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Network Working Group                                          O. Boyaci
Internet-Draft                                            H. Schulzrinne
Intended status: Standards Track                             Columbia U.
Expires: March 12, 2009                                September 8, 2008


      RTP Payload Format for Portable Network Graphics (PNG) image
                        draft-boyaci-avt-png-00

Status of this Memo

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Abstract

   This document describes how to carry Portable Network Graphics (PNG)
   images in RTP packets.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  RTP Header Usage . . . . . . . . . . . . . . . . . . . . .  5
   4.  SDP Parameters . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  Normative References . . . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10


































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1.  Introduction

   PNG is a bitmapped image format that employs lossless data
   compression.  PNG is standartized in ISO/IEC 15948:2003 and the W3C's
   PNG (Second Edition) Recommendation [W3C.REC-PNG-20031110].  Client
   applications may display PNG images carried in RTP as a slideshow.
   Application and desktop sharing systems MAY use PNG images to carry
   screen updates.











































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2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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3.  Payload Format

   For RTP-based transport of PNG images, the standard RTP header is
   followed by the payload data.  There is no payload header.  The
   payload data contains the PNG image.  Each RTP packets carries a
   single PNG image or a fragment of a PNG image.

3.1.  RTP Header Usage

   Marker Bit:  The marker bit is used to indicate the last packet part
      of a PNG image.  PNG images may not fit into a single RTP packet.
      This enables the client to finish decoding the image, where it
      otherwise may need to wait for the next packet to explicitly know
      that.  The M bit is set to one to indicate that the RTP packet
      payload contains a complete PNG image or contains the final
      fragment of a PNG image.

   Timestamp:  For PNG image streams, the RTP timestamp is based on a
      90-kHz clock.  If a PNG image occupies more than one packet, the
      timestamp SHALL be the same on all of those packets.  Furthermore,
      the initial value of the timestamp MUST be random (unpredictable)
      to make known-plaintext attacks on encryption more difficult; see
      RTP [RFC3550].

   The remaining RTP header fields are used as specified in RFC 3550.


























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4.  SDP Parameters

   The MIME media type image/png string is mapped to fields in the
   Session Description Protocol (SDP) as follows:

      The media name in the "m=" line of SDP MUST be image.

      The encoding name in the "a=rtpmap" line of SDP MUST be png (the
      MIME subtype).










































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5.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550].














































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6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [W3C.REC-PNG-20031110]
              Duce, D., "Portable Network Graphics (PNG) Specification
              (Second Edition)", World Wide Web Consortium
              Recommendation REC-PNG-20031110, November 2003,
              <http://www.w3.org/TR/2003/REC-PNG-20031110>.





































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Authors' Addresses

   Omer Boyaci
   Columbia University
   Dept. of Computer Science
   1214 Amsterdam Avenue
   New York, NY  10027
   US

   Email: boyaci@cs.columbia.edu


   Henning Schulzrinne
   Columbia University
   Dept. of Computer Science
   1214 Amsterdam Avenue
   New York, NY  10027
   US

   Email: schulzrinne@cs.columbia.edu































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Full Copyright Statement

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