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Independent Submission H. van Helvoort, Ed.
Request for Comments: 7347 Huawei Technologies
Category: Informational J. Ryoo, Ed.
ISSN: 2070-1721 ETRI
H. Zhang
Huawei Technologies
F. Huang
Philips
H. Li
China Mobile
A. D'Alessandro
Telecom Italia
September 2014
Pre-standard Linear Protection Switching in
MPLS Transport Profile (MPLS-TP)
Abstract
The IETF Standards Track solution for MPLS Transport Profile
(MPLS-TP) Linear Protection is provided in RFCs 6378, 7271, and 7324.
This document describes the pre-standard implementation of MPLS-TP
Linear Protection that has been deployed by several network operators
using equipment from multiple vendors. At the time of publication,
these pre-standard implementations were still in operation carrying
live traffic.
The specified mechanism supports 1+1 unidirectional/bidirectional
protection switching and 1:1 bidirectional protection switching. It
is purely supported by the MPLS-TP data plane and can work without
any control plane.
van Helvoort, et al. Informational [Page 1]
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Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7347.
Copyright Notice
Copyright (c) 2014 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
to this document.
van Helvoort, et al. Informational [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Linear Protection-Switching Overview . . . . . . . . . . . . 6
4.1. Protection Architecture Types . . . . . . . . . . . . . . 6
4.1.1. 1+1 Architecture . . . . . . . . . . . . . . . . . . 6
4.1.2. 1:1 Architecture . . . . . . . . . . . . . . . . . . 6
4.1.3. 1:n Architecture . . . . . . . . . . . . . . . . . . 7
4.2. Protection Switching Type . . . . . . . . . . . . . . . . 7
4.3. Protection Operation Type . . . . . . . . . . . . . . . . 7
5. Protection-Switching Trigger Conditions . . . . . . . . . . . 8
5.1. Fault Conditions . . . . . . . . . . . . . . . . . . . . 8
5.2. External Commands . . . . . . . . . . . . . . . . . . . . 8
5.2.1. End-to-End Commands . . . . . . . . . . . . . . . . . 8
5.2.2. Local Commands . . . . . . . . . . . . . . . . . . . 9
6. Protection-Switching Schemes . . . . . . . . . . . . . . . . 10
6.1. 1+1 Unidirectional Protection Switching . . . . . . . . . 10
6.2. 1+1 Bidirectional Protection Switching . . . . . . . . . 11
6.3. 1:1 Bidirectional Protection Switching . . . . . . . . . 12
7. APS Protocol . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. APS PDU Format . . . . . . . . . . . . . . . . . . . . . 13
7.2. APS Transmission . . . . . . . . . . . . . . . . . . . . 16
7.3. Hold-Off Timer . . . . . . . . . . . . . . . . . . . . . 17
7.4. WTR Timer . . . . . . . . . . . . . . . . . . . . . . . . 17
7.5. Command Acceptance and Retention . . . . . . . . . . . . 18
7.6. Exercise Operation . . . . . . . . . . . . . . . . . . . 18
8. Protection-Switching Logic . . . . . . . . . . . . . . . . . 19
8.1. Principle of Operation . . . . . . . . . . . . . . . . . 19
8.2. Equal Priority Requests . . . . . . . . . . . . . . . . . 21
8.3. Signal Degrade of the Protection Transport Entity . . . . 22
9. Protection-Switching State Transition Tables . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . 24
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Operation Examples of the APS Protocol . . . . . . . 26
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1. Introduction
The IETF Standards Track solution for MPLS Transport Profile
(MPLS-TP) Linear Protection is provided in [RFC6378], [RFC7271], and
[RFC7324].
This document describes the pre-standard implementation of MPLS-TP
Linear Protection that has been deployed by several network operators
using equipment from multiple vendors. At the time of publication,
these pre-standard implementations were still in operation carrying
live traffic.
This implementation was considered in the MPLS WG; however, a
different path was chosen.
This document may be useful in the future if a vendor or operator is
trying to interwork with a different vendor or operator who has
deployed the pre-standard implementation, and it provides a permanent
record of the pre-standard implementation. It is also worth noting
that the experience gained during deployment of the implementations
of this document was used to refine [RFC7271].
MPLS-TP is defined as the transport profile of MPLS technology to
allow its deployment in transport networks. A typical feature of a
transport network is that it can provide fast protection switching
for end-to-end transport paths and transport path segments. The
protection-switching time is generally required to be less than 50 ms
to meet the strict requirements of services such as voice, private
line, etc.
The goal of a linear protection-switching mechanism is to satisfy the
requirement of fast protection switching for an MPLS-TP network.
Linear protection switching means that, for one or more working
transport entities (working paths), there is one protection transport
entity (protection path), which is disjoint from any of the working
transport entities, ready to take over the service transmission when
a working transport entity has failed.
This document specifies a 1+1 unidirectional protection-switching
mechanism for a unidirectional transport entity (either point to
point or point to multipoint) as well as a bidirectional point-to-
point transport entity and a 1+1/1:1 bidirectional protection-
switching mechanism for a point-to-point bidirectional transport
entity. Since bidirectional protection switching needs the
coordination of the two endpoints of the transport entity, this
document also specifies the Automatic Protection Switching (APS)
protocol, which is used for this purpose.
van Helvoort, et al. Informational [Page 4]
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The linear protection mechanism described in this document is
applicable to both Label Switched Paths (LSPs) and Pseudowires (PWs).
