8. Media Streaming Protocols and Standards

This section briefly describes the network protocols for media streaming over the Internet. In addition, we highlight some of the current popular specifications and standards for video streaming, including 3GPP and ISMA.

8.1 Protocols for Video Streaming over the Internet

This section briefly highlights the network protocols for video streaming over the Internet. First, we review the important Internet protocols of IP, TCP, and UDP. This is followed by the media delivery and control protocols.

Internet Protocols: TCP, UDP, IP

The Internet was developed to connect a heterogeneous mix of networks that employ different packet switching technologies. The Internet Protocol (IP) provides baseline best-effort network delivery for all hosts in the network: providing addressing, best-effort routing, and a global format that can be interpreted by everyone. On top of IP are the end-to-end transport protocols, where Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the most important. TCP provides reliable byte-stream services. It guarantees delivery via retransmissions and acknowledgements. On the other hand, UDP is simply a user interface to IP, and is therefore unreliable and connectionless. Additional services provided by UDP include checksum and port-numbering for demultiplexing traffic sent to the same destination. Some of the differences between TCP and UDP that affects streaming applications are:

TCP operates on a byte stream while UDP is packet oriented.
TCP guarantees delivery via retransmissions, but because of the retransmissions its delay is unbounded. UDP does not guarantee delivery, but for those packets delivered their delay is more predictable (i.e. one-way delay) and smaller.
TCP provides flow control and congestion control. UDP provides neither. This provides more flexibility for the application to determine the appropriate flow control and congestion control procedures.
TCP requires a back-channel for the acknowledgements. UDP does not require a back-channel.

Web and data traffic are delivered with TCP/IP because guaranteed delivery is far more important than delay or delay jitter. For media streaming the uncontrollable delay of TCP is unacceptable and compressed media data is usually transmitted via UDP/IP despite control information, which is usually transmitted via TCP/IP.

Media Delivery and Control Protocols

The IETF has specified a number of protocols for media delivery, control, and description over the Internet.

Media Delivery

The Real-time Transport Protocol (RTP) and Real-time Control Protocol (RTCP) are IETF protocols designed to support streaming media. RTP is designed for data transfer and RTCP for control messages. Note that these protocols do not enable real-time services, only the underlying network can do this, however they provide functionalities that support real-time services. RTP does not guarantee QoS or reliable delivery, but provides support for applications with time constraints by providing a standardized framework for common functionalities such as time stamps, sequence numbering, and payload specification. RTP enables detection of lost packets. RTCP provides feedback on quality of data delivery. It provides QoS feedback in terms of number of lost packets, inter-arrival jitter, delay, etc. RTCP specifies periodic feedback packets, where the feedback uses no more than 5 % of the total session bandwidth and where there is at least one feedback message every 5 seconds. The sender can use the feedback to adjust its operation, e.g. adapt its bit rate. The conventional approach for media streaming is to use RTP/UDP for the media data and RTCP/TCP or RTCP/UDP for the control. Often, RTCP is supplemented by another feedback mechanism that is explicitly designed to provide the desired feedback information for the specific media streaming application. Other useful functionalities facilitated by RTCP include inter-stream synchronization and round-trip time measurement.

Media Control

Media control is provided by either of two session control protocols: Real-Time Streaming Protocol (RTSP) or Session Initiation Protocol (SIP). RTSP is commonly used in video streaming to establish a session. It also supports basic VCR functionalities such as play, pause, seek and record. SIP is commonly used in voice over IP (VoIP), and it is similar to RTSP, but in addition it can support user mobility and a number of additional functionalities.

Media Description and Announcement

The Session Description Protocol (SDP) provides information describing a session, for example whether it is video or audio, the specific codec, bit rate, duration, etc. SDP is a common exchange format used by RTSP for content description purposes, e.g., in 3G wireless systems. It has also been used with the Session Announcement Protocol (SAP) to announce the availability of multicast programs.

8.2 Video Streaming Standards and Specifications

Standard-based media streaming systems, as specified by the 3^rd Generation Partnership Project (3GPP) for media over 3G cellular [45] and the Internet Streaming Media Alliance (ISMA) for streaming over the Internet [46], employ the following protocols:

Media encoding
- MPEG-4 video and audio (AMR for 3GPP), H.263
Media transport
- RTP for data, usually over UDP/IP
- RTCP for control messages, usually over UDP/IP
Media session control
- RTSP
Media description and announcement
- SDP

The streaming standards do not specify the storage format for the compressed media, but the MP4 file format has been widely used. One advantage of MP4 file format is the ability to include "hint tracks" that simplify various aspects of streaming by providing hints such as packetization boundaries, RTP headers and transmission times.