In this paper we present the results of an experimental analysis of IPSec performance. IPSec serves as a security service to protect a communication session over unreliable networks using several cryptographic functions. The different IPSec features used to obtain confidentiality, authentication, replay protection, and integrity introduce performance costs brought about by the increased processing overhead. The performance parameters of interest are delay and throughput. However, there is a tradeoff between performance and security. The results of the performance tests indicate the vulnerability of a Denial-of-Service attack against a gateway running IPSec due to resource exhaustion.