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Layer 4 Switching

Layer 4 switching is a term that has almost as many meanings as it has people using the term. In the context of this paper it refers to the ability to multiplex connections received from end-users to back-end servers. This can be implemented in an ethernet switch such as the Alteon Networks ACESwitch. It can also be done in a host such as the Linux Virtual Server, Cisco LocalDirector, F5 BIG/ip and an element of IBM WebSphere13.

A Virtual Service is the point of contact for by end-users and is typically advertised through DNS. A virtual server is defined by: the IP address that clients will use to access the service; the port that clients will connect to and a protocol, either UDP/IP or TCP/IP. The virtual service is assigned a scheduling algorithm which allocates incoming connections to the back-end servers. The scheduling algorithms available will depend on the implementation. In the case of TCP/IP all packets for the life of the connection will be forwarded to the same back-end server so the integrity of the connection between the client and the back-end server is maintained. Many implementations have a feature that allows subsequent TCP/IP connections or UDP/IP datagrams from a host or network to be forwarded to the same back-end server. This is useful for applications such as HTTPS where the encryption used relies on the integrity of a handshake made between the client and a server, hence, clients need to consistently hit the same back-end server.

When a packet is to be forwarded to a back-end server several mechanisms are commonly employed. As a guide the mechanisms implemented by the Linux Virtual Server Project are detailed here.


next up previous contents
Next: DNS Methods Up: Technologies Previous: IP Address Takeover   Contents
Horms 2001-11-23