Segment Routing Point-to-Multipoint Policy

A Multi-point service delivery can be realized with P2MP trees 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)". A Segment Routing P2MP policy, a variant of the SR Policy , defines the Root and Leaf nodes of a P2MP tree. One or more Candidate Paths define optional constraints and/or optimization objectives for the tree. A PCE computes the tree from the Root node to the set of Leaf nodes via a set of Replication nodes based on a SR P2MP Policy. The PCE then instantiates the P2MP tree in the SR domain by signaling Replication segments to Root, Replication and Leaf nodes using protocols such as PCEP, BGP, NetConf etc. The Replication segments of a P2MP tree can be instantiated for SR-MPLS and SRv6 dataplanes.

An SR P2MP policy is a variant of an SR policy and is used to instantiate SR P2MP trees. A SR P2MP Policy is identified by the tuple <Root, Tree-ID>, where: Root: The address of Root node of a P2MP trees instantiated by the SR P2MP Policy Tree-ID: A identifier that is unique in context of the Root. This is an unsigned 32-bit number. A SR P2MP Policy is defined by following elements: Leaf nodes: A set of nodes that terminate the P2MP trees. Candidate Paths: See below. A SR P2MP policy is provisioned on a PCE to compute and instantiate P2MP trees. A PCE computes the P2MP tree instances of a policy and instantiates Replication segments at Root, Replication and Leaf nodes of the trees. The Root and Tree-ID of the SR P2MP policy are mapped to Replication-ID element of the Replication segment identifier. A SR P2MP Policy has one or more Candidate paths. Each candidate path has optional topological/resource constraints and/or optimization objectives that determine the P2MP trees computed for that Candidate path. The Root node selects the active Candidate path based on the tie breaking rules amongst the candidate-paths as specified in. A Candidate path has zero or more P2MP tree instances. Instance-ID is the identifier of an instance of a Candidate path. This is an unsigned 16-bit number which is unique in context of the SR P2MP policy of the Candidate path. The identifier of Replication segments used to instantiate an instance is <Root-ID, Tree-ID, Instance-ID, Node-ID>. The PCE designates an Active instance of a Candidate Path at the Root node of SR P2MP policy by signalling this state in the protocol used to instantiate the Replication segment of the instance. The Tree-SID ( below) is an identifier of a P2MP tree instance in the forwarding plane. It is instantiated in the forwarding plane at Root node, intermediate Replication nodes and Leaf nodes of the P2MP tree of an instance. The Tree-SID of the active instance of the active Candidate path SHOULD be used as Binding SID of the SR P2MP policy. The Root node steers an incoming packet of a Multi-point service into a SR P2MP policy in one of two ways: Based on a local policy-based routing at the Root node. This packet is carried by the active instance of the active Candidate path of the policy. Based on the Tree-SID (Binding SID) in the incoming packet.

A P2MP tree in a SR domain connects a Root to a set of Leaf nodes via a set of intermediate Replication nodes. It consists of a Replication segment at the Root stitched to zero or more Replication segments at intermediate Replication nodes, and eventually the Replication segments at Leaf nodes. The Replication SID of the Replication segment at Root node is called the Tree-SID. The Tree-SID SHOULD also be the Replication-SID of Replication segments at Replication and Leaf nodes. The Replication segments at Replication and Leaf nodes MAY have Replication-SIDs that are not same as the Tree-SID. A Replication Segment MAY be shared by P2MP tree instances, e.g. for protection. A shared Replication Segment MAY be identified with zero Root-ID address (0.0.0.0 for IPv4 and :: for IPv6) and a Replication-ID that is unique in context of Node address where the Replication segment is instantiated. A shared Replication Segment MUST NOT be associated with a SR P2MP tree. For SR-MPLS, a PCE MAY decide not to instantiate Replication segments at Leaf nodes of a P2MP tree if it is known a priori that Multi-point services mapped to the P2MP tree can be identified using a context that is globally unique in SR domain. In this case, Replication nodes upstream to the Leaf nodes effectively implement Penultimate-Hop Pop (PHP) behavior to pop Tree-SID from a packet. A Multi-point service context assigned from "Domain-wide Common Block" (DCB) is an example of a globally unique context. A packet steered into a P2MP tree instance is replicated by the Replication segment at Root node to its downstream nodes. A replicated packet has the Replication-SID of the Replication segment at a downstream node. A downstream node could be a Leaf node or an intermediate Replication Node. In the latter case, the packet is replicated through Replication segments till it reaches all the Leaf nodes.

