Satellite Network Performance Measurements Using Simulated Multi-User Internet Traffic

As a number of diverse satellite systems (both Low Earth Orbit and Geostationary systems) are being designed and deployed, it becomes increasingly important to be able to test these systems under realistic traffic loads. While software simulations can provide valuable input into the system design process, it is crucial that the physical system be tested so that actual network devices can be employed and tuned. These tests need to utilize traffic patterns that closely mirror the expected user load, without the need to actually deploy an end-user network for the test. In this paper, we present trafgen. trafgen uses statistical information about the characteristics of sampled network traffic to emulate the same type of traffic over the test network. This paper compares sampled terrestrial network traffic with emulated satellite network traffic over the NASA ACTS satellite. Introduction When designing and deploying new network technologies and infrastructures, designers must be able to test the characteristics of the new system under a realistic load. Although software simulation of the system can provide valuable information about the expected system characteristics and point out problems early in the design process, it's extremely difficult to correctly model all the subtleties of a working network and its usage patterns. A complementary approach that often proves beneficial is to test the new network (or a subset thereof) under actual traffic. Unfortunately, it's often difficult to generate suitably-accurate network traffic to conduct such tests, particularly when that traffic is the product of the interactive behavior of many users. In this paper we present trafgen [5], a software system capable of creating TCP/IP traffic flows that statistically mirror those of an observed network. Trafgen takes as input the traffic characteristics of a network with usage patterns similar to the ones expected for the network to be tested. trafgen then randomly initiates TCP-based data flows that reproduce the input pattern, traffic types, and connection data sizes of the measured network, subject to an overall scaling factor. We describe here a system which can replicate a network of interest, provided that a suitable description of the expected traffic patterns can be obtained. This paper describes a preliminary series of experiments run over the NASA ACTS satellite network. We gathered network statistics from an actual Internet Service Provider and built a version of trafgen which emulates this traffic. We then conducted a series of experiments at different multiples of the sampled traffic load over the satellite network to test the behavior of trafgen and verify that it preserves the characteristics of the original terrestrial traffic. The following sections describe the trafgen program, the TCP traffic library tcplib [4] on which it is built, the satellite network that we constructed to test the program, the experiments that we conducted, and an analysis of the results. How Trafgen Models a Network A single trafgen program is able to model a network in which TCP data is generated by a computer running trafgen and absorbed by a second computer. A simple example of this is seen in Figure 1. Computer A Trafgen Computer B Discard Data Network Figure 1: A simple network model using trafgen trafgen does not directly model bi-directional communications such as the request-response nature of HTTP connections. However, trafgen can model the aggregate behavior of many such conversations by generating data that correctly emulates the characteristics of both the requests and responses. If the network over which trafgen is running is a shared medium (as in Ethernet in which the requests and responses would travel over the same physical channel), a single trafgen program can emulate both the requests and responses. However, in most wide-area network links the traffic flow in one direction uses a separate channel than the traffic in the reverse direction. This is true for the satellite network used in the experiments reported in this paper. For these networks it is necessary to have an instance of trafgen at both ends of the network to emulate requests and responses originating at either end of the media, as shown in figure 2. Computer A Trafgen Computer B Discard Data