Protocol Independent Multicast Light (PIM Light)

PIM Light Interface describes PIM neighbor discovery via Hello messages. notes 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 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 that does not necessitate PIM neighbor establishment. An example is a Bit Index Explicit Replication (BIER) network connecting multiple PIM domains, where PIM Join/Prune messages are tunneled via BIER as specified in . A 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 or 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 on which PLIs are deployed MUST ensure that there is no multicast packet duplication, such as multiple upstream routers sending the same multicast stream to a single downstream router. For example, an 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 that a single DR forwards the PIM Join message from the receiver to the source.

Message Types Supported by PIM Light The "PIM Message Types" registry lists the message types supported by PIM. PIM Light only supports the following message types in that registry:

type 1 (Register)
type 2 (Register Stop)
type 3 (Join/Prune)
type 8 (Candidate RP Advertisement)
type 13.0 (PIM Packed Null-Register)
type 13.1 (PIM Packed Register-Stop)
Any future PIM message types where the destination is a unicast IP address

No other message types are supported by PIM Light; other message types MUST NOT be processed if received on a PLI.

Considerations for the Absence of Hello Message Because Hello messages are not processed in a PIM Light domain, the considerations in the subsections below should be taken into account.

Join Attribute Since a PLI does not use PIM Hello messages, it also does not support the Join Attribute option in PIM Hello as specified in . As such, PIM Light is unaware of its neighbor's capability to process Join Attributes and SHOULD NOT send a Join message containing a Join Attribute. There are two cases in which a PLI can support a Join Attribute:

The neighbors on the PLI are known via configuration to be capable of processing the attribute.
Internet-Drafts and 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 Internet-Drafts and RFCs.

DR Election 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 domains as in the figure below, 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, they MUST establish PIM adjacency as per to prevent multicast stream duplication and to ensure DR election at the edge of the BIER domain. For example, DR election could be between router D and F in the figure below. When the Join or Prune message arrives from a PIM domain to the downstream BIER edge router, it can be forwarded over the BIER tunnel to the upstream BIER edge router only via the DR.

PIM Assert 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, which can potentially cause packet duplication. PIM Assert is used to select a single transmitter when such duplication is detected. According to , 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 scenario with PIM Light over a BIER domain, a 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 post-processing as described in Appendix A.1 of . 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.

PLI Configuration 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 PLIs on interfaces. Join/Prune messages not received from a PIM neighbor MUST be dropped unless PLI is enabled on the interface. In some cases, a PLI may be enabled automatically via an underlying mechanism on a logical interface. For example, in a BIER domain, a logical interface can connect two or more BIER edge routers as per .

Failures in PLR Domain Because Hello messages are not processed on the PLI, PLI 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. This results in 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 on which PIM Light is configured 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, the PLI is configured automatically between the BIER Edge Routers (BERs) as in the figure below. 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. ]]>

Reliable Transport Mechanism for PIM Light defines a reliable transport mechanism called PIM Over Reliable Transport (PORT) for PIM transmission of Join/Prune messages, using either TCP or SCTP as the transport protocol. Both TCP and SCTP use destination port number 8471. SCTP is explained in and 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; the router MUST include the PIM-over-SCTP-Capable Hello Option in its Hello messages 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 to determine which peer is doing an active transport open to the neighbor and which peer is doing passive transport open, as per . When the router is using SCTP, the Connection ID is not used to determine the active and passive peer since SCTP can handle call collision. Because PIM Light lacks Hello messages, the PLI can be configured with the Connection ID (i.e., the IPv4 or IPv6 address used to establish the SCTP or TCP connection). For PIM Light using the TCP PORT option, each end of the PLI must be explicitly and correctly configured as being either active transport open or passive transport open to ensure that call collision is avoided.

PIM Variants Not Supported The following PIM variants are not supported with PIM Light and not covered by this document:

PIM - Dense Mode (PIM-DM)
Bidirectional PIM (BIDIR-PIM)