Low Latency 100-Gbps Ethernet IP Core User Guide

ID 683160
Date 4/15/2021
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3.2.9. Congestion and Flow Control Using Pause Frames

If you turn on flow control by setting Flow control mode to the value of Standard flow control or Priority-based flow control in the LL 100GbE parameter editor, the LL 100GbE IP core provides flow control to reduce congestion at the local or remote link partner. When either link partner experiences congestion, the respective transmit control sends pause frames. The pause frame instructs the remote transmitter to stop sending data for the duration that the congested receiver specified in an incoming XOFF frame. In the case of priority-based flow control, the pause frame applies to only the indicated priority class.

When the IP core receives the XOFF pause control frame, if the following conditions all hold, the IP core behavior depends on the flow control scheme.

  • In priority-based flow control, the IP core informs the TX client of the received XOFF frame. The IP core continues to process packets it receives on the TX client interface. The IP core tracks the pause quanta countdown internally and informs the TX client when the pause period is ended.
    Note: In the priority-based flow control scheme, the client is responsible to throttle the flow of data to the TX client interface after the IP core informs the client of the received XOFF frame.
  • In standard flow control, the IP core TX MAC does not process TX packets and in fact, prevents the client from sending additional data to the TX client interface, for the duration of the pause quanta of the incoming pause frame.

The IP core responds to an incoming XOFF pause control frame if the following conditions all hold:

  • The relevant cfg_enable bit of the RX_PAUSE_ENABLE register has the value of 1.
  • In the case of standard flow control, bit [0] of the TX_XOF_EN register also has the value of 1.
  • Address matching is positive.

The pause quanta can be configured in the pause quanta register of the device sending XOFF frames. If the pause frame is received in the middle of a frame transmission, the transmitter finishes sending the current frame and then suspends transmission for a period specified by the pause quanta. Data transmission resumes when a pause frame with quanta of zero is received or when the timer has expired. The pause quanta received overrides any counter currently stored. When more than one pause quanta is sent, the value of the pause is set to the last quanta received.

XOFF pause frames stop the remote transmitter. XON pause frames let the remote transmitter resume data transmission.

One pause quanta fraction is equivalent to 512 bit times. The duration of a pause quantum is a function of the client interface datapath width ( 512 bits) and the system clock (clk_txmac) frequency ( 390.625 MHz).

Figure 24. The XOFF and XON Pause Frames for Standard Flow Control
XOFF Frame XON Frame
START[7:0] START[7:0]
PREAMBLE[47:0] PREAMBLE[47:0]
SFD[7:0] SFD[7:0]
DESTINATION ADDRESS[47:0] = 0x010000C28001 1 DESTINATION ADDRESS[47:0] = 0x010000C28001
SOURCE ADDRESS[47:0] SOURCE ADDRESS[47:0]
TYPE[15:0] = 0x8808 TYPE[15:0] = 0x8808
OPCODE[15:0] = 0x001 (standard FC) OPCODE[15:0] = 0x001 (standard FC)
PAUSE QUANTA[15:0] = 0xP1, 0xP2 2 PAUSE QUANTA[15:0] = 0x0000
PAD[335:0] PAD[335:0]
CRC[31:0] CRC[31:0]
Figure 25. The XOFF and XON Pause Frames for Priority Flow Control
XOFF Frame XON Frame
START[7:0] START[7:0]
PREAMBLE[47:0] PREAMBLE[47:0]
SFD[7:0] SFD[7:0]
DESTINATION ADDRESS[47:0] = 0x010000C28001 1 DESTINATION ADDRESS[47:0] = 0x010000C28001
SOURCE ADDRESS[47:0] SOURCE ADDRESS[47:0]
TYPE[15:0] = 0x8808 TYPE[15:0] = 0x8808
OPCODE[15:0] = 0x0101 (PFC) OPCODE[15:0] = 0x0101 (PFC)
PRIORITY_ENABLE[15:0] 3 PRIORITY_ENABLE[15:0] 4
TIME0[15:0] 5 TIME0[15:0] = 0x0000
... ...
TIME7[15:0] 5 TIME7[15:0] = 0x0000
PAD[207:0] PAD[207:0]
CRC[31:0] CRC[31:0]
1 This is a multicast destination address.
2 The bytes P1 and P2 are filled with the value configured in the TX_PAUSE_QUANTA register.
3 Bit [n] has the value of 1 if the TIMEn field is valid.
4 Bit [n] has the value of 1 if the XON request applies to priority queue n.
5 The TIMEn field is filled with the value available in the TX_PAUSE_QUANTAregister when the TX_PAUSE_QNUMBER register holds the value of n.

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