Related to this, an active research topic is the problem of resource reservation in advance (both in IP [#!Degermark:95a!#], and in ATM [#!Bos:98b!#,#!Ferrari:97a!#,#!Hafid:98a!#,#!Schill:97a!#]). It has been questioned whether reservations in advance are needed at all [#!Bake:97!#] and rather than getting deeply involved in this discussion, we take the line that reservations (whether immediate or in advance) are only needed when resources are scarce and when resources are scarce, there exists a class of applications for which advance reservations are useful.
The criterion here is that the cost of making the reservation should be outweighed by the benefits derived from it. Analogous arguments can be found in different aspects of life. For example, it is worth the cost to make advance reservations for a seat in a multi-hop plane trip, but it probably makes no sense to reserve in advance for a similar multi-hop trip by subway. Similarly, it is useful to reserve in advance a table for an important dinner in an expensive restaurant, while it would be nonsensical to reserve in advance for McDonalds. Instead of a multi-hop plane trip, one can think of an important multi-hop video broadcast and instead of a table for dinner in a restaurant it is possible to think of bandwidth for an important video conference.
The point is that there exists a class of applications for which advance reservation may be useful in a resource-scarce environment. If this is the case, we should at least enable those applications to make reservations for the resources they require in advance.
We define advance reservation in a general way as the reservation of resources for future time periods during which these resources will be allocated to the requesting application according to a policy (which in the most general case may depend on time, load in the network etc.). Here also, there are many ways to offer advance reservation and allocation of resources. To be as general as possible we should try not to restrict the user in any way in declaring his or her own policy regarding the advance reservation.