IPX/SPX Programming Manual

Addressing
HP NonStop IPX/SPX Programming Manual—528022-001
2-6
Supported NetWare Protocols
The SPXII packet header fields include:
The minimum SPXII packet size is 42 bytes, which consists of the 30-byte IPX header
plus the 12-byte SPXII header. For SPX, the maximum packet size is 576 bytes
(42-byte header information plus 534 bytes of data). For SPXII, the maximum packet
size is negotiated—it can be up to the maximum Send Data Unit (SDU) size for the
underlying LAN protocol.
SAP
The SAP protocol allows service nodes, such as file servers, print servers, and
database servers, to register their services and addresses in SAP tables maintained by
each server. The registration is done via SAP broadcasts every 60 seconds. If the
broadcast of a specific service stops, all other servers purge the service from their SAP
tables within about three minutes.
Figure 2-4. SPXII Packet Header Format
Conn.
(Connection)
Control
specifies whether the packet is a system packet, an
acknowledge data packet (ACK packet), or a data packet.
Data Stream Type 0xFE is used for an End-of-Connection packet and 0xFF is
used for an End-of-Connection acknowledgment.
Source
Connection ID
contains the connection identification number assigned by SPXII
to the packet originator.
Destination
Connection ID
contains the connection identification number assigned by SPXII
to the packet receiver.
Sequence
Number
tracks the number of data packets sent by the connection and
acknowledged by the receiver.
Acknowledge
Number
the sequence number of the next data packet expected by
SPXII. Received packets that have a sequence number lower
than the acknowledge number are assumed to be duplicates
and are dropped.
Allocation
Number
used for flow control.
VST0204.vsd
1 Byte 1 Byte 2 Bytes2 Bytes2 Bytes2 Bytes2 Bytes
. .
.
Conn
Control
Data
Stream
Type
Source
Connection ID
Destination
Connection ID
Sequence
Number
Acknowledge
Number
Allocation
Number
Data