In my use of 9P within the Linux and IBM communities there have been several requests for extensions to better support Linux environments or features not currently a part of the protocol (locking, caching, batch operations, storage networking, etc.) As such, I'd like to work towards a specification of the protocol which can accomodate these extensions in as non-intrusive a way as possible (the counter-example being 9P2000.u which while negotiated ends up complicating clients and servers who wish to use it).
The core protocol (size prefix, operation, tag) remains the same. The existing 9P2000 operations remain the same, 9P2000.u will be depricated by the new extension model. Extensions will exist in a complimentary namespace -- that is to say operation identifiers will not overlap with existing operations, and all extensions will use new operations as opposed to changes to existing operations. Alternate extension op-codes spaces may be negotiated by optional prefixes during Tversion negotiation -- but every effort will be made to keep operations within the existing namespace.
The extensions are OPTIONAL, servers which don't wish to implement them (or any subset of them) simply returns Rerror -- well behaved clients will fall back to the core 9P2000 operations.
OPERATION CODE RESERVATION
To facilitate complimentary extensions, we'll use a wiki page 9P2010 Operations to coordinate op-code reservation for specific extensions. It will be the definitive registration resource for the time being.