SPRING Working Group                                             X. Geng
Internet-Draft                                                   M. Chen
Intended status: Standards Track                            F. Yang, Ed.
Expires: 19 August 2022                              Huawei Technologies
                                                            P. Camarillo
                                                     Cisco Systems, Inc.
                                                               G. Mishra
                                                            Verizon Inc.
                                                        15 February 2022


                     SRv6 for Redundancy Protection
             draft-ietf-spring-sr-redundancy-protection-01

Abstract

   Redundancy Protection is a generalized protection mechanism to
   achieve the high reliability of service transmission in Segment
   Routing network.  The mechanism inherits the "Live-Live" methodology,
   targeting to enhance the functionalities of Segment Routing over
   IPv6.  Inspired by DetNet Packet Replication and Packet Elimination
   functions, two new Segments are introduced to provide replication and
   elimination functions on specific network nodes by leveraging SRv6
   Segment programming capabilities.

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 .

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
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   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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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 19 August 2022.




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Copyright Notice

   Copyright (c) 2022 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 (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Terminology and Conventions . . . . . . . . . . . . . . .   3
   3.  Redundancy Protection in Segment Routing Scenario . . . . . .   4
   4.  Segment to Support Redundancy Protection  . . . . . . . . . .   5
     4.1.  Redundancy Segment  . . . . . . . . . . . . . . . . . . .   5
     4.2.  Merging Segment . . . . . . . . . . . . . . . . . . . . .   6
   5.  Meta Data to Support Redundancy Protection  . . . . . . . . .   7
   6.  Segment Routing Policy to Support Redundancy Protection . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Redundancy Protection is a generalized protection mechanism to
   achieve the high reliability of service transmission in Segment
   Routing network.  Specifically, packets of flows are replicated at a
   network node into two or more copies, which are transported via
   different and disjoint paths in parallel.  When copies of packets are
   received and merged at one network node, the redundant packets are
   determined and further eliminated to guarantee only one copy of
   packets is transmitted.  The mechanism inherits the "Live-Live"
   methodology, targeting to enhance the functionalities of Segment
   Routing over IPv6 [RFC8986].  Inspired by DetNet [RFC8655] Packet
   Replication and Packet Elimination Functions, two new Segments are
   introduced to provide the replication and elimination functions on



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   specific network nodes by leveraging SRv6 Segment programming
   capabilities.  As it is unnecessary to perform switchover of
   recieving packets between different paths, redundancy protection can
   facilitate to achieve zero packet loss target when failure on either
   path happens.

   Redundancy protection provides ultra reliable protection to many
   services, for example Cloud VR/Game, IPTV service and other type of
   video services, high value private line service etc.  In this
   document, redundancy protection is applied to point-to-point service.
   The mechanism for point-to-multipoint service stays out of the scope
   of this document.

   Segment Routing (SR) leverages the source routing paradigm.  An
   ingress node steers a packet through an ordered list of instructions,
   called "segments".  A segment can be associated to an arbitrary
   processing of the packet in the node identified by the segment.

   This document extends the Segment Routing capabilities to support the
   redundancy protection in an SRv6 environment, including the
   definitions of two new Segments, meta data encapsulation, and a
   variation of Segment Routing Policy.

2.  Terminology


2.1.  Requirements Language

   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.

2.2.  Terminology and Conventions

   SR: Segment Routing

   URLLC: Ultra-Reliable Low-Latency Communication

   VR: Virtual Reality

   Red node: Redundancy node

   Mer node: Merging node

   FID: Flow IDentification




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   SN: Sequence Number

3.  Redundancy Protection in Segment Routing Scenario

              |                                              |
              |<--------------- SRv6 Domain ---------------->|
              |                                              |
              |                    +-----+                   |
              |              +-----+  R3 +-----+             |
              |              |     +-----+     |             |
           +-----+        +--+--+           +--+--+       +-----+
    -------+  R1 +--------+ Red |           | Mer +-------+  R2 +-------
           +-----+        +--+--+           +--+--+       +-----+
                            |      +-----+     |
                            +------+  R4 +-----+
                                   +-----+

   Figure 1: Example Scenario of Redundancy Protection in SRv6 Domain

   This figure shows an example of redundancy protection used in SRv6
   domain.  R1, R2, R3, R4, Red and Mer are SR-capable nodes.  When a
   flow is sent into SRv6 domain, the process is:

   1) R1 receives the traffic flow and encapsulates packets with a list
   of segments destined to R2, which is instantiated as an ordered list
   of SRv6 SIDs.