The APS protocol specified in this document is based on the same
principles and behavior of the APS protocol designed for Synchronous
Optical Network (SONET) [T1.105.01] / Synchronous Digital Hierarchy
(SDH) [G.841], Optical Transport Network (OTN) [G.873.1], and
Ethernet [G.8031] and provides commonality with the established
operation models utilized in transport network technologies (e.g.,
SDH/SONET, OTN, and Ethernet).
2. Conventions Used in This Document
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].
3. Acronyms
This document uses the following acronyms:
APS Automatic Protection Switching
DNR Do not Revert
EXER Exercise
G-ACh Generic Associated Channel
FS Forced Switch
LO Lockout of Protection
LSP Label Switched Path
MPLS-TP MPLS Transport Profile
MS Manual Switch
MS-P Manual Switch to Protection transport entity
MS-W Manual Switch to Working transport entity
NR No Request
OAM Operations, Administration, and Maintenance
OTN Optical Transport Network
PDU Protocol Data Unit
PW Pseudowire
RR Reverse Request
SD Signal Degrade
SD-P Signal Degrade on Protection transport entity
SD-W Signal Degrade on Working transport entity
SDH Synchronous Digital Hierarchy
SF Signal Fail
SF-P Signal Fail on Protection transport entity
SF-W Signal Fail on Working transport entity
SONET Synchronous Optical Network
WTR Wait to Restore
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4. Linear Protection-Switching Overview
To guarantee the protection-switching time for a working transport
entity, its protection transport entity is always preconfigured
before the failure occurs. Normally, traffic will be transmitted and
received on the working transport entity. Switching to the
protection transport entity is usually triggered by link or node
failure, external commands, etc. Note that external commands are
often used in transport networks by operators, and they are very
useful in cases of service adjustment, path maintenance, etc.
4.1. Protection Architecture Types
4.1.1. 1+1 Architecture
In the 1+1 architecture, the protection transport entity is
associated with a working transport entity. The normal traffic is
permanently bridged onto both the working transport entity and the
protection transport entity at the source endpoint of the protected
domain. The normal traffic on working and protection transport
entities is transmitted simultaneously to the destination sink
endpoint of the protected domain, where a selection between the
working and protection transport entity is made based on
predetermined criteria, such as signal fail and signal degrade
indications.
4.1.2. 1:1 Architecture
In the 1:1 architecture, the protection transport entity is
associated with a working transport entity. When the working
transport entity is determined to be impaired, the normal traffic
MUST be transferred from the working to the protection transport
entity at both the source and sink endpoints of the protected domain.
The selection between the working and protection transport entities
is made based on predetermined criteria, such as signal fail and
signal degrade indications from the working or protection transport
entity.
The bridge at the source endpoint can be realized in two ways: it is
either a selector bridge or a broadcast bridge. With a selector
bridge, the normal traffic is connected either to the working
transport entity or the protection transport entity. With a
broadcast bridge, the normal traffic is permanently connected to the
working transport entity, and in case a protection switch is active,
it is also connected to the protection transport entity. The
broadcast bridge is recommended to be used in revertive mode only.
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4.1.3. 1:n Architecture
Details for the 1:n protection-switching architecture are out of
scope of this document and will be provided in a different document
in the future.
It is worth noting that the APS protocol defined here is capable of
supporting 1:n operations.
4.2. Protection Switching Type
The linear protection-switching types can be a unidirectional
switching type or a bidirectional switching type.
o Unidirectional switching type: Only the affected direction of the
working transport entity is switched to the protection transport
entity; the selectors at each endpoint operate independently.
This switching type is recommended to be used for 1+1 protection
in this document.
o Bidirectional switching type: Both directions of the working
transport entity, including the affected direction and the
unaffected direction, are switched to the protection transport
entity. For bidirectional switching, the APS protocol is required
to coordinate the two endpoints so that both have the same bridge
and selector settings, even for a unidirectional failure. This
type is applicable for 1+1 and 1:1 protection.
4.3. Protection Operation Type
The linear protection operation types can be a non-revertive
operation type or a revertive operation type.
o Non-revertive operation: The normal traffic will not be switched
back to the working transport entity even after a protection
switching cause has cleared. This is generally accomplished by
replacing the previous switch request with a "Do not Revert (DNR)"
request, which has a low priority.
o Revertive operation: The normal traffic is restored to the working
transport entity after the condition(s) causing the protection
switching has cleared. In the case of clearing a command (e.g.,
Forced Switch), this happens immediately. In the case of clearing
a defect, this generally happens after the expiry of a "Wait to
Restore (WTR)" timer, which is used to avoid chattering of
selectors in the case of intermittent defects.
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5. Protection-Switching Trigger Conditions
5.1. Fault Conditions
Fault conditions mean the requests generated by the local Operations,
Administration, and Maintenance (OAM) function.
o Signal Fail (SF): If an endpoint detects a failure by an OAM
function or other mechanism, it will submit a local signal failure
(local SF) to the APS module to request a protection switch. The
local SF could be on the working transport entity (Signal Fail on
Working transport entity (SF-W)) or the protection transport
entity (Signal Fail on Protection transport entity (SF-P)).
o Signal Degrade (SD): If an endpoint detects signal degradation by
an OAM function or other mechanism, it will submit a local signal
degrade (local SD) to the APS module to request a protection
switching. The local SD could be on the working transport entity
(Signal Degrade on Working transport entity (SD-W)) or the
protection transport entity (Signal Degrade on Protection
transport entity (SD-P)).
5.2. External Commands
The external command issues an appropriate external request to the
protection process.
