Segment Routing Point-to-Multipoint Policy

RFC 9524 defines a Replication segment which enables an SR node to replicate traffic to multiple downstream nodes in an SR domain . A 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 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 service delivery can be efficiently realized with a P2MP tree in a Segment Routing 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 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) 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 and Leaf nodes. A Path Computation Element (PCE) is one example of such a controller. In other cases, a P2MP tree instance can be installed using NETCONF/YANG or Command Line Interface(CLI) on the Root, Replication and the Leaf nodes. The Replication segments of a P2MP tree instance can be instantiated for SR-MPLS and SRv6 data planes, enabling efficient packet replication within an SR domain. This document updates Replication-ID portion of a Replication segment identifier specified in Section 2 of .

This section defines terms used frequently in this document. Refer to Terminology section of for definition of Replication segment and other terms associated with it and the definition of Root, Leaf and Bud node. 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) of 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) of a candidate path is constructed by stitching Replication segments between Root and Leaf nodes of an SR P2MP Policy. Its topology is determined by 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.

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

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.

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 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 ) or the Root node or 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 protocol.

An SR P2MP Policy has one or more CPs. The tuple <Protocol-Origin, Originator, Discriminator>, as specified in Section 2.6 of , uniquely identifies a candidate path in the context of an SR P2MP Policy. The semantics of Procotol-Origin, Originator and Discriminator fields of the identifier are same as in Section 2.3, 2.4 and 2.5 of respectively. The Root node of the SR P2MP Policy selects the active candidate path based on the tie breaking rules defined in Section 2.9 of . A CP may include topological and/or resource constraints and optimization objectives which 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 topology based on the constraints and/or optimization objectives of the CP. A candidate path can have more than one PTIs, for e.g during Make-Before-Break (see ) 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 of a CP are active at same time, and that CP is the active CP of 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 is unique in context of the SR P2MP Policy of the candidate path. PTIs are instantiated using Replication segments. Section 2 of specifies 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 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, Replication-ID of a Replication segment is a 32-bit number as per Section 2 of RFC 9524. 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 and across different Policies. Even within a given CP, two PTIs of that CP, say during 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 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 SR P2MP Policy by signalling 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, 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 from the Root node to instantiate the PTIs are outside the scope of this document.

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 .Note, even if the Tree-SID is the Replication-SID of all the Replication segments of a PTI, the LOC portion of the SRv6 SID differs for the Root node, the intermediate Replication 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 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. 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, 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 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 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. provide information about the traffic mapped to an SR P2MP Policy and the active CP and active PTI of the Policy. 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 active CPand the packet is replicated with the Replication-SIDs of the downstream nodes. For local policy-based forwarding with SR-MPLS, the TTL the Root node SHOULD set the TTL in encapsulating MPLS header so that the replicated packet can reach the furthest Leaf node. The Root MAY set the TTL in 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 provides guidance to set the IPv6 Hop Limit of the encapsulating IPv6 header.

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.

A specific service is identified by a service context in a packet. A PTI is usually associated with one and only one multi-point service. On a Leaf node of such a multi-point service, the transport identifier which is the Tree-SID or Replication-SID of the Replication segment at a Leaf 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 services may have differing sets of end-points, and b) downstream allocation of service context cannot be encoded in packets replicated in the core. A PTI can be associated with one or more multi-point services on the Root and Leaf nodes. In SR-MPLS deployments, if it is known a priori that multi-point services mapped to an SR-MPLS PTI can be uniquely identified with their service label, a controller MAY opt not to 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 service context allocated from an upstream assigned label or Domain-wide Common Block (DCB), as specified in , 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 is not feasible because the Tree-SID is carried in IPv6 Destination Address field of outer IPv6 header. If two or more multi-point services are mapped to one SRv6 PTI, an SRV6 SID representing the service context is assigned by the Root node or assigned from DCB. This SRv6 SID MUST be encoded as the last segment in the Segment List of the Segment Routing Header by the Root node to derive the packet processing context (PPC) for the service as described in Section 2.2 of at a Leaf node.

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), Instance-ID set to 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, the shared Replication segment identifier conforms with the updated Replication-ID definition in . 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 SR domain. The procedures to share a P2MP tree across PTIs are outside the scope of this document.

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. 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 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. For SRv6, the LOC of 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 .

A controller is instantiated or provisioned with 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.

An SR P2MP Policy is provisioned on a controller by an entity which can be an operator, a network node or a machine, by specifying the addresses of the Root, the set of Leaf nodes and the candidate paths. In this case, the Policy and its CPs are instantiated on the Root node using a signalling protocol. An SR P2MP Policy, its Leaf nodes and the CPs may also be provisioned on the Root node and then instantiated on the controller using a signalling protocol. The procedures and mechanisms for provisioning and instantiate 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 . Other constraints and optimization objectives MAY be used for P2MP tree computation.

A controller performs the following functions in general: Topology Discovery: A controller discovers network topology across Interior Gateway Protocol (IGP) areas, 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.

A controller computes one or more PTIs for CPs of an SR P2MP Policy. A CP may not have any PTI 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 . A controller SHOULD modify a PTI of a candidate path on detecting a change in the network topology, if the change affects the tree instance, or when a better path can be found based on the new network state. Alternatively, the controller MAY decide 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.

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 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) . 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 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. recommends the Tree-SID to be used as the 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, 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 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.

After computing P2MP trees, the controller instantiates the Replication segments that compose the PTIs in the SR domain using signalling protocols such as PCEP , BGP or other mechanisms such as NETCONF/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. when the Replication SID conflicts with another SID on the node. The node SHOULD report this, preferably with a reason for the failure, using a signalling 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 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 instantiating a PTI after a failure. These are out of scope of this document. A PTI should be instantiated within a reasonable time especially if it is the active PTI of a 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 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 Make-Before-Break procedure.

A network link, node or replication branch on a PTI can be protected using SR Policies . The backup SR Policies are associated with replication branches of a Replication 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 is imposed on the downstream Replication SID of a replication branch to steer the traffic on the backup path. It is also possible to use node local Loop-Free Alternate or TI-LFA protection and Micro-Loop or SR Micro-Loop prevention mechanisms to protect link/nodes of a PTI.

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.

This document makes no request of IANA.

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 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 and . 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 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 or apply IACLs on nodes provisioning SIDs, permits attackers from outside the SR domain to send packets to the receivers of Multi-point 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 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 etc. For SRv6, describes an exception for Parameter Problem Message, code 2 ICMPv6 Error messages. If an attacker is able to inject a packet into Multi-point service with source address of a node and with an extension header using 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.

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

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

Consider the following topology:

In these examples, the Node-SID of a node Rn is N-SIDn and Adjacency-SID from node Rm to node Rn is A-SIDmn. Interface between Rm and Rn is Lmn. For SRv6, the reader is expected to be familiar with SRv6 Network Programming to follow the examples. 2001:db8::/32 is an IPv6 block allocated by a RIR to the operator 2001:db8:0::/48 is dedicated to the internal address space 2001:db8:cccc::/48 is dedicated to the internal SRv6 SID space We assume a location expressed in 64 bits and a function expressed in 16 bits node k has a classic IPv6 loopback address 2001:db8::k/128 which is advertised in the IGP node k has 2001:db8:cccc:k::/64 for its local SID space. Its SIDs will be explicitly assigned from that block node k advertises 2001:db8:cccc:k::/64 in its IGP Function :1:: (function 1, for short) represents the End function with Penultimate Segment Pop (PSP) support Function :Cn:: (function Cn, for short) represents the End.X function to node n Function :C1n: (function C1n for short) represents the End.X function to node n with Ultimate Segment Decapsulation (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 following SR P2MP Policy at Root R1 with Tree-ID 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 Replication segments i.e. Replication-ID of these Replication segments is <Root, Tree-ID, Instance-ID>. All Replication segments use the Tree-SID T-SID1 as Replication-SID. For SRv6, assume the Replication-SID at node k, bound to an End.Replicate function, is 2001:db8:cccc:k:fa::/128.

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 and R7. Replication segment at R1 replicates to R2. Replication segment at R2 replicates to R6 and R7. Note nodes R3, R4 and R5 do not have any Replication segment state for the tree.

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

: Replication-SID: T-SID1 Replication State: R2: L12> ]]> Replication to R2 steers a packet directly to the node on interface L12. Replication segment at R2:

: Replication-SID: T-SID1 Replication State: R2: R6: R7: ]]> R2 is a Bud node. It performs role of 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-SID: T-SID1 Replication State: R6: ]]> Replication segment at R7:

: Replication-SID: T-SID1 Replication State: R7: ]]> When a packet is steered into the active instance candidate path 1 of SR P2MP Policy at R1: Since R1 is directly connected to R2, R1 performs PUSH operation with just <T-SID1> label for the replicated copy and sends it to R2 on interface L12. R2, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload. For replication to R6, R2 performs a PUSH operation of N-SID6, to send <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 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, to send <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 and the packet is then sent to R7 with <T-SID1> in the label stack. R6, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload. R7, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload.

