這種問題建議還是直接看RFC最準，各家實做或教科書都還是有可能曲解原意的
節錄如下
8.1.1.7
The branch parameter value MUST be unique across space and time for
all requests sent by the UA. The exceptions to this rule are CANCEL
and ACK for non-2xx responses. As discussed below, a CANCEL request
will have the same value of the branch parameter as the request it
cancels. As discussed in Section 17.1.1.3, an ACK for a non-2xx
response will also have the same branch ID as the INVITE whose
response it acknowledges.
17.2.3
The branch parameter in the topmost Via header field of the request
is examined. If it is present and begins with the magic cookie
"z9hG4bK", the request was generated by a client transaction
compliant to this specification. Therefore, the branch parameter
will be unique across all transactions sent by that client. The
request matches a transaction if:
1. the branch parameter in the request is equal to the one in the
top Via header field of the request that created the
transaction, and
2. the sent-by value in the top Via of the request is equal to the
one in the request that created the transaction, and
3. the method of the request matches the one that created the
transaction, except for ACK, where the method of the request
that created the transaction is INVITE.
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