Network Working Group                                      D. Voyer, Ed.
Internet-Draft                                               Bell Canada
Intended status: Standards Track                             C. Filsfils
Expires: 21 February 2022                                      R. Parekh
                                                     Cisco Systems, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                Z. Zhang
                                                        Juniper Networks
                                                          20 August 2021


        SR Replication Segment for Multi-point Service Delivery
              draft-ietf-spring-sr-replication-segment-05

Abstract

   This document describes the SR Replication segment for Multi-point
   service delivery.  A SR Replication segment allows a packet to be
   replicated from a Replication Node to downstream nodes.

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
   and may be updated, replaced, or obsoleted by other documents at any
   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 21 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Replication Segment . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  SR-MPLS data plane  . . . . . . . . . . . . . . . . . . .   4
     2.2.  SRv6 data plane . . . . . . . . . . . . . . . . . . . . .   5
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Illustration of a Replication Segment  . . . . . . .   8
     A.1.  SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.2.  SRv6  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   We define a new type of segment for Segment Routing [RFC8402], called
   Replication segment, which allows a node (henceforth called as
   Replication Node) to replicate packets to a set of other nodes
   (called Downstream Nodes) in a Segment Routing Domain.  Replication
   segments provide building blocks for Point-to-Multipoint Service
   delivery via SR Point-to-Multipoint (SR P2MP) policy.  A Replication
   segment can replicate packet to directly connected nodes or to
   downstream nodes (without need for state on the transit routers).
   This document focuses on the Replication segment building block.  The
   use of one or more stitched Replication segments constructed for SR
   P2MP Policy tree is specified in [I-D.ietf-pim-sr-p2mp-policy].










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2.  Replication Segment

   In a Segment Routing Domain, a Replication segment is a logical
   construct which connects a Replication Node to a set of Downstream
   Nodes.  A Replication segment is a local segment instantiated at a
   Replication node.  It can be either provisioned locally on a node or
   programmed by a PCE.  Replication segments apply equally to both SR-
   MPLS and SRv6 instantiations of Segment Routing.

   A Replication segment is identified by the tuple <Replication-ID,
   Node-ID>, where:

   *  Replication-ID: An identifier for a Replication segment that is
      unique in context of the Replication Node.

   *  Node-ID: The address of the Replication Node that the Replication
      segment is for.  Note that the root of a multi-point service is
      also a Replication Node.

   In simplest case, Replication-ID can be a 32-bit number, but it can
   be extended or modified as required based on specific use of a
   Replication segment.  When the PCE signals a Replication segment to
   its node, the <Replication-ID, Node-ID> tuple identifies the segment.
   Examples of such signaling and extension are described in
   [I-D.ietf-pim-sr-p2mp-policy].

   A Replication segment includes the following elements:

   *  Replication SID: The Segment Identifier of a Replication segment.
      This is a SR-MPLS label or a SRv6 SID [RFC8402].

   *  Downstream Nodes: Set of nodes in Segment Routing domain to which
      a packet is replicated by the Replication segment.

   *  Replication State: See below.

   The Downstream Nodes and Replication State of a Replication segment
   can change over time, depending on the network state and leaf nodes
   of a multi-point service that the segment is part of.

   Replication SID identifies the Replication segment in the forwarding
   plane.  At a Replication node, the Replication SID is the equivalent
   of Binding SID [I-D.ietf-spring-segment-routing-policy] of a Segment
   Routing Policy.

   Replication State is a list of replication branches to the Downstream
   Nodes.  In this document, each branch is abstracted to a <Downstream
   Node, Downstream Replication SID> tuple.



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   In a branch tuple, <Downstream Node> represents the reachability from
   the Replication Node to the Downstream Node.  In its simplest form,
   this MAY be specified as an interface or nexthop if downstream node
   is adjacent to the Replication Node.  The reachability may be
   specified in terms of Flex-Algo path (including the default algo)
   [I-D.ietf-lsr-flex-algo], or specified by an SR explicit path
   represented either by a SID-list (of one or more SIDs) or by a
   Segment Routing Policy [I-D.ietf-spring-segment-routing-policy].

   A packet is steered into a Replication segment at a Replication Node
   in two ways:

   *  When the Active Segment [RFC8402] is a locally instantiated
      Replication SID

   *  By the root of a multi-point service based on local configuration
      outside the scope of this document.

   In either case, the packet is replicated to each Downstream node in
   the associated Replication state.

   If a Downstream Node is an egress (aka leaf) of the multi-point
   service, i.e. no further replication is needed, then that leaf node's
   Replication segment will not have any Replication State and the
   operation is NEXT.  At an egress node, the Replication SID MAY be
   used to identify that portion of the multi-point service.  Notice
   that the segment on the leaf node is still referred to as a
   Replication segment for the purpose of generalization.

