Stateless SRv6 Point-to-Multipoint Path
draft-chen-pim-srv6-p2mp-path-03

Document Type Active Internet-Draft (individual)
Authors Huaimo Chen  , Mike McBride  , Yanhe Fan  , Xuesong Geng  , Mehmet Toy  , Aijun Wang  , Lei Liu  , Xufeng Liu 
Last updated 2021-07-11
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Network Working Group                                            H. Chen
Internet-Draft                                                M. McBride
Intended status: Experimental                                  Futurewei
Expires: January 12, 2022                                         Y. Fan
                                                            Casa Systems
                                                                 X. Geng
                                                                  Huawei
                                                                  M. Toy
                                                                 Verizon
                                                                 A. Wang
                                                           China Telecom
                                                                  L. Liu
                                                                 Fujitsu
                                                                  X. Liu
                                                          Volta Networks
                                                           July 11, 2021

                Stateless SRv6 Point-to-Multipoint Path
                    draft-chen-pim-srv6-p2mp-path-03

Abstract

   This document describes a solution for a SRv6 Point-to-Multipoint
   (P2MP) Path/Tree to deliver the traffic from the ingress of the path
   to the multiple egresses/leaves of the path in a SR domain.  There is
   no state stored in the core of the network for a SR P2MP path like a
   SR Point-to-Point (P2P) path in this solution.

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

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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 January 12, 2022.

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
   (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
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   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.  Overview of P2MP Multicast Tree . . . . . . . . . . . . . . .   3
   3.  Encoding P2MP Multicast Tree  . . . . . . . . . . . . . . . .   5
   4.  Procedures/Behaviors  . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Procedure/Behavior on Ingress Node  . . . . . . . . . . .   7
     4.2.  Procedure/Behavior on Transit Node  . . . . . . . . . . .   8
     4.3.  Procedure/Behavior on Egress Node . . . . . . . . . . . .  10
   5.  Stateless SRv6 P2MP Path for Ingress  . . . . . . . . . . . .  10
   6.  Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Global Protection . . . . . . . . . . . . . . . . . . . .  12
     6.2.  Local Protection  . . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Example IPv6 Header using G-SRv6 . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The Segment Routing (SR) for unicast or Point-to-Point (P2P) path is
   described in [RFC8402].  For SR multicast or Point-to-Multipoint
   (P2MP) path/tree, it may be implemented through using multiple SR P2P
   paths.  The function of a SR P2MP path/tree from an ingress node to

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   multiple (say n) egress/leaf nodes is implemented by n SR P2P paths.
   These n P2P paths are from the ingress to those n egress/leaf nodes
   of the P2MP path/tree.  This solution may waste some network
   resources such as link bandwidth.

   An alternative solution proposed in
   [I-D.shen-spring-p2mp-transport-chain] uses a number of P2MP chain
   tunnels to implement a P2MP path/tree from an ingress to n egress/
   leaf nodes.  Each P2MP chain tunnel is a tunnel from the ingress to a
   leaf node as its tail end and may have some leaf nodes as its bud
   nodes along the tunnel.  This alternative solution improves the usage
   of network resources over the solution above using pure P2P paths.
   However, these two solutions are based on SR P2P paths.

   A solution for a SR P2MP path/tree using a P2MP multicast tree is
   proposed in [I-D.ietf-pim-sr-p2mp-policy].  For a SR P2MP path/tree
   from an ingress/root to multiple egress/leaf nodes, a multicast P2MP
   tree is created to deliver the traffic from the ingress/root to the
   egress/leaf nodes.  The state of the tree is instantiated in the
   forwarding plane by a controller such as PCE at Root node,
   intermediate Replication nodes and Leaf nodes of the tree.  This is
   not consistent with the SR principles in which no state is stored at
   the core of the network.