   2) When the packet flow arrives at Red node, known as Redundancy
   node, each packet is replicated into two or more copies.  Each copy
   of the packet is encapsulated with a new segment list, which
   represents different disjoint forwarding paths.

   3) Meta data information such as flow identification (FID) and
   sequence number (SN) is used to facilitate the packet elimination on
   Merging node (Mer).  Flow identification identifies the specific
   flow, and sequence number distinguishes the packet sequence of a
   flow.  Meta data is either carried in the packet before it arrives at
   redundancy node, or added to each of the replicas at redundancy node.

   4) The multiple replicas go through different paths until the Mer
   node, known as Merging node.  The first received copy of each flow
   packet is transmitted from Merging node to R2, and the redundant
   packets are eliminated.

   5) When there is any failures or packet loss in one path, the service
   transmission continues through the other path non-disruptively.





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   6) Sometimes, out-of-order packets may occur since service packets
   are recovered from different forwarding paths.  In this case, the
   merging node or other network nodes behind merging node is desired to
   include a reordering function, which is implementation specific and
   out of the scope of this document.

   In this example, service protection is supported by utilizing two
   packet flows transmitted over two forwarding paths.  It is noted that
   there is no limitation of the number of replicas.  For a
   unidirectional flow, Red node supports replication function, and Mer
   node supports elimination function.  Reordering function MAY be
   required in combination of elimination function on merging node.  To
   minimize the jitter caused by random packet loss, the disjoint paths
   are recommended to have similar path forwarding delay.

4.  Segment to Support Redundancy Protection

   To achieve the packet replication and elimination functions,
   Redundancy Segment and Merging Segment, as well as the related SRv6
   Endpoint Behavior are introduced.

4.1.  Redundancy Segment

   Redundancy Segment is the identifier of packets which need to be
   replicated on redundancy node.  It is a variation of Binding SID
   (BSID) to associate with a Redundancy Policy, instantiation of which
   provides segment lists of different disjoint paths.  Similar to the
   relationship between BSID and SR Policy
   [I-D.ietf-spring-segment-routing-policy], the use of Redundancy
   Segment would trigger the Redundancy Policy instantiation on
   redundancy node.

   Redundancy Segment is associated with service instructions,
   indicating the following operations:

   *  Steers the packet into the corresponding redundancy policy

   *  Encapsulates flow identification and sequence number in packets if
      the two information is not carried in packets

   *  Packet replication and segment encapsulation based on the
      information of redundancy policy, e.g., the number of replication
      copies, an ordered list of segments with a topological instruction








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   In the case of SRv6, a new behavior End.R for Redundancy Segment is
   defined.  An instance of a redundancy SID is associated with a
   redundancy policy B and a source address A.  In the following
   description, End.R behavior is specified in the encapsulation mode.
   The End.R behavior in the insertion mode is for further study.

   When an SRv6-capable node (N) receives an IPv6 packet whose
   destination address matches a local IPv6 address instantiated as an
   SRv6 SID (S), and S is a Redundancy SID, N does:

S01. When an SRH is processed {
S02.   If (Segments Left>0)   {
S03.     Decrement IPv6 Hop Limit by 1
S04.     Decrement Segments Left by 1
S05.     Update IPv6 DA with Segment List[Segments Left]
S06.     Add flow identification and sequence number if indicated*
S07.     Duplicate the packets (as number of active SID lists in B)
S08.     Push the new IPv6 headers to each replica. The IPv6 header
         contains an SRH with the SID list in B
S09.     Set the outer IPv6 SA to A
S10.     Set the outer IPv6 DA to the first SID of new SRH SL
S11.     Set the outer Payload Length, Traffic Class, Flow Label,
         Hop Limit and Next-Header fields
S12.     Submit the packet to the egress IPv6 FIB lookup
         for transmission to the new destination
S13.   }
S14. }
* Adding flow identification and sequence number is an optional behavior
for Redundancy Segment. The instruction execution is determined and
explicitly indicated by SR policy or Segment itself.

4.2.  Merging Segment

   Merging Segment is associated with service instructions, indicates
   the following operations:

   *  Packet merging and elimination: forward the first received packets
      and eliminate the redundant packets

   In order to eliminate the redundant packet of a flow, merging node
   utilizes sequence number to evaluate the redundant status of a
   packet.  Note that implementation specific mechanism could be applied
   to control the amount of state monitored on sequence number, so that
   system memory usage can be limited at a reasonable level.

