Subject: [AVT] rtpmap and fmtp for telephone-event

From: "Murat Artun" <[email-masked]>
Date: Mon, 25 Sep 2006 10:43:11 +0300

Hello,

Both in RFC2833 and draft-ietf-avt-rfc2833bis-15 it was specified that
SDP usage is recommended for telephone-event payload format, or I have
interpreted it to be so. In RFC 2833 it was said that: "Receivers MAY
indicate which named events they can handle, for example, by using the
Session Description Protocol (RFC 2327 [7])." In addition, latest
draft indicates in section "Relationship to SDP" that mapping is
recommended.

Considering these, can we say that it is possible for an IP phone that
announces a codec type specification for the telephone-event omits
using fmtp? If this is possible, will the other IP phone which
receives this SDP, interpret it as to be that the remote end is ready
to receive only 0-15?

Thanks.
-- 
M u r at  A r t u n, MSc.
   Software Engineer

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

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From: "Tom-PT Taylor" <[email-masked]>
Date: Wed, 27 Sep 2006 00:39:31 -0400

The key passage is in section 2.5.1.1 of 2833bis:

    The sender SHOULD indicate what events it supports, using the
    optional "events" parameter associated with the telephone-events
    media type.  If the sender receives an "events" parameter from the
    receiver, it MUST restrict the set of events it sends to those listed
    in the received "events" parameter.  For backward compatibility, if
    no "events" parameter is received, the sender SHOULD assume support
    for the DTMF events 0-15 but for no other events.

So it is possible for the IP phone to omit the fmtp, but there should be 
a very good reason for doing so. And as you say, if the fmtp attribute 
isn't there, the other end has good reason to expect support of DTMF 
tone events.