Extreme Networks Px Series - Manual (Page 14)

1-4 Px Series Application Switch Installation and Configuration Guide
Figure 1-2: Single client-server transaction using layer 4
As soon as the first request from the client is received at the application switch, the
application switch uses the server-selection policy configured for the VIP to select the
server and immediately sends out the NAT-ed request to the real server. The client and
server continue the connection establishment protocol using the application switch in
the middle, NAT-ing the traffic. After the connection is established, an HTTP request is
sent and the server responds.
Layer 7 Load Balancing and Content Analysis
To make server-selection decisions based on cookies or the URL being requested by the
client, the application switch must actually look inside the client request. Because this
data request is only sent out after a connection is established, the Px series application
switch must first act as a proxy for the server by acting as the endpoint of the TCP/IP
connection from the client. This process is called layer 7 load balancing.
The Px series application switch delays the establishment of a connection to a server
until the first 1500 bytes of actual data (the HTTP request) is received from the client.
The application switch then takes the content being requested, along with the domain to
which the request pertains, and applies policy rules. Based on the outcome of the policy
decision, the application switch establishes a TCP connection with the real server
WS_013
Client Real Server
SYN
SYN/ACK
ACK
DATA
(http request)
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