5.2.1. End-to-End Commands
These commands are applied to both local and remote nodes. When the
APS protocol is present, these commands, except the Clear command,
are signaled to the far end of the connection. In bidirectional
switching, these commands affect the bridge and selector at both
ends.
o Lockout of Protection (LO): This command is used to provide the
operator a tool for temporarily disabling access to the protection
transport entity.
o Manual Switch (MS): This command is used to provide the operator a
tool for temporarily switching normal traffic to the working
transport entity (Manual Switch to Working transport entity (MS-
W)) or to the protection transport entity (Manual Switch to
Protection transport entity (MS-P)), unless a higher priority
switch request (i.e., LO, FS, or SF) is in effect.
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o Forced Switch (FS): This command is used to provide the operator a
tool for temporarily switching normal traffic from the working
transport entity to the protection transport entity, unless a
higher priority switch request (i.e., LO or SF-P) is in effect.
o Exercise (EXER): Exercise is a command to test if the APS
communication is operating correctly. The EXER command SHALL NOT
affect the state of the protection selector and bridge.
o Clear: This command between management and the local protection
process is not a request sent by APS to other endpoints. It is
used to clear the active near-end external command or WTR state.
5.2.2. Local Commands
These commands apply only to the near end (local node) of the
protection group. Even when an APS protocol is supported, they are
not signaled to the far end.
o Freeze: This command freezes the state of the protection group.
Until the freeze is cleared, additional near-end commands are
rejected, and condition changes and received APS information are
ignored. When the Freeze command is cleared, the state of the
protection group is recomputed based on the condition and received
APS information.
Because the freeze is local, if the freeze is issued at one end
only, a failure of protocol can occur as the other end is open to
accept any operator command or fault condition.
o Clear Freeze: This command clears the local freeze.
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6. Protection-Switching Schemes
6.1. 1+1 Unidirectional Protection Switching
+-----------+ +-----------+
| |---------------------------------------| |
| -+---------------------------------------+- |
| / |---------------------------------------| \ |
| / | Working transport entity | \ |
--+-------> | | --------+->
| \ | | |
| \ |---------------------------------------| |
| -+---------------------------------------| |
| source |---------------------------------------| sink |
+-----------+ Protection transport entity +-----------+
(normal condition)
+-----------+ +-----------+
| |---------------------------------------| |
| -+------------------XX-------------------+ |
| / |---------------------------------------| |
| / | Working transport entity (failure) | |
--|-------> | | --------+->
| \ | | / |
| \ |---------------------------------------| / |
| -+---------------------------------------+- |
| source |---------------------------------------| sink |
+-----------+ Protection transport entity +-----------+
(failure condition)
Figure 1: 1+1 Unidirectional Linear Protection Switching
1+1 unidirectional protection switching is the simplest protection
switching mechanism. The normal traffic is permanently bridged on
both the working and protection transport entities at the source
endpoint of the protected domain. In the normal condition, the sink
endpoint receives traffic from the working transport entity. If the
sink endpoint detects a failure on the working transport entity, it
will switch to receive traffic from the protection transport entity.
1+1 unidirectional protection switching is recommended to be used for
unidirectional transport.
Note that 1+1 unidirectional protection switching does not use the
APS coordination protocol since it only performs protection switching
based on the local request.
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6.2. 1+1 Bidirectional Protection Switching
+-----------+ +-----------+
| |---------------------------------------| |
| -+<--------------------------------------+- |
| / +-------------------------------------->+ \ |
| sink / /|---------------------------------------|\ \ sink |
<-+-------/ / | Working transport entity | --\-------+->
--+--------> | | <------+--
| source \ | | / source|
| \|---------------------------------------| / |
| +-------------------------------------->| / |
| |<--------------------------------------+- |
| APS <...................................................> APS |
| |---------------------------------------+ |
+-----------+ Protection transport entity +-----------+
(normal condition)
+-----------+ +-----------+
| |---------------------------------------| |
| +<----------------XX--------------------+- |
| +-------------------------------------->+ \ |
| /|---------------------------------------| \ |
| source / | Working transport entity (failure) | \ source|
--+--------> | | \<-----+--
<-+------- \ | | --/------+->
| sink \ \|---------------------------------------| / / sink |
| \ +-------------------------------------->+- / |
| --+<--------------------------------------+-/ |
| APS <...................................................> APS |
| |---------------------------------------+ |
+-----------+ Protection transport entity +-----------+
(failure condition)
Figure 2: 1+1 Bidirectional Linear Protection Switching
In 1+1 bidirectional protection switching, for each direction, the
normal traffic is permanently bridged on both the working and
protection transport entities at the source endpoint of the protected
domain. In the normal condition, for each direction, the sink
endpoint receives traffic from the working transport entity.
If the sink endpoint detects a failure on the working transport
entity, it will switch to receive traffic from the protection
transport entity. It will also send an APS message to inform the
sink endpoint on the other direction to switch to receive traffic
from the protection transport entity.
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The APS mechanism is necessary to coordinate the two endpoints of the
transport entity and to implement 1+1 bidirectional protection
switching even for a unidirectional failure.
6.3. 1:1 Bidirectional Protection Switching
+-----------+ +-----------+
| |---------------------------------------| |
| -+<--------------------------------------+- |
| / +-------------------------------------->+ \ |
| sink / /|---------------------------------------|\ \ source|
<-+-------/ / | Working transport entity | \ <-------+--
--+--------> | | ---------+->
| source | | sink |
| |---------------------------------------| |
| | | |
| | | |
| APS <...................................................> APS |
| |---------------------------------------| |
+-----------+ Protection transport entity +-----------+
(normal condition)
+-----------+ +-----------+
| |---------------------------------------| |
| | \/ | |
| | /\ | |
| |---------------------------------------| |
| source | Working transport entity (failure) | sink |
--+-------> | | --------+->
<-+------- \ | | / <------+--
| sink \ \ |---------------------------------------| / / source|
| \ -+-------------------------------------->+- / |
| --+<--------------------------------------+-- |
| APS <...................................................> APS |
| |---------------------------------------+ |
+-----------+ Protection transport entity +-----------+
(failure condition)
Figure 3: 1:1 Bidirectional Linear Protection Switching
In 1:1 bidirectional protection switching, for each direction, the
source endpoint sends traffic on either the working transport entity
or the protection transport entity. The sink endpoint receives the
traffic from the same transport entity on which the source endpoint
sends the traffic.
