s place? The list can go on and on. The intention here is not to be exhaustive, but simply to put things into the overall computational-load context and to show the magnitude and importance of the interoper- ability problem. IPsec IPsec is the current de facto standard in the security of IP-protocol-based networks as it provides mul- tiple mechanisms that ensure confidentiality, integrity, and authentication. The book IPsec: The New Security Standard for the Internet, Intranets, and Virtual Private Networks by Naganand Doraswamy and Dan Harkins (Upper Saddle River, New Jersey: Prentice-Hall, 1999) is an outstanding source of tutorial information. It also contains numerous references to the multiple RFC documents that detail all aspects of IPsec. Here we will limit ourselves only to some generalities to put things into a net- work-processing context. IPsec establishes security relationships between end devices, between end devices and gateways (such as firewalls), or between gateways connecting two LANs from different sites with each other over the insecure WAN. It is designed to function in one of two modes (transport and tunneling), and it allows the use of two basic packet-forming protocols in either of these two modes with the inten- tion of securing IP datagram traffic: Authentication Header (AH) and Encapsulating Security Payload (ESP). The landscape is also complemented with a sophisticated key-generation-and-sharing hand- shake protocol, which is called Internet Key Exchange (IKE). Several cryptographic techniques are used in the heart of IPsec's protocols (along the directions we briefly described earlier). Support is available for several symmetric (such as DES, Triple DES, IDEA, and RC4), asymmetric (such as RSA and DSS), and hashing (such as SHA-1 and MD5) algo- rithms all operating with different key sizes, which are configurable to suit security and performance requirements. The fundamental idea behind the IETF's effort that created IPsec was that IPsec-com- pliant devices designed by different manuf