   A node can be a bud node, i.e. it is a Replication Node and a leaf
   node of a multi-point service at the same time
   [I-D.ietf-pim-sr-p2mp-policy].

   There MUST not be any topological SID after a Replication SID in a
   packet.  Otherwise, the behavior at Downstream nodes of a Replication
   segment is undefined and outside the scope of this document.

2.1.  SR-MPLS data plane

   When the Active Segment is a Replication SID, the processing results
   in a POP operation and lookup of the associated Replication state.
   For each replication in the Replication state, the operation is a
   PUSH of the downstream Replication SID and an optional segment list
   on to the packet which is forwarded to the Downstream node.  For leaf
   nodes the inner packet is forwarded as per local configuration.






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   When the root of a multi-point service steers a packet to a
   Replication segment, it results in a replication to each Downstream
   node in the associated replication state.  The operation is a PUSH of
   the replication SID and an optional segment list on to the packet
   which is forwarded to the downstream node.

2.2.  SRv6 data plane

   In SRv6 [RFC8986], the "Endpoint with replication" behavior
   (End.Replicate for short) replicates a packet and forwards the packet
   according to a Replication state.

   When processing a packet destined to a local Replication-SID, the
   packet is replicated to Downstream nodes in the associated
   Replication state.  For replication, the outer header is re-used, and
   the Downstream Replication SID is written into the outer IPv6 header
   destination address.If required, an optional segment list is used to
   encapsulate the replicated packet via H.Encaps.  For a leaf node, the
   packet is decapsulated and the inner packet is forwarded as per local
   configuration.

   When the root of a multi-point service steers a packet into a
   Replication segment, for each replication, H.Encaps is used to
   encapsulate the packet with the segment list to the Downstream node .

   An End.Replicate SID MUST only appear as the ultimate SID in a SID-
   list.  An implementation that receives a packet destined to a locally
   instantiated End.Replicate SID that is not the ultimate segment
   SHOULD reply with ICMP Parameter Problem error (Erroneous header
   field encountered) and discard the packet.

3.  Use Cases

   In the simplest use case, a single Replication segment includes the
   root node of a multi-point service and the egress/leaf nodes of the
   service as all the Downstream Nodes.  This achieves Ingress
   Replication [RFC7988] that has been widely used for MVPN [RFC6513]
   and EVPN [RFC7432] BUM (Broadcast, Unknown and Multicast) traffic.

   Replication segments can also be used as building blocks for
   replication trees when Replication segments on the root, intermediate
   Replication Nodes and leaf nodes are stitched together to achieve
   efficient replication.  That is specified in
   [I-D.ietf-pim-sr-p2mp-policy].







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4.  IANA Considerations

   This document requires registration of End.Replicate behavior in
   "SRv6 Endpoint Behaviors" sub-registry of "Segment Routing
   Parameters" top-level registry.

              +=======+=====+===================+===========+
              | Value | Hex | Endpoint behavior | Reference |
              +=======+=====+===================+===========+
              | TBD   | TBD |   End.Replicate   | [This.ID] |
              +-------+-----+-------------------+-----------+

                  Table 1: IETF - SRv6 Endpoint Behaviors

5.  Security Considerations

   There are no additional security risks introduced by this design.

6.  Acknowledgements

   The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
   Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene and Joel
   Halpern for their valuable inputs.

7.  Contributors


   Clayton Hassen Bell Canada Vancouver Canada

   Email: clayton.hassen@bell.ca

   Kurtis Gillis Bell Canada Halifax Canada

   Email: kurtis.gillis@bell.ca

   Arvind Venkateswaran Cisco Systems, Inc.  San Jose US

   Email: arvvenka@cisco.com

   Zafar Ali Cisco Systems, Inc.  US

   Email: zali@cisco.com

   Swadesh Agrawal Cisco Systems, Inc.  San Jose US

   Email: swaagraw@cisco.com

   Jayant Kotalwar Nokia Mountain View US



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   Email: jayant.kotalwar@nokia.com

   Tanmoy Kundu Nokia Mountain View US

   Email: tanmoy.kundu@nokia.com

   Andrew Stone Nokia Ottawa Canada

   Email: andrew.stone@nokia.com

   Tarek Saad Juniper Networks Canada

   Email:tsaad@juniper.net

8.  References

8.1.  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-13, 28 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-13.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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [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>.