   This document describes a new solution for a SRv6 Point-to-Multipoint
   (P2MP) Path/Tree to deliver the traffic from the ingress of the path
   to the multiple egresses/leaves of the path in a SR domain.  This
   solution uses a P2MP multicast tree without storing its state in the
   core of the network for a SR P2MP path/tree like a SR P2P path.  For
   distinguishing a SRv6 P2MP path/tree used in the other solutions with
   storing some states in the core, a new name, called stateless SRv6
   P2MP path/tree, is used in the solution in this document.  Even
   though SRv6 P2MP path/tree and stateless SRv6 P2MP path/tree are used
   interchangably in the document, they both mean stateless SRv6 P2MP
   path/tree.

2.  Overview of P2MP Multicast Tree

   For a SR P2P path from its ingress to its egress, a segment list for
   the path is provided to the ingress.  The ingress pushes the list
   into a packet, and the packet is delivered to the egress according to
   the segment list without any state in the core of the network.

   For a SR P2MP path from its ingress to multiple egress/leaf nodes, a
   segment list for the P2MP path is provided to the ingress.  The
   ingress pushes the list into a packet, and the packet is delivered to
   the multiple egress/leaf nodes according to the segment list without
   any state in the core of the network.

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   Figure 1 shows a SR P2MP path from ingress/root R to four egress/leaf
   nodes L1, L2, L3 and L4.  Nodes P1, P2, P3 and P4 are the transit
   nodes of the P2MP path.

   Suppose that X-m is the segment identifier (SID) of node X.  X-m is
   an adjacent SID or node SID.  For simplicity, we assume X-m is a node
   SID in the illustrations below.  R-m, P1-m, P2-m, P3-m, P4-m, L1-m,
   L2-m, L3-m and L4-m are the SIDs of the nodes on the SR P2MP path.
   They are multicast SIDs or replication SIDs in general.

   A multicast SID is a SID from a multicast SID block.  In a SR domain
   supporting SR multicast, each node has a multicast node SID, which is
   globally significant; each adjacency of a node has a multicast
   adjacency SID, which is locally significant.  A multicast SID of a
   node on a SR P2MP path is associated with the SIDs of the next hop
   (or say downstream) nodes.  When the node receives a packet with its
   multicast SID, it duplicates and sends the packet to each of the next
   hop nodes according to their SIDs.

   If node P on a SR P2MP path has B (B > 1) next hop nodes along the
   path, the SID of node P, P-m, MUST be a multicast SID when it is in
   the segment list for the P2MP path.  The SIDs of the B next hop nodes
   just follow P-m in the segment list.  When node P receives the packet
   with P-m, it duplicates and sends the packet to each of the B next
   hop nodes along the P2MP path.

                      [L1]                  R Ingress/Root
                      /                    Li Egress/Leaf
                     /                     Pi Transit Node
                    /
                  [P2]------[L2]
                  /
                 /
                /
     [R]------[P1]                [L3]
                \                /
                 \              /
                  \            /
                  [P3]------[P4]------[L4]

            Figure 1: SR P2MP Path from R to L1, L2, L3 and L4

   <P1-m, P2-m, P3-m, L1-m, L2-m, P4-m, L3-m, L4-m> is a segment list
   for the SR P2MP path in Figure 1 to be pushed into a packet at
   ingress/root R.  Node P1 has 2 next hop nodes P2 and P3 along the
   P2MP path.  The next hop nodes' SIDs P2-m and P3-m follow P1-m, which
   is P1's multicast SID.  When P1 receives a packet transported by the

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   P2MP path, it duplicates and sends the packet to the next hop nodes
   P2 and P3 according to P1-m, P2-m and P3-m.

   The number of branches or next hops from node P1 is a value of one
   argument in P1-m, called N-Branches.  The value of N-Branches in P1-m
   is 2.  With this information, node P1 duplicates and sends the packet
   to 2 next hop nodes P2 and P3, which are indicated by the 2 SIDs P2-m
   and P3-m following P1-m.

   The number of SIDs of the nodes under node P1 is a value of another
   argument in P1-m, called N-SIDs.  The value of N-SIDs in P1-m is 7,
   indicating that there are 7 SIDs following P1-m in the segment list.

   There are 2 branches or next hops (i.e., L1 and L2) from node P2 and
   2 SIDs (i.e., L1-m and L2-m) of the nodes under node P2.  The values
   of N-Branches and N-SIDs in P2-m are 2 and 2.  with this information,
   before sending the packet to node P2, node P1 pushes the SIDs under
   node P2 into the packet (i.e., the packet has a new segment list with
   the SIDs under node P2.  The new segment list replaces the old one in
   the packet).

   There are 1 branch or next hop (i.e., P4) from node P3 and 3 SIDs
   (i.e., P4-m, L3-m and L4-m) of the nodes under node P3.  The values
   of N-Branches and N-SIDs in P3-m are 1 and 3 respectively.  with this
   information, before sending the packet to node P3, node P1 pushes the
   SIDs under node P3 into the packet.

   Each node on the SR P2MP path sends the packet to its next hop nodes
   according to the segment list and no state is stored in any transit
   node (i.e., the core of the network).  The packet is delivered to the
   egress/leaf nodes from the ingress.

3.  Encoding P2MP Multicast Tree

   For each sub-tree STi of a SR P2MP path from the ingress node of the
   P2MP path, suppose that

   o  the multicast SID of the next hop node NHi is mSIDi;

   o  there are Bi branches (i.e., outgoing interfaces) to the next hop
      node BNHj (j = 1, ..., Bi) from node NHi along the sub-tree, the
      multicast SID of BNHj is mSIDij;

   o  the number of branches (i.e., outgoing interfaces) under the node
      BNHj (j = 1, ..., Bi) is BBj; and the number of SIDs of the nodes
      under each of the Bi branches from node BNHj is NSj (j = 1, ...,
      Bi).

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   Sub-tree STi is encoded as segment list

         < mSIDi,  mSIDi1, ..., mSIDiBi,   SegSeq1, ...,  SegSeqBi  >,
           \___/  \____________________/   \______/      \________/
   SIDs of  NHi    Bi branches/next-hops   sub-tree       sub-tree
                   BNHj of node NHi        from BNH1      from BNHBi

   where mSIDi contains the number of branches, Bi, in its N-Branches
   field, and the number of SIDs under mSIDi in its N-SIDs field; mSIDij
   (j = 1, ..., Bi) contains the number of branches, BBj, in its
   N-Branches field and the number of SIDs, NSj, in its N-SIDs field;
   SegSeqj (j = 1, ..., Bi) is the SID sequence in the segment list
   encoding the sub-trees from node BNHj.

   For the P2MP path in Figure 1 from ingress node R to egress nodes L1,
   L2, L3 and L4, there is one sub-tree from R.

   For this sub-tree,

   o  the next hop node is P1 and the multicast SID of P1 is P1-m;

   o  there are 2 branches to the next hop nodes P2 and P3 from node P1
      along the sub-tree; the number of SIDs of the nodes under P1 is 7;
      the multicast SIDs of P2 and P3 are P2-m and P3-m respectively;

   o  the numbers of SIDs of the nodes under these two branches are 2
      and 3 respectively.  The SIDs of the nodes under P2 are L1-m and
      L2-m.  The SIDs of the nodes under P3 are P4-m, L3-m and L4-m.

   The sub-tree is encoded as segment list

         < P1-m,  P2-m, P3-m,          L1-m, L2-m,  P4-m, L3-m, L4-m >,
           \__/  \___________/         \________/   \______________/
   SIDs of  P1   2 branches/next-hops   sub-tree     sub-tree
                 P2 and P3 of node P1   from P2      from P3
       where
         P1-m's N-Branches field is set to 2 and its N-SIDs field to 7;
         P2-m's N-Branches field is set to 2 and its N-SIDs field to 2;
         P3-m's N-Branches field is set to 1 and its N-SIDs field to 3;

         L1-m and L2-m are the SID sequence in the segment list encoding
         the sub-trees from P2;

         P4-m, L3-m and L4-m are the SID sequence in the segment list
         encoding the sub-trees from P3; and

         P4-m's N-Branches field is set to 2 and its N-SIDs field to 2.

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   Figure 2 shows in details the segment list, which is an encoding of
   the P2MP multicast tree for the SR P2MP path from R to L1, L2, L3 and
   L4.

                              N-Branches   N-SIDs
 +---------------------------+-----------+-----------+--------------+
 | P1's Multicast SID Locator|     2     |     7     |  Arguments   | P1-m
 +---------------------------+-----------+-----------+--------------+
 | P2's Multicast SID Locator|     2     |     2     |  Arguments   | P2-m
 +---------------------------+-----------+-----------+--------------+
 | P3's Multicast SID Locator|     1     |     3     |  Arguments   | P3-m
 +---------------------------+-----------+-----------+--------------+
 | L1's Multicast SID Locator|     0     |     0     |  Arguments   | L1-m
 +---------------------------+-----------+-----------+--------------+
 | L2's Multicast SID Locator|     0     |     0     |  Arguments   | L2-m
 +---------------------------+-----------+-----------+--------------+
 | P4's Multicast SID Locator|     2     |     2     |  Arguments   | P4-m
 +---------------------------+-----------+-----------+--------------+
 | L3's Multicast SID Locator|     0     |     0     |  Arguments   | L3-m
 +---------------------------+-----------+-----------+--------------+
 | L4's Multicast SID Locator|     0     |     0     |  Arguments   | L4-m
 +---------------------------+-----------+-----------+--------------+

        Figure 2: Encoding of P2MP Multicast Tree from R to L1 - L4

   SID P1-m indicates that there are 2 branches and 7 SIDs under P1.
   SID P2-m indicates that there are 2 branches and 2 SIDs under P2.
   SID P3-m indicates that there are 1 branch and 3 SIDs under P3.  SIDs
   L1-m and L2-m indicate that there is no branch under them.  SID P4-m
   indicates that there are 2 branches and 2 SIDs under P4.  L3-m and
   L4-m indicate that there is no branch under them.

4.  Procedures/Behaviors

   This section describes the procedures or behaviors on the ingress,
   transit and egress/leaf node of a SR P2MP path to deliver a packet
   received from the path to its destinations.

4.1.  Procedure/Behavior on Ingress Node

   For a packet to be transported by a SR P2MP Path, the ingress of the
   P2MP path duplicates the packet for each sub-tree of the SR P2MP path
   branching from the ingress, pushes the segment list encoding the sub-
   tree into the packet by executing H.Encaps
   [I-D.ietf-spring-srv6-network-programming] and sends the packet to
   the next hop node along the sub-tree.

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   Regarding to the finite size of the segment list, a sub-tree can be
   "split" into multiple sub-trees such that each of the sub-trees can
   be encoded in the segment list of the finite size.

   For example, there is one sub-tree from the ingress R of the SR P2MP
   path in Figure 1 via next hop node P1 towards egress/leaf nodes L1,
   L2, L3 and L4.

   For this sub-tree, the ingress R duplicates the packet, set the
   destination address (DA) to P1-m (i.e., multicast SID of node P1),
   pushes the segment list without P1-m (i.e., <P2-m, P3-m, L1-m, L2-m,
   P4-m, L3-m, L4-m>) encoding the sub-tree in a Segment Routing Header
   (SRH) of the packet by executing H.Encaps and sends the packet to DA
   (i.e., node P1).  The contents of the multicast SIDs P1-m, P2-m,
   P3-m, L1-m, L2-m, P4-m, L3-m, L4-m are shown in Figure 2.

   Suppose that the duplicated packet is Pkt0 for the sub-tree.  The
   execution of H.Encaps pushes an IPv6 header (i.e., SRH) to Pkt0 and
   sets some fields in the header to produce an encapsulated packet
   Pkt'.  Pkt' is represented in the following:

       Pkt' = (SA=R, DA=P1-m)( L4-m, L3-m,..., P3-m,P2-m; SL=7)Pkt0
                              \________________________/
              corresponds to: <P2-m,P3-m, ..., L3-m,L4-m>

   where DA=P1-m means that the destination address (DA) is set to P1-m;
   SA=R means that the source address (SA) is set to R; SL=7 means that
   the number of Segments Left (SL) is 7.

