Information Technology Reference
In-Depth Information
Figure8-3
Ethernet, IEEE 802.3, and IPX Encapsulation Formats
Ethernet
Destination address
Standard IEEE 802.3
Destination address
IPX
Destination address
Source address
Source address
Source address
Type
Length
Length
802.2 header
Upper-layer
data
IPX data
802.2 data
CRC
CRC
CRC
To route packets in an internetwork, IPX uses a dynamic routing protocol called the Routing Information
Protocol (RIP). Like XNS, RIP was derived from work done at Xerox for the XNS protocol family.
In addition to the difference in encapsulation mechanisms, Novell added a protocol called the Service
Advertising Protocol (SAP) to its IPX protocol family. SAP allows nodes that provide services (such as
file servers and print servers) to advertise their addresses and the services that they provide.
Novell also supports IBM logical unit (LU) 6.2 network addressable units (NAUs). LU 6.2 allows
peer-to-peer connectivity across IBM communication environments. Using NetWare's LU 6.2 capability,
NetWare nodes can exchange information across an IBM network. NetWare packets are encapsulated
within LU 6.2 packets for transit across the IBM network.
The Transport Layer
Sequenced Packet Exchange (SPX) is the most commonly used NetWare transport protocol. Novell
derived this protocol from the XNS Sequenced Packet Protocol (SPP). As with the Transmission Control
Protocol (TCP) and many other transport protocols, SPX is a reliable, connection-oriented protocol that
supplements the datagram service provided by Layer 3 protocols.
SPX is noted by Novell's documentation as follows: “SPX (Sequenced Packet Exchange) is a protocol
within IPXODI. SPX is derived from Novell's IPX using the Xerox Sequenced Packet Protocol. It
enhances the IPX protocol by supervising data sent out across the network.”
Novell also offers Internet Protocol (IP) support in the form of User Datagram Protocol (UDP)/IP
encapsulation of other Novell packets, such as SPX/IPX packets. IPX datagrams are encapsulated inside
UDP/IP headers for transport across an IP-based internetwork. NetWare 5.0 runs native IP, but the
previous versions can run IP in the form mentioned previously or NetWare/IP.
Upper-Layer Protocols
NetWare supports a wide variety of upper-layer protocols, but several are somewhat more popular than
others. The NetWare shell runs in clients (often called workstations in the NetWare community) and
intercepts application I/O calls to determine whether they require network access for satisfaction. If they
do, the NetWare shell packages the requests and sends them to lower-layer software for processing and
network transmission. If they do not require network access, they are simply passed to local I/O
resources. Client applications are unaware of any network access required for completion of application
calls. NetWare remote procedure call (NetWare RPC) is another more general redirection mechanism
supported by Novell.
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