rfc:rfc8698
Internet Engineering Task Force (IETF) X. Zhu Request for Comments: 8698 Cisco Systems Category: Experimental R. Pan ISSN: 2070-1721 Intel Corporation
M. Ramalho
AcousticComms
S. Mena
Cisco Systems
February 2020
Network-Assisted Dynamic Adaptation (NADA): A Unified Congestion Control
Scheme for Real-Time Media
Abstract
This document describes Network-Assisted Dynamic Adaptation (NADA), a
novel congestion control scheme for interactive real-time media
applications such as video conferencing. In the proposed scheme, the
sender regulates its sending rate, based on either implicit or
explicit congestion signaling, in a unified approach. The scheme can
benefit from Explicit Congestion Notification (ECN) markings from
network nodes. It also maintains consistent sender behavior in the
absence of such markings by reacting to queuing delays and packet
losses instead.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.
This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8698.
Copyright Notice
Copyright (c) 2020 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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. Terminology
3. System Overview
4. Core Congestion Control Algorithm
4.1. Mathematical Notations
4.2. Receiver-Side Algorithm
4.3. Sender-Side Algorithm
5. Practical Implementation of NADA
5.1. Receiver-Side Operation
5.1.1. Estimation of One-Way Delay and Queuing Delay
5.1.2. Estimation of Packet Loss/Marking Ratio
5.1.3. Estimation of Receiving Rate
5.2. Sender-Side Operation
5.2.1. Rate-Shaping Buffer
5.2.2. Adjusting Video Target Rate and Sending Rate
5.3. Feedback Message Requirements
6. Discussions and Further Investigations
6.1. Choice of Delay Metrics
6.2. Method for Delay, Loss, and Marking Ratio Estimation
6.3. Impact of Parameter Values
6.4. Sender-Based vs. Receiver-Based Calculation
6.5. Incremental Deployment
7. Reference Implementations
8. Suggested Experiments
9. IANA Considerations
10. Security Considerations
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Network Node Operations
A.1. Default Behavior of Drop-Tail Queues
A.2. RED-Based ECN Marking
A.3. Random Early Marking with Virtual Queues
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
Interactive real-time media applications introduce a unique set of challenges for congestion control. Unlike TCP, the mechanism used for real-time media needs to adapt quickly to instantaneous bandwidth changes, accommodate fluctuations in the output of video encoder rate control, and cause low queuing delay over the network. An ideal scheme should also make effective use of all types of congestion signals, including packet loss, queuing delay, and explicit congestion notification (ECN) [RFC3168] markings. The requirements for the congestion control algorithm are outlined in [RMCAT-CC]. The requirements highlight that the desired congestion control scheme should 1) avoid flow starvation and attain a reasonable fair share of bandwidth when competing against other flows, 2) adapt quickly, and 3) operate in a stable manner.
This document describes an experimental congestion control scheme called Network-Assisted Dynamic Adaptation (NADA). The design of NADA benefits from explicit congestion control signals (e.g., ECN markings) from the network, yet also operates when only implicit congestion indicators (delay and/or loss) are available. Such a unified sender behavior distinguishes NADA from other congestion control schemes for real-time media. In addition, its core congestion control algorithm is designed to guarantee stability for path round-trip times (RTTs) below a prescribed bound (e.g., 250 ms with default parameter choices). It further supports weighted bandwidth sharing among competing video flows with different priorities. The signaling mechanism consists of standard Real-time Transport Protocol (RTP) timestamp [RFC3550] and Real-time Transport Control Protocol (RTCP) feedback reports. The definition of the desired RTCP feedback message is described in detail in [RTCP-FEEDBACK] so as to support the successful operation of several congestion control schemes for real-time interactive media.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
3. System Overview
Figure 1 shows the end-to-end system for real-time media transport that NADA operates in. Note that there also exist network nodes along the reverse (potentially uncongested) path that the RTCP feedback reports traverse. Those network nodes are not shown in the figure for the sake of brevity.
+---------+ r_vin +--------+ +--------+ +----------+
| Media |<--------| RTP | |Network | | RTP |
| Encoder |========>| Sender |=======>| Node |====>| Receiver |
+---------+ r_vout +--------+ r_send +--------+ +----------+
/|\ |
| |
+---------------------------------+
RTCP Feedback Report
Figure 1: System Overview
Media encoder with rate control capabilities: Encodes raw media
(audio and video) frames into a compressed bitstream that is later
packetized into RTP packets. As discussed in [RFC8593], the
actual output rate from the encoder r_vout may fluctuate around
the target r_vin. Furthermore, it is possible that the encoder
can only react to bit rate changes at rather coarse time
intervals, e.g., once every 0.5 seconds.
RTP sender: Responsible for calculating the NADA reference rate
based on network congestion indicators (delay, loss, or ECN
marking reports from the receiver), for updating the video encoder
with a new target rate r_vin and for regulating the actual sending
rate r_send accordingly. The RTP sender also generates a sending
timestamp for each outgoing packet.
RTP receiver: Responsible for measuring and estimating end-to-end
delay (based on sender timestamp), packet loss (based on RTP
sequence number), ECN marking ratios (based on [RFC6679]), and
receiving rate (r_recv) of the flow. It calculates the aggregated
congestion signal (x_curr) that accounts for queuing delay, ECN
markings, and packet losses. The receiver also determines the
mode for sender rate adaptation (rmode) based on whether the flow
has encountered any standing non-zero congestion. The receiver
sends periodic RTCP reports back to the sender, containing values
of x_curr, rmode, and r_recv.
Network node with several modes of operation: The system can work
with the default behavior of a simple drop-tail queue. It can
also benefit from advanced Active Queue Management (AQM) features
such as Proportional Integral Controller Enhanced (PIE) [RFC8033],
Flow Queue Controlling Queue Delay (FQ-CoDel) [RFC8290], ECN
marking based on Random Early Detection (RED) [RFC7567], and Pre-
Congestion Notification (PCN) marking using a token bucket
algorithm [RFC6660]. Note that network node operation is out of
scope for the design of NADA.
