Internet DRAFT - draft-ietf-ledbat-congestion
draft-ietf-ledbat-congestion
LEDBAT WG S. Shalunov
Internet-Draft G. Hazel
Intended status: Experimental BitTorrent Inc
Expires: March 29, 2013 J. Iyengar
Franklin and Marshall College
M. Kuehlewind
University of Stuttgart
September 25, 2012
Low Extra Delay Background Transport (LEDBAT)
draft-ietf-ledbat-congestion-10.txt
Abstract
LEDBAT is an experimental delay-based congestion control algorithm
that seeks to utilize the available bandwidth on an end-to-end path
while limiting the consequent increase in queueing delay on that
path. LEDBAT uses changes in one-way delay measurements to limit
congestion that the flow itself induces in the network. LEDBAT is
designed for use by background bulk-transfer applications to be no
more aggressive than standard TCP congestion control (as specified in
RFC5681) and to yield in the presence of competing flows, thus
limiting interference with the network performance of competing
flows.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 29, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
3. LEDBAT Congestion Control . . . . . . . . . . . . . . . . . . 6
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Preliminaries . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Receiver-Side Operation . . . . . . . . . . . . . . . . . 7
3.4. Sender-Side Operation . . . . . . . . . . . . . . . . . . 7
3.4.1. An Overview . . . . . . . . . . . . . . . . . . . . . 7
3.4.2. The Complete Sender Algorithm . . . . . . . . . . . . 8
3.5. Parameter Values . . . . . . . . . . . . . . . . . . . . . 11
4. Understanding LEDBAT Mechanisms . . . . . . . . . . . . . . . 13
4.1. Delay Estimation . . . . . . . . . . . . . . . . . . . . . 13
4.1.1. Estimating Base Delay . . . . . . . . . . . . . . . . 13
4.1.2. Estimating Queueing Delay . . . . . . . . . . . . . . 13
4.2. Managing the Congestion Window . . . . . . . . . . . . . . 14
4.2.1. Window Increase: Probing For More Bandwidth . . . . . 14
4.2.2. Window Decrease: Responding To Congestion . . . . . . 14
4.3. Choosing The Queuing Delay Target . . . . . . . . . . . . 14
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Framing and ACK Frequency Considerations . . . . . . . . . 15
5.2. Competing With TCP Flows . . . . . . . . . . . . . . . . . 15
5.3. Competing With Non-TCP Flows . . . . . . . . . . . . . . . 16
5.4. Fairness Among LEDBAT Flows . . . . . . . . . . . . . . . 16
6. Open Areas for Experimentation . . . . . . . . . . . . . . . . 17
6.1. Network Effects and Monitoring . . . . . . . . . . . . . . 17
6.2. Parameter Values . . . . . . . . . . . . . . . . . . . . . 18
6.3. Filters . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.4. Framing . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Measurement Errors . . . . . . . . . . . . . . . . . 21
A.1. Clock Offset . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. Clock Skew . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
TCP congestion control [RFC5681] seeks to share bandwidth at a
bottleneck link equitably among flows competing at the bottleneck,
and it is the predominant congestion control mechanism used on the
Internet. However, not all applications seek an equitable share of
network throughput. "Background" applications, such as software
updates or file-sharing applications, seek to operate without
interfering with the performance of more interactive and delay-
and/or bandwidth-sensitive "foreground" applications. Standard TCP
congestion control, as specified in [RFC5681], may be too aggressive
for use with such background applications.
LEDBAT is an experimental delay-based congestion control mechanism
that reacts early to congestion in the network, thus enabling
"background" applications to use the network while avoiding
interference with the network performance of competing flows. A
LEDBAT sender uses one-way delay measurements to estimate the amount
of queueing on the data path, controls the LEDBAT flow's congestion
window based on this estimate, and minimizes interference with
competing flows by adding low extra queueing delay on the end-to-end
path.
Delay-based congestion control protocols, such as TCP-Vegas
[Bra94][Low02], are generally designed to achieve more, not less
throughput than standard TCP, and often outperform TCP under
particular network settings. For further discussion on Lower-than-
Best-Effort (BE) transport protocols see [RFC6297]. In contrast,
LEDBAT is designed to be no more aggressive than TCP [RFC5681];
LEDBAT is a "scavenger" congestion control mechanism that seeks to
utilize all available bandwidth and yields quickly when competing
with standard TCP at a bottleneck link.
