Routing Area Working Group                               H. Gredler, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                               S. Amante
Expires: October 05, 2013                   Level 3 Communications, Inc.
                                                               T. Scholl
                                                                L. Jalil
                                                           April 5, 2013

                    Advertising MPLS labels in IGPs


   Historically MPLS label distribution was driven by session oriented
   protocols.  In order to obtain a particular routers label binding for
   a given destination FEC one needs to have first an established
   session with that node.

   This document describes a mechanism to distribute FEC/label mappings
   through flooding protocols.  Flooding protocols publish their objects
   for an unknown set of receivers, therefore one can efficiently scale
   label distribution for use cases where the receiver of label
   information is not directly connected.

   Application of this technique are found in the field of backup (LFA)
   computation, Label switched path stitching, egress protection,
   traffic engineering and egress ASBR link selection.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on October 05, 2013.

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   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Motivation and Applicability . . . . . . . . . . . . . . . . .  3
   3.  Use cases for IGP label distribution . . . . . . . . . . . . .  3
     3.1.  Increase LFA backup coverage using 'Directed Forwarding' .  3
     3.2.  Egress ASBR Link Selection . . . . . . . . . . . . . . . .  5
     3.3.  Tail end protection of BGP service routes  . . . . . . . .  6
     3.4.  Explicit Path Routing through Label Stacking . . . . . . .  7
     3.5.  Stitching MPLS Label Switched Path Segments  . . . . . . .  8
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   MPLS label allocations are predominantly distributed by using the LDP
   [RFC5036], RSVP [RFC5151] or labeled BGP [RFC3107] protocol.  All of
   those protocols have in common that they are session oriented, which
   means that in order to learn the Label Information database of a
   particular router one needs to have a direct control-plane session
   using the given protocol.

   There are a couple of interesting use cases where the consumer of a
   MPLS label allocation may not be adjacent to the router having
   allocated the label.  Bringing up an explicit session using existing
   label distribution protocols between the non-adjacent label allocator
   and the label consumer is the existing remedy for this dilemma.

   For LDP protection routing LDP next next hop labels [NNHOP] have been
   proposed to provide the 2 hop neighborhood labels.  While the 2 hop
   neighborhood provides good backup coverage for the typical network
   operator topology it is inadequate for some sparse for example ring
   like topologies.

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   Depending on the application, retrieval and setup of forwarding state
   of such >1 hop label allocations may only be transient.  As such
   configuring and un-configuring the explicit session is an operational
   burden and therefore should be avoided.

2.  Motivation and Applicability

   It may not be immediate obvious, however introduction of Remote LFA
   [I-D.ietf-rtgwg-remote-lfa] technology has implied important changes
   for an IGP implementation.  Previously the IGP had a one-way
   communication path with the LDP module.  The IGP supplies tracking
   routes and LDP selects the best neighbor based upon FEC to tracking
   routes exact matching results.  Remote LFA changes that relationship
   such that there is a bi-directional communication path between the
   IGP and LDP. Now the IGP needs to learn about if a label switched
   path to a given destination prefix has been established and what the
   ingress label for getting there is.  The IGP needs to push that label
   for the tracking routes of destinations beyond a remote LFA neighbor.

   Since the IGP is now aware of label switched paths and it does create
   forwarding state based on label information it makes sense to
   distribute label switched paths by the IGP as well.

3.  Use cases for IGP label distribution

   This section lists example use cases which illustrate IGP
   distribution of MPLS label switched paths.

3.1.  Increase LFA backup coverage using 'Directed Forwarding'

   Deployment of Loop free alternate backup technology [RFC5286] results
   in backup graphs whose coverage is highly dependent on the underlying
   Layer-3 topology.  Typical network deployments provide backup
   coverage less than 100 percent (see RFC 6571 Section 4.3 for Results
   [RFC6571]) for IGP destination prefixes.

