Refer to Figure 5-3 on page 212 and the following subsections for a summary of the elements of the Data Link Layer.
Unless blocked by the Data Link Layer, the Transaction Layer passes down the Header, Data, and Digest information for each TLP to be sent.
Each TLP is assigned a 12-bit Sequence Number using current NEXT_TRANSMIT_SEQ count.
A check is made to see if the acceptance of new TLPs from the Transaction Layer should be blocked. The transmitter performs a modulo 4096 subtraction of the ACKD_SEQ count from the NEXT_TRANSMIT_SEQ count to see if the result is >= 2048d. If it is, further TLPs are blocked until incoming ACK/NAK DLLPs render the equation untrue.
The NEXT_TRANSMIT_SEQ ...