It is the version of Novels’ Sequenced Packet Protocol (SPP).
It is a transport layer protocol and is used by client server applications. This
protocol provides a packet delivery system for third party applications.
Figure given below illustrate the structure of SPX packet.
Connection Control Flag
|
DataStream Type
|
Source Connection ID
|
|
Destination Connection ID
|
|
Sequence Number
|
|
Acknowledge Number
|
|
Allocation Number
|
|
0-534 bytes of data
|
Connection Control Flag: Four flags are used to
control bidirectional data flow of SPX connection. If its value is 1 flag is
said to be set and if its value is 0 it is said to not set. Following are four
flags:
Bit No.
|
Signal
|
Description
|
Bit 4
|
Eom
|
End of Message
|
Bit 5
|
Att
|
Attention Bit, not used by SPX
|
Bit 6
|
Ack
|
Acknowledge Required
|
Bit 7
|
Sys
|
Transport Control
|
DataStream Type: This is used to specify the data
within the packet.
0-253: Ignored by
SPX.
254: End of Connection.
255: End of connection Acknowledgement.
254: End of Connection.
255: End of connection Acknowledgement.
Source Connection ID: It is a 16 bit number that
is assigned by SPX for the identification of connection.
Destination Connection ID: It is a reference id
that is used to identify the target end of the transport connection.
Sequence Number: It is a 16 bit number that is
used for the identification of transmitted packets. It provides complete
information about a message that a message is decomposed in how many packets
and which packet number is this.
Acknowledgement Number: 16 bit number used to
indicate next expected packet.
Allocation Number: It is a 16 Bit that indicates that
how many sent packets are not acknowledged.
Connection Control Flag:
Bit 2 for Size Negotiation.
Bit 3 for SPX-II Type
Bit 2 for Size Negotiation.
Bit 3 for SPX-II Type
DataStream Type:
252 –Orderly release request.
253 –Orderly release Acknowledgement.
252 –Orderly release request.
253 –Orderly release Acknowledgement.
It also contains additional 2-Byte Extended Acknowledgement
field at the end.
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