RTP Payload Format for Real-Time Pointers
Extracted elements (16)
The payload format is explicitly not designed as a general mouse event transmission mechanism; it intentionally omits functionalities beyond pointer coordinate and icon signaling to keep the format simple and economical.
Transmitting pointer coordinates separately from video is motivated by the conflicting resolution requirements: viewgraphs demand high spatial resolution while pointer movements require high temporal resolution. Encoding both in a single video stream would require at least S-VHS quality, making compression expensive and uncompressed transmission bandwidth-prohibitive.
The payload format has no payload-specific header and reuses the standard RTP header fields (SSRC, CSRC, sequence number, timestamp) exactly as specified in RFC 1889, ensuring compatibility with existing RTP infrastructure.
Applications MAY ignore non-zero PIN values on reception and display a default pointer icon instead.
RTCP SHOULD be used as defined in RFC 1889 when transmitting real-time pointer payloads.
The association between PIN values and pointer icon pictures MUST be established out-of-band. Applications which only support a single pointer icon SHOULD set the PIN field to zero.
The RTP payload type (PT) for this format is not assigned by this document; it is expected to be assigned by the applicable RTP profile, or a payload type in the dynamic range SHALL be chosen.
The use of the L, M, and R special-effects flag bits MUST be established out-of-band; applications MAY ignore these bits entirely.
The RTP Marker (M) bit is set to one when the pointer icon is changed in that packet, giving receivers a hint that the displayed icon should be updated.
The RTP payload for real-time pointers consists of a single 32-bit word appended to the standard RTP header. It contains L, M, R flag bits, a 12-bit x-coordinate, a 3-bit PIN field, and a 12-bit y-coordinate, with two MBZ (must be zero) bits separating the fields. There is no payload-specific header.
The RTP timestamp field for this payload uses a 90 kHz clock, measuring the sampling time of the pointer location.
This document registers the MIME media type video/pointer for use with RTP-transported real-time pointer coordinates. Required and optional parameters are both None.
RTP packets using this payload format are subject to the security considerations of the RTP specification (RFC 1889). This payload type does not exhibit significant non-uniformity in receiver-side computational complexity, so it does not pose a denial-of-service threat through processing asymmetry.
The L (left), M (middle), and R (right) bits are 1-bit flags encoding pointer special effects; their semantics are application-defined and must be negotiated out-of-band.
The PIN (Pointer Icon Number) field is 3 bits wide and selects among pointer icons. PIN = 0 designates the default pointer icon.
The x and y pointer coordinates are each encoded as 12-bit unsigned fixed-point fractions measured from the upper-left corner of the display window, representing values between 0 and (1 - 2^-12) of the corresponding edge length.