IPX/SPX Programming Manual

Addressing
HP NonStop IPX/SPX Programming Manual—528022-001
2-2
Supported NetWare Protocols
protocols (SPX, SPXII, NCP, and SAP) use IPX to transport messages to their
destination. IPX, SPX, NCP, and SAP do not use a checksum; therefore, they cannot
guarantee end-to-end data integrity. Higher-level protocols and applications must
ensure the reliable transmission of the data. SPXII permits the use of a checksum,
which you can enable or disable using the setsockopt or setsockopt_nw routines
described in Section 5, Library Routines.
IPX uses the RIP routing tables to find the fastest route on which to send packets. IPX
uses the services of a LAN driver to deliver messages from one LAN adapter on a
network to another.
The following subsections describe the format of the IPX packet, IPX addresses and
sockets, and how IPX packets are formed in the NonStop IPX/SPX subsystem.
IPX Packet Format
The IPX packet header is 30 bytes long. The format of an IPX packet is shown in
Figure 2-2.
The following describes the fields of the IPX packet header:
Figure 2-2. IPX Packet Format
Checksum is set to FFFF(-1) by IPX implementations. Optionally,
computation or checking of this field can be turned ON or
OFF (see setsockopt, setsockopt_nw on page 5-49 in
Section 5).
Transport Control monitors the number of bridges or routers that a packet
crosses through.
Packet Length is the total length of the IPX header plus data. This length
cannot exceed 576 bytes (a 30-byte header plus 546 bytes
of data).
Packet Type specifies the higher-level protocol associated with the
packet. IPX uses this field and the socket number in the
destination address to identify the process for which an
incoming packet is intended (see IPX Addresses later in
this section). Table 2-1 lists the possible values of the IPX
packet type field and the associated higher-level protocols.
Bytes
Check-
Sum
VST0202.vsd
22
Bytes
12
Bytes
12
Bytes
Packet
Length DataSource AddressDestination Address
. . .
Packet Type (1 Byte)
Transport Control (1 Byte)