| Internet-Draft | PIM PFM Pre-join Sub-TLV | October 2026 |
| Gopal & Venaas | Expires 24 April 2027 | [Page] |
The PIM Flooding Mechanism and Source Discovery (PFM-SD) extension enables PIM routers to announce active multicast sources. The Group Source Info (GSI) TLV extends PFM-SD by allowing Sub-TLVs to be included with a source announcement.¶
This document defines the Pre-join Sub-TLV, which carries the PIM Router-ID of a router selected by the First-Hop Router (FHR). The selected router can establish multicast forwarding state before data arrives, reducing join latency.¶
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It is useful to establish a multicast path before a receiver requests the traffic to reduce join latency. For example, in a spine-leaf topology, a leaf could use deterministic hashing, as described in the deterministic ECMP draft [I-D.ietf-pim-deterministic-ecmp], to select a spine for a given (S,G) from the set of spines it knows about. A spine, however, may not know the identities of all the other spines and therefore may be unable to make the same selection independently. This document instead uses the PIM Flooding Mechanism (PFM) to distribute the choice made by the First-Hop Router (FHR).¶
The PFM-SD extension defined in [RFC8364] enables PIM routers to flood announcements of active multicast sources to all PIM neighbors. The Group Source Info (GSI) TLV, defined in [I-D.ietf-pim-pfm-forwarding-enhancements], extends PFM-SD by allowing routers to include Sub-TLVs alongside each source announcement. The GSI TLV applies to a single (S,G) entry.¶
For example, in a leaf-and-spine topology, the FHR can include the selected spine's Router-ID in a PFM message. PFM distributes the announcement to routers in the domain. The selected spine can then establish (S,G) state toward the source before a leaf has receiver interest. When a receiver becomes active, its leaf router can send a PIM (S,G) Join toward the source, using the path toward the spine announced in the PFM message.¶
This document defines one Sub-TLV for use within the GSI TLV. The Pre-join Sub-TLV identifies a router for which other routers establish the (S,G) path in advance.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The format of the Pre-join Sub-TLV is:¶
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD1 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
The FHR selects the router for pre-joining using an implementation-specific method and includes that router's Router-ID in the Pre-join Sub-TLV of the GSI TLV for the (S,G). A router that receives the GSI TLV and finds that the Router-ID matches its own PIM Router-ID is the selected router.¶
When local policy permits pre-joining, the selected router MUST use the active Pre-join Sub-TLV as an additional input when determining JoinDesired(S,G), as defined in Section 4.5.5 of RFC 7761. This can make JoinDesired(S,G) true even when the router has no local receivers. The router then follows the RFC 7761 upstream state machine to determine its state and whether to send or refresh a PIM Join(S,G) toward RPF'(S,G). The Pre-join Sub-TLV does not by itself require unconditional transmission of a Join, and does not create a second PIM join. The pre-join indication is tracked separately from the PIM forwarding state used to determine JoinDesired(S,G).¶
For a given (S,G), receipt of a GSI TLV with a Pre-join Sub-TLV containing a different Router-ID replaces the previously selected Router-ID. The newly selected router applies the procedure above. The previously selected router removes only the pre-join reason for maintaining its upstream (S,G) state. It MUST continue to follow the RFC 7761 upstream state machine if JoinDesired(S,G) remains true; otherwise, it sends a PIM Prune(S,G) and transitions to NotJoined as specified in RFC 7761 Section 4.5.5.¶
The Pre-join Sub-TLV has no separate holdtime. Its validity is tied to the holdtime of the GSI TLV in which it was most recently received. Receipt of a refreshed GSI TLV for the same (S,G) that contains the same Pre-join Sub-TLV refreshes that validity. If a refreshed GSI TLV for the same (S,G) omits the Pre-join Sub-TLV, the previously advertised Pre-join Sub-TLV remains valid only until its existing holdtime expires; the refresh does not extend that holdtime. Omission of the (S,G) from a PFM message likewise does not withdraw the stored information. A GSI holdtime of zero or expiration of the applicable holdtime ends the pre-join reason. When it ends, the router removes only that reason and sends a PIM Prune(S,G) only if JoinDesired(S,G) is false, following RFC 7761 Section 4.5.5.¶
For general PIM message security, see [RFC7761]. For PFM-specific security considerations, see [RFC8364]. The security considerations of [I-D.ietf-pim-pfm-forwarding-enhancements] also apply to Sub-TLVs carried within the GSI TLV.¶
This document defines a Sub-TLV that carries a PIM Router-ID. An incorrect or spoofed Router-ID in a PFM message could cause a router to establish unnecessary (S,G) state or could affect path selection.¶
The Pre-join Sub-TLV can cause a router to establish state toward a source even in the absence of local receivers. Incorrect or spoofed Router-ID information can therefore result in unnecessary state and resource use, or affect path selection. PIM message authentication and integrity considerations apply.¶
All registries referenced in this section are under the "Protocol Independent Multicast (PIM) Parameters" registry group.¶
This document requests the assignment of one new Sub-TLV type in the "PIM Flooding Mechanism Group Source Info Sub-TLV Types" registry, which was created by [I-D.ietf-pim-pfm-forwarding-enhancements]. Assignments are to be made according to the "IETF Review" policy as defined in [RFC8126].¶
PIM Flooding Mechanism Group Source Info Sub-TLV Types Type Name Reference ------- ------------------------------ --------------- TBD1 Pre-join [This document]¶