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IETF RFC 7271
Last modified on Thursday, June 12th, 2014
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Internet Engineering Task Force (IETF) J. Ryoo, Ed.
Request for Comments: 7271 ETRI
Updates: 6378 E. Gray, Ed.
Category: Standards Track Ericsson
ISSN: 2070-1721 H. van Helvoort
Huawei Technologies
A. D'Alessandro
Telecom Italia
T. Cheung
ETRI
E. Osborne
June 2014
MPLS Transport Profile (MPLS-TP) Linear Protection to Match the
Operational Expectations of Synchronous Digital Hierarchy,
Optical Transport Network, and Ethernet Transport Network Operators
 Abstract
This document describes alternate mechanisms to perform some of the
functions of MPLS Transport Profile (MPLS-TP) linear protection
defined in RFC 6378, and also defines additional mechanisms. The
purpose of these alternate and additional mechanisms is to provide
operator control and experience that more closely models the behavior
of linear protection seen in other transport networks.
This document also introduces capabilities and modes for linear
protection. A capability is an individual behavior, and a mode is a
particular combination of capabilities. Two modes are defined in
this document: Protection State Coordination (PSC) mode and Automatic
Protection Switching (APS) mode.
This document describes the behavior of the PSC protocol including
priority logic and state machine when all the capabilities associated
with the APS mode are enabled.
This document updates RFC 6378 in that the capability advertisement
method defined here is an addition to that document.
Ryoo, et al. Standards Track PAGE 1 
RFC 7271 MPLS-TP LP for ITU-T June 2014
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in 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/RFC 7271.
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. Code Components extracted from this document must
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Capability 1: Priority Modification . . . . . . . . . . . . . 6
4.1. Motivation for Swapping Priorities of FS and SF-P . . . . 6
4.2. Motivation for Raising the Priority of SFc . . . . . . . 7
4.3. Motivation for Introducing the Freeze Command . . . . . . 7
4.4. Procedures in Support of Priority Modification . . . . . 8
5. Capability 2: Non-revertive Behavior Modification . . . . . . 8
6. Capability 3: Support of the MS-W Command . . . . . . . . . . 8
6.1. Motivation for adding MS-W . . . . . . . . . . . . . . . 8
6.2. Terminology to Support MS-W . . . . . . . . . . . . . . . 9
6.3. Behavior of MS-P and MS-W . . . . . . . . . . . . . . . . 9
6.4. Equal-Priority Resolution for MS . . . . . . . . . . . . 10
7. Capability 4: Support of Protection against SD . . . . . . . 10
7.1. Motivation for Supporting Protection against SD . . . . . 10
7.2. Terminology to Support SD . . . . . . . . . . . . . . . . 10
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RFC 7271 MPLS-TP LP for ITU-T June 2014
7.3. Behavior of Protection against SD . . . . . . . . . . . . 11
7.4. Equal-Priority Resolution . . . . . . . . . . . . . . . . 12
8. Capability 5: Support of EXER Command . . . . . . . . . . . . 13
9. Capabilities and Modes . . . . . . . . . . . . . . . . . . . 14
9.1. Capabilities . . . . . . . . . . . . . . . . . . . . . . 14
9.1.1. Sending and Receiving the Capabilities TLV . . . . . 15
9.2. Modes . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2.1. PSC Mode . . . . . . . . . . . . . . . . . . . . . . 16
9.2.2. APS Mode . . . . . . . . . . . . . . . . . . . . . . 16
10. PSC Protocol in APS Mode . . . . . . . . . . . . . . . . . . 17
10.1. Request Field in PSC Protocol Message . . . . . . . . . 17
10.2. Priorities of Local Inputs and Remote Requests . . . . . 17
10.2.1. Equal-Priority Requests . . . . . . . . . . . . . . 18
10.3. Acceptance and Retention of Local Inputs . . . . . . . . 20
11. State Transition Tables in APS Mode . . . . . . . . . . . . . 20
11.1. State Transition by Local Inputs . . . . . . . . . . . . 23
11.2. State Transition by Remote Messages . . . . . . . . . . 25
11.3. State Transition for 1+1 Unidirectional Protection . . . 27
12. Provisioning Mismatch and Protocol Failure in APS Mode . . . 27
13. Security Considerations . . . . . . . . . . . . . . . . . . . 28
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
14.1. MPLS PSC Request Registry . . . . . . . . . . . . . . . 29
14.2. MPLS PSC TLV Registry . . . . . . . . . . . . . . . . . 29
14.3. MPLS PSC Capability Flag Registry . . . . . . . . . . . 29
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
16.1. Normative References . . . . . . . . . . . . . . . . . . 30
16.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. An Example of an Out-of-Service Scenario . . . . . . 32
Appendix B. An Example of a Sequence Diagram Showing
the Problem with the Priority Level of SFc . . . . . 33
Appendix C. Freeze Command . . . . . . . . . . . . . . . . . . . 34
Appendix D. Operation Examples of the APS Mode . . . . . . . . . 35
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RFC 7271 MPLS-TP LP for ITU-T June 2014
1. Introduction
Linear protection mechanisms for the MPLS Transport Profile (MPLS-TP)
are described in RFC 6378 [RFC 6378] to meet the requirements
described in RFC 5654 [RFC 5654].
This document describes alternate mechanisms to perform some of the
functions of linear protection, and also defines additional
mechanisms. The purpose of these alternate and additional mechanisms
is to provide operator control and experience that more closely
models the behavior of linear protection seen in other transport
networks, such as Synchronous Digital Hierarchy (SDH), Optical
Transport Network (OTN), and Ethernet transport networks. Linear
protection for SDH, OTN, and Ethernet transport networks is defined
in ITU-T Recommendations G.841 [G841], G.873.1 [G873.1], and G.8031
[G8031], respectively.
The reader of this document is assumed to be familiar with [RFC 6378].
The alternative mechanisms described in this document are for the
following capabilities:
1. Priority modification,
2. non-revertive behavior modification,
and the following capabilities have been added to define additional
mechanisms:
3. support of the Manual Switch to Working path (MS-W) command,
4. support of protection against Signal Degrade (SD), and
5. support of the Exercise (EXER) command.
The priority modification includes raising the priority of Signal
Fail on Protection path (SF-P) relative to Forced Switch (FS), and
raising the priority level of Clear Signal Fail (SFc) above SF-P.
Non-revertive behavior is modified to align with the behavior defined
in RFC 4427 [RFC 4427] as well as to follow the behavior of linear
protection seen in other transport networks.
Support of the MS-W command to revert traffic to the working path in
non-revertive operation is covered in this document.
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Support of the protection-switching protocol against SD is covered in
this document. The specifics for the method of identifying SD are
out of the scope for this document and are treated similarly to
Signal Fail (SF) in [RFC 6378].
Support of the EXER command to test if the Protection State
Coordination (PSC) communication is operating correctly is also
covered in this document. Without actually switching traffic, the
EXER command tests and validates the linear protection mechanism and
PSC protocol including the aliveness of the priority logic, the PSC
state machine, the PSC message generation and reception, and the
integrity of the protection path.
This document introduces capabilities and modes. A capability is an
individual behavior. The capabilities of a node are advertised using
the method given in this document. A mode is a particular
combination of capabilities. Two modes are defined in this document:
PSC mode and Automatic Protection Switching (APS) mode.
Other modes may be defined as new combinations of the capabilities
defined in this document or through the definition of additional
capabilities. In either case, the specification defining a new mode
will be responsible for documenting the behavior, the priority logic,
and the state machine of the PSC protocol when the set of
capabilities in the new mode is enabled.
This document describes the behavior, the priority logic, and the
state machine of the PSC protocol when all the capabilities
associated with the APS mode are enabled. The PSC protocol behavior
for the PSC mode is as defined in [RFC 6378].
This document updates [RFC 6378] by adding a capability advertisement
mechanism. It is recommended that existing implementations of the
PSC protocol be updated to support this capability. Backward
compatibility with existing implementations that do not support this
mechanism is described in Section 9.2.1.
