Point-to-Multipoint Transport Using Chain Replication in Segment Routing
draft-shen-spring-p2mp-transport-chain-00

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Internet Engineering Task Force                               Yimin Shen
Internet-Draft                                             Zhaohui Zhang
Intended status: Standards Track                        Juniper Networks
Expires: August 6, 2020                                 February 3, 2020

Point-to-Multipoint Transport Using Chain Replication in Segment Routing
               draft-shen-spring-p2mp-transport-chain-00

Abstract

   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.

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Table of Contents

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

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
   independently.

   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 the tail end and some transit leaf nodes
   along the path, resembling a chain.  A transit leaf node replicates a
   packet only once for local processing off the chain, and forwards the

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   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] and
   [RFC8174].

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 for 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].

4.  P2MP Transport Using Chain Replication

   In this document, a P2MP transport scheme 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.

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   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 replication.

       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
   replication".

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

4.1.  Bud Segment

   On a 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] Detect whether this leaf node is a transit or tail-end leaf
      node, based on whether the bud-SID is the last SID of a P2MP
      chain.

      [2] If this is a transit leaf node, replicate the packet to
      produce a copy P1.

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

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         [2.2] 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.

      [3] If this is a tail-end leaf node, perform a NEXT operation on
      the bud-SID for P, and process the packet locally.

   In [2.2], when the transit leaf node processes P1 locally, all the
   SIDs of the P2MP chain are not useful.  Hence, they are removed
   before the processing.

   Bud segments are global segments of leaf nodes.  They are routable
   segments via topological shortest-paths.  Only one bud segment is
   needed per leaf node, and per SR-MPLS or SRv6.  Bud-SIDs are
   allocated from SRGB (SR global block).

   In SR-MPLS, bud-SIDs are 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, there are two cases:

         (a.1) The packet should have no service label, but only P2MP
         chain labels in MPLS header.  In [1], the bud segment SHOULD
         detect whether the leaf node is a transit or tail-end leaf node
         based on the S-bit (bottom of stack) of the bud-SID label.  If
         the S-bit is 0, the leaf node is a transit leaf node.  If the
         S-bit is 1, it is a tail-end leaf node.  In [2.2], the bud
         segment SHOULD simply pop the entire MPLS header.

         (a.2) The packet may have service label(s) after P2MP chain
         labels in MPLS header, e.g. a bridge domain label, a source
         Ethernet segment label, etc.  In this case, the bud segment
         MUST have a way to identify the position of the last P2MP chain
         label.  This document introduces an "end-of-chain" (EoC) label
         to facilitate the process.  An EoC label is a label which is
         known to all root nodes and leaf nodes in a network.  It MUST
         have a globally common value, via configuration on these nodes.
         When a root node constructs an MPLS header for a packet, the
         EoC label MUST be pushed immediately before P2MP chain labels,
         making it the next label after the last P2MP chain label.
         Thus, in [1], the bud segment SHOULD detect whether the leaf
         node is a transit or tail-end leaf node based on whether the
         next label in the current MPLS header is the EoC label.  If so,
         the leaf node is a tail-end leaf node.  Otherwise, it is a
         transit leaf node.  In [2.2], the bud segment SHOULD pop labels
         until the EoC label is popped.  In [3], the bud segment SHOULD

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         pop the bud-SID label and the next label, which is the EoC
         label.

      (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 [1], the bud segment SHOULD detect
      whether it is the last P2MP chain SID based on the SRH.  If the
      SRH does not exist or the Segments Left in the SRH is 0, the leaf
      node is a tail-end leaf node.  Otherwise, it is a transit leaf
      node.  In [2.2] and [3], 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 transport schemes, 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 on a P2MP
   chain.  If a leaf node does not advertise a bud segment, it MUST be
   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 transport scheme is performed
   by a controller or a root node based on path computation (Section 5).
   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
   routers.

   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, ...>, <bud-SID of Ln>

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   Where:

   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  Ln's bud-SID is the last SID of the list, if the sub-path from
      Ln-1 to Ln is partial or empty, or if an EoC label is needed in
      SR-MPLS.  It is optional in other cases.

   The above sub-paths are regular point-to-point paths.  The SIDs in
   the sub-paths 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 an EoC label is needed, the EoC label
   SHOULD 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:

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         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 L1 -> R4 -> L2 matches the shortest
         path from L1 to L2, the bud-SID of L2 is 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

            bud-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
         matches the shortest path from R to L3, the bud-SID of L3 is
         used to represent this sub-path.  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 transport scheme {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 fact, 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:

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      - 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.

      - 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 typically
   not the same.

   The path computation may achieve more predictable results by dividing
   leaf nodes into groups based on their geographical or administrative
   location.  Thus, paths MAY be computed in a manner that each P2MP
   chain is used to reach only a given group, while the number of P2MP
   chains to reach all the leaf nodes of the group is minimized.

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
   document.

7.  IANA Considerations

   This document 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.

8.  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.

9.  Acknowledgements

   This document leverages work done by Alexander Arseniev and Ron
   Bonica.

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10.  References

10.1.  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,
              <https://www.rfc-editor.org/info/rfc8660>.

   [SRv6-SRH]
              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.

   [SRv6-Programming]
              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.

10.2.  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,
              <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>.

Authors' Addresses

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

   Email: yshen@juniper.net

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

   Email: zzhang@juniper.net

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