The primary responsibility of the PCI Express Data Link Layer is to assure that integrity is maintained when TLPs move between two devices. It also has link initialization and power management responsibilities, including tracking of the link state and passing messages and status between the Transaction Layer above and the Physical Layer below.
In performing its role, the Data Link Layer exchanges traffic with its neighbor using Data Link Layer Packets (DLLPs). DLLPs originate and terminate at the Data Link Layer of each device, without involvement of the Transaction Layer. DLLPs and TLPs are interleaved on the link. Figure 4-12 on page 198 depicts the transmission of a DLLP from one device to another.