Implementations are expected to be configured to support a specific
set of capabilities (a mode) and to reject messages that indicate the
use of a different set of capabilities (a different mode). Thus, the
capability advertisement is not a negotiation but a verification that
peers are using the same mode.
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 RFC 2119 [RFC 2119].
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RFC 7271 MPLS-TP LP for ITU-T June 2014
3. Acronyms
This document uses the following acronyms:
APS Automatic Protection Switching
DNR Do-not-Revert
EXER Exercise
FS Forced Switch
LO Lockout of protection
MS Manual Switch
MS-P Manual Switch to Protection path
MS-W Manual Switch to Working path
MPLS-TP MPLS Transport Profile
NR No Request
OC Operator Clear
OTN Optical Transport Network
PSC Protection State Coordination
RR Reverse Request
SD Signal Degrade
SD-P Signal Degrade on Protection path
SD-W Signal Degrade on Working path
SDH Synchronous Digital Hierarchy
SF Signal Fail
SF-P Signal Fail on Protection path
SF-W Signal Fail on Working path
SFc Clear Signal Fail
SFDc Clear Signal Fail or Degrade
WTR Wait-to-Restore
4. Capability 1: Priority Modification
[RFC 6378] defines the priority of FS to be higher than that of SF-P.
That document also defines the priority of Clear SF (SFc) to be low.
This document defines the priority modification capability whereby
the relative priorities of FS and SF-P are swapped, and the priority
of Clear SF (SFc) is raised. In addition, this capability introduces
the Freeze command as described in Appendix C. The rationale for
these changes is detailed in the following subsections from both the
technical and network operational aspects.
4.1. Motivation for Swapping Priorities of FS and SF-P
Defining the priority of FS higher than that of SF-P can result in a
situation where the protected traffic is taken out of service. When
the protection path fails, PSC communication may stop as a result.
In this case, if any input that is supposed to be signaled to the
other end has a higher priority than SF-P, then this can result in an
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unpredictable protection-switching state. An example scenario that
may result in an out-of-service situation is presented in Appendix A
of this document.
According to Section 2.4 of [RFC 5654], it MUST be possible to operate
an MPLS-TP network without using a control plane. This means that
the PSC communication channel is very important for the transfer of
external switching commands (e.g., FS), and these commands should not
rely on the presence of a control plane. In consequence, the failure
of the PSC communication channel has higher priority than FS.
In other transport networks (such as SDH, OTN, and Ethernet transport
networks), the priority of SF-P has been higher than that of FS. It
is therefore important to offer network operators the option of
having the same behavior in their MPLS-TP networks so that they can
have the same operational protection-switching behavior to which they
have become accustomed. Typically, an FS command is issued before
network maintenance jobs (e.g., replacing optical cables or other
network components). When an operator pulls out a cable on the
protection path, by mistake, the traffic should continue to be
protected, and the operator expects this behavior based on his/her
experience with traditional transport network operations.
4.2. Motivation for Raising the Priority of SFc
The priority level of SFc defined in [RFC 6378] can cause traffic
disruption when a node that has experienced local signal fails on
both the working and the protection paths is recovering from these
failures.
A sequence diagram highlighting the problem with the priority level
of SFc as defined in [RFC 6378] is presented in Appendix B.
4.3. Motivation for Introducing the Freeze Command
With the priority swapping between FS and SF-P, the traffic is always
moved back to the working path when SF-P occurs in Protecting
Administrative state. In case network operators need an option to
control their networks so that the traffic can remain on the
protection path even when the PSC communication channel is broken,
the Freeze command can be used. Freeze is defined to be a "local"
command that is not signaled to the remote node. The use of the
Freeze command is described in Appendix C.
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4.4. Procedures in Support of Priority Modification
When the modified priority order specified in this document is in
use, the list of local requests in order of priority SHALL be as
follows (from highest to lowest):
o Clear Signal Fail
o Signal Fail on Protection path
o Forced Switch
o Signal Fail on Working path
This requires modification of the PSC Control Logic (including the
state machine) relative to that described in [RFC 6378]. Sections 10
and 11 present the PSC Control Logic when all capabilities of APS
mode are enabled.
5. Capability 2: Non-revertive Behavior Modification
Non-revertive operation of protection switching is defined in
[RFC 4427]. In this operation, the traffic does not return to the
working path when switch-over requests are terminated.
However, the PSC protocol defined in [RFC 6378] supports this
operation only when recovering from a defect condition: it does not
support the non-revertive function when an operator's switch-over
command, such as FS or Manual Switch (MS), is cleared. To be aligned
with the behavior in other transport networks and to be consistent
with [RFC 4427], a node should go into the Do-not-Revert (DNR) state
not only when a failure condition on the working path is cleared, but
also when an operator command that requested switch-over is cleared.
This requires modification to the PSC Control Logic (including the
state machine) relative to that described in [RFC 6378]. Sections 10
and 11 present the PSC Control Logic when all capabilities of APS
mode are enabled.
6. Capability 3: Support of the MS-W Command
6.1. Motivation for adding MS-W
Changing the non-revertive operation as described in Section 5
introduces the necessity of a new operator command to revert traffic
to the working path in the DNR state. When the traffic is on the
protection path in the DNR state, a Manual Switch to Working (MS-W)
command is issued to switch the normal traffic back to the working
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RFC 7271 MPLS-TP LP for ITU-T June 2014
path. According to Section 4.3.3.6 (Do-not-Revert State) in
[RFC 6378], "To revert back to the Normal state, the administrator
SHALL issue a Lockout of protection command followed by a Clear
command." However, using the Lockout of protection (LO) command
introduces the potential risk of an unprotected situation while the
LO is in effect.
The "Manual switch-over for recovery LSP/span" command is defined in
[RFC 4427]. Requirement 83 in [RFC 5654] states that the external
commands defined in [RFC 4427] MUST be supported. Since there is no
support for this external command in [RFC 6378], this functionality
should be added to PSC. This support is provided by introducing the
MS-W command. The MS-W command, as described here, corresponds to
the "Manual switch-over for recovery LSP/span" command.
6.2. Terminology to Support MS-W
[RFC 6378] uses the term "Manual Switch" and its acronym "MS". This
document uses the term "Manual Switch to Protection path" and "MS-P"
to have the same meaning, while avoiding confusion with "Manual
Switch to Working path" and its acronym "MS-W".
Similarly, we modify the name of "Protecting Administrative" state
(as defined in [RFC 6378]) to be "Switching Administrative" state to
include the case where traffic is switched to the working path as a
result of the external MS-W command.
6.3. Behavior of MS-P and MS-W
MS-P and MS-W SHALL have the same priority. We consider different
instances of determining the priority of the commands when they are
received either in succession or simultaneously.
o When two commands are received in succession, the command that is
received after the initial command SHALL be cancelled.
o If two nodes simultaneously receive commands that indicate
opposite operations (i.e., one node receives MS-P and the other
node receives MS-W) and transmit the indications to the remote
node, the MS-W SHALL be considered to have a higher priority, and
the MS-P SHALL be cancelled and discarded.
Two commands, MS-P and MS-W, are transmitted using the same Request
field value but SHALL indicate in the Fault Path (FPath) value the
path from which the traffic is being diverted. When traffic is
switched to the protection path, the FPath field value SHALL be set
to 1, indicating that traffic is being diverted from the working
path. When traffic is switched to the working path, the FPath field
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value SHALL be set to 0, indicating that traffic is being diverted
from the protection path. The Data Path (Path) field SHALL indicate
where user data traffic is being transported (i.e., if the working
path is selected, then Path is set to 0; if the protection path is
selected, then Path is set to 1).
When an MS command is in effect at a node, any subsequent MS or EXER
command and any other lower-priority requests SHALL be ignored.
6.4. Equal-Priority Resolution for MS
[RFC 6378] defines only one rule for the equal-priority condition in
Section 4.3.2 as "The remote message from the far-end LER is assigned
a priority just below the similar local input." In order to support
the Manual Switch behavior described in Section 6.3, additional rules
for equal-priority resolution are required. Since the support of
protection against signal degrade also requires a similar equal-
priority resolution, the rules are described in Section 7.4.
Support of this function requires changes to the PSC Control Logic
(including the state machine) relative to that shown in [RFC 6378].
Sections 10 and 11 present the PSC Control Logic when all
capabilities of APS mode are enabled.
7. Capability 4: Support of Protection against SD
7.1. Motivation for Supporting Protection against SD
In the MPLS-TP Survivability Framework [RFC 6372], both SF and SD
fault conditions can be used to trigger protection switching.
[RFC 6378], which defines the protection-switching protocol for
MPLS-TP, does not specify how the SF and SD are detected, and
specifies the protection-switching protocol associated with SF only.
The PSC protocol associated with SD is covered in this document, but
the specifics for the method of identifying SD is out of scope for
the protection protocol in the same way that SF detection and MS or
FS command initiation are out of scope.
7.2. Terminology to Support SD
In this document, the term Clear Signal Fail or Degrade (SFDc) is
used to indicate the clearance of either a degraded condition or a
failure condition.
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The second paragraph of Section 4.3.3.2 (Unavailable State) in
[RFC 6378] shows the intention of including Signal Degrade on
Protection path (SD-P) in the Unavailable state. Even though the
protection path can be partially available under the condition of
SD-P, this document follows the same state grouping as [RFC 6378] for
SD-P.
The bulleted item on the Protecting Failure state in Section 3.6 of
[RFC 6378] includes the degraded condition in the Protecting Failure
state. This document follows the same state grouping as [RFC 6378]
for Signal Degrade on Working path (SD-W).
7.3. Behavior of Protection against SD
To better align the behavior of MPLS-TP networks with that of other
transport networks (such as SDH, OTN, and Ethernet transport
networks), we define the following:
o The priorities of SD-P and SD-W SHALL be equal.
o Once a switch has been completed due to SD on one path, it will
not be overridden by SD on the other path (first come, first
served behavior), to avoid protection switching that cannot
improve signal quality.
The SD message indicates that the transmitting node has identified
degradation of the signal or integrity of the packet received on
either the working path or the protection path. The FPath field
SHALL identify the path that is reporting the degraded condition
(i.e., if the protection path, then FPath is set to 0; if the working
path, then FPath is set to 1), and the Path field SHALL indicate
where the data traffic is being transported (i.e., if the working
path is selected, then Path is set to 0; if the protection path is
selected, then Path is set to 1).
When the SD condition is cleared and the protected domain is
recovering from the situation, the Wait-to-Restore (WTR) timer SHALL
be used if the protected domain is configured for revertive behavior.
The WTR timer SHALL be started at the node that recovers from a local
degraded condition on the working path.
Protection switching against SD is always provided by a selector
bridge duplicating user data traffic and feeding it to both the
working path and the protection path under SD condition. When a
local or remote SD occurs on either the working path or the
protection path, the node SHALL duplicate user data traffic and SHALL
feed it to both the working path and the protection path. The packet
duplication SHALL continue as long as any SD condition exists in the
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protected domain. When the SD condition is cleared, in revertive
operation, the packet duplication SHALL continue in the WTR state and
SHALL stop when the node leaves the WTR state; while in non-revertive
operation, the packet duplication SHALL stop immediately.
The selector bridge with the packet duplication under SD condition,
which is a non-permanent bridge, is considered to be a 1:1 protection
architecture.
Protection switching against SD does not introduce any modification
to the operation of the selector at the sink node described in
[RFC 6378]. The selector chooses either the working or protection
path from which to receive the normal traffic in both 1:1 and 1+1
architectures. The position of the selector, i.e., which path to
receive the traffic, is determined by the PSC protocol in
bidirectional switching or by the local input in unidirectional
switching.
7.4. Equal-Priority Resolution
In order to support the MS behavior described in Section 6.3 and the
protection against SD described in Section 7.3, it is necessary to
expand rules for treating equal-priority inputs.
For equal-priority local inputs, such as MS and SD, apply a simple
first-come, first-served rule. Once a local input is determined as
the highest priority local input, then a subsequent equal-priority
local input requesting a different action, i.e., the action results
in the same PSC Request field but different FPath value, will not be
presented to the PSC Control Logic as the highest local request.
Furthermore, in the case of an MS command, the subsequent local MS
command requesting a different action will be cancelled.
If a node is in a remote state due to a remote SD (or MS) message, a
subsequent local input having the same priority but requesting a
different action to the PSC Control Logic will be considered as
having lower priority than the remote message and will be ignored.
For example, if a node is in remote Switching Administrative state
due to a remote MS-P, then any subsequent local MS-W SHALL be ignored
and automatically cancelled. If a node is in remote Unavailable
state due to a remote SD-P, then any subsequent local SD-W input will
be ignored. However, the local SD-W SHALL continue to appear in the
Local Request Logic as long as the SD condition exists, but it SHALL
NOT be the top-priority global request, which determines the state
transition at the PSC Control Logic.
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Cases where two end-points of the protected domain simultaneously
receive local triggers of the same priority that request different
actions may occur (for example, one node receives SD-P and the other
receives SD-W). Subsequently, each node will receive a remote
message with the opposing action indication. To address these cases,
we define the following priority resolution rules:
o When MS-W and MS-P occur simultaneously at both nodes, MS-W SHALL
be considered as having higher priority than MS-P at both nodes.
o When SD-W and SD-P occur simultaneously at both nodes, the SD on
the standby path (the path from which the selector does not select
the user data traffic) is considered as having higher priority
than the SD on the active path (the path from which the selector
selects the user data traffic) regardless of its origin (local or
remote message). Therefore, no unnecessary protection switching
is performed, and the user data traffic continues to be selected
from the active path.
In the preceding paragraphs, "simultaneously" refers to the case a
sent SD (or MS) request has not been confirmed by the remote end in
bidirectional protection switching. When a local node that has
transmitted an SD message receives an SD (or MS) message that
indicates a different value of Path field from the value of Path
field in the transmitted SD (or MS) message, both the local and
remote SD requests are considered to occur simultaneously.
The addition of support for protection against SD requires
modification to the PSC Control Logic (including the state machine)
relative to that described in [RFC 6378]. Sections 10 and 11 present
the PSC Control Logic when all capabilities of APS mode are enabled.
8. Capability 5: Support of EXER Command
The EXER command is used to verify the correct operation of the PSC
communication, such as the aliveness of the Local Request Logic, the
integrity of the PSC Control Logic, the PSC message generation and
reception mechanism, and the integrity of the protection path. EXER
does not trigger any actual traffic switching.
The command is only relevant for bidirectional protection switching,
since it is dependent upon receiving a response from the remote node.
The EXER command is assigned lower priority than any switching
message. It may be used regardless of the traffic usage of the
working path.
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When a node receives a remote EXER message, it SHOULD respond with a
Reverse Request (RR) message with the FPath and Path fields set
according to the current condition of the node. The RR message SHALL
be generated only in response to a remote EXER message.
This command is documented in R84 of [RFC 5654].
If EXER commands are input at both ends, then a race condition may
arise. This is resolved as follows:
o If a node has issued EXER and receives EXER before receiving RR,
it MUST treat the received EXER as it would an RR, and it SHOULD
NOT respond with RR.
The following PSC Requests are added to the PSC Request field to
support the Exercise command (see also Section 14.1):
(3) Exercise - indicates that the transmitting end-point is
exercising the protection channel and mechanism. FPath and Path
are set to the same value of the No Request (NR), RR, or DNR
message whose transmission is stopped by EXER.
(2) Reverse Request - indicates that the transmitting end-point is
responding to an EXER command from the remote node. FPath and
Path are set to the same value of the NR or DNR message whose
transmission is stopped by RR.
The relative priorities of EXER and RR are defined in Section 10.2.
9. Capabilities and Modes
9.1. Capabilities
A Capability is an individual behavior whose use is signaled in a
Capabilities TLV, which is placed in Optional TLVs field inside the
PSC message shown in Figure 2 of [RFC 6378]. The format of the
Capabilities TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = Capabilities | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value = Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of Capabilities TLV
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The value of the Type field is 1.
The value of the Length field is the length of the Flags field in
octets. The length of the Flags field MUST be a multiple of 4 octets
and MUST be the minimum required to signal all the required
capabilities.
Section 4 to Section 8 discuss five capabilities that are signaled
using the five most significant bits; if a node wishes to signal
these five capabilities, it MUST send a Flags field of 4 octets. A
node would send a Flags field greater than 4 octets only if it had
more than 32 Capabilities to indicate. All unused bits MUST be set
to zero.
If the bit assigned for an individual capability is set to 1, it
indicates the sending node's intent to use that capability in the
protected domain. If a bit is set to 0, the sending node does not
intend to use the indicated capability in the protected domain. Note
that it is not possible to distinguish between the intent not to use
a capability and a node's complete non-support (i.e., lack of
implementation) of a given capability.
This document defines five specific capabilities that are described
in Section 4 to Section 8. Each capability is assigned bit as
follows:
0x80000000: priority modification
0x40000000: non-revertive behavior modification
0x20000000: support of MS-W command
0x10000000: support of protection against SD
0x08000000: support of EXER command
If all the five capabilities should be used, a node SHALL set the
Flags field to 0xF8000000.
9.1.1. Sending and Receiving the Capabilities TLV
A node MUST include its Capabilities TLV in every PSC message that it
transmits. The transmission and acceptance of the PSC message is
described in Section 4.1 of [RFC 6378].
When a node receives a Capabilities TLV, it MUST compare the Flags
value to its most recent Flags value transmitted by the node. If the
two are equal, the protected domain is said to be running in the mode
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indicated by that set of capabilities (see Section 9.2). If the sent
and received Capabilities TLVs are not equal, this indicates a
Capabilities TLV mismatch. When this happens, the node MUST alert
the operator and MUST NOT perform any protection switching until the
operator resolves the mismatch between the two end-points.
9.2. Modes
A mode is a given set of Capabilities. Modes are shorthand;
referring to a set of capabilities by their individual values or by
the name of their mode does not change the protocol behavior. This
document defines two modes -- PSC and APS. Capabilities TLVs with
other combinations than the one specified by a mode are not supported
in this specification.
9.2.1. PSC Mode
PSC mode is defined as the lack of support for any of the additional
capabilities defined in this document -- that is, a Capabilities set
of 0x0. It is the behavior specified in [RFC 6378].
There are two ways to declare PSC mode. A node can send no
Capabilities TLV at all since there are no TLV units defined in
[RFC 6378], or it can send a Capabilities TLV with Flags value set to
0x0. In order to allow backward compatibility between two end-points
-- one which supports sending the Capabilities TLV, and one which
does not, the node that has the ability to send and process the PSC
mode Capabilities TLV MUST be able to both send the PSC mode
Capabilities TLV and send no Capabilities TLV at all. An
implementation MUST be configurable between these two options.
9.2.2. APS Mode
APS mode is defined as the use of all the five specific capabilities,
which are described in Sections 4 to 8 in this document. APS mode is
indicated with the Flags value of 0xF8000000.
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10. PSC Protocol in APS Mode
This section and the following section define the behavior of the PSC
protocol when all of the aforementioned capabilities are enabled,
i.e., APS mode.
10.1. Request Field in PSC Protocol Message
This document defines two new values for the "Request" field in the
PSC protocol message that is shown in Figure 2 of [RFC 6378] as
follows:
(2) Reverse Request
(3) Exercise
See also Section 14.1 of this document.
10.2. Priorities of Local Inputs and Remote Requests
Based on the description in Sections 3 and 4.3.2 in [RFC 6378], the
priorities of multiple outstanding local inputs are evaluated in the
Local Request Logic, where the highest priority local input (highest
local request) is determined. This highest local request is passed
to the PSC Control Logic that will determine the higher-priority
input (top-priority global request) between the highest local request
and the last received remote message. When a remote message comes to
the PSC Control Logic, the top-priority global request is determined
between this remote message and the highest local request that is
present. The top-priority global request is used to determine the
state transition, which is described in Section 11. In this
document, in order to simplify the description on the PSC Control
Logic, we strictly decouple the priority evaluation from the state
transition table lookup.
The priorities for both local and remote requests are defined as
follows from highest to lowest:
o Operator Clear (Local only)
o Lockout of protection (Local and Remote)
o Clear Signal Fail or Degrade (Local only)
o Signal Fail on Protection path (Local and Remote)
o Forced Switch (Local and Remote)
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o Signal Fail on Working path (Local and Remote)
o Signal Degrade on either Protection path or Working path (Local
and Remote)
o Manual Switch to either Protection path or Working path (Local and
Remote)
o WTR Timer Expiry (Local only)
o WTR (Remote only)
o Exercise (Local and Remote)
o Reverse Request (Remote only)
o Do-Not-Revert (Remote only)
o No Request (Remote and Local)
Note that the "Local only" requests are not transmitted to the remote
node. Likewise, the "Remote only" requests do not exist in the Local
Request Logic as local inputs. For example, the priority of WTR only
applies to the received WTR message, which is generated from the
remote node. The remote node that is running the WTR timer in the
WTR state has no local request.
The remote SF and SD on either the working path or the protection
path and the remote MS to either the working path or the protection
path are indicated by the values of the Request and FPath fields in
the PSC message.
The remote request from the remote node is assigned a priority just
below the same local request except for NR and equal-priority
requests, such as SD and MS. Since a received NR message needs to be
used in the state transition table lookup when there is no
outstanding local request, the remote NR request SHALL have a higher
priority than the local NR. For the equal-priority requests, see
Section 10.2.1.
10.2.1. Equal-Priority Requests
As stated in Section 10.2, the remote request from the remote node is
assigned a priority just below the same local request. However, for
equal-priority requests, such as SD and MS, the priority SHALL be
evaluated as described in this section.
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For equal-priority local requests, the first-come, first-served rule
SHALL be applied. Once a local request appears in the Local Request
Logic, a subsequent equal-priority local request requesting a
different action, i.e., the action results in the same Request value
but a different FPath value, SHALL be considered to have a lower
priority. Furthermore, in the case of an MS command, the subsequent
local MS command requesting a different action SHALL be rejected and
cleared.
When the priority is evaluated in the PSC Control Logic between the
highest local request and a remote request, the following equal-
priority resolution rules SHALL be applied:
o If two requests request the same action, i.e., the same Request
and FPath values, then the local request SHALL be considered to
have a higher priority than the remote request.
o When the highest local request comes to the PSC Control Logic, if
the remote request that requests a different action exists, then
the highest local request SHALL be ignored and the remote request
SHALL remain to be the top-priority global request. In the case
of an MS command, the local MS command requesting a different
action SHALL be cancelled.
o When the remote request comes to the PSC Control Logic, if the
highest local request that requests a different action exists,
then the top-priority global request SHALL be determined by the
following rules:
* For MS requests, the MS-W request SHALL be considered to have a
higher priority than the MS-P request. The node that has the
local MS-W request SHALL maintain the local MS-W request as the
top-priority global request. The other node that has the local
MS-P request SHALL cancel the MS-P command and SHALL generate
"Operator Clear" internally as the top-priority global request.
* For SD requests, the SD on the standby path (the path from
which the selector does not select the user data traffic) SHALL
be considered to have a higher priority than the SD on the
active path (the path from which the selector selects the user
data traffic) regardless of its origin (local or remote
message). The node that has the SD on the standby path SHALL
maintain the local SD on the standby path request as the top-
priority global request. The other node that has local SD on
the active path SHALL use the remote SD on the standby path as
the top-priority global request to lookup the state transition
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table. The differentiation of the active and standby paths is
based upon which path had been selected for the user data
traffic when each node detected its local SD.
