Which attributes are mandatory in server's request to a client (step 3 of STUN-ICE interaction)?
Chapter "2.2. Connectivity Checks" of RFC 5245 tells about "4-way handshake".
step1: client L sends a request to the server R with USE-CANDIDATE attribute,
step2: server R sends a response to the client L,
step3: server R sends a request to the client L,
step4: client L sends a response to the server R.
The mandatory attributes of step2 are known:
MAPPED-ADDRESS, XOR-MAPPED-ADDRESS, USERNAME, MESSAGE-INTEGRITY, FINGERPRINT.
Additionally, transaction ID in this response must be equal to transaction ID of initial request of the server L.
My question is: which attributes are mandatory in the step3: when server R sends your request to the client L ? And should transaction ID on step3 correspond IDs on step1 and 2 ?