



Active Queue Management and Packet Scheduling (aqm)             G. White
Internet-Draft                                                 CableLabs
Intended status: Informational                                    R. Pan
Expires: August 15, 2016                                   Cisco Systems
                                                       February 12, 2016


                A PIE-Based AQM for DOCSIS Cable Modems
                      draft-ietf-aqm-docsis-pie-02

Abstract

   Cable modems based on the DOCSIS(R) specification provide broadband
   Internet access to over one hundred million users worldwide.  In some
   cases, the cable modem connection is the bottleneck (lowest speed)
   link between the customer and the Internet.  As a result, the impact
   of buffering and bufferbloat in the cable modem can have a
   significant effect on user experience.  The CableLabs DOCSIS 3.1
   specification introduces requirements for cable modems to support an
   Active Queue Management (AQM) algorithm that is intended to alleviate
   the impact that buffering has on latency sensitive traffic, while
   preserving bulk throughput performance.  In addition, the CableLabs
   DOCSIS 3.0 specifications have also been amended to contain similar
   requirements.  This document describes the requirements on Active
   Queue Management that apply to DOCSIS equipment, including a
   description of the "DOCSIS-PIE" algorithm that is required on DOCSIS
   3.1 cable modems.

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
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   This Internet-Draft will expire on August 15, 2016.







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Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview of DOCSIS AQM Requirements . . . . . . . . . . . . .   3
   3.  The DOCSIS MAC Layer and Service Flows  . . . . . . . . . . .   3
   4.  DOCSIS-PIE vs. PIE  . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Latency Target  . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Departure rate estimation . . . . . . . . . . . . . . . .   5
     4.3.  Enhanced burst protection . . . . . . . . . . . . . . . .   6
     4.4.  Expanded auto-tuning range  . . . . . . . . . . . . . . .   7
     4.5.  Trigger for exponential decay . . . . . . . . . . . . . .   7
     4.6.  Drop probability scaling  . . . . . . . . . . . . . . . .   7
     4.7.  Support for explicit congestion notification  . . . . . .   8
   5.  Implementation Guidance . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  DOCSIS-PIE Algorithm definition  . . . . . . . . . .   9
     A.1.  DOCSIS-PIE AQM Constants and Variables  . . . . . . . . .  10
       A.1.1.  Configuration parameters  . . . . . . . . . . . . . .  10
       A.1.2.  Constant values . . . . . . . . . . . . . . . . . . .  10
       A.1.3.  Variables . . . . . . . . . . . . . . . . . . . . . .  10
       A.1.4.  Public/system functions:  . . . . . . . . . . . . . .  11
     A.2.  DOCSIS-PIE AQM Control Path . . . . . . . . . . . . . . .  11
     A.3.  DOCSIS-PIE AQM Data Path  . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   A recent resurgence of interest in Active Queue Management, arising
   from a recognition of the inadequacies of drop tail queuing in the
   presence of loss-based congestion control algorithms, has resulted in
   the development of new algorithms that appear to provide very good



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   congestion feedback to current TCP algorithms, while also having
   operational simplicity and low complexity.  One of these algorithms
   has been selected as a requirement for cable modems built according
   to the DOCSIS 3.1 specification [DOCSIS_3.1].  The Data Over Cable
   Service Interface Specifications (DOCSIS) define the broadband
   technology deployed worldwide for Ethernet and IP service over hybrid
   fiber-coaxial cable systems.  The most recent revision of the DOCSIS
   technology, version 3.1, was published in October 2013 and provides
   support for up to 10 Gbps downstream (toward the customer) and 1 Gbps
   upstream (from the customer) capacity over existing cable networks.
   Previous versions of the DOCSIS technology did not contain
   requirements for AQM.  This document outlines the high-level AQM
   requirements for DOCSIS systems, discusses some of the salient
   features of the DOCSIS MAC layer, and describes the DOCSIS-PIE
   algorithm - largely by comparing it to its progenitor, the
   [I-D.ietf-aqm-pie] algorithm.

2.  Overview of DOCSIS AQM Requirements

   CableLabs' DOCSIS 3.1 specification [DOCSIS_3.1] mandates that cable
   modems implement a specific variant of the Proportional Integral
   controller Enhanced (PIE) [I-D.ietf-aqm-pie] active queue management
   algorithm.  This specific variant is provided for reference in
   Appendix A, and simulation results comparing it to drop tail queuing
   and other AQM options are given in [CommMag] and [DOCSIS-AQM].  In
   addition, CableLabs' DOCSIS 3.0 specification [DOCSIS_3.0] has been
   amended to recommend that cable modems implement the same algorithm.
   Both specifications allow that cable modems can optionally implement
   additional algorithms, that can then be selected for use by the
   operator via the modem's configuration file.

   These requirements on the cable modem apply to upstream transmissions
   (i.e. from the customer to the Internet).

   Both specifications also include requirements (mandatory in DOCSIS
   3.1 and recommended in DOCSIS 3.0) that the Cable Modem Termination
   System (CMTS) implement active queue management for downstream
   traffic, however no specific algorithm is defined for downstream use.

3.  The DOCSIS MAC Layer and Service Flows

   The DOCSIS Media Access Control (sub-)layer provides tools for
   configuring differentiated Quality of Service for different
   applications by the use of Packet Classifiers and Service Flows.

   Each Service Flow has an associated Quality of Service (QoS)
   parameter set that defines the treatment of the packets that traverse
   the Service Flow.  These parameters include (for example) Minimum



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   Reserved Traffic Rate, Maximum Sustained Traffic Rate, Peak Traffic
   Rate, Maximum Traffic Burst, and Traffic Priority.  Each upstream
   Service Flow corresponds to a queue in the cable modem, and each
   downstream Service Flow corresponds to a queue in the CMTS.  The
   DOCSIS AQM requirements mandate that the CM and CMTS implement the
   AQM algorithm (and allow it to be disabled if need be) on each
   Service Flow queue independently.

   Packet Classifiers can match packets based upon several fields in the
   packet/frame headers including the Ethernet header, IP header, and
   TCP/UDP header.  Matched packets are then queued in the associated
   Service Flow queue.

   Each cable modem can be configured with multiple Packet Classifiers
   and Service Flows.  The maximum number of such entities that a cable
   modem supports is an implementation decision for the manufacturer,
   but modems typically support 16 or 32 upstream Service Flows and at
   least that many Packet Classifiers.  Similarly the CMTS supports
   multiple downstream Service Flows and multiple Packet Classifiers per
   cable modem.

   It is typical that upstream and downstream Service Flows used for
   broadband Internet access are configured with a Maximum Sustained
   Traffic Rate.  This QoS parameter rate-shapes the traffic onto the
   DOCSIS link, and is the main parameter that defines the service
   offering.  Additionally, it is common that upstream and downstream
   Service Flows are configured with a Maximum Traffic Burst and a Peak
   Traffic Rate.  These parameters allow the service to burst at a
   higher (sometimes significantly higher) rate than is defined in the
   Maximum Sustained Traffic Rate for the amount of bytes configured in
   Maximum Traffic Burst, as long as the long-term average data rate
   remains at or below the Maximum Sustained Traffic Rate.

   Mathematically, what is enforced is that the traffic placed on the
   DOCSIS link in the time interval (t1,t2) complies with the following
   rate shaping equations:

      TxBytes(t1,t2) <= (t2-t1)*R/8 + B

      TxBytes(t1,t2) <= (t2-t1)*P/8 + 1522

   for all values t2>t1, where:

      R = Maximum Sustained Traffic Rate (bps)

      P = Peak Traffic Rate (bps)

      B = Maximum Traffic Burst (bytes)



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   The result of this configuration is that the link rate available to
   the Service Flow varies based on the pattern of load.  If the load
   that the Service Flow places on the link is less than the Maximum
   Sustained Traffic Rate, the Service Flow "earns" credit that it can
   then use (should the load increase) to burst at the Peak Traffic
   Rate.  This dynamic is important since these rate changes
   (particularly the decrease in data rate once the traffic burst credit
   is exhausted) can induce a step increase in buffering latency.

4.  DOCSIS-PIE vs. PIE

   There are a number of differences between the version of the PIE
   algorithm that is mandated for cable modems in the DOCSIS
   specifications and the version described in [I-D.ietf-aqm-pie].
   These differences are described in the following subsections.

4.1.  Latency Target

   The latency target (aka delay reference) is a key parameter that
   affects, among other things, the tradeoff in performance between
   latency-sensitive applications and bulk TCP applications.  Via
   simulation studies, a value of 10ms was identified as providing a
   good balance of performance.  However, it is recognized that there
   may be service offerings for which this value doesn't provide the
   best performance balance.  As a result, this is provided as a
   configuration parameter that the operator can set independently on
   each upstream service flow.  If not explicitly set by the operator,
   the modem will use 10 ms as the default value.

4.2.  Departure rate estimation

   The PIE algorithm utilizes a departure rate estimator to track
   fluctuations in the egress rate for the queue and to generate a
   smoothed estimate of this rate for use in the drop probability
   calculation.  This estimator may be well suited to many link
   technologies, but is not ideal for DOCSIS upstream links for a number
   of reasons.

   First, the bursty nature of the upstream transmissions, in which the
   queue drains at line rate (up to ~100 Mbps for DOCSIS 3.0 and ~1 Gbps
   for DOCSIS 3.1) and then is blocked until the next transmit
   opportunity, results in the potential for inaccuracy in measurement,
   given that the PIE departure rate estimator starts each measurement
   during a transmission burst and ends each measurement during a
   (possibly different) transmission burst.  For example, in the case
   where the start and end of measurement occur within a single burst,
   the PIE estimator will calculate the egress rate to be equal to the
   line rate, rather than the average rate available to the modem.



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   Second, the latency introduced by the DOCSIS request-grant mechanism
   can result in some further inaccuracy.  In typical conditions, the
   request-grant mechanism can add between ~4 ms and ~8 ms of latency to
   the forwarding of upstream traffic.  Within that range, the amount of
   additional latency that affects any individual data burst is
   effectively random, being influenced by the arrival time of the burst
   relative to the next request transmit opportunity, among other
   factors.

   Third, in the significant majority of cases, the departure rate,
   while variable, is controlled by the modem itself via the pair of
   token bucket rate shaping equations described in Section 3.
   Together, these two equations enforce a maximum sustained traffic
   rate, a peak traffic rate, and a maximum traffic burst size for the
   modem's requested bandwidth.  The implication of this is that the
   modem, in the significant majority of cases, will know precisely what
   the departure rate will be, and can predict exactly when transitions
   between peak rate and maximum sustained traffic rate will occur.
   Compare this to the PIE estimator, which would be simply reacting to
   (and smoothing its estimate of) those rate transitions after the
   fact.

   Finally, since the modem is already implementing the dual token
   bucket traffic shaper, it contains enough internal state to calculate
   predicted queuing delay with a minimum of computations.  Furthermore,
   these computations only need to be run every drop probability update
   interval, as opposed to the PIE estimator, which runs a similar
   number of computations on each packet dequeue event.

   For these reasons, the DOCSIS-PIE algorithm utilizes the
   configuration and state of the dual token bucket traffic shaper to
   translate queue depth into predicted queuing delay, rather than
   implementing the departure rate estimator defined in PIE.

4.3.  Enhanced burst protection

   The PIE [I-D.ietf-aqm-pie] algorithm has two states, INACTIVE and
   ACTIVE.  During the INACTIVE state, AQM packet drops are suppressed.
   The algorithm transitions to the ACTIVE state when the queue exceeds
   1/3 of the buffer size.  Upon transition to the ACTIVE state, PIE
   includes a burst protection feature in which the AQM packet drops are
   suppressed for the first 150ms.  Since DOCSIS-PIE is predominantly
   deployed on consumer broadband connections, a more sophisticated
   burst protection was developed in order to provide better performance
   in the presence of a single TCP session.

   Where the PIE algorithm has two states, DOCSIS-PIE has three.  The
   INACTIVE and ACTIVE states in DOCSIS-PIE are identical to those



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   states in PIE.  The QUIESCENT state is a transitional state between
   INACTIVE and ACTIVE.  The DOCSIS-PIE algorithm transitions from
   INACTIVE to QUIESCENT when the queue exceeds 1/3 of the buffer size.
   In the QUIESCENT state, packet drops are immediately enabled, and
   upon the first packet drop, the algorithm transitions to the ACTIVE
   state (where drop probability is reset to zero for the 150ms duration
   of the burst protection as in PIE).  From the ACTIVE state, the
   algorithm transitions to QUIESCENT if the drop_probability has
   decayed to zero and the queuing latency has been less than half of
   the LATENCY_TARGET for two update intervals.  The algorithm then
   fully resets to the INACTIVE state if this "quiet" condition exists
   for the duration of the BURST_RESET_TIMEOUT (1 second).  One end
   result of the addition of the QUIESCENT state is that a single packet
   drop can occur relatively early on during an initial burst, whereas
   all drops would be suppressed for at least 150ms of the burst
   duration in PIE.  The other end result is that if traffic stops and
   then resumes within 1 second, DOCSIS_PIE can directly drop a single
   packet and then re-enter burst protection, whereas PIE would require
   that the buffer exceed 1/3 full.

4.4.  Expanded auto-tuning range

   The PIE algorithm scales the PI coefficients based on the current
   drop probability.  The DOCSIS-PIE algorithm extends this scaling to
   drop probabilities below 1e-4.

4.5.  Trigger for exponential decay

   The PIE algorithm includes a mechanism by which the drop probability
   is allowed to decay exponentially (rather than linearly) when it is
   detected that the buffer is empty.  In the DOCSIS case, recently
   arrived packets may reside in buffer due to the request-grant latency
   even if the link is effectively idle.  As a result, the buffer may
   not be identically empty in the situations for which the exponential
   decay is intended.  To compensate for this, we trigger exponential
   decay when the buffer occupancy is less than 5ms * Peak Traffic Rate.

4.6.  Drop probability scaling

   The DOCSIS-PIE algorithm scales the calculated drop probability based
   on the ratio of the packet size to a constant value of 1024 bytes
   (representing approximate average packet size).  While [RFC7567] in
   general recommends against this type of scaling, we note that DOCSIS-
   PIE is expected to predominantly be used to manage upstream queues in
   residential broadband deployments, where we believe the benefits
   outweigh the disadvantages.  As a safeguard to prevent a flood of
   small packets from starving flows that use larger packets, DOCSIS-PIE
   limits the scaled probability to a defined maximum value of 0.85.



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4.7.  Support for explicit congestion notification

   DOCSIS-PIE does not include support for explicit congestion
   notification.  Cable modems are essentially IEEE 802.1d Ethernet
   bridges and so are not designed to modify IP header fields.
   Additionally, the packet processing pipeline in a cable modem is
   commonly implemented in hardware.  As a result, introducing support
   for ECN would have engendered a more significant redesign of cable
   modem data paths, and implementations would have been difficult or
   impossible to modify in the future.  At the time of the development
   of DOCSIS-PIE, which coincided with the development of modem chip
   designs, the benefits of ECN marking relative to packet drop were
   considered to be relatively minor, there was considerable discussion
   about differential treatment of ECN capable packets in the AQM drop/
   mark decision, and there were some initial suggestions that a new ECN
   approach was needed.  Due to this uncertainty, we chose not to
   include support for ECN.

5.  Implementation Guidance

   The AQM space is an evolving one, and it is expected that continued
   research in this field may in the future result in improved
   algorithms.

   As part of defining the DOCSIS-PIE algorithm, we split the pseudocode
   definition into two components, a "data path" component and a
   "control path" component.  The control path component contains the
   packet drop probability update functionality, whereas the data path
   component contains the per-packet operations, including the drop
   decision logic.

   It is understood that some aspects of the cable modem implementation
   may be done in hardware, particularly functions that handle packet-
   processing.

   While the DOCSIS specifications don't mandate the internal
   implementation details of the cable modem, modem implementers are
   strongly advised against implementing the control path functionality
   in hardware.  The intent of this advice is to retain the possibility
   that future improvements in AQM algorithms can be accommodated via
   software updates to deployed devices.

6.  IANA Considerations

   This document has no actions for IANA.






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7.  Security Considerations

   This document describes an active queue management algorithm based on
   [I-D.ietf-aqm-pie] for implementation in DOCSIS cable modem devices.
   This algorithm introduces no specific security exposures.

8.  Informative References

   [CommMag]  White, G., "Active queue management in DOCSIS 3.1
              networks", IEEE Communications Magazine vol.53, no.3,
              pp.126-132, March 2015.

   [DOCSIS-AQM]
              White, G., "Active Queue Management in DOCSIS 3.x Cable
              Modems", May 2014, <http://www.cablelabs.com/wp-
              content/uploads/2014/06/DOCSIS-AQM_May2014.pdf>.

   [DOCSIS_3.0]
              CableLabs, "DOCSIS 3.0 MAC and Upper Layer Protocols
              Specification", December 2015, <http://www.cablelabs.com/
              wp-content/uploads/specdocs/
              CM-SP-MULPIv3.0-I29-151210.pdf>.

   [DOCSIS_3.1]
              CableLabs, "DOCSIS 3.1 MAC and Upper Layer Protocols
              Specification", December 2015, <http://www.cablelabs.com/
              wp-content/uploads/specdocs/
              CM-SP-MULPIv3.1-I08-151210.pdf>.

   [I-D.ietf-aqm-pie]
              Pan, R., Natarajan, P., and F. Baker, "PIE: A Lightweight
              Control Scheme To Address the Bufferbloat Problem", draft-
              ietf-aqm-pie-03 (work in progress), November 2015.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <http://www.rfc-editor.org/info/rfc7567>.

Appendix A.  DOCSIS-PIE Algorithm definition

   PIE defines two functions organized here into two design blocks:

   1.  Control path block, a periodically running algorithm that
       calculates a drop probability based on the estimated queuing
       latency and queuing latency trend.





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   2.  Data path block, a function that occurs on each packet enqueue:
       per-packet drop decision based on the drop probability.

   It is desired to have the ability to update the Control path block
   based on operational experience with PIE deployments.

A.1.  DOCSIS-PIE AQM Constants and Variables

A.1.1.  Configuration parameters

   o  LATENCY_TARGET.  AQM Latency Target for this Service Flow

   o  PEAK_RATE.  Service Flow configured Peak Traffic Rate, expressed
      in Bytes/sec.

   o  MSR.  Service Flow configured Max. Sustained Traffic Rate,
      expressed in Bytes/sec.

   o  BUFFER_SIZE.  The size (in bytes) of the buffer for this Service
      Flow.

A.1.2.  Constant values

   o  A = 0.25, B = 2.5.  Weights in the drop probability calculation

   o  INTERVAL = 16 ms.  Update interval for drop probability.

   o  BURST_RESET_TIMEOUT = 1 s.

   o  MAX_BURST = 142 ms (150 ms - 8 ms (update error))

   o  MEAN_PKTSIZE = 1024 bytes

   o  MIN_PKTSIZE = 64 bytes

   o  PROB_LOW = 0.85

   o  PROB_HIGH = 8.5

   o  LATENCY_LOW = 5 ms

   o  LATENCY_HIGH = 200 ms.

A.1.3.  Variables

   o  drop_prob_. The current packet drop probability.

   o  accu_prob_. accumulated drop prob. since last drop



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   o  qdelay_old_. The previous queue delay estimate.

   o  burst_allowance_. Countdown for burst protection, initialize to 0

   o  burst_reset_. counter to reset burst

   o  aqm_state_. AQM activity state encoding 3 states:

         INACTIVE - queue staying below 1/3 full, suppress AQM drops

         QUIESCENT - transition state

         ACTIVE - normal AQM drops (after burst protection period)

   o  queue_. Holds the pending packets.

A.1.4.  Public/system functions:

   o  drop(packet).  Drops/discards a packet

   o  random().  Returns a uniform r.v. in the range 0 ~ 1

   o  queue_.is_full().  Returns true if queue_ is full

   o  queue_.byte_length().  Returns current queue_ length in bytes,
      including all MAC PDU bytes without DOCSIS MAC overhead

   o  queue_.enque(packet).  Adds packet to tail of queue_

   o  msrtokens().  Returns current token credits (in bytes) from the
      Max Sust.  Traffic Rate token bucket

   o  packet.size().  Returns size of packet

A.2.  DOCSIS-PIE AQM Control Path

   The DOCSIS-PIE control path performs the following:

   o  Calls control_path_init() at service flow creation

   o  Calls calculate_drop_prob() at a regular INTERVAL (16ms)

   ================
   //  Initialization function
   control_path_init() {
       drop_prob_ = 0;
       qdelay_old_ = 0;
       burst_reset_ = 0;



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       aqm_state_ = INACTIVE;
   }

   //  Background update, occurs every INTERVAL
   calculate_drop_prob() {

       if (queue_.byte_length() <= msrtokens()) {
           qdelay = queue_.byte_length() / PEAK_RATE;
       } else {
           qdelay = ((queue_.byte_length() - msrtokens()) / MSR \
                     +  msrtokens() / PEAK_RATE);
       }

       if (burst_allowance_ > 0) {
           drop_prob_ = 0;
           burst_allowance_ = max(0, burst_allowance_ - INTERVAL);
       } else {
           p = A * (qdelay - LATENCY_TARGET) + \
               B * (qdelay - qdelay_old_);
           // Since A=0.25 & B=2.5, can be implemented
           // with shift and add

           if (drop_prob_ < 0.000001) {
               p /= 2048;
           } else if (drop_prob_ < 0.00001) {
               p /= 512;
           } else if (drop_prob_ < 0.0001) {
               p /= 128;
           } else if (drop_prob_ < 0.001) {
               p /= 32;
           } else if (drop_prob_ < 0.01) {
               p /= 8;
           } else if (drop_prob_ < 0.1) {
               p /= 2;
           } else if (drop_prob_ < 1) {
               p /= 0.5;
           } else if (drop_prob_ < 10) {
               p /= 0.125;
           } else {
               p /= 0.03125;
           }

           if ((drop_prob_ >= 0.1) && (p > 0.02)) {
               p = 0.02;
           }
           drop_prob_ += p;

           /* some special cases */



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           if (qdelay < LATENCY_LOW && qdelay_old_ < LATENCY_LOW) {
               drop_prob_ *= 0.98;    // exponential decay
           } else if (qdelay > LATENCY_HIGH) {
               drop_prob_ += 0.02;   // ramp up quickly
           }

           drop_prob_ = max(0, drop_prob_);
           drop_prob_ = min(drop_prob_, \
                        PROB_LOW * MEAN_PKTSIZE/MIN_PKTSIZE);
       }

       // check if all is quiet
       quiet = (qdelay < 0.5 * LATENCY_TARGET)
               && (qdelay_old_ < 0.5 * LATENCY_TARGET)
               && (drop_prob_ == 0)
               && (burst_allowance_ == 0);

       // Update AQM state based on quiet or !quiet
       if ((aqm_state_ == ACTIVE) && quiet) {
           aqm_state_ = QUIESCENT;
           burst_reset_ = 0;
       } else if (aqm_state_ == QUIESCENT) {
           if (quiet) {
               burst_reset_ += INTERVAL ;
               if (burst_reset_ > BURST_RESET_TIMEOUT) {
                   burst_reset_ = 0;
                   aqm_state_ = INACTIVE;
               }
           } else {
               burst_reset_ = 0;
           }
       }

       qdelay_old_ = qdelay;

   }

A.3.  DOCSIS-PIE AQM Data Path

   The DOCSIS-PIE data path performs the following:

   o  Calls enque() in response to an incoming packet from the CMCI

   ================
   enque(packet) {
       if (queue_.is_full()) {
           drop(packet);
           accu_prob_ = 0;



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       } else if (drop_early(packet, queue_.byte_length())) {
           drop(packet);
       } else {
           queue_.enque(packet);
       }
   }

   ////////////////
   drop_early(packet, queue_length) {

       // if still in burst protection, suppress AQM drops
       if (burst_allowance_ > 0) {
           return FALSE;
       }

       // if drop_prob_ goes to zero, clear accu_prob_
       if (drop_prob_ == 0) {
           accu_prob_ = 0;
       }

       if (aqm_state_ == INACTIVE) {
           if (queue_.byte_length() < BUFFER_SIZE/3) {
               // if queue is still small, stay in
               // INACTIVE state and suppress AQM drops
               return FALSE;
           } else {
               // otherwise transition to QUIESCENT state
               aqm_state_ = QUIESCENT;
           }
       }

       //The CM can quantize packet.size to 64, 128, 256, 512, 768,
       // 1024, 1280, 1536, 2048 in the calculation below
       p1 = drop_prob_ * packet.size() / MEAN_PKTSIZE;
       p1 = min(p1, PROB_LOW);

       accu_prob_ += p1;

       // Suppress AQM drops in certain situations
       if ( (qdelay_old_ < 0.5 * LATENCY_TARGET && drop_prob_ < 0.2)
             || (queue_.byte_length() <= 2 * MEAN_PKTSIZE) ) {
           return FALSE;
       }

       if (accu_prob_ < PROB_LOW) {  // avoid dropping too fast due
            return FALSE;            // to bad luck of coin tosses...
       } else if (accu_prob_ >= PROB_HIGH) { // ...and avoid droppping
           drop = TRUE;                      // too slowly



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       } else {                        //Random drop
           double u = random();        // 0 ~ 1
           if (u > p1)
              return FALSE;
           else
               drop = TRUE;
       }

       // at this point, drop == TRUE, so packet will be dropped.

       // reset accu_prob_
       accu_prob_ = 0;

       // If in QUIESCENT state, packet drop triggers
       // ACTIVE state and start of burst protection
       if (aqm_state_ == QUIESCENT) {
           aqm_state_ = ACTIVE;
           burst_allowance_ = MAX_BURST;
       }
       return TRUE;
   }

Authors' Addresses

   Greg White
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027-9750
   USA

   Email: g.white@cablelabs.com


   Rong Pan
   Cisco Systems
   510 McCarthy Blvd
   Milpitas, CA  95134
   USA

   Email: ropan@cisco.com











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