Interoperability with NetWare

NWLink provides a comprehensive set of transport and network layer protocols that allow for integration with the NetWare environment. Table 24.11 lists the subprotocols and components and shows their function and associated drivers.

Table 24.11 NWLink Protocols

Figure 24.7 illustrates NWLink in the Windows 2000 Professional architecture and the files in which each protocol is implemented.

Figure 24.7 NWLink in the Windows 2000 Professional Architecture

IPX

IPX is a protocol that provides connectionless datagram transfer services and controls addressing and routing packets of data within and between network segments. With connectionless transmission, a session does not need to be set up each time packets are transmitted; packets are simply sent out on the wire. This requires less overhead than connection-oriented transmission in which a session must be established each time packets are transmitted. Therefore, connectionless transmission is best when data is generated in intermittent short bursts.

Because IPX is a connectionless protocol, it does not provide for flow control or acknowledgment that the receiving station has received the datagram packet. Instead, individual datagram packets travel independently to their destination. There is no guarantee that packets arrive at their destination or that they arrive in sequence. However, because transmission on LANs is relatively error-free, IPX is efficient in delivering data on LANs.

NWLink enables application programming for Windows Sockets and remote procedure calls (RPC) over Sockets. IPX supports Socket IDs for use by Sockets applications. IPX enables NetBIOS, Named Pipes, Mailslot, Network Dynamic Data Exchange (NetDDE), RPC over NetBIOS, and RPC over Named Pipes programming over NBIPX. NWLink also supports other applications that use IPX, through direct hosting. Direct hosting is a feature that allows CIFS-capable computers to communicate over IPX, bypassing the NetBIOS layer. Direct hosting can lower overhead and increase throughput.

SPX

SPX is a transport protocol that offers connection-oriented services over IPX. Although connection-oriented service requires overhead for session setup, after a session is established, connection-oriented service requires less overhead for data transmission than connectionless service. Therefore, it works best for applications that require a continuous connection. SPX provides reliable delivery through sequencing and acknowledgments and verifies successful packet delivery to any network destination by requesting a verification from the destination upon receipt. SPX can track data transmissions consisting of a series of separate packets. If an acknowledgment request brings no response within a specified time, SPX retransmits the request up to eight times. If no response is received, SPX assumes the connection has failed.

SPX also provides a packet burst mechanism. Packet burst , also known as burst mode , allows the transfer of multiple data packets without requiring that each packet be sequenced and acknowledged individually. By allowing multiple packets to be acknowledged once, burst mode can reduce network traffic on most IPX networks. Additionally, the packet burst mechanism monitors for dropped packets and retransmits only the missing packets. In Windows 2000 Professional, burst mode is enabled by default.

SPXII

SPXII improves over SPX in the following ways.

SPXII allows for more outstanding unacknowledged packets than SPX    In SPX there can be only one outstanding unacknowledged packet at any time, but in SPXII there can be as many outstanding packets as negotiated by the networked peers at connection setup time.

SPXII allows for larger packets    SPX has a maximum packet size of 576 bytes, while SPXII can use the maximum packet size of the underlying LAN. For example, on an Ethernet network SPXII can use 1,518 bytes.

RIP

NWLink uses RIP over IPX (RIPX) to implement route and router discovery services used by SPX and NBIPX. RIP determines the forwarding media access control (MAC) address for outbound traffic.

NWLink includes the RIP protocol for Windows-based clients and for computers running Windows 2000 Server that do not have the Routing and Remote Access service installed. These computers do not forward packets the way that routers do, but they use RIP to determine where to send packets. RIP clients, such as workstations, can locate the optimal route to an IPX network number by broadcasting a RIP GetLocalTarget request. Each router that can reach the destination responds to the GetLocalTarget route request. Based on the RIP responses from the local routers, the sending station chooses the best router to use to forward the IPX packet.

SAP

SAP is the protocol used to distribute the names and addresses of services running on IPX nodes.

SAP clients use SAP broadcasts only when bindery-based or NDS queries fail and send the following types of messages:

• SAP GetNearestServer request for the name and address of the nearest server of a specific type.

• SAP general service request for the names and addresses of all services, or of all services of a specific type.

NetBIOS

The NWLink NetBIOS protocol is automatically installed with Client Service. To facilitate the operation of NetBIOS-based applications on an IPX internetwork, NetBIOS over IPX provides standard NetBIOS services such as the following:

• Datagrams. Single packets sent without acknowledgment, for example, broadcasts.

• Sessions. Multiple packets sent with acknowledgments between two endpoints.

• Name management. Registering, querying, and releasing NetBIOS names.

An additional module, NWLnkNB, formats NetBIOS-level requests and passes them to NWLink for transmission. NWLnkNB includes the following performance enhancements:

• PiggyBackAck. An acknowledgment of previous frames within the response frames.

• A sliding window acknowledgment mechanism. A dynamic window–sizing algorithm that allows burst mode to adjust the number of frames that it can send.

Alternately, computers can communicate directly over IPX, through direct hosting. For more information about direct hosting, see Client Service and Gateway Service earlier in this chapter.

Forwarder

The forwarder component is installed with NWLink but is used only when the Windows 2000–based server is used as an IPX router running the Routing and Remote Access service. It operates in kernel mode.

When the IPX router software is activated, the forwarder component works with the IPX Router Manager component of Routing and Remote Access and the filtering component to forward packets. The forwarder component obtains configuration information from the IPX Router Manager and stores a table of the best routes. When it receives an incoming packet, it passes it to the filtering driver to check for input filters. When it receives an outgoing packet, it first passes it to the filtering driver. Assuming that no outgoing filters prevent the packet from being transmitted, the packet is passed back, and the forwarder component forwards the packet over the appropriate interface.