Real-time transmission protocol details

Real-time transmission protocol RTP

1. RTP protocol:
RTP (Real-Time Transport Protocol) was originally used in 1970s to try to transfer audio files, divided into several parts to transmit voice, time signs and queue numbers. After a series of developments, the first version of RTP was released by a laboratory in the United States in August 1991. By the 1996 s of this century, a standard version was formed. Many famous companies such as Netscape claim that "Netscape livemedia" is based on the RTP protocol .. Microsoft also claims that their "netmeeting" also supports the RTP protocol.
RTP is defined as the transmission protocol for transmitting real-time data such as audio, video, and analog data. It was originally designed for data transmission multicast, but it is also used for unicast. Compared with the traditional high-reliability data transmission transport layer protocol, it focuses more on real-time data transmission. Services provided by this Protocol include the time load identifier, data sequence, timestamp, and transmission control. RTP and the Auxiliary Control Protocol RTCP work together to obtain some control information about data transmission.

2. How the RTP protocol works:
As mentioned above, an acute problem that threatens multimedia data transmission is unpredictable data arrival time. However, streaming media transmission requires timely arrival of data for playback and playback. The RTP protocol provides time tags, serial numbers, and other structures for controlling the exile of timely data.
In the concept of stream, "Time tag" is the most important information. The sender sets the time tag in the data packet according to the real-time sampling. After receiving the data packet, the receiver restores the original timely data according to the time tag at the correct rate. Different media formats have different attributes. However, RTP itself is not responsible for synchronization. RTP is only a Transport Layer Protocol. In order to simplify the transport layer processing and improve the efficiency of this layer. Move some transport layer protocol functions (such as traffic control) to the application layer. Synchronization is completed by the application layer protocol. It does not have the complete functions of the transport layer protocol, and does not provide any mechanism to ensure real-time data transmission. It does not support resource reservation or service quality. RTP packets do not even contain the length and packet boundary descriptions. At the same time, the RTP protocol datagram text and control packets use different adjacent ports, which greatly improves the Protocol flexibility and simplicity of processing.
The RTP protocol and UDP protocol work together to complete the transport layer protocol function. UDP only transmits data packets regardless of the time sequence of data packets. RTP protocol data units are carried by UDP groups. When carrying RTP data packets, sometimes a frame of data is divided into several packets with the same time tag, you can know that the time tag is not necessary. The multiplexing of UDP allows the RTP protocol to meet the needs of multimedia sessions by supporting explicit multi-point delivery.
Although the RTP protocol is a transport layer protocol, it is not implemented as a separate layer in the OSI architecture. RTP provides services based on a specific application. RTP only provides the Protocol framework. developers can fully expand the protocol according to the specific requirements of the application. At present, RTP is designed and researched to meet the needs of multi-user Multimedia conferences. It is also suitable for the storage of continuous data, interactive Distributed Simulation and some control and measurement applications. RTP-based experiments and commercial products are also emerging.

Real-time transmission control protocol RTCP protocol

1. RTCP protocol:
RTCP (Real-Time transpor and Control Protocol) is a service control protocol designed and used together with RTP for traffic control and congestion control.

2. How does the RTCP protocol work:
When an application starts a RTP session, two ports are used: one for RTP and the other for RTCP. RTP itself does not provide a reliable transmission mechanism for transmitting data packets in order, nor does it provide traffic control or congestion control. It relies on RTCP to provide these services. Some RTCP packets are issued between RTP sessions in a periodic manner to transmit functions such as listening service quality and exchanging session user information. The RTCP package contains statistics such as the number of sent data packets and the number of lost data packets. Therefore, the server can dynamically change the transmission rate or even the type of the payload. RTP and RTCP can be used together to optimize transmission efficiency with effective feedback and minimal overhead. Therefore, RTP is particularly suitable for transmitting real-time online data. Based on the data transmission feedback between users, you can develop a traffic control policy. When you interact with user information, you can develop a session control policy.
The RTCP protocol processor defines five types of packets as needed --
Rr: Cycler report
SR: Sender report
Sdes: source description items.
Bye: indicates end of participant.
APP: application specific functions
They are used to receive, analyze, generate, and send control packets.

