BIER Extension Headers

Background specifies BIER encapsulation header as following:

The header is fixed format, making it difficult to extend for additional functionalities. describes In-situ Operations, Administration, and Maintenance (IOAM) functionality for BIER, and proposes a way to extend the BIER header to encode IOAM data as well as other potential extension headers. considers Generic Delivery Functions (GDFs, e.g., fragmentation, security, IOAM) that can be applied to various layers (e.g., MPLS, IPv6, BIER) and proposes a slightly different extension header mechanism that is aligned with and work for both MPLS, BIER, and potentially other layers. is not adopted in the MPLS WG. is the candidate solution, and the main difference is that the "next header" concept is no longer used, which means that IPv6 extension headers for GDFs can not be used as is. However, other than that it takes a different way to get to the ancillary data used for the functions, the data itself and the handling can still be generic. To align BIER and MPLS for a generic solution, this document now takes the approach in for discussions and progressing in the BIER WG.

Specifications

BIER Encapsulation with Extension Headers The following figure illustrates a BIER header with extension headers. A TBD value for the "proto" field in the BIER header indicates that some BIER Extension Headers follow the BIER header and precede the BIER payload. An extension header could be an IPv6 one for a GDF like IOAM, even though this is not an IPv6 protocol layer.

R: The "R" flag bit is reserved. It MUST be set to 0 on transmission and ignored on reception. H: If the "H" flag bit is set, it indicates the presence of at least one extension header that needs to be processed hop by hop even before a BFER is reached.

Extension Top Header The Extension Top Header (ETH) encoding is as following:

A BIER capable router is smart enough to interpret the data afer the base BIER header only according to the Proto field, so there is no need to reserve a nibble at the begining of ETH to distinguish from an IP header.

Extension Header Format The format of a BIER Extension Header (EH) mimics the MPLS Post-Stack Network Action Header (assuming the MPLS format will be updated to put its equivalent of EH-DATA at the end of the first word):

EH-OP: 7-bit OP Code for the extension. RR: 2-bit reserved field. EH-LEN: 7-bit unsigned integer for the Extension Header Length in 4-octet units, not including the first four octets. EH-Data:Data for the extension header. The first two octets are not counted in the EH-Len.

Security Considerations To be provided.

IANA Considerations The following IANA actions are requested. Assign a "BIER Extension Header" value from the "BIER Next Protocol Identifiers" registry for the "proto" field in a BIER header. Create a "BIER Extension Header OP Codes" registry for the "EH-OP" field in a BIER specific extension header.