Information Technology Reference
In-Depth Information
Figure8-2
IPX Packet Format
Checksum
Packet length
Transport control
Packet type
Destination network
Destination node
Destination socket
Source network
Source node
Source socket
Upper-layer data
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 that 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. With the use of NetWare Links State Protocol (NLSP), an IPX
packet can travel up to 127 hops to reach a destination.
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 (32-bit field), Destination node (48-bit field), and Destination socket
(16-bit field) —Fields that specify destination information.
Source network (32-bit field), Source node (48-bit field), and Source socket (16-bit
field) —Fields that specify source information.
Upper-layer data —Information for upper-layer processes. This section of the packet is also
referred to as the Higher Level Protocol Headers headers of the higher level NetWare protocols such
as NCP or SPX. These headers occupy the data position of the IPX packet.
Although IPX was derived from XNS, it has several unique features. From the standpoint of routing, the
encapsulation mechanisms of these two protocols represent 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.
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 .
Note
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