wdiff rfc9960.original rfc9960.txt

Network Working Group

Internet Engineering Task Force (IETF) R. Parekh, Ed.
Internet-Draft
Request for Comments: 9960 Arrcus
Updates: 9524 (if approved) D. Voyer, Ed.
Intended status:
Category: Standards Track C. Filsfils
Expires: 8 March 2026
ISSN: 2070-1721 Cisco Systems, Inc.
H. Bidgoli
Nokia
Z. Zhang
Juniper Networks
4 September 2025
April 2026

Segment Routing Point-to-Multipoint Policy
draft-ietf-pim-sr-p2mp-policy-22

Abstract

The Point-to-Multipoint (P2MP) Policy enables creation of P2MP trees
for efficient multi-point multipoint packet delivery in a Segment Routing (SR)
domain. This document specifies the architecture, signaling, and
procedures for SR P2MP Policies with Segment Routing over MPLS (SR-
MPLS) and Segment Routing over IPv6 (SRv6). It defines the SR P2MP
Policy construct, candidate paths (CP) (CPs) of an SR P2MP Policy Policy, and the
instantiation of the P2MP tree instances (PTIs) of a candidate path CP using
Replication segments. Additionally, it describes the required
extensions for a controller to support P2MP path computation and
provisioning. This document updates RFC 9524.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 8 March 2026.
https://www.rfc-editor.org/info/rfc9960.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Requirements Language
2. SR P2MP Policy . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. SR P2MP Policy identification . . . . . . . . . . . . . . 4 Identification
2.2. Components of an SR P2MP Policy . . . . . . . . . . . . . 5
2.3. Candidate Paths and P2MP Tree instances . . . . . . . . . 5 Instances
3. Steering traffic Traffic into an SR P2MP Policy . . . . . . . . . . . 7
4. P2MP tree instance . . . . . . . . . . . . . . . . . . . . . 8 Tree Instance
4.1. Replication segments Segments at Leaf Nodes . . . . . . . . . . . 8
4.2. Shared Replication segments . . . . . . . . . . . . . . . 9 Segments
4.3. Packet forwarding Forwarding in a P2MP tree instance . . . . . . . . . 9 Tree Instance
5. Using a controller Controller to build Build a P2MP Tree . . . . . . . . . . . 10
5.1. SR P2MP Policy on a controller . . . . . . . . . . . . . 10 Controller
5.2. Controller Functions . . . . . . . . . . . . . . . . . . 10
5.3. P2MP Tree Compute . . . . . . . . . . . . . . . . . . . . 11
5.4. SID Management . . . . . . . . . . . . . . . . . . . . . 11
5.5. Instantiating P2MP tree instance Tree Instance on nodes . . . . . . . . 12 Nodes
5.6. Protection . . . . . . . . . . . . . . . . . . . . . . . 13
5.6.1. Local Protection . . . . . . . . . . . . . . . . . . 13
5.6.2. Path Protection . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1.
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2.
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Illustration of the SR P2MP Policy and P2MP Tree . . . . 18
A.1. P2MP Tree with non-adjacent Non-Adjacent Replication Segments . . . . 20
A.1.1. SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . 20
A.1.2. SRv6 . . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. P2MP Tree with adjacent Adjacent Replication Segments . . . . . . 23
A.2.1. SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . 23
A.2.2. SRv6 . . . . . . . . . . . . . . . . . . . . . . . . 25
Acknowledgements
Contributors
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27

1. Introduction

RFC 9524

[RFC9524] defines a Replication segment which that enables an SR a Segment
Routing (SR) node to replicate traffic to multiple downstream nodes
in an SR domain [RFC8402]. A P2MP Point-to-Multipoint (P2MP) service can
be realized by a single Replication segment spanning from the ingress
node to the egress nodes of the service. This effectively achieves
ingress replication replication, which is inefficient since the traffic of the
P2MP service may traverse the same set of nodes and links in the SR
domain on its path from the ingress node to the egress nodes.

A Multi-point multipoint service delivery can be efficiently realized with a P2MP
tree in a Segment Routing domain . domain. A P2MP tree spans from a Root node
to a set of Leaf nodes via intermediate Replication nodes. It
consists of a Replication segment at the Root node, stitched to one
or more Replication segments at Leaf nodes and intermediate
Replication nodes. A Bud node [RFC9524] is a node that is both a
Replication node and a Leaf node. Any mention of "Leaf node(s)" in
this document should be considered as referring to "Leaf or Bud
node(s)".

An SR P2MP Policy defines the Root and Leaf nodes of a P2MP tree. It
has one or more candidate paths (CP) (CPs) provisioned with optional
constraints and/or optimization objectives.

A controller computes P2MP tree instances of the candidate paths
using the constraints and objectives specified in the candidate path.
The controller then instantiates a P2MP tree instance in the SR
domain by signaling Replication segments to the Root, Replication Replication,
and Leaf nodes. A Path Computation Element (PCE) [RFC4655] is one
example of such a controller. In other cases, a P2MP tree instance
can be installed using NETCONF/YANG the Network Configuration Protocol (NETCONF) /
YANG or the Command Line Interface(CLI) Interface (CLI) on the Root, Replication Replication,
and the Leaf nodes.

The Replication segments of a P2MP tree instance can be instantiated
for SR-MPLS [RFC8660] and SRv6 [RFC8986] data planes, enabling
efficient packet replication within an SR domain.

This document updates the Replication-ID portion of a the Replication
segment identifier (Replication-SID) specified in Section 2 of
[RFC9524].

1.1. Terminology

This section defines terms used frequently in this document. Refer
to the Terminology section of [RFC9524] for definition the definitions of
Replication segment and other terms associated with it and the definition
definitions of Root, Leaf Leaf, and Bud node. nodes.

SR P2MP Policy: An SR P2MP Policy is a framework to construct P2MP
trees in an SR domain by specifying a Root and Leaf nodes.

Tree-ID: An identifier of an SR P2MP Policy in context of the Root
node.

