The net use command is an example of a connection-oriented communication, as illustrated in Figure 6.2.
Figure 6.2 Connection-oriented Network Traffic
When a user types net use at the command line to connect to a shared resource, NBF must first locate the server by sending UI-frames, and then initialize the link. This is handled by the redirector when it makes a connection to the NBF drivers via the Transport Driver Interface (TDI) boundary. NBF begins the sequence by generating a NetBIOS Find Name frame. Once the server is found, a session is set up with UC Class-II frames following the standard 802.2 protocol (802.2 governs the overall flow of data).
The client computer sends an SABME (Set Asynchronous Balance Mode Extended) frame, and the server returns a UA (Unnumbered Acknowledgment) frame. Then the client sends an RR (Receive Ready) frame, notifying the server that it is ready to receive I-frames whose sequence number is currently 0. The server acknowledges this frame.
Once the LLC-level session is established, additional NetBEUI-level information is exchanged. The client sends a Session Initialize frame, and then the server responds with a Session Confirm frame. At this point, the NetBEUI-level session is ready to handle application-level frames (Server Message Blocks, or SMBs).
Reliable transfer is achieved with link-oriented frames by numbering the I-frames. This allows the receiving computer to determine whether the frames were lost and in what order they were received.
NBF uses two techniques to improve performance for connection-oriented traffic: use of adaptive sliding windows and use of link timers. These techniques are described in the next two sections.
NBF uses an adaptive sliding window algorithm to improve performance while reducing network congestion and providing flow control. A sliding window algorithm allows a sender to dynamically tune the number of LLC frames sent before an acknowledgment is requested. Figure 6.3 shows frames traveling through a two-way pipe.
Figure 6.3 Adaptive Sliding Window
If the sender could feed only one frame into the pipe and then had to wait for an acknowledgment (ACK), the sender's pipe would be underused. If the sender can send multiple frames before an ACK is returned, the sender can keep the pipe full, thereby using the full bandwidth of the pipe. The frames would travel forward, and then ACKs for the received frames would travel back. The number of frames that the sender is allowed to send before it must wait for an ACK is referred to as the send window. In general, NBF has no receive window, unless it detects that the remote is a version of IBM LAN Server, which never polls; in this case, NBF uses a receive window based on the value of MaximumIncomingFrames in the Registry.
The adaptive sliding window protocol tries to determine the best sizes for the send window for the current network conditions. Ideally, the windows should be big enough so that maximum throughput can be realized. However, if the window gets too big, the receiver could get overloaded and drop frames. For big windows, dropped frames cause significant network traffic because more frames have to be retransmitted. Lost frames might be a problem on slow links or when frames have to pass over multiple hops to find the receiving station. Lost frames coupled with large send windows generate multiple retransmissions. This traffic overhead might make an already congested network worse. By limiting the send window size, traffic is throttled, and congestion control is exercised.
NBF uses three timers: the response timer (T1), the acknowledgment timer (T2), and the inactivity timer (Ti). These timers help regulate network traffic and are controlled by the values of the DefaultT1Timeout, DefaultT2Timeout, and DefaultTiTimeout Registry entries, respectively.
The response timer is used to determine how long the sender should wait before it assumes the I-frame is lost. After T1 milliseconds, NBF sends an RR frame that has not been acknowledged and doubles the value for T1. If the RR frame is not acknowledged after the number of retries defined by the value of LLCRetries, the link is dropped.
Where the return traffic does not allow the receiver to send an I-frame within a legitimate time period, the acknowledgment timer begins, and then the ACK is sent. The value for this timer is set by the T2 variable, with a default value of 150 milliseconds. If the sender has to wait until the T2 timer starts in order to receive a response, the link might be underused while the sender waits for the ACK. This rare situation can occur over slow links. On the other hand, if the timer value is too low, the timer starts and sends unnecessary ACKs, generating excess traffic. NBF is optimized so that the last frame the sender wants to send is sent with the POLL bit turned on. This forces the receiver to send an ACK immediately.
The inactivity timer, Ti, is used to detect whether the link has gone down. The default value for Ti is 30 seconds. If Ti milliseconds pass without activity on the link, NBF sends an I-frame for polling. This is then ACKed, and the link is maintained.
Note Remember that T2 <= T1 <= Ti.