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Relaxing RSVP Loop Checking
draft-balls-ccamp-relax-loop-check-01

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Steve Balls , Jonathan Hardwick , Cyril Margaria
Last updated 2012-05-23
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draft-balls-ccamp-relax-loop-check-01
CCAMP                                                      S. Balls, Ed.
Internet-Draft                                               J. Hardwick
Updates: 3209 (if approved)                                   Metaswitch
Intended status: Informational                               C. Margaria
Expires: November 24, 2012                        Nokia Siemens Networks
                                                            May 23, 2012

                      Relaxing RSVP Loop Checking
                 draft-balls-ccamp-relax-loop-check-01

Abstract

   This specification relaxes the rules governing loop checking within
   RSVP.  These were originally defined in [RFC3209] and are too strict
   for the requirements of today's data planes.

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-
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   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."

   This Internet-Draft will expire on November 24, 2012.

Copyright Notice

   Copyright (c) 2012 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
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   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.

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1.  Introduction

   Generalized MPLS (GMPLS) Traffic Engineering (TE) Label Switched
   Paths (LSPs) are prohibited from passing through a single node more
   than once.  Today's data planes are such that allowing certain loops
   is required for some LSPs to be possible.

1.1.  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 [RFC2119].

2.  General Overview

   With today's data planes it is acceptable for a single data flow
   (LSP) to pass through a single control plane node on more than one
   occasion on the path from source to destination.  Currently control
   plane protocols will prevent such a path being managed in the control
   plane as they explicitly prevent routing loops.  It is desirable for
   such LSPs to be able to be managed in the same way as non-looping
   LSPs.

2.1.  Example in WDM networks

   In WDM networks it can be necessary to route the data via an
   additional box in order to fulfil regeneration or wavelength
   conversion requirements.  For example, consider the following simple
   example.

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                  +-----+          +-----+          +-----+
                  |     |  Link 1  |     |  Link 2  |     |
                  |  A  |----------|  B  |----------|  C  |
                  |     |          |     |          |     |
                  +-----+          +-----+          +-----+
                                     | |
                                     | |
                              Link 3 | | Link 4
                                     | |
                                     | |
                                   +-----+
                                   |     |
                                   |  D  |
                                   |     |
                                   +-----+

                                 Figure 1

   If node B cannot perform wavelength conversion but Link 1 and Link 2
   do not have a common free wavelength then the only way to set up a
   path from node A to node C will be via node D. This requires two
   passes through node B which to RSVP looks like a loop.

2.2.  Example using Connectivity Matrices

   In any type of network a specific node may have connectivity
   restrictions that limit the output ports available given the input
   ports.  For example, given the above network, where node B has the
   following connectivity restrictions.

                                  +-------+
                                  |       |
                                1 |   B   | 2
                              ----|-\   /-|----
                                  | |   | |
                                  | |   | |
                                  +-------+
                                    |   |
                                  3 |   | 4
                                    |   |

                                 Figure 2

   As in the above example, the only way to set up a path from node A to

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   node C will be via node D. This requires two passes through node B
   which to RSVP looks like a loop.

2.3.  Example with additional label restrictions

   Connections between ports on a node may be restricted based on
   labels.  Consider the following network.

                 +-----+          +-----+          +-----+
                 |     |  Link 1  |     |  Link 2  |     |
                 |  A  |----------|  B  |----------|  C  |
                 |     |          |     |          |     |
                 +-----+          +-----+          +-----+
                    \               |
                     \              |
                      \      Link 3 |
                       \            |
                        \         +-----+
                         \        |     |
                  Link 4  +-------|  D  |
                                  |     |
                                  +-----+

                                 Figure 3

   This network has the following properties.

   o  Node A is electro-optical outputting Lambda 1 and can switch
      Lambda 2.

   o  Node D can convert between Lambda 1 and Lambda 2.

   o  Link 1 and Link 3 have Lambda 1 available.

   o  All links have Lambda 2 available.

   To setup a path from A to C in this network, the LSP must pass
   through Link 1 twice: once using Lambda 1 and once using Lambda 2.
   This results in the path A-B-D-A-B-C being taken which requires two
   passes through node A.

3.  Existing workaround

   In current networks it is possible to support such paths either

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   through management configuration at each node, or splitting the path
   into two or more signalling sessions.  In the above examples this can
   be achieved with one session from A to D, and a second session from D
   to C. It would also require management on node D to join the data
   paths together.  It is desirable that a single signalling session can
   be used to set up such paths, thus only requiring management input at
   the ingress.

