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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                               Yimin Shen
Internet-Draft                                             Zhaohui Zhang
Intended status: Standards Track                        Juniper Networks
Expires: October 3, 2020                                  Rishabh Parekh
                                                           Cisco Systems
                                                          Hooman Bidgoli
                                                             Yuji Kamite
                                                      NTT Communications
                                                           April 1, 2020

Point-to-Multipoint Transport Using Chain Replication in Segment Routing


   This document specifies a point-to-multipoint (P2MP) transport
   mechanism based on chain replication.  It can be used in segment
   routing to achieve traffic optimization.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on October 3, 2020.

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Table of Contents

   1.  Introduction
   2.  Specification of Requirements
   3.  Applicability
   4.  P2MP Transport Using Chain Replication
     4.1.  Bud Segment
     4.2.  P2MP Chain
     4.3.  Example
   5.  Path Computation for P2MP Chains
   6.  IGP and BGP-LS Extensions for Bud Segment
   7.  Bud Segments for Special Processing
   8.  IANA Considerations
   9.  Security Considerations
   10. Acknowledgements
   11. Contributors
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Authors' Addresses

1.  Introduction

   The Segment Routing Architecture [RFC8402] describes segment routing
   (SR) and its instantiation in two data planes, i.e. MPLS and IPv6.
   In SR, point-to-multipoint (P2MP) transport is currently achieved by
   using ingress replication, where a point-to-point (P2P) SR tunnel is
   constructed from a root node to each leaf node, and every ingress
   packet is replicated and sent via a bundle of such P2P SR tunnels to
   all the leaf nodes.  Although this approach provides P2MP
   reachability, it does not consider traffic optimization across the
   tunnels, as the path of each tunnel is computed or decided

   An alternative approach would be to use P2MP-tree based transport.
   Such approach can achieve maximum traffic optimization, but it relies
   a controller or path computation element (PCE) to dynamically
   provision and manage "replication segments" on branch nodes.  The
   replication segments are essentially per-P2MP-tree (i.e. per-tunnel)
   state on transit routers.  Therefore, this approach is not fully
   aligned with SR's principles of single-point (i.e. ingress router)
   provisioning and stateless core.

   This document introduces a new solution for P2MP transport in SR,
   based on "chain replication".  In this solution, P2MP transport is
   achieved by constructing a set of "P2MP chain tunnels" (or simply
   "P2MP chains") from a root node to leaf nodes.  Each P2MP chain is a
   tunnel with a leaf node at tail end and some transit leaf nodes along
   the path, resembling a chain.  The leaf node at the tail end behaves
   as a normal receiver.  Each transit leaf node replicates a packet
   once for local processing off the chain, and also forwards the
   original packet down the chain.  The root node replicates and sends
   packets via the set of P2MP chains to all the leaf nodes.

   As a P2MP chain can reach multiple leaf nodes, it is considered to be
   more efficient than the multiple P2P tunnels which would be needed in
   ingress replication to reach these leaf nodes.  Compared with ingress
   replication and the P2MP-tree based approach, this solution provides
   a middle ground by achieving a certain level of traffic optimization,
   while aligning with the fundamental principles of SR, including
   single-point provisioning and stateless core.  The solution can be
   used to improve P2MP transport efficiency in general, and to achieve
   maximum traffic optimization in certain types of topologies.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119] and

3.  Applicability

   The P2MP transport mechanism in this document is generally applicable
   to all networks.  However, it benefits more for certain types of
   topologies than others.  These topologies include ring topologies,
   linear topologies, topologies with leaf nodes concentrated in
   geographical sites which can be modeled as leaf groups, etc.

   The mechanism is transparent to all transit routers.  Leaf nodes
   intended to take advantage of the mechanism will need to support the
   new forwarding behavior specified in this document.  For other leaf
   nodes, the mechanism has a backward compatibility to allow them to be
   reached by P2P tunnels using ingress replication.  Path computation
   and P2MP chain construction will need to be supported by a controller
   or root nodes, depending on where they are performed.

