MPLS Working Group Huub van Helvoort, Ed.
Internet Draft Huawei Technologies
Intended status: Standards Track Jeong-dong Ryoo, Ed.
ETRI
Haiyan Zhang
Huawei Technologies
Feng Huang
Alcatel-Lucent Shanghai Bell
Han Li
China Mobile
Alessandro D'Alessandro
Telecom Italia
Expires: January 13, 2012 July 13, 2011
Linear Protection Switching in MPLS-TP
draft-zulr-mpls-tp-linear-protection-switching-03.txt
Abstract
This document specifies a linear protection switching mechanism for
MPLS-TP. This mechanism supports 1+1 unidirectional/bidirectional
protection switching and 1:1 bidirectional protection switching. It
is purely supported by MPLS-TP data plane, and can work without any
control plane.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunications Union Telecommunications
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
Zhang, et al. Expires January 13, 2012 [Page 1]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 13, 2012.
Copyright Notice
Copyright (c) 2011 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 BSD License.
Zhang, et al. Expires January 13, 2012 [Page 2]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Table of Contents
1. Introduction ................................................ 4
2. Linear protection switching overview ........................ 5
2.1. Protection Architecture Types .......................... 5
2.2. Protection Switching Types ............................. 6
2.3. Protection Operation Types ............................. 6
3. Protection switching trigger conditions ..................... 7
3.1. Fault Conditions ....................................... 7
3.2. External commands ...................................... 7
4. Protection Switching Schemes ................................ 8
4.1. 1+1 unidirectional protection switching ................ 8
4.2. 1+1 bidirectional protection switching ................. 9
4.3. 1:1 bidirectional protection switching ................ 10
5. APS Protocol ............................................... 11
5.1. APS PDU Format ........................................ 11
5.2. APS transmission ...................................... 14
5.3. Hold-off timer ........................................ 15
6. Protection switching logic ................................. 16
7. Protection Switching State Transition Table ................ 18
7.1. State transition for 1:1 bidirectional switching with
revertive mode ............................................. 20
7.2. State transition for 1:1 bidirectional switching with non-
revertive mode ............................................. 23
7.3. State transition for 1+1 bidirectional switching with
revertive mode ............................................. 27
7.4. State transition for 1+1 bidirectional switching with non-
revertive mode ............................................. 30
7.5. State transition for 1+1 unidirectional switching with
revertive mode ............................................. 35
7.6. State transition for 1+1 unidirectional switching with non-
revertive mode ............................................. 36
8. Security Considerations .................................... 38
9. IANA Considerations ........................................ 38
10. Acknowledgments ........................................... 38
APPENDIX A: Operation Examples of APS Protocol ................ 39
11. References ................................................ 46
11.1. Normative References ................................. 46
11.2. Informative References ............................... 46
Zhang, et al. Expires January 13, 2012 [Page 3]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
1. Introduction
MPLS-TP is defined as transport profile of MPLS technology to fulfill
the deployment in transport network. A typical feature of transport
network is that it can provide fast protection switching for end-to-
end or segments. The protection switching time is generally required
to be less than 50ms according to the strictest requirement of
services such as voice, private line, etc.
The goal of linear protection switching mechanism is to satisfy the
requirement of fast protection switching for MPLS-TP network. Linear
protection switching means that, for one or more working transport
entities, there is one protection transport entity, which is disjoint
from any of working transport entities, ready for taking over the
service transmission when a working transport entity failed.
This document specifies 1+1 unidirectional protection switching
mechanism for unidirectional transport entity (either point-to-point
or point-to-multipoint) as well as bidirectional point-to-point
transport entity, and 1+1/1:1 bidirectional protection switching
mechanism for 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 APS
(Automatic Protection Switching) protocol details which is used for
this purpose.
The linear protection mechanism described in this document is
applicable to both LSPs and PWs.
The APS protocol specified in this document is based on the same
principles and behavior of the APS protocol designed for SONET/SDH
networks (i.e., it is mature and proven) and provides commonality
with the established operation models utilized in other transport
network technologies (e.g., SDH/SONET and OTN).
It is also worth noting that multi-vendor implementations of the APS
protocol described in this document already exist.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunications Union Telecommunications
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
Zhang, et al. Expires January 13, 2012 [Page 4]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
2. Linear protection switching overview
To guarantee the protection switching time, for a working transport
entity, its protection transport entity is always pre-configured
before the failure occurs. Normally, the normal traffic will be
transmitted and received on the working transport entity. The
switching to protection transport entity is usually triggered by
link/node failure, external commands, etc. Note that external
commands are often used in transport network by operators, and they
are very useful in cases of service adjustment, path maintenance, etc.
2.1. Protection Architecture Types
- 1+1 architecture
In the 1+1 architecture, a protection transport entity is associated
with the 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 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.
- 1:1 architecture
In the 1:1 architecture, a protection transport entity is associated
with the 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 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 also to
the protection transport entity. Broadcast bridge is recommended to
be used in revertive mode only.
- 1:n architecture
Zhang, et al. Expires January 13, 2012 [Page 5]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Details for the 1:n protection switching architecture will be
provided in a future version of this draft.
It is worth noting that the APS protocol defined here is ready to
support 1:n operations.
2.2. Protection Switching Types
The linear protection switching types can be a unidirectional
switching type or a bidirectional switching type.
- Unidirectional switching type: Only the affected direction of
working transport entity is switched to 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.
- Bidirectional switching type: Both directions of working transport
entity, including the affected direction and the unaffected
direction, are switched to protection transport entity. For
bidirectional switching, automatic protection switching (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.
2.3. Protection Operation Types
The linear protection operation types can be a non-revertive
operation type or a revertive operation type.
- 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.
- 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
of 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.
Zhang, et al. Expires January 13, 2012 [Page 6]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
3. Protection switching trigger conditions
3.1. Fault Conditions
Fault conditions mean the requests generated by the local OAM
function.
- Signal Failure (SF): If an endpoint detects a failure by OAM
function or other mechanism, it will submit a local signal failure
(local SF) to APS module to request a protection switching. The
local SF could be on working transport entity or protection
transport entity.
- Signal Degrade (SD): If an endpoint detects signal degrade by OAM
function or other mechanism, it will submit a local signal failure
(local SD) to APS module to request a protection switching. The
local SD could be on working transport entity or protection
transport entity.
3.2. External commands
The external command issues an appropriate external request on to the
protection process:
- Lockout of Protection (LO): This command is used to provide
operator a tool for temporarily disabling access to the protection
transport entity.
- Manual switch (MS): This command is used to provide operator a
tool for temporarily switching normal traffic to working transport
entity (MS-W) or protection transport entity (MS-P), unless a
higher priority switch request (i.e., LP, FS, or SF) is in effect.
- Forced switch (FS): This command is used to provide operator a
tool for temporarily switching normal traffic from working
transport entity to protection transport entity, unless a higher
priority switch request (i.e., LP) is in effect.
- Exercise (EXER): Exercise is a command to test if the APS
communication is operating correctly. The signal is chosen so as
not to modify the selector.
- Clear: This command between management and 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.
Zhang, et al. Expires January 13, 2012 [Page 7]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
4. Protection Switching Schemes
4.1. 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 protection domain. In normal condition, the sink
endpoint receives traffic from working transport entity. If the sink
endpoint detects a failure on working transport entity, it will
switch to receive traffic from protection transport entity. 1+1
unidirectional protection switching is recommended to be used for
unidirectional transport entity.
Note that 1+1 unidirectional protection switching does not need APS
coordination protocol since it only perform protection switching
based on the local request.
Zhang, et al. Expires January 13, 2012 [Page 8]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
4.2. 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
protection domain. In normal condition, for each direction, the sink
endpoint receives traffic from working transport entity.
If the sink endpoint detects a failure on the working transport
entity, it will switch to receive traffic from protection transport
entity. It will also send an APS message to inform the sink endpoint
Zhang, et al. Expires January 13, 2012 [Page 9]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
on another direction to switch to receive traffic from protection
transport entity.
APS mechanism is necessary to coordinate the two endpoints of
transport entity and implement 1+1 bidirectional protection switching
even for a unidirectional failure.
4.3. 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 working transport entity or
Zhang, et al. Expires January 13, 2012 [Page 10]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
protection transport entity. The sink endpoint receives the traffic
from the transport entity where the source endpoint sends on.
In normal condition, for each direction, the source endpoint and sink
endpoint send and receive traffic from working transport entity.
If the sink endpoint detects a failure on the working transport
entity, it will switch to send and receive traffic from 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 protection transport entity.
APS mechanism is necessary to coordinate the two endpoints of
transport entity and implement 1:1 bidirectional protection switching
even for a unidirectional failure.
5. APS Protocol
5.1. APS PDU Format
APS packets MUST be sent over a G-ACh as defined in [RFC5586].
The format of APS PDU is specified in the 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| Y.1731 Channel Type (0xXX) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MEL | Version | OpCode | Flags | TLV Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| APS Specific Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End TLV |
+-+-+-+-+-+-+-+-+
Figure 4 APS PDU format
The following values shall be used for APS PDU:
o The Y.1731 Channel Type is set as defined in [BHH MPLS-TP OAM];
o MEL: set as defined in [BHH MPLS-TP OAM];
o Version: 0x00
o OpCode: 0d39 (=0x27)
Zhang, et al. Expires January 13, 2012 [Page 11]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
o Flags: 0x00
o TLV Offset: 4
o END TLV: 0x00
The format of the APS-specific information is defined in the 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 |
| State |A|B|D|R| Signal | Signal | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 APS specific information format
- Request/State
The 4 bits indicate the protection switching request type. See Figure
6 for the code of each request/state type.
In case that there are multiple protection switching requests, only
the protection switching request with the highest priority will be
processed.
Zhang, et al. Expires January 13, 2012 [Page 12]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
+------------------------------------+---------------+
| Request/State | code/priority |
+------------------------------------+---------------+
|Lockout of Protection (LO) | 1111 (highest)|
+------------------------------------+---------------+
|Signal Fail for Protection (SF-P) | 1110 |
+------------------------------------+---------------+
|Forced Switch (FS) | 1101 |
+------------------------------------+---------------+
|Signal Fail for Working (SF-W) | 1011 |
+------------------------------------+---------------+
|Signal Degrade | 1001 |
+------------------------------------+---------------+
|Manual Switch | 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
- Protection type (Pr.Type)
The 4 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
- Requested signal
Zhang, et al. Expires January 13, 2012 [Page 13]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
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
- 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
- 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
- Reserved
This field should be set to zero.
5.2. APS transmission
The APS message should be transported on protection transport entity
by encapsulated with the protection transport entity label. If an
endpoint receives APS-specific information from the working entity,
it should ignore this information, and should detect the Failure of
Protocol defect (see Section 6).
A new APS packet must be transmitted immediately when a change in the
transmitted status occurs. The first three APS packets should 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
Zhang, et al. Expires January 13, 2012 [Page 14]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
interval of the first three APS packets should be 3.3ms. APS packets
after the first three should be transmitted with the interval of 5
seconds.
If no valid APS-specific information is received, the last valid
received information remains applicable.
5.3. 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 protection group 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/SD) on the
transport entity that currently carries 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 will be
started. When the hold-off timer expires, it will be checked whether
a defect still exists on the transport entity that started the timer.
If it does, that defect will 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.
Zhang, et al. Expires January 13, 2012 [Page 15]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
6. Protection switching logic
+-------------+ 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 is passed on to the "APS process logic". Note
that an accepted Clear command, clearance of SF(-P) or expiration of
WTR timer shall not be processed by the local request logic, but
Zhang, et al. Expires January 13, 2012 [Page 16]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
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:
- The ''B'' field mismatch due to incompatible provisioning;
- The reception of APS message from the working entity due to
working/protection configuration mismatch;
- No match in sent ''Requested traffic'' and received ''requested
signal'' for more than 50 ms;
- 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:
- If "D" bit mismatches, the bidirectional side will fall back to
unidirectional switching.
- 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.
- 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 should be ignored, and the
last valid received information remains applicable.
The linear protection switching algorithm commences immediately
every time one of the input signals changes, i.e., when the status of
any local request changes, or when a 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 a new APS specific
information (if necessary), or detect the failure of protocol defect
if the protection switching is not completed within 50 ms.
The state transition is calculated in the ''APS process logic'' based
on the highest local request, the request of the last received
Zhang, et al. Expires January 13, 2012 [Page 17]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
''Request/State'' information, and state transition tables defined in
Section 7, as follows:
- If the highest local request is Clear, clearance of SF(-P) or of
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.
- If the highest local request is neither Clear, nor clearance of
SF(-P) 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.
i) If the highest local request has higher or equal priority, it
is used with the state transition table for local requests
defined in Section 7 to determine the final state; otherwise
ii) The request of the last received ''Request/State'' information is
used with the state transition table for far end requests
defined in Annex A 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).
7. Protection Switching State Transition Table
In this section, state transition tables for the following protection
switching configurations are described.
- 1:1 bidirectional (revertive mode, non-revertive mode);
- 1+1 bidirectional (revertive mode, non-revertive mode);
- 1+1 unidirectional (revertive mode, non-revertive mode).
Note that any other global or local request which 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:
Zhang, et al. Expires January 13, 2012 [Page 18]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
- No request: No Request is the state entered by the local priority
under all conditions where no local protection switching requests
(including wait-to-restore and do-not-revert) are active. NR can
also indicates 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.
- Lockout, Signal Fail(P): The access by the normal traffic to the
protection transport entity is NOT allowed, due to the SF detected
on the protection entity or due to the lockout of protection
command applied. 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).
- Forced Switch, Signal Fail(W), Signal Degrade(W), Signal
Degrade(P), Manual Switch: 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 behaviour of the specific switching trigger.
- Wait to Restore: In revertive operation, after the clearing of an
SF or SD on working transport entity, maintains normal traffic as
selected from the protection transport entity until a wait-to-
restore timer expires or another request with higher priority,
including a clear command, is received. This is used to prevent
frequent operation of the selector in the case of intermittent
failures.
- Do not revert: In non-revertive operation, this is used to
maintain a normal traffic to be selected from the protection
transport entity.
- Exercise: Exercise of the APS protocol.
- Reverse Request: The near end will enter and signal Reverse
Request only in response to an EXER from the far end.
Zhang, et al. Expires January 13, 2012 [Page 19]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.1. State transition for 1:1 bidirectional switching with revertive
mode
Table 7.1 - State transition by local requests (1:1, bidirectional,
revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 20]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Table 7.2 - State transition by far end requests (1:1, bidirectional,
revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 21]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 22]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.2. State transition for 1:1 bidirectional switching with non-revertive
mode
Table 7.3 - State transition by local requests (1:1, bidirectional,
non-revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 23]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 24]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Table 7.4 - State transition by far end requests (1:1, bidirectional,
non-revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 25]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 26]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.3. State transition for 1+1 bidirectional switching with revertive
mode
Table 7.5 - State transition by local requests (1+1, bidirectional,
revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 27]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Table 7.6 - State transition by far end requests (1+1, bidirectional,
revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 28]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 29]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.4. State transition for 1+1 bidirectional switching with non-revertive
mode
Table 7.7 - State transition by local requests (1+1, bidirectional,
non-revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 30]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 31]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Table 7.8 - State transition by far end requests (1+1 bidirectional,
non-revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 32]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 33]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 34]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.5. State transition for 1+1 unidirectional switching with revertive
mode
Table 7.9 - State transition by local requests (1+1, unidirectional,
revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 35]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
7.6. State transition for 1+1 unidirectional switching with non-
revertive mode
Table 7.10 - State transition by local requests (1+1, unidirectional,
non-revertive mode)
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 36]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
[See the PDF form of this document]
Zhang, et al. Expires January 13, 2012 [Page 37]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
8. Security Considerations
To be added in a future version of the document.
9. IANA Considerations
To be added in a future version of the document.
10. Acknowledgments
The authors would like to thank Hao Long, Vincenzo Sestito, Italo
Busi, Igor Umansky for their input to and review of the current
document.
Zhang, et al. Expires January 13, 2012 [Page 38]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
APPENDIX A: Operation Examples of 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 protection 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 SF(r=1, b=1) message.
Zhang, et al. Expires January 13, 2012 [Page 39]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
(3) Upon receiving SF(r=1, b=1), node Z sets selector and bridge to
protection entity. As there is no local request in node Z, node Z
generates NR(r=1, b=1) message.
(4) Node A confirms that the far end is also selecting protection
entity.
(5) Node A detects clearing of SF condition, starts the WTR timer,
and sends WTR(r=1, b=1) message.
(6) At expiration of the WTR timer, node A sets selector and bridge
to working entity and sends NR(r=0, b=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 protection domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
Zhang, et al. Expires January 13, 2012 [Page 40]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
(2) Nodes A and Z detect local Signal Fail conditions on the working
entity, set selector and bridge to protection entity, and generate
SF(r=1, b=1) messages.
(3) Upon receiving SF(r=1, b=1), each node confirms that the far end
is also selecting protection entity.
(4) Each node detects clearing of SF condition, and sends NR(r=1, b=1)
message as the last received APS message was SF.
(5) Upon receiving NR(r=1, b=1), each node starts the WTR timer and
sends WTR(r=1, b=1).
(6) At expiration of the WTR timer, each node sends NR(r=1, b=1) as
the last received APS message was WTR.
(7) Upon receiving NR(r=1, b=1), each node sets selector and bridge
to working entity and sends NR(r=0, b=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
Zhang, et al. Expires January 13, 2012 [Page 41]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
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 protection domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Nodes A and Z detect local Signal Fail conditions on the working
entity , set selector and bridge to protection entity, and generate
SF(r=1, b=1) messages.
(3) Upon receiving SF(r=1, b=1), each node confirms that the far end
is also selecting protection entity.
(4) Each node detects clearing of SF condition, and sends NR(r=1, b=1)
message as the last received APS message was SF.
(5) Upon receiving NR(r=1, b=1), each node starts the WTR timer and
sends WTR(r=1, b=1).
(6) At expiration of the WTR timer in node A, node A sends NR(r=1,
b=1) as the last received APS message was WTR.
(7) At node Z, the received NR(r=1, b=1) is ignored as the local WTR
has a higher priority.
(8) At expiration of the WTR timer in node Z, node Z node sets
selector and bridge to working entity, and sends NR(r=0, b=0) message.
Zhang, et al. Expires January 13, 2012 [Page 42]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
(9) Upon receiving NR(r=0, b=0), node A sets selector and bridge to
working entity and sends NR(r=0, b=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
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 protection 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 SF(r=1, b=1) message.
(3) Upon receiving SF(r=1, b=1), node Z sets selector and bridge to
protection entity. As there is no local request in node Z, node Z
generates NR(r=1, b=1) message.
Zhang, et al. Expires January 13, 2012 [Page 43]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
(4) Node A confirms that the far end is also selecting protection
entity.
(5) Node A detects clearing of SF condition, and sends DNR(r=1, b=1)
message.
(6) Upon receiving DNR(r=1, b=1), node Z also generates DNR(r=1, b=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 SF-P(r=0, b=0) message.
(8) Upon receiving SF-P(r=0, b=0), node A sets selector and bridge to
working entity, and generates NR(r=0, b=0) message.
(9) Node Z detects clearing of SF condition, and sends NR(r=0, b=0)
message.
Exmaple 5. 1:1 bidirectional protection switching (non-revertive mode)
- Bidirectional SF on working followed by bidirectional SF on
protection
Zhang, et al. Expires January 13, 2012 [Page 44]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
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 protection domain is operating without any defect, and the
working entity is used for delivering the normal traffic.
(2) Nodes A and Z detect local Signal Fail conditions on the working
entity, set selector and bridge to protection entity, and generate
SF(r=1, b=1) messages.
(3) Upon receiving SF(r=1, b=1), each node confirms that the far end
is also selecting protection entity.
(4) Each node detects clearing of SF condition, and sends NR(r=1, b=1)
message as the last received APS message was SF.
(5) Upon receiving NR(r=1, b=1), each node sends DNR(r=1, b=1).
(6) Signal Fail occurs on the protection entity in both directions.
Selector and bridge of each node selects the working entity. Each
node generates SF-P(r=0, b=0) message.
(7) Upon receiving SF-P(r=0, b=0), each node confirms that the far
end is also selecting working entity
(8) Each node detects clearing of SF condition, and sends NR(r=0, b=0)
message.
Zhang, et al. Expires January 13, 2012 [Page 45]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
11. References
11.1. Normative References
[RFC5317] Bryant,S., Andersson,L., ''Joint Working Team (JWT) Report
on MPLS Architectural Considerations for a Transport
Profile'', RFC 5317, February 2009
[RFC5586] Bocci,M., Vigoureux,M., and Bryant,S., MPLS Generic
Associated Channel'', RFC 5586, June 2009
[RFC5654] Niven-Jenkins,B., Brungard,D., and Betts,M., ''Requirements
of an MPLS Transport Profile'', RFC 5654, September 2009
[BHH MPLS-TP OAM] I. Busi., H. van Helvoort and J. He., "MPLS-TP OAM
based on Y.1731", draft-bhh-mpls-tp-oam-y1731-05 (work in
progress), July 12, 2010
11.2. Informative References
[MPLS-TP Survive Frmk] Sprecher,N., and Farrel,A., ''Multiprotocol
Label Switching Transport Profile Survivability Framework",
draft-ietf-mpls-tp-survive-fwk-03(work in progress),
November 2009
Author's Addresses
Huub van Helvoort (Editor)
Huawei Technologies Co., Ltd.
Email: hhelvoort@huawei.com
Jeong-dong Ryoo (Editor)
ETRI
Email: ryoo@etri.re.kr
Haiyan Zhang
Huawei Technologies Co., Ltd.
Email: zhanghaiyan@huawei.com
Feng Huang
Alcatel-Lucent Shanghai Bell
Email: feng.f.huang@alcatel-sbell.com.cn
Zhang, et al. Expires January 13, 2012 [Page 46]
Internet-Draft Linear Protection Switching in MPLS-TP July 2011
Han Li
China Mobile
Email: lihan@chinamobile.com
Alessandro D'Alessandro
Telecom Italia
Email: alessandro.dalessandro@telecomitalia.it
Zhang, et al. Expires January 13, 2012 [Page 47]