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Versions: 00 01 02 03                                                   
MPLS Working Group                                             H. Zhang
Internet Draft                                                    J. He
Intended status: Standards Track                                 Huawei

                                                                  H. Li
                                                           China Mobile
                                                       October 25, 2010
Expires: April 2011



               SD-Triggered Protection Switching in MPLS-TP
                        draft-zhl-mpls-tp-sd-03.txt


Abstract

   In MPLS-TP survivability framework, a fault condition includes both
   Signal Failure (SF) and Signal Degrade (SD) that can be used to
   trigger protection switching. This document describes Signal Degrade
   (SD) detection and the consequent protection switching mechanism.

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
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   This Internet-Draft will expire on April 25, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.



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Internet-Draft       draft-zhl-mpls-tp-sd-03.txt          October 2010


   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.

Table of Contents


   1. Introduction.................................................2
   2. Terminology..................................................3
   3. SD-Triggered Protection Switching Architecture...............4
   4. SD-Triggered Protection Solution Analysis....................4
      4.1. SD-Triggered Protection Based on OAM packets............5
         4.1.1. Detection of SD....................................5
         4.1.2. APS Protocol.......................................5
      4.2. Broadcast Bridge with Special APS Request Priority......6
         4.2.1. Protection Switching...............................6
         4.2.2. APS Protocol.......................................7
      4.3. Broadcast Bridge with Extra Protection Switching Coordination
       ............................................................8
         4.3.1. Protection Switching...............................8
         4.3.2. APS Protocol.......................................8
      4.4. Analysis................................................9
   5. Security Considerations......................................9
   6. IANA Considerations..........................................9
   7. Acknowledgments..............................................9
   8. References..................................................10
      8.1. Normative References...................................10
      8.2. Informative References.................................10
   Author's Addresses.............................................10

1. Introduction

   In packet transport network, protection switching can be triggered by
   fault conditions and external manual commands. The fault conditions
   include Signal Failure (SF) and Signal Degrade (SD). SF on a
   transport entity can be detected by fault management OAM functions;
   SD on a transport entity can be detected by performance monitoring
   OAM functions.

   The SD condition used for protection switching mostly depends on
   packet loss ratio. For some services that are sensitive to time and
   synchronization, the SD condition may depend on packet loss ratio,
   packet delay and delay variation.



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   In MPLS-TP OAM tools, the packet loss measurement (LM) is used to
   measure the amount of the lost service packets and compute the loss
   ratio of service packet. The packet Delay Measurement (DM) is used to
   measure the packet delay and delay variation by sending periodic DM
   packets during the diagnostic interval. The LM and DM mechanisms are
   out of scope of this document.

   The detection of SD (e.g. packet loss) should be based on service
   packets. But in some situation, there is no normal traffic on
   working/protection transport entity, which makes the detection of SD
   based on service packets impossible and causes switch flapping.

   This document describes the mechanism for SD detection and consequent
   protection switching in 1:1 and 1:n protection architecture.

2. Terminology

   The reader is assumed to be familiar with the terminology in MPLS-TP.
   The relationship between ITU-T and IETF terminologies on MPLS-TP can
   be found in [Rosetta stone].

       MPLS-TP: MPLS Transport Profile

       SF: Signal Failure

       SF-W: SF for working entity

       SF-P: SF for protection entity

       SD: Signal Degrade

       SD-W: SD for working entity

       SD-P: SD for protection entity

       APS: Automatic Protection Switching

       OAM: Operations, Administration and Maintenance

       CC/CV: Continuity Check/Connectivity Verification

       LM: Loss Measurement

       DM: Delay Measurement





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3. SD-Triggered Protection Switching Architecture

   The SD-triggered protection switching complies with general
   protection architecture.

   In case of MPLS-TP 1+1 protection architecture, the normal traffic is
   permanently sent on both working and protection entities by the
   permanent bridge at the source. Therefore, the detection of SD or the
   clearing of SD on both working and protection entities can be based
   on the characteristics of the normal traffic.

   In case of MPLS-TP 1:1 and 1:n protection architecture, the selector
   bridge is usually adopted at the source end. The normal traffic is
   transported on the working entity together with the OAM of the
   working entity, while on the protection entity only the OAM of the
   protection entity is transported alone so that it may not be possible
   to detect SD. If SD is detected on a working entity, protection
   switching is occurred, and the traffic is transported on the
   protection entity together with OAM. On the working entity OAM
   packets is transported alone, and the detected SD may be cleared even
   if the working entity is still degraded. This will result in another
   switch of traffic back to the working entity, and SD may be detected
   again, etc.

