[EMAIL PROTECTED] wrote: > > Can anyone enlighten me on this? >From "Troubleshooting Campus Networks"
"LLC supports three modes of service. Connectionless mode, also known as datagram service and LLC Type 1, is a simple service that does not use sequence numbers or acknowledgments and does not require a connection to be established before data can be exchanged. LLC Type 2 is a connection-oriented service that guarantees delivery of data using sequence numbers and acknowledgments. The third mode of service, LLC Type 3, is acknowledged, but connectionless. LLC Type 3 lets network layers exchange data that is acknowledged at the LLC layer without establishing a connection first. LLC Type 3 is not common and is not covered in this book. LLC Type 1 Frame Format The following example shows protocol analyzer output for an LLC Type 1 header. Because LLC Type 1 is connectionless, there is little analysis required. LLC: ----- LLC Header ----- LLC: LLC: DSAP Address = E0, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = E0, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI The Individual/Group (IG) bit in the DSAP is 0 in this example, which means that the frame is destined to an individual SAP. A frame addressed to a group SAP is destined to more than one process in the receiving system. If you see the IG bit set to Group, you should find out which station is setting the bit and why. The use of a group SAP is a possible misconfiguration or bug that could lead to communication problems in recipient devices. The Command/Response (CR) bit in the SSAP is 0 in the example, which means that the frame is a command. An LLC command designation simply means that the frame was not in response to a previous frame; that is, it was originated spontaneously by the sending station. When a station responds, it sets the CR bit to 1 to designate a response. The Control field for the frame in the example specifies an Unnumbered Information frame (UI frame). This means that there is essentially no processing at the LLC layer. LLC moves the incoming data into the specified SAP without any special attention. LLC Type 2 Frame Format NetBEUI and Systems Network Architecture (SNA) use LLC Type 2. NetBEUI is an implementation of the NetBIOS session layer running directly over a data-link layer, without a network or transport layer. SNA is a complex architecture and set of protocols developed by IBM in the early 1970s. SNA originally ran on serial links and then later Token Ring and Ethernet LANs. Because SNA and NetBEUI are becoming less common, the use of LLC Type 2 is diminishing. However, it is still important to learn LLC Type 2 because it shares many characteristics with the quintessential High-Level Data Link Control (HDLC) family of WAN protocols. HDLC influenced the development of LLC Type 2, IBM's Synchronous Data Link Control (SDLC), and the X.25 link access procedure (LAP) protocols. The Link Access Procedure on the D Channel (LAPD) protocol, also known as ITU-T Q.921, is still very common on ISDN circuits. So a quick explanation of LLC Type 2 is warranted because it will help you troubleshoot WANs as well as legacy Ethernet networks that still use LLC Type 2. NOTE Cisco's HDLC is a customized version of HDLC and is not similar to LLC Type 2. Cisco's HDLC is connectionless and is similar to LLC Type 1. Because LLC Type 2 is connection-oriented, a connection setup must precede the transmission of data. A connection tear-down process occurs at the end of an LLC session. During a session, each frame is numbered and acknowledged. The session setup follows this sequence: 1. Station A -> Station B LLC C DSAP=F0 SSAP=F0 SABME P 2. Station B -> Station A LLC R DSAP=F0 SSAP=F0 UA F 3. Station A -> Station B LLC C DSAP=F0 SSAP=F0 RR NR=0 P 4. Station B -> Station A LLC R DSAP=F0 SSAP=F0 RR NR=0 F In Frame 1, Station A tells Station B to Set Asynchronous Balanced Mode Extended (SABME), which means that either side can initiate a conversation and send data without permission from the other side. In the olden days of mainframes and terminals, a lowly device like a terminal could not send data until it was polled. With the advent of mini and microcomputers, communication became more democratic. Station authority was more balanced and either station could send asynchronously with respect to the other. The original Asynchronous Balanced Mode allowed the transmission of 8 frames without an acknowledgment. To improve performance, the 8 frames were later extended to 128 frames. The extension to 128 frames is called Asynchronous Balanced Mode Extended. When Station A in the example makes the SABME request, it also sets the poll bit (that's the letter P at the end of the line in Frame 1). The poll bit has nothing to do with polling terminals and simply means, "Answer immediately; don�t wait until I�ve sent the full 128 frames that are possible." Station B replies to the SABME with an Unnumbered Acknowledgment (UA) in Frame 2. The final bit (F) is set to indicate that this response is complying with the request to answer immediately. Following the SABME and UA, Station A says in Frame 3 that its Receiver is Ready (RR) and that it expects to receive protocol data unit #0 from Station B. Next Receive (NR) = 0. In Frame 4, Station B says its receiver is also ready and that it expects to receive protocol data unit #0 from Station A. After the session establishment, analysis of LLC Type 2 is simply a matter of following the conversation from the SABME to the Disconnect Mode (DM), the last frame in a normal LLC session. You can track the sequence numbers and acknowledgments by viewing the Now Sending (NS) sequence number and the Next Receive (NR) acknowledgment number for each side of the conversation. There are two error commands that might occur: 1 Reject (REJ) -- A station sends a REJ when it receives an unexpected sequence number. 2 Frame Reject (FRMR) -- A station sends an FRMR when it receives an invalid frame or sequence number. REJs are normal; they simply ask for retransmission when a frame is lost. An LLC connection spans hubs, switches, and possibly routers. (To reduce the chance of LLC timeouts, you can optionally terminate an LLC connection at a router.) A frame could get lost as the result of congestion in switches, routers, or hosts, or on shared network segments. A recipient can send an REJ frame to indicate that there's a missing sequence number so that the sender retransmits. FRMRs, on the other hand, are abnormal and probably indicate a bug in a device driver or other serious problem with system memory or processing in the device to which the FRMR is sent. The FRMR could also be the result of an error in the station sending the FRMR." Priscilla Oppenheimer > > Many thx indeed > > > ------------------------------------------------------------------------ > For more information about Barclays Capital, please > visit our web site at http://www.barcap.com. > > > Internet communications are not secure and therefore the > Barclays > Group does not accept legal responsibility for the contents of > this > message. Although the Barclays Group operates anti-virus > programmes, > it does not accept responsibility for any damage whatsoever > that is > caused by viruses being passed. Any views or opinions > presented are > solely those of the author and do not necessarily represent > those of the > Barclays Group. 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