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.Thislayer deals with IP addresses, which are logical addresses, and routingbetween separate networks.A number of protocols operate at this level.They include Internet Protocol (IP), Internet Protocol version 6 (IPv6),ARP, Routing Information Protocol (RIP), Open Shortest Path First(OSPF), Internet Group Management Protocol (IGMP), and InternetControl Message Protocol (ICMP).Protocols such as RIP and OSPFallow the determination of the shortest routes to particular destinations.3.Host-to-Host (Transport) The Host-to-Host layer has the samefunctionality as the Transport layer of the OSI model.Like the OSImodel, it is responsible for ensuring reliable transmission of data basedon the end-to-end communication established by its lower layer.TCPand UDP are found at this layer.4.Application This layer corresponds to the top three layers of the OSImodel: Session, Presentation, and Application.However, the Session layerdoes not map very cleanly to the Application layer;TCP, for example,creates sessions by means of a three-way handshake between hosts.Manyprotocols are found at this layer, including HTTP, Post Office Protocolversion 3 (POP3), Dynamic Host Configuration Protocol (DHCP),SMTP, and others.Like the OSI Reference Model, the DoD model is a good conceptual modelto use for troubleshooting.Because we know the protocols that are implementedat each layer, it is relatively easy to narrow down where the problem originated.Again, the most effective way to troubleshoot a communications problem is tostart at the bottom of the model and work your way up.www.syngress.com189_XP_06.qxd 11/12/01 9:26 AM Page 308308 Chapter 6 " Windows XP NetworkingWindows XP Networking ArchitectureSince the first version of NT, Microsoft has provided a modular network archi-tecture that also employs layers.In this layered, modular approach, Microsoftimplements a specific type of layer called a boundary layer.There are threeboundary layers: the Application Programming Interface (API) boundary layer, theTransport Device Interface (TDI) boundary layer, and the Network Device InterfaceSpecification (NDIS) boundary layer.These three boundary layers serve to provideinterfaces to the operating-specific components found within Microsoft s imple-mentation of networking.For example, Microsoft s implementation of TCP isfound between the NDIS and TDI boundary layers.The modular approach, combined with the use of boundary layers, has anumber of advantages.One is that it is relatively easy for a third-party vendor tocreate a networking component to integrate with Microsoft s networking compo-nent.For example, a network card vendor need not be concerned itself with theparticular details of Microsoft s implementation of TCP/IP.It need only concernitself with creating a driver that uses the methods specified by the NDIS boundary.Furthermore, the same driver will make it possible for the network adaptor to useall of the installed protocols, or a vendor such as Novell can more easily create aclient component to enable communication with a NetWare server.Likewise, if software developers use the methods specified by the APIboundary, they need not be concerned about creating separate methods foraccessing the hard drive and the network, since from the point of view of theapplication there is no difference between data that is on the network and datathat is local to the computer.Between the API and the TDI boundary layers,Microsoft implements the network Server redirector, which responds to requestsfrom other workstations on the network, and the Workstation redirector, whichmakes requests on the network, as file system drivers.Additional file systemdrivers make it possible to access the NTFS and FAT partitions as well as CD-ROMs.The manner in which Microsoft implements Network redirectors as filesystem drivers explains why Windows 98 computers are able to access files storedon an NTFS partition across the network.When the Server redirector receives arequest, it simply redirects the request to the NTFS file system driver forretrieval.Once the NTFS driver retrieves the data, the Server redirector can sendthe data over the network to the Windows 98 computer requesting the data.Given that MS implements the network redirectors as file system drivers andintegrates network functionality so tightly into the operating system, you caneasily appreciate that networking is a core function of Windows XP.Now that wewww.syngress.com189_XP_06 [ Pobierz całość w formacie PDF ]