4. Core Congestion Control Algorithm
Like TCP-Friendly Rate Control (TFRC) [FLOYD-CCR00] [RFC5348], NADA
is a rate-based congestion control algorithm. In its simplest form,
the sender reacts to the collection of network congestion indicators
in the form of an aggregated congestion signal and operates in one of
two modes:
Accelerated ramp up: When the bottleneck is deemed to be
underutilized, the rate increases multiplicatively with respect to
the rate of previously successful transmissions. The rate
increase multiplier (gamma) is calculated based on the observed
round-trip time and target feedback interval, so as to limit self-
inflicted queuing delay.
Gradual rate update: In the presence of a non-zero aggregate
congestion signal, the sending rate is adjusted in reaction to
both its value (x_curr) and its change in value (x_diff).
This section introduces the list of mathematical notations and describes the core congestion control algorithm at the sender and receiver, respectively. Additional details on recommended practical implementations are described in Sections 5.1 and 5.2.
4.1. Mathematical Notations
This section summarizes the list of variables and parameters used in
the NADA algorithm. Table 2 also includes the default values for
choosing the algorithm parameters to represent either a typical
setting in practical applications or a setting based on theoretical
and simulation studies. See Section 6.3 for some of the discussions
on the impact of parameter values. Additional studies in real-world
settings suggested in Section 8 could gather further insight on how
to choose and adapt these parameter values in practical deployment.
+------------+------------------------------------------------+
| Notation | Variable Name |
+============+================================================+
| t_curr | Current timestamp |
+------------+------------------------------------------------+
| t_last | Last time sending/receiving a feedback message |
+------------+------------------------------------------------+
| delta | Observed interval between current and previous |
| | feedback reports: delta = t_curr-t_last |
+------------+------------------------------------------------+
| r_ref | Reference rate based on network congestion |
+------------+------------------------------------------------+
| r_send | Sending rate |
+------------+------------------------------------------------+
| r_recv | Receiving rate |
+------------+------------------------------------------------+
| r_vin | Target rate for video encoder |
+------------+------------------------------------------------+
| r_vout | Output rate from video encoder |
+------------+------------------------------------------------+
| d_base | Estimated baseline delay |
+------------+------------------------------------------------+
| d_fwd | Measured and filtered one-way delay |
+------------+------------------------------------------------+
| d_queue | Estimated queuing delay |
+------------+------------------------------------------------+
| d_tilde | Equivalent delay after non-linear warping |
+------------+------------------------------------------------+
| p_mark | Estimated packet ECN marking ratio |
+------------+------------------------------------------------+
| p_loss | Estimated packet loss ratio |
+------------+------------------------------------------------+
| x_curr | Aggregate congestion signal |
+------------+------------------------------------------------+
| x_prev | Previous value of aggregate congestion signal |
+------------+------------------------------------------------+
| x_diff | Change in aggregate congestion signal w.r.t. |
| | its previous value: x_diff = x_curr - x_prev |
+------------+------------------------------------------------+
| rmode | Rate update mode: (0 = accelerated ramp up; 1 |
| | = gradual update) |
+------------+------------------------------------------------+
| gamma | Rate increase multiplier in accelerated ramp- |
| | up mode |
+------------+------------------------------------------------+
| loss_int | Measured average loss interval in packet count |
+------------+------------------------------------------------+
| loss_exp | Threshold value for setting the last observed |
| | packet loss to expiration |
+------------+------------------------------------------------+
| rtt | Estimated round-trip time at sender |
+------------+------------------------------------------------+
| buffer_len | Rate-shaping buffer occupancy measured in |
| | bytes |
+------------+------------------------------------------------+
Table 1: List of Variables
+-----------+-------------------------------------------+---------+
| Notation | Parameter Name | Default |
| | | Value |
+===========+===========================================+=========+
| PRIO | Weight of priority of the flow | 1.0 |
+-----------+-------------------------------------------+---------+
| RMIN | Minimum rate of application supported by | 150 |
| | media encoder | Kbps |
+-----------+-------------------------------------------+---------+
| RMAX | Maximum rate of application supported by | 1.5 |
| | media encoder | Mbps |
+-----------+-------------------------------------------+---------+
| XREF | Reference congestion level | 10 ms |
+-----------+-------------------------------------------+---------+
| KAPPA | Scaling parameter for gradual rate update | 0.5 |
| | calculation | |
+-----------+-------------------------------------------+---------+
| ETA | Scaling parameter for gradual rate update | 2.0 |
| | calculation | |
+-----------+-------------------------------------------+---------+
| TAU | Upper bound of RTT in gradual rate update | 500 ms |
| | calculation | |
+-----------+-------------------------------------------+---------+
| DELTA | Target feedback interval | 100 ms |
+-----------+-------------------------------------------+---------+
| LOGWIN | Observation window in time for | 500 ms |
| | calculating packet summary statistics at | |
| | receiver | |
+-----------+-------------------------------------------+---------+
| QEPS | Threshold for determining queuing delay | 10 ms |
| | buildup at receiver | |
+-----------+-------------------------------------------+---------+
| DFILT | Bound on filtering delay | 120 ms |
+-----------+-------------------------------------------+---------+
| GAMMA_MAX | Upper bound on rate increase ratio for | 0.5 |
| | accelerated ramp up | |
+-----------+-------------------------------------------+---------+
| QBOUND | Upper bound on self-inflicted queuing | 50 ms |
| | delay during ramp up | |
+-----------+-------------------------------------------+---------+
| MULTILOSS | Multiplier for self-scaling the | 7.0 |
| | expiration threshold of the last observed | |
| | loss (loss_exp) based on measured average | |
| | loss interval (loss_int) | |
+-----------+-------------------------------------------+---------+
| QTH | Delay threshold for invoking non-linear | 50 ms |
| | warping | |
+-----------+-------------------------------------------+---------+
| LAMBDA | Scaling parameter in the exponent of non- | 0.5 |
| | linear warping | |
+-----------+-------------------------------------------+---------+
| PLRREF | Reference packet loss ratio | 0.01 |
+-----------+-------------------------------------------+---------+
| PMRREF | Reference packet marking ratio | 0.01 |
+-----------+-------------------------------------------+---------+
| DLOSS | Reference delay penalty for loss when | 10 ms |
| | packet loss ratio is at PLRREF | |
+-----------+-------------------------------------------+---------+
| DMARK | Reference delay penalty for ECN marking | 2 ms |
| | when packet marking is at PMRREF | |
+-----------+-------------------------------------------+---------+
| FPS | Frame rate of incoming video | 30 |
+-----------+-------------------------------------------+---------+
| BETA_S | Scaling parameter for modulating outgoing | 0.1 |
| | sending rate | |
+-----------+-------------------------------------------+---------+
| BETA_V | Scaling parameter for modulating video | 0.1 |
| | encoder target rate | |
+-----------+-------------------------------------------+---------+
| ALPHA | Smoothing factor in exponential smoothing | 0.1 |
| | of packet loss and marking ratios | |
+-----------+-------------------------------------------+---------+
Table 2: List of Algorithm Parameters and Their Default Values
4.2. Receiver-Side Algorithm
The receiver-side algorithm can be outlined as below:
On initialization:
set d_base = +INFINITY
set p_loss = 0
set p_mark = 0
set r_recv = 0
set both t_last and t_curr as current time in milliseconds
On receiving a media packet:
obtain current timestamp t_curr from system clock
obtain from packet header sending time stamp t_sent
obtain one-way delay measurement: d_fwd = t_curr - t_sent
update baseline delay: d_base = min(d_base, d_fwd)
update queuing delay: d_queue = d_fwd - d_base
update packet loss ratio estimate p_loss
update packet marking ratio estimate p_mark
update measurement of receiving rate r_recv
On time to send a new feedback report (t_curr - t_last > DELTA):
calculate non-linear warping of delay d_tilde if packet loss
exists
calculate current aggregate congestion signal x_curr
determine mode of rate adaptation for sender: rmode
send feedback containing values of: rmode, x_curr, and r_recv
update t_last = t_curr
In order for a delay-based flow to hold its ground when competing against loss-based flows (e.g., loss-based TCP), it is important to distinguish between different levels of observed queuing delay. For instance, over wired connections, a moderate queuing delay value on the order of tens of milliseconds is likely self-inflicted or induced by other delay-based flows, whereas a high queuing delay value of several hundreds of milliseconds may indicate the presence of a loss- based flow that does not refrain from increased delay.
If the last observed packet loss is within the expiration window of loss_exp (measured in terms of packet counts), the estimated queuing delay follows a non-linear warping:
/ d_queue, if d_queue < QTH
|
d_tilde = < (1)
| (d_queue-QTH)
\ QTH exp(-LAMBDA ---------------) , otherwise
QTH
In Equation (1), the queuing delay value is unchanged when it is below the first threshold QTH; otherwise, it is scaled down following a non-linear curve. This non-linear warping is inspired by the delay-adaptive congestion window backoff policy in [BUDZISZ-AIMD-CC] so as to "gradually nudge" the controller to operate based on loss- induced congestion signals when competing against loss-based flows. The exact form of the non-linear function has been simplified with respect to [BUDZISZ-AIMD-CC]. The value of the threshold QTH should be carefully tuned for different operational environments so as to avoid potential risks of prematurely discounting the congestion signal level. Typically, a higher value of QTH is required in a noisier environment (e.g., over wireless connections or where the video stream encounters many time-varying background competing traffic) so as to stay robust against occasional non-congestion- induced delay spikes. Additional insights on how this value can be tuned or auto-tuned should be gathered from carrying out experimental studies in different real-world deployment scenarios.
The value of loss_exp is configured to self-scale with the average packet loss interval loss_int with a multiplier MULTILOSS:
loss_exp = MULTILOSS * loss_int.
Estimation of the average loss interval loss_int, in turn, follows Section 5.4 of "TCP Friendly Rate Control (TFRC): Protocol Specification" [RFC5348].
In practice, it is recommended to linearly interpolate between the warped (d_tilde) and non-warped (d_queue) values of the queuing delay during the transitional period lasting for the duration of loss_int.
The aggregate congestion signal is:
/ p_mark \^2 / p_loss \^2
x_curr = d_tilde + DMARK*|--------| + DLOSS*|--------| (2)
\ PMRREF / \ PLRREF /
Here, DMARK is prescribed a reference delay penalty associated with ECN markings at the reference marking ratio of PMRREF; DLOSS is prescribed a reference delay penalty associated with packet losses at the reference packet loss ratio of PLRREF. The value of DLOSS and DMARK does not depend on configurations at the network node. Since ECN-enabled active queue management schemes typically mark a packet before dropping it, the value of DLOSS SHOULD be higher than that of DMARK. Furthermore, the values of DLOSS and DMARK need to be set consistently across all NADA flows sharing the same bottleneck link so that they can compete fairly.
In the absence of packet marking and losses, the value of x_curr
reduces to the observed queuing delay d_queue. In that case, the
NADA algorithm operates in the regime of delay-based adaptation.
Given observed per-packet delay and loss information, the receiver is also in a good position to determine whether or not the network is underutilized and then recommend the corresponding rate adaptation mode for the sender. The criteria for operating in accelerated ramp- up mode are:
- No recent packet losses within the observation window LOGWIN; and
- No buildup of queuing delay: d_fwd-d_base < QEPS for all previous
delay samples within the observation window LOGWIN.
Otherwise, the algorithm operates in graduate update mode.
4.3. Sender-Side Algorithm
The sender-side algorithm is outlined as follows:
On initialization:
set r_ref = RMIN
set rtt = 0
set x_prev = 0
set t_last and t_curr as current system clock time
On receiving feedback report:
obtain current timestamp from system clock: t_curr
obtain values of rmode, x_curr, and r_recv from feedback report
update estimation of rtt
measure feedback interval: delta = t_curr - t_last
if rmode == 0:
update r_ref following accelerated ramp-up rules
else:
update r_ref following gradual update rules
clip rate r_ref within the range of minimum rate (RMIN) and
maximum rate (RMAX).
set x_prev = x_curr
set t_last = t_curr
In accelerated ramp-up mode, the rate r_ref is updated as follows:
QBOUND
gamma = min(GAMMA_MAX, ------------------) (3)
rtt+DELTA+DFILT
r_ref = max(r_ref, (1+gamma) r_recv)
(4)
The rate increase multiplier gamma is calculated as a function of the upper bound of self-inflicted queuing delay (QBOUND), round-trip time (rtt), and target feedback interval (DELTA); it is bound on the filtering delay for calculating d_queue (DFILT). It has a maximum value of GAMMA_MAX. The rationale behind Equations (3)-(4) is that the longer it takes for the sender to observe self-inflicted queuing delay buildup, the more conservative the sender should be in increasing its rate and, hence, the smaller the rate increase multiplier.
In gradual update mode, the rate r_ref is updated as:
x_offset = x_curr - PRIO*XREF*RMAX/r_ref (5)
x_diff = x_curr - x_prev (6)
delta x_offset
r_ref = r_ref - KAPPA*-------*------------*r_ref
TAU TAU
x_diff
- KAPPA*ETA*---------*r_ref (7)
TAU
The rate changes in proportion to the previous rate decision. It is affected by two terms: the offset of the aggregate congestion signal from its value at equilibrium (x_offset) and its change (x_diff). The calculation of x_offset depends on the maximum rate of the flow (RMAX), its weight of priority (PRIO), as well as a reference congestion signal (XREF). The value of XREF is chosen so that the maximum rate of RMAX can be achieved when the observed congestion signal level is below PRIO*XREF.
At equilibrium, the aggregated congestion signal stabilizes at x_curr = PRIO*XREF*RMAX/r_ref. This ensures that when multiple flows share the same bottleneck and observe a common value of x_curr, their rates at equilibrium will be proportional to their respective priority levels (PRIO) and the range between minimum and maximum rate. Values of the minimum rate (RMIN) and maximum rate (RMAX) will be provided by the media codec, for instance, as outlined by [RMCAT-CC-RTP]. In the absence of such information, the NADA sender will choose a default value of 0 for RMIN and 3 Mbps for RMAX.
As mentioned in the sender-side algorithm, the final rate is always clipped within the dynamic range specified by the application:
r_ref = min(r_ref, RMAX) (8)
r_ref = max(r_ref, RMIN) (9)
The above operations ignore many practical issues such as clock synchronization between sender and receiver, the filtering of noise in delay measurements, and base delay expiration. These will be addressed in Section 5.
5. Practical Implementation of NADA
5.1. Receiver-Side Operation
The receiver continuously monitors end-to-end per-packet statistics in terms of delay, loss, and/or ECN marking ratios. It then aggregates all forms of congestion indicators into the form of an equivalent delay and periodically reports this back to the sender. In addition, the receiver tracks the receiving rate of the flow and includes that in the feedback message.
5.1.1. Estimation of One-Way Delay and Queuing Delay
The delay estimation process in NADA follows an approach similar to that of earlier delay-based congestion control schemes, such as Low Extra Delay Background Transport (LEDBAT) [RFC6817]. For experimental implementations, instead of relying on RTP timestamps and the transmission time offset RTP header extension [RFC5450], the NADA sender can generate its own timestamp based on the local system clock and embed that information in the transport packet header. The NADA receiver estimates the forward delay as having a constant base delay component plus a time-varying queuing delay component. The base delay is estimated as the minimum value of one-way delay observed over a relatively long period (e.g., tens of minutes), whereas the individual queuing delay value is taken to be the difference between one-way delay and base delay. By re-estimating the base delay periodically, one can avoid the potential issue of base delay expiration, whereby an earlier measured base delay value is no longer valid due to underlying route changes or a cumulative timing difference introduced by the clock-rate skew between sender and receiver. All delay estimations are based on sender timestamps with a recommended granularity of 100 microseconds or finer.
The individual sample values of queuing delay should be further filtered against various non-congestion-induced noise, such as spikes due to a processing "hiccup" at the network nodes. Therefore, in addition to calculating the value of queuing delay using d_queue = d_fwd - d_base, as expressed in Section 5.1, the current implementation further employs a minimum filter with a window size of 15 samples over per-packet queuing delay values.
5.1.2. Estimation of Packet Loss/Marking Ratio
The receiver detects packet losses via gaps in the RTP sequence numbers of received packets. For interactive real-time media applications with stringent latency constraints (e.g., video conferencing), the receiver avoids the packet reordering delay by treating out-of-order packets as losses. The instantaneous packet loss ratio p_inst is estimated as the ratio between the number of missing packets over the number of total transmitted packets within the recent observation window LOGWIN. The packet loss ratio p_loss is obtained after exponential smoothing:
p_loss = ALPHA*p_inst + (1-ALPHA)*p_loss (10)
The filtered result is reported back to the sender as the observed packet loss ratio p_loss.
The estimation of the packet marking ratio p_mark follows the same procedure as above. It is assumed that ECN marking information at the IP header can be passed to the receiving endpoint, e.g., by following the mechanism described in [RFC6679].
5.1.3. Estimation of Receiving Rate
It is fairly straightforward to estimate the receiving rate r_recv.
NADA maintains a recent observation window with a time span of LOGWIN
and simply divides the total size of packets arriving during that
window over the time span. The receiving rate (r_recv) can be either
calculated at the sender side based on the per-packet feedback from
the receiver or included as part of the feedback report.
5.2. Sender-Side Operation
Figure 2 provides a detailed view of the NADA sender. Upon receipt
of an RTCP feedback report from the receiver, the NADA sender
calculates the reference rate r_ref as specified in Section 4.3. It
further adjusts both the target rate for the live video encoder r_vin
and the sending rate r_send over the network based on the updated
value of r_ref and rate-shaping buffer occupancy buffer_len.
The NADA sender behavior stays the same in the presence of all types of congestion indicators: delay, loss, and ECN marking. This unified approach allows a graceful transition of the scheme as the network shifts dynamically between light and heavy congestion levels.
+----------------+
| Calculate | <---- RTCP report
| Reference Rate |
-----------------+
| r_ref
+------------+-------------+
| |
\|/ \|/
+-----------------+ +---------------+
| Calculate Video | | Calculate |
| Target Rate | | Sending Rate |
+-----------------+ +---------------+
| /|\ /|\ |
r_vin | | | |
\|/ +-------------------+ |
+----------+ | buffer_len | r_send
| Video | r_vout -----------+ \|/
| Encoder |--------> |||||||||=================>
+----------+ -----------+ RTP packets
Rate-Shaping Buffer
Figure 2: NADA Sender Structure
5.2.1. Rate-Shaping Buffer
The operation of the live video encoder is out of the scope of the design for the congestion control scheme in NADA. Instead, its behavior is treated as a black box.
A rate-shaping buffer is employed to absorb any instantaneous mismatch between the encoder rate output r_vout and the regulated sending rate r_send. Its current level of occupancy is measured in bytes and is denoted as buffer_len.
A large rate-shaping buffer contributes to higher end-to-end delay, which may harm the performance of real-time media communications. Therefore, the sender has a strong incentive to prevent the rate- shaping buffer from building up. The mechanisms adopted are:
- To deplete the rate-shaping buffer faster by increasing the
sending rate r_send; and
- To limit incoming packets of the rate-shaping buffer by reducing
the video encoder target rate r_vin.
5.2.2. Adjusting Video Target Rate and Sending Rate
If the level of occupancy in the rate-shaping buffer is accessible at the sender, such information can be leveraged to further adjust the target rate of the live video encoder r_vin as well as the actual sending rate r_send. The purpose of such adjustments is to mitigate the additional latencies introduced by the rate-shaping buffer. The amount of rate adjustment can be calculated as follows:
r_diff_v = min(0.05*r_ref, BETA_V*8*buffer_len*FPS) (11)
r_diff_s = min(0.05*r_ref, BETA_S*8*buffer_len*FPS) (12)
r_vin = max(RMIN, r_ref - r_diff_v) (13)
r_send = min(RMAX, r_ref + r_diff_s) (14)
In Equations (11) and (12), the amount of adjustment is calculated as proportional to the size of the rate-shaping buffer but is bounded by 5% of the reference rate r_ref calculated from network congestion feedback alone. This ensures that the adjustment introduced by the rate-shaping buffer will not counteract with the core congestion control process. Equations (13) and (14) indicate the influence of the rate-shaping buffer. A large rate-shaping buffer nudges the encoder target rate slightly below (and the sending rate slightly above) the reference rate r_ref. The final video target rate (r_vin) and sending rate (r_send) are further bounded within the original range of [RMIN, RMAX].
Intuitively, the amount of extra rate offset needed to completely drain the rate-shaping buffer within the duration of a single video frame is given by 8*buffer_len*FPS, where FPS stands for the reference frame rate of the video. The scaling parameters BETA_V and BETA_S can be tuned to balance between the competing goals of maintaining a small rate-shaping buffer and deviating from the reference rate point. Empirical observations show that the rate- shaping buffer for a responsive live video encoder typically stays empty and only occasionally holds a large frame (e.g., when an intra- frame is produced) in transit. Therefore, the rate adjustment introduced by this mechanism is expected to be minor. For instance, a rate-shaping buffer of 2000 bytes will lead to a rate adjustment of 48 Kbps given the recommended scaling parameters of BETA_V = 0.1 and BETA_S = 0.1, and the reference frame rate of FPS = 30.
5.3. Feedback Message Requirements
The following list of information is required for NADA congestion control to function properly:
Recommended rate adaptation mode (rmode): A 1-bit flag indicating
whether the sender should operate in accelerated ramp-up mode
(rmode=0) or gradual update mode (rmode=1).
Aggregated congestion signal (x_curr): The most recently updated
value, calculated by the receiver according to Section 4.2. This
information can be expressed with a unit of 100 microseconds
(i.e., 1/10 of a millisecond) in 15 bits. This allows a maximum
value of x_curr at approximately 3.27 seconds.
Receiving rate (r_recv): The most recently measured receiving rate
according to Section 5.1.3. This information is expressed with a
unit of bits per second (bps) in 32 bits (unsigned int). This
allows a maximum rate of approximately 4.3 Gbps, approximately
1000 times the streaming rate of a typical high-definition (HD)
video conferencing session today. This field can be expanded
further by a few more bytes if an even higher rate needs to be
specified.
The above list of information can be accommodated by 48 bits, or 6 bytes, in total. They can be either included in the feedback report from the receiver or, in the case where all receiver-side calculations are moved to the sender, derived from per-packet information from the feedback message as defined in [RTCP-FEEDBACK]. Choosing the feedback message interval DELTA is discussed in Section 6.3. A target feedback interval of DELTA = 100 ms is recommended.
6. Discussions and Further Investigations
This section discusses the various design choices made by NADA, potential alternative variants of its implementation, and guidelines on how the key algorithm parameters can be chosen. Section 8 recommends additional experimental setups to further explore these topics.
6.1. Choice of Delay Metrics
The current design works with relative one-way delay (OWD) as the
main indication of congestion. The value of the relative OWD is
obtained by maintaining the minimum value of observed OWD over a
relatively long time horizon and subtracting that out from the
observed absolute OWD value. Such an approach cancels out the fixed
difference between the sender and receiver clocks. It has been
widely adopted by other delay-based congestion control approaches
such as [RFC6817]. As discussed in [RFC6817], the time horizon for
tracking the minimum OWD needs to be chosen with care; it must be
long enough for an opportunity to observe the minimum OWD with zero
standing queue along the path, and it must be sufficiently short
enough to timely reflect "true" changes in minimum OWD introduced by
route changes and other rare events and to mitigate the cumulative
impact of clock rate skew over time.
The potential drawback in relying on relative OWD as the congestion signal is that when multiple flows share the same bottleneck, the flow arriving late at the network experiencing a non-empty queue may mistakenly consider the standing queuing delay as part of the fixed path propagation delay. This will lead to slightly unfair bandwidth sharing among the flows.
Alternatively, one could move the per-packet statistical handling to
the sender instead and use relative round-trip time (RTT) in lieu of
relative OWD, assuming that per-packet acknowledgments are available.
The main drawback of an RTT-based approach is the noise in the
measured delay in the reverse direction.
Note that the choice of either delay metric (relative OWD vs. RTT) involves no change in the proposed rate adaptation algorithm. Therefore, comparing the pros and cons regarding which delay metric to adopt can be kept as an orthogonal direction of investigation.
6.2. Method for Delay, Loss, and Marking Ratio Estimation
Like other delay-based congestion control schemes, performance of
NADA depends on the accuracy of its delay measurement and estimation
module. Appendix A of [RFC6817] provides an extensive discussion on
this aspect.
The current recommended practice of applying minimum filter with a window size of 15 samples suffices in guarding against processing delay outliers observed in wired connections. For wireless connections with a higher packet delay variation (PDV), more sophisticated techniques on denoising, outlier rejection, and trend analysis may be needed.