8.2.  Informative References

   [I-D.filsfils-spring-srv6-net-pgm-illustration]
              Filsfils, C., Garvia, P. C., Li, Z., Matsushima, S.,
              Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
              J. Leddy, "Illustrations for SRv6 Network Programming",



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              Work in Progress, Internet-Draft, draft-filsfils-spring-
              srv6-net-pgm-illustration-04, 30 March 2021,
              <https://www.ietf.org/archive/id/draft-filsfils-spring-
              srv6-net-pgm-illustration-04.txt>.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", Work in Progress,
              Internet-Draft, draft-ietf-lsr-flex-algo-17, 6 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-flex-algo-
              17.txt>.

   [I-D.ietf-pim-sr-p2mp-policy]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy", Work
              in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-
              policy-02, 19 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-pim-sr-p2mp-
              policy-02.txt>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7988]  Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
              Replication Tunnels in Multicast VPN", RFC 7988,
              DOI 10.17487/RFC7988, October 2016,
              <https://www.rfc-editor.org/info/rfc7988>.

Appendix A.  Illustration of a Replication Segment

   This section illustrates an example of a single Replication segment.
   Examples showing Replication segment stitched together to form P2MP
   tree (based on SR P2MP policy) are in [I-D.ietf-pim-sr-p2mp-policy].

   Consider the following topology:

                                  R3------R6
                                 /         \
                         R1----R2----R5-----R7
                                 \         /
                                  +--R4---+




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                             Figure 1: Figure 1

A.1.  SR-MPLS

   In this example, the Node-SID of a node Rn is N-SIDn and Adjacency-
   SID from node Rm to node Rn is A-SIDmn.  Interface between Rm and Rn
   is Lmn.

   Assume a Replication segment identified with R-ID at Replication Node
   R1 and downstream Nodes R2, R6 and R7.  The Replication SID at node n
   is R-SIDn.  A packet replicated from R1 to R7 has to traverse R4.

   The Replication segment state at nodes R1, R2, R6 and R7 is shown
   below.  Note nodes R3, R4 and R5 do not have state for the
   Replication segment.

   Replication segment at R1:

   Replication segment <R-ID,R1>:
    Replication SID: R-SID1
    Replication State:
      R2: <R-SID2->L12>
      R6: <N-SID6, R-SID6>
      R7: <N-SID4, A-SID47, R-SID7>

   Replication to R2 steers packet directly to R2 on interface L12.
   Replication to R6, using N-SID6, steers packet via IGP shortest path
   to that node.  Replication to R7 is steered via R4, using N-SID4 and
   then adjacency SID A-sID47 to R7.

   Replication segment at R2:

   Replication segment <R-ID,R2>:
    Replication SID: R-SID2
    Replication State:
      R2: <Leaf>

   Replication segment at R6:

   Replication segment <R-ID,R6>:
    Replication SID: R-SID6
    Replication State:
      R6: <Leaf>

   Replication segment at R7:






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   Replication segment <R-ID,R7>:
    Replication SID: R-SID7
    Replication State:
      R7: <Leaf>

   When a packet is steered into the Replication segment at R1:

   *  Since R1 is directly connected to R2, R1 performs PUSH operation
      with just <R-SID2> label for the replicated copy and sends it to
      R2 on interface L12.  R2, as Leaf, performs NEXT operation, pops
      R-SID2 label and delivers the payload.

   *  R1 performs PUSH operation with <N-SID6, R-SID6> label stack for
      the replicated copy to R6 and sends it to R2, the nexthop on IGP
      shortest path to R6.  R2 performs CONTINUE operation on N-SID6 and
      forwards it to R3.  R3 is the penultimate hop for N-SID6; it
      performs penultimate hop popping, which corresponds to the NEXT
      operation and the packet is then sent to R6 with <R-SID6> in the
      label stack.  R6, as Leaf, performs NEXT operation, pops R-SID6
      label and delivers the payload.

   *  R1 performs PUSH operation with <N-SID4, A-SID47, R-SID7> label
      stack for the replicated copy to R7 and sends it to R2, the
      nexthop on IGP shortest path to R4.  R2 is the penultimate hop for
      N-SID4; it performs penultimate hop popping, which corresponds to
      the NEXT operation and the packet is then sent to R4 with
      <A-SID47, R-SID1> in the label stack.  R4 performs NEXT operation,
      pops A-SID47, and delivers packet to R7 with <R-SID7> in the label
      stack.  R7, as Leaf, performs NEXT operation, pops R-SID7 label
      and delivers the payload.

A.2.  SRv6

   For SRv6 , we use SID allocation scheme, reproduced below, from
   Illustrations for SRv6 Network Programming
   [I-D.filsfils-spring-srv6-net-pgm-illustration]

   *  2001:db8::/32 is an IPv6 block allocated by a RIR to the operator

   *  2001:db8:0::/48 is dedicated to the internal address space

   *  2001:db8:cccc::/48 is dedicated to the internal SRv6 SID space

   *  We assume a location expressed in 64 bits and a function expressed
      in 16 bits

   *  Node k has a classic IPv6 loopback address 2001:db8::k/128 which
      is advertised in the IGP



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   *  Node k has 2001:db8:cccc:k::/64 for its local SID space.  Its SIDs
      will be explicitly assigned from that block

   *  Node k advertises 2001:db8:cccc:k::/64 in its IGP

   *  Function :1:: (function 1, for short) represents the End function
      with PSP support

   *  Function :Cn:: (function Cn, for short) represents the End.X
      function from to Node n

   Each node k has:

   *  An explicit SID instantiation 2001:db8:cccc:k:1::/128 bound to an
      End function with additional support for PSP

   *  An explicit SID instantiation 2001:db8:cccc:k:Cj::/128 bound to an
      End.X function to neighbor J with additional support for PSP

   *  An explicit SID instantiation 2001:db8:cccc:k:Fk::/128 bound to an
      End.Replcate function

   Assume a Replication segment identified with R-ID at Replication Node
   R1 and downstream Nodes R2, R6 and R7.  The Replication SID at node
   k, bound to an End.Replcate function, is 2001:db8:cccc:k:Fk::/128.  A
   packet replicated from R1 to R7 has to traverse R4.

   The Replication segment state at nodes R1, R2, R6 and R7 is shown
   below.  Note nodes R3, R4 and R5 do not have state for the
   Replication segment.

   Replication segment at R1:

   Replication segment <R-ID,R1>:
    Replication SID: 2001:db8:cccc:1:F1::0
    Replication State:
      R2: <2001:db8:cccc:2:F2::0->L12>
      R6: <2001:db8:cccc:6:F6::0>
      R7: <2001:db8:cccc:4:C7::0, 2001:db8:cccc:7:F7::0>

   Replication to R2 steers packet directly to R2 on interface L12.
   Replication to R6, using 2001:db8:cccc:6:F6::0, steers packet via IGP
   shortest path to that node.  Replication to R7 is steered via R4,
   using End.X SID 2001:db8:cccc:4:C7::0 at R4 to R7.

   Replication segment at R2:





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   Replication segment <R-ID,R2>:
    Replication SID: 2001:db8:cccc:2:F2::0
    Replication State:
      R2: <Leaf>

   Replication segment at R6:

   Replication segment <R-ID,R6>:
    Replication SID: 2001:db8:cccc:6:F6::0
    Replication State:
      R6: <Leaf>

   Replication segment at R7:

   Replication segment <R-ID,R7>:
    Replication SID: 2001:db8:cccc:7:F7::0
    Replication State:
      R7: <Leaf>

   When a packet, (A,B2), is steered into the Replication segment at R1:

   *  Since R1 is directly connected to R2, R1 creates encapsulated
      replicated copy (2001:db8::1, 2001:db8:cccc:2:F2::0) (A, B2), and
      sends it to R2 on interface L12.  R2, as Leaf, removes outer IPv6
      header and delivers the payload.

   *  R1 creates encapsulated replicated copy (2001:db8::1,
      2001:db8:cccc:6:F6::0) (A, B2) then forwards the resulting packet
      on the shortest path to 2001:db8:cccc:6::/64.  R2 and R3 forward
      the packet using 2001:db8:cccc:6::/64.  R6, as Leaf, removes outer
      IPv6 header and delivers the payload.

   *  R1 creates encapsulated replicated copy (2001:db8::1,
      2001:db8:cccc:4:C7::0) (2001:db8:cccc:7:F7::0; SL=1) (A, B2) and
      sends it to R2, the nexthop on IGP shortest path to
      2001:db8:cccc:4::/64.  R2 forwards packet to R4 using
      2001:db8:cccc:4::/64.  R4 executes End.X function on
      2001:db8:cccc:4:C7::0, performs PSP action, removes SRH and sends
      resulting packet (2001:db8::1, 2001:db8:cccc:7:F7::0) (A, B2) to
      R7.  R7, as Leaf, removes outer IPv6 header and delivers the
      payload.

Authors' Addresses








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   Daniel Voyer (editor)
   Bell Canada
   Montreal
   Canada

   Email: daniel.voyer@bell.ca


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Rishabh Parekh
   Cisco Systems, Inc.
   San Jose,
   United States of America

   Email: riparekh@cisco.com


   Hooman Bidgoli
   Nokia
   Ottawa
   Canada

   Email: hooman.bidgoli@nokia.com


   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net















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