Candidate path: A candidate path (CP) (CP): A CP of the SR P2MP Policy defines topological
or resource constraints and optimization objectives that are used
to compute and construct P2MP tree instances.

P2MP tree instance: A P2MP tree instance (PTI) (PTI): A PTI of a candidate path is constructed
by stitching Replication segments between the Root and Leaf nodes
of an SR P2MP Policy. Its topology is determined by the
constraints and optimization objective of the candidate path.

Instance-ID: An identifier of a P2MP tree instance in context of the
SR P2MP Policy.

Tree-SID: The Replication-SID of the Replication segment at the Root
node of a P2MP tree instance.

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. SR P2MP Policy

An SR P2MP Policy is used to instantiate P2MP trees between a Root and
Leaf nodes in an SR domain. Note, Note that multiple SR P2MP Policies can
have identical Root node nodes and identical set sets of Leaf nodes. An SR
P2MP Policy has one or more candidate paths [RFC9256].

2.1. SR P2MP Policy identification Identification

An SR P2MP Policy is uniquely identified by the tuple <Root, Tree-
ID>, where:

* Root: The IP address of the Root node of P2MP trees instantiated
by the SR P2MP Policy.

* Tree-ID: A 32-bit unsigned integer that uniquely identifies the SR
P2MP Policy in the context of the Root node.

2.2. Components of an SR P2MP Policy

An SR P2MP Policy consists of the following elements:

* Leaf nodes: A set of nodes that terminate the P2MP trees of the SR
P2MP Policy.

* candidate Candidate paths: A set of possible paths that define constraints
and optimization objectives for P2MP tree instances of the SR P2MP
Policy.

An SR P2MP Policy and its CPs are provisioned on a controller (see
Section 5) or the Root node or both both, depending upon the provisioning
model. After provisioning, the Policy and its CPs are instantiated
on the Root node or the controller by using a signalling signaling protocol.

2.3. Candidate Paths and P2MP Tree instances Instances

An SR P2MP Policy has one or more CPs. The tuple <Protocol-Origin,
Originator, Discriminator>, as specified in Section 2.6 of [RFC9256],
uniquely identifies a candidate path in the context of an SR P2MP
Policy. The semantics of Procotol-Origin, Originator Protocol-Origin, Originator, and
Discriminator fields of the identifier are the same as in Section Sections
2.3,
2.4 2.4, and 2.5 of [RFC9256] [RFC9256], respectively.

The Root node of the SR P2MP Policy selects the active candidate path
based on the tie breaking tiebreaking rules defined in Section 2.9 of [RFC9256].

A CP may include topological and/or resource constraints and
optimization objectives which that influence the computation of the PTIs of
the CP.

A candidate path has zero or more PTIs. A candidate path does not
have a PTI when the controller cannot compute a P2MP tree from the
netowrk
network topology based on the constraints and/or optimization
objectives of the CP. A candidate path can have more than one PTIs,
for e.g PTI,
e.g., during the Make-Before-Break (see Section 5.3) procedure to
handle a network state change. However, one and only one PTI MUST be
the active instance of the CP. If more than one PTIs PTI of a CP are is
active at same time, and that CP is the active CP of the SR P2MP
Policy, then duplicate traffic may be delivered to the Leaf nodes.

A PTI is identified by an Instance-ID. This is an unsigned 16-bit
number which that is unique in context of the SR P2MP Policy of the
candidate path.

PTIs are instantiated using Replication segments. Section 2 of
[RFC9524] specifies the Replication-ID of the Replication segment
identifier tuple as a variable length field that can be modified as
required based on the use of a Replication segment. However, length
is an imprecise indicator of the actual structure of the Replication-
ID. This document updates the Replication-ID of a Replication
segment identifier of RFC 9524 [RFC9524] to be the tuple: <Root, Tree-ID,
Instance-ID, Node-ID>, where <Root, Tree-ID> identifies the SR P2MP
Policy and Instance-ID identifies the PTI within that SR P2MP Policy.
This results in the Replication segments used to instantiate a PTI
being identified by the tuple: <Root, Tree-ID, Instance-ID, Node-ID>.
In the simplest case, the Replication-ID of a Replication segment is
a 32-bit number as per Section 2 of RFC 9524. [RFC9524]. For this use case,
the Root MUST be zero (0.0.0.0 for IPv4 and :: for IPv6) and the
Instance-ID MUST be zero and the 32-bit Tree-ID effectively make the
Replication segment identifier <[0.0.0.0 or ::], Tree-ID, 0, Node-
ID>.

PTIs may have different tree topologies due to possibly differing
constraints and optimization objectives of the CPs in an SR P2MP
policy
Policy and across different Policies. policies. Even within a given CP, two
PTIs of that CP, say during the Make-Before-Break procedure, are
likely to have different tree topologies due to a change in the
network state. Since the PTIs may have different tree topologies,
their replication states also differ at various nodes in the SR
domain.
Therefore Therefore, each PTI has its own Replication segment and a
unique Replication-SID at a given node in the SR domain.

A controller designates an active instance of a CP at the Root node
of the SR P2MP Policy by signalling signaling this state through the protocol
used to instantiate the Replication segment of the instance.

This document focuses on the use of a controller to compute and
instantiate PTIs of SR P2MP Policy CPs. It is also feasible to
provision an explicit CP in an SR P2MP Policy with a static tree
topology using NETCONF/YANG or CLI. Note, Note that a static tree topology
will not adapt to any changes in the network state of an SR domain.
The explicit CPs may be provisioned on the controller or the Root
node. When an explicit CP is provisioned on the controller, the
controller bypasses the compute stage and directly instantiates the
PTIs in the SR domain. When an explicit CP is provisioned on the
Root node, the Root node instantiates the PTIs in the SR domain. The
exact procedures for provisioning an explicit CP and the signalling signaling
from the Root node to instantiate the PTIs are outside the scope of
this document.