10.3. Acceptance and Retention of Local Inputs
A local input indicating a defect, such as SF-P, SF-W, SD-P, and
SD-W, SHALL be accepted and retained persistently in the Local
Request Logic as long as the defect condition exists. If there is
any higher-priority local input than the local defect input, the
higher-priority local input is passed to the PSC Control Logic as the
highest local request, but the local defect input cannot be removed
but remains in the Local Request Logic. When the higher-priority
local input is cleared, the local defect will become the highest
local request if the defect condition still exists.
The Operator Clear (OC) command, SFDc, and WTR Timer Expiry are not
persistent. Once they appear to the Local Request Logic and complete
all the operations in the protection-switching control, they SHALL
disappear.
The LO, FS, MS, and EXER commands SHALL be rejected if there is any
higher-priority local input in the Local Request Logic. If a new
higher-priority local request (including an operator command) is
accepted, any previous lower-priority local operator command SHALL be
cancelled. When any higher-priority remote request is received, a
lower-priority local operator command SHALL be cancelled. The
cancelled operator command is cleared. If the operators wish to
renew the cancelled command, then they should reissue the command.
11. State Transition Tables in APS Mode
When there is a change in the highest local request or in remote PSC
messages, the top-priority global request SHALL be evaluated, and the
state transition tables SHALL be looked up in the PSC Control Logic.
The following rules are applied to the operation related to the state
transition table lookup.
o If the top-priority global request, which determines the state
transition, is the highest local request, the local state
transition table in Section 11.1 SHALL be used to decide the next
state of the node. Otherwise, the remote state transition table
in Section 11.2 SHALL be used.
o If in remote state, the highest local defect condition (SF-P,
SF-W, SD-P, or SD-W) SHALL always be reflected in the Request and
FPath fields.
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o For the node currently in the local state, if the top-priority
global request is changed to OC or SFDc, causing the next state to
be Normal, WTR, or DNR, then all the local and remote requests
SHALL be re-evaluated as if the node is in the state specified in
the footnotes to the state transition tables, before deciding the
final state. If there are no active requests, the node enters the
state specified in the footnotes to the state transition tables.
This re-evaluation is an internal operation confined within the
local node, and the PSC messages are generated according to the
final state.
o The WTR timer is started only when the node that has recovered
from a local failure or degradation enters the WTR state. A node
that is entering into the WTR state due to a remote WTR message
does not start the WTR timer. The WTR timer SHALL be stopped when
any local or remote request triggers the state change out of the
WTR state.
The extended states, as they appear in the table, are as follows:
N Normal state
UA:LO:L Unavailable state due to local LO command
UA:P:L Unavailable state due to local SF-P
UA:DP:L Unavailable state due to local SD-P
UA:LO:R Unavailable state due to remote LO message
UA:P:R Unavailable state due to remote SF-P message
UA:DP:R Unavailable state due to remote SD-P message
PF:W:L Protecting Failure state due to local SF-W
PF:DW:L Protecting Failure state due to local SD-W
PF:W:R Protecting Failure state due to remote SF-W message
PF:DW:R Protecting Failure state due to remote SD-W message
SA:F:L Switching Administrative state due to local FS command
SA:MW:L Switching Administrative state due to local MS-W command
SA:MP:L Switching Administrative state due to local MS-P command
SA:F:R Switching Administrative state due to remote FS message
SA:MW:R Switching Administrative state due to remote MS-W message
SA:MP:R Switching Administrative state due to remote MS-P message
WTR Wait-to-Restore state
DNR Do-not-Revert state
E::L Exercise state due to local EXER command
E::R Exercise state due to remote EXER message
Each state corresponds to the transmission of a particular set of
Request, FPath, and Path fields. The table below lists the message
that is generally sent in each particular state. If the message to
be sent in a particular state deviates from the table below, it is
noted in the footnotes of the state transition tables.
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State Request(FPath,Path)
------- ------------------------------------
N NR(0,0)
UA:LO:L LO(0,0)
UA:P:L SF(0,0)
UA:DP:L SD(0,0)
UA:LO:R highest local request(local FPath,0)
UA:P:R highest local request(local FPath,0)
UA:DP:R highest local request(local FPath,0)
PF:W:L SF(1,1)
PF:DW:L SD(1,1)
PF:W:R highest local request(local FPath,1)
PF:DW:R highest local request(local FPath,1)
SA:F:L FS(1,1)
SA:MW:L MS(0,0)
SA:MP:L MS(1,1)
SA:F:R highest local request(local FPath,1)
SA:MW:R NR(0,0)
SA:MP:R NR(0,1)
WTR WTR(0,1)
DNR DNR(0,1)
E::L EXER(0,x), where x is the existing Path value
when Exercise command is issued.
E::R RR(0,x), where x is the existing Path value
when RR message is generated.
Some operation examples of APS mode are shown in Appendix D.
In the state transition tables below, the letter 'i' stands for
"ignore" and is an indication to remain in the current state and
continue transmitting the current PSC message
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11.1. State Transition by Local Inputs
| OC | LO | SFDc | SF-P | FS | SF-W |
--------+-----+---------+------+--------+--------+--------+
N | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
UA:LO:L | (1) | i | i | i | i | i |
UA:P:L | i | UA:LO:L | (1) | i | i | i |
UA:DP:L | i | UA:LO:L | (1) | UA:P:L | SA:F:L | PF:W:L |
UA:LO:R | i | UA:LO:L | i | UA:P:L | i | PF:W:L |
UA:P:R | i | UA:LO:L | i | UA:P:L | i | PF:W:L |
UA:DP:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
PF:W:L | i | UA:LO:L | (2) | UA:P:L | SA:F:L | i |
PF:DW:L | i | UA:LO:L | (2) | UA:P:L | SA:F:L | PF:W:L |
PF:W:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
PF:DW:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
SA:F:L | (3) | UA:LO:L | i | UA:P:L | i | i |
SA:MW:L | (1) | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
SA:MP:L | (3) | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
SA:F:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
SA:MW:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
SA:MP:R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
WTR | (4) | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
DNR | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
E::L | (5) | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
E::R | i | UA:LO:L | i | UA:P:L | SA:F:L | PF:W:L |
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(Continued)
| SD-P | SD-W | MS-W | MS-P | WTRExp | EXER
--------+---------+---------+---------+---------+--------+------
N | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i | E::L
UA:LO:L | i | i | i | i | i | i
UA:P:L | i | i | i | i | i | i
UA:DP:L | i | i | i | i | i | i
UA:LO:R | UA:DP:L | PF:DW:L | i | i | i | i
UA:P:R | UA:DP:L | PF:DW:L | i | i | i | i
UA:DP:R | UA:DP:L | PF:DW:L | i | i | i | i
PF:W:L | i | i | i | i | i | i
PF:DW:L | i | i | i | i | i | i
PF:W:R | UA:DP:L | PF:DW:L | i | i | i | i
PF:DW:R | UA:DP:L | PF:DW:L | i | i | i | i
SA:F:L | i | i | i | i | i | i
SA:MW:L | UA:DP:L | PF:DW:L | i | i | i | i
SA:MP:L | UA:DP:L | PF:DW:L | i | i | i | i
SA:F:R | UA:DP:L | PF:DW:L | i | i | i | i
SA:MW:R | UA:DP:L | PF:DW:L | SA:MW:L | i | i | i
SA:MP:R | UA:DP:L | PF:DW:L | i | SA:MP:L | i | i
WTR | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | (6) | i
DNR | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i | E::L
E::L | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i | i
E::R | UA:DP:L | PF:DW:L | SA:MW:L | SA:MP:L | i | E::L
NOTES:
(1) Re-evaluate to determine the final state as if the node is in
the Normal state. If there are no active requests, the node
enters the Normal State.
