Towards scalable network emulation Rob Simmonds * , Brian W. Unger Department of Computer Science, University of Calgary, Calgary, Alta., Canada Accepted 12 May 2002 Abstract The Internet protocol traffic and network emulator (IP-TNE) enables real hosts and a real network to interact with a virtual network. It combines a real-time network simulator with a mechanism to capture packets from and write packets to a real network. Packets generated by external hosts interact with synthetic traffic within the virtual network, providing a controlled environment for testing real Internet applications. IP-TNE can also generate simulated traffic internally enabling its use as a sophisticated workload generator for stress testing real Web servers. This paper focuses on two issues related to the scalability of network emulators, such as IP-TNE. The scalability of the virtual network within the emulator and the scalability of the real-time I/O interface used to interoperate with the physical network. For the scalability of the virtual network, parallel discrete event simulation techniques are employed. The scalability of the real-time interfaces requires handling varying amounts of network I/O and mapping packets into the simulator efficiently. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Network Emulation; Parallel computing; Scalability 1. Introduction A network emulator enables Internet applications run- ning on general purpose computing devices, such as servers, workstations, personal computers, personal digital assist- ants, games consoles, etc. to interact over a virtual network. It provides a configurable and controlled test environment for unmodified real networked devices and applications. Several forms of network emulator exist. Some model the virtual network as a simple delay line and can be configured to drop packets with some probability. These can be useful for simple studies and require little or no additional hardware. Other emulators use real-time net- work simulators to model a virtual network in detail. These allow test networks to be constructed to suit the experiment being performed and enable real network traffic passing through the emulator to interact with synthetically generated traffic within the virtual network. This paper considers scalability issues faced by these network simulation based emulation tools. In particular, this paper examines the Internet protocol traffic and network emulator (IP-TNE) [26], one of the few simulation based network emulators and the first to utilize parallel simulation technologies to improve its scalability. Fig. 1 shows a typical in-line emulation test environment using IP-TNE. The emulator runs on a multi-processor computer with one or more high bandwidth network interfaces. Packets generated by hosts connected to a local area network (LAN) are captured by IP-TNE packet reader and inserted into the virtual network modeled by IP-TNE network simulation engine. Packets are inserted at real-time endpoints (EPs), with one real-time EP representing each real host involved in the experiment. By adding appropriate entries to the routing tables on the real hosts involved in the experiment, either specific traffic streams or all traffic passing between the real hosts will traverse the virtual network modeled within IP-TNE. Fig. 2 shows an emulator configuration for testing a server system, such as a Web server. Simulated hosts within IP-TNE produce data streams that are routed through the virtual network and out to the real network connecting to the Web server. Using an emulator in this way has many advantages [28]. All of the effects of data streams traversing a wide area network are captured with an easily configurable network simulator. Also, since streams are generated using ‘fake’ TCP state machines, it is possible to generate far Computer Communications 26 (2003) 264–277 www.elsevier.com/locate/comcom 0140-3664/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0140-3664(02)00141-X * Corresponding author. Tel.: þ1-403-220-5103; fax: þ 1-403-284-4707. E-mail addresses: simmonds@cpsc.ucalgary.ca (R. Simmonds), unger@cpsc.ucalgary.ca (B.W. Unger).