More sophisticated methods in packet loss ratio calculation, such as that adopted by [FLOYD-CCR00], will likely be beneficial. These alternatives are part of the experiments this document proposes.
6.3. Impact of Parameter Values
In the gradual rate update mode, the parameter TAU indicates the upper bound of round-trip time (RTT) in the feedback control loop. Typically, the observed feedback interval delta is close to the target feedback interval DELTA, and the relative ratio of delta/TAU versus ETA dictates the relative strength of influence from the aggregate congestion signal offset term (x_offset) versus its recent change (x_diff), respectively. These two terms are analogous to the integral and proportional terms in a proportional-integral (PI) controller. The recommended choice of TAU = 500 ms, DELTA = 100 ms, and ETA = 2.0 corresponds to a relative ratio of 1:10 between the gains of the integral and proportional terms. Consequently, the rate adaptation is mostly driven by the change in the congestion signal with a long-term shift towards its equilibrium value driven by the offset term. Finally, the scaling parameter KAPPA determines the overall speed of the adaptation and needs to strike a balance between responsiveness and stability.
The choice of the target feedback interval DELTA needs to strike the right balance between timely feedback and low RTCP feedback message counts. A target feedback interval of DELTA = 100 ms is recommended, corresponding to a feedback bandwidth of 16 Kbps with 200 bytes per feedback message -- approximately 1.6% overhead for a 1 Mbps flow. Furthermore, both simulation studies and frequency-domain analysis in [IETF-95] have established that a feedback interval below 250 ms (i.e., more frequently than 4 feedback messages per second) will not break up the feedback control loop of NADA congestion control.
In calculating the non-linear warping of delay in Equation (1), the current design uses fixed values of QTH for determining whether to perform the non-linear warping. Its value should be carefully tuned for different operational environments (e.g., over wired vs. wireless connections) so as to avoid the potential risk of prematurely discounting the congestion signal level. It is possible to adapt its value based on past observed patterns of queuing delay in the presence of packet losses. It needs to be noted that the non-linear warping mechanism may lead to multiple NADA streams stuck in loss- based mode when competing against each other.
In calculating the aggregate congestion signal x_curr, the choice of DMARK and DLOSS influence the steady-state packet loss/marking ratio experienced by the flow at a given available bandwidth. Higher values of DMARK and DLOSS result in lower steady-state loss/marking ratios but are more susceptible to the impact of individual packet loss/marking events. While the value of DMARK and DLOSS are fixed and predetermined in the current design, this document also encourages further explorations of a scheme for automatically tuning these values based on desired bandwidth sharing behavior in the presence of other competing loss-based flows (e.g., loss-based TCP).
6.4. Sender-Based vs. Receiver-Based Calculation
In the current design, the aggregated congestion signal x_curr is calculated at the receiver, keeping the sender operation completely independent of the form of actual network congestion indications (delay, loss, or marking) in use.
Alternatively, one can shift receiver-side calculations to the sender, whereby the receiver simply reports on per-packet information via periodic feedback messages as defined in [RTCP-FEEDBACK]. Such an approach enables interoperability amongst senders operating on different congestion control schemes but requires slightly higher overhead in the feedback messages. See additional discussions in [RTCP-FEEDBACK] regarding the desired format of the feedback messages and the recommended feedback intervals.
6.5. Incremental Deployment
One nice property of NADA is the consistent video endpoint behavior irrespective of network node variations. This facilitates gradual, incremental adoption of the scheme.
Initially, the proposed congestion control mechanism can be implemented without any explicit support from the network and relies solely on observed relative one-way delay measurements and packet loss ratios as implicit congestion signals.
When ECN is enabled at the network nodes with RED-based marking, the
receiver can fold its observations of ECN markings into the
calculation of the equivalent delay. The sender can react to these
explicit congestion signals without any modification.
Ultimately, networks equipped with proactive marking based on the
level of token bucket metering can reap the additional benefits of
zero standing queues and lower end-to-end delay and work seamlessly
with existing senders and receivers.
7. Reference Implementations
The NADA scheme has been implemented in both ns-2 [NS-2] and ns-3 [NS-3] simulation platforms. The implementation in ns-2 hosts the calculations as described in Section 4.2 at the receiver side, whereas the implementation in ns-3 hosts these receiver-side calculations at the sender for the sake of interoperability. Extensive ns-2 simulation evaluations of an earlier draft version of this document are recorded in [ZHU-PV13]. An open-source implementation of NADA as part of an ns-3 module is available at [NS3-RMCAT]. Evaluation results of this document based on ns-3 are presented in [IETF-90] and [IETF-91] for wired test cases as documented in [RMCAT-EVAL-TEST]. Evaluation results of NADA over Wi- Fi-based test cases as defined in [WIRELESS-TESTS] are presented in [IETF-93]. These simulation-based evaluations have shown that NADA flows can obtain their fair share of bandwidth when competing against each other. They typically adapt fast in reaction to the arrival and departure of other flows and can sustain a reasonable throughput when competing against loss-based TCP flows.
[IETF-90] describes the implementation and evaluation of NADA in a lab setting. Preliminary evaluation results of NADA in single-flow and multi-flow test scenarios are presented in [IETF-91].
A reference implementation of NADA has been carried out by modifying the WebRTC module embedded in the Mozilla open-source browser. Presentations from [IETF-103] and [IETF-105] document real-world evaluations of the modified browser driven by NADA. The experimental setting involves remote connections with endpoints over either home or enterprise wireless networks. These evaluations validate the effectiveness of NADA flows in recovering quickly from throughput drops caused by intermittent delay spikes over the last-hop wireless connections.
8. Suggested Experiments
NADA has been extensively evaluated under various test scenarios,
including the collection of test cases specified by [RMCAT-EVAL-TEST]
and the subset of Wi-Fi-based test cases in [WIRELESS-TESTS].
Additional evaluations have been carried out to characterize how NADA
interacts with various AQM schemes such as RED, Controlling Queue
Delay (CoDel), and Proportional Integral Controller Enhanced (PIE).
Most of these evaluations have been carried out in simulators. A few
key test cases have been evaluated in lab environments with
implementations embedded in video conferencing clients. It is
strongly recommended to carry out implementation and experimentation
of NADA in real-world settings. Such exercises will provide insights
on how to choose or automatically adapt the values of the key
algorithm parameters (see list in Table 2) as discussed in Section 6.
