SR Replication Segment for Multi-point Service Delivery

1.1. Terminology

• Multi-point Service: A service that has multiple endpoints. A packet is delivered to all the endpoints.¶
• Replication Segment: A segment in SR domain that replicates packets.¶
• Replication Node: A node in SR domain which replication packets based on Replication Segment¶
• Downstream Nodes: A Replication Node replicates packets to a set of Downstream Nodes¶
• Replication-ID: Identifier of a Replication Segment at Replication Node¶
• Replication State: This is state of Replication Segment at a Replication Node. It is conceptually a list of replication branches to Downstream nodes. The list can be empty.¶
• Replication SID: Data plane identifier of a Replication Segment. This is a SR-MPLS label or SRv6 SID.¶

2.1. SR-MPLS data plane

When the Active Segment is a Replication SID, the processing results in a POP operation and lookup of the associated Replication state. For each replication in the Replication state, the operation is a PUSH of the downstream Replication SID and an optional segment list on to the packet to steer the packet to the Downstream node.¶

For Leaf/Bud nodes local delivery off tree is per local configuration. For some usages, this may involve looking at the next SID for example to get the necessary context.¶

When the root of a multi-point service steers a packet to a Replication segment, it results in a replication to each Downstream node in the associated replication state. The operation is a PUSH of the replication SID and an optional segment list on to the packet which is forwarded to the downstream node.¶

SIDs MAY be added before the downstream SR-MPLS Replication SID in order to guide a packet from a non-adjacent SR node to a Replication Node. A Replication Node MAY replicate a packet to a non-adjacent Downstream Node using SIDs it inserts in the copy preceding the downstream Replication SID. The Downstream Node may be leaf node of the Replication Segment, or another Replication Node, or both in case of bud node. A Replication Node MAY use an Anycast SID or BGP PeerSet SID in segment list to send a replicated packet to one downstream Replication node in an Anycast set if and only if all nodes in the set have an identical Replication SID and reach the same set of receivers.. For some use cases, there MAY be SIDs after the Replication SID in the segment list of a packet. These SIDs are used only by the Leaf/Bud nodes to forward a packet off the tree independent of the Replication SID. Coordination regarding the absence or presence and value of context information for Leaf/Bud nodes is outside the scope of this document.¶

2.2. SRv6 data plane

In SRv6 [RFC8986], the “Endpoint with replication” behavior (End.Replicate for short) replicates a packet and forwards the packet according to a Replication state.¶

When processing a packet destined to a local Replication SID, the packet is replicated according to the associated replication state to Downstream nodes and/or locally delivered off tree when this is a Leaf/Bud node. IPv6 Hop Limit MUST be decremented and MUST be non-zero to replicate an incoming packet. For replication, the outer header is re-used, and the Downstream Replication SID is written into the outer IPv6 header destination address. If required, an optional segment list may be used on some branches using H.Encaps.Red (while some other branches may not need that). Note that this H.Encaps.Red is independent from the replication segment – it is just used to steer the replicated packet on a traffic engineered path to a Downstream node. The pen-ultimate segment in encapsulating IPv6 header will execute USD flavor of End/End.X behavior and forward the inner (replicated) packet to the Downstream node.¶

The above also applies when the Replication segment is for the Root node, whose upstream node has placed the Replication-SID in the header. A local application (e.g. MVPN/EVPN) may also apply H.Encaps.Red and then steer the resulting traffic into the segment. Again note that the H.Encaps.Red is independent of the Replication segment – it is the action of the application (e.g. MVPN/EVPN service). If the service is on a Root node, the two H.Encaps mentioned, one for the service and other in the previous paragraph for replication to Downstream node SHOULD be combined for optimization (to avoid extra IPv6 encapsulation).¶

For Leaf/Bud nodes local delivery off the tree is per Replication SID or next SID (if present in SRH). For some usages, this may involve getting the necessary context either from the next SID (e.g., MVPN with shared tree) or from the replication SID itself (e.g., MVPN with non-shared tree). In both cases, the context association is achieved with signaling and is out of scope of this document¶