   As mentioned above, in case of 1:1 and 1:n protection architecture,
   the normal traffic will be either on the working transport entity or
   on the protection transport entity. This makes it impossible to
   detect SD on the standby entity. Consequently SD will be detected
   only after a protection switch and flapping may happen.

4. SD-Triggered Protection Solution Analysis

   In case of 1:1 and 1:n protection architecture, there are the
   following solutions to implement the SD-triggered protection
   switching without flapping.

   - The detection of SD is based on OAM packets; (Section 4.1)

   - The broadcast bridge has to be applied to enable SD detection in
      SD-triggered protection, together with

      - the special APS request priority (Section 4.2), or

      - the extra protection switching coordination (Section 4.3).

      The broadcasting is applied only in case protection switching is
      active.


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4.1. SD-Triggered Protection Based on OAM packets

4.1.1. Detection of SD

   In case of 1:1 and 1:n protection architecture, for the transport
   entity on which there is no normal traffic, the detection of SD can
   be based on OAM packets, e.g.

   - OAM packets for Test

   The Test OAM packets can be used to simulate the characteristics of
   the normal traffic and replace the normal service. With the
   performance monitoring OAM packets (LM) the SD condition of the
   transport entity can be detected.

   - OAM packets for CC/CV

   The CC/CV OAM packets can be used instead of normal service in packet
   loss measurement. That is, the performance monitoring OAM packets (LM)
   will count CC/CV packets and the loss measurement is based on CC/CV
   packets, which will be used as the input of SD condition of the
   transport entity where there is no normal service.

   The above-mentioned SD-triggered protection switching mechanism based
   on OAM packets (test or CC/CV) is general and flexible without
   constraint to the bridge type at source end, that may be selector
   bridge or broadcast bridge.

   The performance measurement by Test OAM packets is accurate. But the
   usage of test packets on the protection entity defeats the objective
   of the 1:1 and 1:n architectures, which is to have the protection
   entity bandwidth available for best effort traffic during the time
   there is no fault or degradation of the working transport entity. The
   test packets consume now this bandwidth. For the case of the 1:1
   protection, this makes the bandwidth usage of this 1:1 architecture
   similar to the bandwidth usage of the 1+1 architecture.

   OAM packets for CC/CV are sent in fixed transmission period (3.3ms,
   10ms, 1s, etc.) which doesn't exactly reflect the condition of real
   services. It is applicable only in places without strict requirement
   for SD measurement. The detection of SD by CC/CV is not recommended
   when the packet loss is caused by congestion.

4.1.2. APS Protocol

   In APS request types, similar to the definition of SF-W (SF for
   working entity) and SF-P (SF for protection entity), a definition of


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   SD-W (SD for working entity) and SD-P (SD for protection entity) is
   required to prevent flapping.

   The priority of SD-P and SD-W shall be fixed as SD-P > SD-W similar
   to SF-P > SF-W. In this case a protection switching based on SD
   detection on the working entity shall not be initiated if there
   exists also an SD condition on the protection transport entity.

4.2. Broadcast Bridge with Special APS Request Priority

   SD-triggered protection based on normal traffic can be implemented by
   adopting broadcast bridge at source end with the special APS request
   priority.

4.2.1. Protection Switching


            +----------+                         +---------+
            |          |                         |         |
   ---->----|---+------+------------>------------+---\     |
   Source   |   |      |      Working Entity     |    \    |     Sink
            |   |  /   |                         |     +---|---->----
            |   | /    |                         |         |
            |   +-  ---+------------>------------+---      |
            |          |     Protection Entity   |         |
            |          |                         |         |
            +----------+                         +---------+

          Figure 1 Normal Condition in 1:1 Protection Architecture

   In the normal state, the normal traffic is sent only on the working
   transport entity and only the SD condition of the working transport
   entity can be evaluated (see Figure 1).















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            +----------+                         +---------+
            |          |                         |         |
   ---->----|---+------+--------SD-->------------+---      |
   Source   |   |      |      Working Entity     |         |     Sink
            |   |      |                         |     +---|---->----
            |   |      |                         |    /    |
            |   +-+----+------------>------------+---/     |
            |          |     Protection Entity   |         |
            |          |                         |         |
            +----------+                         +---------+

        Figure 2 SD on Working Entity in 1:1 Protection Architecture

   When SD is detected on the working transport entity, the sink end
   sends SD-W indication to the source end and the selector at the sink
   end switches to the protection transport entity. The broadcast bridge
   at the source end will then send the normal traffic on both working
   and protection transport entities and the performance of both working
   and protection transport entities can be monitored for SD (see Figure
   2).

