Congestion Control Parameter Settings

Intel® Ethernet 800 Series Linux Flow Control

Configuration Guide for RDMA Use Cases

ID	Date	Version	Classification
635330	11/13/2025		Public

The Intel® Ethernet 800 Series supports multiple congestion control algorithms for RDMA transports. These algorithms are configurable via configfs, and additional tuning is available through module parameters when using out-of-tree irdma drivers.

Algorithm	Transport	Description
DCQCN	RoCEv2	Combines ECN and PFC to provide scalable, lossless congestion control.
DCTCP	RoCEv2, iWARP	Uses ECN feedback to adjust transmission rate with fine granularity.
TIMELY	RoCEv2, iWARP	Latency-based algorithm that adjusts rate based on RTT measurements.
TCP New Reno plus ECN	iWARP	Traditional TCP congestion control enhanced with ECN support.

Note: Always consult the README_irdma.txt for the latest information on congestion control support.

To access congestion control settings:

Navigate to the configfs directory after loading the RDMA driver: cd /sys/kernel/config/irdma

Create a new directory for each RDMA device you want to configure. Note: Use ibv_devices for a list of RDMA devices.
For example, to create configfs entries for the rdmap<interface> device:
mkdir rdmap<interface>

List available congestion control attributes: cd rdmap<interface> for f in *; do echo -n "$f: "; cat "$f"; done;
iWARP mode:
- iw_dctcp_enable: Enables DCTCP congestion control.
- iw_ecn_enable: Enables ECN marking for TCP New Reno.
- iw_timely_enable: Enables latency-based TIMELY algorithm.
RoCEv2 mode:
- roce_dcqcn_enable: Enables DCQCN congestion control.
- roce_dctcp_enable: Enables DCTCP congestion control.
- roce_timely_enable: Enables latency-based TIMELY algorithm.

Enable or disable the desired algorithms.
1. To enable an algorithm: echo 1 > <attribute>
  For example, to add ECN marker processing to the default TCP New Reno iWARP congestion control algorithm:
  echo 1 > /sys/kernel/config/irdma/rdmap<interface>/iw_ecn_enable
2. To disable an algorithm: echo 0 > <attribute>
  For example: echo 0 > /sys/kernel/config/irdma/rdmap<interface>/iw_ecn_enable
3. To read the current status: cat <attribute>
  Default values:
  - iwarp_dctcp_en: off
  - iwarp_timely_en: off
  - iwarp_ecn_en: ON
  - roce_timely_en: off
  - roce_dctcp_en: off
  - roce_dcqcn_en: off

Remove the configfs directory created above: rmdir /sys/kernel/config/irdma/rdmap<interface> Note:The driver will not unload until you remove the configfs directory.

Advanced Congestion Control Knobs

The following module parameters are available for RoCEv2 DCQCN tuning on Intel® Ethernet 800 Series adapters.

These parameters are only supported when using out-of-tree irdma drivers. They are not available with inbox drivers.

Additionally, ensure that roce_ena is set to true for these parameters to take effect.

Parameter	Description
dcqcn_enable	Enables the DCQCN algorithm for RoCEv2. Note: roce_ena must also be set to True.
dcqcn_cc_cfg_valid	Indicates that all DCQCN parameters are valid and should be updated in the registers or QP context.
dcqcn_min_dec_factor	Minimum percentage (1-100) by which the transmit rate can be reduced upon receiving a CNP.
dcqcn_min_rate	Minimum transmit rate in Mbps.
dcqcn_F	Number of times to stay in each stage of bandwidth recovery.
dcqcn_T	Microseconds to wait before increasing CWND in DCQCN mode.
dcqcn_B	Number of bytes to transmit before updating CWND in DCQCN mode.
dcqcn_rai_factor	Number of MSS to add to CWND in additive increase mode.
dcqcn_hai_factor	Number of MSS to add to CWND in hyperactive increase mode.
dcqcn_rreduce_mperiod	Minimum time between two consecutive rate reductions for a flow, triggered only if a CNP is received.