RSVP


Traditional data traffic is relatively tolerant of delays and retransmissions. When you are ftp’ing a file down from a server, of course you want it to go as fast as possible. However, if a few frames get delayed along the way, they will eventually either arrive or be retransmitted, and in the end you will have your file. This type of traffic is defined as elastic, meaning that it is tolerant of delay. Now, consider a video or voice transmission. If you are watching a video and one of the frames gets delayed, you don’t really want it later. You either have it in time to show in the video transmission, or you skip it and don’t want it later. This type of traffic is defined as inelastic, meaning that it is not tolerant of transmission delays.

Resource Reservation Protocol (RSVP) allows end stations to communicate with routers and routers to communicate with each other to guarantee passage of these inelastic transmissions. This guarantee is secured by reserving bandwidth on every link between the source and destination of the transmission. Of course, data traffic will likely still be using these same links, which can lead to a conflict. Part of the RSVP design should include defining just how much (what percentage) of a particular link can be reserved for inelastic transmission. By default, 75 percent of the bandwidth on an interface is reservable by RSVP, although this can be changed.

Another design consideration when deploying RSVP is the type of interface or network. RSVP works better across some data-link networks than across others. Serial line interfaces generally work very well, but some LAN interfaces pose more of a challenge. A router with Ethernet interfaces, for example, may not be able to reliably deliver a set bandwidth across the Ethernet LAN. As you recall, with Ethernet, all devices have an equal chance to use the network (CSMA/CD), and the router has no special advantage in this competition. Therefore, RSVP across Ethernet is less reliable than a serial line, simply because the router does not directly control the traffic on Ethernet the same way it controls the traffic on a serial connection.




CCDA. Cisco Certified Design Associate Study Guide
CCDA: Cisco Certified Design Associate Study Guide, 2nd Edition (640-861)
ISBN: 0782142001
EAN: 2147483647
Year: 2002
Pages: 201

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