   By closer examining the coverage gaps from the referenced production
   network topologies, it becomes obvious that most topologies lacking
   backup coverage are close to ring shaped topologies (Figure 1).

   Remote LFA [I-D.ietf-rtgwg-remote-lfa] has introduced the notion of a
   "remote" LFA neighbor.  This helper router which is both in P and Q
   space could forward the traffic to the final destination.  Router 'H'
   is in P space, however due to the actual metric allocation router 'H'
   is not in Q space.

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              |  D  |
             /       \
            / M1      \ M4 >= (M1 + M2 + M3)
           /           \
    +-----+             +-----+
    | PLR |             |  H  |
    +-----+             +-----+
           \           /
            \ M2      / M3
             \       /
              |  E  |

   The protection router (PLR) evaluates for a primary path to
   destination 'D' if {E -> H -> D} is a viable backup path.  Because
   the metric M4 {H -> D} is higher than the sum of the original primary
   path and the path from router 'H' to the PLR, this particular path
   would result in a loop and therefore is rejected.

   Now consider that router 'H' would advertise a label for FEC 'D',
   which has the semantics that H will POP the label and forward to the
   destination node 'D'. This is done irrespective of the underlying IGP
   metric 'M4' it is a 'strict forwarding' label.  The PLR router can
   now construct a label stack where the outermost label provides
   transport to router 'H'. The next label on the MPLS stack is the IGP
   learned 'strict forwarding label' label.  Note that the label 'strict
   forwarding' semantics are similar to a 1-hop ERO (Explicit route
   object). The Remote 'LFA' calculation would need to get changed, such
   that even if a node is not in PQ space, but rather in P space, it may
   get used as a backup neighbor if it advertises a strict forwarding
   label to the final destination.  A recursive version of the algorithm
   is applicable as well as long a node in P space has some non looping
   LSP path to the final destination.  The PLR router can now program a
   backup path irrespective of the undesirable underlying layer-3

   Using existing tunnels for backup routing has been previously
   described in [I-D.bryant-ipfrr-tunnels].  Section 5.2.3 'Directed
   forwarding' describes an option to insert a single MPLS label between
   the tunnel and the payload.  Traffic may thereby be directed to a

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   particular neighbor.  The mechanism described in this document, is an
   MPLS specific manifestation of 'Directed forwarding'.

3.2.  Egress ASBR Link Selection

   In the topology described in Figure 2.  router 'S' is facing a
   dilemma.  Router S receives a BGP route from all of its 4 upstream
   routers.  Using existing mechanism the provider owning AS1 can
   control the loading of its direct links *to* its ASBR1 and ASBR2,
   however it cannot control the load of the links beyond the ASBRs,
   except manually tweaking the eBGP import policy and filtering out a
   certain prefix.  It would be more desirable to have visibility of all
   four BGP paths and be able to control the loading of those four paths
   using Weighted ECMP.  Note that the computation of the 'Weight'
   percentage and the component doing this computation (Router embedded
   or SDN) is outside the scope of this document.

   If all the ASes would be under one common administrative control then
   the network operator could deploy a forwarding hierarchy by using
   [RFC3107] to learn about the remote-AS BGP nexthop addresses and
   associated labels.  An ingress router 'S' would then stack the
   transport label to its local egress ASBR and the remote ASBR supplied
   label.  In reality it is hard to convince a peering AS to deploy
   another protocol just in order to easier control the egress load on
   the WAN links for the ingress AS.

