1.3 Overview

  • Article

This document specifies proprietary extensions to the Real-Time Transport Protocol (RTP) and the audio video profile (AVP). RTP provides end-to-end delivery services for data with real-time characteristics. The AVP defines the AV-specific interpretations of profile-dependent fields. RTP extensions define packet formats to convey additional information and behavioral changes to enhance host security. These extensions include:

  • Dominant speaker notification: Extends the standard RTP "Active Contributor" mechanism, the contributing source (CSRC) list, by assigning a special meaning to the first element of the list.

  • Synchronization Source (SSRC)/Sequence Number change throttling: Limits the number of times the SSRC or sequence number of a participant can change over a short period in time. The intention of this limit is to avoid attacks that seek to artificially increase resource usage on a host by flooding it with values that could force a costly re-initialization of receiver components.

  • Bandwidth estimation: Defines a new mechanism to estimate and communicate the bandwidth on a channel. One host sends two or more "probe packets", and the other host can use the time interval between them to estimate the bandwidth, which is then communicated back through a Real-Time Transport Control Protocol (RTCP) profile extension.

  • Packet loss notification: Defines an RTCP profile extension that allows a receiver to quickly notify the sender of the loss of a specific packet. The sender can then use this information to hasten recovery, such as by generating a new I-frame or Super P-frame (SP-frame) in the case of a video stream encapsulated through extensions described in [MS-RTVPF].

  • Application Sharing: Defines an application sharing profile, described in [MS-RTASPF], to support desktop/application sharing over RTP.

  • Video Preference: Defines an RTCP profile extension that allows a receiver to request a sender to change the video resolution in the middle of the call, such as by generating a new I-frame of the desired resolution of a video stream encapsulated through extensions described in [MS-RTVPF].

  • Policy Server Bandwidth: Defines an RTCP profile extension that allows a host to send its bandwidth provisioned by the policy server, as obtained through the Traversal Using Relay NAT (TURN) protocol to the remote host.

  • TURN Server Bandwidth: Defines an RTCP profile extension that allows a host to send its bandwidth provisioned by the TURN server as obtained through the TURN protocol to the remote host.

  • SDES PRIV extension for media quality: Defines a private Source Descriptions for Media Streams (SDES) extension for sending the media quality from the CSRC or SSRC to a media receiver. The receiver can use this information to show which source is causing quality issues.

  • Receiver-side audio healer report: Defines an RTCP profile extension that allows a host to send its receiver-side audio healer metrics, local network receive quality, and FEC distance request to the sender to help the sender drive the audio forward error correction (FEC).

  • Receiver-side bandwidth limit: Defines an RTCP profile extension that allows a host to send its receiver-side bandwidth limit request to the remote host to let the remote host know the maximum bandwidth it is capable of receiving.

  • Peer info exchange: Defines an RTCP profile extension that enables a host to send its inbound and outbound network bandwidth throughput limit to the remote host.

  • Picture loss indication: Defines an RTCP feedback message that enables a receiver to indicate there is a packet loss to the sender.

  • Video source request: Defines an RTCP feedback message that enables a receiver to request video from a specified video source.

  • Dominant speaker history notification: Defines an RTCP feedback message that enables a mixer to notify the receivers the history of dominant speakers.