IS-IS Point-to-Multipoint operation

The core functionality of the protocol outlined in this document consists of an additional Subnetwork dependent function per Section 8 of ISO/IEC 10589:2002. In that regard, the next section can be understood as new section "8.5 Point-to-multipoint networks". The outlined protocol is remotely similar to the derelict section "8.3 ISO 8208 subnetworks" in that multiple point-to-point adjacencies are established on an interface. Point-to-multipoint operation of IS-IS is thus not a new idea; in fact section 6.2 already mentions "multipoint links." This document re-enables the concept.

In place of ISO 8473 call management establishing sessions, this document establishes pairwise "pseudocircuits" between two IS on a common medium. Multiple such pseudocircuits can normally exist on one P2MP interface. Each pseudocircuit operates, aside from subnetwork attachment procedures, exactly as a PtP link. It should be noted that while the Multicast autodiscovery mechanism requires broadcast capability, no other part of the protocol does. The P2MP interface type can be used on non-transitive and/or non-broadcast interfaces.

An implementation maintains a set of pseudocircuits per P2MP interface. Each pseudocircuit is uniquely identified by the local interface and peer's SNPA address. Each participating system MUST use a consistent SNPA address per local interface. A change in SNPA address results in all neighbors treating the interface as distinct from the previous one. A system MAY support multiple SNPA addresses per interface by treating them as distinct interfaces. Unnumbered media are not supported by this protocol. Each participant on a link MUST have an unique SNPA address on that link. As pseudocircuits are dynamic in nature, they must be created and destroyed as needed.

Pseudocircuits are represented in LSPs as a regular PtP circuit would be. As a result, their treatment in SPF calculations is also identical to PtP circuits.

In scenarios where pseudocircuits do not form a full mesh of all participants on a medium, the best path for a packet may be through the same interface as the one it was received on. Systems implementing this specification MUST therefore support forwarding packets on the same interface that they were received on. This applies only to interfaces configured for P2MP operation.

The discovery machinery produces as output a "candidate neighbor list," which is a list of possible neighbor's SNPAs and is maintained per P2MP interface. Additional information (metric, etc.) MAY be associated with the SNPA, but this is not part of the discovery mechanism. The candidate neighbor list is conceptual in nature; adding and removing entries results in pseudocircuit creation and deletion. It need not be implemented as an actual list. The list is the union of the result of each of the following sources of neighbor information. A neighbor may be listed by multiple sources: it MUST NOT be removed while any source still lists it. The IS-IS implementation SHOULD provide user configuration to disable individual mechanisms and/or filter candidate neighbors both as a security and as misconfiguration preventing measure.

A list of neighbor IS MAY be configured by the user, providing possible neighbor's SNPAs on an interface.

Lower protocol layers (VPLS, IEEE 802.11) may be able to provide a list of attached neighbors. This list MAY be fed into the candidate neighbor list.

For broadcast capable networks, this document defines an autodiscovery mechanism based on multicasting Hello packets. This mechanism MAY be disabled by the user, but MUST be implemented for all lower layer link types with (limited or full) broadcast capability. Multicast autodiscovery P2MP Hello PDUs are distinguished by not containing a RFC5303 Three-Way Adjacency TLV. Receiving such a Hello places the sending IS on the candidate list for as long as the PDU's holdtime indicates. A system MAY implement a receive-only passive multicast autodiscovery mode where it adds candidates (forms pseudocircuits) on receiving P2MP Hello PDUs, but does not send such PDUs itself. If either passive or full multicast autodiscovery is enabled, receiving a P2MP Hello PDU with a Three-Way Adjacency TLV also adds the neighbor to the candidate list.

Since there is no underlying protocol layer partitioning the link into actual PtP circuits in this case, additional handling is required on bringing up the individual pseudocircuit adjacencies. To prevent different pseudocircuits from "bleeding" into each other, implementations of this protocol MUST enable on all P2MP pseudocircuits, with changes as follows: Received IIH PDUs on P2MP pseudocircuits without the Point-to-Point Three-Way Adjacency option MUST be discarded. The TLV is not optional on pseudocircuit adjacencies but rather mandatory.

