Web OS 10.0 Application Guide422 n Chapter 16: Persistence 212777-A, February 2002Overview of PersistenceIn a typical SLB environment, traffic comes from various client networks across the Internet tothe virtual server IP address on the Web switch. The switch then load balances this trafficamong the available real servers.In any authenticated Web-based application, it is necessary to provide a persistent connectionbetween a client and the Web server to which it is connected. Because HTTP does not carryany state information for these applications, it is important for the browser to be mapped to thesame real server for each HTTP request until the transaction is complete. This ensures that theclient traffic is not load balanced mid-session to a different real server, forcing the user torestart the entire transaction.Persistence-based SLB enables the network administrator to configure the network to redirectrequests from a client to the same real server that initially handled the request. Persistence is animportant consideration for administrators of e-commerce Web sites, where a server may havedata associated with a specific user that is not dynamically shared with other servers at the site.In Web OS, persistence can be based on the following characteristics: source IP address, cook-ies, and Secure Sockets Layer (SSL) session ID.Using Source IP AddressUntil recently, the only way to achieve TCP/IP session persistence was to use the source IPaddress as the key identifier. There are two major conditions which cause problems when ses-sion persistence is based on a packet’s IP source address: Many clients sharing the same source IP address (proxied clients): Proxied clientsappear to the switch as a single source IP address and do not take advantage of SLB on theswitch. When many individual clients behind a firewall use the same proxied source IPaddress, requests are directed to the same server, without the benefit of load balancing thetraffic across multiple servers. Persistence is supported without the capability of effec-tively distributing traffic load.Also, persistence is broken if you have multiple proxy servers behind the Web switch per-forming SLB. The Web switch changes the client’s address to different proxy addresses asattempts are made to load balance client requests. Single client sharing a pool of source IP addresses: When individual clients share apool of source IP addresses, persistence for any given request cannot be assured. Althougheach source IP address is directed to a specific server, the source IP address itself is ran-domly selected, thereby making it impossible to predict which server will receive therequest. SLB is supported, but without persistence for any given client.
