Scenario: NetCache deployed with high-latency, high-bandwidth links

About this scenario

This company has a site in Seoul, South Korea, and another in San Francisco, California. These two sites are connected by a satellite (high-latency) link. The Seoul site accesses the Internet through the San Francisco site.

This company provides Web caching service only.

Goal for deploying NetCache appliances

NetCache appliances are deployed on each side of the satellite link. The goal is to optimize TCP transmission over the satellite link to achieve persistent connections and higher throughput.

Deployment Illustrated

The following illustration shows the deployment of NetCache appliances in this multisite company.

Optimization of TCP connections over the satellite link

TCP connections over the satellite link were optimized as follows:

NetCache appliances were deployed at each end of the satellite link.
Cache-to-cache communication over the link was set up by specifying in the configuration of each NetCache appliance in Seoul that the L4 switch in San Francisco is a parent. Cache-to-cache communication over the satellite link keeps the TCP connection open. The benefits of this persistent connection are as follows:
- TCP connection setup overhead is reduced.
- TCP connection setup is reduced, eliminating the needless use of slow start. The result is that response time is significantly increased.

Because NetCache implements selective acknowledgments, NetCache can better handle dropped segments, resulting in increased throughput.

How higher throughput was achieved

Higher throughput was also achieved by configuring TCP large windows on each NetCache appliance at each site. There are two ways you can achieve higher throughput when configuring TCP large windows in the Appliance Manager:

On all the NetCache appliances at one site, when configuring TCP large windows, specify the subnet on which the appliances at the other site are located and the window size. Repeat this process on the NetCache appliances at the other site.
On all the NetCache appliances at one site, when configuring TCP large windows, specify each NetCache appliance at the other site and the window size.
The advantage of this method is that you enable TCP large windows only for the traffic being sent over the satellite link between the caches. If you specify a subnet, you enable TCP large windows for any other devices on the subnet that use TCP large windows.

Limitations of this type of deployment

The combination of persistent connections and TCP large windows works well when the load over the satellite link remains fairly low. The use of TCP large windows is problematic when a large number of connections between NetCache appliances is required. The reason is that each connection can use a large amount of memory. NetCache safeguards against this problem by establishing an upper limit on the number of TCP large window connections.