Advertising MPLS labels in IGPs
draft-gredler-rtgwg-igp-label-advertisement-02

Abstract

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 trough 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, traffic engineering and egress ASBR link selection.

Requirements Language

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].

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on August 24, 2013.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Motivation and Applicability
• 2.1. Explicit One hop tunnels
• 2.2. Egress ASBR Link Selection
• 2.3. Explicit Path Routing through Label Stacking
• 2.4. Stitching MPLS Label Switched Path Segments
• 3. Acknowledgements
• 4. IANA Considerations
• 5. Security Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• 6.3. References
• Author's Address

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.

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 now creates forwarding state based on label information it may make sense to distribute label by the IGP as well. This section lists example applications of IGP distribution of MPLS labels.

2.1. Explicit One hop tunnels

Deployment of Loop free alternate backup technology RFC 5286 [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 [coverage-gap-analysis].

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.

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

        -------------traffic-flow--------->
        <-----------signaling-flow---------

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

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

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