3Com CoreBuilder 3500, Implementation Manual

Page: 422 / 592

422 C
HAPTER
15: IPX R
OUTING
IPX Packet Format
An IPX packet consists of a 30-byte header followed by packet data. The
packet header contains network, node, and socket addresses for both the
destination and the source.
Figure 75 shows the IPX packet format.
Figure 75 IPX Packet Format
The IPX packet contains the following elements:
■
Checksum — A 16-bit checksum that is set to 1s.
■
Packet length — A 2-byte field that indicates the packet’s length in
bytes. This length, which includes both header and data, must be at
least 30 bytes.
■
Transport control — A 1-byte field that indicates how many routers
a packet has passed through on its way to its destination. Packets are
discarded when this value reaches 16. A network node sets this field
to 0 before sending the IPX packet.
■
Packet type — A 1-byte field that specifies the upper-layer protocol
that receives the packet.
■
Destination network — A 4-byte field that contains the network
number of the destination node. When a sending node sets this field
to 0, the system routes the packet as if the sending and destination
nodes were on the same local segment.
Checksum
Packet length
Transport control
Destination network
Destination node
Destination socket
Source network
Source node
Source socket
Packet data
Packet type
(2 bytes)
(2 bytes)
(1 byte)
(2 bytes)
(2 bytes)
(6 bytes)
(4 bytes)
(4 bytes)
(6 bytes)
(1 byte)
Packet header