RTP Payload Format for Portable Network Graphics (PNG) image
Columbia University
Dept. of Computer Science
1214 Amsterdam Avenue
New York NY
10027
US
boyaci@cs.columbia.edu
Columbia University
Dept. of Computer Science
1214 Amsterdam Avenue
New York NY
10027
US
schulzrinne@cs.columbia.edu
This document describes how to carry Portable Network Graphics (PNG) images in RTP packets.
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 .
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.
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 .
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.
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.
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 .
The remaining RTP header fields are used as specified in RFC 3550.
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).
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification .