SPX (Sequential Packet Exchange) Protocols

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.

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

DataStream Type:
252 –Orderly release request.
253 –Orderly release Acknowledgement.

It also contains additional 2-Byte Extended Acknowledgement field at the end.