MPLS Working Group H. Zhang
Internet Draft J. He
Intended status: Standards Track
H. Li
China Mobile
Expires: April 2010 October 19, 2009
SD-Triggered Protection Switching in MPLS-TP
draft-zhl-mpls-tp-sd-00.txt
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Abstract
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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.
Table of Contents
1. Introduction.................................................2
2. Terminology..................................................3
3. SD-Triggered Protection Switching............................4
3.1. General Operation.......................................4
3.1.1. General protection architecture....................4
3.1.2. APS Protocol.......................................4
3.2. SD-Triggered Protection Based on other Traffic (i.e. non
normal traffic)..............................................5
3.2.1. Detection of SD....................................5
3.2.2. Protection Switching and APS Protocol..............6
3.3. SD-Triggered Protection Based on Normal Traffic.........6
3.3.1. Broadcast Bridge with special APS request priority.6
3.3.2. Broadcast Bridge with extra protection switching
coordination..............................................7
4. Analysis.....................................................7
5. Security Considerations......................................7
6. IANA Considerations..........................................7
7. Acknowledgments..............................................8
8. References...................................................9
8.1. Normative References....................................9
8.2. Informative References..................................9
Author's Addresses..............................................9
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.
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
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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) must 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
3.1. General Operation
3.1.1. 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 original traffic.
In case of MPLS-TP revertive 1:1 and 1:n protection architecture:
- If the protection switching is not active, normal traffic is
transported together with OAM on the working entity while on the
protection entity OAM is transported alone or together with the
possible extra traffic.
- If the protection switching is active, only OAM is transported on
the working entity. Normal traffic is transported on the
protection entity together with OAM. The extra traffic originally
transported on protection entity is disrupted.
In case of MPLS-TP non-revertive 1:1 and 1:n protection architecture:
- If the protection switching is not active, normal traffic is
transported together with OAM on the working entity while on the
protection entity OAM is transported alone or together with the
possible extra traffic.
- If the protection switching is active, OAM is transported alone or
possibly together with extra traffic on the working entity. Normal
traffic is transported on the protection entity together with OAM
traffic. That is, the normal traffic is switched from the working
entity to the protection entity and the extra traffic is switched
from the protection entity to the working entity.
As mentioned above, in case of 1:1 and 1:n protection architecture,
there may be no normal traffic on working/protection transport entity,
which makes the detection of SD impossible and consequently switch
flapping may happen.
3.1.2. 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.
3.2. SD-Triggered Protection Based on other Traffic (i.e. non normal
traffic)
3.2.1. 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 other service packets, e.g.
- Extra traffic
The extra traffic can be used instead of normal traffic in packet
loss measurement. That is, the performance monitoring OAM packets (LM)
will count the number of extra service packets and the loss
measurement is based on extra service, which will be used as the
input of SD condition of the transport entity that lacks normal
service.
- 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 other traffic is general and flexible without constraint on the
bridge type by using the extra traffic, OAM packets (test or CC/CV).
However, the characteristics of the extra traffic are usually not the
same as that of the normal traffic so that the performance
measurement is not exactly reflecting the condition of real service.
OAM packets for CC/CV are sent in fixed transmission period (3.3ms,
10ms, 1s, etc.) which as well doesn't reflect the condition of real
services. It is applicable in places without strict requirement for
SD measurement.
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The performance measurement by Test OAM packets is accurate. But the
usage of test packets on 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.
3.2.2. Protection Switching and APS Protocol
For the SD-triggered protection based on other traffic, if a
broadcast bridge is used, the other traffic used to detect SD is
required on the protection transport entity only.
The priority of SD-P and SD-W shall be fixed as SD-P > SD-W similar
to SF-P > SF-W. Therefore, a protection switching based on SD
detection shall not be initiated if there exists also an SD condition
on the protection transport entity. The priority of SD-P and SD-W can
be flexible as well based on the actual degraded degree of signal.
Therefore, traffic is sent on the transport entity with better SD
condition if both SD-W and SD-P exist.
3.3. SD-Triggered Protection Based on Normal Traffic
In case of 1:1 and 1:n protection architecture, a broadcast bridge
can be applied to assist SD detection in SD-triggered protection.
3.3.1. Broadcast Bridge with special APS request priority
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.
When SD is detected on the working transport entity, 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.
If SD is detected on protection transport entity as well, i.e. SD-W
and SD-P exist simultaneously, normal traffic is selected at the sink
still from the protection transport entity to avoid flapping between
protection and working states.
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In this case the priority of SD-W and SD-P in the APS protocol is
fixed as SD-W > SD-P to avoid flapping of between protection and
working states.
3.3.2. Broadcast Bridge with extra protection switching coordination
The SD-W based protection switch action described in section 3.3.1 is
performed under the assumption that a SD condition on a transport
entity is a rare condition, and it is thus unlikely that SD on
protection will co-exist with SD on working. When such assumption is
not considered to be reasonable, the operation of the selector may be
modified as described hereafter.
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 send the normal traffic on both working and
protection transport entity after receiving SD-W indication sent by
the sink end. 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 is sent 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.
SD-triggered protection switching based normal traffic is simple and
efficient by applying a specific broadcast bridge but with a minor
modification to the priority order of APS request (i.e. SD-W > SD-P)
or to the operation order.
4. Analysis
In section 3.2 and 3.3, the different mechanisms are described. They
may be suitable in differenct application scenarios with different
requirements for simplicity, accuracy, flexibility.
[More analysis may be needed in a future version of this draft]
5. Security Considerations
To be added in a future version of the document.
6. IANA Considerations
No new IANA considerations.
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7. Acknowledgments
To be added in a future version of the document.
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8. References
8.1. Normative References
[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 Survive Frmk] Sprecher,N., and Farrel,a., "Multiprotocol
Label Switching Transport Profile Survivability Framework",
draft-ietf-mpls-tp-survive-fwk-00(work in progress), April
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-03(work in progress), August
2009
Author's Addresses
Haiyan Zhang
Huawei Technologies Co., Ltd.
Phone: +86-755-28972333
Email: zhanghaiyan@huawei.com
Jia He
Huawei Technologies Co., Ltd.
Phone: +86-755-28972333
Email: hejia@huawei.com
Han Li
China Mobile Communications Corporation
Email: lihan@chinamobile.com
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