network protocols handbook.pdf

217Protocols GuideLAN - Other ProtocolsProtocol NameLLC: Logic Link Control (IEEE802.2)Protocol DescriptionLogic Link Control (LLC) is the IEEE 802.2 LAN protocol thatspecifies an implementation of the LLC sublayer of the datalink layer. IEEE 802.2 LLC is used in IEEE802.3 (Ethernet) andIEEE802.5 (Token Ring) LANs to perform these functions:a. Managing the data-link communicationb. Link Addressingc. Defining Service Access Points (SAPs)d. SequencingThe LLC provides a way for the upper layers to deal with anytype of MAC layer (e.g. Ethernet - IEEE 802.3 CSMA/CD or TokenRing IEEE 802.5 Token Passing).LLC originated from the High-Level Data-Link Control (HDLC)and uses a subclass of the HDLC specification. LLC definesthree types of operation for data communication:Type 1: Connectionless. The connectionless operation is basicallysending with no guarantee of receiving;Type 2: Connection Oriented. The Type 2 Connection-Orientedoperation for the LLC layer provides these 4 services: Connectionestablishment, Confirmation and acknowledgement thatdata has been received, Error recovery by requesting receivedbad data to be resent, Sliding Windows (Modulus: 128), which isa method of increasing the rate of data transfer.Type 3: Acknowledgement with connectionless service.The Type 1 connectionless service of LLC specifies a staticframeformat and allows network protocols to run on it. Networkprotocols that fully implement a transport layer will generally useType 1 service.The Type 2 connection-oriented service of LLC provides reliabledata transfer. It is used in LAN environments that do not invokenetwork and transport layer protocols.Protocol StructureLogic Link Control Layer (LLC) Header:8 16 24 or 32bit VariableDSAP SSAP Control LLC information1 8bitI/GAddress bitsI/G Individual/group address may be: 0 IndividualDSAP; 1 Group DSAP.• SSAP - The source service access point structure isas follows:1 8bitC/RAddress bitsC/R Command/response: 0 Command; 1 Response.• Control - The structure of the control field is as follows:1 8 9 16bitInformation 0 N(S) P/F N(R)Supervisory 1 0 SS XXXX P/F N(R)Unnumbered 1 1 MM P/F MMMN(S) Transmitter send sequence number.N(R) Transmitter receive sequence number.P/F Poll/final bit. Command LLC PDU transmission/responseLLC PDU transmission.S Supervisory function bits:00 RR (receive ready).01 REJ (reject).10 RNR (receive not ready).X Reserved and set to zero.M Modifier function bits.• LLC information - LLC data or higher layer protocols.Related protocolsIEEE 802.3, 802.5Sponsor SourceLLC is defined by IEEE (http://www.ieee.org) in the 802.2 specifications.Referencehttp://standards.ieee.org/getieee802/download/802.2-1998.pdfIEEE 802.2 specification.https://secure.linuxports.com/howto/intro_to_networking/c5048.htmIEEE 802.2: Logic Link Control Layer• DSAP - The destination service access point structureis as follows: