-----Original Message-----
From: Catherine Marselli [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, June 12, 2001 10:47 AM
To: [EMAIL PROTECTED]
Subject: [Sip-implementors] RTP in a SIP session
>Dear all,
>
>In a SIP session, at what time should the User Agent be able to listening
>to the RTP port: Once the session is established (invite, ok 200, ack) or
>at any time during the message exchanges ?
It was agreed upon at previous IETFs that a UAC should be prepared to
receive media after sending the INVITE. This is to avoid media clipping,
since in the case of a phone, the called party will pick up the phone and
say "hello" at the same time, generating media at nearly the same instant
the 200 OK is sent. It is highly likely that the 200 OK will arrive after
the media is received. If the UAC is not prepared to receive the media
before the 200 OK, this media is lost, and thus the "clipping".
Some have argued that this is why early media is needed; specifically, that
the UAS should rather send a 183 with SDP immediately in order to enable
media at that time. During the SIP interim meeting some time back, we
discussed this, and concluded that there is a difference between early media
(media sent by the UAS BEFORE the call is accepted), and clipping. We will
solve the early media problem separately, since its not always needed, but
the clipping problem is common and needs to be solved without requiring
early media. Thus the decision to listen for media after sending INVITE.
Others have argued that this introduces security concerns (anyone can send
me media!). However, they are no worse than accepting media after the 183.
So, it was agreed that if you were concerned about security, the right thing
is to esablish a truly secure, authenticated media stream (ala SRTP).
-Jonathan R.
---
Jonathan D. Rosenberg, Ph.D. 72 Eagle Rock Ave.
Chief Scientist First Floor
dynamicsoft East Hanover, NJ 07936
[EMAIL PROTECTED] FAX: (973) 952-5050
http://www.jdrosen.net PHONE: (973) 952-5000
http://www.dynamicsoft.com
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