Internet Engineering Task Force (IETF) J. Ryoo
Request for Comments: 8234 T. Cheung
Updates: 7271 ETRI
Category: Standards Track H. van Helvoort
ISSN: 2070-1721 Hai Gaoming BV
I. BusiG. Wen
August 2017 Updates to MPLS Transport Profile (MPLS-TP) Linear Protection in
Automatic Protection Switching (APS) Mode
This document contains updates to MPLS Transport Profile (MPLS-TP)
linear protection in Automatic Protection Switching (APS) mode
defined in RFC 7271. The updates provide rules related to the
initialization of the Protection State Coordination (PSC) Control
Logic (in which the state machine resides) when operating in APS mode
and clarify the operation related to state transition table lookup.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
MPLS Transport Profile (MPLS-TP) linear protection in Automatic
Protection Switching (APS) mode is defined in RFC 7271 [RFC7271]. It
defines a set of alternate and additional mechanisms to perform some
of the functions of linear protection described in RFC 6378
[RFC6378]. The actions performed at initialization of the Protection
State Coordination (PSC) Control Logic are not described in either
[RFC7271] or [RFC6378]. Although it is a common perception that the
state machine starts at the Normal state, this is not explicitly
specified in any of the documents and various questions have been
raised by implementers and in discussions on the MPLS working group
mailing list concerning the detailed actions that the PSC Control
Logic should take.
The state machine described in [RFC7271] operates under the
assumption that both end nodes of a linear protection domain start in
the Normal state. In the case that one node reboots while the other
node is still in operation, various scenarios may arise resulting in
problematic situations. This document resolves all the problematic
cases and minimizes traffic disruptions related to initialization,
including both cold and warm reboots that require re-initialization
of the PSC Control Logic.
This document contains updates to the MPLS-TP linear protection in
APS mode defined in [RFC7271]. The updates provide rules related to
initialization of the PSC Control Logic (in which the state machine
resides) when operating in APS mode. The updates also include
modifications to the state transition table defined in Section 11.2
of [RFC7271]. The changes in the state transition table have been
examined to make sure that no new problems are introduced.
This document does not introduce backward compatibility issues with
implementations of [RFC7271]. In case a node implementing this
document restarts, the new state changes will not cause problems at
the remote node implementing [RFC7271], and the two ends will
converge to the same local and remote states. In case a node
implementing [RFC7271] restarts, the two ends behave as they do
This document also provides some clarifications on the operation
related to state transition table lookup.
The reader of this document is assumed to be familiar with [RFC7271].
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the following abbreviations:
APS Automatic Protection Switching
E::R Exercise state due to remote EXER message
MS-P Manual Switch to Protection path
MS-W Manual Switch to Working path
MPLS-TP MPLS Transport Profile
NR No Request
PF:DW:R Protecting Failure state due to remote SD-W message
PF:W:L Protecting Failure state due to local SF-W
PF:W:R Protecting Failure state due to remote SF-W message
PSC Protection State Coordination
RR Reverse Request
SA:MP:R Switching Administrative state due to remote MS-P message
SA:MW:R Switching Administrative state due to remote MS-W message
SD Signal Degrade
SD-W Signal Degrade on Working path
SF Signal Fail
SF-P Signal Fail on Protection path
SF-W Signal Fail on Working path
UA:P:L Unavailable state due to local SF-P
This section specifies the actions that will be performed at the
initialization of the PSC Control Logic and the modifications of the
state transition table defined in Section 11.2 of [RFC7271]. Some
clarifications on the operation related to state transition table
lookup are also provided.
4.1. Initialization Behavior
This section defines initialization behavior that is not described in
When the PSC Control Logic is initialized, the following actions MUST
o Stop the WTR timer if it is running.
o Clear any operator command in the Local Request Logic.
o If an SF-W or SF-P exists as the highest local request, the node
being initialized starts at the PF:W:L or UA:P:L state,
o If the node being initialized has no local request:
* If the node being initialized does not remember the active path
or if the node being initialized remembers the working path as
the active path, the node starts at the Normal state.
* Else (the node being initialized remembers the protection path
as the active path), the node starts at the WTR state sending
NR(0,1) or at the DNR state sending DNR(0,1) depending on the
configuration that allows or prevents automatic reversion to
the Normal state.
o In case any local SD exists, the local SD MUST be considered as an
input to the Local Request Logic only after the local node has
received the first protocol message from the remote node and
completed the processing (i.e., updated the PSC Control Logic and
decided which action, if any, is to be sent to the PSC Message
o If the local node receives an EXER message as the first protocol
message after initialization and the remote EXER becomes the top-
priority global request, the local node MUST set the position of
the bridge and selector according to the Path value in the EXER
message and transit to the E::R state.
In the case of no local request, remembering the active path
minimizes traffic switchovers when the remote node is still in
operation. This approach does not cause a problem even if the
remembered active path is no longer valid due to any local input that
occurred at the remote node while the initializing node was out of
Note that in some restart scenarios (e.g., cold rebooting), no valid
SF/SD indications may be present at the input of the Local Request
Logic. In this case, the PSC Control Logic restarts as if no local
requests are present. If a valid SF/SD indication is detected later,
the PSC Control Logic is notified and state change is triggered.
4.2. State Transition Modification
In addition to the initialization behavior described in Section 4.1,
four cells of the remote state transition table need to be changed to
make two end nodes converge after initialization. State transition
by remote message as defined in Section 11.2 of [RFC7271] is modified
as follows (only modified cells are shown):
| MS-W | MS-P | WTR | EXER | RR | DNR | NR
N | | | (13)| | | DNR |
PF:W:R | | | | | | DNR |
PF:DW:R | | | | | | DNR |
The changes in two rows of remote protecting failure states lead to
the replacement of note (10) with DNR; therefore, note (10) is no
longer needed. The resultant three rows read:
| MS-W | MS-P | WTR | EXER | RR | DNR | NR
N | SA:MW:R | SA:MP:R | (13)| E::R | i | DNR | i
PF:W:R | SA:MW:R | SA:MP:R | (9) | E::R | i | DNR | (11)
PF:DW:R | SA:MW:R | SA:MP:R | (9) | E::R | i | DNR | (11)
In the tables above, the letters 'i' and 'N' stand for "ignore" and
"Normal state", respectively. Other abbreviations can be found in
4.3. Operation Related to State Transition Table Lookup
In addition to the rules related to the state transition table lookup
listed in Section 11 of [RFC7271], the following rule is also applied
to the operation related to the state transition table lookup:
o When the local SF-P is cleared and the priorities of the local and
remote requests are re-evaluated, the last received remote message
may no longer be valid due to the previous failure of the
protection path. Therefore, the last received message MUST be
treated as if it were NR and only the local request shall be
The last paragraph in Section 11 of [RFC7271] is modified as follows:
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.
In the state transition tables below, the letter 'i' is the
"ignore" flag; if it is set, it means that the top-priority
global request is ignored.
If re-evaluation is triggered, the ignore flag is checked. If it
is set, the state machine will transit to the supposed state, which
can be Normal or DNR as indicated in the footnotes to the state
transition table in Section 11.1 of [RFC7271]. If the ignore flag
is not set, the state machine will transit to the state indicated
in the cell of the state transition table.
If re-evaluation is not triggered, the ignore flag is checked. If
it is set, the state machine will remain in the current state, and
the current PSC message continues to be transmitted. If the ignore
flag is not set, the state machine will transit to the state
indicated in the cell of the state transition table.
5. Security Considerations
No specific security issue is raised in addition to those ones
already documented in [RFC7271]. Note that tightening the
description of the initializing behavior may help to protect networks
from restart attacks.
6. IANA Considerations
This document does not require any IANA actions.