Negotiation of Generic Image Attributes in the Session Description Protocol (SDP)
RFC 6236

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Internet Engineering Task Force (IETF)                      I. Johansson
Request for Comments: 6236                                   Ericsson AB
Category: Standards Track                                        K. Jung
ISSN: 2070-1721                            Samsung Electronics Co., Ltd.
                                                                May 2011

               Negotiation of Generic Image Attributes in
                 the Session Description Protocol (SDP)

Abstract

   This document proposes a new generic session setup attribute to make
   it possible to negotiate different image attributes such as image
   size.  A possible use case is to make it possible for a low-end hand-
   held terminal to display video without the need to rescale the image,
   something that may consume large amounts of memory and processing
   power.  The document also helps to maintain an optimal bitrate for
   video as only the image size that is desired by the receiver is
   transmitted.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6236.

Johansson & Jung             Standards Track                    [Page 1]
RFC 6236                 Image Attributes in SDP                May 2011

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  5
   3.  Specification of the 'imageattr' SDP Attribute . . . . . . . .  5
     3.1.  Attribute Syntax . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Overall View of Syntax . . . . . . . . . . . . . . . .  5
     3.2.  Considerations . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.1.  No imageattr in First Offer  . . . . . . . . . . . . . 11
       3.2.2.  Different Payload Type Numbers in Offer and Answer . . 11
       3.2.3.  Asymmetry  . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.4.  sendonly and recvonly  . . . . . . . . . . . . . . . . 12
       3.2.5.  Sample Aspect Ratio  . . . . . . . . . . . . . . . . . 13
       3.2.6.  SDPCapNeg Support  . . . . . . . . . . . . . . . . . . 13
       3.2.7.  Interaction with Codec Parameters  . . . . . . . . . . 14
       3.2.8.  Change of Display in Middle of Session . . . . . . . . 16
       3.2.9.  Use with Layered Codecs  . . . . . . . . . . . . . . . 16
       3.2.10. Addition of Parameters . . . . . . . . . . . . . . . . 16
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  A High-Level Example . . . . . . . . . . . . . . . . . . . 16
     4.2.  Detailed Examples  . . . . . . . . . . . . . . . . . . . . 17
       4.2.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . 17
       4.2.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . 18
       4.2.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . 19
       4.2.4.  Example 4  . . . . . . . . . . . . . . . . . . . . . . 20
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 22

Johansson & Jung             Standards Track                    [Page 2]
RFC 6236                 Image Attributes in SDP                May 2011

1.  Introduction

   This document proposes a new SDP attribute to make it possible to
   negotiate different image attributes, such as image size.  The term
   image size is defined here, as it may differ from the physical screen