PIM Message Type Space Extension and Reserved Bits

The PIM version 2 messages share a common message header format defined in the PIM Sparse Mode specification. The common header definition contains eight Reserved bits. While all message types use this common header, there is no document formally specifying that these bits are to be used per message type. This document refers to the bits specified as Reserved in the common PIM header as PIM message type Flag Bits, or simply Flag Bits, and it specifies that they are to be separately used on a per-message-type basis. It creates a registry containing the per-message-type usage. This document Updates and by defining the use of the currently Reserved field in the PIM common header. This document further updates and , along with , , and , by specifying the use of the currently Reserved bits for each PIM message. The currently defined PIM message types are in the range from 0 to 15. That type space is almost exhausted. Message type 15 was reserved by for type space extension. In , this document specifies the use of the Flag Bits for message types 13, 14 and 15 in order to extend the PIM type space. This document Obsoletes .

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 when, and only when, they appear in all capitals, as shown here.

The common PIM header is defined in section 4.9 of . This document updates the definition of the Reserved field and refers to that field as PIM message type Flag Bits, or simply Flag Bits. The new common header format is as below.

The Flag Bits field is defined in . All other fields remain unchanged.

Unless otherwise specified, all the Flag Bits for each PIM type are Reserved . They MUST be set to zero on transmission, and they MUST be ignored upon receipt. The specification of a new PIM type MUST indicate whether the bits should be treated differently. When defining Flag Bits, it is helpful to have a well-defined way of referring to a particular bit. The most significant of the Flag Bits, the bit immediately following the type field is referred to as bit 7. The least significant, the bit right in front of the checksum field is referred to as bit 0. This is shown in the diagram below.

PIM message type 4 (Bootstrap) defines Flag Bit 7 as No-Forward. The usage of the bit is defined in that document. The remaining Flag Bits are Reserved.

PIM message type 10 (DF Election) specifies that the four most significant Flag Bits (bits 4-7) are to be used as a Subtype. The usage of those bits is defined in that document. The remaining Flag Bits are Reserved.

PIM message type 12 (PFM) defines Flag Bit 7 as No-Forward. The usage of the bit is defined in that document. The remaining Flag Bits are Reserved.

These types and the corresponding Flag Bits are defined in .

This document defines types 13, 14 and 15, such that each of these types has 16 subtypes, providing a total of 48 subtypes available for future PIM extensions. This is achieved by defining a new SubType field (see Figure 3) using the four most significant Flag Bits (bits 4-7). The notation type.subtype is used to reference these new extended types. The remaining four Flag Bits (bits 0-3) are Reserved to be used by each extended type (abbreviated as FB below).

This document clarifies the use of the Flag Bits in the common PIM header and it extends the PIM type space. As such, there is no impact on security or changes to the considerations in and .

This document updates the PIM Message Types registry to indicate which Flag Bits are defined for use by each of the PIM message types. The Registry should now reference this document instead of . The Registration Policy remains IETF Review . Assignments into this registry MUST define any non-default usage (see ) of the Flag Bits in addition to defining the Type. The updated PIM Message Types registry is shown below.

The Unassigned types above, as explained in , use the extended type notation of type.subtype. Each extended type only has 4 Flag Bits available. New extended message types should be assigned conscutively, starting with 13.0, then 13.1, etc.