Internetwork Packet Exchange (IPX)
| Novell originally developed IPX as the native protocol for the NetWare 3.x and 4.x operating systems. In 1998, NetWare 5 changed the IPX legacy by making IP the native NetWare protocol; however, it still uses IPX. The IPX design has as its basis the Internet Datagram Protocol (IDP) of the XNS protocol. Although Novell recommends pure IP on your network, small networks might benefit from using IPX only because IPX requires no special address resolution protocols (it can assign addresses dynamically). IP is better suited for large IP-based networks attached to the Internet, to WAN links, or where IP is the exclusively required protocol. If you don't require IP for any of these reasons, and you can use an IPX network, you might find that an IPX implementation is easier to administer. Because IPX automatically distributes subnet addresses, whereas you must manually configure IP, IPX is easier to manage than IP.
In contrast to the Microsoft NetBEUI (NetBIOS Extended User Interface) protocol, IPX/SPX is routable. It should be noted, however, that Microsoft provides for sending NetBEUI over TCP/IP, which is a routable protocol. IPX was a good solution for small and mid- sized networks for many years . Several major network operating systems and most desktop operating systems support the IPX protocol, mostly to accommodate legacy clients and servers. If you want to connect a Windows server and a NetWare server, you can use either IPX (and Microsoft's NWLink) or TCP/IP. Several other network operating systems, from Windows NT and Windows 2000 to Windows 2003 servers, include the NWLink protocol to provide interoperability with NetWare and NetWare-compatible applications and devices. Although older NetWare networks ( specifically NetWare 3.x) still use IPX as an internetwork protocol, Microsoft uses IPX differently than Novell due to NetBIOS naming. In a Windows NT network using IPX, you must configure all routers to broadcast NetBIOS on all segments if you want services to be locatable. Novell isolates traffic per segment and uses IPX RIP/SAP or NDS to find its resources. Because of this difference, Microsoft's use of IPX is inefficient as a routed protocol. Yet you must consider that Microsoft provides NWLink only (1) to enable data exchanges between older NetWare systems, which may indeed reside on a single network segment, and (2) to provide an upgrade path so that you can migrate your older NetWare servers (and the data they hold) to more modern Windows platforms. The IPX protocol uses the ODI (Open Data-link Interface) specification for DOS and Windows workstations on the network. ODI allows workstations or servers to use multiple protocols on the same network. Each workstation can use a combination of protocols on the same network card. This allows your workstation to communicate with the NetWare network and other systems, such as a mainframe computer or an Internet connection, concurrently. In addition, the ODI specification provides a modular way of installing network drivers so that when you replace a network adapter, you need change only the LAN card driver.
Similar to IP, IPX is an internetworking protocol that provides datagram services. IPX is a connectionless datagram service, which means after the computer sends a datagram, there are no guarantees that it was delivered. One application for which you might use simple IPX is that of broadcast messages, such as error notifications and time synchronization. IPX performs dynamic route selections based on tables of network reachability information compiled by RIP. IPX Packet CommunicationsThe IPX protocol divides data into packets to send across the network. Packets are a specific size and contain a certain amount of information. A packet contains the data designated for transmission, in addition to the necessary addressing information. NetWare uses three main types of addressing:
IPX also can send groups of packets, called bursts , without requiring the recipient to acknowledge each packet. NetWare added burst-mode technology to enhance the IPX protocol when used over WAN links. Burst mode lets a workstation make one request for a file. After it receives the request, the server responds with a continuous stream of packets and then requires only a single acknowledge response from the recipient after it has received the packet burst. Burst mode improves network throughput and greatly reduces the amount of traffic on the network, two important factors for improving performance over WAN links. Burst mode improves performance in these environments:
IPX Packet StructureThe Transport Control Field contains 8 bits, the last field of which (0) is the packet type. It identifies the data contents of the data portion of the IPX packet. Field 8 provides for protocol multiplexing, which enables other protocols to reside on top of IPX and helps IPX determine which of the client protocols to send the packet to. Table 32.1 defines the Packet Type Assignments for IPX. Table 32.1. IPX Packet Type Assignments
An IPX protocol packet can be 30 to 65,535 bytes long. Table 32.2 outlines the packet structure for the IPX protocol. Ethernet networks have a default packet size of 1,500 bytes, and Token-Ring networks have a default packet size of 4,202 bytes. Table 32.2. IPX Protocol Packet Structure
The IPX datagram structure includes a network address and a node address. You normally assign the network address to the network when you install the first NetWare server or router on a segment. Each of the four frame types can be installed and bound to the same Network Interface Card (NIC) in a server, establishing a different network address for each frame type. All subsequent NetWare server installations on that network must correspond to the network address of each frame type assigned by the first server. A router that is set up to route IPX also must have a corresponding network address for each frame type that it uses. The node address is the MAC address assigned to the NIC. A socket address is included to identify a running process in a computer. An example of an IPX address is 1AB47E3F 0080D4287DE1 0121. IPX Frame TypesThe IPX protocol has four Ethernet frame types and two Token-Ring frame types. The default frame type for NetWare 4.x on an Ethernet network is IEEE_802.2, more commonly known as Ethernet 802.2. Table 32.3 identifies the frame types for both Ethernet and Token-Ring networks. Table 32.3. Ethernet and Token-Ring Frame Types
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