consider multicast support in IOAM

[haoyusong]

A scheme described in draft [https://datatracker.ietf.org/doc/draft-song-multicast-telemetry/] requires some augments and modificactions to the IOAM DEX option header and IOAM data field format:

1. A bit in flag field needs to be assigned to indicate this is for multicast
2. An branch index optional field needs to be included in the IOAM DEX option header
3. In export data, the branch ID is proposed to be encoded with the existing node ID data field

We need to make sure the new augmentation does no conflict with the main IOAM Data and IOAM DEX specification.

[shwethab]

Few question:

1. is the intent of adding

• flag bit multicast indication leading to parsing node-id differently and
• adding branch index in node id to discover the multicast topology

2. why not use a multicast namespace and namespace specific data to carry the branch index?

[haoyusong]

The current draft describes the requirements and provides a possible solution. Other solutions are possible. For example, I'd like to see the details for the suggestion 2 above.
The intention here is to raise the awareness of the multicast case and ensure the related specifications can reasonably support it.

For the first question:

1. A dedicate flag may be used to indicate some different operation is needed. For example, the need of the branch ID is something new.
2. Branch ID is used to reconstruct the multicast topology.