 SO much as 5 seconds between answering the phone and talking. Extending SIP: The SIP Toolkit In the Internet, establishment time increases dramatically if the session requires a certain QoS and/or a certain level of security. Establishing a secure channel between two end users and providing QoS (using RSVP for instance) takes time. But even if an extremely fast resource reservation mechanism that could fulfill a variety of applications requirements was available, it would still be impossible to know beforehand whether the network will grant the neces- sary session QoS or not. If the session is established and the network does not grant the necessary QoS, the session fails. In an audio session, this would mean that a user answers a SIP phone that is ringing and finds no call established-a case of ghost ringing. This problem can be resolved either by changing the user's expectations or meeting the requirements. In the early days of cellular telephony, it was the former. Users of the fixed telephony network were accustomed to very short delays between the moment they finished dialing a telephone number and the first rings on the caller's phone. Cellular users had to wait signifi- cantly longer for their calls to be established. However, they were willing to accept this delay in exchange for a new service feature: mobility. A typical user of SIP devices might accept longer establishment delays because he or she can trade off a little patience for a wide range of new services. As we all know, sometimes it is necessary to simply fulfill the user's requirements. Users who only want voice sessions are most likely to refuse to use a new technology (SIP) if it performs worse than the one they already For these occasions, the SIP toolkit includes a way to meet preconditions for a session, such as security and QoS, before alerting the user. This mech- anism ensures that everything is ready for establishing the session when the user agrees to participate in it. How It's Done A new method called preCOnditions MET (COMET) was 