BROCADE IP PRIMER

User Datagram Protocol (UDP)When it is finished communicating, the client will send a TCP FIN (request toclose) message to the server. The server will send an ACK to acknowledge theFIN. The server will send a message to its application to close the session, andthen it will send a TCP FIN/ACK segment to the client. Finally, the client willreply with an ACK segment to the server. It's as if the client is saying, “I want tostop now, okay?” The server replies with, “Okay,” followed by, “I'm stoppingnow, okay?” And finally, the client responds with, “Okay.” And this closes thesession. Any more communication between these two devices would result ina new 3-way handshake and, thus, a new session.In summary, the way TCP closes a session looks like this:1. Client sends TCP FIN to server2. Server sends TCP ACK to client, to acknowledge the FIN3. Server sends TCP FIN/ACK to client4. Client sends TCP ACK to serverTCP FINTCP ACKTCP FIN/ACKTCP ACKUser Datagram Protocol (UDP)UDP is a connectionless protocol. This means that there is no 3-way handshake,acknowledgments, windows, nor flow control. If the receiver gets thesegment, great. If it doesn't, too bad. We talked about this in Chapter 1, but itbears repeating. The reason that connectionless protocols are used is thatthey're very, very speedy.Think about TCP. You spend three segments just starting the session. Youspend untold number of segments sending acknowledgments. You spend timewaiting for acknowledgments. It even takes four segments just to finish. Oh,sure, your data gets there, and you know it gets there, but at a cost.In reality, in a perfect world, every protocol would be connectionless. It's fastand it's based on trust. How do I know my packet got to my destination?Because I sent it! That's how I know! Sadly, of course, networks just aren't thatreliable. To be fair, networks are a lot more reliable now than they used to be,and on a well-designed Ethernet switched network, UDP works great! Still,always keep this in mind: UDP is fast, but you might lose a few segments; TCPis slower, but it is safe.Brocade IP Primer 59