Layer Fourth Exchange

What is layer Fourth exchange
　　
Although people are still weighing what the third tier means, the fourth exchange has sounded the latest notes in the Internet arena. Third-tier switching is routing, so what is the fourth-tier exchange?
　　
The fourth layer of the OSI model is the transport layer. The transport layer is responsible for end-to-end communication, which is to coordinate communication between the network source and the target system. In the IP protocol stack, this is the protocol layer where TCP (a transport Protocol) and UDP (User Packet protocol) reside.
　　
In layer fourth, TCP and UDP headers contain port numbers, which can uniquely differentiate which application protocols each packet contains (for example, HTTP, FTP, and so on). The endpoint system uses this information to differentiate the data in the packet, especially the port number, which enables a receiving computer to determine the type of IP packet it receives and hand it over to the appropriate high-level software. The combination of port number and device IP address is often referred to as "socket".
　　
The port number between 1 and 255 is reserved, they are called "well-known" ports, that is, in all host TCP/IP protocol stack implementations, these port numbers are the same. In addition to the "familiar" port, standard UNIX services are allocated in 256 to 1024 port ranges, and custom applications typically allocate port numbers above 1024. The additional information provided by the TCP/UDP port number can be used by the network switch, which is the basis of layer fourth exchange.
　　
The second, third and fourth exchange can be used in the campus network well. The second layer exchanges the user and the network, directs the business flow in the subnet, the third tier exchange or the router transfers the packet from one subnet to another subnet, and the layer fourth exchange passes the packet to the Terminal server. Layer Fourth exchange is an important factor in the network infrastructure, which increases the server capacity with the increase of network bandwidth.
　　
From an operational point of view, layer fourth Exchange is solid because it controls the packet from the source to the host. On the other hand, routers or third-tier swaps, which handle only a single package, do not know where the previous package came from or what the next package would be. They simply detect the TCP port number in the packet header and establish a priority queue based on the application. Routers determine the route of a package based on the nodes available to the link and network. The fourth layer is to determine the interval based on the available server and performance.
　　
The primary role of layer fourth exchange is to improve the reliability and scalability of personal server and group servers.
　　
If the server is not up to speed, even a network with the fastest switching does not fully guarantee end-to-end performance. High priority businesses can be expected to block in this QoS-enabled network because of lower-priority business queues in the server. In worse cases, the server may even lose the ability to recycle the business.
　　
The purpose of the fourth tier exchange, designed on the server, is to extend the performance and business flow management capabilities of the second and third tiers of the server and application in the past.
　　
Advantages of Layer Fourth Exchange
　　
The fourth layer Exchange uses the header information of the third layer and the fourth Layer information packet, according to the application interval to identify the traffic flow, the entire interval segment of the traffic flow to the appropriate application server for processing.
　　
Each open interval is associated with a particular server, and for tracking servers, the fourth tier Exchange enables multiple servers to support special applications that linearly enhance overall performance as the server increases. At the same time, layer fourth exchange improves application reliability by reducing reliance on any particular server.
　　
Layer fourth switching also requires End-to-end QoS to improve the ability of the second and third tiers to exchange packets and packets of QoS transmissions. For example, a network business flow from a business or critical application from a high-level user can be assigned to the fastest I/O system and CPU, while the normal business is assigned to a poorly performing machine.
　　
Level Fourth Exchange brief
　　
Layer Fourth switching functions are like virtual IP, pointing to the physical server. It transmits a variety of business compliance protocols, with HTTP, FTP, NFS, Telnet, or other protocols. These services require a complex load balancing algorithm based on the physical server. In the IP world, the business type is determined by the terminal TCP or UDP port address, and the application interval in layer fourth switching is determined by the source and terminal IP addresses, TCP, and UDP ports.
　　
A virtual IP address (VIP) is set up for each server group for search in layer fourth exchange, and each group of servers supports some application. Each application server address stored in the Domain name server (DNS) is a VIP, not a real server address.
　　
When a user applies for an application, a VIP connection request (such as a TCPSYN packet) with the target server group is sent to the server switch. The server switch selects the best server in the group, replaces the VIP in the terminal address with the IP of the actual server, and passes the connection request to the server. In this way, all packets in the same interval are mapped by the server switch and transmitted between the user and the same server.