DLLPs (Data Link Layer Packets) are tiny, 8-byte control packets that:

• Travel only between two directly connected devices on a PCIe Link.
• Are never routed through Switches, nor passed up to the Transaction Layer.
• Are used for link management functions, including:
  • Flow Control Updates (credits)
  • Acknowledgements / NAKs
  • Power Management Messages

Because they’re so small, they introduce minimal overhead while keeping the link reliable and synchronized.

🧠 DLLP Life Cycle

1. DLLP Assembly (Transmit Side)

• Starts at the Data Link Layer of the transmitter.
• DLLP Core (the 8-byte payload) is created (e.g., flow control update).
• A 16-bit CRC is appended for error detection.
• The Physical Layer then:
  • Adds Start-of-Frame (STP) and End-of-Frame (END) characters (Gen1/Gen2)
  • Encodes the bitstream (8b/10b for Gen1/Gen2, 128b/130b for Gen3+)
  • Sends it across all lanes using differential signaling.

2. DLLP Disassembly (Receive Side)

• The Physical Layer:

  • Decodes the bitstream
  • Removes STP/END framing characters

• The resulting DLLP is forwarded to the Data Link Layer, which:

  • Checks the CRC for errors
  • Consumes the packet — it does not go up to the Transaction Layer
  • Takes an action based on the DLLP type (e.g., update credit counters)

🎯 Key Points

• Transparent to Software: The Transaction Layer and software do not see DLLPs — everything is automatic.
• Always 8 Bytes: Small size = minimal bandwidth cost.
• Stay Local: Only exist between two directly connected devices.
• Essential for Reliability: DLLPs keep the link flow-controlled and error-free.