   A 'strict forwarding' paradigm would solve this problem: An Egress
   ASBR (e.g.  ASBR 1 and 2) allocates a strict forwarding label toward
   all of its peering ASes and advertises it into its local IGP. The
   forwarding state of all those labels is to POP off the label and
   forward to the respective interface.  The ingress router 'S' then
   builds a MPLS label stack by combining its local transport label to
   ASBR1 or ASBR2 with the IGP learned label pointing to the remote-AS

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                            :      AS3
                            :   +-------+
           AS1             _:___+ ASBR3 |
                          / :   +-------+
                 +-------+  :
                 | ASBR1 |  :      AS4
                 +-------+  :   +-------+
                /         \_:___+ ASBR4 |
               /            :   +-------+
              /             :
       +-----+              :
       |  S  |              :
       +-----+              :      AS5
              \             :   +-------+
               \           _:___+ ASBR5 |
                \         / :   +-------+
                 +-------+  :
                 | ASBR2 |  :      AS6
                 +-------+  :   +-------+
                          \_:___+ ASBR6 |
                            :   +-------+

   ASBR {1,2} may want to periodically check the liveliness state to the
   endpoint of the label (ASBR {3,4,5,6}) which they are advertising.
   BFD Echo mode [RFC5880] is suitable technology to ensure liveliness
   state of undirectional links.

3.3.  Tail end protection of BGP service routes

   [I-D.minto-2547-egress-node-fast-protection] describes how PE routers
   advertising their labeled routes could get protected from node-
   failures.  This is a local repair technology being dependent upon
   successful construction of a LFA path from any PLR to the 'protector
   PE' in a network.

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                                 //                     \\
                                //                       \\
   +------+   +------+   +------+   +------+   +------+   +------+
   | CE1  +---+PEingr+---+PEprot+---+  P   +---+ PLR  +-X-+PEegr +
   +------+   +------+   +------+   +------+   +------+   +------+
                 \\              \                       /  //
                                   \                   /
                                    \    +------+     /
                                     \___+  CE2 +____/

   Assume a primary LDP LSP from the 'ingress PE' router to the 'egress
   PE' router.  Now consider the FRR calculation from the 'PLR' router
   if its direct link to the 'egress PE' router fails (X) or the entire
   'egress PE' goes down.  The 'PLR' cannot find a LFA path to local-
   repair the traffic to the 'protector PE'.  This is because the
   'protector PE' router has not yet converged, and hence would want to
   forward the traffic to the original PE egress router, such that a
   temporal forwarding loop would be established.

   Using IGP advertisement of MPLS Labels the 'protector PE' router can
   advertise a Label which identifies backup traffic such that arriving
   traffic, can be forwarded using a context specific forwarding table,
   rather than the main LSP transit table.  The advertised context label
   is a unidirectional pointer to the 'egress PE' router.  The LFA
   calculation of the PLR gets augmented such that it considers
   advertised labels pointing to the original tail-end of the LSP. The
   network learns thereby an egress LSP point which is is as good as the
   original egress LSP point.

3.4.  Explicit Path Routing through Label Stacking

   IGP advertised strict forwarding labels can be utilized for
   constructing simple EROs via virtue of the MPLS label stack.  In a
   classical traffic engineering problem (Figure 4) is illustrated.  The
   best IGP path between {S,D} is {S, R3, R4, D}.  Unfortunately this
   path is congested.  It turns out that the links {S, R1}, {R1, R4} and
   {R2, R4} do have some spare capacity.  In the past a C-SPF
   calculation would have passed the ERO {S, R1, R4, R2, D} down to RSVP
   for signaling.  The conceptional problem with RSVP signaled paths is
   that they cannot by shared with other nodes in the network.  Hence
   all potential ingress routers need to setup their "private" ERO path
   and allocate network signaling resources and forwarding state.

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              +----+         +----+
              | R1 +---------+ R2 |
              +----+    2    +----+
             /      \           |  \
            / 2      \          |   \ 2
           /          \         |    \
    +-----+            \        |     +-----+
    |  S  |             \ 5     | 5   |  D  |
    +-----+              \      |     +-----+
           \              \     |    /
            \ 1            \    |   / 1
             \              \   |  /
              +----+         +----+
              | R3 +---------+ R4 |
              +----+    1    +----+

   Consider now every router along the path does advertise a strict
   forwarding label for its direct neighbor.  Router S could now
   construct a couple of paths for avoiding the hot links without
   explicitly signaling them.

   o  {S, R1, R2, D}

   o  {S, R1, R4, D}

   o  {S, R1, R4, R2, D}

   Note that not every hop in the ERO needs to be unique label in the
   label stack.  This is undesired as existing forwarding hardware
   technology has got upper limits how much labels can get pushed on the
   label stack.  In fact an existing tunnel (for example LDP tunnel {S,
   R1, R2} can be reused for certain path segments.

