[asterisk-users] T.38 - Which endpoint shall reINVITE ? caller or callee ?

[Steve Underwood]

Hi Olivier,

Olivier wrote:
>
>     T.38 says that if the call starts in audio mode it is the called end
>     which should initiate a re-invite to change from audio to T.38. This
>     makes sense, as that is the end which has the best chance of figuring
>     out if a FAX machine answers the call. In practice many T.38
>     implementations will send out a re-invite when they are the calling
>     side, so any practical implementation has to allow for this.
>     Clashes are
>     possible, if both ends send re-invite, and this is not always handled
>     properly
>
>
> Yesterday, with 2 consecutive sendings on the same setup (same fax 
> file, same ATAs, same servers), on the first try, I've seen the 
> reINVITE coming from callee on from the caller on the second try.
> I don't remember I changed anything between both tries (though I may 
> have done without noticing this).
That is what typically what happens when the calling end doesn't obey 
the spec. It comes down to a race for who initiates the re-invite first. 
If you are lucky the two ends sort themselves out. If you are unlucky 
you end up with both ends re-inviting, and you may get a call failure.

Regards,
Steve