Protocol Independent Multicast Light (PIM Light)

RFC section 4.3.1 describes the PIM neighbor discovery via Hello messages. In section 4.5 it describes that if a router receives a Join/Prune message from a particular IP source address and it has not seen a PIM Hello message from that source address, then the Join/Prune message SHOULD be discarded without further processing. In certain scenarios, it is desirable to establish multicast states between two layer-3 adjacent routers without forming a PIM neighborship. This can be necessary for various reasons, such as signaling multicast states upstream between multiple PIM domains over a network that is not optimized for PIM or does not necessitate PIM Neighbor establishment. For example, in a Bit Index Explicit Replication (BIER) networks connecting multiple PIM domains, where PIM Join/Prune messages are tunneled via BIER as specified in . A PIM Light Interface (PLI) accepts Join/Prune messages from an unknown PIM router without requiring a PIM Hello message from the router. The absence of Hello messages on a PLI means there is no mechanism to discover neighboring PIM routers or their capabilities, nor to execute basic algorithms such as Designated Router (DR) election . Consequently, the PIM Light router does not create any general-purpose state for neighboring PIM routers and only processes Join/Prune messages from downstream routers in its multicast routing table. Processing these Join/Prune messages will introduce multicast states in a PIM Light router. Due to these constraints, a PLI should be deployed in very specific scenarios where PIM-SM is not suitable. The applications or the networks that PLIs are deployed on MUST ensure there is no multicast packet duplication, such as multiple upstream routers sending the same multicast stream to a single downstream router. As an example the implementation should ensure that DR election is done on upstream Redundant PIM routers that are at the edge of the PIM Light Domain to ensure a single Designated Router to forward the PIM Join message from reviver to the Source.

IANA , lists the PIM supported message types. PIM Light only supports the following message types from the table "PIM Message Types" type 3 (Join/Prune) from the ALL-PIM-ROUTERS message types listed in . type 1 (Register) type 2 (Register Stop) type 8 (Candidate RP Advertisement) type 13 (PIM Packed Null-Register) type 13.1 (PIM Packed Register-Stop) Any future PIM message types that use unicast destination IP. No other message types are supported for PIM Light and MUST NOT be process if received on a PLI.

In a PIM Light domain, the following considerations should be taken into account due to the lack of processing Hello messages.

Since a PLI does not process PIM Hello messages, it also does not support the join attributes option in PIM Hello as specified in . As such, PIM Light is unaware of its neighbor's capability to process join attributes and it SHOULD NOT process a join message containing it. For a PLI to send and process a join attributes there can be two cases: It must be configured with appropriate join attribute type that the PLI is capable of processing as per table. Separate IETF drafts or RFCs may dictate that certain join attributes are allowed to be used without explicit configuration of the PLI in certain scenarios. The details are left to those drafts or RFCs.

Due to the absence of Hello messages, DR Election is not supported on a PIM Light router. The network design must ensure DR Election occurs within the PIM domain, assuming the PIM Light domain interconnects PIM domains.

For instance, in a BIER domain connecting two PIM networks, a PLI can be used between BIER edge routers solely for multicast state communication and transmit only PIM Join/Prune messages. If there are redundant PIM routers at the edge of the BIER domain, to prevent multicast stream duplication, they MUST establish PIM adjacency as per to ensure DR election at the edge of BIER domain. An example DR election could be DR election between router D and F in above figure. When the Join or Prune message arrives from a PIM domain to the down stream BIER edge router, it can be forwarded over the BIER tunnel to the upstream BIER edge router only via the designated router.

In scenarios where multiple PIM routers peer over a shared LAN or a Point-to-Multipoint medium, more than one upstream router may have valid forwarding state for a packet, potentially causing packet duplication. PIM Assert is used to select a single transmitter when such duplication is detected. According to section 4.6, PIM Assert should only be accepted from a known PIM neighbor. In PIM Light implementations, care must be taken to avoid duplicate streams arriving from multiple upstream PIM Light routers to a single downstream PIM Light router. If network design constraints prevent this, the implemented network architecture must take measures to avoid traffic duplication. For example, in a PIM Light over a BIER domain scenario, downstream IBBR (Ingress BIER Border Router) in a BIER domain can identify the nearest EBBRs (Egress BIER Border Routers) to the source using the Shortest Path First (SPF) algorithm with a post-processing as described in Appendix A.1. If the downstream IBBR identifies two EBBRs, it can select one using a unique IP selection algorithm, such as choosing the EBBR with the lowest or highest IP address. If the selected EBBR goes offline, the downstream router can use the next EBBR based on the IP selection algorithm, which is beyond the scope of this document.

Since a PLI doesn't require PIM Hello Messages and PIM neighbor adjacency is not checked for arriving Join/Prune messages, there needs to be a mechanism to enable PLI on interfaces. If a router supports PIM Light, only when PLI is enabled on an interface, arriving Join/Prune messages MUST be processed, otherwise they MUST be dropped. While on some logical interfaces PLI maybe enabled automatically or via an underlying mechanism, as an example the logical interface connecting two or more BIER edge routers in a BIER subdomain .

Because the Hello messages are not processed on the PLI, PIM Light Interface failures may not be discovered in a PIM Light domain and multicast routes will not be pruned toward the source on the PIM Light domain, leaving the upstream routers continuously sending multicast streams until the outgoing interface (OIF) expires. Other protocols can be used to detect these failures in the PIM Light domain and they can be implementation specific. As an example, the interface that PIM Light is configured on can be protected via Bidirectional Forwarding Detection (BFD) or similar technology. If BFD to the far-end PLI goes down, and the PIM Light Router is upstream and has an OIF for a multicast route <S,G>, PIM must remove that PLI from its OIF list.

]]> In another example, where the PLI is configured automatically between the BIER Edge Routers (BER), when the downstream BIER Edge Router (DBER) is no longer reachable on the upstream BIER Edge Router (UBER), the UBER which is also a PIM Light Router can prune the <S,G> advertised toward the source on the PIM domain to stop the transmission of the multicast stream.

defines a reliable transport mechanism for PIM transmission of Join/Prune messages, using either TCP or SCTP as transport protocol. For TCP, PIM over reliable transport (PORT) uses port 8471 which is assigned by IANA. SCTP is explained in , and it is used as a second option for PORT. mentions that when a router is configured to use PIM over TCP on a given interface, it MUST include the PIM-over-TCP-Capable Hello Option in its Hello messages for that interface. The same is true for SCTP and the router must include PIM-over-SCTP-Capable Hello Option in its Hello messsage on that interface. These Hello options contain a Connection ID which is an IPv4 or IPv6 address used to establish the SCTP or TCP connection. For PORT using TCP, the connection ID is used for determining which peer is doing an active transport open to the neighbor and which peer is doing passive transport open, as per section 4 of When the router is using SCTP, the Connection ID IP address comparison need not be done since the SCTP protocol can handle call collision. PIM Light lacks Hello messages, the PLI can be configured with the Connection ID IPv4 or IPv6 addresses used to establish the SCTP or TCP connection. For PIM Light using TCP PORT option each end of the PLI must be explicitly and correct configured as being active transport open or passive transport open to ensure handle call collision is avoided.

The following PIM variants are not supported with PIM Light and not covered by this document: Protocol Independent Multicast - Dense Mode (PIM-DM) Bidirectional Protocol Independent Multicast (BIDIR-PIM)