3. Steering traffic Traffic into an SR P2MP Policy

The Replication-SID of the Replication segment at the Root node is
referred to as the Tree-SID of a PTI. It is RECOMMENDED that the
Tree-SID is also used as the Replication-SID for the Replication
segments at the intermediate Replication nodes and the Leaf nodes of
the PTI as it simplifies operations and troubleshooting. However,
the Replication-SIDs of the Replication segments at the intermediate
Replication nodes and the Leaf nodes MAY differ from the Tree-SID.
For SRv6, Replication-SID is the FUNCT portion of the SRv6 SID segment ID
(SID) [RFC8986] [RFC9524].Note, [RFC9524]. Note that even if the Tree-SID is the
Replication-SID of all the Replication segments of a PTI, the LOC locator
(LOC) portion of the SRv6 SID [RFC8986] differs for the Root node,
the intermediate Replication
nodes nodes, and the Leaf nodes of the PTI.

An SR P2MP Policy has a Binding SID (BSID). The BSID is used to
steer traffic into an SR Policy, as described below, when the Root
node is not the ingress node of the SR domain where the traffic
arrives. The packets are steered from the ingress node to the Root
node using a segment list with the BSID as the last segment in the
list. In this case, it is RECOMMENDED that the BSID of an SR P2MP
Policy SHOULD be constant throughout the lifetime of the Policy policy so
the steering of traffic to the Root node remains unchanged. The BSID
of an SR P2MP Policy MAY be the Tree-SID of the active P2MP instance
of the active CP of the Policy. policy. In this case, the BSID of an SR P2MP
Policy changes when the active CP or the active PTI of the SR P2MP
Policy changes. Note, Note that the BSID is not required to steer traffic
into an SR P2MP Policy when the Root node of an SR P2MP Policy is
also the ingress node of the SR domain where the traffic arrives.

The Root node can steer an incoming packet into an SR P2MP Policy in
one of following methods:

* Local policy-based Local-policy-based forwarding: The Root node maps the incoming
packet to the active PTI of the active CP of an SR P2MP Policy
based on local forwarding policy policy, and it is replicated with the
encapsulated Replication-SIDs of the downstream nodes. The
procedures to map an incoming packet to an SR P2MP Policy are out
of scope of this document. It is RECOMMENDED that an
implementation provide a mechanism to examine the result of
application of the local forwarding policy i.e. policy, i.e., provide
information about the traffic mapped to an SR P2MP Policy and the
active CP and active PTI of the Policy. policy.

* Tree-SID based Tree-SID-based forwarding: The Binding SID, which may be the Tree-
SID of the active PTI, in an incoming packet is used to map the
packet to the active PTI. The Binding SID in the incoming packet
is replaced with the Tree-SID of the active PTI of the active CPand CP,
and the packet is replicated with the Replication-SIDs of the
downstream nodes.

For local policy-based local-policy-based forwarding with SR-MPLS, the TTL for the Root
node SHOULD set the TTL in the encapsulating MPLS header so that the
replicated packet can reach the furthest Leaf node. The Root MAY set
the TTL in the encapsulating MPLS header from the payload. In this
case, the TTL may not be sufficient for the replicated packet to
reach the furthest node. For SRv6, Section 2.2 of [RFC9524] provides
guidance to set the IPv6 Hop Limit of the encapsulating IPv6 header.

4. P2MP tree instance Tree Instance

A P2MP tree instance within an SR domain establishes a forwarding
structure that connects a Root node to a set of Leaf nodes via a
series of intermediate Replication nodes. The tree consists of:

* A Replication segment at the Root node.

* Zero or more Replication segments at intermediate Replication
nodes.

* Replication segments at the Leaf nodes.

4.1. Replication segments Segments at Leaf Nodes

A specific service is identified by a service context in a packet. A
PTI is usually associated with one and only one multi-point multipoint service.
On a Leaf node of such a multi-point multipoint service, the transport
identifier
identifier, which is the Tree-SID or Replication-SID of the
Replication segment at a Leaf node node, is also associated with the
service context because it is not always feasible to separate the
transport and service context with efficient replication in core
since a) multi-point multipoint services may have differing sets of end-points, endpoints and
b) downstream allocation of a service context cannot be encoded in
packets replicated in the core.

A PTI can be associated with one or more multi-point multipoint services on the
Root and Leaf nodes. In SR-MPLS deployments, if it is known a priori
that multi-point multipoint services mapped to an SR-MPLS PTI can be uniquely
identified with their service label, a controller MAY opt not to not
instantiate Replication segments at Leaf nodes. In such cases,
Replication nodes upstream of the Leaf nodes can remove the Tree-SID
from the packet before forwarding it. A multi-point multipoint service context
allocated from an upstream assigned label or Domain-wide Common Block
(DCB), as specified in [RFC9573], is an example of a globally unique
context that facilitates this optimization.

In SRv6 deployments, Replication segments of a PTI MUST be
instantiated on Leaf nodes of the tree since PHP like behavior like
Penultimate Hop Popping (PHP) is not feasible because the Tree-SID is
carried in the IPv6 Destination Address field of the outer IPv6
header. If two or more multi-point multipoint services are mapped to one SRv6
PTI, an SRV6 SID representing the service context is assigned by the
Root node or assigned from the DCB. This SRv6 SID MUST be encoded as
the last segment in the Segment List of the Segment Routing Header
[RFC8754] by the Root node to derive the packet processing context
(PPC) for the service service, as described in Section 2.2 of [RFC9524] [RFC9524], at a
Leaf node.

4.2. Shared Replication segments Segments

A Replication segment MAY be shared across different PTIs. One
simple use of a shared Replication segment is for local protection on
a Replication node. A shared Replication segment can protect
Replication segments of different PTIs against an adjacency or path
failure to the common downstream node of these Replication segments.

A shared Replication segment MUST be identified using a Root set to
zero (0.0.0.0 for IPv4 and :: for IPv6), an Instance-ID set to zero zero,
and a Tree-ID that is unique within the context of the node where the
Replication segment is instantiated. The Root is zero because a
shared Replication segment is not associated with a particular SR
P2MP Policy or a PTI. Note, Note that the shared Replication segment
identifier conforms with the updated Replication-ID definition in
Section 2.3.

