This section describes the possible sources of errors that may occur in delivery of TLPs from a transmitter to a receiver across a Link. The ACK/NAK protocol guarantees reliable delivery of TLPs despite the unlikely event that these errors occur. Below is a bullet list of errors and the related error correction mechanism the protocol uses to resolve the error:
Problem:
CRC error occurs in transmission of a TLP (see "Transmitter's Response to a NAK DLLP" on page 224 and "Receiver Schedules a NAK" on page 233.)
Solution:
Receiver detects LCRC error and schedules a NAK DLLP with Sequence Number = NEXT_RCV_SEQ count - 1. Transmitter replays TLPs.
Problem:
One or more TLPs are lost en route to the receiver.
Solution:
The receiver performs a sequence number check on all received TLPs. The receiver expects TLPs to arrive with each TLP that has an incremented 12-bit Sequence Number from that in the previous TLP. If one or more TLPs are lost en route, a TLP will have a Sequence Number issued later than expected Sequence Number reflected in the NEXT_RCV_SEQ count. The receiver schedules a NAK DLLP with a Sequence Number = NEXT_RCV_SEQ count - 1. Transmitter replays the Replay Buffer contents.
Problem:
Receiver returns an ACK DLLP, but it is corrupted en route to the transmitter. The remote Transmitter detects a CRC error in the DLLP (DLLP is covered by 16-bit CRC, see "ACK/NAK DLLP Format" on page 219). In fact, the transmitter does not know that the malformed DLLP just received is supposed to be an ACK DLLP. All it knows is that the packet is a DLLP.
Solution:
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Case 1:
The Transmitter discards the DLLP. A subsequent ACK DLLP received with a later Sequence Number causes the transmitter Replay Buffer to purge all TLPs with equal and earlier generated Sequence Numbers. The transmitter never knew that anything went wrong.
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Case 2:
The Transmitter discards the DLLP. A subsequent NAK DLLP received with a later generated Sequence Number causes the transmitter Replay Buffer to purge TLPs with equal to an earlier Sequence Numbers. The transmitter then replay all TLPs with later Sequence Numbers till the last TLP in the Replay Buffer. The transmitter never knew that anything went wrong.
Problem:
ACK or NAK DLLP for received TLPs are not returned by the receiver by the proper ACKNAK_LATENCY_TIMER time-out. The associated TLPs remain in the transmitter Replay Buffer.
Solution:
The REPLAY_TIMER times-out and the transmitter replays its Replay Buffer.
Problem:
The Receiver returns a NAK DLLP but it is corrupted en route to the transmitter. The remote Transmitter detects a CRC error in the DLLP. In fact, the transmitter does not know that the DLLP received is supposed to be an NAK DLLP. All it knows is that the packet is a DLLP.
Solution:
The Transmitter discards the DLLP. The receiver discards all subsequently received TLPs and awaits the replay. Given that the NAK was rejected by the transmitter, it's REPLAY_TIMER expires and triggers the replay.
Problem:
Due to an error in the receiver, it is unable to schedule an ACK or NAK DLLP for a received TLP.
Solution:
The transmitter REPLAY_TIMER will expire and result in TLP replay.
