Coyote Point Systems E350GX, Installation And Administration Manual

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Introducing Equalizer
Equalizer Installation and Administration Guide 23
request. This is necessary since the client sent its original request to the cluster IP and will not recognize the
server’s IP address as a response to its request -- instead, it will drop the packet.
4. NAT can also be enabled for packets that originate on the servers behind Equalizer and are destined for subnets
other than the subnet on which the servers reside -- on Equalizer, this is called outbound NAT . This is usually
required in dual network mode when reserved IP addresses (e.g., 10.x.x.x, 192.168.x.x) are being used on the
internal interface, so that the recipients do not see reserved IP addresses in packets originating from the servers.
When the global
outbound NAT
option is enabled, Equalizer translates the source IP in packets from the servers
that are not part of a client connection to the Equalizer’s Default VLAN IP address (the external interface IP
address on the E250GX and legacy ‘si’ systems), or to the address specified in the server’s
Outbound NAT
tab.
Enabling
outbound NAT,
as a result, has a performance cost since Equalizer is examining every outbound
packet.
Note that when Equalizer receives a packet that is not destined for a virtual cluster IP address, a failover IP address,
a client IP address on an open connection, or one of its own IP addresses, Equalizer passes the packet through to the
destination network unaltered.
For more information:
• about setting NAT and spoofing options, see “Working with Virtual Clusters” on page 112.
• about using reserved, non-routing IP addresses with Equalizer, see Appendix C, ”Using Reserved IP
Addresses” on page 271.
Maintaining Persistent Sessions and Connections
The persistence of session data is important when a client and server need to refer to data previously generated
again and again as they interact over more than one transaction, possibly more than one connection. Whenever a
client places an item in a shopping cart, for example, session data (the item in the cart, customer information, etc.) is
created that potentially needs to persist across many individual TCP connections before the data is no longer needed
and the session is complete.
It’s important to note that session persistence is managed by the server application, not Equalizer. Equalizer provides
server persistence so that a persistent connection between a particular client and a particular server can be
maintained; this supports a client-server session where session data is being maintained on the server for the life of
the connection. In other words, whether you need to enable persistence on Equalizer depends on the application you
are load balancing.
Equalizers have no knowledge of the fact that the user has placed something in a shopping cart, logged into a web
application, requested a file from shared storage, or made a "post" in a front end presentation server that has been
written to a database. Basically, a "state" has been created in the load balanced application of which Equalizer is not
aware. What Equalizer
does know is that a specific client has been load balanced to a specific server in one of its
virtual clusters. With this knowledge, Equalizer can track that information and send that client back to the same
server they were connected the first time.
Equalizer provides server or connection persistence using cookies in Layer 7 HTTP and HTTPS clusters, and using
the client IP address in Layer 4 TCP and UDP clusters. The following sections explain connection persistence
provided by Equalizer, and its relationship to session persistence.
Cookie-Based Persistence (Layer 7)
Equalizer can use cookie-based persistent connections for Layer 7 HTTP and HTTPS clusters. In cookie-based
persistence, Equalizer "stuffs" a cookie into the server's response header on its way back to the client. This cookie
Note – When Equalizer is in single network mode, outbound NAT should be disabled . Since Equalizer
resides on a single subnet, outbound NAT is not needed, and may cause unexpected behavior. See “Adding
Equalizer to Your Network” on page 29 for a description of Equalizer’s network modes.