In the rest of this document, we refer to [RFC5681]-specified
congestion control as "standard TCP".
2.1. Design Goals
LEDBAT congestion control seeks to achieve the following goals:
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1. to utilize end-to-end available bandwidth, and to maintain low
queueing delay when no other traffic is present,
2. to add limited queuing delay to that induced by concurrent flows,
and
3. to yield quickly to standard TCP flows that share the same
bottleneck link.
2.2. Applicability
LEDBAT is a "scavenger" congestion control mechanism that is
motivated primarily by for background bulk-transfer applications,
such as large file transfers (as with file-sharing applications) and
software updates. It can be used with any application that seeks to
minimize its impact on the network and on other interactive delay-
and/or bandwidth-sensitive network applications. LEDBAT is expected
to work well when the sender and/or receiver is connected via a
residential access network.
LEDBAT can be used as part of a transport protocol or as part of an
application, as long as the data transmission mechanisms are capable
of carrying timestamps and acknowledging data frequently. LEDBAT can
be used with TCP, SCTP, and DCCP, with appropriate extensions where
necessary, and with proprietary application protocols, such as those
built on top of UDP for P2P applications.
When used with an ECN-capable framing protocol, LEDBAT should react
to an ECN mark as it would to a loss, as specified in [RFC3168].
LEDBAT is designed to reduce build-up of a standing queue by long-
lived LEDBAT flows at a link with a tail-drop FIFO queue, so as to
avoid persistently delaying other flows sharing the queue. If Active
Queue Management (AQM) is configured to drop or ECN-mark packets
before the LEDBAT flow starts reacting to persistent queue build-up,
LEDBAT reverts to standard TCP behavior rather than yielding to other
TCP flows. However, such an AQM is still desirable since it keeps
queuing delay low, achieving an outcome that is in line with LEDBAT's
goals. Additionally, a LEDBAT transport that supports ECN enjoys the
advantages that an ECN-capable TCP enjoys over an ECN-agnostic TCP;
avoiding losses and possible retransmissions. Weighted Fair Queuing
(WFQ), as employed by some home gateways, seeks to isolate and
protect delay-sensitive flows from delays due to standing queues
built up by concurrent long-lived flows. Consequently, while it
prevents LEDBAT from yielding to other TCP flows, it again achieves
an outcome that is in line with LEDBAT's goals [Sch10].
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3. LEDBAT Congestion Control
3.1. Overview
A standard TCP sender increases its congestion window until a loss
occurs [RFC5681] or an ECN mark is received [RFC3168], which, in the
absence of any AQM and link errors in the network, occurs only when
the queue at the bottleneck link on the end-to-end path overflows.
Since packet loss or marking at the bottleneck link is expected to be
preceded by an increase in the queueing delay at the bottleneck link,
LEDBAT congestion control uses this increase in queueing delay as an
early signal of congestion, enabling it to respond to congestion
earlier than standard TCP, and enabling it to yield bandwidth to a
competing TCP flow.
LEDBAT employs one-way delay measurements to estimate queueing delay.
When the estimated queueing delay is less than a pre-determined
target, LEDBAT infers that the network is not yet congested, and
increases its sending rate to utilize any spare capacity in the
network. When the estimated queueing delay becomes greater than the
pre-determined target, LEDBAT decreases its sending rate as a
response to potential congestion in the network.
3.2. Preliminaries
A LEDBAT sender uses a congestion window (cwnd) to gate the amount of
data that the sender can send into the network in one roundtrip time
(RTT). A sender MAY maintain its cwnd in bytes or in packets; this
document uses cwnd in bytes. LEDBAT requires that each data segment
carries a "timestamp" from the sender, based on which the receiver
computes the one-way delay from the sender and sends this computed
value back to the sender.
In addition to the LEDBAT mechanism described below, we note that a
slow start mechanism can be used as specified in [RFC5681]. Since
slow start leads to faster increase in the window than that specified
in LEDBAT, conservative congestion control implementations employing
LEDBAT may skip slow start altogether and start with an initial
window of INIT_CWND * MSS. (INIT_CWND is described later in
Section 3.5.)
The term "MSS", or the sender's Maximum Segment Size, used in this
document refers to the size of the largest segment that the sender
can transmit. The value of MSS can be based on the path MTU
discovery [RFC4821] algorithm and/or on other factors.
