man > tc-fq(8)

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 pacing.  FQ does flow separation, and is able to
       respect  pacing requirements 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_PACING_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
       directly sets the appropriate Departure Time for each skb.

       Dequeueing  happens in a round-robin fashion.  A special FIFO queue is reserved for high pri‐
       ority 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 window 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  allowed  to  dequeue  at
       once. A larger value means a longer time period before 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 using 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 assigned  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 algorithms 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)