3.5.  Stitching MPLS Label Switched Path Segments

   One of the shortcomings of existing traffic-engineering solutions is
   that existing label switched paths cannot get advertised and shared
   by many ingress routers in the network.  In the example network
   (Figure 5) a LSP with an ERO of {R4, R2, R6} has been established in
   order to utilize two unused north / south links.  The only way to
   attract traffic to that LSP is to advertise the LSP as a forwarding
   adjacency.  This causes loss of the original path information which
   might be interesting for a potential router which might wants to use
   this LSP for backup purposes.  A computing router would need to have
   all underlying fate-sharing and bandwidth utilization information.

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              +----+         +----+         +----+
              | R1 +---------+ R2 +---------+ R5 |
              +----+    2    +----+    2    +----+
             /      \           |  \              \
            / 2      \          |   \              \ 2
           /          \         |    \              \
     +----+            \        |     \              +----+
     | S  |             \ 5     | 5    \ 5           | D  |
     -----+              \      |       \            +----+
           \              \     |        \          /
            \ 1            \    |         \        / 1
             \              \   |          \      /
              +----+         +----+         +----+
              | R3 +---------+ R4 |---------+ R6 |
              +----+    1    +----+    1    +----+

   The IGP on R4 can now advertise the LSP segment by advertising its
   ingress label and optionally pass the original ERO, such that any
   upstream router can do their fate-sharing computations.  Potential
   ingress routers now can use this LSP as a segment of the overall LSP.
   Furthermore ingress routers can combine label advertisements from
   different routers along the path.  For example router S could stacks
   its LDP path to R2 {S, R1, R2} plus the IGP learned RSVP LSP {R4, R5,
   R6} plus a strict forwarding label {R6, D}.

4.  Acknowledgements

   Many thanks to Yakov Rehkter, Ina Minei, Stephane Likowski and Bruno
   Decraene for their useful comments.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   This document does not introduce any change in terms of IGP security.
   It simply proposes to flood existing information gathered from other
   protocols via the IGP.

7.  References

7.1.  Normative References

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

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 2001.

   [RFC5036]  Andersson, L., Minei, I. and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

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   [RFC5151]  Farrel, A., Ayyangar, A. and JP. Vasseur, "Inter-Domain
              MPLS and GMPLS Traffic Engineering -- Resource Reservation
              Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
              5151, February 2008.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC6571]  Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N. and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, June 2012.

7.2.  Informative References

              Bryant, S., Filsfils, C., Previdi, S. and M. Shand, "IP
              Fast Reroute using tunnels", Internet-Draft draft-bryant-
              ipfrr-tunnels-03, November 2007.

              Bryant, S., Filsfils, C., Previdi, S., Shand, M. and S.
              Ning, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-
              remote-lfa-01, December 2012.

              Jeganathan, J. and H. Gredler, "2547 egress PE Fast
              Failure Protection", Internet-Draft draft-minto-2547
              -egress-node-fast-protection-01, October 2012.

   [NNHOP]    Chen, E., Shen, N. and A. Tian, "Discovering LDP Next-
              Nexthop Labels", November 2005, <

Authors' Addresses

   Hannes Gredler, editor
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089


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   Shane Amante
   Level 3 Communications, Inc.
   1025 Eldorado Blvd
   Broomfield, CO 80021


   Tom Scholl
   Seattle, WA


   Luay Jalil
   1201 E Arapaho Rd.
   Richardson, TX 75081


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