tc-fq

FQ(8)                                Linux                               FQ(8)

NAME
       FQ - Fair Queue traffic policing

SYNOPSIS
       tc  qdisc  ...  fq  [  limit PACKETS ] [ flow_limit PACKETS ] [ quantum
       BYTES ] [ initial_quantum BYTES ] [ maxrate RATE ] [ buckets NUMBER ] [
       orphan_mask NUMBER ] [ pacing | nopacing ] [ ce_threshold TIME ]

DESCRIPTION
       FQ (Fair Queue) is a classless packet scheduler meant to be mostly used
       for locally generated traffic.  It is designed to achieve per flow pac-
       ing.   FQ  does flow separation, and is able to respect pacing require-
       ments set by TCP stack.  All packets belonging to a socket are  consid-
       ered as a 'flow'.  For non local packets (router workload), packet hash
       is used as fallback.

       An application can specify a maximum pacing rate using the  SO_MAX_PAC-
       ING_RATE  setsockopt  call.   This  packet scheduler adds delay between
       packets to respect rate limitation set on each socket. Note that  after
       linux-4.20,  linux  adopted  EDT  (Earliest Departure Time) and TCP di-
       rectly sets the appropriate Departure Time for each skb.

       Dequeueing happens in a round-robin fashion.  A special FIFO  queue  is
       reserved  for  high  priority packets ( TC_PRIO_CONTROL priority), such
       packets are always dequeued first.

       FQ is non-work-conserving.

       TCP pacing is good for flows having idle times, as the congestion  win-
       dow  permits  TCP  stack  to  queue a possibly large number of packets.
       This removes the 'slow start after idle' choice,  badly  hitting  large
       BDP  flows  and  applications  delivering  chunks of data such as video
       streams.

PARAMETERS
   limit
       Hard limit on the real queue size. When  this  limit  is  reached,  new
       packets  are  dropped.  If the value is lowered, packets are dropped so
       that the new limit is met. Default is 10000 packets.

   flow_limit
       Hard limit on the maximum number of packets queued per  flow.   Default
       value is 100.

   quantum
       The credit per dequeue RR round, i.e. the amount of bytes a flow is al-
       lowed to dequeue at once. A larger value means a longer time period be-
       fore the next flow will be served.  Default is 2 * interface MTU bytes.

   initial_quantum
       The initial sending rate credit, i.e. the amount of bytes a new flow is
       allowed to dequeue initially.  This is specifically meant to allow  us-
       ing  IW10  without  added  delay.   Default is 10 * interface MTU, i.e.
       15140 for 'standard' ethernet.

   maxrate
       Maximum sending rate of a flow.   Default  is  unlimited.   Application
       specific setting via SO_MAX_PACING_RATE is ignored only if it is larger
       than this value.

   buckets
       The size of the hash table used for flow lookups. Each  bucket  is  as-
       signed  a  red-black  tree  for  efficient collision sorting.  Default:
       1024.

   orphan_mask
       For packets not owned by a socket,  fq  is  able  to  mask  a  part  of
       skb->hash  and  reduce  number  of buckets associated with the traffic.
       This is a DDOS prevention mechanism, and the default is  1023  (meaning
       no more than 1024 flows are allocated for these packets)

   [no]pacing
       Enable or disable flow pacing. Default is enabled.

   ce_threshold
       sets a threshold above which all packets are marked with ECN Congestion
       Experienced. This is useful for DCTCP-style  congestion  control  algo-
       rithms that require marking at very shallow queueing thresholds.

EXAMPLES
       #tc qdisc add dev eth0 root fq ce_threshold 4ms
       #tc -s -d qdisc show dev eth0
       qdisc  fq  8001:  dev  eth0  root refcnt 2 limit 10000p flow_limit 100p
       buckets 1024 orphan_mask  1023  quantum  3028b  initial_quantum  15140b
       low_rate_threshold 550Kbit refill_delay 40.0ms ce_threshold 4.0ms
        Sent 72149092 bytes 48062 pkt (dropped 2176, overlimits 0 requeues 0)
        backlog 1937920b 1280p requeues 0
         flows 34 (inactive 17 throttled 0)
         gc 0 highprio 0 throttled 0 ce_mark 47622 flows_plimit 2176

SEE ALSO
       tc(8), socket(7)

AUTHORS
       FQ was written by Eric Dumazet.

iproute2                         10 Sept 2015                            FQ(8)