Nortel Networks WEB OS 212777 - Manual (Page 422)

Web OS 10.0 Application Guide
422 n Chapter 16: Persistence
212777-A, February 2002
Overview of Persistence
In a typical SLB environment, traffic comes from various client networks across the Internet to
the virtual server IP address on the Web switch. The switch then load balances this traffic
among the available real servers.
In any authenticated Web-based application, it is necessary to provide a persistent connection
between a client and the Web server to which it is connected. Because HTTP does not carry
any state information for these applications, it is important for the browser to be mapped to the
same real server for each HTTP request until the transaction is complete. This ensures that the
client traffic is not load balanced mid-session to a different real server, forcing the user to
restart the entire transaction.
Persistence-based SLB enables the network administrator to configure the network to redirect
requests from a client to the same real server that initially handled the request. Persistence is an
important consideration for administrators of e-commerce Web sites, where a server may have
data 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 Address
Until recently, the only way to achieve TCP/IP session persistence was to use the source IP
address 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:
n Many clients sharing the same source IP address (proxied clients):
Proxied clients
appear to the switch as a single source IP address and do not take advantage of SLB on the
switch. When many individual clients behind a firewall use the same proxied source IP
address, requests are directed to the same server, without the benefit of load balancing the
traffic 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 as
attempts are made to load balance client requests.
n Single client sharing a pool of source IP addresses:
When individual clients share a
pool of source IP addresses, persistence for any given request cannot be assured. Although
each 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 the
request. SLB is supported, but without persistence for any given client.