The 254-session limit is based on a key variable in the NetBIOS architecture called the Local Session Number (LSN). This is a 1-byte number (0 to 255) with several numbers reserved for system use. When two computers establish a session via NBF, there is an exchange of LSNs.
The LSNs on the two computers might be different. They do not have to match, but a computer always uses the same LSN for a given session. This number is assigned when a program issues a call for a Network Control Block (CALL NCB). The number is actually shared between the two computers in the initial frame sent from the calling computer to the listening computer. Figure 13.7 shows this session-creation frame exchange.
Figure 13.7 Multicast of NameQuery
The initial frame is a NameQuery frame. In earlier implementations of NBF, this frame was broadcast onto the network. All computers read the frame and check to see if they have the name in their name space and if there is a LISTEN NCB pending on the name. If there is a LISTEN NCB pending, the computer assigns a new LSN for itself. It adds the LSN to the response frame, satisfying the LISTEN NCB which now contains just the LSN used on that computer. Even though each computer knows the LSN of the other, the information is not used. The more important information for the two communicating partners is the network addresses that are part of the frames. As the frames are exchanged, each partner picks up the address of the other in the source address component of the frame received. The NBF protocol keeps the network address of the remote partner so that subsequent frames can be addressed directly.
Note
This process applies only to NBF connections. NetBIOS connections established by using NetBIOS over TCP/IP (NetBT) are handled differently.
Windows NT has to use the same NameQuery frame to establish connections with remote computers via NBF; otherwise, it would not be able to talk to existing workstations and servers. The NameQuery frame transmitted must contain the 1-byte-wide LSN to be used.
For example, suppose a computer running Windows NT Server and NBF assigns a session number of 1 to identify the session between itself and Computer A and also assigns the number 1 for the simultaneous session with Computer B. The computer running Windows NT Server can identify the different sessions because it uses the computer network address as part of the TDI handle to further identify each session. When the computer running Windows NT Server sends a session frame to either Computer A or Computer B, it uses the network address to direct the frame across the network. Therefore, Computer A does not receive the frames addressed to Computer B, and vice versa. However, if the computer running Windows NT Server and NBF establishes another session with Computer A, it must use a session number other than 1 to uniquely identify the second session.
A Windows NT Server computer running NBF can do this only when it is able to determine beforehand which adapter the session connection is going to be made to. Consider the following three scenarios:
1. A client connecting to a Windows NT computer running NBF.
When a client connects to a computer running NBF, NBF can inspect the incoming frame to determine the remote adapter's address and assign a session number for that adapter.
2. NBF connecting to a remote client with a unique NetBIOS name.
When NBF is connecting to a remote client, it first sends a FINDNAME frame. A response from the remote client means that the name is a unique name. NBF can then look at the remote adapter address in the response and assign a session number for that adapter because it knows that only that remote client owns this name.
3. NBF connecting to a remote client with a group NetBIOS name.
If the FINDNAME response indicates that the connection is being made to a group name, NBF has no way of knowing which adapter belonging to the group will respond when it tries to connect, and so it has to assign a session number on a global basis, just as NetBEUI did for all connections.
These scenarios show that NBF has no limit on sessions, unless it is establishing connections to group names, in which case the old NetBEUI limit still applies. To be accurate, if you have n group name connections, then you can have 254-n connections to any given remote client. If n is 0, then you can have a full 254 connections to a remote. If n is 253, you can still have 1 connection to each remote, but if n is 254, then no connections can be made until one of the existing ones is disconnected.