In the normal condition, for each direction, the source and sink
endpoints send and receive traffic from the working transport entity.
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If the sink endpoint detects a failure on the working transport
entity, it will switch to send and receive traffic from the
protection transport entity. It will also send an APS message to
inform the sink endpoint on another direction to switch to send and
receive traffic from the protection transport entity.
The APS mechanism is necessary to coordinate the two endpoints of the
transport entity and implement 1:1 bidirectional protection switching
even for a unidirectional failure.
7. APS Protocol
This APS protocol is based upon the APS protocol defined in
Section 11 of [G.8031]. See that reference for further definition of
the Protocol Data Unit (PDU) fields and protocol details beyond the
description in this document.
7.1. APS PDU Format
APS packets MUST be sent over a Generic Associated Channel (G-ACh) as
defined in [RFC5586].
The format of APS PDU is specified in Figure 4 below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|0 0 0 0|0 0 0 0 0 0 0 0| Channel Type (=0x7FFA) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MEL | Version | OpCode | Flags | TLV Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| APS Specific Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End TLV |
+-+-+-+-+-+-+-+-+
Figure 4: APS PDU Format
The following values MUST be used for APS PDU:
o Channel Type: The Channel Type MUST be configurable by the
implementation. During deployment, the local system administrator
provisioned the value 0x7FFA. This is a code point value in the
range of experimental Channel Types as described in RFC 5586,
Section 10.
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o Maintenance Entity group Level (MEL): The MEL value to set and
check MUST be configurable. The DEFAULT value MUST be "111".
With co-routed bidirectional transport paths, the configured MEL
MUST be the same in both directions.
o Version: 0x00
o OpCode: 0x27 (=0d39)
o Flags: 0x00
o TLV Offset: 4
o End TLV: 0x00
The format of the APS-specific information is defined in Figure 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Request|Pr.Type| Requested | Bridged | | |
| / |-+-+-+-| | |T| Reserved(0)|
| State |A|B|D|R| Signal | Signal | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: APS-Specific Information Format
All bits defined as "Reserved" MUST be transmitted as 0 and ignored
on reception.
o Request/State:
The four bits indicate the protection-switching request type. See
Figure 6 for the code of each request/state type.
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In case that there are multiple protection-switching requests,
only the protection-switching request with the highest priority
MUST be processed.
+------------------------------------+---------------+
| Request/State | Code/Priority |
+------------------------------------+---------------+
|Lockout of Protection (LO) | 1111 (highest)|
+------------------------------------+---------------+
|Signal Fail on Protection (SF-P) | 1110 |
+------------------------------------+---------------+
|Forced Switch (FS) | 1101 |
+------------------------------------+---------------+
|Signal Fail on Working (SF-W) | 1011 |
+------------------------------------+---------------+
|Signal Degrade (SD) | 1001 |
+------------------------------------+---------------+
|Manual Switch (MS) | 0111 |
+------------------------------------+---------------+
|Wait to Restore (WTR) | 0101 |
+------------------------------------+---------------+
|Exercise (EXER) | 0100 |
+------------------------------------+---------------+
|Reverse Request (RR) | 0010 |
+------------------------------------+---------------+
|Do Not Revert (DNR) | 0001 |
+------------------------------------+---------------+
|No Request (NR) | 0000 (lowest) |
+------------------------------------+---------------+
Figure 6: Protection-Switching Request Code/Priority
o Protection Type (Pr.Type):
The four bits are used to specify the protection type.
A: reserved (set by default to 1)
B: 0 - 1+1 (permanent bridge)
1 - 1:1 (no permanent bridge)
D: 0 - Unidirectional switching
1 - Bidirectional switching
R: 0 - Non-revertive operation
1 - Revertive operation
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o Requested Signal:
This byte is used to indicate the traffic that the near-end
requests to be carried over the protection entity.
value = 0: Null traffic
value = 1: Normal traffic 1
value = 2~255: Reserved
o Bridged Signal:
This byte is used to indicate the traffic that is bridged onto the
protection entity.
value = 0: Null traffic
value = 1: Normal traffic 1
value = 2~255: Reserved
o Bridge Type (T):
This bit is used to further specify the type of non-permanent
bridge for 1:1 protection switching.
value = 0: Selector bridge
value = 1: Broadcast bridge
o Reserved:
This field MUST be set to zero.
7.2. APS Transmission
The APS message MUST be transported on the protection transport
entity by encapsulation with the protection transport entity label
(the label of the LSP used to transport protection traffic). If an
endpoint receives APS-specific information from the working transport
entity, it MUST ignore this information and MUST report the failure
of protocol defect (see Section 8.1) to the operator.
A new APS packet MUST be transmitted immediately when a change in the
transmitted status occurs. The first three APS packets MUST be
transmitted as fast as possible only if the APS information to be
transmitted has been changed so that fast protection switching is
possible, even if one or two APS packets are lost or corrupted. The
interval of the first three APS packets SHOULD be 3.3 ms. APS
packets after the first three MUST be transmitted with the interval
of 5 seconds.
