> From: Isaac Dudy > > Suppose UA sends INVITE to FQDN which is resolved to 2 different IP > addresses. > The UA tries the first IP, but there is no response since the > server is > down so the UA tries the second IP and succeeds (the UA got 2xx > response). > Now the UA has to send ACK but it MUST do resolving again.
I believe that that statement is not correct. See para. 4 of section 13.2.2.4 of RFC 3261: "The header fields of the ACK are constructed in the same way as for any request sent within a dialog ..." In this case, it would only be sensible if the ACK was sent using the route set established by the INVITE/200. (I admit that para. 5 of that section contradicts my analysis: "Once the ACK has been constructed, the procedures of [4][essentially, re-resolution of the URI -- DRW] are used to determine the destination address, port and transport." But following that rule contradicts para. 4 and leads to the operational difficulties you described above.) Dale
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