Pseudocircuits are destroyed when their PtP state machine has transitioned into "Down" state and they are no longer listed as candidates by any discovery mechanism. The conditions for pseudocircuit removal are thus: PtP adjacency timeout functionality (IS-IS section 8.2.6) has moved the pseudocircuit to Down state, or it never moved out of Down state The holdtime of any multicast autodiscovery P2MP Hello PDUs has expired. Manual configuration or lower layer autodiscovery no longer lists the neighbor. As long as a pseudocircuit is present, its PtP state machine will, as expected for PtP circuits, trigger transmission of further Hello PDUs, even when it is in Down state.

All PDU types that are not Hello PDUs, e.g. L1/L2 LSP PDUs, L1/L2 CSNP/PSNP PDUs, FS-LSP, FS-CSNP and FS-PSNP PDUs are processed on P2MP interfaces by dispatching them to an individual pseudocircuit by looking up the PDU's source address in the interface's list of P2MP pseudocircuits. This includes all functions defined in Section 7 of ISO/IEC 10589:2002 and PDUs added in and . While possible, this document does not suggest sending such PDUs to multicast destinations. All systems MUST send these PDUs to unicast lower layer destinations so that they are received only by the pseudocircuit's neighbor system. However, systems SHOULD NOT check the destination address on receipt to allow future optimisations.

Received LAN and P2P Hello PDUs MUST be discarded when received on an interface configured for P2MP operation. The system MAY notify the operator of such a misconfiguration. TODO: hitless migration possible?

P2MP Hello PDUs are divided in two categories depending on the presence of a RFC5303 Three-way Adjacency TLV.

For each P2MP interface that has the multicast autodiscovery mechanism enabled, a system periodically sends P2MP Hello PDUs without Three-way Adjacency TLV to a lower layer specific multicast address. The periodic timer SHOULD be configurable and is subject to jitter per Section 10.1 of ISO/IEC 10589:2002. On receipt, such PDUs are not processed on any pseudocircuit's PtP state machine. They are processed on pseudocircuit management level as described in : If no pseudocircuit matches the PDU's lower layer source address, one is created for that address The pseudocircuit is held in existence at least until the PDU's holdtime expires. No information other than the neighbor's SNPA is carried from the PDU onto the pseudocircuit, thus most TLVs that are normally valid in Hellos become pointless. However, all TLVs that affect PDU processing (in particular authentication TLVs) are processed normally. On IEEE 802 lower layers that support 48bit MAC addresses, the AllIntermediateSystems multicast address (09-00-2B-00-00-05) is used as destination when sending these PDUs.

P2MP Hello PDUs with a RFC5303 Three-way Adjacency TLV are both processed and generated by the PtP Hello state machine (Section 8.2 of ISO/IEC 10589:2002) of their associated pseudocircuit. This implies that received PDUs are dispatched by looking up their source SNPA address among the pseudocircuits associated with the receiving interface (possibly creating a pseudocircuit). For Hello PDUs sent by the pseudocircuit's PtP Hello state machine, the destination SNPA address is set to the pseudocircuit's neighbor SNPA address. TLV validity and processing is exactly as for PtP Hellos.

(Note this section follows ISO 10589 section 9, particularly in numbering bits starting at 1 for the LSB.)

Figure 1: P2MP Hello PDU format MSB LSB MSB LSB 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 +---+---+---+---+---+---+---+---+ - + - + - + - + - + - + - + - + | 0x83 (Protocol Discriminator) | +---+---+---+---+---+---+---+---+ | Length Indicator | +---+---+---+---+---+---+---+---+ | 0x01 (Version/ID Extension) | +---+---+---+---+---+---+---+---+ | ID Length | +---+---+---+---+---+---+---+---+ | R R R | TBD-PDU (PDU Type)| +---+---+---+---+---+---+---+---+ | 0x01 (Version) | +---+---+---+---+---+---+---+---+ | R R R R R R R R | +---+---+---+---+---+---+---+---+ | Maximum Area Addresses | +---+---+---+---+---+---+---+---+ | R R R R R R |CircTyp| +---+---+---+---+---+---+---+---+ | Source ID | : (ID Length) : : : +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Holding Time | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | PDU Length | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ : Variable Length Fields (TLVs) : : : All fields function exactly as for Point-to-Point IS-IS hello PDUs, as specified in Section 9.7 of ISO/IEC 10589:2002. Note the Local Circuit ID field is absent. Its function is replaced by .

IANA is requested to allocate a value from the "IS-IS PDU Registry" as follows: TBD-PDU P2MP-HELLO-PDU This document Applicability of sub-TLVs for this PDU (per "TLV Codepoints Registry") is per the "IIH" column.

TODO: YANG model.

TODO.