AQM working group                                 T. Hoeiland-Joergensen
Internet-Draft                                       Karlstad University
Intended status: Informational                               P. McKenney
Expires: May 14, 2015                        IBM Linux Technology Center
                                                                 D. Taht
                                                               J. Gettys
                                                              E. Dumazet
                                                            Google, Inc.
                                                       November 10, 2014



   This memo presents the FQ-CoDel hybrid packet scheduler/AQM
   algorithm, a critical tool for fighting bufferbloat and reducing
   latency across the Internet.

   FQ-CoDel mixes packets from multiple flows and reduces the impact of
   head of line blocking from bursty traffic.  It provides isolation for
   low-rate traffic such as DNS, web, and videoconferencing traffic.  It
   improves utilisation across the networking fabric, especially for
   bidirectional traffic, by keeping queue lengths short; and it can be
   implemented in a memory- and CPU-efficient fashion across a wide
   range of hardware.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 14, 2015.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The FQ-CoDel algorithm is a combined packet scheduler and AQM
   developed as part of the bufferbloat-fighting community effort.  It
   is based on a modified Deficit Round Robin (DRR) queue scheduler,
   with the CoDel AQM algorithm operating on each sub-queue.  This
   document describes the combined algorithm; reference implementations
   are available for ns2 and ns3 and it is included in the mainline
   Linux kernel as the FQ-CoDel queueing discipline.

   The rest of this document is structured as follows: This section
   gives some concepts and terminology used in the rest of the document,
   and gives a short informal summary of the FQ-CoDel algorithm.
   Section 2 gives an overview of the CoDel algorithm.  Section 3 covers
   the DRR portion.  Section 4 defines the parameters and data
   structures employed by FQ-CoDel.  Section 5 describes the working of
   the algorithm in detail.  Section 6 describes implementation
   considerations, and section 7 lists some of the limitations of using
   flow queueing.  Finally section 11 concludes.

1.1.  Terminology and concepts

   Flow: A flow is typically identified by a 5-tuple of source IP,
   destination IP, source port, destination port, and protocol.  It can
   also be identified by a superset or subset of those parameters, or by
   mac address, or other means.

   Queue: A queue of packets represented internally in FQ-CoDel.  In
   most instances each flow gets its own queue; however because of the
   possibility of hash collisions, this is not always the case.  In an
   attempt to avoid confusion, the word 'queue' is used to refer to the
   internal data structure, and 'flow' to refer to the actual stream of
   packets being delivered to the FQ-CoDel algorithm.

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   Scheduler: A mechanism to select which queue a packet is dequeued

   CoDel AQM: The Active Queue Management algorithm employed by FQ-

   DRR: Deficit round-robin scheduling.

   Quantum: The maximum amount of bytes to be dequeued from a queue at

1.2.  Informal summary of FQ-CoDel

   FQ-CoDel is a _hybrid_ of DRR [DRR] and CODEL [CODEL2012], with an
   optimisation for sparse flows similar to SQF [SQF2012] and DRR++
   [DRRPP].  We call this "Flow Queueing" rather than "Fair Queueing" as
   flows that build a queue are treated differently than flows that do

   FQ-CoDel stochastically classifies incoming packets into different
   sub-queues by hashing the 5-tuple of IP protocol number and source
   and destination IP and port numbers, perturbed with a random number
   selected at initiation time (although other flow classification
   schemes can optionally be configured instead).  Each queue is managed
   by the CoDel queueing discipline.  Packet ordering within a queue is
   preserved, since queues have FIFO ordering.

   The FQ-CoDel algorithm consists of two logical parts: the scheduler
   which selects which queue to dequeue a packet from, and the CoDel AQM
   which works on each of the queues.  The subtleties of FQ-CoDel are
   mostly in the scheduling part, whereas the interaction between the
   scheduler and the CoDel algorithm are fairly straight forward:

   At initialisation, each queue is set up to have a separate set of
   CoDel state variables.  By default, 1024 queues are created.  The
   current implementation supports anywhere from one to 64K separate
   queues, and each queue maintains the state variables throughout its
   lifetime, and so acts the same as the non-FQ CoDel variant would.
   This means that with only one queue, FQ-CoDel behaves essentially the
   same as CoDel by itself.

   On dequeue, FQ-CoDel selects a queue from which to dequeue by a two-
   tier round-robin scheme, in which each queue is allowed to dequeue up
   to a configurable quantum of bytes for each iteration.  Deviations
   from this quantum is maintained as a deficit for the queue, which
   serves to make the fairness scheme byte-based rather than a packet-
   based.  The two-tier round-robin mechanism distinguishes between
   "new" queues (which don't build up a standing queue) and "old"

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   queues, that have queued enough data to be around for more than one
   iteration of the round-robin scheduler.

   This new/old queue distinction has a particular consequence for
   queues that don't build up more than a quantum of bytes before being
   visited by the scheduler: Such queues are removed from the list, and
   then re-added as a new queue each time a packet arrives for it, and
   so will get priority over queues that do not empty out each round
   (except for a minor modification to protect against starvation,
   detailed below).  Exactly how much data a flow has to send to keep
   its queue in this state is somewhat difficult to reason about,
   because it depends on both the egress link speed and the number of
   concurrent flows.  However, in practice many things that are
   beneficial to have prioritised for typical internet use (ACKs, DNS
   lookups, interactive SSH, HTTP requests, ARP, ICMP, VoIP) _tend_ to
   fall in this category, which is why FQ-CoDel performs so well for
   many practical applications.  However, the implicitness of the
   prioritisation means that for applications that require guaranteed
   priority (for instance multiplexing the network control plane over
   the network itself), explicit classification is still needed.

   This scheduling scheme has some subtlety to it, which is explained in
   detail in the remainder of this document.

2.  CoDel

   CoDel is described in the the ACM Queue paper, CODEL [CODEL2012], and
   Van Jacobson's IETF84 presentation CODELDRAFT [CODELDRAFT].  The
   basic idea is to control queue length, maintaining sufficient
   queueing to keep the outgoing link busy, but avoiding building up the
   queue beyond that point.  This is done by preferentially dropping
   packets that remain in the queue for "too long".

   When each new packet arrives, its arrival time is stored with it.
   Later, when it is that packet's turn to be dequeued, CoDel computes
   its sojourn time (the current time minus the arrival time).  If the
   sojourn time for packets being dequeued exceeds the _target_ time for
   a time period of at least the (current value of) _interval_, one or
   more packets will be dropped (or marked, if ECN is enabled) in order
   to signal the source endpoint to reduce its send rate.  If the
   sojourn still remains above the target time, the value of interval be
   lowered, and additional packet drops will occur on a schedule
   computed from an inverse-square-root control law until either (1) the
   queue becomes empty or (2) a packet is encountered with a sojourn
   time that is less than the target time.  Upon exiting the dropping
   mode, CoDel caches the last calculated interval (applying varying
   amounts of hysteresis to it), to be used as the starting point on
   subsequent re-entries into dropping mode.

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   The _target_ time is normally set to about five milliseconds, and the
   initial _interval_ is normally set to about 100 milliseconds.  This
   approach has proven to be quite effective in a wide variety of

   CoDel drops packets at the head of a queue, rather than at the tail.

3.  Flow Queueing

   FQ-CoDel's DRR scheduler is byte-based, employing a deficit round-
   robin mechanism between queues.  This works by keeping track of the
   current byte _deficit_ of each queue.  This deficit is initialised to
   the configurable quantum; each time a queue gets a dequeue
   opportunity, it gets to dequeue packets, decreasing the deficit by
   the packet size for each packet, until the deficit runs into the
   negative, at which point it is increased by one quantum, and the
   dequeue opportunity ends.

