In the PCI Express (PCIe) Data Link Layer, the Ack/Nak protocol is essential for ensuring reliable packet delivery. However, requiring a receiver to send a dedicated Acknowledge (Ack) packet back to the transmitter for every single Transaction Layer Packet (TLP) it receives would create a massive amount of unnecessary overhead.
To solve this problem, the PCIe specification utilizes an internal countdown called the AckNak_LATENCY_TIMER to coalesce (combine) Acks, dramatically improving link efficiency by validating multiple packets at once.
Here is how receivers use this timer to delay acknowledgements and keep the link running efficiently.
How the Timer Operates
The AckNak_LATENCY_TIMER dictates exactly how long a receiver is allowed to wait before it must acknowledge a good packet.
- Starting the Clock: The timer begins running anytime a receiver successfully processes a valid TLP that it has not yet acknowledged.
- Holding the Ack: While the timer is ticking, the receiver is allowed to continue receiving and accepting more good TLPs without sending any Acks back to the transmitter.
- Resetting the Clock: The timer is only reset and halted when an Ack or Nak DLLP is finally scheduled and sent. It will immediately start counting again the moment the next unacknowledged, good TLP arrives.
Sending the Coalesced Ack
The receiver is not required to act until the AckNak_LATENCY_TIMER finally expires. Once the timeout is reached, the receiver generates and sends just a single Ack DLLP to the transmitter.
Crucially, the Sequence Number placed inside this delayed Ack is strictly the number of the last good TLP the receiver successfully processed. The receiver determines this number by taking its expected NEXT_RCV_SEQ (NRS) count and subtracting one.
Why Coalescing Works
This clever delay tactic allows a single Ack to represent and validate multiple successful TLPs at once.
This technique works flawlessly because of the strict “golden rule” of the PCIe Link Layer: TLPs must always be successfully received in their exact sequential order.
Because ordering is strictly enforced, the transmitter inherently knows that if it receives an Ack for sequence number 7, not only did packet 7 arrive safely, but all the packets sent before it must have arrived perfectly as well. The transmitter can then safely purge all of those acknowledged TLPs from its Replay Buffer in one bulk action.
Summary By utilizing the AckNak_LATENCY_TIMER to delay the acknowledgement process, a receiver avoids clogging the link with constant, dedicated local traffic. This significantly reduces overhead and ensures that more of the link’s bandwidth is dedicated to moving actual data payloads rather than just link management messages.