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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. Busi
                                                                G. Wen
                                                   Huawei Technologies
                                                           August 2017

  Updates to MPLS Transport Profile (MPLS-TP) Linear Protection in
             Automatic Protection Switching (APS) Mode

Abstract

 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
 http://www.rfc-editor.org/info/rfc8234.

Ryoo, et al. Standards Track [Page 1] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

Copyright Notice

 Copyright (c) 2017 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
 3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . .   4
 4.  Updates . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.1.  Initialization Behavior . . . . . . . . . . . . . . . . .   5
   4.2.  State Transition Modification . . . . . . . . . . . . . .   6
   4.3.  Operation Related to State Transition Table Lookup  . . .   6
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
   7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

Ryoo, et al. Standards Track [Page 2] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

1. Introduction

 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
 today.

 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].

Ryoo, et al. Standards Track [Page 3] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

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.

3. Abbreviations

 This document uses the following abbreviations:

 APS     Automatic Protection Switching
 DNR     Do-not-Revert
 E::R    Exercise state due to remote EXER message
 EXER    Exercise
 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
 WTR     Wait-to-Restore

4. Updates

 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.

Ryoo, et al. Standards Track [Page 4] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

4.1. Initialization Behavior

 This section defines initialization behavior that is not described in
 [RFC7271].

 When the PSC Control Logic is initialized, the following actions MUST
 be performed:

 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,
    respectively.

 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
    Generator).

 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
 operation.

Ryoo, et al. Standards Track [Page 5] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

 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
 Section 3.

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
    evaluated.

Ryoo, et al. Standards Track [Page 6] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

 The last paragraph in Section 11 of [RFC7271] is modified as follows:

——–

Old text:

——–

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.

——–

New text:

——–

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.

Ryoo, et al. Standards Track [Page 7] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

7. References

7.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.

 [RFC7271]  Ryoo, J., Ed., Gray, E., Ed., van Helvoort, H.,
            D'Alessandro, A., Cheung, T., and E. Osborne, "MPLS
            Transport Profile (MPLS-TP) Linear Protection to Match the
            Operational Expectations of Synchronous Digital Hierarchy,
            Optical Transport Network, and Ethernet Transport Network
            Operators", RFC 7271, DOI 10.17487/RFC7271, June 2014,
            <https://www.rfc-editor.org/info/rfc7271>.

 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

 [RFC6378]  Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
            N., and A. Fulignoli, Ed., "MPLS Transport Profile
            (MPLS-TP) Linear Protection", RFC 6378,
            DOI 10.17487/RFC6378, October 2011,
            <https://www.rfc-editor.org/info/rfc6378>.

Acknowledgements

 The authors would like to thank Joaquim Serra for raising the issue
 related to initialization of the PSC Control Logic at the very
 beginning.  The authors would also like to thank Adrian Farrel and
 Loa Andersson for their valuable comments and suggestions on this
 document.

Ryoo, et al. Standards Track [Page 8] RFC 8234 Updates to MPLS-TP LP in APS Mode August 2017

Authors' Addresses

 Jeong-dong Ryoo
 ETRI

 Email: ryoo@etri.re.kr

 Taesik Cheung
 ETRI

 Email: cts@etri.re.kr

 Huub van Helvoort
 Hai Gaoming BV

 Email: huubatwork@gmail.com

 Italo Busi
 Huawei Technologies

 Email: Italo.Busi@huawei.com

 Guangjuan Wen
 Huawei Technologies

 Email: wenguangjuan@huawei.com

Ryoo, et al. Standards Track [Page 9]