7.3 Different types of Queue objects

A queue object is a general class of object capable of holding and possibly marking or discarding packets as they travel through the simulated topology. Configuration Parameters used for queue objects are:

limit_

The queue size in packets.

blocked_

Set to false by default, this is true if the queue is blocked (unable to send a packet to its downstream neighbor).

unblock_on_resume_

Set to true by default, indicates a queue should unblock itself at the time the last packet packet sent has been transmitted (but not necessarily received).

Other queue objects derived from the base class Queue are drop-tail, FQ, SFQ, DRR, RED, CBQ, CoDel, and SFQ-CoDel queue objects. Each are described as follows:

• Drop-tail objects: Drop-tail objects are a subclass of Queue objects that implement simple FIFO queue. There are no methods, configuration parameter, or state variables that are specific to drop-tail objects.

• FQ objects: FQ objects are a subclass of Queue objects that implement Fair queuing. There are no methods that are specific to FQ objects. Configuration Parameters are:

  secsPerByte_

  There are no state variables associated with this object.

• SFQ objects: SFQ objects are a subclass of Queue objects that implement Stochastic Fair queuing. There are no methods that are specific to SFQ objects. Configuration Parameters are:

  maxqueue_

  buckets_

  There are no state variables associated with this object.

• DRR objects: DRR objects are a subclass of Queue objects that implement deficit round robin scheduling. These objects implement deficit round robin scheduling amongst different flows ( A particular flow is one which has packets with the same node and port id OR packets which have the same node id alone). Also unlike other multi-queue objects, this queue object implements a single shared buffer space for its different flows. Configuration Parameters are:

  buckets_

  Indicates the total number of buckets to be used for hashing each of the flows.

  blimit_

  Indicates the shared buffer size in bytes.

  quantum_

  Indicates (in bytes) how much each flow can send during its turn.

  mask_

  mask_, when set to 1, means that a particular flow consists of packets having the same node id (and possibly different port ids), otherwise a flow consists of packets having the same node and port ids.

• RED objects: RED objects are a subclass of Queue objects that implement random early-detection gateways. The object can be configured to either drop or ``mark'' packets. There are no methods that are specific to RED objects. Configuration Parameters are:

  bytes_

  Set to "true" to enable ``byte-mode'' RED, where the size of arriving packets affect the likelihood of marking (dropping) packets.

  queue-in-bytes_

  Set to "true" to measure the average queue size in bytes rather than packets. Enabling this option also causes thresh_ and maxthresh_ to be automatically scaled by mean_pktsize_ (see below).

  thresh_

  The minimum threshold for the average queue size in packets.

  maxthresh_

  The maximum threshold for the average queue size in packets.

  mean_pktsize_

  A rough estimate of the average packet size in bytes. Used in updating the calculated average queue size after an idle period.

  q_weight_

  The queue weight, used in the exponential-weighted moving average for calculating the average queue size.

  wait_

  Set to true to maintain an interval between dropped packets.

  linterm_

  As the average queue size varies between "thresh_" and "maxthresh_", the packet dropping probability varies between 0 and "1/linterm".

  setbit_

  Set to "true" to mark packets by setting the congestion indication bit in packet headers rather than drop packets.

  drop-tail_

  Set to true to use drop-tail rather than randomdrop when the queue overflows or the average queue size exceeds "maxthresh_". For a further explanation of these variables, see [2].

  None of the state variables of the RED implementation are accessible.

• CBQ objects: CBQ objects are a subclass of Queue objects that implement class-based queueing.

  $cbq insert class
  Insert traffic class class into the link-sharing structure associated with link object cbq.

  $cbq bind cbqclass id1 [$id2]
  Cause packets containing flow id id1 (or those in the range id1 to id2 inclusive) to be associated with the traffic class cbqclass.

  $cbq algorithm alg
  Select the CBQ internal algorithm. <alg> may be set to one of: "ancestor-only", "top-level", or "formal".

• CBQ/WRR objects: CBQ/WRR objects are a subclass of CBQ objects that implement weighted round-robin scheduling among classes of the same priority level. In contrast, CBQ objects implement packet-by-packet round-robin scheduling among classes of the same priority level. Configuration Parameters are:

  maxpkt_

  The maximum size of a packet in bytes. This is used only by CBQ/WRR objects in computing maximum bandwidth allocations for the weighted round-robin scheduler.

• CoDel objects: CoDel objects are a subclass of Queue objects that implement the Controlled Delay (CoDel) active queue manager. Configuration Parameters are:

  interval_

  The CoDel measurement interval. This is typically set to a value that is on the order of the worst-case RTT of connections utilizing the queue.

  target_

  The CoDel latency target. This is an upper bound on acceptable standing queue delay.

• SFQ-CoDel objects: SFQ-CoDel objects are a subclass of Queue objects that implement the Stochastic Flow Queuing - Controlled Delay queue manager. Configuration Parameters are:

  interval_

  The CoDel measurement interval. This is typically set to a value that is on the order of the worst-case RTT of connections utilizing the queue.

  target_

  The CoDel latency target. This is an upper bound on acceptable standing queue delay.

  maxbins_

  The number of SFQ "bins" implemented by the SFQ-CoDel queue.

  quantum_

  The deficit-round-robin quantum used for dequeuing packets from the SFQ structure.

CBQCLASS OBJECTS
CBQClass objects implement the traffic classes associated with CBQ objects.

$cbqclass setparams parent okborrow allot maxidle prio level
Sets several of the configuration parameters for the CBQ traffic class (see below).

