Running RSVP on an ATM network

On the Internet the IP protocol is used which means that every packet is switched independently from one another, because IP is connectionless. ATM is a connection oriented protocol. On an ATM network, for a session, a virtual path and a virtual channel are established from the sender to the receiver, which means that every IP packet is going to follow the same route as every other IP packet until it reaches its destination. The establishment of this virtual circuit is called signalling.

The advantage of using such a technique is that once the circuit has been established, the switch just has to look at the ATM cell header. The virtual path identifier (VPI) and the virtual path identifier (VCI) used by the cell are found in the header. The switch reads from its switching table which VPI/VCI it has to use to forward the cell. As the (VPI,VCI) pair is encoded in only 20 bits, this makes the switching process very fast.

One could think that running RSVP on top of ATM is going to decrease the performance. However, when RSVP is asked to make a reservation for a session, it can establish the connection at the same time. The signalling can hence be done by the RSVP agent, which has enough information to set up the connection and to set the different ATM QoS (quality of service) parameters. The QoS parameters of RSVP and ATM are indeed very similar and reservations made by RSVP can be easily translated into ATM service classes.

Figure: RSVP agent controlling an ATM switch

Figure  shows a typical situation:

1. A reservation request is sent; the switch forwards it to the control architecture.
2. The RSVP process makes the reservation and asks the Ariel server to create an ATM connection with the appropriate QoS.
3. The Ariel server creates the connection which will be used by the data flow.
4. The control architecture forwards the request.

Once a connection is established, the data flow will not go through the control architecture. Only control messages are sent to it.

Therefore, it is interesting to run RSVP on top of ATM. This is a new approach since RSVP is a protocol running on top of IP. Thanks to Ariel, we can make the RSVP agent interact directly with the ATM layer.