Billy,
I know others have been thinking about whether we need the full To and From URLs (or
a MD5 hash of them) in the Replaces header. I'm not convinced myself, but there is
one possible scenario that could cause trouble. A RFC2543 UAC which is redirected
directly to another RFC2543 UAS (no proxies in between) could result in a call
without any tags (To tags are not mandatory in RFC2543 unless there are more than
one Via headers, ie a proxy in the path). Now, we are down to relying only on the
uniqueness of the Call-ID. Of course, any bis UA will use From and To tags.
The only other argument that I've heard that could sway me is that if this actual
situation was encountered in the real world, the result would be disaster. If a
Gateway replaces the wrong session, then the wrong session will be dropped and two
parties that had no intention of talking will suddenly be connected - a very hard
thing to explain to your customers!
Wouldn't a MD5 hash of specific parts of the To and From headers (user, host, port,
and tags, with rules for default handling) answer your concerns about URL escaping
and header size and eliminate the possibility of the wrong session being replaced?
My main concern is that we finish this discussion now so that we can finalize the
attended transfer call flows now that REFER seems to be stable.
Thanks,
Alan Johnston
WorldCom
sip:[EMAIL PROTECTED]
Billy Biggs wrote:
> Dvir Oren ([EMAIL PROTECTED]):
>
> > Are the tags in 'to' and 'from' plus the call-id sufficient to find a
> > call-leg? It appears from the replaces draft that this is all is
> > needed. However, at least in my implementation, I also need the 'to'
> > and 'from' themselves, and not only the tags.
>
> It's unclear to my why you feel so strongly about needing the full to
> and from headers.
>
> In almost all cases, the call-id alone should be sufficient to
> identify an active call. For Replaces, the only cases I can think of
> where the call-id might be insufficient are:
>
> a) Two UAs answer the same call and both are accepted. When an INVITE
> arrives with a replaces, there is a possibility that both UAs
> receive the INVITE (some transparent proxy may have decided to fork
> it). In this case, the UAs each check to see who's tags are
> listed to determine if they should honour the Replaces.
>
> b) Some broken UA makes many calls under the same call-id. In this
> case, checking the tags is a cheap way of identifying which call
> was intended.
>
> Clearly in these cases, having the tags is useful, and having the full
> to and from would be more 'correct' (but AFAICT not, as you claim,
> required). My reason for just using the tags is a practical one: if
> the full to and from are used, the escaping and unescaping gets much
> worse for very little gain, and we've also greatly increased the length
> of the header.
>
> I worry about:
> Replaces: [EMAIL PROTECTED];[EMAIL PROTECTED]%3bmobileid%3d9775
> ;to-tag=88f787f
>
> from:
> Refer-To: sip:[EMAIL PROTECTED]?
> [EMAIL PROTECTED][EMAIL PROTECTED]
> %253bmobileid%253d9775%3dto-tag%3d88f787f
>
> Where double-escape code decoding is required.
>
> --
> Billy Biggs [EMAIL PROTECTED]
> http://www.billybiggs.com/ [EMAIL PROTECTED]
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