   The mechanism is applicable to both SR-MPLS [RFC8660] and SRv6
   [SRv6-SRH], [SRv6-Programming].

   The mechanism does not create any state of P2MP tunnel or P2MP tree
   on routers.  Therefore, if leaf nodes need to know the service level
   context (e.g. source, VPN) of a P2MP stream, they must rely on the
   information contained in an inner header.  In SR-MPLS, service labels
   may be allocated from a domain-wide common block (DCB) to serve as
   globally unique context indicators.  In SRv6, a root node's IP
   address or an upstream-assigned context indicator may be encoded in
   the source address of IPv6 header, or a downstream-assigned context
   indicator may be encoded in the ARG portion of a service SID.

4.  P2MP Transport Using Chain Replication

   In this document, a P2MP stream associated with a root node and a set
   of leaf nodes is denoted as {root node, leaf nodes}. It is achieved
   by using a bundle of P2MP chains covering all the leaf nodes.  Each
   P2MP chain is a tunnel starting from the root node and reaching one
   or multiple leaf nodes along the path.  The tail-end node of the P2MP
   chain is a leaf node, called a "tail-end" leaf node.  Each leaf node
   traversed by the P2MP chain is called a "transit" leaf node.  As a
   special case, a P2MP chain may have no transit leaf node, but only a
   tail-end leaf node, essentially becoming a P2P tunnel of ingress

       R ------ R1 ------ R2 ------ L1 ------ R3 ------ L2 ------ L3

                          R  : root node
                          Li : leaf node
                          Ri : transit router

                                 Figure 1

   A tail-end leaf node and a transit leaf nodes have different
   behaviors when processing a received packet.  In particular, a tail-
   end leaf node processes the packet as a normal receiver.  A transit
   leaf node not only processes the packet as a receiver, but also
   forwards it downstream along the P2MP chain, hence acting as a "bud
   node".  To achieve this, the transit leaf node needs to replicate the
   packet, producing two packets, one for forwarding and the other for
   local processing.  Such packet replication happens on every transit
   leaf node along a P2MP chain.  Therefore, it is called "chain

   This document introduces a new type of segments, called "bud
   segments", to facilitate the above packet processing on transit leaf
   nodes.  The segment ID (SID) of a bud segment is a "bud-SID".

4.1.  Bud Segment

   On a transit leaf node, a bud segment represents the following
   instructions for forwarding hardware to execute on a received packet
   P.  They apply when the active SID of the packet P is the bud-SID of
   this bud segment.

      [1] Replicate the packet P to generate a copy P1.

      [2] For P, perform a NEXT operation on the bud-SID, make the next
      SID active, and forward the packet based on that SID.

      [3] For P1, perform a sequence of NEXT operations on the bud-SID
      and all the subsequent SIDs of the P2MP chain, and process the
      packet locally.  (The SIDs of the P2MP chain are not useful for
      processing P1 locally.  Hence, they are removed before the

   Bud segments are global segments of leaf nodes.  They are routable
   segments via topological shortest-paths.  Bud-SIDs are allocated from
   SRGB (SR global block).  Only one bud segment is needed per leaf
   node, and per SR-MPLS or SRv6.  It is used only when the leaf node is
   a transit leaf node on a P2MP chain.

   In SR-MPLS, bud-SIDs are labels, and penultimate hop popping (PHP)
   MUST be disabled for bud-SID labels.  In SRv6, bud-SIDs are IPv6
   addresses explicitly associated with bud segments.  Therefore, the
   above instructions [1] to [3] are achieved in different ways in SR-
   MPLS and SRv6:

      (a) In SR-MPLS, the packet may have a service label(s) after P2MP
      chain labels in MPLS header, e.g. a VPN label, a bridge domain
      label, a source Ethernet segment label, etc.  Therefore, the bud
      segment MUST have a way to identify the position of the last P2MP
      chain label, in order to execute [3] above.  This document
      introduces an "end-of-chain" (EoC) label to facilitate the
      process.  The EoC label is an extended special-purpose label
      (ESPL) [RFC 7274] with value TDB.  When a root node constructs an
      MPLS header for a packet, if the packet has a service label(s),
      the root node MUST push the Extension Label (XL, value 15) and the
      EoC label, after pushing the service label(s) and before push P2MP
      chain labels.  Hence, [XL, EoC] serves as a recognizable pattern
      to indicate the end of the P2MP chain labels.  If the packet does
      not have a service label(s), the root node SHOULD NOT push [XL,
      EoC] to the MPLS header.  In any case, in [3] above, the bud
      segment MUST pop labels until [XL, EoC] are popped or all labels
      have been popped.

      (b) In SRv6, the packet is encapsulated with an outer IPv6 header
      corresponding to the P2MP chain, optionally followed by a segment
      routing header (SRH) containing the SIDs of the P2MP chain, and
      followed by an inner header (of IPv4, IPv6, MPLS, layer-2, etc.)
      associated with a service.  In [3] above, the bud segment SHOULD
      simply remove the outer IPv6 header and the SRH (if any), and
      leave the packet with the inner header to local processing.

   Bud segments are shared by all P2MP streams, i.e. all combinations of
   {root node, leaf nodes}. A leaf node SHOULD advertise a bud segment
   for SR-MPLS, if its forwarding hardware supports the above SR-MPLS
   processing.  Likewise, it SHOULD advertise a bud segment for SRv6, if
   its forwarding hardware supports the above SRv6 processing.  The
   advertisement may be via IGP (ISIS, OSPF) or BGP-LS.  The
   advertisement allows the leaf node to be considered as a transit leaf
   node on a P2MP chain.  If a leaf node does not advertise a bud
   segment, it can only be considered as a tail-end leaf node on a P2MP
   chain, or reached via a P2P tunnel using ingress replication.

   Bud segments are generic purpose segments.  They may also be used in
   cases other than P2MP transport, such as traffic monitoring.  These
   use cases are out of the scope of this document.

4.2.  P2MP Chain

   Construction of P2MP chains for a P2MP stream is performed by a
   controller or the root node based on path computation (Section 5).
   This decides the number of P2MP chains to use, and the set of leaf
   nodes that each P2MP chain reaches.  In general, if the leaf nodes of
   the P2MP stream cannot be covered by using a single P2MP chain,
   multiple P2MP chains MUST be used, and the root node MUST replicate
   ingress packets over the P2MP chains.

   The path of a P2MP chain is a single path traversing one or multiple
   transit leaf nodes and terminating at a tail-end leaf node.  Between
   the root node and the first transit leaf node, and between two
   consecutive leaf nodes, there may be none, one, or multiple transit

   The path is then translated to a SID list to be programmed on the
   root node.  In the SID list, each transit leaf node has its bud-SID
   in a corresponding position.  Given a P2MP chain to a set of leaf
   nodes in the order of L1, L2, ..., Ln, the SID list may be
   represented as:

   <SID_11, SID_12, ...>, bud-SID of L1, ..., <SID_i1, SID_i2, ...>,
   bud-SID of Li, ..., <SID_n1, SID_n2, ...>


   o  <SID_11, SID_12, ...> is the sub-path from the root node to L1.

   o  <SID_i1, SID_i2, ...> is the sub-path from Li-1 to Li.

   o  <SID_n1, SID_n2, ...> is the sub-path from Ln-1 to Ln.  There is
      no need for Ln's bud-SID to be at the end of the SID list, because
      the tail-end leaf node does not perform a chain replication.

   Each of the above sub-paths is a regular point-to-point path.  The
   SIDs in the sub-path are regular SIDs, such as adjacency-SIDs, node-
   SIDs, binding-SIDs, etc.  There is no SID specific to the given P2MP
   chain.  A sub-path from Li-1 to Li may have an empty SID list, if the
   sub-path takes the shortest path indicated by the bud-SID of Li.

   The root node then uses the SID list in packet encapsulation.  Note
   that in the SR-MPLS case where the EoC label is needed, [XL, EoC]
   MUST be pushed to an MPLS header, before the SID list is pushed.

4.3.  Example

   In the following example, P2MP transport is needed from the root node
   R, to leaf nodes L1, L2, L3 and L4.

                R ------ R1 -------------------- R2 ------- L1
                          |                       |      /
                          |                       |    /
                          |                       |  /
                         R3 -------------------- R4 ------- L2
                          |                       |
                          |                       |
                          |                       |
                         R5 -------------------- R6 ------- L3
                          |                       |      /
                          |                       |    /
                          |                       |  /
                         R7 -------------------- R8 ------- L4

                                 Figure 2

   Path computation results in two P2MP chains:

      P2MP chain 1:

         Path: R -> R1 -> R2 -> L1 -> R4 -> L2, where L1 is a transit
         leaf node, and L2 is the tail-end leaf node.

         Assuming that the sub-path R -> R1 -> R2 -> L1 is not the
         shortest path from R to L1, so that an explicit sub-path must
         be used.  Also assuming that the sub-path L1 -> R4 -> L2 is the
         shortest path from L1 to L2, so that the node-SID of L2 can be
         used to represent this sub-path.  The segment list applied to
         packets on R is:

            adj-SID 100 - link from R to R1

            adj-SID 200 - link from R1 to R2

            adj-SID 300 - link from R2 to L1

            bud-SID 1000 - L1

            node-SID 2000 - L2

      P2MP chain 2:

         Path: R -> R1 -> R3 -> R5 -> R6 -> L3 -> R8 -> L4, where L3 is
         a transit leaf node, and L4 is the tail-end leaf node.

         Assuming that the sub-path R -> R1 -> R3 -> R5 -> R6 -> L3 is
         the shortest path from R to L3, so that the bud-SID of L3 can
         be used to represent this sub-path.  Also assuming that the
         sub-path L3 -> R8 -> L4 is not the shortest path from L3 to L4,
         so that an explicit sub-path must be used.  The segment list
         applied to packets on R is:

            bud-SID 3000 - L3

            adj-SID 600 - link from L3 to R8

            adj-SID 700 - link from R8 to L4

            bud-SID 4000 - L4

5.  Path Computation for P2MP Chains

   Path computation for the P2MP chains of a P2MP stream {root node,
   leaf nodes} lies in the responsibility of a controller or the root
   node.  This document does not enforce a particular computation
   algorithm.  In general, any P2P path computation algorithm may be
   extended to serve the purpose.

   The path computation may consider general metric for shortest paths,
   or traffic engineering (TE) constraints for TE paths.  This document
   recommends the following constraints to be considered as well:

      - The maximum hop count of path.  This SHOULD be based on the
      maximum delay allowed for a packet to accumulate before reaching a
      tail-end leaf node.  It may be used to restrict the length of each
      P2MP chain.

      - The maximum length of SID list.  This SHOULD be based on the
      maximum header size which a root node may apply to a packet.  This
      is typically a limit of forwarding hardware.  Note that a SID list
      is translated from a computed path.  Hence, the length of the SID
      list and the hop count of the path are generally not the same.

      - Maximum leaf nodes per P2MP chain.  This may be used to restrict
      the length of each P2MP chain.

      - Maximum hops between consecutive leaf nodes on a P2MP chain.
      This may be used prevent a P2MP chain from attempting leaf nodes
      which should ideally be reached by separate P2MP chains.

      - Maximum times that a node or link may be traversed by a P2MP
      chain.  This may be used to prevent a P2MP chain from congesting a
      node or link.

