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
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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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