$cbqclass parent cbqcl|none
specify the parent of this class in the link-sharing tree. The parent may be specified as ``none'' to indicate this class is a root.

$cbqclass newallot a
Change the link allocation of this class to the specified amount (in range 0.0 to 1.0). Note that only the specified class is affected.

$cbqclass install-queue q
Install a Queue object into the compound CBQ or CBQ/WRR link structure. When a CBQ object is initially created, it includes no internal queue (only a packet classifier and scheduler).

Configuration Parameters are:

okborrow_

is a boolean indicating the class is permitted to borrow bandwidth from its parent.

allot_

is the maximum fraction of link bandwidth allocated to the class expressed as a real number between 0.0 and 1.0.

maxidle_

is the maximum amount of time a class may be required to have its packets queued before they are permitted to be forwarded

priority_

is the class' priority level with respect to other classes. This value may range from 0 to 10, and more than one class may exist at the same priority. Priority 0 is the highest priority.

level_

is the level of this class in the link-sharing tree. Leaf nodes in the tree are considered to be at level 1; their parents are at level 2, etc.

extradelay_

increase the delay experienced by a delayed class by the specified time

QUEUE-MONITOR OBJECTS
QueueMonitor Objects are used to monitor a set of packet and byte arrival, departure and drop counters. It also includes support for aggregate statistics such as average queue size, etc.

$queuemonitor
reset all the cumulative counters described below (arrivals, departures, and drops) to zero. Also, reset the integrators and delay sampler, if defined.

$queuemonitor set-delay-samples delaySamp_
Set up the Samples object delaySamp_ to record statistics about queue delays. delaySamp_ is a handle to a Samples object i.e the Samples object should have already been created.

$queuemonitor get-bytes-integrator
Returns an Integrator object that can be used to find the integral of the queue size in bytes.

$queuemonitor get-pkts-integrator
Returns an Integrator object that can be used to find the integral of the queue size in packets.

$queuemonitor get-delay-samples
Returns a Samples object delaySamp_ to record statistics about queue delays.
There are no configuration parameters specific to this object.
State Variables are:

size_

Instantaneous queue size in bytes.

pkts_

Instantaneous queue size in packets.

parrivals_

Running total of packets that have arrived.

barrivals_

Running total of bytes contained in packets that have arrived.

pdepartures_

Running total of packets that have departed (not dropped).

bdepartures_

Running total of bytes contained in packets that have departed (not dropped).

pdrops_

Total number of packets dropped.

bdrops_

Total number of bytes dropped.

bytesInt_

Integrator object that computes the integral of the queue size in bytes. The sum_ variable of this object has the running sum (integral) of the queue size in bytes.

pktsInt_

Integrator object that computes the integral of the queue size in packets. The sum_ variable of this object has the running sum (integral) of the queue size in packets.

QUEUEMONITOR/ED OBJECTS
This derived object is capable of differentiating regular packet drops from early drops. Some queues distinguish regular drops (e.g. drops due to buffer exhaustion) from other drops (e.g. random drops in RED queues). Under some circumstances, it is useful to distinguish these two types of drops.
State Variables are:

epdrops_

The number of packets that have been dropped ``early''.

ebdrops_

The number of bytes comprising packets that have been dropped ``early''.

Note: because this class is a subclass of QueueMonitor, objects of this type also have fields such as pdrops_ and bdrops_. These fields describe the total number of dropped packets and bytes, including both early and non-early drops.

QUEUEMONITOR/ED/FLOWMON OBJECTS
These objects may be used in the place of a conventional QueueMonitor object when wishing to collect per-flow counts and statistics in addition to the aggregate counts and statistics provided by the basic QueueMonitor.

$fmon classifier cl
This inserts (read) the specified classifier into (from) the flow monitor object. This is used to map incoming packets to which flows they are associated with.

$fmon dump
Dump the current per-flow counters and statistics to the I/O channel specified in a previous attach operation.

$fmon flows
Return a character string containing the names of all flow objects known by this flow monitor. Each of these objects are of type QueueMonitor/ED/Flow.

$fmon attach chan
Attach a tcl I/O channel to the flow monitor. Flow statistics are written to the channel when the dump operation is executed.

Configuration Parameters are:

enable_in_

Set to true by default, indicates that per-flow arrival state should be kept by the flow monitor. If set to false, only the aggregate arrival information is kept.

enable_out_

Set to true by default, indicates that per-flow departure state should be kept by the flow monitor. If set to false, only the aggregate departure information is kept.

enable_drop_

Set to true by default, indicates that per-flow drop state should be kept by the flow monitor. If set to false, only the aggregate drop information is kept.

enable_edrop_

Set to true by default, indicates that per-flow early drop state should be kept by the flow monitor. If set to false, only the aggregate early drop information is kept.

QUEUEMONITOR/ED/FLOW OBJECTS
These objects contain per-flow counts and statistics managed by a QueueMonitor/ED/Flowmon object. They are generally created in an OTcl callback procedure when a flow monitor is given a packet it cannot map on to a known flow. Note that the flow monitor's classifier is responsible for mapping packets to flows in some arbitrary way. Thus, depending on the type of classifier used, not all of the state variables may be relevant (e.g. one may classify packets based only on flow id, in which case the source and destination addresses may not be significant). State Variables are:

src_

The source address of packets belonging to this flow.

dst_

The destination address of packets belonging to this flow.

flowid_

The flow id of packets belonging to this flow.