There are two types of ACK which has different handling mechanism.
There is no retransmissions for ACK. ACK is always sent in response to
"response".
ACK for 200 OK is end-to-end and is sent whenever 200 OK is received
by the application. This is Route constructed from Record-Route of 200
OK and request-uri modified to the Contact.
ACK for final failure response is hop-to-hop and is sent whenever
retransmitted final failure response is received for the interval of
Timer-D (Usually 64*T1). The later case is usually handled by the
transaction layer and not by the application as the first final
response received terminates the dialog in the application.
On 2/27/07, John Williams <[EMAIL PROTECTED]> wrote:
> Hello,
>
> I have several questions about retransmissions in a UAC, especially ACK
> retransmission :
>
> 1. When an ACK request must be retransmitted ? and how ?
>
> I've been looking in RFC 3261 in 17.1.1.3 for 3xx/6xx ACK and in
> 13.2.2.4 for 2xx ACK. ACK construction is detailed but I did not found
> the retransmission process. In 13.2.2.4 :
>
> The ACK MUST be passed to the client transport every time a
> retransmission of the 2xx final response that triggered the ACK
> arrives.
>
> Ok, but this is not really an ACK retransmission as it is triggered by
> the arrival of another 2xx response.
>
> The UAC core considers the INVITE transaction completed 64*T1 seconds
> after the reception of the first 2xx response. At this point all the
> early dialogs that have not transitioned to established dialogs are
> terminated. Once the INVITE transaction is considered completed by
> the UAC core, no more new 2xx responses are expected to arrive.
>
> Ok, that says when ACK retransmissions must be stopped, but not when
> (and how) retransmissions are triggered. I think they can be triggered
> like regular requests (exponential T1 timer), but there is no response
> to stop ACK retransmission... There must be something I misunderstood.
>
> 2. Is it necessary to retransmit an ACK if a transport error occured
> while sending the first ACK ?
>
> That would be the trigger I am looking for.
>
> 3. More generally, is it necessary to retransmit a request if a
> transport error occured (creating a new transport) ?
>
> I think it is not necessary, but I am not sure.
>
>
> Thanks,
>
> john
>
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