What is the fourth layer exchange

What is the fourth layer exchange

Although it is still weighing what it means to the third layer switching, the fourth layer exchange has already sounded the latest in networking notes. The third layer switching is routing, then what is the fourth layer of the exchange?

The fourth layer of OSI model is the transport layer. Transport layer is responsible for end to end communication, that is the source and target systems in the network communication between the co-ordination. In the IP protocol stack is TCP (a transport protocol) and UDP (user datagram protocol) protocol layer is located.

In the fourth layer, TCP and UDP header contains the port number (portnumber), they can be uniquely identified what each packet contains the application protocol (eg HTTP, FTP, etc.). Endpoint system uses this information to distinguish between packets of data, particularly the port number to a receiver computer system before it can determine the IP packet type, and gave it to the appropriate high-level software. IP address port number and device combination often called "jack (socket)".

1 and the port number 255 is reserved, they called the "well known" ports, that is, in all host TCP / IP protocol stack implementation, the port number is the same. In addition to "known" ports, the standard UNIX distribution service port in the 256-1024 range, custom application normally assigned port numbers above 1024. TCP / UDP port number to provide the additional information can be exploited for network switches, which is the basis for the exchange of the fourth floor.

Exchange of the fourth layer and the second layer, the difference between the third layer switching

Second, third and the fourth floors of the exchange in the campus network can have a good application. The second layer switching and network connected users, the guidelines in the subnet traffic, the third layer or the router to packet switching from one subnet to another subnet, the fourth layer of the package reached the terminal server exchange. The fourth tier is the network infrastructure in the exchange of important factors, it makes the server capacity increases with the network bandwidth.

From the operational point of view, the fourth layer exchange is strong, as it ended to package control in places from the source side of the interval. On the other hand, router, or third exchange, only for a single packet for processing, do not know where he comes from a bag, do not know the situation of the next packet. They are just testing the TCP packet header port number, depending on the application to establish priority queue. Routers and network is available under the link packet routing decision nodes. The fourth tier is the performance of the available server and on the basis of first identifying the interval.

The main function of the fourth layer is to increase the exchange of individual servers and groups server reliability and scalability.

If the server could not keep pace, even with the most rapid exchange of the network can not fully ensure that the end of the performance. One can imagine such a high priority of the business QoS-enabled network server in a low priority because of the queue and obstruction of business. In the worse case, the server will even lose the ability to handle business cycle.

Design of the fourth layer on the server that is extended past the purpose of the exchange server and application in the second and third floor exchange performance and business flow management.

Advantage of the fourth layer switching

Exchange of the fourth layer a third layer and fourth layer of information in the packet header information, identifying service flow according to the interval, the business section of the entire range of flow assigned to the appropriate application server for processing.

Each open interval associated with a particular server, for tracking server, the fourth layer switching allows multiple servers to support specific applications, increases linearly with the server's overall performance enhancement. Meanwhile, the fourth layer exchange by reducing the dependence on any particular server, while improving application reliability.

The fourth tier exchange also requires end to end QoS, the second and third tiers to improve the exchange of a QoS packet then a packet transmission capabilities. For example, users from high-level business or important applications to network traffic, can be assigned to the fastest I / O system and CPU, while the ordinary business allocated to the poor performance of the machine.

Brief exchange of the fourth layer

Switching function is like the fourth level virtual IP, point to the physical server. It transfers a wide range of business subject to the agreement, there are HTTP, FTP, NFS, Telnet or other agreement. These business based on the physical server, require complex load balancing algorithm. In the IP world, business type from the TCP or UDP port address of the terminal to determine, in the fourth floor of the exchange zone by the source and the terminal IP address, TCP and UDP port joint decision.

Exchange in the fourth layer for each server group for setting up the search using virtual IP address (VIP), each server supports a specific application. In the domain name server (DNS) for each application server to store address is VIP, but not the real server address.

When a user for application, a group of VIP with the target server connection requests (such as a TCPSYN package) distributed server switch. Server switches in the group select the best server, the terminal address in the VIP IP replaced with the actual server, the server will pass the connection request. In this way, the same interval all packets by the server switches to map between the user and the same server for transmission.







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