PIM Message Type Space Extension and Reserved Bits

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Table of Contents

1. Introduction

The PIM version 2 messages share a common message header format defined in the PIM Sparse Mode specification [RFC7761]. 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 [RFC7761] 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 [RFC7761] and [RFC3973] by defining the use of the currently Reserved field in the PIM common header. This document further updates [RFC7761] and [RFC3973], along with [RFC5015], [RFC5059], [RFC6754], and [RFC8364], by specifying the use of the currently reserved bits for each PIM message.¶

The originally defined PIM message types were in the range from 0 to 15. Message type 15 had been reserved by [RFC6166] for type space extension. In Section 5, this document specifies the use of the Flag Bits for message types 13, 14, and 15 in order to extend the PIM type space. The type space extension in [RFC6166] was made obsolete by [RFC8736]. This document obsoletes [RFC8736].¶

4. Flag Bit Definitions

Unless otherwise specified, all the flag bits for each PIM type are Unassigned [RFC8126]. 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.¶

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |7 6 5 4 3 2 1 0|           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5. PIM Type Space Extension

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, abbreviated as FB below) are to be defined by each extended type.¶

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |Subtype|  FB   |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7. IANA Considerations

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 registration policy remains IETF Review [RFC8126]. Assignments into this registry MUST define any non- default usage (see Section 4) of the flag bits in addition to the type.¶

Because this document obsoletes RFC 8736, IANA is asked to change all registration information that references [RFC8736] to instead reference [this document].¶

The updated "PIM Message Types" registry is shown below.¶

The unassigned types above, as explained in Section 5, use the extended type notation of type.subtype. Each extended type only has 4 flag bits available. New extended message types should be assigned consecutively, starting with 13.0, then 13.1, etc.¶

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