There MAY be SIDs preceding the SRv6 Replication SID in order to guide a packet from a non-adjacent SR node to a Replication Node via an explicit path. A Replication Node MAY steer a replicated packet on an explicit path to a non-adjacent Downstream Node using SIDs it inserts in the copy preceding the downstream Replication SID. The Downstream Node may be leaf node of the Replication Segment, or another Replication Node, or both in case of bud node. For SRv6, as described in above paragraphs, the insertion of SIDs prior to Replication SID entails a new IPv6 encapsulation with SRH, but this can be optimized on Root node or for compressed SRv6 SIDs. Note that locator of Replication SID is sufficient to guide a packet on IGP shortest path, for default or Flex algo, between non-adjacent nodes. A Replication Node MAY use an Anycast SID or BGP PeerSet SID in segment list to send a replicated packet to one downstream Replication node in an Anycast set if and only if all nodes in the set have an identical Replication SID and reach the same set of receivers. There MAY be SIDs after the Replication SID in the SRH of a packet. These SIDs are used to provide additional context for processing a packet locally at the node where the Replication SID is the Active Segment. Coordination regarding the absence or presence and value of context information for Leaf/Bud nodes is outside the scope of this document.¶

Replication State:
{
  Node-Role: {Head, Transit, Leaf, Bud};
  # On Leaf, replication list is zero length
  Replication-List:
  {
    Downstream Node: <Node-Identifier>;
    Downstream Replication SID: R-SID;
    # Segment-List maybe be empty
    Segment-List: [SID-1, .... SID-N];
  }
}

S01. Replicate(RS, packet)
S02. {
S03.    For each Replication R in RS.Replication-List {
S04.       Make a copy of the packet
S05.       Set IPv6 DA = RS.R-SID
S06.       If RS.Segment-List is not empty {
S07.         # Head node MAY optimize below encap and
S08.         # the encap of packet in a single encap
S09.         Execute H.Encaps or H.Encaps.Red with RS.Segment-List
             on packet copy #RFC 8986 Section 5.1, 5.2
S10.       }
S11.       Submit the packet to the egress IPv6 FIB lookup and
           transmission to the new destination
S12.   }
S13. }

S01.   Lookup FUNCT portion of S to get Replication State RS
S02.   If (IPv6 Hop Limit <= 1) {
S03.     Discard the packet
S04.     # ICMP Time Exceeded is not permitted (Section 2.2.3 below)
S05.   }
S06.   If RS is not found {
S07.     Discard the packet
S08.   }
S09.   Decrement IPv6 Hop Limit by 1
S10.   If (IPv6 NH == SRH and SRH TLVs present) {
S11.     Process SRH TLVs if allowed by local configuration
S12.   }
S13.   Call Replicate(RS, packet)
S14.   If (RS.Node-Role == Leaf or RS.Node-Role == Bud) {
S15.     If (IPv6 NH == SRH and Segments Left > 0) {
S16.       Derive packet processing context(PPC)
           from Segment List[Segments Left - 1]
S17.     } Else {
S18.       Derive packet processing context(PPC)
           from FUNCT of Replication SID
S19.     }
S20.     Remove the outer IPv6 header with all its extension headers
S21.     Process the packet in context of PPC
S21.   }

4.1. Cisco implementation

Cisco Implementation uses Replication Segments defined in this draft as a basis for PCE to compute and establish P2MP trees in SR domain to provide multi-point services. The implementation, based on latest version of this draft, is in production and supports all MUST and SHOULD clauses for SR-MPLS Replication segments. The documentation is available at Cisco documentation and the point of contact is Rishabh Parekh (riparekh@cisco.com).¶

4.2. Nokia implementation

Nokia has implemented replication SID as defined in this draft to establish P2MP tree in segment routing domain. The implementation supports SR-MPLS encapsulation and has all the Must and SHOULD clause in this draft. The implementation is at general availability maturity and is compliant with the latest version of the draft. The documentation for implementation can be found at Nokia help and the point of contact is hooman.bidgoli@nokia.com.¶

9.1. 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>.