A P2MP tree can be instantited by a Path Computation Element (PCE). This section outlines a high-level architecture for such an approach.

A SR P2MP policy can be instantiated and maintained in a using a Path Computation Element (PCE).

North-bound APIs on a PCE can be used to: Create SR P2MP policy: CreateSRP2MPPolicy<Root, Tree-ID> Delete SR P2MP policy: DeleteSRP2MPPolicy<Root, Tree-ID> Modify SR P2MP policy Leaf Set: SRP2MPPolicyLeafSetModify<Root, Tree-ID, {Leaf Set}> Create a Candidate Path for SR P2MP policy: CreateSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>> Delete a Candidate Path for SR P2MP policy: DeleteSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>> Update a Candidate Path for SR P2MP policy: UpdateSRP2MPCandidatePath<Root, Tree-ID, <CP-ID>, Preference, Constraints, Optimization, ...> CP-ID is identifier of a Candidate Path within a SR P2MP policy. One possible identifier is the tuple <Protocol-Origin, originator, discriminator> as specified in . Note these are conceptual APIs. Actual implementations may offer different APIs as long as they provide same functionality. For example, API might allow symbolic name to be assigned for a P2MP policy or APIs might allow individual Leaf nodes to be added or deleted from a policy instead of an update operation.

Interaction with a PCE can be via PCEP, REST, Netconf, gRPC, CLI. A YANG model shall be be developed for this purpose as well.

An entity (an operator, a network node or a machine) provisions a SR P2MP policy by specifying the addresses of the root (R) and set of leaves {L} as well as Traffic Engineering (TE) attributes of Candidate paths via a suitable North-Bound API. The PCE computes one or more instances of P2MP trees of a candidate path. The PCE MAY compute P2MP trees for all Candidate paths. If tree computation is successful, PCE instantiates the P2MP tree instance(s) using Replication segments on Root, Replication, and Leaf nodes. A Candidate path may not have any instance of P2MP tree if PCE cannot compute a tree. Candidate path constraints shall include link color affinity, bandwidth, disjointness (link, node, SRLG), delay bound, link loss, flexible algorithm etc. Candidate path shall be optimized based on IGP or TE metric or link latency. Other constraints and optimization objectives MAY be used for P2MP tree computation. The Tree SID of an instance of a Candidate path of a SR P2MP policy can be either dynamically allocated by the PCE or statically assigned by entity provisioning the SR P2MP policy. Ideally, same Tree-SID SHOULD be used for Replication segments at Root, Replication, and Leaf nodes. Different Tree-SIDs MAY be used at Replication Node(s) if it is not feasible to use same Tree SID. A PCE can modify a P2MP tree of a Candidate path on detecting a change in the network topology or in case a better path can be found based on the new network state. In this case, the PCE MAY create a new instance of a P2MP tree and remove the old instance of the tree from the network in order to minimize traffic loss. A PCE shall be capable of computing paths across multiple IGP areas or levels as well as Autonomous Systems (ASs).

A PCE shall learn network topology, TE attributes of link/node as well as SIDs via dynamic routing protocols (IGP and/or BGP-LS). It may be possible for entities to pass topology information to PCE via north-bound API.

It shall be possible for a node to advertise SR P2MP tree capability via IGP, BGP-LS and/or PCEP. Similarly, a PCE can also advertise its P2MP tree computation capability via IGP, BGP-LS and/or PCEP. Capability advertisement allows a network node to dynamically choose one or more PCE(s) to obtain services pertaining to SR P2MP policies, as well a PCE to dynamically identify SR P2MP tree capable nodes.

A PCE MUST compute a P2MP tree such that there are no loops in the tree at steady state (Section 2 of ). An OPTIONAL algorithm to compute a loop free tree is listed below, Given SR P2MP Polciy with Root (R) and Leaf node set (LS), a Candidate path of the policy with constraints(C) and optimization objective(O), and Constrained Shortest Path First(CSPF) algorithm to compute a path between a pair of nodes:

= {} S02. For each Leaf(L) in LS { S03. Path P = Compute CSPF(R, L, C, O) S04. Add P to PS S05. } S06. Tree = Merge(PS)]]> Notes: Specification of CSPF algorithm is outside the scope of this document Path Set Merge function merges individual paths resulting in a tree of Root, intermediate Replication and Leaf nodes. The specfication of this function is outside the scope of this document. A PCE MAY implement other tree computation algorithm(s) which MUST guarantee loop prevention or loop detection and mitigation at steady state.

Once a PCE computes a P2MP tree for an instance of a Candidate path of a SR P2MP policy, it needs to instantiate the tree on the relevant network nodes via Replication segments. The PCE can use various protocols to program the Replication segments as described below.

PCE Protocol (PCEP) has been traditionally used: For a head-end to obtain paths from a PCE. A PCE to instantiate SR policies. PCEP protocol can be stateful in that a PCE can have a stateful control of an SR policy on a head-end which has delegated the control of the SR policy to the PCE. PCEP shall be extended to provision and maintain SR P2MP trees in a stateful fashion.

BGP has been extended to instantiate and report SR policies. It shall be extended to instantiate and maintain P2MP trees for SR P2MP policies.

A network link, node or path on the instance of a P2MP tree can be protected using SR policies computed by PCE. The backup SR policies shall be programmed in forwarding plane in order to minimize traffic loss when the protected link/node fails. It is also possible to use node local Loop-Free Alternate protection mechanisms (LFA) to protect link/nodes of P2MP tree.

It is possible for PCE create a disjoint backup tree for providing end-to-end path protection.

This document makes no request of IANA.

This document describes how a P2MP tree can be created in an SR domain by stitching Replication Segments together. Some security considerations for Replication Segments outlined in are also appicable to this document. Following is a brief reminder of the same. An SR domain needs protection from outside attackers as described in . 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 Multipoint services that use the SR P2MP trees 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 Multipoint services that use the SR P2MP trees provisioned within the domain. Incorrect provisioning of Replication segments by a PCE that computes SR P2MP tree instance 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 P2MP tree instance 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 Multipoint 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

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. This document re-uses SID allocation scheme, reproduced below, from Illustrations in SRv6 Network Programming 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 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 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 PCE 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 PCE is responsible for computing a P2MP tree instance of the Candidate Path. In this example, we assume one active instance of P2MP tree with Instance-ID I-ID1. Assume PCE instantiates P2MP trees 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 PCE computes a P2MP tree instance with Root node R1, Intermediate and Leaf node R2, and Leaf nodes R6 and R7. The PCE 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 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 packet via IGP shortest path to that node. Replication to R7, using N-SID7, steers 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 a SR-TE 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 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 packet via IGP shortest path to that node. Replication to R7, via SR-TE 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 PCE computes a P2MP tree with Root node R1, Intermediate and Leaf node R2, Intermediate nodes R3 and R5, and Leaf nodes R6 and R7. The PCE instantiates the P2MP tree instance 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 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 packet directly to the node on L23. Replication to R5, steers packet directly to the node on L25. Replication segment at R3:

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

: Replication-SID: T-SID1 Replication State: R7: L57> ]]> Replication to R7, steers 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 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 packet directly to the node on L23. Replication to R5, steers 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 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 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.