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3.3. Receiver-Side Operation
A LEDBAT receiver calculates the one-way delay from the sender to the
receiver based on its own system time and timestamps in the received
data packets. The receiver then feeds the computed one-way delay
back to the sender in the next acknowledgement. A LEDBAT receiver
operates as follows:
on data_packet:
remote_timestamp = data_packet.timestamp
acknowledgement.delay = local_timestamp() - remote_timestamp
# fill in other fields of acknowledgement
acknowlegement.send()
A receiver may choose to delay sending an ACK and may combine
acknowledgements for more than one data packet into a single ACK
packet, as is with delayed ACKs in standard TCP, for example. In
such cases, the receiver MAY bundle all the delay samples into one
ACK packet and MUST transmit the samples in the order generated.
When multiple delay samples are bundled within a single ACK, the
sender applies these bundled delay samples at once during its cwnd
adjustment (discussed in the next section). Since the sender's
adjustment may be sensitive to the order in which the delay samples
are applied, the computed delay samples should be available to the
sender in the order they were generated at the receiver.
3.4. Sender-Side Operation
3.4.1. An Overview
As a first approximation, a LEDBAT sender operates as shown below;
the complete algorithm is specified later in Section 3.4.2. TARGET
is the maximum queueing delay that LEDBAT itself may introduce in the
network, and GAIN determines the rate at which the cwnd responds to
changes in queueing delay; both constants are specified later.
off_target is a normalized value representing the difference between
the measured current queueing delay and the pre-determined TARGET
delay. off_target can be positive or negative, consequently, cwnd
increases or decreases in proportion to off_target.
on initialization:
base_delay = +INFINITY
on acknowledgement:
current_delay = acknowledgement.delay
base_delay = min(base_delay, current_delay)
queuing_delay = current_delay - base_delay
off_target = (TARGET - queuing_delay) / TARGET
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cwnd += GAIN * off_target * bytes_newly_acked * MSS / cwnd
The simplified mechanism above ignores multiple delay samples in an
acknowledgment, noise filtering, base delay expiration, and sender
idle times, which we now take into account in our complete sender
algorithm below.
3.4.2. The Complete Sender Algorithm
update_current_delay() maintains a list of one-way delay
measurements, of which a filtered value is used as an estimate of the
current end-to-end delay. update_base_delay() maintains a list of
one-way delay minima over a number of one-minute intervals, to
measure and to track changes in the base delay of the end-to-end
path. Both of these lists are maintained per LEDBAT flow.
We note this algorithm assumes that slight random fluctuations exist
in inter-packet arrival times at the bottleneck queue, to allow a
LEDBAT sender to correctly measure the base delay. See section
Section 5.4 for a more complete discussion.
The full sender-side algorithm is given below:
on initialization:
# cwnd is the amount of data that is allowed to be
# outstanding in an RTT and is defined in bytes.
# CTO is the Congestion Timeout value.
create current_delays list with CURRENT_FILTER elements
create base_delays list with BASE_HISTORY number of elements
initialize elements in base_delays to +INFINITY
initialize elements in current_delays according to FILTER()
last_rollover = -INFINITY # More than a minute in the past
flightsize = 0
cwnd = INIT_CWND * MSS
CTO = 1 second
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on acknowledgment:
# flightsize is the amount of data outstanding before this ACK
# was received and is updated later;
# bytes_newly_acked is the number of bytes that this ACK
# newly acknowledges, and it MAY be set to MSS.
for each delay sample in the acknowledgment:
delay = acknowledgement.delay
update_base_delay(delay)
update_current_delay(delay)
queuing_delay = FILTER(current_delays) - MIN(base_delays)
off_target = (TARGET - queuing_delay) / TARGET
cwnd += GAIN * off_target * bytes_newly_acked * MSS / cwnd
max_allowed_cwnd = flightsize + ALLOWED_INCREASE * MSS
cwnd = min(cwnd, max_allowed_cwnd)
cwnd = max(cwnd, MIN_CWND * MSS)
flightsize = flightsize - bytes_newly_acked
update_CTO()
on data loss:
# at most once per RTT
cwnd = min (cwnd, max (cwnd/2, MIN_CWND * MSS))
if data lost is not to be retransmitted:
flightsize = flightsize - bytes_not_to_be_retransmitted
if no ACKs are received within a CTO:
# extreme congestion, or significant RTT change.
# set cwnd to 1MSS and backoff the congestion timer.
cwnd = 1 * MSS
CTO = 2 * CTO
update_CTO()
# implements an RTT estimation mechanism using data
# transmission times and ACK reception times,
# which is used to implement a congestion timeout (CTO).
# If implementing LEDBAT in TCP, sender SHOULD use
# mechanisms described in RFC 6298 [RFC6298],
# and the CTO would be the same as the RTO.
update_current_delay(delay)
# Maintain a list of CURRENT_FILTER last delays observed.
delete first item in current_delays list
append delay to current_delays list
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update_base_delay(delay)
# Maintain BASE_HISTORY delay-minima.
# Each minimum is measured over a period of a minute.
# 'now' is the current system time
if round_to_minute(now) != round_to_minute(last_rollover)
last_rollover = now
delete first item in base_delays list
append delay to base_delays list
else
base_delays.tail = MIN(base_delays.tail, delay)
The LEDBAT sender seeks to extract the actual delay estimate from the
current_delay samples by implementing FILTER() to eliminate any
outliers. Different types of filters MAY be used for FILTER() --- a
NULL filter, that does not filter at all, is a reasonable candidate
as well, since LEDBAT's use of a linear controller for cwnd increase
and decrease may allow it to recover quickly from errors induced by
bad samples. Another example of a filter is the Exponentially-
Weighted Moving Average (EWMA) function, with weights that enable
agile tracking of changing network delay. A simple MIN filter
applied over a small window (much smaller than BASE_HISTORY) may also
provide robustness to large delay peaks, as may occur with delayed
ACKs in TCP. Care should be taken that the filter used, while
providing robustness to noise, remains sensitive to persistent
congestion signals.
We note that when multiple delay samples are bundled within a single
ACK, the sender's resulting cwnd may be slightly different than when
the samples are sent individually in separate ACKs. The cwnd is
adjusted based on the total number of bytes ACKed and the final
filtered value of queueing_delay, irrespective of the number of delay
samples in an ACK.
To implement an approximate minimum over the past few minutes, a
LEDBAT sender stores BASE_HISTORY separate minima---one each for the
last BASE_HISTORY-1 minutes, and one for the running current minute.
At the end of the current minute, the window moves---the earliest
minimum is dropped and the latest minimum is added. If the
connection is idle for a given minute, no data is available for the
one-way delay and, therefore, a value of +INFINITY has to be stored
in the list. If the connection has been idle for BASE_HISTORY
minutes, all minima in the list are thus set to +INFINITY and
measurement begins anew. LEDBAT thus requires that during idle
periods, an implementation must maintain the base delay list.
LEDBAT restricts cwnd growth after a period of inactivity. When the
sender is application-limited, the sender's cwnd is clamped down
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using max_allowed_cwnd to a little more than flightsize. To be TCP-
friendly, LEDBAT halves its cwnd on data loss.
LEDBAT uses a congestion timeout (CTO) to avoid transmitting data
during periods of heavy congestion, and to avoid congestion collapse.
A CTO is used to detect heavy congestion indicated by loss of all
outstanding data or acknowledgments, resulting in reduction of the
cwnd to 1 MSS and an exponential backoff of the CTO interval. This
backoff of the CTO value avoids sending more data into an overloaded
queue, and also allows the sender to cope with sudden changes in the
RTT of the path. The function of a CTO is similar to that of an
retransmission timeout (RTO) in TCP [RFC6298], but since LEDBAT
separates reliability from congestion control, a retransmission need
not be triggered by a CTO. LEDBAT, however does not preclude a CTO
from triggering retransmissions, as could be the case if LEDBAT
congestion control were to be used with TCP framing and reliability.
The CTO is a gating mechanism that ensures exponential backoff of
sending rate under heavy congestion, and it may be implemented with
or without a timer. An implementation choosing to avoid timers may
consider using a "next-time-to-send" variable, set based on the CTO,
to control the earliest time a sender may transmit without receiving
any ACKs. A maximum value MAY be placed on the CTO, and if placed,
it MUST be at least 60 seconds.
3.5. Parameter Values
TARGET MUST be 100 milliseconds or less, and this choice of value is
explained further in Section 4.3. Note that using the same TARGET
value across LEDBAT flows enables equitable sharing of the bottleneck
bandwidth. A flow with a higher TARGET value than other competing
LEDBAT flows may get a larger share of the bottleneck bandwidth. It
is possible to consider the use of different TARGET values for
implementing a relative priority between two competing LEDBAT flows
by setting a higher TARGET value for the higher-priority flow.
