Subject: Re: [sipcore] INVITE 3xx and early media [was OPTIONS 3xx: To fork or not to fork?]

From: [email-masked]
Date: Mon, 8 Mar 2010 10:28:54 +0800

I'd like to talk about how our equipment handle this issue(I still think 
the early media issue of forking is exist).

Logically, we define two logical point(can be in one physical equipment):
1. Caller's Gateway, such as MGCF(of IMS) for PSTN.
2. Forking Equipment.

As Forking Equipment has no way to know the contents of early media. So it 
is not proper for the Forking Equipment to do any filter of early medias. 
So, we make the Forking Equipment transfer all early medias in different 
dialogs to the caller direction.

The Caller's Gateway is the equipment familiar with caller's capability 
and its preference. So, if caller can accept multiple early medias, the 
Caller's Gateway just render the all; if caller can only afford one early 
media, the Caller's Gateway just send one early media(Usually, our 
Caller's Gateway send its own ringing backtone, insteand of choosing one).

In fact, if some early medias are used as collecting DTMF for call 
re-dispatch or code, the Caller's Gateway may still discard them and cause 
problem. While caller's capability may cause more basic problem of 
forking, such as caller can only afford three forking dialog, but it is 
the fourth one has real call answer. Such problems caused by UAC's 
capability can only be handled as best effort, IMO.

Thanks,

Gao
 
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