It is possible for different PTIs to share a P2MP tree at a
Replication node. This allows a common sub-tree to be shared across
PTIs whose tree topologies are identical in some portion of a an SR
domain. The procedures to share a P2MP tree across PTIs are outside
the scope of this document.

4.3. Packet forwarding Forwarding in a P2MP tree instance Tree Instance

When a packet is steered into a PTI, the Replication segment at the
Root node performs packet replication and forwards copies to
downstream nodes.

* Each replicated packet carries the Replication-SID of the
Replication segment at the downstream node.

* A downstream node can be either:

- A Leaf node, in which case the replication process terminates. terminates,
or

- An intermediate Replication node, which further replicates the
packet through its associated Replication segments until it
reaches all Leaf nodes.

A Replication node and a downstream node can be non-adjacent. In
this case case, the replicated packet has to traverse a path to reach the
downstream node. For SR-MPLS, this is achieved by inserting one or
more SIDs before the downstream Replication SID. Replication-SID. For SRv6, the LOC
[RFC8986] of the downstream Replication-SID can guide the packet to
the downstream node or an optional segment list may be used to steer
the replicated packet on a specific path to the downstream node. For
details of SRv6 replication to non-adjacent downstream node and IPv6
Hop Limit considerations, refer to Section 2.2 of [RFC9524].

5. Using a controller Controller to build Build a P2MP Tree

A controller is instantiated or provisioned with the SR P2MP Policy
and its candidate paths to compute and instantiate PTIs in an SR
domain. The procedures for provisioning or instantiation of these
constructs on a controller are outside the scope of this document.

5.1. SR P2MP Policy on a controller Controller

An SR P2MP Policy is provisioned on a controller by an entity which that
can be an operator, a network node node, or a machine, machine by specifying the
addresses of the Root, the set of Leaf nodes nodes, and the candidate
paths. In this case, the Policy policy and its CPs are instantiated on the
Root node using a signalling signaling protocol. An SR P2MP Policy, its Leaf nodes
nodes, and the CPs may also be provisioned on the Root node and then
instantiated on the controller using a signalling signaling protocol. The
procedures and mechanisms for provisioning and instantiate instantiation of an SR
P2MP Policy and its CPS on a controller or a Root node are outside
the scope of this document.

The possible set of constraints and optimization objective of a CP
are described in Section 3 of
[I-D.filsfils-spring-sr-policy-considerations]. [SR-POLICY]. Other constraints and
optimization objectives MAY be used for P2MP tree computation.

5.2. Controller Functions

A controller performs the following functions in general:

* Topology Discovery: A controller discovers network topology across
Interior Gateway Protocol (IGP) areas, levels levels, or Autonomous
Systems (ASes).

* Capability Exchange: A controller discovers a node's capability to
participate in SR P2MP as well as advertise its capability to
support SR P2MP.

5.3. P2MP Tree Compute

A controller computes one or more PTIs for CPs of an SR P2MP Policy.
A CP may not have any PTI PTIs if a controller cannot compute a P2MP tree
for it.

A controller MUST compute a P2MP tree such that there are no loops in
the tree at steady state as required by [RFC9524].

A controller SHOULD modify a PTI of a candidate path on detecting a
change in the network topology, topology if the change affects the tree
instance,
instance or when a better path can be found based on the new network
state. Alternatively, the controller MAY decide to implement a Make-
Before-Break approach to minimize traffic loss. The controller can
do this by creating a new PTI, activating the new instance once it is
instantiated in the network, and then removing the old PTI.

5.4. SID Management

The controller assigns the Replication-SIDs for the Replication
segments of the PTI.

The Replication-SIDs of a PTI of a CP of an SR P2MP Policy can be
either dynamically assigned by the controller or statically assigned
by the entity provisioning the SR P2MP Policy.

For SR-MPLS, a Replication-SID may be assigned from the SR Local
Block (SRLB) or the SR Global Block (SRGB) [RFC8402]. It is
RECOMMENDED to assign a Replication-SID from the SRLB since
Replication segments are local to each node of the PTI. It is NOT
RECOMMENDED to allocate a Replication-SID from the SRBG SRGB since this
block is globally significant the SR domain any it may get depleted
if significant number of PTIs are instantiated in the SR domain.

Section 3 recommends that the Tree-SID to be used as the Replication-SIDs Replication-
SIDs for all the Replication segments of a PTI. It may be feasible
to allocate the same Tree-SID value for all the Replication segments
if the blocks used for allocation are not identical on all the nodes
of the PTI, PTI or if the particular Tree-SID value in the block is
assigned to some other SID on some node.

A BSID is also assigned for the SR P2MP Policy. The controller MAY
decide to not assign a BSID and allow the Root node of the SR P2MP
Policy to assign the BSID. It is RECOMMENDED to assign the BSID of
an SR P2MP Policy from the SRLB for SR-MPLS.

The controller MAY be provisioned with a reserved block or multiple
reserved blocks for assigning Replication-SIDs and/or the BSIDs for
SR P2MP Policies. a A single block maybe be reserved for the whole SR
domain, or dedicated blocks can be reserved for each node or a group
of nodes in the SR domain. These blocks MAY overlap with either the
SRBG, SRLB
SRGB, the SRLB, or both. The procedures for provisioning these
reserved blocks and procedures for deconflicting assignments from
these reserved blocks with overlapping SRLB or SRGB blocks are
outside the scope of this document.

A controller may not be aware of all the assignments of SIDs from the
SRGB or the SRLB of the SR domain. If reserved blocks are not used,
the assignment of Replication-SIDs or BSIDs of SR P2MP Policies from
these blocks may conflict with other SIDs.

5.5. Instantiating P2MP tree instance Tree Instance on nodes Nodes

After computing P2MP trees, the controller instantiates the
Replication segments that compose the PTIs in the SR domain using
signalling
signaling protocols such as PCEP [I-D.ietf-pce-sr-p2mp-policy], the Path Computation Element
Communication Protocol (PCEP) [SR-P2MP-PING], BGP
[I-D.ietf-idr-sr-p2mp-policy] [P2MP-BGP], or
other mechanisms such as NETCONF/
YANG [I-D.hb-spring-sr-p2mp-policy-yang] , NETCONF/YANG [SR-P2MP-YANG], etc. The
procedures for the instantiation of the Replication segments in an SR
domain are outside the scope of this document.