ALLOWED_INCREASE SHOULD be 1, and it MUST be greater than 0. An
ALLOWED_INCREASE of 0 results in no cwnd growth at all, and an
ALLOWED_INCREASE of 1 allows and limits the cwnd increase based on
flightsize in the previous RTT. An ALLOWED_INCREASE greater than 1
MAY be used when interactions between LEDBAT and the framing protocol
provide a clear reason for doing so.
GAIN MUST be set to 1 or less. A GAIN of 1 limits the maximum cwnd
ramp-up to the same rate as TCP Reno in Congestion Avoidance. While
this document specifies the use of the same GAIN for both cwnd
increase (when off_target is greater than zero) and decrease (when
off_target is less than zero), implementations MAY use a higher GAIN
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for cwnd decrease than for the increase; our justification follows.
When a competing non-LEDBAT flow increases its sending rate, the
LEDBAT sender may only measure a small amount of additional delay and
decrease the sending rate slowly. To ensure no impact on a competing
non-LEDBAT flow, the LEDBAT flow should decrease its sending rate at
least as quickly as the competing flow increases its sending rate. A
higher decrease-GAIN MAY be used to allow the LEDBAT flow to decrease
its sending rate faster than the competing flow's increase rate.
The size of the base_delays list, BASE_HISTORY, SHOULD be 10. If the
actual base delay decreases, due to a route change for instance, a
LEDBAT sender adapts immediately, irrespective of the value of
BASE_HISTORY. If the actual base delay increases however, a LEDBAT
sender will take BASE_HISTORY minutes to adapt and may wrongly infer
a little more extra delay than intended (TARGET) in the meanwhile. A
value for BASE_HISTORY is thus a tradeoff: a higher value may yield a
more accurate measurement when the base delay is unchanging, and a
lower value results in a quicker response to actual increase in base
delay.
A LEDBAT sender uses the current_delays list to maintain only delay
measurements made within a RTT amount of time in the past, seeking to
eliminate noise spikes in its measurement of the current one-way
delay through the network. The size of this list, CURRENT_FILTER,
may be variable, and depends on the FILTER() function as well as the
number of successful measurements made within a RTT amount of time in
the past. The sender should seek to gather enough delay samples in
each RTT so as to have statistical confidence in the measurements.
While the number of delay samples required for such confidence will
vary depending on network conditions, we recommend that the sender
SHOULD use at least 4 samples in each RTT, unless the number of
samples is lower due to a small congestion window. Thus, subject to
congestion window constraints, the number of delay samples in each
RTT SHOULD be at least 4. The value of CURRENT_FILTER will depend on
the filter being employed, but CURRENT_FILTER MUST be limited such
that samples in the list are not older than an RTT in the past.
INIT_CWND and MIN_CWND SHOULD both be 2. An INIT_CWND of 2 should
help seed FILTER() at the sender when there are no samples at the
beginning of a flow, and a MIN_CWND of 2 allows FILTER() to use more
than a single instantaneous delay estimate while not being too
aggressive. Slight deviations may be warranted, for example, when
these values of INIT_CWND and MIN_CWND interact poorly with the
framing protocol. However, INIT_CWND and MIN_CWND MUST be no larger
than the corresponding values specified for TCP [RFC5681].
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4. Understanding LEDBAT Mechanisms
This section describes the delay estimation and window management
mechanisms used in LEDBAT.
4.1. Delay Estimation
LEDBAT estimates congestion in the direction of the data flow, and to
avoid measuring additional delay from e.g. queue build-up on the
reverse path (or ACK path) or reordering, LEDBAT uses one-way delay
estimates. LEDBAT assumes measurements are done with data packets,
thus avoiding the need for separate measurement packets and avoiding
the pitfall of measurement packets being treated differently from the
data packets in the network.
End-to-end delay can be decomposed into transmission (or
serialization) delay, propagation (or speed-of-light) delay, queueing
delay, and processing delay. On any given path, barring some noise,
all delay components except for queueing delay are constant. To
observe an increase in the queueing delay in the network, a LEDBAT
sender separates the queueing delay component from the rest of the
end-to-end delay, as described below.
4.1.1. Estimating Base Delay
Since queuing delay is always additive to the end-to-end delay,
LEDBAT estimates the sum of the constant delay components, which we
call "base delay", to be the minimum delay observed on the end-to-end
path.
To respond to true changes in the base delay, as can be caused by a
route change, LEDBAT uses only recent measurements in estimating the
base delay. The duration of the observation window itself is a
tradeoff between robustness of measurement and responsiveness to
change---a larger observation window increases the chances that the
true base delay will be detected (as long as the true base delay is
unchanged), whereas a smaller observation window results in faster
response to true changes in the base delay.
4.1.2. Estimating Queueing Delay
Assuming that the base delay is constant (in the absence of any route
changes), the queueing delay is represented by the variable component
of the measured end-to-end delay. LEDBAT measures queueing delay as
simply the difference between an end-to-end delay measurement and the
current estimate of base delay. The queueing delay should be
filtered (depending on the usage scenario) to eliminate noise in the
delay estimation, such as due to spikes in processing delay at a node
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on the path.
4.2. Managing the Congestion Window
LEDBAT uses a simple linear controller to determine the sending rate
as a function of the delay estimate, where the response of the sender
is proportional to the difference between the current queueing delay
estimate and the target.
4.2.1. Window Increase: Probing For More Bandwidth
When the queuing delay is smaller than a delay target value, as
specified by the TARGET parameter in this document, a LEDBAT sender
will increase its congestion window proportionally to the relative
difference between the current queueing delay and the delay target.
As the current queuing delay gets closer to TARGET, LEDBAT's window
growth gets slower. To compete fairly with concurrent TCP flows, we
set the highest rate of LEDBAT's window growth (when the current
queueing delay estimate is zero) to be the same as TCP's (increase of
one packet per RTT). In other words, a LEDBAT flow thus never ramps
up faster than a competing TCP flow over the same path. The TARGET
value specifies the maximum extra queuing delay that LEDBAT will
induce. If the current queuing delay equals the TARGET value, LEDBAT
tries to maintain this extra delay. This is done by a very slow
increase rate (1 packet all couple of RTTs) in this state.
4.2.2. Window Decrease: Responding To Congestion
When a sender's queueing delay estimate is higher than the target,
the LEDBAT flow's rate should be reduced. LEDBAT's linear controller
allows the sender to decrease the window proportional to the
difference between the target and the current queueing delay.
Unlike TCP-like loss-based congestion control, LEDBAT seeks to avoid
losses and so a LEDBAT sender is not expected to normally rely on
losses to determine the sending rate. However, when data loss does
occur, LEDBAT must respond as standard TCP does; even if the queueing
delay estimates indicate otherwise, a loss is assumed to be a strong
indication of congestion. Thus, to deal with severe congestion when
packets are dropped in the network, and to provide a fallback against
incorrect queuing delay estimates, a LEDBAT sender halves its
congestion window when a loss event is detected. As with TCP New-
Reno, LEDBAT reduces its cwnd by half at most once per RTT.
4.3. Choosing The Queuing Delay Target
The International Telecommunication Union's (ITU's) Recommendation
G.114 defines a one-way delay of 150 ms to be acceptable for most
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user voice applications [g114]. Thus the delay induced by LEDBAT
must be well below 150 ms to limit its impact on concurrent delay-
sensitive traffic sharing the same bottleneck queue. A target that
is too low, on the other hand, increases the sensitivity of the
sender's algorithm to noise in the one-way delays and in the delay
measurement process, and may lead to reduced throughput for the
LEDBAT flow and to under-utilization of the bottleneck link.
Our recommendation of 100 ms or less as the target is a tradeoff
between these considerations. Anecdotal evidence indicates that this
value works well---LEDBAT has been implemented and successfully
deployed with a target value of 100 ms in two Bittorrent
implementations: as the exclusive congestion control mechanism in
BitTorrent Delivery Network Accelerator (DNA), and as an experimental
mechanism in uTorrent [uTorrent].
5. Discussion
5.1. Framing and ACK Frequency Considerations
While the actual framing and wire format of the protocols using
LEDBAT are outside the scope of this document, we briefly consider
the data framing and ACK frequency needs of LEDBAT mechanisms.
To compute the data path's one-way delay, our discussion of LEDBAT
assumes a framing that allows the sender to timestamp packets and for
the receiver to convey the measured one-way delay back to the sender
in ACK packets. LEDBAT does not require this particular method, but
it does require unambiguous delay estimates using data and ACK
packets.
