OK, after some research I think I found the answer.
The keywords you should have been looking for are "strong name" (or "strongName") and/or permutation, since it seems that with the RPC request they send out the permuatation strong name (that MD5 hash), so that you can possibly distinguish on the server side from which permutation the request was send.
The core function is Util.computeStrongName
, it computes an MD5 hash (d'oh) of the provided byte array, with the added catch:
/*
* Include the lengths of the contents components in the hash, so that the
* hashed sequence of bytes is in a one-to-one correspondence with the
* possible arguments to this method.
*/
From there, I tracked back to the linkers and then to PermutationResult which is feeding Util.computeStrongName
via this function:
/**
* The compiled JavaScript code as UTF8 bytes.
*/
byte[][] getJs();
Eh, I hope that was at least a bit helpful ;) If this still doesn't answer your question (or you were looking for something different), try in trunk/user/src/com/google/gwt/user/client/rpc
(start in RpcRequestBuilder.java) and/or trunk/user/src/com/google/gwt/rpc