RTSP
1. RTSP protocol:
The RTSP (Real Time Streaming Protocol) Protocol defines how to effectively transmit multimedia data over an IP network. It is a multimedia description protocol from a client to a server.
RTSP is an application layer protocol that is very similar to HTTP. Each release and media file is also defined as RTSP upl. The publishing information of a media file is written into a file called a media publishing file, which will be described later. This file describes the encoder, language, rtsp uls, address, and port number with several other parameters. The published file can be obtained by email or HTTP on the client.
RTSP was proposed by RealNetworks, Netscape, and Columbia University. It is developed from the practice and experience of RealNetworks's "RealAudio" and Netscape's "livemedia. The first RTSP protocol was officially submitted by IETF in August 9, 1996 as the internet standard, after which this Protocol has undergone many significant changes. It is widely used now. Apple, ibmnetscape, apple, IBM, Silicon Graphics, vxtreme, sun, and other companies all claim that their online players support the RTSP protocol, however, Microsoft has always insisted that it does not support this agreement and does not know how long the situation will last.

2. Features of the RTSP protocol:
RTSP is an application layer protocol. It is designed with RTP and RSVP to Fully Stream Services.
RTSP is flexible and can be used on multiple operating systems. It allows clients to interact with service platforms of different vendors.
RTSP is based on RTP and RTCP in the architecture. It uses RTP for data transmission. It transmits streaming media data over the network to the client.
RTSP can maintain a fixed connection between the user's computer and the transmission stream service server. It is used for communication between viewers and Unicast (unicast) servers and allows two-way communication. Viewers can communicate with streaming media servers.
Provides operations similar to "VCR", such as pause, fast forward, reverse, and jump. The operated resource object can be a live stream or a storage fragment.
RTSP is set and also provides the selection of transmission channels, such as using UDP or multi-point UDP or TCP.

Resource Reservation Protocol (RSVP)
1. RSVP protocol:
The RSVP (resorce Reservation Protocol) Resource Reservation Protocol is not a routing protocol, but a signaling protocol in an IP network. It is combined with the routing protocol to achieve the quality of service (QoS) for network transmission). RSVP was proposed to support Internet integrated services. This is a technology that solves the QoS (Service Quality) Problem in IP communication and is used to ensure Point-to-End transmission bandwidth.

2. How the RSVP protocol works:
RSVP uses control datagram, which includes the information that needs to be checked by the router (sometimes updated) during transmission to a specific address, if the router needs to determine whether to check the content of the datagram, perform a syntax analysis on the content of the Upper-layer data. The cost of this analysis is not small. The current situation is that the network terminal uses it to apply for resources from the network. The signal indicating "application" contains the following information: business type? User Type? When? How much bandwidth is required? Other reference information? After the network receives the class information, it will assign a priority code for the connection based on the actual situation. When the user uses the priority code for information transmission, the network does not need to re-analyze and identify the business, in terms of another angle, the use of RSVP can reduce the network delay in information processing to a certain extent, improve the efficiency of network nodes, and improve the quality of service (QoS) of information transmission ). Real-time applications use RSVP to maintain necessary resources in the transmission path to ensure that requests arrive.
RSVP is a significant step forward for IP routers to provide better service quality. Traditionally, an IP router is only responsible for group forwarding and knows the address of the neighboring router through the routing protocol. RSVP, like the signaling protocol of the circuit switching system, provides quality assurance for each node (IP router) through which a data stream is notified. As soon as the RSVP Protocol emerged, it was widely recognized and basically solved the problem of resource reservation.

We have discussed four similar real-time media control protocols. Here I will give a general description:
RTP is a real-time data transmission protocol. It provides time signs, serial numbers, and other methods that can ensure processing time during real-time data transmission. It relies on RVSP to ensure service quality standards.

RTCP is the control part of RTP and is used to ensure service quality and member management.

RTSP is used to start and guide streaming media data from the Streaming Media Server. It can also be called the "online video recorder control protocol". It provides remote control, and the specific data transmission is handed over to RTP.

RSVP is a Resource Reservation Protocol on the Internet. It uses RSVP to reserve some network resources (that is, bandwidth), which can provide QoS for streaming media transmission to a certain extent. It does not transmit data.