Using IP multicast technology to transmit video information (1)

With the rapid development of the global Internet), the number of Internet users is growing exponentially. The ratio of Internet-dominated Data Communication in the total amount of communication services is increasing rapidly, internet services have become the most rapidly developing and competitive multimedia communication industry. The dramatic increase in Internet transmission and processing capabilities has led to an increasing number of online applications, especially the development and maturity of video and audio compression technologies, making the online video and audio business one of the most important services on the Internet.
On-demand video/audio (VOD), videophone, video conferencing, and other video/audio services on the Internet are characterized by large data volumes, high latency sensitivity, and long duration. Therefore, the minimum time and space are used to transmit and solve the problems of high network utilization, fast transmission speed and real-time performance required by the video/audio service, we need to adopt forwarding technology and QoS service assurance mechanism different from traditional unicast and broadcast mechanisms. IP multicast technology is the key technology to solve these problems.
I. Overview of basic knowledge of IP multicast technology
1. Concepts of IP multicast technology
IP multicast is also called multi-site broadcast or multicast) technology, which allows one or more host multicast sources to send a single packet to multiple hosts once) TCP/IP network technology. Multicast is one of the effective ways to save network bandwidth as a single-point-to-multi-point communication. In Network Audio/Video Broadcast applications, when a node signal needs to be transmitted to multiple nodes, whether it is through repeated point-to-point communication or broadcast, network bandwidth is a serious waste. Only multicast is the best choice. Multicast enables one or more multicast sources to only send data packets to a specific multicast group. Only the host that joins the multicast group can receive data packets. Currently, IP multicast technology is widely used in network audio/video broadcast, AOD/VOD, network video conferencing, multimedia distance education, and "push" technology such as stock quotations) and Virtual Reality games.
2. IP multicast addresses and Multicast Groups
IP multicast communication must depend on the IP multicast address. In IPv4, It is a Class d ip address ranging from 224.0.0.0 to 239.255.255.255, it is divided into three types: Local Link multicast address, reserved multicast address, and management permission multicast address. The local link multicast address ranges from 224.0.0.0 ~ 224.0.0.255 is the address reserved for the routing protocol and other purposes. The Router does not forward IP packets in this range. The reserved multicast address is 224.0.1.0 ~ 238.255.255.255, which can be used on a global scale such as the Internet) or network protocol. The management permission multicast address is 239.0.0.0 ~ 239.00000000255, which can be used within an organization. It is similar to a private IP address and cannot be used on the Internet. It can restrict the multicast range.
All Hosts that use the same IP multicast address to receive multicast packets constitute a host group, also known as multicast groups. Members of a multicast group change at any time. A host can join or leave a multicast group at any time. The number and geographic location of multicast group members are not limited, A host can also belong to several multicast groups. In addition, a host that does not belong to a multicast group can also send packets to the multicast group.
3. multicast distribution tree
In order to send multicast data to all the receiving hosts, the multicast distribution tree is used to describe the path of IP multicast in the network. The multicast distribution tree has two basic types: the source tree and the sharing tree.
The source tree uses multicast sources as the root of the source tree. The branches of the source tree form a distribution tree that reaches the receiving host through the network, because the source tree runs through the network in the shortest path, so it is also called the Shortest Path Tree SPT ).
The shared tree uses one of some selectable multicast routes in the multicast network as the public root of the shared tree. This root is called the convergence point RP ). The shared tree can be divided into one-way shared tree and two-way shared tree. One-way shared tree means that multicast data streams must pass through the shared tree from the root to the multicast receiver. A two-way shared tree means that multicast data streams do not pass through the shared tree.

4. Reverse path forwarding
RPF) is the basis of the multicast data forwarding process in the Multicast Routing Protocol. Its working mechanism is that when multicast information passes through the active tree, the multicast router checks the source address of the multicast data packet to determine whether the interface that the multicast data packet passes through is on the source branch. If yes, the RPF check succeeds, multicast data packets are forwarded. If the RPF check fails, the multicast data packet is discarded.
5. Internet multicast trunk MBONE) NetworkNetwork
An Internet multicast backbone (MBONE) network is composed of a series of interconnected subnet hosts and routers that support IP multicast. It can be viewed as a virtual network built on the upper layer of the Internet physical network. In this virtual network, the multicast information stream sent from the multicast source can be directly transmitted between vro groups that support IP multicast, point-to-Point Tunneling Technology is required between multicast router groups and non-multicast router groups.
Ii. IP multicast routes and their Protocols
1. Basic Types of IP multicast routes
A common idea of multicast routing is to construct an extended distribution tree between multicast group members. The IP multicast traffic on a specific "sending source, target group" is transmitted from the sending source to the recipient through this extension tree, the extension tree connects all the hosts in the multicast group. Different IP Multicast Routing Protocols use different technologies to construct these multicast extension trees. Once this tree is constructed, all multicast traffic will be transmitted through it.
According to the distribution of multicast group members in the network, IP multicast routing protocols can be divided into the following two basic types. First, assume that multicast group members are densely distributed in the network. That is to say, most subnets in the network contain at least one multicast group member, and the network bandwidth is large enough, this multicast routing protocol is called "intensive Mode" Dense-Mode). It relies on broadcast technology to push data to all routers in the network. Intensive mode Routing protocols include the Distance Vector Multicast Routing Protocol DVMRP: Distance Vector Multicast Routing Protocol) and the Multicast Open Shortest Path First Protocol MOSPF: Multicast Open Shortest Path) and Dense Mode Independent Multicast Protocol PIM-DM: Protocol-Independent Multicast-Dense Mode) and so on.
The second type of multicast routing is assumed that the multicast group members are sparse and scattered in the network, and the network cannot provide sufficient transmission bandwidth, for example, a large number of users in many different regions are connected over ISDN lines on the Internet. In this case, broadcast will waste a lot of unnecessary network bandwidth, which may cause serious network performance problems. Therefore, the Multicast Routing Protocol in Sparse Mode must rely on technologies with routing selection capabilities to establish and maintain multicast trees. Sparse Mode mainly includes the Core Tree-Based Multicast Protocol CBT: Core Based Tree) and Sparse Mode Independent Protocol Multicast PIM-SM: Protocol-Independent Multicast-Sparse Mode ).