In this paper, we propose a simulation framework, TranSim, that reduces the rate at which packet-events are generated, in order to accelerate large-scale simulation of IP networks with TCP/UDP traffic. Conceptually, we transform an IP network into an alternate network that generates a smaller number of packet events, carry out simulation in the "transformed" network, and then extrapolate simulation results for the original network from those obtained in the "transformed" network. We formally prove that if the network invariant - the bandwidthdelay product -is preserved, the network dynamics, such as the queue dynamics and the packet dropping probability at each link, and TCP dynamics, such as the congestion window, RTTs, and rate dynamics, remain unchanged in the process of network transformation. We have implemented TranSim in ns-2, and conducted an empirical study to evaluate it against packet level simulation, with respect to the capability of capturing transient, packet level network dynamics, the reduction in the execution time and the memory usage, and the discrepancy in the system throughput. The simulation results indicate maximally two orders of magnitude improvement in the execution time and the performance improvement becomes more prominent as the network size increases (in terms of the number of nodes, the number of flows, the complexity of topology, and link capacity) or as the degree of downsizing increases. The memory usage incurred in TranSim is comparable to that in packet level simulation. The error discrepancy between TranSim and packet level simulation, on the other hand, is between 1-10 % in a wide variety of network topologies, inclusive of randomly generated topologies, and traffic loads with various AQM strategies.