7. Protocol Version Negotiation

A router MUST start each transport connection by issuing either a Reset Query or a Serial Query. This query will tell the cache which version of this protocol the router implements.

If a cache which supports version 1 receives a query from a router which specifies version 0, the cache MUST downgrade to protocol version 0 [RFC6810] or send a version 1 Error Report PDU with Error Code 4 ("Unsupported Protocol Version") and terminate the connection.

If a router which supports version 1 sends a query to a cache which only supports version 0, one of two things will happen:

1. The cache may terminate the connection, perhaps with a version 0 Error Report PDU. In this case, the router MAY retry the connection using protocol version 0.

2. The cache may reply with a version 0 response. In this case, the router MUST either downgrade to version 0 or terminate the connection.

In any of the downgraded combinations above, the new features of version 1 will not be available, and all PDUs will have 0 in their version fields.

If either party receives a PDU containing an unrecognized Protocol Version (neither 0 nor 1) during this negotiation, it MUST either downgrade to a known version or terminate the connection, with an Error Report PDU unless the received PDU is itself an Error Report PDU.

The router MUST ignore any Serial Notify PDUs it might receive from the cache during this initial startup period, regardless of the Protocol Version field in the Serial Notify PDU. Since Session ID and Serial Number values are specific to a particular protocol version, the values in the notification are not useful to the router. Even if these values were meaningful, the only effect that processing the notification would have would be to trigger exactly the same Reset Query or Serial Query that the router has already sent as part of the not-yet-complete version negotiation process, so there is nothing to be gained by processing notifications until version negotiation completes.

Caches SHOULD NOT send Serial Notify PDUs before version negotiation completes. Routers, however, MUST handle such notifications (by ignoring them) for backwards compatibility with caches serving protocol version 0.

Once the cache and router have agreed upon a Protocol Version via the negotiation process above, that version is stable for the life of the session. See Section 5.1 for a discussion of the interaction between Protocol Version and Session ID.

If either party receives a PDU for a different Protocol Version once the above negotiation completes, that party MUST drop the session; unless the PDU containing the unexpected Protocol Version was itself an Error Report PDU, the party dropping the session SHOULD send an Error Report with an error code of 8 ("Unexpected Protocol Version").