Additional experiments are suggested for the following scenarios, preferably over real-world networks:
- Experiments reflecting the setup of a typical WAN connection.
- Experiments with ECN marking capability turned on at the network
for existing test cases.
- Experiments with multiple NADA streams bearing different user-
specified priorities.
- Experiments with additional access technologies, especially over
cellular networks such as 3G/LTE.
- Experiments with various media source contents, including audio
only, audio and video, and application content sharing (e.g.,
slideshows).
9. IANA Considerations
This document has no IANA actions.
10. Security Considerations
The rate adaptation mechanism in NADA relies on feedback from the receiver. As such, it is vulnerable to attacks where feedback messages are hijacked, replaced, or intentionally injected with misleading information resulting in denial of service, similar to those that can affect TCP. Therefore, it is RECOMMENDED that the RTCP feedback message is at least integrity checked. In addition, [RTCP-FEEDBACK] discusses the potential risk of a receiver providing misleading congestion feedback information and the mechanisms for mitigating such risks.
The modification of the sending rate based on the sender-side rate-
shaping buffer may lead to temporary excessive congestion over the
network in the presence of an unresponsive video encoder. However,
this effect can be mitigated by limiting the amount of rate
modification introduced by the rate-shaping buffer, bounding the size
of the rate-shaping buffer at the sender, and maintaining a maximum
allowed sending rate by NADA.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
<https://www.rfc-editor.org/info/rfc5348>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[BUDZISZ-AIMD-CC]
Budzisz, L., Stanojevic, R., Schlote, A., Baker, F., and
R. Shorten, "On the Fair Coexistence of Loss- and Delay-
Based TCP", IEEE/ACM Transactions on Networking, vol. 19,
no. 6, pp. 1811-1824, DOI 10.1109/TNET.2011.2159736,
December 2011,
<https://doi.org/10.1109/TNET.2011.2159736>.
[FLOYD-CCR00]
Floyd, S., Handley, M., Padhye, J., and J. Widmer,
"Equation-based congestion control for unicast
applications", ACM SIGCOMM Computer Communications Review,
vol. 30, no. 4, pp. 43-56, DOI 10.1145/347057.347397,
October 2000, <https://doi.org/10.1145/347057.347397>.
[IETF-103] Zhu, X., Pan, R., Ramalho, M., Mena, S., Jones, P., Fu,
J., and S. D'Aronco, "NADA Implementation in Mozilla
Browser", IETF 103, November 2018,
<https://datatracker.ietf.org/meeting/103/materials/
slides-103-rmcat-nada-implementation-in-mozilla-browser-
00>.
[IETF-105] Zhu, X., Pan, R., Ramalho, M., Mena, S., Jones, P., Fu,
J., and S. D'Aronco, "NADA Implementation in Mozilla
Browser and Draft Update", IETF 105, July 2019,
<https://datatracker.ietf.org/meeting/105/materials/
slides-105-rmcat-nada-update-02.pdf>.
[IETF-90] Zhu, X., Ramalho, M., Ganzhorn, C., Jones, P., and R. Pan,
"NADA Update: Algorithm, Implementation, and Test Case
Evaluation Results", IETF 90, July 2014,
<https://tools.ietf.org/agenda/90/slides/slides-90-rmcat-
6.pdf>.
[IETF-91] Zhu, X., Pan, R., Ramalho, M., Mena, S., Ganzhorn, C.,
Jones, P., and S. D'Aronco, "NADA Algorithm Update and
Test Case Evaluations", IETF 91, November 2014,
<https://www.ietf.org/proceedings/interim/2014/11/09/
rmcat/slides/slides-interim-2014-rmcat-1-2.pdf>.
[IETF-93] Zhu, X., Pan, R., Ramalho, M., Mena, S., Ganzhorn, C.,
Jones, P., D'Aronco, S., and J. Fu, "Updates on NADA",
IETF 93, July 2015,
<https://www.ietf.org/proceedings/93/slides/slides-93-
rmcat-0.pdf>.
[IETF-95] Zhu, X., Pan, R., Ramalho, M., Mena, S., Jones, P., Fu,
J., D'Aronco, S., and C. Ganzhorn, "Updates on NADA:
Stability Analysis and Impact of Feedback Intervals", IETF
95, April 2016,
<https://www.ietf.org/proceedings/95/slides/slides-95-
rmcat-5.pdf>.
[NS-2] "ns-2", December 2014,
<http://nsnam.sourceforge.net/wiki/index.php/Main_Page>.
[NS-3] "ns-3 Network Simulator", <https://www.nsnam.org/>.
[NS3-RMCAT]
Fu, J., Mena, S., and X. Zhu, "Simulator of IETF RMCAT
congestion control protocols", November 2017,
<https://github.com/cisco/ns3-rmcat>.
[RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in
RTP Streams", RFC 5450, DOI 10.17487/RFC5450, March 2009,
<https://www.rfc-editor.org/info/rfc5450>.
[RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
Pre-Congestion Notification (PCN) States in the IP Header
Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
DOI 10.17487/RFC6660, July 2012,
<https://www.rfc-editor.org/info/rfc6660>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.
[RFC8593] Zhu, X., Mena, S., and Z. Sarker, "Video Traffic Models
for RTP Congestion Control Evaluations", RFC 8593,
DOI 10.17487/RFC8593, May 2019,
<https://www.rfc-editor.org/info/rfc8593>.
[RMCAT-CC] Jesup, R. and Z. Sarker, "Congestion Control Requirements
for Interactive Real-Time Media", Work in Progress,
Internet-Draft, draft-ietf-rmcat-cc-requirements-09, 12
December 2014, <https://tools.ietf.org/html/draft-ietf-
rmcat-cc-requirements-09>.
[RMCAT-CC-RTP]
Zanaty, M., Singh, V., Nandakumar, S., and Z. Sarker,
"Congestion Control and Codec interactions in RTP
Applications", Work in Progress, Internet-Draft, draft-
ietf-rmcat-cc-codec-interactions-02, 18 March 2016,
<https://tools.ietf.org/html/draft-ietf-rmcat-cc-codec-
interactions-02>.
