Enhanced sequenced packet exchange (SPXII) protocol

Data packet format

``SPXII data packet'' shows the fields and values of an SPXII data packet.

The value in the Length field reflects the actual size of the data packet. A data packet can vary from an SPXII header alone to an SPXII header plus the maximum negotiated data size (for a range of total packet size of 576 to 65,535 bytes).

The ACK bit in the Connection Control field is optional. For NetWare in a UNIX^® environment, the SPXII driver sets the ACK bit when any of the following conditions occur:

The packet has the EOM bit set in Connection Control field.
No more data is currently on the Streams write queue.
This packet fills the receiver's last available buffer (according to the window size in the receiver's last ACK).

The Sequence Number field tracks the number of data packets the sender has transmitted. The first data packet has a sequence of zero. Once the driver has placed that packet on the sending queue, the driver increments the count so that the next data packet will have a sequence number of 1.

SPXII data packet

The Acknowledge Number field tracks the number of data packets the sender has received from the other endpoint. It is incremented on the reception of a packet and contains the value of the sequence number expected in the next data packet.

For example, the Acknowledge Number field would have the following values under the described conditions:

0: No data packets have been sent by the other endpoint.
4: Four data packets have been sent by the other endpoint (0,1,2,3) and the next data packet to arrive will have a Sequence Number of 4.

The Allocation Number indicates the current receive window size of the sender. Actual window size is calculated by subtracting the acknowledge number from the allocation number and adding one: (Alloc# - Ack#) + 1.