ACK is dropped.

Hello everyone,
    i fell in a problem today. i developed a proxy based on nist-sip1.2.
today, i tested the proxy with a new sip phone and found the ACK message is
dropped by sip stack. by going through the related code, i found the reason
of dropping ACK is that the ACK and its INVITE have the same branchids in
via header.
    in the beginning, i supposed it's a problem of the new sip phone. but
when i looked up rfc3261, i found the following information from rfc3261
section17.1.1.3: "The ACK MUST contain a single Via header field, and this
MUST be equal to the top Via header field of the original request.". there
is also an example:

   For example, consider the following request:

   INVITE sip:[email protected] SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
   To: Bob <sip:[email protected]>
   From: Alice <sip:[email protected]>;tag=88sja8x
   Max-Forwards: 70
   Call-ID: 987asjd97y7atg
   CSeq: 986759 INVITE

   The ACK request for a non-2xx final response to this request would
   look like this:

   ACK sip:[email protected] SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
   To: Bob <sip:[email protected]>;tag=99sa0xk
   From: Alice <sip:[email protected]>;tag=88sja8x
   Max-Forwards: 70
   Call-ID: 987asjd97y7atg
   CSeq: 986759 ACK
    so i am confused. anyone can help me?

--
Best Regards,

Jie Shen

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