Not a header but an important parameter.
a) Used by both client and server.
b) Unique across space and time for all requests sent by UA.
c) Exceptions to this rule are CANCEL and ACK for non-2xx responses.
i) CANCEL requests will have the same value of the branch parameter as the request it cancels.
ii) ACK for a non-2xx response will also have the same Branch ID as the INVITE whose response it acknowledeges.