PIM Light

RFC section 4.3.1 describes the PIM neighbor discovery via Hello messages. It also describes that PIM Join/Prune are not accepted from a router unless a Hello message has been heard from that router. In some scenarios it is desirable to communicate and build multicast states between two directly or non directly attach routers without establishing a PIM neighborship. There could be many reasons for this desired, but one example is the desired to signal multicast states upstream, between two or more PIM Domains via a network or medium that is not optimized for PIM or does not require PIM Neighbor establishment. An example is a BIER network connecting multiple PIM domains. In these BIER networks PIM Join/prune messages are tunneled via bier as per . A PIM Light Interface (PLI) ONLY accepts Join/Prune messages from an unknown PIM router and it accepts these messages it without receiving a PIM Hello message form the router. Lack of Hello Messages on a PLI means there is no mechanism to learn about the neighboring PIM routers on each interface and their capabilities or run some of the basic algorithms like DR Priority between the routers. As such the router doesn't create any General-Purpose state for neighboring PIM and it only accepts and installs each Join message from upstream routers in its multicast routing table. Because of this, a PLI needs to be created in very especial cases and the application that is using these PLIs should ensure there is no multicast duplication of packets. As an example, multiple upstream routers sending the same multicast stream to a single downstream router.

As per IANA pim currently supports 12 message types, PIM Light only supports message type 3 (Join/Prune). All other message types are not supported for PIM Light and should not be process if recived on a PLR interface.

Lack of register message on PLR means that, the Source, DR, RP all need to be in a common PIM domain and can not be connected over PLR. PLR will only processes join/prune regardless of if the join/prune is <S,G> or <*,G>.

The following should be considered on a PLR domain since hello messages are not processed.

Since PLI does not process the pim hello message, processing of the join attributes option in pim hello as per is also not supported, leaving PLRs unaware if their neighbors have the capability of processing the join attribute. A PLR that does not understand the type 1 Encoded-source Address, should not process a join message that contains it. Otherwise the PLR can process the Join Attribute accordingly.

Since DR selection is not supported on the PLR because of lack of hello messages, the network design should ensure that DR selection is achieve on the PIM domain, assuming the PLR domain is connecting PIM domains. As an example, in a BIER domain which is connecting 2 PIM networks, a PLI can be used between the BIER edge routers. The PLI will be only used for multicast states communication, by transmitting ONLY PIM Join/prunes over the BIER domain. In this case to ensure there is no multicast stream duplication the PIM routers attached on each side of the BIER domain might want to establish PIM Adjacency via to ensure DR selection on the edge of the BIER router, while PLI is used in the BIER domain, between BIER edge routers. When the Join or Prune message arrives from a PIM domain to the down stream BIER edge router, it can be send over the BIER tunnel to the upstream BIER edge router only via the selected designated router.

Since a PLI doesn't require PIM Hello Messages and PIM neighbor adjacency is not checked for join/prune messages, there needs to be a mechanism to enable PLI on interfaces for security purpose, while on some other interfaces this may be enabled automatically. An example of the latter is the logical interface for a BIER sub-domain . If a system explicitly needs a PLI to be configured, then this system should not accepts the Join/Prune messages on interfaces that the PLI is not configured on, and it should drop these messages on a non PLI interface. If the system automatically enables PLI on some special interfaces, as an example interfaces facing a BIER domain, then it should accept Join/Prune messages on these interfaces only.

Because the hello messages are not processed on the PLI, some failures may not be discovered in PLI domain and multicast routes will not be pruned toward the source on the PIM domain, leaving the upstream routers continuously sending multicast streams. Other protocols can be used to detect these failures in the PLR domain and they can be implementation specific. As an example, the interface that PLR is configured on can be protected via BFD or similar technology. If BFD to the far-end PLR goes down, and the PLR is upstream and is an OIF for a multicast route <S,G>, PIM should remove that PLR from its OIF list. In addition if upstream PLR is configured automatically, as an example in BIER case, when the downstream BFR is no longer reachable, the upstream PLR can prune the <S,G> advertised by that BFR, toward the source to stop the transmission of the multicast stream.