   This means that if one queue contains packets of size quantum/3, and
   another contains quantum-sized packets, the first queue will dequeue
   three packets each time it gets a turn, whereas the second only
   dequeues one.  This means that flows that send small packets are not
   penalised by the difference in packet sizes; rather, the DRR scheme
   approximates a (single-)byte-based fairness queueing.  The size of
   the quantum determines the scheduling granularity, with the tradeoff
   from too small a quantum being scheduling overhead.  For small
   bandwidths, lowering the quantum from the default MTU size can be

   Unlike DRR there are two sets of flows - a "new" list for flows that
   have not built a queue recently, and an "old" list for flow-building

4.  FQ-CoDel Parameters and Data Structures

   This section goes into the parameters and data structures in FQ-

4.1.  Parameters

4.1.1.  Interval

   The _interval_ parameter 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 end-points sufficient time to react.

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   The default interval value is 100 ms.

4.1.2.  Target

   The _target_ parameter has the same semantics as CoDel.  It is the
   acceptable minimum standing/persistent queue delay for each FQ-CoDel
   Queue.  This minimum delay is identified by tracking the local
   minimum queue delay that packets experience.

   The default target value is 5 ms, but this value SHOULD be tuned to
   be at least the transmission time of a single MTU-sized packet at the
   prevalent egress link speed (which for e.g. 1Mbps and MTU 1500 is
   ~15ms).  It should otherwise be set to on the order of 5-10% of the
   configured interval.

4.1.3.  Packet limit

   Routers do not have infinite memory, so some packet limit MUST be

   The _limit_ parameter is the hard limit on the real queue size,
   measured in number of packets.  This limit is a global limit on the
   number of packets in all queues; each individual queue does not have
   an upper limit.  When the limit is reached and a new packet arrives
   for enqueue, a packet is dropped from the head of the largest queue
   (measured in bytes) to make room for the new packet.

   The default packet limit is 10240 packets, which is suitable for up
   to 10GigE speeds.  In practice, the hard limit is rarely, if ever,
   hit, as drops are performed by the CoDel algorithm long before the
   limit is hit.  For platforms that are severely memory constrained, a
   lower limit can be used.

4.1.4.  Quantum

   The _quantum_ parameter is the number of bytes each queue gets to
   dequeue on each round of the scheduling algorithm.  The default is
   set to 1514 bytes which corresponds to the Ethernet MTU plus the
   hardware header length of 14 bytes.

   In TSO-enabled systems, where a "packet" consists of an offloaded
   packet train, it can presently be as large as 64K bytes.  In GRO-
   enabled systems, up to 17 times the TCP max segment size (or 25K

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4.1.5.  Flows

   The _flows_ parameter sets the number of sub-queues 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.

   This parameter can be set only at load time since memory has to be
   allocated for the hash table in the current implementation.

   The default value is 1024.

4.1.6.  ECN

   ECN is _enabled_ by default.  Rather than do anything special with
   misbehaved ECN flows, FQ-CoDel relies on the packet scheduling system
   to minimise their impact, thus unresponsive packets in a flow being
   marked with ECN can grow to the overall packet limit, but will not
   otherwise affect the performance of the system.

   It can be disabled by specifying the _noecn_ parameter.

4.2.  Data structures

4.2.1.  Internal sub-queues

   The main data structure of FQ-CoDel is the array of sub-queues, which
   is instantiated to the number of queues specified by the _flows_
   parameter at instantiation time.  Each sub-queue consists simply of
   an ordered list of packets with FIFO semantics, two state variables
   tracking the queue deficit and total number of bytes enqueued, and
   the set of CoDel state variables.  Other state variables to track
   queue statistics can also be included: for instance, the Linux
   implementation keeps a count of dropped packets.

   Queue space is shared: there's a global limit on the number of
   packets the queues can hold, but not one per queue.

