SR Replication Segment for Multi-point Service Delivery

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 which is forwarded to the Downstream node. For leaf nodes the inner packet is forwarded as per local configuration.¶

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.¶

There MAY be SIDs preceding the 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. An Anycast SID or BGP PeerSID MUST NOT appear in segment list preceding a Replication SID. There MAY be SIDs after the Replication SID in the segment list 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. The processing of SIDs following the Replication SID MUST NOT forward the SR-MPLS packet to another node.¶

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 to Downstream nodes and/or locally delivered off tree (when this is a bud/leaf node) according to the associated replication state. 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 traffic on a traffic engineered path to a Downstream node. If SRv6 SID compression is possible [I-D.ietf-spring-srv6-srh-compression], the Replication node SHOULD use a CSID container with Downstream Replication SID as the Last uSID in the container instead of H.Encaps.Red.¶

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 the local delivery on a bud/leaf node, the action associated with Replication-SID is "look at next SID in SRH". The next SID could be a SID with End.DTMC4/6 or End.DT2M local behavior (equivalent of MVPN/EVPN PMSI label in case of tunnel sharing across multiple VPNs). There may also not be a next SID (e.g. MVPN/EVPN with one tunnel per VPN), in which case the Replication-SID is then equivalent to End.DTM4/6 or End.DT2M. Note that decapsulation is not an inherent action of a Replication segment even on a bud/leaf node.¶

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. An Anycast SID or BGP PeerSID MUST NOT appear in segment list preceding a Replication SID. 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. The processing of SIDs following the Replication SID MUST NOT forward the SRv6 packet to some other node. The restrictions described in this paragraph apply to both un-compressed and compressed SRv6 encapsulation.¶

4.1. Cisco implmentation

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 multipoint 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 documentaion 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 encap 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>.

[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>.

[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>.

9.2. Informative References

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

Filsfils, C., Garvia, P. C., 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://www.ietf.org/archive/id/draft-filsfils-spring-srv6-net-pgm-illustration-04.txt>.

[I-D.ietf-lsr-flex-algo]

Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", Work in Progress, Internet-Draft, draft-ietf-lsr-flex-algo-25, 6 October 2022, <https://www.ietf.org/archive/id/draft-ietf-lsr-flex-algo-25.txt>.

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

(editor), D. V., Filsfils, C., Parekh, R., Bidgoli, H., and Z. Zhang, "Segment Routing Point-to-Multipoint Policy", Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-policy-05, 2 July 2022, <https://www.ietf.org/archive/id/draft-ietf-pim-sr-p2mp-policy-05.txt>.

[I-D.ietf-spring-srv6-srh-compression]

Cheng, W., Filsfils, C., Li, Z., Decraene, B., Cai, D., Voyer, D., Clad, F., Zadok, S., James Guichard, N., Aihua, L., Raszuk, R., and C. Li, "Compressed SRv6 Segment List Encoding in SRH", Work in Progress, Internet-Draft, draft-ietf-spring-srv6-srh-compression-02, 11 July 2022, <https://www.ietf.org/archive/id/draft-ietf-spring-srv6-srh-compression-02.txt>.

[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>.

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.¶

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 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.Replcate 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.Replcate 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.¶

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.¶