4.2.  Procedure/Behavior on Transit Node

   When a transit node of a SR P2MP path receives a packet transported
   by the P2MP path, the DA of the packet is a multicast SID of the node
   and the packet contains a segment list for the sub-trees under the
   transit node.  The DA and the segment list comprise the information
   for encoding the sub-trees.

   For example, when node P1 receives a packet transported by the SR
   P2MP path in Figure 1, the packet's DA is P1-m (which is a multicast
   SID of node P1) and the segment list in the packet is <P2-m, P3-m,
   L1-m, L2-m, P4-m, L3-m, L4-m>.

   The N-Branches field (which has value of n) of the DA indicates that
   there are n branches (or say sub-trees) under the transit node.  The
   N-SIDs field of the DA indicates the number of SIDs for these n sub-
   trees under the transit node.  The multicast SIDs of the next hop
   nodes of these n sub-trees are the first n multicast SIDs of the
   segment list in the packet.

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   For example, the N-Branches field (which has value of 2) of DA = P1-m
   indicates that there are 2 branches (or say sub-trees) under node P1.
   The N-SIDs field (which has value of 7) of the DA = P1-m indicates
   that there are 7 SIDs for these 2 sub-trees under node P1.

   The first multicast SID (P2-m) of the segment list is the SID of the
   next hop node (P2) of the first sub-tree; The second multicast SID
   (P3-m) of the segment list is the SID of the next hop node (P3) of
   the second sub-tree.

   After the multicast SIDs of the next hop nodes, there are n blocks of
   SIDs for those n sub-trees.  The N-SIDs field (which has value of B1)
   of the first multicast SID of the next hop nodes indicates that there
   are B1 SIDs in the first block for the first sub-tree; the N-SIDs
   field (which has value of B2) of the second multicast SID of the next
   hop nodes indicates that there are B2 SIDs in the second block for
   the second sub-tree after the first block; and so on.

   For example, there are 2 blocks of SIDs for the 2 sub-trees under
   node P1 after the multicast SIDs P2-m and P3-m of the next hop nodes
   P2 and P3.  The N-SIDs field of P2-m (the first multicast SID of the
   next hop nodes) has value of 2, indicating that there are 2 SIDs in
   the first block for the first sub-tree, which are L1-m and L2-m.

   The N-SIDs field of P3-m (the second multicast SID of the next hop
   nodes) has value of 3, indicating that there are 3 SIDs in the second
   block for the second sub-tree after the first block, which are P4-m,
   L3-m and L4-m.

   The transit node duplicates the packet without top header for each
   sub-tree under it and adds a new header with a new segment list built
   from the SID block for the sub-tree to the duplicated packet by
   executing H.Encaps.  It sets the DA of the packet to the multicast
   SID of the next hop node along the sub-tree and sends the packet to
   the DA.

   For example, node P1 duplicates the packet for the first sub-tree
   towards L1 and L2 and adds a new header with a new segment list
   <L1-m, L2-m>.  It sets DA = P2-m (multicast SID of next hop P2), and
   sends the packet to the DA (i.e., P2).

   Suppose that the duplicated packet is Pkt0 for the sub-tree.  The
   execution of H.Encaps pushes a new IPv6 header (i.e., SRH) to Pkt0
   and sets some fields in the header to produce an encapsulated packet
   Pkt'.  Pkt' is represented in the following:

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       Pkt' = (SA=P1, DA=P2-m)( L2-m, L1-m;  SL=2)Pkt0.
                               \__________/
             corresponds to:   <L1-m, L2-m>

   where DA=P2-m means that the destination address (DA) is set to P2-m;
   SA=P1 means that the source address (SA) is set to P1; SL=2 means
   that the number of Segments Left (SL) is 2.

   Node P1 duplicates the packet for the second sub-tree via P3 towards
   L3 and L4 and adds a new header with a new segment list <P4-m, L3-m,
   L4-m>.  It sets DA = P3-m (multicast SID of next hop P3), and sends
   the packet to the DA (i.e., P3).

4.3.  Procedure/Behavior on Egress Node

   When an egress node of a SR P2MP path receives a packet transported
   by the P2MP path, the DA of the packet is a SID of the egress node.
   The egress node sends the packet to its destination accordingly.  If
   the SID is a multicast SID of the egress, the N-Branches field and
   N-SIDs field are all zeros.