4.2.2.  New and old queues lists

   FQ-CoDel maintains two lists of active queues, called "new" and "old"
   queues.  Each list is an ordered list containing references to the
   array of sub-queues.  When a packet is added to a queue that is not
   currently active, that queue becomes active by being added to the
   list of new queues.  Later on, it is moved to the list of old queues,
   from which it is removed when it is no longer active.  This behaviour
   is the source of some subtlety in the packet scheduling at dequeue
   time, explained below.

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5.  The FQ-CoDel scheduler and AQM interactions

   This section describes the operation of the FQ-CoDel scheduler and
   AQM.  It is split into two parts explaining the enqueue and dequeue

5.1.  Enqueue

   The packet enqueue mechanism consists of three stages: classification
   into a sub-queue, timestamping and bookkeeping, and optionally
   dropping a packet when the total number of enqueued packets goes over
   the maximum.

   When a packet is enqueued, it is first classified into the
   appropriate sub-queue.  By default, this is done by hashing on the
   5-tuple of IP protocol, and source and destination IP and port
   numbers, permuted by a random value selected at initialisation time,
   and taking the hash value modulo the number of sub-queues.  However,
   an implementation MAY also specify a configurable classification
   scheme along a wide variety of other possible parameters such as mac
   address, diffserv, firewall and flow specific markings, etc. (the
   Linux implementation does so in the form of the 'tc filter' command).

   If a custom filter fails, classification failure results in the
   packet being dropped and no further action taken.  By design the
   standard filter cannot fail.

   Additionally, the default hashing algorithm presently deployed does
   decapsulation of some common packet types (6in4, IPIP, GRE 0), mixes
   IPv6 IP addresses thoroughly, and uses Jenkins hash on the result.

   Once the packet has been successfully classified into a sub-queue, it
   is handed over to the CoDel algorithm for timestamping.  It is then
   added to the tail of the selected queue, and the queue's byte count
   is updated by the packet size.  Then, if the queue is not currently
   active (i.e. if it is not in either the list of new or the list of
   old queues), it is added to the end of the list of new queues, and
   its deficit is initiated to the configured quantum.  Otherwise it is
   added to the old queue list.

   Finally, the total number of enqueued packets is compared with the
   configured limit, and if it is _above_ this value (which can happen
   since a packet was just enqueued), a packet is dropped from the head
   of the queue with the largest current byte count.  Note that this in
   most cases means that the packet that gets dropped is different from
   the one that was just enqueued, and may even be from a different

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5.2.  Dequeue

   Most of FQ-CoDel's work is done at packet dequeue time.  It consists
   of three parts: selecting a queue from which to dequeue a packet,
   actually dequeuing it (employing the CoDel algorithm in the process),
   and some final bookkeeping.

   For the first part, the scheduler first looks at the list of new
   queues; for each queue in that list, if that queue has a negative
   deficit (i.e. it has already dequeued at least a quantum of bytes),
   its deficit is increased by one quantum, and the queue is put onto
   the end of the list of old queues, and the routine selects the next
   queue and starts again.

   Otherwise, that queue is selected for dequeue.  If the list of new
   queues is empty, the scheduler proceeds down the list of old queues
   in the same fashion (checking the deficit, and either selecting the
   queue for dequeuing, or increasing the deficit and putting the queue
   back at the end of the list).

   After having selected a queue from which to dequeue a packet, the
   CoDel algorithm is invoked on that queue.  This applies the CoDel
   control law, and may discard one or more packets from the head of
   that queue, before returning the packet that should be dequeued (or
   nothing if the queue is or becomes empty while being handled by the
   CoDel algorithm).

   Finally, if the CoDel algorithm did not return a packet, the queue is
   empty, and the scheduler does one of two things: if the queue
   selected for dequeue came from the list of new queues, it is moved to
   the end of the list of old queues.  If instead it came from the list
   of old queues, that queue is removed from the list, to be added back
   (as a new queue) the next time a packet arrives that hashes to that
   queue.  Then (since no packet was available for dequeue), the whole
   dequeue process is restarted from the beginning.

   If, instead, the scheduler _did_ get a packet back from the CoDel
   algorithm, it updates the byte deficit for the selected queue before
   returning the packet as the result of the dequeue operation.

   The step that moves an empty queue from the list of new queues to the
   list of old queues before it is removed is crucial to prevent
   starvation.  Otherwise the queue could reappear (the next time a
   packet arrives for it) before the list of old queues is visited; this
   can go on indefinitely even with a small number of active flows, if
   the flow providing packets to the queue in question transmits at just
   the right rate.  This is prevented by first moving the queue to the

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   list of old queues, forcing a pass through that, and thus preventing

   The resulting migration of queues between the different states is
   summarised in the following state diagram:

 +-----------------+                +--------------------+
 |                 |     Empty      |                    |
 |     Empty       |<---------------+        Old         +-----+
 |                 |                |                    |     |
 +-------+---------+                +--------------------+     |
         |                             ^              ^        |Quantum
         |Arrival                      |              |        |Exceeded
         v                             |              |        |
 +-----------------+                   |              |        |
 |                 |     Empty or      |              |        |
 |      New        +-------------------+              +--------+
 |                 |  Quantum exceeded

6.  Implementation considerations

6.1.  Probability of hash collisions

   Since the Linux FQ-CoDel implementation by default uses 1024 hash
   buckets, the probability that (say) 100 VoIP sessions will all hash
   to the same bucket is something like ten to the power of minus 300.
   Thus, the probability that at least one of the VoIP sessions will
   hash to some other queue is very high indeed.

   Conversely, the probability that each of the 100 VoIP sessions will
   get its own queue is given by (1023!/(924!*1024^99)) or about 0.007;
   so not all that probable.  The probability rises sharply, however, if
   we are willing to accept a few collisions.  For example, there is
   about an 86% probability that no more than two of the 100 VoIP
   sessions will be involved in any given collision, and about a 99%
   probability that no more than three of the VoIP sessions will be
   involved in any given collision.  These last two results were
   computed using Monte Carlo simulations: Oddly enough, the mathematics
   for VoIP-session collision exactly matches that of hardware cache

6.2.  Memory Overhead

   FQ-CoDel can be implemented with a very low memory footprint (less
   than 64 bytes per queue on 64 bit systems).  These are the data
   structures used in the Linux implementation:

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struct codel_vars {
    u32             count;
    u32             lastcount;
    bool            dropping;
    u16             rec_inv_sqrt;
    codel_time_t    first_above_time;
    codel_time_t    drop_next;
    codel_time_t    ldelay;

struct fq_codel_flow {
    struct sk_buff    *head;
    struct sk_buff    *tail;
    struct list_head  flowchain;
    int               deficit;
    u32               dropped; /* number of drops (or ECN marks) on this flow */
    struct codel_vars cvars;

   The master table managing all queues looks like this:

  struct fq_codel_sched_data {
      struct tcf_proto *filter_list;  /* optional external classifier */
      struct fq_codel_flow *flows;    /* Flows table [flows_cnt] */
      u32             *backlogs;      /* backlog table [flows_cnt] */
      u32             flows_cnt;      /* number of flows */
      u32             perturbation;   /* hash perturbation */
      u32             quantum;        /* psched_mtu(qdisc_dev(sch)); */
      struct codel_params cparams;
      struct codel_stats cstats;
      u32             drop_overlimit;
      u32             new_flow_count;

      struct list_head new_flows;     /* list of new flows */
      struct list_head old_flows;     /* list of old flows */

6.3.  Per-Packet Timestamping

   The CoDel portion of the algorithm requires per-packet timestamps be
   stored along with the packet.  While this approach works well for
   software-based routers, it may be impossible to retrofit devices that
   do most of their processing in silicon and lack space or mechanism
   for timestamping.

   Also, while perfect resolution is not needed, timestamping functions
   in the core OS need to be efficient as they are called at least once
   on each packet enqueue and dequeue.

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6.4.  Other forms of "Fair Queueing"

   Much of the scheduling portion of FQ-CoDel is derived from DRR and is
   substantially similar to DRR++. SFQ-based versions have also been
   produced and tested in ns2.  Other forms of Fair Queueing, such as
   WFQ or QFQ, have not been thoroughly explored.

6.5.  Differences between CoDel and FQ-CoDel behaviour

   CoDel can be applied to a single queue system as a straight AQM,
   where it converges towards an "ideal" drop rate (i.e.  one that
   minimises delay while keeping a high link utilisation), and then
   optimises around that control point.

   The scheduling of FQ-CoDel mixes packets of competing flows, which
   acts to pace bursty flows to better fill the pipe.  Additionally, a
   new flow gets substantial "credit" over other flows until CoDel finds
   an ideal drop rate for it.  However, for a new flow that exceeds the
   configured quantum, more time passes before all of its data is
   delivered (as packets from it, too, are mixed across the other
   existing queue-building flows).  Thus, FQ-CoDel takes longer (as
   measured in time) to converge towards an ideal drop rate for a given
   new flow, but does so within fewer delivered _packets_ from that

   Finally, the flow isolation FQ-CoDel provides means that the CoDel
   drop mechanism operates on the flows actually building queues, which
   results in packets being dropped more accurately from the largest
   flows than CoDel alone manages.  Additionally, flow isolation
   radically improves the transient behaviour of the network when
   traffic or link characteristics change (e.g. when new flows start up
   or the link bandwidth changes); while CoDel itself can take a while
   to respond, fq_codel doesn't miss a beat.

7.  Limitations of flow queueing

   While FQ-CoDel has been shown in many scenarios to offer significant
   performance gains, there are some scenarios where the scheduling
   algorithm in particular is not a good fit.  This section documents
   some of the known cases which either may require tweaking the default
   behaviour, or where alternatives to flow queueing should be

7.1.  Fairness between things other than flows

   In some parts of the network, enforcing flow-level fairness may not
   be desirable, or some other level of fairness may be more important.
   An example of this can be an Internet Service Provider that may be

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   more interested in ensuring fairness between customers than between
   flows.  Or a hosting or transit provider that wishes to ensure
   fairness between connecting Autonomous Systems or networks.  Another
   issue can be that the number of simultaneous flows experienced at a
   particular link can be too high for flow-based fairness queueing to
   be effective.

   Whatever the reason, in a scenario where fairness between flows is
   not desirable, reconfiguring FQ-CoDel to match on a different
   characteristic can be a way forward.  The implementation in Linux can
   leverage the powerful packet matching mechanism of the _tc_ subsystem
   to use any available packet field to partition packets into virtual
   queues, to for instance match on address or subnet source/destination
   pairs, application layer characteristics, etc.

   Furthermore, as commonly deployed today, FQ-CoDel is used with three
   or more tiers of classification: priority, best effort and
   background, based on diffserv markings.  Some products do more
   detailed classification, including deep packet inspection and
   destination-specific filters to achieve their desired result.

7.2.  Flow bunching by opaque encapsulation

   Where possible, FQ-CoDel will attempt to decapsulate packets before
   matching on the header fields for the flow hashing.  However, for
   some encapsulation techniques, most notably encrypted VPNs, this is
   not possible.  If several flows are bunched into one such
   encapsulated tunnel, they will be seen as one flow by the FQ-CoDel
   algorithm.  This means that they will share a queue, and drop
   behaviour, and so flows inside the encapsulation will not benefit
   from the implicit prioritisation of FQ-CoDel, but will continue to
   benefit from the reduced overall queue length from the CoDel
   algorithm operating on the queue.  In addition, when such an
   encapsulated bunch competes against other flows, it will count as one
   flow, and not assigned a share of the bandwidth based on how many
   flows are inside the encapsulation.

