In the PCI Express (PCIe) Data Link Layer, the Ack/Nak protocol is a two-way street. While the receiver evaluates incoming Transaction Layer Packets (TLPs) and schedules Acknowledgements (Acks), the transmitter must constantly manage its memory and timers based on the responses it gets back.
When a valid Ack DLLP successfully returns to the transmitting device, the hardware executes a rapid sequence of events to clear its pipeline and keep data flowing efficiently. Here is a look at exactly how a transmitter processes an incoming Ack.
Verifying Forward Progress
Before a transmitter alters its buffers or timers, it must first verify that the incoming Ack represents actual “forward progress”.
To do this, the transmitter relies on an internal 12-bit register called the ACKD_SEQ register, which stores the Sequence Number of the most recently received valid Ack or Nak. The transmitter compares the Sequence Number found inside the newly arrived Ack against the value currently sitting in the ACKD_SEQ register.
If the incoming sequence number is logically later than the number stored in the register, the transmitter officially knows that forward progress has been made. It immediately updates the ACKD_SEQ register with this new sequence number to serve as the new baseline.
Purging the Replay Buffer
Once forward progress is confirmed, the transmitter can finally clear out its Replay Buffer.
Because the golden rule of PCIe requires that all TLPs be successfully received in their exact sequential order, the transmitter knows that if it receives an Ack for packet 7, packet 7 and all the packets before it must have arrived perfectly.
Because of this sequential guarantee, the transmitter aggressively purges all TLPs from the Replay Buffer whose Sequence Numbers are equal to or earlier than the sequence number contained in the Ack. This mechanism allows a single Ack DLLP to serve as a bulk receipt, acknowledging multiple successful TLPs at once and freeing up valuable buffer space without clogging the link with unnecessary traffic.
Resetting the Watchdog Timers
The final step in processing an Ack is calling off the transmitter’s internal error-recovery watchdogs. Because a valid Ack proves that the link is healthy and packets are arriving safely, the transmitter immediately resets two critical tracking components:
- The REPLAY_NUM Counter: This 2-bit counter tracks how many times the transmitter has been forced to replay failed packets. Upon seeing an Ack that proves forward progress, this counter is instantly reset to zero (
00b). - The REPLAY_TIMER: This countdown clock ensures the transmitter does not wait indefinitely for a response. When the valid Ack arrives, this timer is immediately reset to 0. What happens next depends on the Replay Buffer:
- If the Replay Buffer is now completely empty, the timer rests at 0 and stays halted. It will not begin counting again until the last symbol of a brand new TLP is transmitted.
- If there are still unacknowledged TLPs left in the buffer after the purge, the timer instantly restarts and begins counting again to monitor those remaining packets.
Summary Processing an Ack is a masterclass in hardware efficiency. By utilizing sequential numbering to purge multiple TLPs in a single action and instantly resetting its recovery timers, the transmitter ensures its Replay Buffer never overflows and its data pipeline remains perfectly stable.