Virtual Circuits and Datagrams

Question

I know virtual circuits are connection orientated and datagrams are connectionless. I also found out that VC's have very strict time requirements while datagram networks provide a elastic service.

From this I am assuming that virtual circuits use TCP while datagram networks use UDP. This seems to make the most sense in my head seeing how TCP provides reliable service (VC's need this) while datagram networks are connectionless, leading me to believe that by contrast they would use UDP.

Is my understanding here correct? I tried searching for this online but the sources I found confuse me even more, so I was hoping that I could get a simple yes or no answer here. Thanks for any help.

Answer 1

Actually, your deduction is logical. But you must pay attention that you are talking about transport layer (layer 4 of the OSI model) VC. Although, TCP is usally used to form a VC, sometimes you may find another transport protocol named SCTP that works over IP directly or over UDP (don't be confused, SCTP will manage the reliability of the connection ;)).

On the other hand, VCs exist also on data link and network layers (layers 2/3 of the OSI model). Here the connection is a layer 2/3 connection, e.g. ATM, Frame Relay ... (it has nothing to do with transport layer). So transport protocol may be TCP or UDP or others ...

Hope this clarified the idea :)