3Com 4007, Руководство по внедрению, Страница: 511 / 684

Key Concepts
511
Figure 59 shows the IPX packet format.
Figure 59 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 includes both header and data. The length 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.
■
Destination node
— A 6-byte field that contains the physical address
of the destination node.
■
Destination socket — A 2-byte field that contains the socket address
of the packet’s destination process.
Checksum
Packet length
Transport control
Destination network
Destination node
Destination socket
Source network
Source node
Source socket
Higher level protocal headers
Packet type
(2 bytes)
(2 bytes)
(1 byte)
(2 bytes)
(2 bytes)
(6 bytes)
(4 bytes)
(4 bytes)
(6 bytes)
(1 byte)
Packet header
Packet data
MAC Header
Data
MAC Trailer