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If no valid APS-specific information is received, the last valid
received information remains applicable.
7.3. Hold-Off Timer
In order to coordinate timing of protection switches at multiple
layers, a hold-off timer MAY be required. The purpose is to allow a
server-layer protection switch to have a chance to fix the problem
before switching at a client layer.
Each selector SHOULD have a provisioned hold-off timer. The
suggested range of the hold-off timer is 0 to 10 seconds in steps of
100 ms (accuracy of +/-5 ms).
When a new defect or more severe defect occurs (new SF or SD) on the
active transport entity (the transport entity that currently carries
and selects traffic), this event will not be reported immediately to
protection switching if the provisioned hold-off timer value is non-
zero. Instead, the hold-off timer SHALL be started. When the hold-
off timer expires, it SHALL be checked whether a defect still exists
on the transport entity that started the timer. If it does, that
defect SHALL be reported to protection switching. The defect need
not be the same one that started the timer.
This hold-off timer mechanism SHALL be applied for both working and
protection transport entities.
7.4. WTR Timer
In revertive mode of operation, to prevent frequent operation of the
protection switch due to an intermittent defect, a failed working
transport entity MUST become fault free. After the failed working
transport entity meets this criterion, a fixed period of time SHALL
elapse before a normal traffic signal uses it again. This period,
called a WTR period, MAY be configured by the operator in 1 minute
steps between 5 and 12 minutes; the default value is 5 minutes. An
SF or SD condition will override the WTR. To activate the WTR timer
appropriately, even when both ends concurrently detect clearance of
SF-W and SD-W, when the local state transits from SF-W or SD-W to No
Request (NR) with the requested signal number 1, the previous local
state, SF-W or SD-W, MUST be memorized. If both the local state and
far-end state are NR with the requested signal number 1, the local
state transits to WTR only when the previous local state is SF-W or
SD-W. Otherwise, the local state transits to NR with the requested
signal number 0.
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In revertive mode of operation, when the protection is no longer
requested, i.e., the failed working transport entity is no longer in
SF or SD condition (and assuming no other requesting transport
entities), a local WTR state will be activated. Since this state
becomes the highest in priority, it is indicated on the APS signal
and maintains the normal traffic signal from the previously failed
working transport entity on the protection transport entity. This
state SHALL normally time out and become an NR state. The WTR timer
deactivates earlier when any request of higher priority request
preempts this state.
7.5. Command Acceptance and Retention
The commands Clear, LO, FS, MS, and EXER are accepted or rejected in
the context of previous commands, the condition of the working and
protection entities in the protection group, and (in bidirectional
switching only) the APS information received.
The Clear command MUST be only valid if a near-end LO, FS, MS, or
EXER command is in effect or if a WTR state is present at the near
end and rejected otherwise. This command will remove the near-end
command or WTR state, allowing the next lower-priority condition or
(in bidirectional switching) APS request to be asserted.
Other commands MUST be rejected unless they are higher priority than
the previously existing command, condition, or (in bidirectional
switching) APS request. If a new command is accepted, any previous,
lower-priority command that is overridden MUST be forgotten. If a
higher priority command overrides a lower-priority condition or (in
bidirectional switching) APS request, that other request will be
reasserted if it still exists at the time the command is cleared. If
a command is overridden by a condition or (in bidirectional
switching) APS request, that command MUST be forgotten.
7.6. Exercise Operation
Exercise is a command to test if the APS communication is operating
correctly. It is lower priority than any "real" switch request. It
is only valid in bidirectional switching, since this is the only
place where you can get a meaningful test by looking for a response.
The Exercise command SHALL issue the command with the same requested
and bridged signal numbers of the NR, Reverse Request (RR), or DNR
request that it replaces. The valid response will be an RR with the
corresponding requested and bridged signal numbers. When Exercise
commands are input at both ends, an EXER, instead of RR, MUST be
transmitted from both ends. The standard response to DNR MUST be DNR
rather than NR. When the exercise command is cleared, it MUST be
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replaced with NR or RR if the requested signal number is 0 and DNR or
RR if the requested signal number is 1.
8. Protection-Switching Logic
8.1. Principle of Operation
+-------------+ Persistent +----------+
SF,SD | Hold-off | fault | Local |
----------->| timer logic |----------->| request |
+-------------+ | logic |
Other local requests ----------------->| |
(LO, FS, MS, EXER, Clear) +----------+
|
| Highest
| local request
|
Remote APS V
message +-------+ Remote APS +----------------+
------------->| APS | request/state | APS process |
(received | check |-------------->| logic |
from far end) +-------+ +----------------+
| ^ | |
| | | Signaled |
| | | APS |
| | Txed | |
| | "Requested V |
| | Signal" +-----------+ |
| +-----------------| APS mess. | |
| | generator | |
| +-----------+ |
| | |
V | |
Failure of V |
protocol APS message |
detection V
Set local
bridge/selector
Figure 7: Protection-Switching Logic
Figure 7 describes the protection-switching logic.
One or more local protection-switching requests may be active. The
"local request logic" determines which of these requests is highest
using the order of priority given in Figure 6. This highest local
request information SHALL be passed on to the "APS process logic".
Note that an accepted Clear command, clearance of SF or SD, or
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expiration of the WTR timer SHALL NOT be processed by the local
request logic but SHALL be considered as the highest local request
and submitted to the APS process logic for processing.
