PIM Flooding Mechanism and Source Discovery Enhancements

3.2. PFM optimization Hello option

A PIM router indicates that it supports enhancement mechanisms specified in this document by including the new PFM optimization Hello option (Option TBD3).¶

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       OptionType = TBD3       |           Length = 0          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

OptionType = TBD3

OptionLength = 0

All PIM routers supporting forwarding optimization MUST track whether it is supported by all PIM neighbors on each PIM interface. This tracking is beneficial in heterogeneous networks where only certain routers support the optimization.¶

Additionally, for each unique Router-ID received by a PIM router in a PIM domain, the router MUST maintain a set of interfaces where the following two conditions are met: 1. The neighbor with this Router-ID is the only PIM neighbor on this interface and, 2. the neighbor is advertising the PFM optimization option TBD3 on this interface. This set is referred to as the PFM_OPT_IF set for each Router-ID. PFM message exchange is optimized on the interfaces belonging to PFM_OPT_IF for each Router-ID and is discussed in Section 3.4.¶

3.3. Sample PFM Topology

                         Router C
                            │
                            │ LAN 1
                            │
     Router A --------------------------------- Router B
        |                                          |
        |----------------- Link L1 ----------------|
        |                                          |
        |----------------- Link L2 ----------------|
        |                                          |
        |----------------- Link L3 ----------------|
        |__________________________________________|
                            │
                            │  LAN 2
                            |
                         Router D

3.4. PFM message handling with Relaxed-RPF enabled

Consider a topology where two PIM routers maintain multiple PIM neighborships over several links within the same PIM domain, and are the only two routers on these links, either a P2P link, or 2 PIM neighbors on a LAN. On P2P links, each router sees only one neighbor, but on shared LANs, routers may see multiple neighbors. An example of such a topology is illustrated in Figure 1. Between Router A and Router B, there are multiple links - 3 P2P links and 2 shared LANs. Traditionally, each of the routers in Figure 1 will send out PFM messages out over all the links to its neighbor. RPF checks are one of the commonly used ways to prevent loops, hence the recipient router performs an RPF check and accepts only on one link, thereby dropping packets from all the others. Since the sender does not know which link will be chosen as the RPF-source on the neighbor, it cannot choose one of the links, without knowing its neighbor's decision. But this can be optimized as follows.¶

Assume both routers A and B are advertising their respective Router-IDs on all links. When the optimizations specified in Section Section 3.2 are in effect, On both routers A and B, PFM_OPT_IF = {L1, L2, L3}.¶

If the Relaxed-RPF optimization is advertised by both routers, the sender MUST choose one link from their PFM_OPT_IF set and send and forward PFM messages to its neighbor using only that link. On shared LANs, the sender MUST send PFM messages as normal since optimization cannot be applied when there are more than two routers on the network segment. In other words, the scope of optimization is limited to links present in the PFM_OPT_IF set for each Router-ID.¶

For example, referring to Figure 1, if Router A is forwarding or originating a PFM message, it MUST send the message on one link out of Links L1, L2, or L3. Router A also MUST send the message on both LAN 1 and LAN 2 to ensure Routers C and D receive the message. This selective behavior reduces PFM message processing overhead on the Point-to-Point links. The mechanism to choose a link from the PFM_OPT_IF set is left to the implementation.¶

When a router that supports the Relaxed-RPF optimization receives a PFM message, it MUST first determine the expected RPF interface for the message using standard RPF calculations. If the message was received on a link belonging to the PFM_OPT_IF set AND both the sender and receiver support Relaxed-RPF optimization, the receiver MUST accept the message regardless of the RPF check result. In all other cases, the receiver MUST perform normal RPF validation and only accept the message if it arrives on the correct RPF interface.¶

The optimization mechanism relies heavily on a router's insight into whether all neighbors on each PIM interface support the TLV Type TBD3 and/or Relaxed-RPF optimization. All checks can be done at the time when a PFM message is forwarded, but it is possible to perform most checks when there are neighbor changes, so that the processing at forwarding time can be minimized. The following scenarios MUST be handled:¶

Adding a new neighbor on any link:

If the neighbor being added is the first neighbor on this link, the router MUST check whether this neighbor supports the optimization and announces a Router-ID. If both conditions hold TRUE, this router MUST check whether PFM_OPT_IF exists for this Router-ID. This means that the newly added neighbor is also the sole neighbor on at least one other link. Therefore, forwarding optimization MUST be enabled on this link by adding it to the existing PFM_OPT_IF set for that Router-ID. If PFM_OPT_IF does not exist for this Router-ID, it MUST be created, and this link MUST be added to the set. If the neighbor being added is the second neighbor on this link, and forwarding optimization was previously enabled for the first neighbor, it MUST now be disabled for that Router-ID on this link. Hence this link MUST be removed from the PFM_OPT_IF set for the first neighbor's Router-ID.¶

Neighbor removal on a link:

When a PIM neighbor is removed on a link, and there is exactly one remaining neighbor, it MUST be checked whether the remaining neighbor supports the forwarding optimization and is advertising a Router-ID. If all three conditions hold TRUE ((i) sole remaining neighbor that (ii) supports forwarding optimization, and (iii) is advertising a Router-ID), this router must check whether PFM_OPT_IF exists for this Router-ID. If the PFM_OPT_IF set for this Router-ID does not exist, it MUST be created; otherwise, the link MUST be added to the existing set.¶

Neighbor starts/stops advertising Router-ID:

When a PIM neighbor starts advertising a Router-ID on this link, it MUST be checked whether this neighbor also supports the forwarding optimization (TBD3) on this link and whether it is the sole neighbor on this link. If both conditions hold TRUE, this router MUST check whether PFM_OPT_IF exists for this Router-ID. If it does not exist, create PFM_OPT_IF for this Router-ID and this link MUST be added to the set. If PFM_OPT_IF already exists, add this link to the existing set. When a PIM neighbor stops advertising a Router-ID on this link and is still forwarding optimization capable while being the sole neighbor on this link, this link MUST be removed from the PFM_OPT_IF set for this Router-ID. If the PFM_OPT_IF set for this Router-ID becomes empty, it MUST be deleted.¶

Neighbor starts/stops advertising forwarding optimization:

When a PIM neighbor starts advertising the forwarding optimization (TBD3) on this link, it MUST be checked whether this neighbor is the sole neighbor on this link and whether it is advertising its Router-ID on this link. If both conditions hold TRUE, this router MUST check whether PFM_OPT_IF exists for this Router-ID. If it does not exist, create PFM_OPT_IF for this Router-ID and this link MUST be added to the set. If PFM_OPT_IF already exists, add this link to the existing set. When a PIM neighbor stops advertising the forwarding optimization (TBD3) on this link, while it is still advertising a non-zero Router-ID and is the sole neighbor on this link, this link MUST be removed from the PFM_OPT_IF set for this Router-ID. If the PFM_OPT_IF set for this Router-ID becomes empty, it MUST be deleted.¶

The scenarios described above apply during network and configurations changes as well as software upgrades or downgrades, that could lead to changes in neighbor capabilities. These changes will be reflected in Hello messages with the relevant options. It is essential to consistently maintain the PFM_OPT_IF set for each non-zero Router-ID with any such changes.¶