Ubuntu Manpage: CoDel - Fair Queuing (FQ) with Controlled Delay (CoDel)

Provided by: iproute2_6.10.0-2ubuntu1_amd64

NAME

       CoDel - Fair Queuing (FQ) with Controlled Delay (CoDel)

SYNOPSIS

       tc qdisc ... fq_codel [ limit PACKETS ] [ flows NUMBER ] [ target TIME ] [ interval TIME ]
       [ quantum BYTES ] [ ecn | noecn ] [ ce_threshold TIME ] [ ce_threshold_selector VALUE/MASK
       ] [ memory_limit BYTES ]

DESCRIPTION

       FQ_Codel  (Fair Queuing Controlled Delay) is queuing discipline that combines Fair Queuing
       with the CoDel AQM scheme. FQ_Codel uses a stochastic model to classify  incoming  packets
       into different flows and is used to provide a fair share of the bandwidth to all the flows
       using the queue. Each such flow is managed by the  CoDel  queuing  discipline.  Reordering
       within a flow is avoided since Codel internally uses a FIFO queue.

PARAMETERS

limit
has the same semantics as codel and is the hard limit on the real queue size. When this
limit is reached, incoming packets are dropped. Default is 10240 packets.

memory_limit
sets a limit on the total number of bytes that can be queued in this FQ-CoDel instance.
The lower of the packet limit of the limit parameter and the memory limit will be
enforced. Default is 32 MB.

flows
is the number of flows into which the incoming packets are classified. Due to the
stochastic nature of hashing, multiple flows may end up being hashed into the same slot.
Newer flows have priority over older ones. This parameter can be set only at load time
since memory has to be allocated for the hash table. Default value is 1024.

target
has the same semantics as codel and is the acceptable minimum standing/persistent queue
delay. This minimum delay is identified by tracking the local minimum queue delay that
packets experience. Default value is 5ms.

interval
has the same semantics as codel and is used to ensure that the measured minimum delay does
not become too stale. The minimum delay must be experienced in the last epoch of length
interval. It should be set on the order of the worst-case RTT through the bottleneck to
give endpoints sufficient time to react. Default value is 100ms.

quantum
is the number of bytes used as 'deficit' in the fair queuing algorithm. Default is set to
1514 bytes which corresponds to the Ethernet MTU plus the hardware header length of 14
bytes.

ecn | noecn
has the same semantics as codel and can be used to mark packets instead of dropping them.
If ecn has been enabled, noecn can be used to turn it off and vice-a-versa. Unlike codel,
ecn is turned on by default.

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.

ce_threshold_selector
sets a filter so that the ce_threshold feature is applied to only a subset of the traffic
seen by the qdisc. If set, the MASK value will be applied as a bitwise AND to the
diffserv/ECN byte of the IP header, and only if the result of this masking equals VALUE,
will the ce_threshold logic be applied to the packet.

drop_batch
sets the maximum number of packets to drop when limit or memory_limit is exceeded. Default
value is 64.

EXAMPLES

       #tc qdisc add   dev eth0 root fq_codel
       #tc -s qdisc show
       qdisc fq_codel 8002: dev eth0 root refcnt 2 limit 10240p flows 1024 quantum 1514
        target 5.0ms interval 100.0ms ecn
          Sent 428514 bytes 2269 pkt (dropped 0, overlimits 0 requeues 0)
          backlog 0b 0p requeues 0
           maxpacket 256 drop_overlimit 0 new_flow_count 0 ecn_mark 0
           new_flows_len 0 old_flows_len 0

       #tc qdisc add dev eth0 root fq_codel limit 2000 target 3ms interval 40ms noecn
       #tc -s qdisc show
       qdisc fq_codel 8003: dev eth0 root refcnt 2 limit 2000p flows  1024  quantum  1514  target
       3.0ms interval 40.0ms
        Sent 2588985006 bytes 1783629 pkt (dropped 0, overlimits 0 requeues 34869)
        backlog 0b 0p requeues 34869
         maxpacket 65226 drop_overlimit 0 new_flow_count 73 ecn_mark 0
         new_flows_len 1 old_flows_len 3

AUTHORS