4.  Solution

4.1.  Overview

   To support such networks, the rules governing RSVP loop checking are
   relaxed.  No changes to protocol messages are made.

4.2.  Assumptions and limitations

   These changes are only applicable to GMPLS out of band signalling
   when using point to point data links.

4.3.  General Rules

   The following rules govern the changes in behaviour that allow RSVP
   loop checking to be relaxed while still setting up non-looping data
   paths in RSVP.

   o  For each pass through the node, the pair of inbound and outbound
      data interfaces and labels must be different.

   Are there any further rules when are loops OK and when not OK?

4.4.  RRO handling

   Section 4.4.4 of [RFC3209] states that RSVP must reject a Path
   message if the receiving router is already in the RRO.  This is now
   relaxed to allow such a condition provided a different interface-
   label pair is used in each case.  If the router has existing session
   state for a received Path message, and it MUST verify that the newly
   requested data path (input and output interface and label) is
   different from the existing data path(s) for that session, and the
   existing data path(s) is (are) present earlier in the RRO.  If this
   is not the case, the router MUST return a "Routing problem" PathErr
   message with the error value "loop detected".

   In order to carry out this checking correctly, specific interfaces
   and labels SHOULD be recorded in the RRO.  If this is not the case,
   each node can only verify the path is acceptable against local state

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   and should not reject the RRO if the local state is valid.

   It is allowable for local policy to exist to limit the number of
   different paths through a router in a single LSP instance.  If this
   limit is exceeded the router SHOULD return a "Routing problem"
   PathErr message with the error value "loop detected".  This local
   policy is not intended to be advertised in routing.  It is present as
   a backstop to protect against malicious Path messages consuming all
   resources on the router.

4.5.  ERO handling

   Sections 4.3.4.1 and 4.3.5 of [RFC3209] also state that RSVP must
   detect and avoid loops.  This checking is also relaxed to allow loops
   in the cases stated above.  Again, local policy can limit the number
   of different paths through a router in a single LSP instance.  A
   router may "look ahead" in the ERO to determine such local policy
   will be exceeded in advance of it happening and SHOULD return a
   "Routing problem" PathErr message with the error value "loop
   detected" in such a case.

   When calculating or expanding an ERO a router may include multiple
   entries through a single router.  If the ERO contains loose hops that
   form a loop, and a node determines a non-looping route is available,
   it MAY remove the loop from the ERO.

4.6.  Interface handling

   As stated in the general rules, an implementation supporting multiple
   passes through a node must ensure that for each pass the input and
   output interfaces and labels are different.

   Internally, this means that if a Path message is received using a
   different input interface this may no longer mean the LSP has been
   rerouted upstream.  Implementations must check the RRO to determine
   the correct behaviour when processing such a Path message.  Care must
   be taken to handle valid cases where the incoming label can change.

4.7.  Signalling

   For the avoidance of doubt, no new signalling is being defined in
   this draft.

   The behaviour of refresh or error messages is unchanged and should
   therefore be sent along the looped path (if present).  Nodes SHOULD
   NOT shortcut the loop.

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4.8.  Error Handling

   How to behave when receiving a PathErr with error value "loop
   detected" is out of scope of this draft and is a local implementation
   decision.  For example, it may choose to try and recalculate the path
   mandating that the error node is avoided, or does not support
   looping.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   In principle these changes to RSVP pose no security exposures over
   and above [RFC3209].  However, by allowing loops a single LSP can now
   consume multiple resources.  As suggested local policy can limit the
   number of paths and thus the resource a single LSP can consume.

7.  Normative References

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

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

Authors' Addresses

   Steve Balls (editor)
   Metaswitch
   100 Church Street
   Enfield,   EN2 6BQ
   UK

   Phone: +44 208 366 1177
   Email: steve.balls@metaswitch.com

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   Jonathan Hardwick
   Metaswitch
   100 Church Street
   Enfield,   EN2 6BQ
   UK

   Phone: +44 208 366 1177
   Email: jonathan.hardwick@metaswitch.com

   Cyril Margaria
   Nokia Siemens Networks
   St Martin Strasse 76
   Munich,   81541
   Germany

   Phone: +49 89 5159 16934
   Email: cyril.margaria@nsn.com

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