System information

Troubleshooting Novell IPX 8-139
The Network Layer
Figure 8-2 IPX Packet Format
The fields of the IPX packet are as follows:
• Checksum—A 16-bit field that is set to ones.
• Packet length—A 16-bit field that specifies the length, in bytes, of the complete IPX datagram.
IPX packets can be any length up to the media maximum transmission unit (MTU) size. There is
no packet fragmentation.
• Transport control—An 8-bit field that indicates the number of routers the packet has passed
through. When the value of this field reaches 15, the packet is discarded under the assumption
that a routing loop might be occurring.
• Packet type—An 8-bit field that specifies the upper-layer protocol to receive the packet’s
information. Two common values for this field are 5, which specifies Sequenced Packet
Exchange (SPX), and 17, which specifies the NetWare Core Protocol (NCP).
• Destination network, destination node, and destination socket—Specify destination information.
• Source network, source node, and source socket—Specify source information.
• Upper-layer data—Contains information for upper-layer processes.
Although IPX was derived from XNS, it has several unique features. From the standpoint of routing,
the encapsulation mechanisms of these two protocols are the most important difference.
Encapsulation is the process of packaging upper-layer protocol information and data into a frame.
For Ethernet, XNS uses standard Ethernet encapsulation, whereas IPX packets are encapsulated in
Ethernet Version 2.0 or IEEE 802.3 without the IEEE 802.2 information that typically accompanies
these frames. Figure 8-3 illustrates Ethernet, standard IEEE 802.3, and IPX encapsulation.
Note NetWare 4.0 supports encapsulation of IPX packets in standard IEEE 802.3 frames. It also
supports Subnetwork Access Protocol (SNAP) encapsulation, which extends the IEEE 802.2
headers by providing a type code similar to that defined in the Ethernet specification.
Checksum
Packet length
Transport control Packet type
Destination network
Destination node
Destination socket
Source network
Source node
Source socket
Upper-layer data