   bypass links, then it may be able to converge to a smaller ring with
   protection.  Details of this process will be given in a future

   The addition of a new ring node can also be handled incrementally.
   Again, the details of this process will be given in a futre version.

5.  Ring Signaling

   A future version of this document will specify protocol-independent
   details about ring LSP signaling.

6.  Ring OAM

   Each ring node should advertise in its ring node TLV the OAM
   protocols it supports.  Each ring node is expected to run a link-
   level OAM over each ring and bypass link.  This should be an OAM
   protocol that both neighbors agree on.  The default hello time is 3.3

   Each ring node also sends OAM messages over each direction of its
   ring LSP.  This is a multi-hop OAM to check LSP liveness; typically,
   BFD would be used for this.  The node chooses the hello interval; the
   default is once a second.

7.  Security Considerations

   It is not anticipated that either the notion of MPLS rings or the
   extensions to various protocols to support them will cause new
   security loopholes.  As this document is updated, this section will
   also be updated.

8.  Acknowledgments

   Many thanks to Pierre Bichon whose exemplar of self-organizing
   networks and whose urging for ever simpler provisioning led to the
   notion of promiscuous nodes.

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9.  IANA Considerations

   There are no requests as yet to IANA for this document.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

10.2.  Informative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <>.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks, Inc.
   1194 N.  Mathilda Avenue
   Sunnyvale, CA  94089


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   Luis M. Contreras
   Telefonica I+D
   Ronda de la Comunicacion
   Sur-3 building, 3rd floor
   Madrid  28050


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