5.  Stateless SRv6 P2MP Path for Ingress

   A controller such as PCE can compute a stateless SRv6 P2MP path and
   send it to its ingress.  For a packet to be transported by the path,
   the ingress encapsulates the packet with the path and the packet will
   be delivered to the egresses of the path without any states in the
   network core.

   An example architecture using PCE as a controller is illustrated in
   Figure 3.  There is a connection (i.e., PCE session) between the PCE
   and (the PCC running on) each of the PEs, which are possible ingress
   nodes in the network domain.  Note that some of connections between
   the PCE and PEs are not shown in the figure.

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                      +------------------------------------+
                      |                 PCE                |
                      +------------------------------------+
                      /                                    \
                     /                                      \
                    /    ~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~~\~^~
                   /   _(      (P2)---------(P3)-----------(PE2)  )
                  /   (        /              \_______      /      )
                 /  _(        /                _______)____/        )
                / _(         /                /      (_____         )
               /_(          /                /             \       )
              /(           /                /               \      )
   (CE) --- (PE1)--------(P1)-------------(P4)-------------(PE3)    )
              ( \          \                \                      )
              (  \          \                \       Network      )
              (   \          \                \                    )
               (_ (PE5)------(P5)------------(PE4)                 )
                 (                                                )
                  '---._.-.-._.-._.-.-._.-._.-.-._.-.-._.-.-._.-.)

                     Figure 3: Architecture using PCE

   The PCE has the information about the network domain from the IGP or
   BGP (BGP-LS).  The information includes link bandwidth, link colors,
   node SIDs, and so on.  A separate multicast SID could be provisioned
   on every replication node and the PCE gets the SID on the node from
   IGP or BGP.

   The PCE maintains the current status of the network resource usage in
   its local TED (Traffic Engineering Database), and the status of every
   stateless SRv6 P2MP path in its local LSP-DB (Label Switch Path
   Database).

   Upon receiving a request for a stateless SRv6 P2MP path from a user
   or application, the PCE computes a path based on the network resource
   availability stored in the TED.  After a path satisfying the given
   constraints is found, the PCE constructs a stateless SRv6 P2MP path
   using the multicast SIDs of the nodes on the path and encodes the
   structure of the P2MP path/tree into the parameters of the SIDs.  In
   fact, the stateless SRv6 P2MP path is a segment list consisting of
   multicast SIDs with parameter values.

   And then the PCE sends the segment list representing the path to the
   ingress node of the path in a PCEP message such as PCInitiate.  After
   receiving the path from the PCE, the ingress node establishes the
   path by creating a forwarding entry in its FIB.  For every multicast
   packet to be transported by the path, the forwarding entry
   encapsulates the packet with the segment list and the packet will be

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   delivered to the egress nodes of the path along the path without any
   state in the core of the network.

6.  Protection

   Protections for a SR P2MP path can be classified into two types:
   global protection and local protection.

6.1.  Global Protection

   For a primary SR P2MP path from an ingress node R1 to multiple egress
   nodes Li (i = 1, ..., n), a backup SR P2MP path from an ingress node
   R1' to multiple egress nodes Li' (i = 1, ..., n) is set up to provide
   global protection for the primary SR P2MP path.  If R1' is the same
   as R1, the failure of the ingress node R1 of the primary SR P2MP path
   is not protected; otherwise (i.e., R1' and R1 are different and
   connected to the same traffic source), the failure of the ingress
   node R1 is protected.  If Li' is the same as Li (i = 1, ..., n), the
   failure of the egress nodes Li (i = 1, ..., n) of the primary SR P2MP
   path is not protected; otherwise (i.e., Li' and Li are different and
   connected to the same destination), the failure of the egress nodes
   Li is protected.

   When a failure happens on the primary SR P2MP path and is detected by
   the source of the traffic or other entity, the traffic to be
   transported by the primary SR P2MP path is switched to the backup SR
   P2MP path, which sends the traffic from its ingress node R1' to its
   egress nodes Li' (i = 1, ..., n).

