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Streaming media are multimedia that are constantly received by, and normally presented to, an end-user while being delivered by a streaming provider (the term "presented" is used in this article in a general sense that includes audio or video playback). The name refers to the delivery method of the medium rather than to the medium itself. The distinction is usually applied to media that are distributed over telecommunications networks, as most other delivery systems are either inherently streaming (e.g., radio, television) or inherently non-streaming (e.g., books, video cassettes, audio CDs). The verb 'to stream' is also derived from this term, meaning to deliver media in this manner. Internet television is a commonly streamed media.

Streaming Protocols

Designing a network protocol to support streaming media raises many issues, such as:

Datagram protocols, such as the User Datagram Protocol (UDP), send the media stream as a series of small packets. This is simple and efficient; however, there is no mechanism within the protocol to guarantee delivery. It is up to the receiving application to detect loss or corruption and recover data using error correction techniques. If data is lost, the stream may suffer a dropout.

The Real-time Streaming Protocol (RTSP), Real-time Transport Protocol (RTP) and the Real-time Transport Control Protocol (RTCP) were specifically designed to stream media over networks. RTSP runs over a variety of transport protocols, while the latter two are built on top of UDP.

Reliable protocols, such as the Transmission Control Protocol (TCP), guarantee correct delivery of each bit in the media stream. However, they accomplish this with a system of timeouts and retries, which makes them more complex to implement. It also means that when there is data loss on the network, the media stream stalls while the protocol handlers detect the loss and retransmit the missing data. Clients can minimize this effect by buffering data for display. While delay due to buffering is acceptable in video on demand scenarios, users of interactive applications such as video conferencing will experience a loss of fidelity if the delay that buffering contributes to exceeds 200 ms.

Unicast protocols send a separate copy of the media stream from the server to each recipient. Unicast is the norm for most Internet connections, but does not scale well when many users want to view the same program concurrently.

Multicast protocols were developed to reduce the data replication (and consequent server/network loads) that occurs when many recipients receive unicast content streams independently. These protocols send a single stream from the source to a group of recipients. Depending on the network infrastructure and type, multicast transmission may or may not be feasible. One potential disadvantage of multicasting is the loss of video on demand functionality. Continuous streaming of radio or television material usually precludes the recipient's ability to control playback. However, this problem can be mitigated by elements such as caching servers, digital set-top boxes, and buffered media players.

IP Multicast provides a means to send a single media stream to a group of recipients on a computer network. A multicast protocol, usually IGMP, is used to manage delivery of multicast streams to the groups of recipients on a LAN. One of the challenges in deploying IP multicast is that routers and firewalls between LANs must allow the passage of packets destined to multicast groups. If the organization that is serving the content has control over the network between server and recipients (i.e., educational, government, and corporate intranets), then routing protocols such as PIM can be used to deliver stream content to multiple LAN segments.

Peer-to-peer (P2P) protocols arrange for prerecorded streams to be sent between computers. This prevents the server and its network connections from becoming a bottleneck. However, it raises technical, performance, quality, and business issues.

HTTP Live Streaming

HTTP Live Streaming is an HTTP-based media streaming communications protocol implemented by Apple Inc. as part of their QuickTime X and iPhone software systems. It works by breaking the overall stream into a sequence of small HTTP-based file downloads, each download loading one short chunk of an overall potentially unbounded transport stream. As the stream is played, the client may select from a number of different alternate streams containing the same material encoded at a variety of data rates, allowing the streaming session to adapt to the available data rate. At the start of the streaming session, it downloads an extended M3U playlist containing the metadata for the various sub-streams which are available.

Since its requests use only standard HTTP transactions, HTTP Live Streaming is capable of traversing any firewall or proxy server that lets through standard HTTP traffic, unlike UDP-based protocols such as RTP, or adapted versions of HTTP such as RTSP.

Apple have documented HTTP Live Streaming as an Internet-Draft, the first stage in the process of submitting it to the IETF as a proposed Internet standard.