   As merging node needs to maintain the state of flows, a centralized
   controller should have a knowledge of merging nodes capability, and
   never provision the redundancy policy to redundancy node when the



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   computation result goes beyond the flow recovery capability of
   merging node.  The capability advertisement of merging node will be
   specified separately elsewhere, which is not within the scope of this
   document.

   In the case of SRv6, a new behavior End.M for Merging Segment is
   defined.

   When an SRv6-capable node (N) receives an IPv6 packet whose
   destination address matches a local IPv6 address instantiated as an
   SRv6 SID (S), and S is a Merging SID, N does:

 S01. When an SRH is processed {
 S02.  If (Segments Left> or ==0)   {
 S03.    Acquire the sequence number of received packet and
         look it up in table
 S04.      If (this sequence number does not exist in the table) {
 S05.       Store this sequence number in table
 S06.       Remove the outer IPv6+SRH header
 S07.       Decrement IPv6 Hop Limit by 1 in inner SRH
 S08.       Decrement Segments Left by 1 in inner SRH
 S09.       Update IPv6 DA with Segment List[Segments Left] in inner SRH
 S10.       Submit the packet to the egress IPv6 FIB lookup and transmit
 S11.      }
 S12.      ELSE {
 S13.           Drop the packet
 S14.      }
 S15.    }
 S16. }

5.  Meta Data to Support Redundancy Protection

   To support the redundancy protection function, flow identification
   and sequence number are added in the packet and further used at
   merging node when the merging function is executed.  Flow
   identification identifies one specific flow of redundancy protection,
   and is usually allocated from centralized controller to SR ingress
   node or redundancy node in SR network.  Note that flow identification
   can also be allocated and advertised by merging node.  BGP, PCEP or
   Netconf protocols can facilitate the advertisement and distribution
   of flow identification among controller, redundancy node and merging
   node.  Sequence number distinguishes the packets within a flow by
   specifying the order of packets.  Not like the uniqueness of flow
   identification to one specific flow, sequence number keeps changing
   to each packet within a flow.  It is RECOMMENDED to add the sequence
   number in forwarding plane as performance and scalability is
   required.




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   Figure 4 suggests an encapsulation of flow identification and
   sequence number in Segment Routing Header (SRH)[RFC8754] when
   redundancy protection is used in SRv6 network.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Next Header   |  Hdr Ext Len  |  Routing Type | Segments Left |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Last Entry |    Flags      |     Tag (Sequence Number)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~          Merging Segment (Locator+Function+Arg:Flow ID)       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                              ...                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                  Segment List[n] (128 bits)                   ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4 Encapsulation of Flow Identification and Sequence Number

   Since the flow identification is only used at merging node to
   identify the specific flow of redundancy protection, it is
   RECOMMENDED to be encapsulated in the Arguments of Merging Segment in
   SRH.  The length of flow identification is not limited, however in
   practice it is suggested to be 16 bits.

   All the duplicates of the same packet need to be tagged for
   deduplication at the merging node.  For this purpose, we will use a
   sequence number.  It is RECOMMENDED to encode the seq number in the
   Tag field of the SRH, with a length of 16bits.

6.  Segment Routing Policy to Support Redundancy Protection

   Redundancy Policy is a variation of SR Policy to conduct the replicas
   to multiple disjoint paths for redundancy protection.  It extends SR
   policy [I-D.ietf-spring-segment-routing-policy] to include more than
   one active and parallel ordered lists of segments between redundancy
   node and merging node, and all the ordered lists of segments are used
   at the same time to steer each copy of flow into different disjoint
   paths.

7.  IANA Considerations

   This document requires registration of End.R behavior and End.M
   behavior in "SRv6 Endpoint Behaviors" sub-registry of "Segment
   Routing Parameters" registry.




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8.  Security Considerations

   TBD

9.  Acknowledgements

   The authors would like to thank Bruno Decraene, Ron Bonica, James
   Guichard, Jeffrey Zhang, Balazs Varga for their valuable comments and
   discussions.

10.  References

10.1.  Normative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-16, 28 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-16.txt>.

   [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>.

   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

10.2.  Informative References

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.



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Authors' Addresses

   Xuesong Geng
   Huawei Technologies
   China

   Email: gengxuesong@huawei.com


   Mach(Guoyi) Chen
   Huawei Technologies
   China

   Email: mach.chen@huawei.com


   Fan Yang
   Huawei Technologies
   China

   Email: shirley.yangfan@huawei.com


   Pablo Camarillo Garvia
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com


   Gyan Mishra
   Verizon Inc.

   Email: gyan.s.mishra@verizon.com

















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