At 02:59 AM 12/7/01, mlh wrote: >Hi, there, > >I read Todd Lammle's CCNA2.0 study guide and found this sentence: "Remember >that none of the upper >layers know anything about networking or network addresses." I am wondering >if the session layer doesn't >use network address, how can it establish a dialogue with other session >layer in other host?
I would probably disagree with Todd's statement, although it's taken out of context and you haven't given us enough information to say that the statement is definitely "wrong." However, try to picture the numerous OSI pictures you have seen. Most of them show horizontal lines between a layer on one host talking to the same layer on another host. So the session layer talks to the session layer on the other host. That's probably what Todd was getting at. However, the pictures also show vertical lines. A layer calls on a layer below to provide services. Each layer offers services to layers above it. The session layer is an elusive beast that is not implemented much. But one example might help. NetBIOS is a session layer. On a Windows client, when you access a Server Message Block (SMB) server, NetBIOS has the job of setting up a session with the server. Before it can do that, however, it must find the address of the server. If it's a modern Windows network, then SMB and NetBIOS are probably running above TCP/IP and UDP/IP. So NetBIOS sends a DNS or WINS query to find the IP address of the named server. It then sets up a NetBIOS session with the server. Actually, first, the client sets up a TCP connection. TCP has port numbers. The client sends to the well-known TCP port for NetBIOS session (139) and use an ephemeral port on its side. These port numbers could be considered "addresses" at the transport layer. Anyway, back to the question. The statement is at best over-simplified. I recommend you get yourself a sniffer and watch what really happens between layers. (Ethereal is free by the way.) Priscilla >Thank you for your time. > >mlh ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=28465&t=28378 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