A node SHOULD report a successful instantiation of a Replication
segment. The exact procedure for reporting this is outside the scope
of this document.

The instantiation of a Replication segment on a node may fail, for
e.g. e.g.,
when the Replication SID Replication-SID conflicts with another SID on the node. The
node SHOULD report this, preferably with a reason for the failure,
using a signalling signaling protocol. The exact procedure for reporting this
failure is outside the scope of this document.

If the instantiation of a Replication segment on a node fails, the
controller SHOULD attempt to re-instantiate the Replication segment.
There SHOULD be an upper bound on the number of attempts. If the
instantiation of a Replication segment ultimately fails after the
allowed number of attempts, the controller SHOULD generate an alert
via mechanisms like syslog. These alerts SHOULD be rate-limited to
protect the logging facility in case Replication segment
instantiation fails on multiple nodes. The controller MAY decide to
tear down the PTI if the instantiations of some of the Replication
segments of the instance fail. The controller is RECOMMENDED to tear
down the PTI if the instantiation of the Replication segment on the
Root node fails. The controller can employ different strategies to
re-try
retry instantiating a PTI after a failure. These are out of scope of
this document.

A PTI should be instantiated within a reasonable time time, especially if
it is the active PTI of a an SR P2MP Policy. One approach is the
controller instantiates the Replication segments in a batch. For
example, the controller instantiates the Replication segments of the
Leaf nodes and the intermediate Replication nodes first. If all of
these Replication segments are successfully instantiated, the
controller next then proceeds to instantiate the Replication segment at
the Root node. If the Replication segment instantiation at the Root
node succeeds, the controller can immediately activate the instance
if it needs to carry traffic of the SR P2MP Policy. A controller can
adopt a similar approach when instantiating the new PTI for the Make-
Before-Break procedure.

5.6. Protection

5.6.1. Local Protection

A network link, node node, or replication branch on a PTI can be protected
using SR Policies [RFC9256]. The backup SR Policies are associated
with replication branches of a Replication segment, segment and are programmed
in the data plane in order to minimize traffic loss when the
protected link/node fails. The segment list of the backup SR
policy Policy
is imposed on the downstream Replication SID Replication-SID of a replication branch
to steer the traffic on the backup path.

It is also possible to use a node local Loop-Free Alternate [RFC5286]
or TI-LFA [I-D.ietf-rtgwg-segment-routing-ti-lfa] Topology Independent Loop-Free Alternate (TI-LFA) [RFC9855]
protection and a Micro-Loop [RFC5715] or SR Micro-Loop
[I-D.bashandy-rtgwg-segment-routing-uloop] [SR-LOOP]
prevention mechanisms mechanism to protect link/nodes the links/nodes of a PTI.

5.6.2. Path Protection

A controller can create a disjoint backup tree instance for providing
end-to-end tree protection if the topology permits. This can be
achieved by having a backup CP with constraints and/or optimization
objectives that ensure its PTIs are disjoint from the PTIs of the
primary/active CP.

6. IANA Considerations

This document makes has no request of IANA. IANA actions.

7. Security Considerations

This document describes how a PTI can be created in an SR domain by
stitching Replication segments together. Some security
considerations for Replication segments outlined in [RFC9524] are
also applicable to this document. Following is a brief reminder of
the same.

An SR domain needs protection from outside attackers as described in
[RFC8402] [RFC8754]
[RFC8402], [RFC8754], and [RFC8986] . [RFC8986].

Failure to protect the SR MPLS domain by correctly provisioning MPLS
support per interface permits attackers from outside the domain to
send packets to receivers of the Multi-point multipoint services that use the SR
P2MP Policies provisioned within the domain.

Failure to protect the SRv6 domain with inbound Infrastructure Access
Control Lists (IACLs) on external interfaces, combined with failure
to implement BCP 38 [RFC2827] the method described in RFC 2827 [BCP38] or apply IACLs
on nodes provisioning SIDs, permits attackers from outside the SR
domain to send packets to the receivers of Multi-point multipoint services that
use the SR P2MP Policies provisioned within the domain.

Incorrect provisioning of Replication segments by a controller that
computes SR PTI can result in a chain of Replication segments forming
a loop. In this case, replicated packets can create a storm till until
MPLS TTL (for SR-MPLS) or IPv6 Hop Limit (for SRv6) decrements to
zero.

The control plane protocols (like PCEP, BGP, etc.) used to
instantiate Replication segments of SR PTI can leverage their own
security mechanisms such as encryption, authentication filtering filtering,
etc.

For SRv6, [RFC9524] describes an exception for Parameter Problem
Message,
message, code 2 ICMPv6 Error messages. If an attacker is able to
inject a packet into Multi-point a multipoint service with the source address of
a node and with an extension header using an unknown option type
marked as mandatory, then a large number of ICMPv6 Parameter Problem
messages can cause a denial-of-service attack on the source node.

8. Acknowledgements

The authors would like to acknowledge Siva Sivabalan, Mike Koldychev
and Vishnu Pavan Beeram for their valuable inputs.

9. 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

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

Kamran Raza Cisco Systems, Inc. Canada

Email:skraza@cisco.com

Anuj Budhiraja Cisco Systems, Inc. US

Email:abudhira@cisco.com

Mankamana Mishra Cisco Systems, Inc. US
Email:mankamis@cisco.com

10. References

10.1.

8.1. Normative 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>.

[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>.

[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.

[RFC9524] Voyer, D., Ed., Filsfils, C., Parekh, R., Bidgoli, H., and
Z. Zhang, "Segment Routing Replication for Multipoint
Service Delivery", RFC 9524, DOI 10.17487/RFC9524,
February 2024, <https://www.rfc-editor.org/info/rfc9524>.

10.2.

8.2. Informative References

[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-17, 29 June 2024,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-17>.