[RFC4443]

Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, <https://www.rfc-editor.org/info/rfc4443>.

[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>.

[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>.

[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>.

[RFC9259]

Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M. Chen, "Operations, Administration, and Maintenance (OAM) in Segment Routing over IPv6 (SRv6)", RFC 9259, DOI 10.17487/RFC9259, June 2022, <https://www.rfc-editor.org/info/rfc9259>.

9.2. Informative References

[I-D.filsfils-spring-srv6-net-pgm-illustration]

Filsfils, C., Camarillo, P., Li, Z., Matsushima, S., Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and J. Leddy, "Illustrations for SRv6 Network Programming", Work in Progress, Internet-Draft, draft-filsfils-spring-srv6-net-pgm-illustration-04, 30 March 2021, <https://datatracker.ietf.org/doc/html/draft-filsfils-spring-srv6-net-pgm-illustration-04>.

[I-D.ietf-pim-sr-p2mp-policy]

Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z. J. Zhang, "Segment Routing Point-to-Multipoint Policy", Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-policy-05, 2 July 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-pim-sr-p2mp-policy-05>.

[RFC6513]

Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February 2012, <https://www.rfc-editor.org/info/rfc6513>.

[RFC7432]

Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A., Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February 2015, <https://www.rfc-editor.org/info/rfc7432>.

[RFC7942]

Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, <https://www.rfc-editor.org/info/rfc7942>.

[RFC7988]

Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress Replication Tunnels in Multicast VPN", RFC 7988, DOI 10.17487/RFC7988, October 2016, <https://www.rfc-editor.org/info/rfc7988>.

[RFC9350]

Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, February 2023, <https://www.rfc-editor.org/info/rfc9350>.

A.1. SR-MPLS

In this example, 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. The state representation uses "R-SID->Lmn" to represent a packet replication with outgoing replication SID R-SID sent on interface Lmn.¶

Assume a Replication segment identified with R-ID at Replication Node R1 and downstream Nodes R2, R6 and R7. The Replication SID at node n is R-SIDn. A packet replicated from R1 to R7 has to traverse R4.¶

The Replication segment state at nodes R1, R2, R6 and R7 is shown below. Note nodes R3, R4 and R5 do not have state for the Replication segment.¶

Replication segment at R1:¶

Replication segment <R-ID,R1>:
 Replication SID: R-SID1
 Replication State:
   R2: <R-SID2->L12>
   R6: <N-SID6, R-SID6>
   R7: <N-SID4, A-SID47, R-SID7>

Replication to R2 steers packet directly to R2 on interface L12. Replication to R6, using N-SID6, steers packet via IGP shortest path to that node. Replication to R7 is steered via R4, using N-SID4 and then adjacency SID A-sID47 to R7.¶

Replication segment at R2:¶

Replication segment <R-ID,R2>:
 Replication SID: R-SID2
 Replication State:
   R2: <Leaf>

Replication segment at R6:¶

Replication segment <R-ID,R6>:
 Replication SID: R-SID6
 Replication State:
   R6: <Leaf>

Replication segment at R7:¶

Replication segment <R-ID,R7>:
 Replication SID: R-SID7
 Replication State:
   R7: <Leaf>

When a packet is steered into the Replication segment at R1:¶

• Since R1 is directly connected to R2, R1 performs PUSH operation with just <R-SID2> label for the replicated copy and sends it to R2 on interface L12. R2, as Leaf, performs NEXT operation, pops R-SID2 label and delivers the payload.¶
• R1 performs PUSH operation with <N-SID6, R-SID6> label stack for the replicated copy to R6 and sends it to R2, the nexthop on IGP shortest path to R6. R2 performs CONTINUE operation on N-SID6 and forwards it 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 <R-SID6> in the label stack. R6, as Leaf, performs NEXT operation, pops R-SID6 label and delivers the payload.¶
• R1 performs PUSH operation with <N-SID4, A-SID47, R-SID7> label stack for the replicated copy to R7 and sends it to R2, the nexthop on IGP shortest path to R4. R2 is the penultimate hop for N-SID4; it performs penultimate hop popping, which corresponds to the NEXT operation and the packet is then sent to R4 with <A-SID47, R-SID1> in the label stack. R4 performs NEXT operation, pops A-SID47, and delivers packet to R7 with <R-SID7> in the label stack. R7, as Leaf, performs NEXT operation, pops R-SID7 label and delivers the payload.¶

