Implementations are encouraged to follow this protocol for populating the extraData field of mined blocks.
extraData should be an RLP list whose first element is a version identifier encoded as a canonical RLP positive integer. All other items in the list are determined by the version ID.
[ version: P, ... ]
[ version: P, clientIdentity: B ]
One further argument, a raw representation of a string to identify the client (this would usually be a shortened form of the client identifier as returned by the JSON RPC's web3_clientVersion).