(2) In the case that both local input after SFDc and the last
received remote message are NR, the node enters into the WTR
state when the domain is configured for revertive behavior, or
the node enters into the DNR state when the domain is configured
for non-revertive behavior. In all the other cases, where one
or more active requests exist, re-evaluate to determine the
final state as if the node is in the Normal state.
(3) Re-evaluate to determine final state as if the node is in the
Normal state when the domain is configured for revertive
behavior, or as if the node is in the DNR state when the domain
is configured for non-revertive behavior. If there are no
active requests, the node enters either the Normal state when
the domain is configured for revertive behavior or the DNR state
when the domain is configured for non-revertive behavior.
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(4) Remain in the WTR state and send an NR(0,1) message. Stop the
WTR timer if it is running. In APS mode, OC can cancel the WTR
timer and hasten the state transition to the Normal state as in
other transport networks.
(5) If Path value is 0, re-evaluate to determine final state as if
the node is in the Normal state. If Path value is 1,
re-evaluate to determine final state as if the node is in the
DNR state. If there are no active requests, the node enters the
Normal state when Path value is 0, or the DNR state when Path
value is 1.
(6) Remain in the WTR state and send an NR(0,1) message.
11.2. State Transition by Remote Messages
| LO | SF-P | FS | SF-W | SD-P | SD-W |
--------+---------+--------+--------+--------+---------+---------+
N | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
UA:LO:L | i | i | i | i | i | i |
UA:P:L | UA:LO:R | i | i | i | i | i |
UA:DP:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | i | (7) |
UA:LO:R | i | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
UA:P:R | UA:LO:R | i | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
UA:DP:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | i | PF:DW:R |
PF:W:L | UA:LO:R | UA:P:R | SA:F:R | i | i | i |
PF:DW:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | (8) | i |
PF:W:R | UA:LO:R | UA:P:R | SA:F:R | i | UA:DP:R | PF:DW:R |
PF:DW:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | i |
SA:F:L | UA:LO:R | UA:P:R | i | i | i | i |
SA:MW:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
SA:MP:L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
SA:F:R | UA:LO:R | UA:P:R | i | PF:W:R | UA:DP:R | PF:DW:R |
SA:MW:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
SA:MP:R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
WTR | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
DNR | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
E::L | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
E::R | UA:LO:R | UA:P:R | SA:F:R | PF:W:R | UA:DP:R | PF:DW:R |
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(Continued)
| MS-W | MS-P | WTR | EXER | RR | DNR | NR
--------+---------+---------+-----+------+----+------+----
N | SA:MW:R | SA:MP:R | i | E::R | i | i | i
UA:LO:L | i | i | i | i | i | i | i
UA:P:L | i | i | i | i | i | i | i
UA:DP:L | i | i | i | i | i | i | i
UA:LO:R | SA:MW:R | SA:MP:R | i | E::R | i | i | N
UA:P:R | SA:MW:R | SA:MP:R | i | E::R | i | i | N
UA:DP:R | SA:MW:R | SA:MP:R | i | E::R | i | i | N
PF:W:L | i | i | i | i | i | i | i
PF:DW:L | i | i | i | i | i | i | i
PF:W:R | SA:MW:R | SA:MP:R | (9) | E::R | i | (10) | (11)
PF:DW:R | SA:MW:R | SA:MP:R | (9) | E::R | i | (10) | (11)
SA:F:L | i | i | i | i | i | i | i
SA:MW:L | i | i | i | i | i | i | i
SA:MP:L | i | i | i | i | i | i | i
SA:F:R | SA:MW:R | SA:MP:R | i | E::R | i | DNR | N
SA:MW:R | i | SA:MP:R | i | E::R | i | i | N
SA:MP:R | SA:MW:R | i | i | E::R | i | DNR | N
WTR | SA:MW:R | SA:MP:R | i | i | i | i | (12)
DNR | SA:MW:R | SA:MP:R | (13)| E::R | i | i | i
E::L | SA:MW:R | SA:MP:R | i | i | i | i | i
E::R | SA:MW:R | SA:MP:R | i | i | i | DNR | N
NOTES:
(7) If the received SD-W message has Path=0, ignore the message. If
the received SD-W message has Path=1, go to the PF:DW:R state
and transmit an SD(0,1) message.
(8) If the received SD-P message has Path=1, ignore the message. If
the received SD-P message has Path=0, go to the UA:DP:R state
and transmit an SD(1,0) message.
(9) Transition to the WTR state and continue to send the current
message.
(10) Transition to the DNR state and continue to send the current
message.
(11) If the received NR message has Path=1, transition to the WTR
state if the domain is configured for revertive behavior, else
transition to the DNR state. If the received NR message has
Path=0, transition to the Normal state.
Ryoo, et al. Standards Track PAGE 26
RFC 7271 MPLS-TP LP for ITU-T June 2014
(12) If the receiving node's WTR timer is running, maintain the
current state and message. If the WTR timer is not running,
transition to the Normal state.
(13) Transit to the WTR state and send an NR(0,1) message. The WTR
timer is not initiated.
11.3. State Transition for 1+1 Unidirectional Protection
The state transition tables given in Sections 11.1 and 11.2 are for
bidirectional protection switching, where remote PSC protocol
messages are used to determine the protection-switching actions. 1+1
unidirectional protection switching does not require the remote
information in the PSC protocol message and acts upon local inputs
only. The state transition by local inputs in Section 11.1 SHALL be
reused for 1+1 unidirectional protection under the following
conditions:
o The value of Request field in the received remote message is
ignored and always assumed to be no request.
o Replace footnote (4) with "Stop the WTR timer and transit to the
Normal state."
o Replace footnote (6) with "Transit to the Normal state."
o Exercise command is not relevant.
12. Provisioning Mismatch and Protocol Failure in APS Mode
The remote PSC message that is received from the remote node is
subject to the detection of provisioning mismatch and protocol
failure conditions. In APS mode, provisioning mismatches are handled
as follows:
o If the PSC message is received from the working path due to
working/protection path configuration mismatch, the node MUST
alert the operator and MUST NOT perform any protection switching
until the operator resolves this path configuration mismatch.
o In the case that the mismatch happens in the two-bit "Protection
Type (PT)" field, which indicates permanent/selector bridge type
and uni/bidirectional switching type:
Ryoo, et al. Standards Track PAGE 27
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* If the value of the PT field of one side is 2 (i.e., selector
bridge) and that of the other side is 1 or 3 (i.e., permanent
bridge), then this event MUST be notified to the operator and
each node MUST NOT perform any protection switching until the
operator resolves this bridge type mismatch.
* If the bridge type matches but the switching type mismatches,
i.e., one side has PT=1 (unidirectional switching) while the
other side has PT=2 or 3 (bidirectional switching), then the
node provisioned for bidirectional switching SHOULD fall back
to unidirectional switching to allow interworking. The node
SHOULD notify the operator of this event.
o If the "Revertive (R)" bit mismatches, two sides will interwork
and traffic is protected according to the state transition
definition given in Section 11. The node SHOULD notify the
operator of this event.
o If the Capabilities TLV mismatches, the node MUST alert the
operator and MUST NOT perform any protection switching until the
operator resolves the mismatch in the Capabilities TLV.
The following are the protocol failure situations and the actions to
be taken:
o No match in sent "Data Path (Path)" and received "Data Path
(Path)" for more than 50 ms: The node MAY continue to perform
protection switching and SHOULD notify the operator of this event.
o No PSC message is received on the protection path during at least
3.5 times the long PSC message interval (e.g., at least 17.5
seconds with a default message interval of 5 seconds), and there
is no defect on the protection path: The node MUST alert the
operator and MUST NOT perform any protection switching until the
operator resolves this defect.
13. Security Considerations
This document introduces no new security risks. [RFC 6378] points out
that MPLS relies on assumptions about the difficulty of traffic
injection and assumes that the control plane does not have end-to-end
security. [RFC 5920] describes MPLS security issues and generic
methods for securing traffic privacy and integrity. MPLS use should
conform to such advice.
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14. IANA Considerations
14.1. MPLS PSC Request Registry
In the "Generic Associated Channel (G-ACh) Parameters" registry, IANA
maintains the "MPLS PSC Request Registry".
IANA has assigned the following two new code points from this
registry.
Value Description Reference
----- --------------------- ---------------
2 Reverse Request (this document)
3 Exercise (this document)
14.2. MPLS PSC TLV Registry
In the "Generic Associated Channel (G-ACh) Parameters" registry, IANA
maintains the "MPLS PSC TLV Registry".
This document defines the following new value for the Capabilities
TLV type in the "MPLS PSC TLV Registry".
Value Description Reference
------ --------------------- ---------------
1 Capabilities (this document)
14.3. MPLS PSC Capability Flag Registry
IANA has created and now maintains a new registry within the "Generic
Associated Channel (G-ACh) Parameters" registry called "MPLS PSC
Capability Flag Registry". All flags within this registry SHALL be
allocated according to the "Standards Action" procedures as specified
in RFC 5226 [RFC 5226].
The length of each flag MUST be a multiple of 4 octets. This
document defines 4-octet flags. Flags greater than 4 octets SHALL be
used only if more than 32 Capabilities need to be defined. The flags
defined in this document are:
Bit Hex Value Capability Reference
---- ---------- ----------------------------------- ---------------
0 0x80000000 priority modification (this document)
1 0x40000000 non-revertive behavior modification (this document)
2 0x20000000 support of MS-W command (this document)
3 0x10000000 support of protection against SD (this document)
4 0x08000000 support of EXER command (this document)
5-31 Unassigned (this document)
Ryoo, et al. Standards Track PAGE 29
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15. Acknowledgements
The authors would like to thank Yaacov Weingarten, Yuji Tochio,
Malcolm Betts, Ross Callon, Qin Wu, and Xian Zhang for their valuable
comments and suggestions on this document.
We would also like to acknowledge explicit text provided by Loa
Andersson and Adrian Farrel.
16. References
16.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC 5654] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
and S. Ueno, "Requirements of an MPLS Transport Profile",
RFC 5654, September 2009.
[RFC 6378] Weingarten, Y., Bryant, S., Osborne, E., Sprecher, N., and
A. Fulignoli, "MPLS Transport Profile (MPLS-TP) Linear
Protection", RFC 6378, October 2011.
16.2. Informative References
[G8031] International Telecommunication Union, "Ethernet Linear
Protection Switching", ITU-T Recommendation G.8031/Y.1342,
June 2011.
[G841] International Telecommunication Union, "Types and
characteristics of SDH network protection architectures",
ITU-T Recommendation G.841, October 1998.
[G873.1] International Telecommunication Union, "Optical Transport
Network (OTN): Linear protection", ITU-T Recommendation
G.873.1, July 2011.
[RFC 4427] Mannie, E. and D. Papadimitriou, "Recovery (Protection and
Restoration) Terminology for Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 4427, March 2006.
[RFC 5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Ryoo, et al. Standards Track PAGE 30
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[RFC 6372] Sprecher, N. and A. Farrel, "MPLS Transport Profile
(MPLS-TP) Survivability Framework", RFC 6372, September
2011.
Ryoo, et al. Standards Track PAGE 31
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Appendix A. An Example of an Out-of-Service Scenario
The sequence diagram shown is an example of the out-of-service
scenarios based on the priority level defined in [RFC 6378]. The
first PSC message that differs from the previous PSC message is
shown.
A Z
| |
(1) |-- NR(0,0) ------>| (1)
|<----- NR(0,0) ---|
| |
| |
| (FS issued at Z) | (2)
(3) |<------ FS(1,1) --|
|-- NR(0,1) ------>|
| |
| |
(4) | (SF on P(A<-Z)) |
| |
| |
| (Clear FS at Z) | (5)
(6) | X <- NR(0,0) --|
| |
| |
(1) Each end is in the Normal state and transmits NR(0,0) messages.
(2) When a FS command is issued at node Z, node Z goes into local
Protecting Administrative state (PA:F:L) and begins transmission
of an FS(1,1) message.
(3) A remote FS message causes node A to go into remote Protecting
Administrative state (PA:F:R), and node A begins transmitting
NR(0,1) messages.
(4) When node A detects a unidirectional SF-P, node A keeps sending
an NR(0,1) message because SF-P is ignored under the PA:F:R
state.
(5) When a Clear command is issued at node Z, node Z goes into the
Normal state and begins transmission of NR(0,0) messages.
(6) But, node A cannot receive PSC message because of local
unidirectional SF-P. Because no valid PSC message is received
over a period of several successive message intervals, the last
valid received message remains applicable, and the node A
continue to transmit an NR(0,1) message in the PA:F:R state.
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Now, there exists a mismatch between the selector and bridge
positions of node A (transmitting an NR(0,1) message) and node Z
(transmitting an NR(0,0) message). It results in an out-of-service
situation even when there is neither SF-W nor FS.
Appendix B. An Example of a Sequence Diagram Showing the Problem with
the Priority Level of SFc
An example of a sequence diagram showing the problem with the
priority level of SFc defined in [RFC 6378] is given below. The
following sequence diagram depicts the case when the bidirectional
signal fails. However, other cases with unidirectional signal fails
can result in the same problem. The first PSC message that differs
from the previous PSC message is shown.
A Z
| |
(1) |-- NR(0,0) ------>| (1)
|<----- NR(0,0) ---|
| |
| |
(2) | (SF on P(A<->Z)) | (2)
|-- SF(0,0) ------>|
|<------ SF(0,0) --|
| |
| |
(3) | (SF on W(A<->Z)) | (3)
| |
| |
(4) | (Clear SF-P) | (4)
| |
| |
(5) | (Clear SF-W) | (5)
| |
| |
(1) Each end is in the Normal state and transmits NR(0,0) messages.
(2) When SF-P occurs, each node enters into the UA:P:L state and
transmits SF(0,0) messages. Traffic remains on the working
path.
(3) When SF-W occurs, each node remains in the UA:P:L state as SF-W
has a lower priority than SF-P. Traffic is still on the working
path. Traffic cannot be delivered, as both the working path and
the protection path are experiencing signal fails.
Ryoo, et al. Standards Track PAGE 33
RFC 7271 MPLS-TP LP for ITU-T June 2014
(4) When SF-P is cleared, the local "Clear SF-P" request cannot be
presented to the PSC Control Logic, which takes the highest
local request and runs the PSC state machine, since the priority
of "Clear SF-P" is lower than that of SF-W. Consequently, there
is no change in state, and the selector and/or bridge keep
pointing at the working path, which has SF condition.
Now, traffic cannot be delivered while the protection path is
recovered and available. It should be noted that the same problem
will occur in the case that the sequence of SF-P and SF-W events is
changed.
If we further continue with this sequence to see what will happen
after SF-W is cleared:
(5) When SF-W is cleared, the local "Clear SF-W" request can be
passed to the PSC Control Logic, as there is no higher-priority
local input, but it will be ignored in the PSC Control Logic
according to the state transition definition in [RFC 6378].
There will be no change in state or protocol message
transmitted.
As SF-W is now cleared and the selector and/or bridge are still
pointing at the working path, traffic delivery is resumed. However,
each node is in the UA:P:L state and transmitting SF(0,0) messages,
while there exists no outstanding request for protection switching.
Moreover, any future legitimate protection-switching requests, such
as SF-W, will be rejected as each node thinks the protection path is
unavailable.
Appendix C. Freeze Command
The "Freeze" command applies only to the local node of the protection
group and is not signaled to the remote node. This command freezes
the state of the protection group. Until the Freeze is cleared,
additional local commands are rejected, and condition changes and
received PSC information are ignored.
The "Clear Freeze" command clears the local freeze. When the Freeze
command is cleared, the state of the protection group is recomputed
based on the persistent condition of the local triggers.
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 a fault condition.
Ryoo, et al. Standards Track PAGE 34
RFC 7271 MPLS-TP LP for ITU-T June 2014
Appendix D. Operation Examples of the APS Mode
The sequence diagrams shown in this section are only a few examples
of the APS mode operations. The first PSC protocol message that
differs from the previous message is shown. The operation of the
hold-off timer is omitted. The Request, FPath, and Path fields whose
values are changed during PSC message exchange are shown. For an
example, SF(1,0) represents a PSC message with the following field
values: Request=SF, FPath=1, and Path=0. The values of the other
fields remain unchanged from the initial configuration. W(A->Z) and
P(A->Z) indicate the working path and the protection path in the
direction of A to Z, respectively.
Example 1. 1:1 bidirectional protection switching (revertive
operation) - Unidirectional SF case
A Z
| |
(1) |<---- NR(0,0)---->| (1)
| |
| |
(2) | (SF on W(Z->A)) |
|---- SF(1,1)----->| (3)
(4) |<----- NR(0,1)----|
| |
| |
(5) | (Clear SF-W) |
|---- WTR(0,1)---->|
/| |
| | |
WTR timer | |
| | |
\| |
(6) |---- NR(0,1)----->| (7)
(8) |<----- NR(0,0)----|
|---- NR(0,0)----->| (9)
| |
(1) The protected domain is operating without any defect, and the
working path is used for delivering the traffic in the Normal
state.
(2) SF-W occurs in the Z to A direction. Node A enters into the
PF:W:L state and generates an SF(1,1) message. Both the
selector and bridge of node A are pointing at the protection
path.
Ryoo, et al. Standards Track PAGE 35
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(3) Upon receiving an SF(1,1) message, node Z sets both the selector
and bridge to the protection path. As there is no local request
in node Z, node Z generates an NR(0,1) message in the PF:W:R
state.
(4) Node A confirms that the remote node is also selecting the
protection path.
(5) Node A detects clearing of SF condition, starts the WTR timer,
and sends a WTR(0,1) message in the WTR state.
(6) Upon expiration of the WTR timer, node A sets both the selector
and bridge to the working path and sends an NR(0,1) message.
(7) Node Z is notified that the remote request has been cleared.
Node Z transits to the Normal state and sends an NR(0,0)
message.
(8) Upon receiving an NR(0,0) message, node A transits to the Normal
state and sends an NR(0,0) message.
(9) It is confirmed that the remote node is also selecting the
working path.
Ryoo, et al. Standards Track PAGE 36
RFC 7271 MPLS-TP LP for ITU-T June 2014
Example 2. 1:1 bidirectional protection switching (revertive
operation) - Bidirectional SF case - Inconsistent WTR timers
A Z
| |
(1) |<---- NR(0,0)---->| (1)
| |
| |
(2) | (SF on W(A<->Z)) | (2)
|<---- SF(1,1)---->|
| |
| |
(3) | (Clear SF-W) | (3)
|<---- NR(0,1)---->|
(4) |<--- WTR(0,1) --->| (4)
/| |\
| | | |
WTR timer | | WTR timer
| | | |
| | |/
| |<------ NR(0,1)---| (5)
| | |
\| |
(6) |--- NR(0,1)------>|
|<------ NR(0,0)---| (7)
(8) |--- NR(0,0)------>|
| |
(1) Each end is in the Normal state and transmits NR(0,0) messages.
(2) When SF-W occurs, each node enters into the PF:W:L state and
transmits SF(1,1) messages. Traffic is switched to the
protection path. Upon receiving an SF(1,1) message, each node
confirms that the remote node is also sending and receiving the
traffic from the protection path.
(3) When SF-W is cleared, each node transits to the PF:W:R state and
transmits NR(0,1) messages as the last received message is SF-W.
(4) Upon receiving NR(0,1) messages, each node goes into the WTR
state, starts the WTR timer, and sends the WTR(0,1) messages.
(5) Upon expiration of the WTR timer in node Z, node Z sends an
NR(0,1) message as the last received APS message was WTR. When
the NR(0,1) message arrives at node A, node A maintains the WTR
state and keeps sending current WTR messages as described in the
state transition table.
Ryoo, et al. Standards Track PAGE 37
RFC 7271 MPLS-TP LP for ITU-T June 2014
(6) Upon expiration of the WTR timer in node A, node A sends an
NR(0,1) message.
(7) When the NR(0,1) message arrives at node Z, node Z moves to the
Normal state, sets both the selector and bridge to the working
path, and sends an NR(0,0) message.
(8) The received NR(0,0) message causes node A to go to the Normal
state. Now, the traffic is switched back to the working path.
Example 3. 1:1 bidirectional protection switching - R bit mismatch
This example shows that both sides will interwork and the traffic is
protected when one side (node A) is configured as revertive operation
and the other (node Z) is configured as non-revertive operation. The
interworking is covered in the state transition tables.
(revertive) A Z (non-revertive)
| |
(1) |<---- NR(0,0)---->| (1)
| |
| |
(2) | (SF on W(A<->Z)) | (2)
|<---- SF(1,1)---->|
| |
| |
(3) | (Clear SF-W) | (3)
|<---- NR(0,1)---->|
(4) |<----- DNR(0,1)---| (4)
/|-- WTR(0,1)------>|
| |<----- NR(0,1)----| (5)
| | |
WTR timer | |
| | |
| | |
\| |
(6) |--- NR(0,1)------>|
|<------ NR(0,0)---| (7)
(8) |--- NR(0,0)------>|
| |
(1) Each end is in the Normal state and transmits NR(0,0) messages.
(2) When SF-W occurs, each node enters into the PF:W:L state and
transmits SF(l,l) messages. Traffic is switched to the
protection path. Upon receiving an SF(1,1) message, each node
confirms that the remote node is also sending and receiving the
traffic on the protection path.
Ryoo, et al. Standards Track PAGE 38
RFC 7271 MPLS-TP LP for ITU-T June 2014
(3) When SF-W is cleared, each node transits to the PF:W:R state and
transmits NR(0,1) messages as the last received message is SF-W.
(4) Upon receiving NR(0,1) messages, node A goes into the WTR state,
starts the WTR timer, and sends WTR(0,1) messages. At the same
time, node Z transits to the DNR state and sends a DNR(0,1)
message.
(5) When the WTR message arrives at node Z, node Z transits to the
WTR state and sends an NR(0,1) message according to the state
transition table. At the same time, the DNR message arrived at
node Z is ignored according to the state transition table.
Therefore, node Z, which is configured as non-revertive
operation, is operating as if in revertive operation.
(6) Upon expiration of the WTR timer in node A, node A sends an
NR(0,1) message.
(7) When the NR(0,1) message arrives at node Z, node Z moves to the
Normal state, sets both the selector and bridge to the working
path, and sends an NR(0,0) message.
(8) The received NR(0,0) message causes node A to transit to the
Normal state. Now, the traffic is switched back to the working
path.
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RFC 7271 MPLS-TP LP for ITU-T June 2014
Authors' Addresses
Jeong-dong Ryoo (editor)
ETRI
218 Gajeongno
Yuseong-gu, Daejeon 305-700
South Korea
Phone: +82-42-860-5384
EMail: ryoo@etri.re.kr
Eric Gray (editor)
Ericsson
EMail: eric.gray@ericsson.com
Huub van Helvoort
Huawei Technologies
Karspeldreef 4,
Amsterdam 1101 CJ
The Netherlands
Phone: +31 20 4300936
EMail: huub.van.helvoort@huawei.com
Alessandro D'Alessandro
Telecom Italia
via Reiss Romoli, 274
Torino 10148
Italy
Phone: +39 011 2285887
EMail: alessandro.dalessandro@telecomitalia.it
Taesik Cheung
ETRI
218 Gajeongno
Yuseong-gu, Daejeon 305-700
South Korea
Phone: +82-42-860-5646
EMail: cts@etri.re.kr
Eric Osborne
EMail: eric.osborne@notcom.com
Ryoo, et al. Standards Track PAGE 40
RFC TOTAL SIZE: 87586 bytes
PUBLICATION DATE: Thursday, June 12th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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