   If SD is detected on the protection transport entity as well, normal
   traffic will still be selected at the sink from the protection
   transport entity to avoid flapping between protection and working
   states.

4.2.2. APS Protocol

   In APS request types, a SD request code should be defined.

   In case a SD condition affects both transport entities, the first SD
   detected is not overridden by the second SD detected, to avoid
   flapping between protection entity and working entity.

   In case SD is detected simultaneously either as local or far end
   requests on both working and protection transport entities, SD on the
   protection transport entity is considered as having higher priority
   than SD on the working transport entity, and the normal traffic
   continues to be selected from the working transport entity (i.e. no
   unneeded protection switching is performed).

   A new bridge type code in APS protocol need be defined to distinguish
   the broadcast bridge and the selector bridge.





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4.3. Broadcast Bridge with Extra Protection Switching Coordination

   SD-triggered protection based on normal traffic can be implemented by
   adopting broadcast bridge at source end with the extra protection
   switching coordination.

   The SD-W based protection switch action described in section 4.2.1
   assumes that the probability of SD condition occurring on both
   working entity and protection entity at the same time is very small.
   When this assumption is not considered to be reasonable, the
   operation of the selector may be modified as described hereafter.

4.3.1. Protection Switching


            +----------+                         +---------+
            |          |                         |         |
   ---->----|---+------+--------SD-->------------+---\     |
   Source   |   |      |      Working Entity     |    \    |     Sink
            |   |      |                         |     +---|---->----
            |   |      |                         |         |
            |   +-+----+------------>------------+---      |
            |          |     Protection Entity   |         |
            |          |                         |         |
            +----------+                         +---------+

        Figure 3 SD on Working Entity in 1:1 Protection Architecture

   When the sink end detects an SD condition, it does not switch to the
   protection entity immediately. Instead, the broadcast bridge at the
   source end will first send the normal traffic on both working and
   protection transport entity after receiving SD-W indication sent by
   the sink end (see Figure 3). Then sink end is able to detect the SD
   condition of working and protection transport entity.

   If SD is detected on the protection transport entity as well, the
   normal traffic remains broadcasted to both working and protection
   transport entities and is selected from the working transport entity;
   if no SD is detected on the protection transport entity the normal
   traffic is selected from the protection entity.

4.3.2. APS Protocol

   The SD priority is the same as described in 4.1.2: SD-P > SD-W.

   A new bridge type code in APS protocol need be defined to distinguish
   the broadcast bridge and the selector bridge.


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4.4. Analysis

   In section 4.1, 4.2 and 4.3, the different mechanisms for detecting
   SD are described. The mechanism described in 4.2 is preferred because
   of its simplicity, accuracy and flexibility.

5. Security Considerations

   To be added in a future version of the document.

6. IANA Considerations

   IANA is requested to allocate two further APS request codes as
   follows:

      xx  Signal Degradation;

      yy  Selector Bridge for 1:1 case;

      zz  Broadcast Bridge for 1:1 case.

7. Acknowledgments

   The authors would like to thank Huub van Helvoort for his input to
   and review of the current document.























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8. References

8.1. Normative References

   [RFC5654] Niven-Jenkins,B., Brungard,D., and Betts,M., "Requirements
             of an MPLS Transport Profile", RFC5654, September 2009

   [ITU-T Recommendation G.808.1 Amd1] "Generic protection switching -
             Linear trail and subnetwork protection", ITU-T G.808.1
             Amendment 1, January 2009

8.2. Informative References

   [MPLS-TP Framework] Bocci,M., and Bryant,S., "A Framework for MPLS in
             Transport Networks", draft-ietf-mpls-tp-framework-10 (Work
             in progress), February 2010

   [MPLS-TP Survive Frmk] Sprecher,N., and Farrel,a., "Multiprotocol
             Label Switching Transport Profile Survivability Framework",
             draft-ietf-mpls-tp-survive-fwk-03(work in progress),
             November 2009

   [MPLS-TP OAM Requirements] Vigoureux,M., Ward,D., and Betts,M.,
             "Requirements for OAM in MPLS Transport Networks", draft-
             ietf-mpls-tp-oam-requirements-06(work in progress), March
             2010

Author's Addresses

   Haiyan Zhang
   Huawei Technologies Co., Ltd.

   Phone: +86-755-28972293
   Email: zhanghaiyan@huawei.com


   Jia He
   Huawei Technologies Co., Ltd.

   Phone: +86-755-28972293
   Email: hejia@huawei.com


   Han Li
   China Mobile Communications Corporation

   Email: lihan@chinamobile.com


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