Sherry,
Please see responses inline, hope I relayed this correctly.
>What's the common implementation for handle transport error on Invite?
Depends on the transport, have a read of section 17.1.1 for unreliable
(UDP) the UAC will retransmit the request at T1 interval etc.... it is not
re-sent over reliable transport and either timer B fires or the TU detects
transport failure.
>In some implementation, if an error occurred when sending Invite, an ACK
>would be sent. Why?
Whether the INVITE request results in success or failure the client sends
an ACK, this is part of the INVITE transaction 3 way handshake {INVITE,
response, ACK]
>If the above is required by spec, then what if the underlying transport
>(TCP, UDP, or TLS) has an error, should UAC still try to send ACK
>knowing that it would go anywhere anyway?
Depends when the transport error occurred - the client sends an ACK to the
final response 3xx-6xx, so if the transport error occured before then
either TU detects transport failure or timer B will expire and the INVITE
transaction will time out.
Thanks,
Sherry
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