Re: [AVTCORE] What does "same codec+configuration" mean?

Magnus Westerlund <> Tue, 22 March 2016 14:55 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id ED29912D9EF for <>; Tue, 22 Mar 2016 07:55:48 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -4.202
X-Spam-Status: No, score=-4.202 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id cf7iuYg7lY3R for <>; Tue, 22 Mar 2016 07:55:47 -0700 (PDT)
Received: from ( []) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 73C0312D914 for <>; Tue, 22 Mar 2016 07:55:29 -0700 (PDT)
X-AuditID: c1b4fb3a-f79d86d000005b69-e1-56f15cdf7782
Received: from (Unknown_Domain []) by (Symantec Mail Security) with SMTP id CE.A0.23401.FDC51F65; Tue, 22 Mar 2016 15:55:27 +0100 (CET)
Received: from [] ( by ( with Microsoft SMTP Server id; Tue, 22 Mar 2016 15:55:25 +0100
To: =?UTF-8?Q?I=c3=b1aki_Baz_Castillo?= <>, "" <>
References: <>
From: Magnus Westerlund <>
Message-ID: <>
Date: Tue, 22 Mar 2016 15:55:24 +0100
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0
MIME-Version: 1.0
In-Reply-To: <>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Transfer-Encoding: 8bit
X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFtrELMWRmVeSWpSXmKPExsUyM2K7ru79mI9hBht/alm87FnJbjF9n40D k8e5hvfsHkuW/GQKYIrisklJzcksSy3St0vgyrh74wRzwUmhit53XewNjC/5uhg5OSQETCR+ Nv5ghLDFJC7cW8/WxcjFISRwmFFi0o+J7BDOckaJzYuXsoBUCQs4Shy9voEVxBYRiJD48vIv UJwDqChA4scKeZAwm4CFxM0fjWwgNq+AusSeKxvYQWwWAVWJVbv3g40RFYiROP7uHCNEjaDE yZlPwMZwCgRKXLlYChJmBhozc/55RghbXqJ562xmEFtIQFuioamDdQKjwCwk3bOQtMxC0rKA kXkVo2hxanFxbrqRkV5qUWZycXF+nl5easkmRmBIHtzy22oH48HnjocYBTgYlXh4DbZ+CBNi TSwrrsw9xCjBwawkwmsc/TFMiDclsbIqtSg/vqg0J7X4EKM0B4uSOC/bp8thQgLpiSWp2amp BalFMFkmDk6pBkbTItd5rvuPin6XE7n14sCle+y5fb+f62StT7h37VXmFMYmo2tPZa49vsbv 78Wed/XLpRfTfGsL1c+7C0iesvNmKi2WEYx3LqgQOL82YoLU5Y/TA1v+rbLUnLV8clf8ypnH lld8uZykXxBee5o1544mr72/65r8s0LTGE/svde/t+z1ZCnzHF0lluKMREMt5qLiRACMHMTQ RQIAAA==
Archived-At: <>
Subject: Re: [AVTCORE] What does "same codec+configuration" mean?
X-Mailman-Version: 2.1.17
Precedence: list
List-Id: Audio/Video Transport Core Maintenance <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Tue, 22 Mar 2016 14:55:49 -0000

Den 2016-03-11 kl. 21:36, skrev Iñaki Baz Castillo:
> Hi,
> According to BUNDLE spec, two bundled m= lines can share the same PT
> value only if such a PT value points to the same codec name + codec
> configuration.
> What does "codec + configuration" mean? Some codecs have specific
> parameters, some parameters may be optional, and AFAIK there are some
> parameters that don't need to be fully negotiated by both endpoints.
> May I get some examples and be pointed to the appropriate spec?

So if one reads the bundle specification it says the following:

10.1.2.  Payload Type (PT) Value Reuse

    Multiple bundled "m=" lines might represent RTP based media.  As all
    RTP based media associated with a BUNDLE group belong to the same RTP
    session, in order for a given payload type value to be used inside
    more than one bundled "m=" line, all codecs associated with the
    payload type number MUST share an identical codec configuration.
    This means that the codecs MUST share the same media type, encoding
    name, clock rate and any parameter that can affect the codec
    configuration and packetization.
    [I-D.ietf-mmusic-sdp-mux-attributes] lists SDP attributes, whose
    attribute values must be identical for all codecs that use the same
    payload type value.

So identical a=rtpmap and a=fmtp lines would meet this criteria.

However, two different m= blocks configuring a particular PT for a codec 
could possibly use different values for an optional parameter that can 
be received correctly by either configurations.

Codecs that can produce different encoding bit-rates can in many cases 
be used at different bit-rates and fulfil this requirement.

The underlying issue is that what is identical codec configurations are 
codec specific. Thus hard to write a precise definition of it.

Does this help?

Magnus Westerlund

Services, Media and Network features, Ericsson Research EAB/TXM
Ericsson AB                 | Phone  +46 10 7148287
Färögatan 6                 | Mobile +46 73 0949079
SE-164 80 Stockholm, Sweden | mailto: