Class StunChangeRequestAttribute

source code

CHANGE-REQUEST

The CHANGE-REQUEST attribute is used by the client to request that
the server use a different address and/or port when sending the
response.  The attribute is 32 bits long, although only two bits (A
and B) are used:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The meaning of the flags is:

A: This is the "change IP" flag.  If true, it requests the server
   to send the Binding Response with a different IP address than the
   one the Binding Request was received on.

B: This is the "change port" flag.  If true, it requests the
   server to send the Binding Response with a different port than the
   one the Binding Request was received on.