[I-D.filsfils-spring-sr-policy-considerations]
Filsfils, C., Talaulikar, K., Król, P. G., Horneffer, M.,
and

[BCP38] Best Current Practice 38,
<https://www.rfc-editor.org/info/bcp38>.
At the time of writing, this BCP comprises the following:

Ferguson, P. Mattes, "SR Policy Implementation and Deployment
Considerations", Work in Progress, Internet-Draft, draft-
filsfils-spring-sr-policy-considerations-09, 24 April
2022, <https://datatracker.ietf.org/doc/html/draft-
filsfils-spring-sr-policy-considerations-09>.

[I-D.hb-spring-sr-p2mp-policy-yang]
Bidgoli, H., Voyer, D., Parekh, R., Saad, T., and T.
Kundu, "YANG Data Model for p2mp sr policy", Work in
Progress, Internet-Draft, draft-hb-spring-sr-p2mp-policy-
yang-02, 30 October 2020,
<https://datatracker.ietf.org/doc/html/draft-hb-spring-sr-
p2mp-policy-yang-02>.

[I-D.ietf-idr-sr-p2mp-policy] D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.

[P2MP-BGP] Bidgoli, H., Voyer, D., Stone, A., Parekh, R., Krier, S.,
and S. Agrawal, "Advertising p2mp policies in BGP", Work
in Progress, Internet-Draft, draft-ietf-idr-sr-p2mp-
policy-00, 27 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
p2mp-policy-00>.

[I-D.ietf-pce-sr-p2mp-policy]
Bidgoli, H., Voyer, D., Budhiraja, A., Parekh, R., and S.
Sivabalan, "PCEP extensions for P2MP SR Policy", Work in
Progress, Internet-Draft, draft-ietf-pce-sr-p2mp-policy-
11, 19 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
p2mp-policy-11>.

[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
21, 12 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-21>.

[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>.

[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.

[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.

[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.

[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>.

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

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

[RFC9573] Zhang, Z., Rosen, E., Lin, W., Li, Z., and IJ. Wijnands,
"MVPN/EVPN Tunnel Aggregation with Common Labels",
RFC 9573, DOI 10.17487/RFC9573, May 2024,
<https://www.rfc-editor.org/info/rfc9573>.

[RFC9855] Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute Using Segment Routing", RFC 9855,
DOI 10.17487/RFC9855, October 2025,
<https://www.rfc-editor.org/info/rfc9855>.

[SR-LOOP] Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-17, 29 June 2024,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-17>.

[SR-P2MP-PING]
Bidgoli, H., Voyer, D., Budhiraja, A., Parekh, R., and S.
Sivabalan, "PCEP extensions for SR P2MP Policy", Work in
Progress, Internet-Draft, draft-ietf-pce-sr-p2mp-policy-
14, 23 February 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
p2mp-policy-14>.

[SR-P2MP-YANG]
Bidgoli, H., Voyer, D., Parekh, R., Saad, T., and T.
Kundu, "YANG Data Model for p2mp sr policy", Work in
Progress, Internet-Draft, draft-hb-spring-sr-p2mp-policy-
yang-02, 30 October 2020,
<https://datatracker.ietf.org/doc/html/draft-hb-spring-sr-
p2mp-policy-yang-02>.

[SR-POLICY]
Filsfils, C., Talaulikar, K., Król, P. G., Horneffer, M.,
and P. Mattes, "SR Policy Implementation and Deployment
Considerations", Work in Progress, Internet-Draft, draft-
filsfils-spring-sr-policy-considerations-09, 24 April
2022, <https://datatracker.ietf.org/doc/html/draft-
filsfils-spring-sr-policy-considerations-09>.

Appendix A. Illustration of the SR P2MP Policy and P2MP Tree

Consider the following topology:

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

Figure 1: SR Toplogy Topology

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

For SRv6, the reader is expected to be familiar with SRv6 Network
Programming [RFC8986] to follow the examples.

* 2001:db8::/32 is an IPv6 block allocated by a RIR Regional Internet
Registry (RIR) to the operator operator.

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

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

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

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

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

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

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

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

* Function :C1n: (function C1n for short) represents the End.X
function to node n with Ultimate Segment Decapsulation (USD) (USD).

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:C1j::/128 bound to
an End.X function to neighbor J with additional support for USD

Assume a controller is provisioned with the following SR P2MP Policy
at Root R1 with Tree-ID T-ID:

SR P2MP Policy <R1,T-ID>:
Leaf nodes: {R2, R6, R7}
candidate-path 1:
Optimize: IGP metric
Tree-SID: T-SID1

The controller is responsible for computing a PTI of the candidate
path. In this example, we assume one active PTI with Instance-ID
I-ID1. Assume the controller instantiates PTIs by signalling signaling
Replication segments i.e. segments, i.e., the Replication-ID of these Replication
segments is <Root, Tree-ID, Instance-ID>. All Replication segments
use the Tree-SID T-SID1 as the Replication-SID. For SRv6, assume the
Replication-SID at node k, bound to an End.Replicate function, is
2001:db8:cccc:k:fa::/128.

A.1. P2MP Tree with non-adjacent Non-Adjacent Replication Segments

Assume the controller computes a PTI with Root node R1, Intermediate
and Leaf node R2, and Leaf nodes R6 and R7. The controller
instantiates the instance by stitching Replication segments at R1,
R2, R6 R6, and R7. The Replication segment at R1 replicates to R2. The
Replication segment at R2 replicates to R6 and R7. Note that nodes
R3, R4 R4, and R5 do not have any Replication segment state for the
tree.

A.1.1. SR-MPLS

The Replication segment state at nodes R1, R2, R6 R6, and R7 is shown
below.

Replication segment at R1:

Replication segment <R1,T-ID,I-ID1,R1>:
Replication-SID: T-SID1
Replication State:
R2: <T-SID1->L12>

Replication to R2 steers a packet directly to the node on interface
L12.

