Implementing Traffic Generation

Source-level traffic generators for network testbeds (rather than for software simulators) are usually implemented using user-level programs that make use of the socket interface to generate traffic. This is the case for tcplib [DJ91], httperf [MJ98], SURGE [BC98], and other web traffic generators [BD99,CJOS00]. In order to introduce network-level parameters in test-bed experiments, such as a realistic distribution of round-trip times, it is necessary to rely on a layer of simulation either in the end hosts or somewhere in the path of the traffic. For example, Rizzo's dummynet [Riz97] makes it possible to apply arbitrary delays, loss rates and bandwidth constraints on the end systems to specific network flows or collections of network flows (that share a network prefix). The implementation combines event-driven simulation and packet queuing, and sits between the IP and link layers. Dummynet is part of the standard distribution of the FreeBSD operating system. The experiments in this dissertation were performed using an extended version of dummynet that can be controlled from the application layer^2.2.

Kamath et al. [KcLH$^+$02] argue that source-level traffic generation is much more demanding in terms of CPU and memory processing than packet-level replay. While it is indeed true that far more CPU time is needed to simulate endpoint behavior and use network stacks, memory requirements are actually far more stringent for packet-level replay. This is because packet header traces are much longer than their source-level representations. For example, the approach in this dissertation considers the replay of source-level traces that are roughly 100 times smaller than the packet header traces from which they were derived.