| Internet-Draft | PIM Light | November 2021 |
| Bidgoli, et al. | Expires 16 May 2022 | [Page] |
This document specifies a new Protocol Independent Multicast interface which does not need PIM Hello to accept PIM Join/Prunes or PIM Asserts.¶
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It might be desirable to create a PIM interface between routers where only PIM Join/Prunes and Asserts packets are triggered over it without having a full PIM neighbor discovery. As an example, this type of PIM interface can be useful in some scenarios where the multicast state needs to be signaled over a network or medium which is not capable of or has no need for creating full PIM neighborship between its Peer Routers. These type of PIM interfaces are called PIM Light Interfaces (PLI).¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
RFC [RFC7761] section 4.3.1 describes the PIM neighbor discovery via Hello messages. It also describes that PIM Join/Prune or Assert messages are not accepted from a router unless a Hello message has been heard from that router.¶
In some scenarios it is desired to build a multicast state between two directly attach or remote 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 or does not require PIM Neighbor establishment. An example is a BIER network connecting multiple PIM domains and PIM Join/prune messages are tunneled via bier as per [draft-ietf-bier-pim-signaling].¶
A PIM Light Interface (PLI) does accept Join/Prune and Assert messages from a unknown PIM router, 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 there is no DR Priority options communicated between Routers either. As such the router doesn't create any General-Purpose state for neighboring PIM routers and it 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 an example, in a BIER domain which is connecting 2 PIM networks. A PLI can be used to connect edge BIER routers and only multicast states communicated via 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 [RFC7761] to ensure DR selection on the edge of the BIER router while PLI is used in core of the BIER Domain.¶
Since a PLI doesn't require PIM Hello Messages and PIM neighbor adjacency is not checked for join/prune/assert 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 [draft-ietf-bier-pim-signaling].¶