A.2. SRv6

For SRv6 , we use SID allocation scheme, reproduced below, from Illustrations for SRv6 Network Programming [I-D.filsfils-spring-srv6-net-pgm-illustration]¶

• 2001:db8::/32 is an IPv6 block allocated by a Regional Internet Registry (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 from to Node n¶

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:Fk::/128 bound to an End.Replicate function¶

Assume a Replication segment identified with R-ID at Replication Node R1 and downstream Nodes R2, R6 and R7. The Replication SID at node k, bound to an End.Replicate function, is 2001:db8:cccc:k:Fk::/128. A packet replicated from R1 to R7 has to traverse R4.¶

The Replication segment state at nodes R1, R2, R6 and R7 is shown below. Note nodes R3, R4 and R5 do not have state for the Replication segment. The state representation uses "R-SID->Lmn" to represent a packet replication with outgoing replication SID R-SID sent on interface Lmn.¶

Replication segment at R1:¶

Replication segment <R-ID,R1>:
 Replication SID: 2001:db8:cccc:1:F1::0
 Replication State:
   R2: <2001:db8:cccc:2:F2::0->L12>
   R6: <2001:db8:cccc:6:F6::0>
   R7: <2001:db8:cccc:4:C7::0, 2001:db8:cccc:7:F7::0>

Replication to R2 steers packet directly to R2 on interface L12. Replication to R6, using 2001:db8:cccc:6:F6::0, steers packet via IGP shortest path to that node. Replication to R7 is steered via R4, using End.X SID 2001:db8:cccc:4:C7::0 at R4 to R7.¶

Replication segment at R2:¶

Replication segment <R-ID,R2>:
 Replication SID: 2001:db8:cccc:2:F2::0
 Replication State:
   R2: <Leaf>

Replication segment at R6:¶

Replication segment <R-ID,R6>:
 Replication SID: 2001:db8:cccc:6:F6::0
 Replication State:
   R6: <Leaf>

Replication segment at R7:¶

Replication segment <R-ID,R7>:
 Replication SID: 2001:db8:cccc:7:F7::0
 Replication State:
   R7: <Leaf>

When a packet, (A,B2), is steered into the Replication segment at R1:¶

• Since R1 is directly connected to R2, R1 creates encapsulated replicated copy (2001:db8::1, 2001:db8:cccc:2:F2::0) (A, B2), and sends it to R2 on interface L12. R2, as Leaf, removes outer IPv6 header and delivers the payload.¶
• R1 creates encapsulated replicated copy (2001:db8::1, 2001:db8:cccc:6:F6::0) (A, B2) then forwards the resulting packet on the shortest path to 2001:db8:cccc:6::/64. R2 and R3 forward the packet using 2001:db8:cccc:6::/64. R6, as Leaf, removes outer IPv6 header and delivers the payload.¶
• R1 creates encapsulated replicated copy (2001:db8::1, 2001:db8:cccc:4:C7::0) (2001:db8:cccc:7:F7::0; SL=1) (A, B2) and sends it to R2, the nexthop on IGP shortest path to 2001:db8:cccc:4::/64. R2 forwards packet to R4 using 2001:db8:cccc:4::/64. R4 executes End.X function on 2001:db8:cccc:4:C7::0, performs PSP action, removes SRH and sends resulting packet (2001:db8::1, 2001:db8:cccc:7:F7::0) (A, B2) to R7. R7, as Leaf, removes outer IPv6 header and delivers the payload.¶