The remote APS message is received from the far end and is subjected
to the validity check and mismatch detection in "APS check". Failure
of protocol situations are as follows:
o The "B" field mismatch due to incompatible provisioning;
o The reception of the APS message from the working entity due to
working/protection configuration mismatch;
o No match in sent "Requested Signal" and received "Requested
Signal" for more than 50 ms;
o No APS message is received on the protection transport entity
during at least 3.5 times the long APS interval (e.g., at least
17.5 seconds), and there is no defect on the protection transport
entity.
Provided the "B" field matches:
o If the "D" bit mismatches, the bidirectional side will fall back
to unidirectional switching.
o If the "R" bit mismatches, one side will clear switches to WTR and
the other will clear to DNR. The two sides will interwork and the
traffic is protected.
o If the "T" bit mismatches, the side using a broadcast bridge will
fall back to using a selector bridge.
The APS message with invalid information MUST be ignored, and the
last valid received information remains applicable.
The linear protection-switching algorithm SHALL commence immediately
every time one of the input signals changes, i.e., when the status of
any local request changes, or when different APS-specific information
is received from the far end. The consequent actions of the
algorithm are also initiated immediately, i.e., change the local
bridge/selector position (if necessary), transmit new APS-specific
information (if necessary), or detect the failure of protocol defect
if the protection switching is not completed within 50 ms.
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The state transition is calculated in the "APS process logic" based
on the highest local request, the request of the last received
"Request/State" information, and state transition tables defined in
Section 9, as follows:
o If the highest local request is Clear, clearance of SF or SD, or
expiration of WTR, a state transition is calculated first based on
the highest local request and state machine table for local
requests to obtain an intermediate state. This intermediate state
is the final state in case of clearance of SF-P; otherwise,
starting at this intermediate state, the last received far-end
request and the state machine table for far-end requests are used
to calculate the final state.
o If the highest local request is neither Clear nor clearance of SF
or of SD nor expiration of WTR, the APS process logic compares the
highest local request with the request of the last received
"Request/State" information based on Figure 6.
1. If the highest local request has higher or equal priority, it
is used with the state transition table for local requests
defined in Section 9 to determine the final state; otherwise,
2. The request of the last received "Request/State" information
is used with the state transition table for far-end requests
defined in Section 9 to determine the final state.
The "APS message generator" generates APS-specific information with
the signaled APS information for the final state from the state
transition calculation (with coding as described in Figure 5).
8.2. Equal Priority Requests
In general, once a switch has been completed due to a request, it
will not be overridden by another request of the same priority
(first-come, first-served policy). Equal priority requests from both
sides of a bidirectional protection group are both considered valid,
as follows:
o If the local state is NR, with the requested signal number 1, and
the far-end state is NR, with the requested signal number 0, the
local state transits to NR with the requested signal number 0.
This applies to the case when the remote request for switching to
the protection transport entity has been cleared.
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o If both the local and far-end states are NR, with the requested
signal number 1, the local state transits to the appropriate new
state (DNR state for non-revertive mode and WTR state for
revertive mode). This applies to the case when the old request
has been cleared at both ends.
o If both the local and far-end states are RR, with the same
requested signal number, both ends transit to the appropriate new
state according to the requested signal number. This applies to
the case of concurrent deactivation of EXER from both ends.
o In other cases, no state transition occurs, even if equal priority
requests are activated from both ends. Note that if MSs are
issued simultaneously to both working and protection transport
entities, either as local or far-end requests, the MS to the
working transport entity is considered as having higher priority
than the MS to the protection transport entity.
8.3. Signal Degrade of the Protection Transport Entity
Signal degrade on the protection transport entity has the same
priority as signal degrade on the working transport entity. As a
result, if an SD condition affects both transport entities, the first
SD detected MUST NOT be overridden by the second SD detected. If the
SD is detected simultaneously, either as local or far-end requests on
both working and protection transport entities, then the SD on the
standby transport entity MUST be considered as having higher priority
than the SD on the active transport entity, and the normal traffic
signal continues to be selected from the active transport entity
(i.e., no unnecessary protection switching is performed).
In the preceding sentence, "simultaneously" relates to the occurrence
of SD on both the active and standby transport entities at input to
the protection-switching process at the same time, or as long as an
SD request has not been acknowledged by the remote end in
bidirectional protection switching.
9. Protection-Switching State Transition Tables
In this section, state transition tables for the following protection
switching configurations are described.
o 1:1 bidirectional (revertive mode, non-revertive mode);
o 1+1 bidirectional (revertive mode, non-revertive mode);
o 1+1 unidirectional (revertive mode, non-revertive mode).
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Note that any other global or local request that is not described in
state transition tables does not trigger any state transition.
The states specified in the state transition tables can be described
as follows:
o NR: NR is the state entered by the local priority under all
conditions where no local protection-switching requests (including
WTR and DNR) are active. NR can also indicate that the highest
local request is overridden by the far-end request, whose priority
is higher than the highest local request. Normal traffic signal
is selected from the corresponding transport entity.
o LO, SF-P, SD-P: The access by the normal traffic to the protection
transport entity is NOT allowed in this state. The normal traffic
is carried by the working transport entity, regardless of the
fault/degrade condition possibly present (due to the highest
priority of the switching triggers leading to this state).
o FS, SF-W, SD-W, MS-W, MS-P: A switching trigger NOT resulting in
the protection transport entity unavailability is present. The
normal traffic is selected either from the corresponding working
transport entity or from the protection transport entity,
according to the behavior of the specific switching trigger.
o WTR: In revertive operation, after the clearing of an SF-W or SD-
W, this maintains normal traffic as selected from the protection
transport entity until the WTR timer expires or another request
with higher priority, including the Clear command, is received.