6.2.  Local Protection

   Local protection or say Fast Reroute (FRR) of a node and adjacency
   segment on a SR P2P path is proposed in
   [I-D.ietf-rtgwg-segment-routing-ti-lfa] and
   [I-D.ietf-rtgwg-srv6-egress-protection].  It can be applied to FRR of
   a node and adjacency segment on a SR P2MP path in a similar way.  But
   FRR for SR P2MP path is more complicated.

   More details will be added later.

7.  IANA Considerations

   TBD

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8.  Security Considerations

   TBD

9.  Acknowledgements

   The authors would like to thank Acee Lindem, Jeffrey Zhang and Daniel
   Voyer for their valuable comments and suggestions on this draft.

10.  References

10.1.  Normative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-26 (work in
              progress), October 2019.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", draft-ietf-rtgwg-segment-
              routing-ti-lfa-06 (work in progress), February 2021.

   [I-D.ietf-rtgwg-srv6-egress-protection]
              Hu, Z., Chen, H., Chen, H., Wu, P., Toy, M., Cao, C., He,
              T., Liu, L., and X. Liu, "SRv6 Path Egress Protection",
              draft-ietf-rtgwg-srv6-egress-protection-02 (work in
              progress), November 2020.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Garvia, P. C., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", draft-ietf-spring-srv6-
              network-programming-28 (work in progress), December 2020.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

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   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-pim-sr-p2mp-policy]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy",
              draft-ietf-pim-sr-p2mp-policy-02 (work in progress),
              February 2021.

   [I-D.ietf-spring-sr-replication-segment]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "SR Replication Segment for Multi-point Service
              Delivery", draft-ietf-spring-sr-replication-segment-04
              (work in progress), February 2021.

   [I-D.shen-spring-p2mp-transport-chain]
              Shen, Y., Zhang, Z., Parekh, R., Bidgoli, H., and Y.
              Kamite, "Point-to-Multipoint Transport Using Chain
              Replication in Segment Routing", draft-shen-spring-p2mp-
              transport-chain-03 (work in progress), October 2020.

Appendix A.  Example IPv6 Header using G-SRv6

   For simplicity, 64 bits for Common Prefix, 16 bits for Node ID, 8
   bits for the number of branches (N-Branches) and 8 bits for the
   number of SIDs (N-SIDs) are used when G-SRv6 compression method is
   applied for <P1-m, P2-m, P3-m, L1-m, L2-m, P4-m, L3-m, L4-m> at
   ingress node R in Figure 1.  The Destination Address (DA) is
   illustrated below in Figure 4.  It contains the Common Prefix of 64
   bits, node P1's ID of 16 bits, the value 2 for the number of branches
   (N-Branches) of 8 bits, and the value 7 for the number of SIDs
   (N-SIDs) of 8 bits.

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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         2001:db9:0:0 (Common Prefix)                          |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            P1 ID              |       2       |       7       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Destination Address (DA)

   The IPv6 header is shown in Figure 5.  Ingress node R sends a packet
   with the IPv6 header to the DA.

     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              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            P4 ID              |       2       |       2       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            L3 ID              |       0       |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            L4 ID              |       0       |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Padding                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            P2 ID              |       2       |       2       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            P3 ID              |       1       |       3       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            L1 ID              |       0       |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            L2 ID              |       0       |       0       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 5: IPv6 Header

Authors' Addresses

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   Huaimo Chen
   Futurewei
   Boston, MA
   USA

   Email: Huaimo.chen@futurewei.com

   Mike McBride
   Futurewei

   Email: michael.mcbride@futurewei.com

   Yanhe Fan
   Casa Systems
   USA

   Email: yfan@casa-systems.com

   Xuesong Geng
   Huawei

   Email: gengxuesong@huawei.com

   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com

   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing,    102209
   China

   Email: wangaj3@chinatelecom.cn

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   Lei Liu
   Fujitsu

   USA

   Email: liulei.kddi@gmail.com

   Xufeng Liu
   Volta Networks

   McLean, VA
   USA

   Email: xufeng.liu.ietf@gmail.com

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