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

]]> Replication segment at 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-SID: 2001:db8:cccc:2:fa:: Replication State: R2: R6: <2001:db8:cccc:6:fa::> R7: <2001:db8:cccc:7:fa:: -> Policy27>]]> R2 is a Bud node. It performs role of Leaf as well as a transit node replicating to R6 and R7. Replication to 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 outer header and sends the original inner packet to R7. Replication segment at R6:

: Replication-SID: 2001:db8:cccc:6:fa:: Replication State: R6: ]]> Replication segment at R7:

: Replication-SID: 2001:db8:cccc:7:fa:: Replication State: R7: ]]> When a packet (A,B2) is steered into the active instance of candidate path 1 of SR P2MP Policy at R1 using H.Encaps.Replicate behavior: Since R1 is directly connected to R2, R1 sends replicated copy (2001:db8::1, 2001:db8:cccc:2:fa::) (A,B2) to R2 on interface L12. R2, as Leaf removes outer IPv6 header and delivers the payload. R2, as a 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 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 outer IPv6 header and sends (2001:db8::1, 2001:db8:cccc:7:fa::) (A,B2) to R7. R6, as Leaf, removes outer IPv6 header and delivers the payload. R7, as Leaf, removes outer IPv6 header and delivers the payload.

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 and R7. Replication segment at R1 replicates to R2. Replication segment at R2 replicates to R3 and R5. Replication segment at R3 replicates to R6. Replication segment at R5 replicates to R7. Note node R4 does not have any Replication segment state for the tree.

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

: Replication-SID: T-SID1 Replication State: R2: L12> ]]> Replication to R2 steers a packet directly to the node on interface L12. Replication segment at R2:

: Replication-SID: T-SID1 Replication State: R2: R3: L23> R5: L25>]]> R2 is a Bud node. It performs role of Leaf as well as a transit node replicating to R3 and R5. Replication to R3, steers a packet directly to the node on L23. Replication to R5, steers a packet directly to the node on L25. Replication segment at R3:

: Replication-SID: T-SID1 Replication State: R6: L36> ]]> Replication to R6, steers a packet directly to the node on L36. Replication segment at R5:

: Replication-SID: T-SID1 Replication State: R7: L57> ]]> Replication to R7, steers a packet directly to the node on L57. Replication segment at R6:

: Replication-SID: T-SID1 Replication State: R6: ]]> Replication segment at R7:

: Replication-SID: T-SID1 Replication State: R7: ]]> When a packet is steered into the SR P2MP Policy at R1: Since R1 is directly connected to R2, R1 performs PUSH operation with just <T-SID1> label for the replicated copy and sends it to R2 on interface L12. R2, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload. It also performs PUSH operation on T-SID1 for replication to R3 and R5. For replication to R6, R2 sends <T-SID1> label stack to R3 on interface L23. For replication to R5, R2 sends <T-SID1> label stack to R5 on interface L25. R3 performs NEXT operation on T-SID1 and performs a PUSH operation for replication to R6 and sends <T-SID1> label stack to R6 on interface L36. R5 performs NEXT operation on T-SID1 and performs a PUSH operation for replication to R7 and sends <T-SID1> label stack to R7 on interface L57. R6, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload. R7, as Leaf, performs NEXT operation, pops T-SID1 label and delivers the payload.

The Replication segment state at nodes R1, R2, R3, R5, R6 and R7 is shown below. Replication segment at 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-SID: 2001:db8:cccc:2:fa:: Replication State: R2: R3: <2001:db8:cccc:3:fa::->L23> R5: <2001:db8:cccc:5:fa::->L25>]]> R2 is a Bud node. It performs role of Leaf as well as a transit node replicating to R3 and R5. Replication to R3, steers a packet directly to the node on L23. Replication to R5, steers a packet directly to the node on L25. Replication segment at R3:

: Replication-SID: 2001:db8:cccc:3:fa:: Replication State: R6: <2001:db8:cccc:6:fa::->L36> ]]> Replication to R6, steers a packet directly to the node on L36. Replication segment at R5:

: Replication-SID: 2001:db8:cccc:5:fa:: Replication State: R7: <2001:db8:cccc:7:fa::->L57> ]]> Replication to R7, steers a packet directly to the node on L57. Replication segment at R6:

: Replication-SID: 2001:db8:cccc:6:fa:: Replication State: R6: ]]> Replication segment at R7:

: Replication-SID: 2001:db8:cccc:7:fa:: Replication State: R7: ]]> When a packet (A,B2) is steered into the active instance of candidate path 1 of SR P2MP Policy at R1 using H.Encaps.Replicate behavior: Since R1 is directly connected to R2, R1 sends replicated copy (2001:db8::1, 2001:db8:cccc:2:fa::) (A,B2) to R2 on interface L12. R2, as Leaf, removes outer IPv6 header and delivers the payload. R2, as a 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 Leaf, removes outer IPv6 header and delivers the payload. R7, as Leaf, removes outer IPv6 header and delivers the payload.