[RMCAT-EVAL-TEST]
Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating RMCAT Proposals", Work in Progress,
Internet-Draft, draft-ietf-rmcat-eval-test-10, 23 May
2019, <https://tools.ietf.org/html/draft-ietf-rmcat-eval-
test-10>.
[RTCP-FEEDBACK]
Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
Control Protocol (RTCP) Feedback for Congestion Control",
Work in Progress, Internet-Draft, draft-ietf-avtcore-cc-
feedback-message-05, 4 November 2019,
<https://tools.ietf.org/html/draft-ietf-avtcore-cc-
feedback-message-05>.
[WIRELESS-TESTS]
Sarker, Z., Johansson, I., Zhu, X., Fu, J., Tan, W., and
M. Ramalho, "Evaluation Test Cases for Interactive Real-
Time Media over Wireless Networks", Work in Progress,
Internet-Draft, draft-ietf-rmcat-wireless-tests-08, 5 July
2019, <https://tools.ietf.org/html/draft-ietf-rmcat-
wireless-tests-08>.
[ZHU-PV13] Zhu, X. and R. Pan, "NADA: A Unified Congestion Control
Scheme for Low-Latency Interactive Video", Proc. IEEE
International Packet Video Workshop, San Jose, CA, USA,
DOI 10.1109/PV.2013.6691448, December 2013,
<https://doi.org/10.1109/PV.2013.6691448>.
Appendix A. Network Node Operations
NADA can work with different network queue management schemes and does not assume any specific network node operation. As an example, this appendix describes three variants of queue management behavior at the network node, leading to either implicit or explicit congestion signals. It needs to be acknowledged that NADA has not yet been tested with non-probabilistic ECN marking behaviors.
In all three flavors described below, the network queue operates with the simple First In, First Out (FIFO) principle. There is no need to maintain per-flow state. The system can scale easily with a large number of video flows and at high link capacity.
A.1. Default Behavior of Drop-Tail Queues
In a conventional network with drop-tail or RED queues, congestion is inferred from the estimation of end-to-end delay and/or packet loss. Packet drops at the queue are detected at the receiver and contribute to the calculation of the aggregated congestion signal x_curr. No special action is required at the network node.
A.2. RED-Based ECN Marking
In this mode, the network node randomly marks the ECN field in the IP packet header following the Random Early Detection (RED) algorithm [RFC7567]. Calculation of the marking probability involves the following steps on packet arrival:
1. update smoothed queue size q_avg as:
q_avg = w*q + (1-w)*q_avg
2. calculate marking probability p as:
/ 0, if q < q_lo
|
| q_avg - q_lo
p= < p_max*--------------, if q_lo <= q < q_hi
| q_hi - q_lo
|
\ p = 1, if q >= q_hi
Here, q_lo and q_hi correspond to the low and high thresholds of queue occupancy. The maximum marking probability is p_max.
The ECN marking events will contribute to the calculation of an equivalent delay x_curr at the receiver. No changes are required at the sender.
A.3. Random Early Marking with Virtual Queues
Advanced network nodes may support random early marking based on a token bucket algorithm originally designed for Pre-Congestion Notification (PCN) [RFC6660]. The early congestion notification (ECN) bit in the IP header of packets is marked randomly. The marking probability is calculated based on a token bucket algorithm originally designed for PCN [RFC6660]. The target link utilization is set as 90%; the marking probability is designed to grow linearly with the token bucket size when it varies between 1/3 and 2/3 of the full token bucket limit.
Calculation of the marking probability involves the following steps upon packet arrival:
1. meter packet against token bucket (r,b)
2. update token level b_tk
3. calculate the marking probability as:
/ 0, if b-b_tk < b_lo
|
| b-b_tk-b_lo
p = < p_max* --------------, if b_lo <= b-b_tk < b_hi
| b_hi-b_lo
|
\ 1, if b-b_tk >= b_hi
Here, the token bucket lower and upper limits are denoted by b_lo and b_hi, respectively. The parameter b indicates the size of the token bucket. The parameter r is chosen to be below capacity, resulting in slight underutilization of the link. The maximum marking probability is p_max.
The ECN marking events will contribute to the calculation of an
equivalent delay x_curr at the receiver. No changes are required at
the sender. The virtual queuing mechanism from the PCN-based marking
algorithm will lead to additional benefits such as zero standing
queues.
Acknowledgments
The authors would like to thank Randell Jesup, Luca De Cicco, Piers O'Hanlon, Ingemar Johansson, Stefan Holmer, Cesar Ilharco Magalhaes, Safiqul Islam, Michael Welzl, Mirja Kühlewind, Karen Elisabeth Egede Nielsen, Julius Flohr, Roland Bless, Andreas Smas, and Martin Stiemerling for their valuable review comments and helpful input to this specification.
Contributors
The following individuals contributed to the implementation and evaluation of the proposed scheme and, therefore, helped to validate and substantially improve this specification.
Paul E. Jones <paulej@packetizer.com> of Cisco Systems implemented an
early version of the NADA congestion control scheme and helped with
its lab-based testbed evaluations.
Jiantao Fu <jianfu@cisco.com> of Cisco Systems helped with the
implementation and extensive evaluation of NADA both in Mozilla web
browsers and in earlier simulation-based evaluation efforts.
Stefano D'Aronco <stefano.daronco@geod.baug.ethz.ch> of ETH Zurich (previously at Ecole Polytechnique Federale de Lausanne when contributing to this work) helped with the implementation and evaluation of an early version of NADA in [NS-3].
Charles Ganzhorn <charles.ganzhorn@gmail.com> contributed to the
testbed-based evaluation of NADA during an early stage of its
development.
Authors' Addresses
Xiaoqing Zhu Cisco Systems 12515 Research Blvd., Building 4 Austin, TX 78759 United States of America
Email: xiaoqzhu@cisco.com
Rong Pan Intel Corporation 2200 Mission College Blvd Santa Clara, CA 95054 United States of America
Email: rong.pan@intel.com
Michael A. Ramalho AcousticComms Consulting 6310 Watercrest Way Unit 203 Lakewood Ranch, FL 34202-5211 United States of America
Phone: +1 732 832 9723 Email: mar42@cornell.edu URI: http://ramalho.webhop.info/
Sergio Mena Cisco Systems EPFL, Quartier de l'Innovation, Batiment E CH-1015 Ecublens Switzerland
Email: semena@cisco.com
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