This is used to prevent frequent operation of the selector in the
case of intermittent failures.
o DNR: In non-revertive operation, this is used to maintain a normal
traffic to be selected from the protection transport entity.
o EXER: Exercise of the APS protocol.
o RR: The near end will enter and signal Reverse Request only in
response to an EXER from the far end.
[State transition tables are shown at the end of the PDF form of this
document.]
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10. Security Considerations
MPLS-TP is a subset of MPLS and so builds upon many of the aspects of
the security model of MPLS. MPLS networks make the assumption that
it is very hard to inject traffic into a network and equally hard to
cause traffic to be directed outside the network. The control-plane
protocols utilize hop-by-hop security and assume a "chain-of-trust"
model such that end-to-end control-plane security is not used. For
more information on the generic aspects of MPLS security, see
[RFC5920].
This document describes a protocol carried in the G-ACh [RFC5586] and
so is dependent on the security of the G-ACh, itself. The G-ACh is a
generalization of the associated channel defined in [RFC4385]. Thus,
this document relies heavily on the security mechanisms provided for
the associated channel and described in those two documents.
11. Acknowledgements
The authors would like to thank Hao Long, Vincenzo Sestito, Italo
Busi, Igor Umansky, and Andy Malis for their input to and review of
the current document.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
Use over an MPLS PSN", RFC 4385, February 2006.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[G.841] International Telecommunications Union, "Types and
characteristics of SDH network protection architectures",
ITU-T Recommendation G.841, October 1998.
[G.873.1] International Telecommunications Union, "Optical Transport
Network (OTN): Linear protection", ITU-T Recommendation
G.873.1, May 2014.
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[G.8031] International Telecommunications Union, "Ethernet linear
protection switching", ITU-T Recommendation G.8031/Y.1342,
June 2011.
[T1.105.01]
American National Standards Institute, "Synchronous
Optical Network (SONET) - Automatic Protection Switching",
ANSI 0900105.01:2000 (R2010), March 2000.
12.2. Informative References
[RFC6378] Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
Protection", RFC 6378, October 2011.
[RFC7271] Ryoo, J., Gray, E., van Helvoort, H., D'Alessandro, A.,
Cheung, T., and E. Osborne, "MPLS Transport Profile (MPLS-
TP) Linear Protection to Match the Operational
Expectations of Synchronous Digital Hierarchy, Optical
Transport Network, and Ethernet Transport Network
Operators", RFC 7271, June 2014.
[RFC7324] Osborne, E., "Updates to MPLS Transport Profile Linear
Protection", RFC 7324, July 2014.
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Appendix A. Operation Examples of the APS Protocol
The sequence diagrams shown in this section are only a few examples
of the APS operations. The first APS message, which differs from the
previous APS message, is shown. The operation of hold-off timer is
omitted. The fields whose values are changed during APS packet
exchange are shown in the APS packet exchange. They are Request/
State, requested traffic, and bridged traffic. For an example,
SF(0,1) represents an APS packet with the following field values:
Request/State = SF, Requested Signal = 0, and Bridged Signal = 1.
The values of the other fields remain unchanged from the initial
configuration. The signal numbers 0 and 1 refer to null signal and
normal traffic signal, respectively. W(A->Z) and P(A->Z) indicate
the working and protection paths in the direction of A to Z,
respectively.
Example 1. 1:1 bidirectional protection switching (revertive mode) -
Unidirectional SF case
A Z
| |
(1) |---- NR(0,0)----->|
|<----- NR(0,0)----|
| |
| |
(2) | (SF on W(Z->A)) |
|---- SF(1,1)----->| (3)
|<----- NR(1,1)----|
(4) | |
| |
(5) | (Recovery) |
|---- WTR(1,1)---->|
/| |
WTR timer | |
\| |
(6) |---- NR(0,0)----->| (7)
(8) |<----- NR(0,0)----|
| |
(1) The protected domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Signal Fail occurs on the working entity in the Z to A
direction. Selector and bridge of node A select protection
entity. Node A generates an SF(1,1) message.
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(3) Upon receiving SF(1,1), node Z sets selector and bridge to
protection entity. As there is no local request in node Z, node
Z generates an NR(1,1) message.
(4) Node A confirms that the far end is also selecting protection
entity.
(5) Node A detects clearing of the SF condition, starts the WTR
timer, and sends a WTR(1,1) message.
(6) At expiration of the WTR timer, node A sets selector and bridge
to working entity and sends an NR(0,0) message.
(7) Node Z is notified that the far-end request has been cleared and
sets selector and bridge to working entity.
(8) It is confirmed that the far end is also selecting working
entity.
Example 2. 1:1 bidirectional protection switching (revertive mode) -
Bidirectional SF case
A Z
| |
(1) |---- NR(0,0)----->| (1)
|<----- NR(0,0)----|
| |
| |
(2) | (SF on W(Z<->A)) | (2)
|<---- SF(1,1)---->|
(3) | | (3)
| |
(4) | (Recovery) | (4)
|<---- NR(1,1)---->|
(5) |<--- WTR(1,1)---->| (5)
/| |\
WTR timer | | WTR timer
\| |/
(6) |<---- NR(1,1)---->| (6)
(7) |<----- NR(0,0)--->| (7)
(8) | | (8)
(1) The protected domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Nodes A and Z detect local SF conditions on the working entity,
set selector and bridge to protection entity, and generate
SF(1,1) messages.
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(3) Upon receiving SF(1,1), each node confirms that the far end is
also selecting protection entity.
(4) Each node detects clearing of the SF condition and sends an
NR(1,1) message as the last received APS message was SF.
(5) Upon receiving NR(1,1), each node starts the WTR timer and sends
WTR(1,1).
