Define how to map 7-bit priorities into smaller bit-space schemes

[bslassey]

As noted in the proposal introduction, most implementations have fewer priority buckets than can be expressed in 7-bits. Perhaps to avoid interop issues, we could include some language around how this should be handled, for instance by dropping the lowest order bits.

[pmeenan]

We could also just limit it to 4 or 5 bits in the spec and leave the remaining 2-3 bits unused or reserved. Or we could just leave it at 7 bits and remove the comment.

The main purpose for the extra bits was if we wanted to allow for a "group ID" or something like that and round-robin between groups to allow for the case of multiplexing clients over a single connection (and the VPN case). I'm not sure 2-3 bits leaves enough for those use cases but if we agree it is valuable then maybe it is worth including. In theory it could also be used for separating websockets or streaming content into different (equal-weighted) groups from page content in a browser use case.

[bslassey]

I presume some sort of splitting of the space would need to be negotiated, which is fine. What I'm thinking is that if no other negotiation establishes a purpose for the extra bits we should give guidance on how they should be used (or not used).