33.7. RED queue disc¶
33.7.1. Model Description¶
Random Early Detection (RED) is a queue discipline that aims to provide early signals to transport protocol congestion control (e.g. TCP) that congestion is imminent, so that they back off their rate gracefully rather than with a bunch of tail-drop losses (possibly incurring TCP timeout). The model in ns-3 is a port of Sally Floyd’s ns-2 RED model.
Note that, starting from ns-3.25, RED is no longer a queue variant and cannot be installed as a NetDevice queue. Instead, RED is a queue disc and must be installed in the context of the traffic control (see the examples mentioned below).
The RED queue disc does not require packet filters, does not admit child queue discs and uses a single internal queue. If not provided by the user, a DropTail queue operating in the same mode (packet or byte) as the queue disc and having a size equal to the RED MaxSize attribute is created. Otherwise, the capacity of the queue disc is determined by the capacity of the internal queue provided by the user.
33.7.1.1. Adaptive Random Early Detection (ARED)¶
ARED is a variant of RED with two main features: (i) automatically sets Queue weight, MinTh and MaxTh and (ii) adapts maximum drop probability. The model in ns-3 contains implementation of both the features, and is a port of Sally Floyd’s ns-2 ARED model. Note that the user is allowed to choose and explicitly configure the simulation by selecting feature (i) or feature (ii), or both.
33.7.1.2. Feng’s Adaptive RED¶
Feng’s Adaptive RED is a variant of RED that adapts the maximum drop probability. The model in ns-3 contains implementation of this feature, and is a port of ns-2 Feng’s Adaptive RED model.
33.7.1.3. Nonlinear Random Early Detection (NLRED)¶
NLRED is a variant of RED in which the linear packet dropping function of RED is replaced by a nonlinear quadratic function. This approach makes packet dropping gentler for light traffic load and aggressive for heavy traffic load.
33.7.1.4. Explicit Congestion Notification (ECN)¶
This RED model supports an ECN mode of operation to notify endpoints of congestion that may be developing in a bottleneck queue, without resorting to packet drops. Such a mode is enabled by setting the UseEcn attribute to true (it is false by default) and only affects incoming packets with the ECT bit set in their header. When the average queue length is between the minimum and maximum thresholds, an incoming packet is marked instead of being dropped. When the average queue length is above the maximum threshold, an incoming packet is marked (instead of being dropped) only if the UseHardDrop attribute is set to false (it is true by default).
The implementation of support for ECN marking is done in such a way as
to not impose an internet module dependency on the traffic control module.
The RED model does not directly set ECN bits on the header, but delegates
that job to the QueueDiscItem class. As a result, it is possible to
use RED queues for other non-IP QueueDiscItems that may or may not support
the Mark () method.
33.7.1.5. References¶
The RED queue disc aims to be close to the results cited in: S.Floyd, K.Fall http://icir.org/floyd/papers/redsims.ps
ARED queue implementation is based on the algorithm provided in: S. Floyd et al, http://www.icir.org/floyd/papers/adaptiveRed.pdf
Feng’s Adaptive RED queue implementation is based on the algorithm provided in: W. C. Feng et al, http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=752150
NLRED queue implementation is based on the algorithm provided in: Kaiyu Zhou et al, http://www.sciencedirect.com/science/article/pii/S1389128606000879
The addition of explicit congestion notification (ECN) to IP: K. K. Ramakrishnan et al, https://tools.ietf.org/html/rfc3168
33.7.1.6. Attributes¶
The RED queue contains a number of attributes that control the RED policies:
MaxSize
MeanPktSize
IdlePktSize
Wait (time)
Gentle mode
MinTh, MaxTh
Queue weight
LInterm
LinkBandwidth
LinkDelay
UseEcn
UseHardDrop
In addition to RED attributes, ARED queue requires following attributes:
ARED (Boolean attribute. Default: false)
AdaptMaxP (Boolean attribute to adapt m_curMaxP. Default: false)
Target Delay (time)
Interval (time)
LastSet (time)
Top (upper limit of m_curMaxP)
Bottom (lower limit of m_curMaxP)
Alpha (increment parameter for m_curMaxP)
Beta (decrement parameter for m_curMaxP)
RTT
In addition to RED attributes, Feng’s Adaptive RED queue requires following attributes:
FengAdaptive (Boolean attribute, Default: false)
Status (status of current queue length, Default: Above)
FengAlpha (increment parameter for m_curMaxP, Default: 3)
FengBeta (decrement parameter for m_curMaxP, Default: 2)
The following attribute should be turned on to simulate NLRED queue disc:
NLRED (Boolean attribute. Default: false)
Consult the ns-3 documentation for explanation of these attributes.
33.7.1.7. Simulating ARED¶
To switch on ARED algorithm, the attribute ARED must be set to true,
as done in src/traffic-control/examples/adaptive-red-tests.cc:
Config::SetDefault("ns3::RedQueueDisc::ARED", BooleanValue(true));
Setting ARED to true implicitly configures both: (i) automatic setting of Queue weight, MinTh and MaxTh and (ii) adapting m_curMaxP.
NOTE: To explicitly configure (i) or (ii), set ARED attribute to false and follow the procedure described next:
To configure (i); Queue weight, MinTh and MaxTh, all must be set to 0,
as done in src/traffic-control/examples/adaptive-red-tests.cc:
Config::SetDefault("ns3::RedQueueDisc::QW", DoubleValue(0.0));
Config::SetDefault("ns3::RedQueueDisc::MinTh", DoubleValue(0));
Config::SetDefault("ns3::RedQueueDisc::MaxTh", DoubleValue(0));
To configure (ii); AdaptMaxP must be set to true, as done in
src/traffic-control/examples/adaptive-red-tests.cc:
Config::SetDefault("ns3::RedQueueDisc::AdaptMaxP", BooleanValue(true));
33.7.1.8. Simulating Feng’s Adaptive RED¶
To switch on Feng’s Adaptive RED algorithm, the attribute FengAdaptive must be
set to true, as done in examples/traffic-control/red-vs-fengadaptive.cc:
Config::SetDefault("ns3::RedQueueDisc::FengAdaptive", BooleanValue(true));
33.7.1.9. Simulating NLRED¶
To switch on NLRED algorithm, the attribute NLRED must be set to true, as shown below:
Config::SetDefault("ns3::RedQueueDisc::NLRED", BooleanValue(true));
33.7.1.10. Examples¶
The RED queue example is found at src/traffic-control/examples/red-tests.cc.
ARED queue examples can be found at:
src/traffic-control/examples/adaptive-red-tests.cc and
src/traffic-control/examples/red-vs-ared.cc
Feng’s Adaptive RED example can be found at:
examples/traffic-control/red-vs-fengadaptive.cc
NLRED queue example can be found at:
examples/traffic-control/red-vs-nlred.cc
33.7.2. Validation¶
The RED model has been validated and the report is currently stored at: https://gitlab.com/nsnam/ns-3-dev/uploads/6937dac2431a0b61209728ecbfd442ac/report-red-ns3.pdf