RSVP

Intro:

A great deal of information is required to provide real-time service from a network. The most popular setup protocol for providing such information is known as Resource Reservation Protocol (RSVP). RSVP is used to guarantee a certain Quality of Service QOS from a network.

RSVP has several key features incorporated into its design which set it apart from other setup protocols. These features include its receiver-oriented design, and the use of soft-state in routers.

RSVP has many powerful abilities which also distinguish it from other setup protocols, including its ability to support multicast and unicast flows equally as well. Despite RSVPs many positive aspects it also has several negative aspects as well. One of the negative aspects of RSVP is its negative effect on scalability.

RSVP:

RSVP works by having the users send information about how much of the networks resources they will require to each node that they will be connecting through during the transmission. This information is then checked in two ways by each node that supports RSVP.

First there is a Policy control. This is a simple check to see if the user has sufficient privileges to request resources on this node.

The second check is an admission control. This is a test of the nodes resources. The node determines if it is able to fulfill the request at its current load level.

If either of these two checks fails the protocol will try to request through a different node. If the checks pass then the user is given a guarantee of “Quality of Service”. Any packets sent to the user through this node will be processed in such a way that this guarantee is met.

This information is not kept forever in the nodes to allow for the failure in the network. This is called a soft-state. The RSVP state information is updated from time to time and this makes RSVP reliable for use in the Internet.

RSVP Key Features:

RSVP was designed with the idea that many applications have substantially more receivers than senders. To account for this, the designers of RSVP designed this protocol to operate using a receiver oriented approach. The advantage of this design is that the senders do not need to keep track of a large number of receivers, making it the receivers responsibility to monitor resource reservation. The advantage to this approach is quite clear, and very significant.

A second advantage to using RSVP protocol is the soft state nature which is built into the protocol. Soft state design ensures all QOS information is updated in the routers after a certain time interval. So each receiver sends a fresh signal after at a designated constant interval. Deleting the old resource reservation information and replacing it with updated information. This ensures that if a host were to crash the resources allocated would eventually become available to other users.

RSVP Shortcomings:

RSVP’s biggest issue is scalability. As the networks complexity increases the amount of information stored in the soft-state will increase. As this increases the overhead increases. Very soon the networks routers and other nodes will be spending a great deal of time maintaining the information in their states.

The fact that the state info must be updated constantly to account for changes in the network topology also adds to overhead and creates a dangerous balance. If the time between updates is too long a users packets can be forwarded to a dead node for the length of the time between updates, thus killing the quality of service. However if the time between updates is too short the network spends a majority of its time updating the soft-state and thus the total throughput on the network will be reduced greatly.

RSVP Current implementations:

RSVP is currently implemented in several Operating systems. According to a Microsoft document it is included with all Versions of Windows from Windows_95 to Windows_XP. According to an Apple developer brief the BSD implementation of RSVP is included in all builds of Mac OS X server and client.

Competing Technologies:

Because of the shortcomings of RSVP many competing technologies have been developed. There is a system that uses Differentiated Services to create certain quality of service. How this works is by classifying certain classes of packets thus the routers can know what packets need to be sent through faster.

A second way that certain networks can create quality of service is to use ATM. ATM approaches the problem for the server side and does not have some of the benefits that RSVP has because of this, however the overhead is far less because of this fact.

LINKS:

http://www.nwfusion.com/news/tech/2002/0617tech.html

http://www.cs.caltech.edu/~anti/rfc.html