PackMimeHTTP uses ns to model the TCP-level interaction between web clients and servers on the simulated link. To simulate network-level effects of HTTP transfer through the clouds, use DelayBox (see 22). DelayBox is an ns analog to dummynet, often used in network testbeds to delay and drop packets. The delay times model the propagation and queuing delay incurred from the source to the edge of the cloud (or edge of the cloud to destination). Since all HTTP connections in PackMimeHTTP take place between only two ns nodes, there must be an ns object to delay packets in each flow, rather than just having a static delay on the link between the two nodes. DelayBox also models bottleneck links and packet loss on an individual connection basis. Two DelayBox nodes are used as shown in Figure 43.3. One node is placed in front of the web client cloud ns node to handle client-side delays, loss, and bottleneck links. The other DelayBox node is placed in front of the web server cloud ns node to handle the server-side delays, loss, and bottleneck links.
Tom Henderson 2011-11-05