The integrity guard. It adds a Sequence Number and CRC (LCRC) to every TLP, creating a . It uses a retry mechanism for lost packets.

| Space | Purpose | Example | | :--- | :--- | :--- | | | Normal data transfer | DMA from SSD to RAM | | I/O | Legacy (deprecated in newer systems) | Old serial ports | | Configuration | Device discovery & setup | lspci on Linux | | Message | Interrupts (MSI/MSI-X) & power events | Signaling an interrupt without a dedicated pin |

The specification defines and Traffic Classes (TCs) . This allows system designers to prioritize certain types of data. For example, a video streaming packet can be given higher priority (isochronous transfer) than a file download packet, preventing jitter and lag.