network protocols handbook.pdf

293Protocols GuideIBM ProtocolsProtocol NameNetBIOS: Network Basic InputOutput SystemProtocol DescriptionNetwork Basic Input Output System (NetBIOS) was created byIBM. NetBIOS defines a software interface and standard methodsproviding a communication interface between the applicationprogram and the attached medium. NetBIOS, a sessionlayer protocol, is used in various LAN (Ethernet, Token Ring etc)as well as WAN environments such as TCP/IP, PPP and X.25networks.NetBIOS frees the application from having to understand the detailsof the network, including error recovery (in session mode).A NetBIOS request is provided in the form of a Network ControlBlock (NCB) which, among other things, specifies a messagelocation and the name of a destination.NetBIOS provides the session and transport services describedin the Open Systems Interconnection (OSI) model. However, itdoes not provide a standard frame or data format for transmission.A standard frame format is provided by NetBEUI (NetBIOSExtended User Interface), which provides transport and networklayer support of NetBIOS.NetBIOS provides two communication modes: session or datagram.Session mode lets two computers establish a connectionfor a “conversation,” allows larger messages to be handled,and provides error detection and recovery. Datagram mode is“connectionless” (each message is sent independently), messagesmust be smaller, and the application is responsible forerror detection and recovery. Datagram mode also supports thebroadcast of a message to every computer on the LAN.NetBIOS names are 16 bytes long (padded if necessary) andthere are very few restraints on the byte values which can beused. There are three methods of mapping NetBIOS names toIP addresses on small networks that don’t perform routing:1. IP broadcasting - A data packet with the NetBIOS computername is broadcast when an associated address isnot in the local cache. The host with that name returnsits address.2. The lmhosts file - This is a file that maps IP addressesand NetBIOS computer names.3. NBNS - NetBIOS Name Server. A server that maps Net-BIOS names to IP addresses. This service is providedby the nmbd daemon on Linux.Protocol StructureNetBIOS packets have many different formats depending on theservices and message types as well as on the transport protocolsused to carry the NetBIOS packets. NetBIOS has threebasic services: NAME, SESSION and DATAGRAM. As an example,we display the NetBIOS name packet format in the TCP/IP environment:Header (12 bytes)Question Entry (variable)Answer Resource Records (variable)Authority Resource Records (variable)Additional Resource Records (variable)The format of the NetBIOS header is shown below:2 2 1 1 2 2 2 bytesLengthDeliminatorCommandData1Data2Destination name (16 bytes)Source name (16 bytes)XMITCorRSPCor• Len - The length of the NETBIOS header.• Deliminator - A delimiter indicating that subsequentdata is destined for the NetBIOS function.• Command - A specific protocol command that indicatesthe type of function of the frame.• Data 1 - One byte of optional data per specific command.• Data 2 - Two bytes of optional data per specific command.• Xmit/response correlator - Used to associate receivedresponses with transmitted requests.• Destination name/num - In non-session frames thisfield contains the 16-character name.• Source name/num - In non-session frames this fieldcontains the 16-character source name. In sessionframes this field contains a 1 byte source sessionnumber.Related protocolsTCP, SMP, Ethernet, Token Ring, X.25, UDP, IPX, NetBEUI,PPPSponsor SourceNetBIOS and NetBEUI are IBM protocols.Referencehttp://ourworld.compuserve.com/homepages/TimothyDEvans/contents.htmNetBios, NetBEUI, NBF, SMB, CIFS Networkinghttp://www.javvin.com/protocol/rfc1001.pdfPROTOCOL STANDARD FOR A NetBIOS SERVICE ON ATCP/UDP TRANSPORT: CONCEPTS AND METHODShttp://www.javvin.com/protocol/rfc1002.pdfPROTOCOL STANDARD FOR A NetBIOS SERVICE ON ATCP/UDP TRANSPORT: DETAILED SPECIFICATIONS