RTP Payload Format for Portable Network Graphics (PNG) image

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 .