   As an example, the path computation may start with forming a path
   from the root node to the closest leaf node, and extend the path to a
   second leaf node, a third leaf node, and so on.  When any of the
   above limits is hit, the current computation SHOULD end, the path
   SHOULD be saved as a completed P2MP chain, and a new computation
   SHOULD be performed for the rest leaf nodes.  This process SHOULD
   repeat until all the leaf nodes are covered, where a set of paths
   have been computed.

   The path computation is generally deterministic in a ring or linear
   topology.  In an arbitrary topology, deterministic path computation
   may be achieved by dividing leaf nodes into groups based on their
   location, and computing a separate path for each group.  A group may
   even define its leaf nodes as an ordered list of loose hops, so that
   a path will traverse the leaf nodes in the specified order.  During
   the computation of a group, if any of the above limits is hit, the
   computation SHOULD end, the path SHOULD be saved as a completed P2MP
   chain, and a new computation SHOULD be performed for the rest leaf
   nodes of the group.  This process SHOULD repeat until all the leaf
   nodes of the group are covered.  In this case, the group will end up
   using multiple P2MP chains.

6.  IGP and BGP-LS Extensions for Bud Segment

   The protocol extensions of IGP (ISIS and OSPF) and BGP-LS for bud
   segment advertisement will be specified in the next version of this

7.  Bud Segments for Special Processing

   So far, the discussion in this document has been focusing on bud
   segments that are created on a per SR-MPLS or SRv6 basis on each leaf
   node.  These bud segments indicate generic local processing which is
   based on the inner header of a packet.  They are applicable to most
   of the common cases of P2MP transport, and hence are viewed as the
   default bud segments of leaf nodes.

   The concept of bud segment can also be extended to other cases, where
   a transit leaf node needs to perform a special kind of local
   processing for packets, but cannot derive the context of the
   processing from their inner headers.  For example, the node may need
   to forward the packets over one or more interfaces or tunnels to
   downstream device(s), or to process the packets based on a particular
   forwarding table or policy, and so on.  In such cases, a dedicated
   bud segment SHOULD be created for the special kind of local
   processing.  It will serve the general purpose of a bud segment, and
   additionally indicate the context of the special processing.  Note
   that scaling of such bud segments per leaf node SHOULD be a
   consideration in network design, as well as the requirement for a
   controller or ingress router to have the knowledge of various special
   processing scenarios on leaf nodes and use the corresponding bud
   segments in P2MP chain construction.

8.  IANA Considerations

   This document requires IANA to allocate a value from the "Extended
   Special-Purpose MPLS Label Values" registry for the EoC label.

   The document also requires IANA registration and allocation for the
   ISIS, OSPF and BGP-LS extensions for bud segment advertisement.  The
   details will be provided in the next version of this document.

9.  Security Considerations

   This document introduces bud segments for leaf nodes to act as both
   packet receivers and transit routers.  A security attack may target
   on a leaf node by constructing malicious packets with the node's bud-
   SID.  Such kind of attacks can be defeated by restricting bud segment
   distribution and P2MP chain construction within the scope of a
   controller and a given network.

10.  Acknowledgements

   This document leverages work done by Alexander Arseniev and Ron

11.  Contributors

   Alexander Arseniev

   Juniper networks

   Email: aarseniev@juniper.net

   Ron Bonica

   Juniper networks



   Email: rbonica@juniper.net

12.  References

12.1.  Normative References

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,

              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing
              Header", draft-ietf-6man-segment-routing-header (work in
              progress), 2019.

              Filsfils, C., Garvia, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming (work in
              progress), 2019.

12.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

Authors' Addresses

   Yimin Shen
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886

   Email: yshen@juniper.net

   Zhaohui Zhang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886

   Email: zzhang@juniper.net

   Rishabh Parekh
   Cisco Systems
   San Jose, CA

   Email: riparekh@cisco.com

   Hooman Bidgoli

   Email: hooman.bidgoli@nokia.com

   Yuji Kamite
   NTT Communications

   Email: y.kamite@ntt.com