Replication segment at R2:

Replication segment <R1,T-ID,I-ID1,R2>:
Replication-SID: T-SID1
Replication State:
R2: <Leaf>
R6: <N-SID6, T-SID1>
R7: <N-SID7, T-SID1>

R2 is a Bud node. It performs the role of a Leaf as well as a
transit node replicating to R6 and R7. Replication to R6, using
N-SID6, steers a packet via IGP shortest path to that node.
Replication to R7, using N-SID7, steers a packet via IGP shortest
path to R7 via either R5 or R4 based on ECMP hashing.

Replication segment at R6:

Replication segment <R1,T-ID,I-ID1,R6>:
Replication-SID: T-SID1
Replication State:
R6: <Leaf>

Replication segment at R7:

Replication segment <R1,T-ID,I-ID1,R7>:
Replication-SID: T-SID1
Replication State:
R7: <Leaf>

When a packet is steered into the active instance candidate path 1 of
the SR P2MP Policy at R1:

* Since R1 is directly connected to R2, R1 performs the PUSH
operation with just the <T-SID1> label for the replicated copy and
sends it to R2 on interface L12.

* R2, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload. For replication to R6, R2 performs a
PUSH operation of N-SID6, N-SID6 to send the <N-SID6,T-SID1> label stack
to R3. R3 is the penultimate hop for N-SID6; it performs
penultimate hop popping, which corresponds to the NEXT operation operation,
and the packet is then sent to R6 with <T-SID1> in the label
stack. For replication to R7, R2 performs a PUSH operation of N-SID7,
N-SID7 to send the <N-SID7,T-SID1> label stack to R4, one of IGP
ECMP nexthops towards R7. R4 is the penultimate hop for N-SID7;
it performs penultimate hop popping, which corresponds to the NEXT operation
operation, and the packet is then sent to R7 with <T-SID1> in the
label stack.

* R6, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload.

* R7, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload.

A.1.2. SRv6

For SRv6, the replicated packet from R2 to R7 has to traverse R4
using an SR Policy, Policy27. The Policy policy has one SID in the segment
list: End.X function with USD of R4 to R7 . R7. The Replication segment
state at nodes R1, R2, R6 R6, and R7 is shown below.

Policy27: <2001:db8:cccc:4:c17::>

Replication segment at R1:

Replication segment <R1,T-ID,I-ID1,R1>:
Replication-SID: 2001:db8:cccc:1:fa::
Replication State:
R2: <2001:db8:cccc:2:fa::->L12>

Replication to R2 steers a packet directly to the node on interface
L12.

Replication segment at R2:

Replication segment <R1,T-ID,I-ID1,R2>:
Replication-SID: 2001:db8:cccc:2:fa::
Replication State:
R2: <Leaf>
R6: <2001:db8:cccc:6:fa::>
R7: <2001:db8:cccc:7:fa:: -> Policy27>

R2 is a Bud node. It performs the role of a Leaf as well as a
transit node replicating to R6 and R7. Replication to R6, R6 steers a
packet via IGP shortest path to that node. Replication to R7, via an
SR Policy, first encapsulates the packet using H.Encaps and then
steers the outer packet to R4. End.X USD on R4 decapsulates the
outer header and sends the original inner packet to R7.

Replication segment at R6:

Replication segment <R1,T-ID,I-ID1,R6>:
Replication-SID: 2001:db8:cccc:6:fa::
Replication State:
R6: <Leaf>

Replication segment at R7:

Replication segment <R1,T-ID,I-ID1,R7>:
Replication-SID: 2001:db8:cccc:7:fa::
Replication State:
R7: <Leaf>

When a packet (A,B2) is steered into the active instance of candidate
path 1 of the SR P2MP Policy at R1 using H.Encaps.Replicate behavior:

* Since R1 is directly connected to R2, R1 sends the replicated copy
(2001:db8::1, 2001:db8:cccc:2:fa::) (A,B2) to R2 on interface L12.

* R2, as Leaf a Leaf, removes the outer IPv6 header and delivers the
payload. R2, as a bud Bud node, also replicates the packet.

* - For replication to R6, R2 sends (2001:db8::1,
2001:db8:cccc:6:fa::) (A,B2) to R3. R3 forwards the packet
using the 2001:db8:cccc:6::/64 packet to R6.

- For replication to R7 using Policy27, R2 encapsulates and sends
(2001:db8::2, 2001:db8:cccc:4:C17::) (2001:db8::1,
2001:db8:cccc:7:fa::) (A,B2) to R4. R4 performs End.X USD
behavior, decapsulates the outer IPv6 header header, and sends
(2001:db8::1, 2001:db8:cccc:7:fa::) (A,B2) to R7.

* R6, as a Leaf, removes the outer IPv6 header and delivers the
payload.

* R7, as a Leaf, removes the outer IPv6 header and delivers the
payload.

A.2. P2MP Tree with adjacent Adjacent Replication Segments

Assume the controller computes a PTI with Root node R1, Intermediate
and Leaf node R2, Intermediate nodes R3 and R5, and Leaf nodes R6 and
R7. The controller instantiates the PTI by stitching Replication
segments at R1, R2, R3, R5, R6 R6, and R7. The Replication segment at
R1 replicates to R2. The Replication segment at R2 replicates to R3
and R5. The Replication segment at R3 replicates to R6. The
Replication segment at R5 replicates to R7. Note that node R4 does
not have any Replication segment state for the tree.

A.2.1. SR-MPLS

The Replication segment state at nodes R1, R2, R3, R5, R6 R6, and R7 is
shown below.

Replication segment at R1:

Replication segment <R1,T-ID,I-ID1,R1>:
Replication-SID: T-SID1
Replication State:
R2: <T-SID1->L12>

Replication to R2 steers a packet directly to the node on interface
L12.

Replication segment at R2:

Replication segment <R1,T-ID,I-ID1,R2>:
Replication-SID: T-SID1
Replication State:
R2: <Leaf>
R3: <T-SID1->L23>
R5: <T-SID1->L25>

R2 is a Bud node. It performs the role of a Leaf as well as a
transit node replicating to R3 and R5. Replication to R3, R3 steers a
packet directly to the node on L23. Replication to R5, R5 steers a
packet directly to the node on L25.

