---
name: "ng_car"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/ng_car"
---

# ng\_car(4)

Committed Access Rate netgraph node

## NAME

**ng\_car** – Committed Access Rate netgraph node

## SYNOPSIS

**#include \<netgraph/ng\_car.h\>**

## DESCRIPTION

The **car** node type limits traffic flowing through it using:

- Single rate three color marker as described in RFC 2697,

- Two rate three color marker as described in RFC 2698,

- RED-like rate limit algorithm used by Cisco,

- Traffic shaping with RED.

## HOOKS

This node type supports the following hooks:

- *upper*

  Hook leading to upper layer protocols.

- *lower*

  Hook leading to lower layer protocols.

Traffic flowing from *upper* to *lower* is considered **downstream** traffic. Traffic flowing from *lower* to *upper* is considered **upstream** traffic.

## MODES OF OPERATION

Each hook can operate in one of the following modes:

- NG\_CAR\_SINGLE\_RATE

  Single rate three color marker as described in RFC 2697. Committed burst packets are counted as green, extended burst packets are counted as yellow and exceeding packets are counted as red. Committed burst getting refilled with CIR (Committed Information Rate) speed. When it is full, exceeded burst getting refilled.

- NG\_CAR\_DOUBLE\_RATE

  Two rate three color marker as described in RFC 2698. Committed burst packets are counted as green, peak burst packets are counted as yellow and exceeding packets are counted as red. Committed burst getting refilled with CIR speed. Peak burst getting refilled with PIR (Peak Information Rate) speed at the same time.

- NG\_CAR\_RED

  Similar to NG\_CAR\_SINGLE\_RATE, but with different understanding of extended burst. When normal burst exceeded and extended burst is used, packets are counted red with probability equal to part of extended burst consumed. Extended burst getting refilled first. When it is full, committed burst getting refilled. This behavior is similar to RED active queue management algorithm.

  This algorithm is more polite to the TCP traffic than NG\_CAR\_SINGLE\_RATE.

- NG\_CAR\_SHAPE

  Committed burst packets are counted as green, exceeding packets are delayed by queue with RED management and counted as yellow. Packets dropped by queue counted as red. Queue parameters are hardcoded: length 99 packets, min\_th 8 packets, max\_p 100%.

  Traffic shaping is much more polite to the TCP traffic than rate limit on links with bandwidth \* delay product less than 6-8 TCP segments, but it consumes additional system resources for queue processing.

By default, all information rates are measured in bits per second and bursts are measured in bytes. But when NG\_CAR\_COUNT\_PACKETS option is enabled, rates are measured in packets per second and bursts are in packets.

## CONTROL MESSAGES

This node type supports the generic control messages and the following specific messages.

- NGM\_CAR\_SET\_CONF (**setconf**)

  Set node configuration to the specified at *struct ng\_car\_bulkconf*

- NGM\_CAR\_GET\_CONF (**getconf**)

  Return current node configuration as *struct ng\_car\_bulkconf*

  ```
  struct ng_car_hookconf {
  	uint64_t cbs;		/* Committed burst size (bytes) */
  	uint64_t ebs;		/* Exceeded/Peak burst size (bytes) */
  	uint64_t cir;		/* Committed information rate (bits/s) */
  	uint64_t pir;		/* Peak information rate (bits/s) */
  	uint8_t  green_action;	/* Action for green packets */
  	uint8_t  yellow_action;	/* Action for yellow packets */
  	uint8_t  red_action;	/* Action for red packets */
  	uint8_t  mode;		/* single/double rate, ... */
  	uint8_t  opt;		/* color-aware or color-blind */
  };
  /* possible actions (..._action) */
  enum {
      NG_CAR_ACTION_FORWARD = 1,
      NG_CAR_ACTION_DROP,
      NG_CAR_ACTION_MARK
  };
  /* operation modes (mode) */
  enum {
      NG_CAR_SINGLE_RATE = 0,
      NG_CAR_DOUBLE_RATE,
      NG_CAR_RED,
      NG_CAR_SHAPE
  };
  /* mode options (bits for opt) */
  #define NG_CAR_COLOR_AWARE	1
  #define NG_CAR_COUNT_PACKETS	2
  struct ng_car_bulkconf {
  	struct ng_car_hookconf upstream;
  	struct ng_car_hookconf downstream;
  };
  ```

- NGM\_CAR\_GET\_STATS (**getstats**)

  Return node statistics as *struct ng\_car\_bulkstats*

  ```
  struct ng_car_hookstats {
  	uint64_t passed_pkts;	/* Counter for passed packets */
  	uint64_t dropped_pkts;	/* Counter for dropped packets */
  	uint64_t green_pkts;	/* Counter for green packets */
  	uint64_t yellow_pkts;	/* Counter for yellow packets */
  	uint64_t red_pkts;	/* Counter for red packets */
  	uint64_t errors;	/* Counter for operation errors */
  };
  struct ng_car_bulkstats {
  	struct ng_car_hookstats upstream;
  	struct ng_car_hookstats downstream;
  };
  ```

- NGM\_CAR\_CLR\_STATS (**clrstats**)

  Clear node statistics.

- NGM\_CAR\_GETCLR\_STATS (**getclrstats**)

  Atomically return and clear node statistics.

## SHUTDOWN

This node shuts down upon receipt of a NGM\_SHUTDOWN control message, or when all hooks have been disconnected.

## EXAMPLES

Limit outgoing data rate over fxp0 Ethernet interface to 20Mbit/s and incoming packet rate to 5000pps.

```
/usr/sbin/ngctl -f- <<-SEQ
	mkpeer fxp0: car lower lower
	name fxp0:lower fxp0_car
	connect fxp0: fxp0_car: upper upper
	msg fxp0_car: setconf { downstream={ cir=20000000 cbs=2500000 ebs=2500000 greenAction=1 yellowAction=1 redAction=2 mode=2 } upstream={ cir=5000 cbs=100 ebs=100 greenAction=1 yellowAction=1 redAction=2 mode=2 opt=2 } }
SEQ
```

## SEE ALSO

[netgraph(4)](/netgraph/4.md), [ngctl(8)](/ngctl/8.md) J. Heinanen, *A Single Rate Three Color Marker*, RFC 2697. J. Heinanen, *A Two Rate Three Color Marker*, RFC 2698.

## AUTHORS

Nuno Antunes ⟨*nuno.antunes@gmail.com*⟩ Alexander Motin ⟨*mav@FreeBSD.org*⟩

## BUGS

At this moment only DROP and FORWARD actions are implemented.