   Depending on the application, this may or may not be desirable
   behaviour.  In cases where it is not, changing FQ-CoDel's matching to
   not be flow-based (as detailed in the previous subsection above) can
   be a way to mitigate this.

7.3.  Low-priority congestion control algorithms

   Because of the flow isolation that FQ-CoDel provides, low-priority
   congestion control algorithms (or, in general, algorithms that try to
   voluntarily use up less than their fair share of bandwidth) can be
   re-prioritised.  Because a flow experiences very little added latency

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   when the link is congested, such algorithms lack the signal to back
   off that added latency previously afforded them.  As such, existing
   algorithms tend to revert to loss-based congestion control, and will
   consume the fair share of bandwidth afforded to them by the FQ-CoDel
   scheduler.  However, low-priority congestion control mechanisms may
   be able to take steps to continue to be low priority, for instance by
   taking into account the vastly reduced level of delay afforded by an
   AQM, or by using a coupled approach to observing the behaviour of
   multiple flows.

8.  Security Considerations

   There are no specific security exposures associated with FQ-CoDel.
   Some exposures present in current FIFO systems are in fact reduced
   (e.g. simple minded packet floods).

9.  IANA Considerations

   This document has no actions for IANA.

10.  Acknowledgements

   Our deepest thanks to Eric Dumazet (author of FQ-CoDel), Kathie
   Nichols, Van Jacobson, and all the members of the

11.  Conclusions

   FQ-CoDel is a very general, efficient, nearly parameterless active
   queue management approach combining flow queueing with CoDel.  It is
   a critical tool in solving bufferbloat.

   FQ-CoDel's default settings SHOULD be modified for other special-
   purpose networking applications, such as for exceptionally slow
   links, for use in data centres, or on links with inherent delay
   greater than 800ms (e.g. satellite links).

   On-going projects are: improving FQ-CoDel with more SFQ-like
   behaviour for lower bandwidth systems, improving the control law,
   optimising sparse packet drop behaviour, etc..

   In addition to the Linux kernel sources, ns2 and ns3 models are
   available.  Refinements (such as NFQCODEL [1]) are being tested in
   the CeroWrt effort.

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12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC0970]  Nagle, J., "On packet switches with infinite storage", RFC
              970, December 1985.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

              Nichols, K., Jacobson, V., McGregor, A., and J. Iyengar,
              "Controlling Queue Delay", October 2014,

12.2.  Informative References

   [SFQ]      McKenney, P., "Stochastic Fairness Queuing", September
              1990, <

              Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              July 2012, <>.

   [SQF2012]  Bonald, T., Muscariello, L., and N. Ostallo, "On the
              impact of TCP and per-flow scheduling on Internet
              Performance - IEEE/ACM transactions on Networking", April
              2012, <http://perso.telecom-

   [DRR]      Shreedhar, M. and G. Varghese, "Efficient Fair Queueing
              Using Deficit Round Robin", June 1996,

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   [DRRPP]    MacGregor, and Shi, "Deficits for Bursty Latency-critical
              Flows: DRR++", 2000, <

12.3.  URIs


Authors' Addresses

   Toke Hoeiland-Joergensen
   Karlstad University
   Dept. of Computer Science
   Karlstad  65188


   Paul McKenney
   IBM Linux Technology Center
   1385 NW Amberglen Parkway
   Hillsboro, OR  97006


   Dave Taht
   2104 W First street
   Apt 2002
   FT Myers, FL  33901


   Jim Gettys
   Google, Inc.
   21 Oak Knoll Road
   Carlisle, MA  01741


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   Eric Dumazet
   Google, Inc.
   1600 Amphitheater Pkwy
   Mountain View, Ca  94043


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