A simple definition of the fourth layer exchange is that it is a function that determines transmission based not only on the MAC address (the second layer network bridge) or source/destination IP address (the third layer routing) but also on TCP / UDP (the fourth layer) application port number. The fourth layer exchange's function is like a virtual IP, pointing to the physical server. It transmits services that are subject to a variety of protocols, including HTTP, FTP, NFS, Telnet, or other protocols.

The principle of the fourth layer switch

The fourth layer of the OSI model is the transport layer. The transport layer is responsible for end-to-end communication, that is, communication coordinated 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) are.

In the fourth layer, the TCP and UDP headers contain the port number, which uniquely differentiates which application protocols (such as HTTP, FTP, etc.) each packet contains. The endpoint system uses this information to distinguish the data in the packet. Especially the port number can make a computer system with receiving port determine the type of IP packet it receives and sends it to the appropriate high-level software. The combination of port number and device IP address is often referred to as a "socket".

How to choose the appropriate fourth layer exchange?

1. Speed

In order to be effective in the enterprise network, the fourth layer exchange must provide the third layer of the wire-speed router with the comparable performance. That is, the fourth layer switch must operate at all ports at full media speed, even on multiple Gigabit Ethernet connections. Gigabit Ethernet speed is equal to the maximum routing speed of 1488000 packets per second (assuming the worst case, that is, all the packages and the minimum size defined by the network with the length of 64 bytes).

2. Server capacity balance algorithm

Depending on the desired capacity balance interval, the Layer 4 switch has a number of algorithms for assigning the application to the server. There is a simple way to detect the loop's closest connection, detect loop delay, or detect closed loop feedback from the server itself. In all predictions, closed-loop feedback provides the most accurate detection that reflects the server's existing traffic.

3. Table capacity

It should be noted that switches that perform Layer 4 switching require the ability to distinguish and store a large number of sending entries. This is especially true when the switch is at the heart of an enterprise network. Many layer 2 /layer 3 switches tend to send tables that are proportional to the number of network devices. For Layer 4 switch, this number must be multiplied by the number of different application protocols and sessions used in the network. Thus the size of the send table grows rapidly as the number of endpoints and application types grows. Layer 4 switch designers need to consider this growth into the design of their products. Large table capacity is critical to manufacturing high-performance switches that support wire-speed traffic for Layer 4 traffic.

4. Redundancy

The Layer 4 switch has features that support redundant topology. When a network link with a dual link is fault-tolerant, it is possible to build a fully redundant system from one server to a network card, link, and server switch.