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Versions: 00 01 02                                                      
MPLS Working Group                                                L. Han
Internet-Draft                                              China Mobile
Intended status: Standards Track                                 F. Yang
Expires: February 17, 2022                           Huawei Technologies
                                                                 J. Zhao
                                                                   CAICT
                                                         August 16, 2021


     Signal Degrade Indication in Segment Routing over MPLS Network
                        draft-han-mpls-sdi-sr-02

Abstract

   This document describes a typical use case of MPLS-TP, where signal
   degrade defect needs to be correctly detected and transmitted via OAM
   messages within network.  When MPLS-TP evolves to Segment Routing
   MPLS, transit node has no knowledge of labels to be encapsulated in
   MPLS label stack.  Transit node cannot spread OAM messages with
   signal degrade defect indication.  Thus, a solution is proposed in
   this draft.

Requirements Language

   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 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 17, 2022.







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Copyright Notice

   Copyright (c) 2021 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Defect Triggered Procedure  . . . . . . . . . . . . . . .   4
     3.2.  MPLS-TP Solution  . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Problem in SR-MPLS  . . . . . . . . . . . . . . . . . . .   6
   4.  Solution in SR-MPLS . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Background

   In early era of telecommunication, transport network is set up to
   provide voice service.  The connection in network is always
   connection-oriented and circuit switching.  With the rapid increasing
   bandwidth brought by Ethernet, transport network transforms into the
   packet-switched transport network.  Technologies like MPLS/PWE3
   perfectly meet the requirements of supporting both packet-transport
   and circuit-transport.  It led to the work of MPLS Transport Profile
   (MPLS-TP), collaborated between ITU-T and IETF at the first decade of
   the 21st century.

   MPLS-TP is a subset of MPLS.  Features that are not applicable to
   transport network are excluded, and features to meet the requirements
   of transport network, e.g., bidirectional path, deterministic control
   and management, etc., are strictly required.  According to the Joint



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   Working Team consensus, any extension of MPLS-TP would be included in
   MPLS field.

   With the emerge of Segment Routing (SR) and Software Defined Network
   (SDN), MPLS-TP network technologies are adapted as well.  In this
   draft, we recognize one use case where the signal degrade defect can
   be correctly detected and transmitted via MPLS-TP OAM in MPLS-TP, but
   not fulfilled in SR-MPLS.  To fix this problem is the motivation of
   this draft.

   Editor's note: This section gives a historical introduction of MPLS-
   TP, since it has been extensively deployed in packet switched
   transport networks for years.  The intention of this section is to
   help readers understand the unique of requirements from packet
   transport network.  Once the draft becomes RFC, part of this section
   can be moved to Appendix.

2.  Terminology

   MPLS: MultiProtocol Label Switching

   PWE3: Pseudo Wire Emulation Edge to Edge

   MPLS-TP: MultiProtocol Label Switching - Transport Profile

   SR: Segment Routing

   SDN: Software Defined Network

   OAM: Operation, Administration and Maintenance

   SD: Signal Degrade

   BER: Bit Error Rate

   WDM: Wavelength Division Multiplexing

   NMS: Network Management System

   G-ACh: Generic Associated Channel

   PDU: Protocol Data Unit

   CCM: Continuity Check Message

   MEP: Maintenance Entity Group End Point

   MIP: Maintenance Entity Group Intermediate Point



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   AIS: Alarm Indication Signal

3.  Problem Statement

3.1.  Defect Triggered Procedure

   Signal Degrade (SD) describes a status of signal associated data has
   degraded and a degraded defect is active.  Signal degrade of a
   physical link is usually measured and represented by Bit Error Rate
   (BER) value.  Fiber aging, impairment and pollution, optical module
   mismatch or WDM transmission error are the reasons to lead to signal
   degrade.  More information about signal degrade can be found
   in[I-D.yang-mpls-ps-sdi-sr].

   In practice, when physical link degrades in network, signal degrade
   defect is firstly detected and reported by the node.  A specific type
   of alarm is generated and sent to Network Management System (NMS) or
   a SDN controller.  It is a report to management plane and strongly
   required from perspective of network management.  However, the
   problem is the notification to management plane is usually not fast
   enough to assist the network recovery.  It may result in hour or even
   day level of service interruption time.

   As mentioned in [RFC6372], defect may trigger system to perform a
   survivability action, when notification of an issue is reported from
   equipment in a lower layer, system fails to receive an OAM continuity
   check message, or receives of an OAM message reporting a failure
   condition.  Similarly, when signal degrade defect is reported from
   the lower layer, e.g. physical layer, local protection mechanism can
   be triggered within the internal system of nodez.  In case of
   protection switchover selector is at the source or destination node,
   while the signal degrade is happened at intermediate node, an OAM
   message should be transmitted to notify the degrade condition to the
   nodes actually perform the protection switchover.  This action is
   preferred to be triggered by events in the data plane [RFC6372].

3.2.  MPLS-TP Solution

   Generic Associated Channel (G-ACh) [RFC5586] is defined to carry OAM
   messages for MPLS pseudowires, LSPs and sections.  The Generic
   Associated Channel format used in MPLS is shown in Figure 1.  By
   using the generic associated channel and indication of channel type,
   different OAM mechanisms with different formats can be encapsulated
   uniformly as well as independently.







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     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Label                    | EXP |S|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             GAL Label (13)            | TC  |S|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 0 1|Version|   Reserved    |         Channel Type          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1 G-ACh Format in MPLS

   In MPLS-TP, ITU-T G.8113.1 [ITU-T G.8113.1] specifies a large set of
   OAM mechanisms and has been widely deployed in packet transport
   networks.  Figure 2 shows the common OAM PDU format of different OAM
   mechanisms.

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | MEL | Version |    OpCode     |     Flags     |  TLV Offset   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           OAM PDU                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |End TLV|
     +-+-+-+-+

               Figure 2 ITU-T G.8113.1 Common OAM PDU Format

   MEG Level: MEG Level is a 3-bit field.  It contains an integer value
   that identifies the MEG level of OAM PDU.  Value ranges from 0 to 7.

   Version: Version is a 5-bit field.  It contains an integer value that
   identifies the OAM protocol version.  Value is 0 in the current
   version.

   OpCode: OpCode is a 1-octet field.  It contains an OpCode that
   identifies an OAM PDU type.  OpCode is used to identify the remaining
   content of an OAM PDU.  Value for the CCM PDU type is 1.

   Flags: Flags is an 8-bit field.  Use of the bits in this field is
   dependent on the OAM PDU type.

   TLV Offset: TLV Offset is a 1-octet field.  It contains the offset to
   the first TLV in an OAM PDU relative to the TLV Offset field.  The
   value of this field is associated with an OAM PDU type.  When the TLV
   Offset is 0, it points to the first octet following the TLV Offset
   field.



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   End TLV: an all-ZEROes octet value.

   When signal degrade happens in MPLS-TP, an MPLS-TP Alarm Indication
   Signal (AIS) OAM message with active AIS indication is generated and
   transmitted within the OAM maintenance domain.  Maintenance Entity
   Group End Point (MEP), usually also acting as protection switchover
   selector, performs the protection switchover once it receives the AIS
   indication in MPLS-TP OAM message.

3.3.  Problem in SR-MPLS

   When Segment Routing is introduced to MPLS, the nodes except the
   headend have no information of the forwarding path.  If the signal
   degrade is happened on the transit nodes, MPLS-TP AIS OAM message
   cannot be generated because this node has no knowledge of labels
   ought to be encapsulated in MPLS label stack.  Either the label
   information of forwarding path can be obtained on transit node, or
   the defect can be indicated in different messages could help the
   defect spread in network.  It is valuable to keep transit node with
   the capability of reporting defects in SR-MPLS.

4.  Solution in SR-MPLS

   Segment routing is designed to reduce the states in transit nodes,
   any defects like SD defect cannot be indicated in a newly generated
   OAM message on transit node.  Alternative way is to indicate the
   defect in other OAM messages.  Continuity Check Message (CCM) is
   proposed to indicate the signal degrade defect for two reasons.
   Firstly, CCM is designed to be applicable for fault management,
   performance monitoring, or protection switching applications.
   Secondly, consider the merit of CCM's various transmission period,
   the defect indication can be flexibly transmitted according to
   operator's needs.

   One reservation bits in Flag section in CCM OAM PDU message can be
   used as Error Indication (EI) to indicate signal degrade.  Flag
   format with EI extension is shown in Figure 3.

                     MSB                           LSB
                       0
                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     |RDI| EI| Reserved  |  Period   |
                     +---+---+---+---+---+---+---+---+

            Figure 3 CCM OAM PDU Flags Format with EI Extension





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   RDI: Remote Defect Indication, set to 1 to indicate RDI, otherwise it
   is set to 0.

   Period: Indicate the transmission period.

   EI: Error indication, 0 indicates no error, 1 indicates error.

   Reserved: Reserved fields are set to all ZEROes.

   If the node detects the signal degrade defect, EI field is set in CCM
   OAM message and transmitted to other nodes.  Note that, Maintenance
   Entity Group Intermediate Point (MIP) is required to be transparent
   to CCM message in MPLS-TP.  In order to support BER indication on
   each node along the forwarding path, extra configuration and
   intervening implementation to process CCM message would be required
   on MIP.

   Editor's Note: When other OAM mechanisms used in generic associated
   channel (G-ACh), there might be various solutions to transmit signal
   degrade defect, or any other defects detected by transit nodes.  This
   draft introduces a very light-weight solution, which has already been
   implemented and deployed in networks.

5.  IANA Considerations

   This document requests IANA to assign one bit from Flags of MPLS-TP
   OAM PDU format to indicate "Signal Degrade".

6.  Security Considerations

   There are MEP and MIP node defined in OAM mechanisms.  Some types of
   OAM message are defined to be transparent to MIP node, and requires
   no extra configuration or message processing on MIP nodes.  If the
   transit node of SR-MPLS acts as MIP in OAM maintenance domain, this
   MIP node needs to process the OAM messages to indicate the defects.
   At the moment, explicit configuration is required on MIP to have the
   authority to process OAM messages.

7.  Acknowledgements

   The authors want to thank Yuanlong Jiang, Mach Chen, Yongjian Hu for
   their valuable suggestions during the construction of draft.

8.  References







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8.1.  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>.

8.2.  Informative References

   [I-D.yang-mpls-ps-sdi-sr]
              Yang, F., Han, L., and J. Zhao, "Problem Statement of
              Signal Degrade Indication for SR over MPLS", draft-yang-
              mpls-ps-sdi-sr-01 (work in progress), November 2020.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [RFC6372]  Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              DOI 10.17487/RFC6372, September 2011,
              <https://www.rfc-editor.org/info/rfc6372>.

   [ITU-T_G8113.1]
              ITU-T, "ITU-T G.8113.1: Operations, administration
                          and maintenance mechanisms for MPLS-TP in packet
                          transport networks", April 2016.

Authors' Addresses

   Liuyan Han
   China Mobile
   Beijing
   China

   Email: hanliuyan@chinamobile.com


   Fan Yang
   Huawei Technologies
   Beijing
   China

   Email: shirley.yangfan@huawei.com






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   Junfeng Zhao
   CAICT
   Beijing
   China

   Email: zhaojunfeng@caict.ac.cn













































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