RLB (Replication through Local Bitstring) Segment for Multicast Source Routing over IPv6

MSR6(Multicast Source Routing over IPv6) is an IPv6 based multicast source routing (MSR6) solution, defined in , which leverages the benefits of source routing over IPv6 data plane to provide simplified multicast TE and BE service in an IPv6 network without unnecessary multicast tree status and complex control plane protocols. MSR6 needs to reuse the advantages of SRv6 and BIER to implement source routing. MSR6 has two basic modes of forwarding: one is based on Shortest Path First(SPF), which is called MSR6 BE mode; the other is based on traffic engineered, which is called MSR6 TE mode. and have introduced structured segment list by defining arguments in each segment. Based on these existing work, this document defines 3 new types of segment with local bitstring, which could reduce encapsulation overhead and enhance scalability.

MSR6: Multicast Source Routing over IPv6, defined in . MRH: Multicast Routing Header, a new type of Routing Header which is used for MSR6. Multicast domain: A set of network device which could provide P2MP multicast transport. In this document, the multicast domain is an MSR domain, where every nodes support the capability of MSR6. Root Node: Root node is the beginning point of a multicast tree and also the initiation node of a multicast tunnel. It encapsulates the packet with a multicast header. The type of the encapsulation depends on the routing protocol used in the multicast domain. For MSR6 TE, the encapsulation is MSR6 TE header, which is an IPv6 header with MRH. Replication Endpoint: the intermediate node of a multicast tree, which replicates packet and forwards the packet to the downstream nodes. For MSR6, the Replication Node is called Replication Endpoint which can be indicated by the MSR6 Segment and replicate packets according to the multicast source routing information encapsulation in the MSR6 header of the packet. Leaf Node: Leaf node is the end point of a multicast tree and also the decapsulation node of a multicast tunnel. It decapsulates the multicast header in the packet and forwards the packet based on overlay encapsulation. Parent Node: The parent node is the node that does the packet replication, corresponding to the concept of a child node. Child Node: The child node is the downstream node that will receive the packet which has been replicated ,corresponding to the concept of a parent node. End.RLB(Replication through Local Bitstring): Endpoint with the function of Replication through Local Bitstring End.RLB(Replication through Local Bitstring).X: Endpoint with the function of Replication through Local Bitstring and defining output port. LB(Local Bitstring)Segment: Segment with Local Bitstring.

For a specific node, the downstream port can be represented by a specific bit in the local bitstring of the node. If the packet needs to be replicated through the downstream port, the bit MUST be set. Or else, the bit MUST be set as 0.

The segment defined in can represent instruction, topological or service based on the requirement. This document specifies RLB.X segment for multicast path encoding. End.RLB.X is a new type of segment which is used to identify the Replication Endpoint, which is able to replicate the packet based on the local bitstring in the segment. RLB.X segment is used as an IPv6 address, which is 128 bits and follows the SID format defined in , consisting of LOC:FUNCT:ARG. RLB.X segment is advertised by the RLB.X replication endpoint.

Local Bitstring indicates a set of adjacency links that the packet are supposed to be replicated to and Pointer is used to locate the segment the packet will parse in the next replication endpoint. Each replication node maintains the mapping relationship of the bit position and adjacency link in the Local Bitstring Forwarding Table(LBFT).

The replication node does the following when the DA of the packet is a local End.RLB.X SID.

max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1 S13. Replicate the packet based on the local bitstring in the End.RLB.X and look up LBFT S14. Set the Segment Left of the 1st replicated packet to "Pointer" in the argument of the SID S15. Update IPv6 DA with Segment List[Segment Left] S16. Forward the packet through the adjacency link indicated by local bitstring S17. Repeat S14-S17 with "Pointer+n"(n=the number of packets which have been updated) untill all the packets are transmitted through the indicated link R18. }]]>

End.RLB is a new type of segment which is used to identify the Replication Endpoint, which is able to replicate the packet based on the local bitstring in the correponding LB segment. RLB segment is used as an IPv6 address, which is 128 bits and follows the SID format defined in [RFC8986], consisting of LOC:FUNCT:ARG. RLB segment is advertised by the RLB replication endpoint.

When RLB Segment is used, there MUST be a corresponding LB Segment in the segment list. LB Segment is a special segment which is 128-bits containing the local bitstring. That is, LB segment does not take the format of LOC:FUNCT:ARG and need not to define the behavior. There is an argument in End.LB to indicate the next LB segments for the downstream nodes, for example "pointer " defined in .

End.RLB segment and LB segment are used together. Each replication node maintains the mapping relationship of the bit position with the adjacency link and the next hop End.RLB SID in the Local Bitstring Forwarding Table(LBFT).

The replication node does the following when the DA of the packet is a local End.RLB SID.

max_LE) or (Segments Left > Last Entry+1)) { S10. Send an ICMP Parameter Problem to the Source Address with Code 0 (Erroneous header field encountered) and Pointer set to the Segments Left field, interrupt packet processing, and discard the packet. S11. } S12. Decrement IPv6 Hop Limit by 1 S13. Replicate the packet based on the local bitstring in the SID[Segment Left] and look up LBFT S14. Set the Segment Left of the 1st replicated packet to "Pointer" in the argument of the SID S15. Update IPv6 DA with End.RLB indicated by the first bit set in the LBFT S16. Forward the packet through the adjacency link indicated by the first bit set in the LBFT S17. Repeat S14-S17 with "Pointer+n"(n=the number of packets which have been updated) untill all the packets are transmitted through the indicated link R18. }]]>

MSR-R<--|->MSR End<-|-->MSR-L<--| MSR-R: MSR6 Root Node, including Node A; MSR End: MSR6 Replication Endpoint, including Node B and C; MSR-L: MSR6 Leaf Nodes, including Node D, E, F, G;]]> Take End.RLB and End.LB as an example. Acording to the requirement of the service, a multicast tree is calculated as showed in the above figure.

In node A, the packet is encapsulated with an IPv6 header with MRH as an extension header. The segment list in the MRH is as follows, which is composed of LB SID.

Node A will receive the packet with the destination address of the IPv6 header is a local SID for node A and the function is End.RLB.X. Local Bitstring in the segment will be used as an entry to look up LBFT in node A. So the packet will be replicated into 2 packets and sent out through A1 and A2. The destination address of the replicated packet will be set to SID[2] and SID[3] based on the value of the pointer.

(preamble) Node B will receive the packet with the destination address of the IPv6 header is a local SID for node B and the function is End.RLB.X. Local Bitstring in the segment will be used as an entry to look up LBFT in node B. So the packet will be replicated into 2 packets and sent out through B2 and B4. The destination address of the replicated packet will be set to address-D and address-E based on the LBFT.

(preamble) Node C will receive the packet with the destination address of the IPv6 header is a local SID for node C and the function is End.RLB.X. Local Bitstring in the segment will be used as an entry to look up LBFT in node C. So the packet will be replicated into 2 packets and sent out through C4 and C7. The destination address of the replicated packet will be set to address-F and address-G based on the LBFT.

In node A, the packet is encapsulated with an IPv6 header with MRH as an extension header. The segment list in the MRH is as follows, which is composed of LB SID.

Node A will receive the packet with the destination address of the IPv6 header is a local End.RLB for node A, which is SID A. Based on the End.RLB behavior defined in section 4, node A will parse SID[Segment Left] in MRH, which is SID[1]. SID[1] will be used as an entry to look up LBFT in node A. So the packet will be replicated into 2 packets. For the 1st packet, the destination address of the IPv6 header will be set to SID B, the Segment Left will be set to 2 based on the pointer and the packet will be sent out through A1. For the 2nd packet, the destination address of the IPv6 header will be set to SID C, the Segment Left will be set to 2+1 based on the pointer and the packet will be sent out through A2.

Node B will receive the packet with the destination address of the IPv6 header is a local End.RLB for node B, which is SID B. Based on the End.RLB behavior defined in section 4, the node will parse SID[Segment Left] in MRH, which is SID[2]. SID[2] will be used as an entry to look up LBFT in node B. So the packet will be replicated into 2 packets. For the 1st packet, the destination address of the IPv6 header will be set to SID D, the Segment Left will be set to 0 based on the pointer and the packet will be sent out through B2. For the 2nd packet, the destination address of the IPv6 header will be set to SID E, the Segment Left will be set to 0 based on the pointer and the packet will be sent out through B4.

Node C will receive the packet with the destination address of the IPv6 header is a local End.RLB for node C, which is SID C. Based on the End.RLB behavior defined in section 4, the node will parse SID[Segment Left] in MRH, which is SID[3]. SID[3] will be used as an entry to look up LBFT in node C. So the packet will be replicated into 2 packets. For the 1st packet, the destination address of the IPv6 header will be set to SID F, the Segment Left will be set to 0 based on the pointer and the packet will be sent out through C4. For the 2nd packet, the destination address of the IPv6 header will be set to SID G, the Segment Left will be set to 0 based on the pointer and the packet will be sent out through C7.

Node D will receive the packet with the destination address of the IPv6 header is a local End.RLB for node D, which is SID D. Because the Segment Left is 0, the packet will decapsulate the IPv6 header with the MRH. Node E,F,G will do the same.

This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC.