Re: [MMUSIC] RTSP 2.0 SDP issue: Directionality attribute

Magnus Westerlund <> Mon, 18 March 2013 19:16 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 040491F0C52 for <>; Mon, 18 Mar 2013 12:16:39 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -105.774
X-Spam-Status: No, score=-105.774 tagged_above=-999 required=5 tests=[AWL=0.475, BAYES_00=-2.599, HELO_EQ_SE=0.35, RCVD_IN_DNSWL_MED=-4, USER_IN_WHITELIST=-100]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id qiEBf-EhPsYd for <>; Mon, 18 Mar 2013 12:16:29 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id 6EA0421F9097 for <>; Mon, 18 Mar 2013 12:15:31 -0700 (PDT)
X-AuditID: c1b4fb25-b7f366d000004d10-00-514767d16ce0
Received: from (Unknown_Domain []) by (Symantec Mail Security) with SMTP id AE.31.19728.1D767415; Mon, 18 Mar 2013 20:15:30 +0100 (CET)
Received: from [] ( by ( with Microsoft SMTP Server id; Mon, 18 Mar 2013 20:15:29 +0100
Message-ID: <>
Date: Mon, 18 Mar 2013 20:15:29 +0100
From: Magnus Westerlund <>
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20130307 Thunderbird/17.0.4
MIME-Version: 1.0
To: "Dale R. Worley" <>
References: <> <>
In-Reply-To: <>
X-Enigmail-Version: 1.5.1
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit
X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFupnluLIzCtJLcpLzFFi42KZGfG3RvdSunugQfdJJoupyx+zWLw8UebA 5DF5/1dmjyVLfjIFMEVx2aSk5mSWpRbp2yVwZUz+uZG5oFWqYmvLJ6YGxtMiXYycHBICJhJN 694zQdhiEhfurWfrYuTiEBI4yShx+dssZghnOaNEw6vVrCBVvALaEpMnbWMGsVkEVCX2757E CGKzCVhI3PzRyAZiiwoES/x8dYYFol5Q4uTMJ0A2B4eIgKZEx4IckDCzgLDEhfPHwRYLCzhJ fFhyA6xVSCBOYv6zCWDjOQXsJT6v7WKHOE5SYsuLdnaIXj2JKVdbGCFseYnmrbOZIXq1JRqa OlgnMArNQrJ5FpKWWUhaFjAyr2Jkz03MzEkvN9rECAzUg1t+q+5gvHNO5BCjNAeLkjhvuOuF ACGB9MSS1OzU1ILUovii0pzU4kOMTBycUg2Ma+dp6W6LanXpEOHZ1X/O6sulWu5Drx6zPusy v7TguZ7wIeN6J2uTybMZ2FfNWHXp+75A7V95axrSF+i9YJWqbilUeWQ1m6XE/e+x+J+fNI8V 7T/1buXnL2y/1N/f/D1vTlZM9Z4nz7cd8NUuqPU+unPO2ok81/6ePueTWf7fMiH/TLRk3q7P V5VYijMSDbWYi4oTAfZpCdIiAgAA
Subject: Re: [MMUSIC] RTSP 2.0 SDP issue: Directionality attribute
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Multiparty Multimedia Session Control Working Group <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 18 Mar 2013 19:16:39 -0000

Hi Dale,

You are correct that RTSP uses SDP with an different interpretation, the
interpretation that is common to usage in SAP, rather than the one in
O/A. I think you are correct that this should be pointed out, but
belongs in the introduction, rather than down in the definition,
although this appears to be the only section where it really matters.

I thus propose a more generic version of your text in the D. section:

   This appendix describes how an SDP file determines the operation of
   an RTSP session.  Thus, it is worth pointing out that the
   interpretation of the SDP is done in the context of the SDP receiver
   is the one being configured.  This is the same as in SAP [RFC2974],
   this differs from SDP Offer/Answer [RFC3264] where the SDP describes
   what the peer agent is ready to receive and willing to send.

Does this works for you?



On 2013-03-15 15:31, Dale R. Worley wrote:
>> From: Magnus Westerlund <>
>>    "a=recvonly" in a SDP provided to the RTSP client MUST
>>    indicate that media delivery will only occur in the direction from
>>    the RTSP server to the client.
> I know little about RTSP though I've worked with SIP a lot.  I think
> you might want to flag this odd inconsistency between RTSP and SIP:
> When SDP is used in offer-answer mode, the directionality attributes
> indicate directionality *from the point of view of the sender of the
> SDP*.  (RFC 3264 section 5.1) So a SIP offer containing "a=recvonly"
> indicates that the offerer expects to *receive* media.
> When SDP is used in session-announcement mode, the directionality
> attributes indicate directionality *from the point of view of the
> receiver of the SDP*.  (This is not stated directly in RFC 4566,
> rather it says "a=recvonly: This specifies that the tools should be
> started in receive-only mode where applicable." but eventually you can
> track down that "the tools" means the receiver's device.)  So an RTSP
> announcement containing "a=recvonly" indicates that the announcer
> expects to *send* media.
> This difference is not what I would have expected.  Initially, I
> assumed the text in your draft had gotten the directions reversed
> relative to reality.
> You might want to mention this somewhere, e.g., after the first
> paragraph of D.1.5, add the paragraph
>     (Note that RTSP uses SDP in session-announcement mode [RFC 4566],
>     in which the directionality attributes indicate the direction of
>     media flow from the point view of the interpreter of the SDP.
>     This contrasts with SDP in offer-answer mode [RFC 3264], in which
>     the directionality attributes indicate the direction of media flow
>     from the point of view of the generator of the SDP.)
> Dale


Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVM
Ericsson AB                | Phone  +46 10 7148287
Färögatan 6                | Mobile +46 73 0949079
SE-164 80 Stockholm, Sweden| mailto: