25G Ethernet Intel® Stratix® 10 FPGA IP User Guide

ID 683154
Date 9/15/2021
Public
Document Table of Contents

4.1.8. Flow Control

Flow control reduces congestion at the local or remote link partner. When either link partner experiences congestion, the respective transmit control sends pause frames. XOFF Pause frames stop the remote transmitter. XON Pause frames let the remote transmitter resume data transmission. The 25G Ethernet Intel FPGA IP core supports both standard and Priority-based Flow Control (PFC) control frames.
Figure 21. Flow Control Module Conceptual OverviewThe flow control module acts as a buffer between client logic and the TX and RX MAC.

Standard Flow Control (Pause Frame Flow Control):
  • Inhibits the next client frame transmission on the reception of a valid Pause frame.
Priority-based Flow Control (PFC):
  • PFC frame transmission follows a priority-based arbitration scheme, where the Frame Type indication is provided for the usage of external downstream logic.
  • Inhibits the per queue client frame transmission on the reception of a valid PFC frame from the client. Includes per-queue PFC Pause quanta duration indicator
Flow Control includes the following features:
Feature Standard Flow Control Priority-based Flow Control (PFC)
Generation and Transmission
Programmable 1-bit or 2-bit XON/XOFF request mode Supported Supported
In 2-bit request mode, programmable selection of register or signal-based control Supported Supported
Programmable destination and source addresses Supported Supported
Programmable pause quanta Supported Supported
Programmable per-queue XOFF frame separation Supported
Reception and Decode
Programmable destination address for filtering incoming pause and PFC frames Supported Supported
Configurable enable, directing the IP core to ignore incoming flow control frames Supported Supported
Per-queue client frame transmission pause duration indicator Supported
CAUTION:
The 25G Ethernet Intel FPGA IP core supports the flow control feature for either value of the Ready Latency parameter. However, in standard flow control you might experience data delay if you select the value of 3 for this parameter. The IP core might still hold user data packet in its internal buffer if transmission of the IP core stops due to flow control. This issue does not occur in priority-based flow control.

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