Altera SerialLite II Protocol - Manual (Page 66)

66 Altera Corporation
SerialLite II Protocol Reference Manual
Data Link Layer Description
5. Once all buffers are full, no further segments can be transmitted
until the oldest segment is acknowledged and released from its
buffer. Segment transmission is suspended until the duplicated
segment is acknowledged. During this interruption, regular data
packets can resume transmission.
The expected segment ID is associated with the next segment in sequence
to be released from the transmit buffer. When there are a number of
segments waiting to be released, the expected segment ID points to the
oldest segment of the sequence.
Segment Acknowledgement Timer
The near transmitter has a segment acknowledgement timer that
monitors the status of the oldest segment awaiting acknowledgement.
This timer guards against the possibility that the link management packet
carrying the segment acknowledgement encounters errors and gets
discarded. The timer increments when there is a segment present in the
buffer and resets upon a valid ACK or NACK with a matching expected
segment ID. If this counter expires without receiving a valid ACK or
NACK, retransmission starts with the unacknowledged segment
followed by all buffered segments in sequence. If this timer expires three
times for that expected segment without receiving an acknowledgement
(that is, the segment is unsuccessfully retransmitted three times), a link
error is declared.
The timeout limit is specified not in terms of absolute time, but in terms
of the number of columns. This measure allows the value to scale to the
operation frequency.
Retry-on-Error Implementation Rules
The following retry-on-error implementation rules must be followed:
1. Retry-on-error only applies to priority packets.
2. Retry-on-error uses the segment identification field of SPP2.
3. The near transmitter may transmit up to eight segments before
receiving an ACK or NACK.
4. Every segment transmitted requires an acknowledgment from the
far transmitter.
5. The far transmitter must acknowledge good segments with ACK or
bad segments with NACK.