Subject: [MMUSIC] setup:actpass = simultaneous-open?

From: "Alan Bergmin" <[email-masked]>
Date: Fri, 17 Aug 2007 06:38:45 +0200

I have a question concerning RFC4145 (TCP-Based Media Transport in the SDP)

Does a setup:actpass SDP attribute mean that the agent has to
implement a TCP simultaneous-open behavior?

Thanks

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From: Rémi Denis-Courmont <[email-masked]>
Date: Fri, 17 Aug 2007 08:51:52 +0300

> implement a TCP simultaneous-open behavior?

No. actpass means that in his grandeur the offerer is willing to abide to 
whatever the answerer decides as regard the connection direction. The 
answerer MUST decide on either active or passive (see RFC4145, §4.1).

There is a possibility that the answerer decides to be the "active" side, 
sends the TCP SYN packet and the SDP answer. The TCP SYN packet might reach 
the offerer before the SDP answer; in fact, this is very likely to happen, 
because routing a single TCP packet is normally much faster than routing a 
SIP response. Therefore, when "actpass" is used, the offerer, MUST have a 
listening socket ready before it sends the SDP offer.
However, it does not need to implement simultaneous-open: if it turns out that 
the answerer picks "passive", the offerer can close the listening socket, and 
open a new socket to do an active connection.

Simultaneous-open is only needed with ICE-TCP. In fact, it is only legal when 
ICE-TCP is used.

-- 
Rémi Denis-Courmont