Replication segment at R3:

Replication segment <R1,T-ID,I-ID1,R3>:
Replication-SID: T-SID1
Replication State:
R6: <T-SID1->L36>

Replication to R6, R6 steers a packet directly to the node on L36.

Replication segment at R5:

Replication segment <R1,T-ID,I-ID1,R5>:
Replication-SID: T-SID1
Replication State:
R7: <T-SID1->L57>

Replication to R7, R7 steers a packet directly to the node on L57.

Replication segment at R6:

Replication segment <R1,T-ID,I-ID1,R6>:
Replication-SID: T-SID1
Replication State:
R6: <Leaf>

Replication segment at R7:

Replication segment <R1,T-ID,I-ID1,R7>:
Replication-SID: T-SID1
Replication State:
R7: <Leaf>

When a packet is steered into the SR P2MP Policy at R1:

* Since R1 is directly connected to R2, R1 performs the PUSH
operation with just the <T-SID1> label for the replicated copy and
sends it to R2 on interface L12.

* R2, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload. It also performs the PUSH operation on
T-SID1 for replication to R3 and R5. For replication to R6, R2
sends the <T-SID1> label stack to R3 on interface L23. For
replication to R5, R2 sends the <T-SID1> label stack to R5 on
interface L25.

* R3 performs the NEXT operation on T-SID1 and T-SID1, performs a PUSH
operation for replication to R6 R6, and sends the <T-SID1> label
stack to R6 on interface L36.

* R5 performs the NEXT operation on T-SID1 and T-SID1, performs a PUSH
operation for replication to R7 R7, and sends the <T-SID1> label
stack to R7 on interface L57.

* R6, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload.

* R7, as a Leaf, performs the NEXT operation, pops the T-SID1 label label,
and delivers the payload.

A.2.2. SRv6

The Replication segment state at nodes R1, R2, R3, R5, R6 R6, and R7 is
shown below.

Replication segment at R1:

Replication segment <R1,T-ID,I-ID1,R1>:
Replication-SID: 2001:db8:cccc:1:fa::
Replication State:
R2: <2001:db8:cccc:2:fa::->L12>

Replication to R2 steers a packet directly to the node on interface
L12.

Replication segment at R2:

Replication segment <R1,T-ID,I-ID1,R2>:
Replication-SID: 2001:db8:cccc:2:fa::
Replication State:
R2: <Leaf>
R3: <2001:db8:cccc:3:fa::->L23>
R5: <2001:db8:cccc:5:fa::->L25>

Replication segment at R3:

Replication segment <R1,T-ID,I-ID1,R3>:
Replication-SID: 2001:db8:cccc:3:fa::
Replication State:
R6: <2001:db8:cccc:6:fa::->L36>

Replication to R6, R6 steers a packet directly to the node on L36.

Replication segment at R5:

Replication segment <R1,T-ID,I-ID1,R5>:
Replication-SID: 2001:db8:cccc:5:fa::
Replication State:
R7: <2001:db8:cccc:7:fa::->L57>

Replication to R7, R7 steers a packet directly to the node on L57.

Replication segment at R6:

Replication segment <R1,T-ID,I-ID1,R6>:
Replication-SID: 2001:db8:cccc:6:fa::
Replication State:
R6: <Leaf>

Replication segment at R7:

Replication segment <R1,T-ID,I-ID1,R7>:
Replication-SID: 2001:db8:cccc:7:fa::
Replication State:
R7: <Leaf>

When a packet (A,B2) is steered into the active instance of candidate
path 1 of the SR P2MP Policy at R1 using the H.Encaps.Replicate
behavior:

* Since R1 is directly connected to R2, R1 sends the replicated copy
(2001:db8::1, 2001:db8:cccc:2:fa::) (A,B2) to R2 on interface L12.

* R2, as a Leaf, removes the outer IPv6 header and delivers the
payload. R2, as a bud Bud node, also replicates the packet. For
replication to R3, R2 sends (2001:db8::1, 2001:db8:cccc:3:fa::)
(A,B2) to R3 on interface L23. For replication to R5, R2 sends
(2001:db8::1, 2001:db8:cccc:5:fa::) (A,B2) to R5 on interface L25.

* R3 replicates and sends (2001:db8::1, 2001:db8:cccc:6:fa::) (A,B2)
to R6 on interface L36.

* R5 replicates and sends (2001:db8::1, 2001:db8:cccc:7:fa::) (A,B2)
to R7 on interface L57.

* R6, as a Leaf, removes the outer IPv6 header and delivers the
payload.

* R7, as a Leaf, removes the outer IPv6 header and delivers the
payload.

Acknowledgements

The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
and Vishnu Pavan Beeram for their valuable input.

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,
United States of America
Email: arvvenka@cisco.com

Zafar Ali
Cisco Systems, Inc.
United States of America
Email: zali@cisco.com

Swadesh Agrawal
Cisco Systems, Inc.
San Jose,
United States of America
Email: swaagraw@cisco.com

Jayant Kotalwar
Nokia
Mountain View,
United States of America
Email: jayant.kotalwar@nokia.com

Tanmoy Kundu
Nokia
Mountain View,
United States of America
Email: tanmoy.kundu@nokia.com

Andrew Stone
Nokia
Ottawa
Canada
Email: andrew.stone@nokia.com

Tarek Saad
Juniper Networks
Canada
Email: tsaad@juniper.net

Kamran Raza
Cisco Systems, Inc.
Canada
Email: skraza@cisco.com

Anuj Budhiraja
Cisco Systems, Inc.
United States of America
Email: abudhira@cisco.com

Mankamana Mishra
Cisco Systems, Inc.
United States of America
Email: mankamis@cisco.com

Authors' Addresses

Rishabh Parekh (editor)
Arrcus
San Jose,
United States of America
Email: rishabh@arrcus.com

Daniel Voyer (editor)
Cisco Systems, Inc.
Montreal
Canada
Email: davoyer@cisco.com

Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com

Hooman Bidgoli
Nokia
Ottawa
Canada
Email: hooman.bidgoli@nokia.com

Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net