(6) At expiration of the WTR timer, each node sends NR(1,1) as the
last received APS message was WTR.
(7) Upon receiving NR(1,1), each node sets selector and bridge to
working entity and sends an NR(0,0) message.
(8) It is confirmed that the far end is also selecting working
entity.
Example 3. 1:1 bidirectional protection switching (revertive mode) -
Bidirectional SF case - Inconsistent WTR timers
A Z
| |
(1) |---- NR(0,0)----->| (1)
|<----- NR(0,0)----|
| |
| |
(2) | (SF on W(Z<->A)) | (2)
|<---- SF(1,1)---->|
(3) | | (3)
| |
(4) | (Recovery) | (4)
|<---- NR(1,1)---->|
(5) |<--- WTR(1,1)---->| (5)
/| |\
WTR timer | | |
\| | WTR timer
(6) |----- NR(1,1)---->| | (7)
| |/
(9) |<----- NR(0,0)----| (8)
|---- NR(0,0)----->| (10)
(1) The protected domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Nodes A and Z detect local SF conditions on the working entity,
set selector and bridge to protection entity, and generate
SF(1,1) messages.
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(3) Upon receiving SF(1,1), each node confirms that the far end is
also selecting protection entity.
(4) Each node detects clearing of the SF condition and sends an
NR(1,1) message as the last received APS message was SF.
(5) Upon receiving NR(1,1), each node starts the WTR timer and
sends WTR(1,1).
(6) At expiration of the WTR timer in node A, node A sends an
NR(1,1) message as the last received APS message was WTR.
(7) At node Z, the received NR(1,1) is ignored as the local WTR has
a higher priority.
(8) At expiration of the WTR timer in node Z, node Z sets selector
and bridge to working entity and sends an NR(0,0) message.
(9) Upon receiving NR(0,0), node A sets selector and bridge to
working entity and sends an NR(0,0) message.
(10) It is confirmed that the far end is also selecting working
entity.
Example 4. 1:1 bidirectional protection switching (non-revertive
mode) - Unidirectional SF on working followed by unidirectional SF on
protection
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A Z
| |
(1) |---- NR(0,0)----->| (1)
|<----- NR(0,0)----|
| |
| |
(2) | (SF on W(Z->A)) |
|----- SF(1,1)---->| (3)
(4) |<----- NR(1,1)----|
| |
| |
(5) | (Recovery) |
|----- DNR(1,1)--->| (6)
|<--- DNR(1,1)---->|
| |
| |
| (SF on P(A->Z)) | (7)
(8) |<--- SF-P(0,0)----|
|---- NR(0,0)----->|
| |
| |
| (Recovery) | (9)
|<----- NR(0,0)----|
| |
(1) The protected domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Signal Fail occurs on the working entity in the Z to A
direction. Selector and bridge of node A select the protection
entity. Node A generates an SF(1,1) message.
(3) Upon receiving SF(1,1), node Z sets selector and bridge to
protection entity. As there is no local request in node Z, node
Z generates an NR(1,1) message.
(4) Node A confirms that the far end is also selecting protection
entity.
(5) Node A detects clearing of the SF condition and sends a DNR(1,1)
message.
(6) Upon receiving DNR(1,1), node Z also generates a DNR(1,1)
message.
(7) Signal Fail occurs on the protection entity in the A to Z
direction. Selector and bridge of node Z select the working
entity. Node Z generates an SF-P(0,0) message.
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(8) Upon receiving SF-P(0,0), node A sets selector and bridge to
working entity and generates an NR(0,0) message.
(9) Node Z detects clearing of the SF condition and sends an NR(0,0)
message.
Exmaple 5. 1:1 bidirectional protection switching (non-revertive
mode) - Bidirectional SF on working followed by bidirectional SF on
protection
A Z
| |
(1) |---- NR(0,0)----->| (1)
|<----- NR(0,0)----|
| |
| |
(2) | (SF on W(A<->Z)) | (2)
(3) |<---- SF(1,1)---->| (3)
| |
| |
(4) | (Recovery) | (4)
(5) |<---- NR(1,1)---->| (5)
|<--- DNR(1,1)---->|
| |
| |
(6) | (SF on P(A<->Z)) | (6)
(7) |<--- SF-P(0,0)--->| (7)
| |
| |
(8) | (Recovery) | (8)
|<---- NR(0,0)---->|
| |
(1) The protected domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Nodes A and Z detect local SF conditions on the working entity,
set selector and bridge to protection entity, and generate
SF(1,1) messages.
(3) Upon receiving SF(1,1), each node confirms that the far end is
also selecting protection entity.
(4) Each node detects clearing of the SF condition and sends an
NR(1,1) message as the last received APS message was SF.
(5) Upon receiving NR(1,1), each node sends DNR(1,1).
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(6) Signal Fail occurs on the protection entity in both directions.
Selector and bridge of each node selects the working entity.
Each node generates an SF-P(0,0) message.
(7) Upon receiving SF-P(0,0), each node confirms that the far end is
also selecting working entity.
(8) Each node detects clearing of the SF condition and sends an
NR(0,0) message.
Authors' Addresses
Huub van Helvoort (editor)
Huawei Technologies
EMail: huub@van-helvoort.eu
Jeong-dong Ryoo (editor)
ETRI
EMail: ryoo@etri.re.kr
Haiyan Zhang
Huawei Technologies
EMail: zhanghaiyan@huawei.com
Feng Huang
Philips
EMail: feng.huang@philips.com
Han Li
China Mobile
EMail: lihan@chinamobile.com
Alessandro D'Alessandro
Telecom Italia
EMail: alessandro.dalessandro@telecomitalia.it
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