A LEDBAT receiver may send an ACK as frequently as one for every data
packet received or less frequently; LEDBAT does require that the
receiver MUST transmit at least one ACK in every RTT.
5.2. Competing With TCP Flows
LEDBAT is designed to respond to congestion indications earlier than
loss-based standard TCP [RFC5681]. A LEDBAT flow gets more
aggressive as the queueing delay estimate gets lower; since the
queueing delay estimate is non-negative, LEDBAT is most aggressive
when the queueing delay estimate is zero. In this case, LEDBAT ramps
up its congestion window at the same rate as standard TCP [RFC5681].
LEDBAT may reduce its rate earlier than standard TCP and always
halves its congestion window on loss. Thus, in the worst case, where
the delay estimates are completely and consistently off, a LEDBAT
flow falls back to standard TCP behavior, and is no more aggressive
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than standard TCP [RFC5681].
5.3. Competing With Non-TCP Flows
While LEDBAT yields to all high-load flows, both TCP and non-TCP,
LEDBAT may not yield to low-load and latency-sensitive traffic that
do not induce a measurable delay at the bottleneck queue, such as
VoIP traffic. While such flows will experience additional delay due
to any concurrent LEDBAT flows, the TARGET delay sets a limit to the
total amount of additional delay that all the concurrent LEDBAT flows
will jointly induce. If the TARGET delay is higher than what the
bottleneck queue can sustain, the LEDBAT flows should experience loss
and will fall back to standard loss-based TCP behavior. Thus, in the
worst case, LEDBAT will add no more latency than standard TCP when
competing with non-TCP flows. In the common case however, we expect
LEDBAT flows to add TARGET amount of delay, which ought to be within
the delay tolerance for most latency-sensitive applications,
including VoIP applications.
5.4. Fairness Among LEDBAT Flows
The primary design goals of LEDBAT are focussed on the aggregate
behavior of LEDBAT flows when they compete with standard TCP. Since
LEDBAT is designed for background traffic, we consider link
utilization to be more important than fairness amongst LEDBAT flows.
Nevertheless, we now consider fairness issues that might arise
amongst competing LEDBAT flows.
LEDBAT as described so far lacks a mechanism specifically designed to
equalize utilization amongst LEDBAT flows. Anecdotally observed
behavior of existing implementations indicates that a rough
equalization does occur since in most enviroments some amount of
randomness in the inter-packet transmission times exist, as explained
further below.
Delay-based congestion control systems suffer from the possibility of
late-comers incorrectly measuring and using a higher base-delay than
an active flow that started earlier. Consider that a bottleneck is
saturated by a single LEDBAT flow, and the flow therefore maintains
the bottleneck queue at TARGET delay. When a new LEDBAT flow arrives
at the bottleneck, it might incorrectly include the steady queueing
delay in its measurement of the base delay on the path. The new flow
has an inflated estimate of the base delay, and may now effectively
build on top of the existing, already maximal, queueing delay. As
the late-comer flow builds up, the old flow sees the true queueing
delay and backs off, while the late-comer keeps building up, using up
the entire link's capacity, and effectively shutting the old flow
out. This advantage is called the "late-comer's advantage".
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In the worst case, if the first flow yields at the same rate as the
new flow increases its sending rate, the new flow will see constant
end-to-end delay, which it assumes is the base delay, until the first
flow backs off completely. As a result, by the time the second flow
stops increasing its cwnd, it would have added twice the target
queueing delay to the network.
This advantage can be reduced if the first flow yields and empties
the bottleneck queue faster than the incoming flow increases its
occupancy in the queue. In such a case, the late-comer might measure
correctly a delay that is closer to the base delay. While such a
reduction might be achieved through a multiplicative decrease of the
congestion window, this may cause strong fluctuations in flow
throughput during the flow's steady state. Thus we do not recommend
a multiplicative decrease scheme.
We note that in certain use-case scenarios, it is possible for a
later LEDBAT flow to gain an unfair advantage over an existing one
[Car10]. In practice, this concern ought to be alleviated by the
burstiness of network traffic: all that's needed to measure the base
delay is one small gap in transmission schedules between the LEDBAT
flows. These gaps can occur for a number of reasons such as latency
introduced due to application sending patterns, OS scheduling at the
sender, processing delay at the sender or any network node, and link
contention. When such a gap occurs in the first sender's
transmission while the late-comer is starting, base delay is
immediately correctly measured. With a small number of LEDBAT flows,
system noise may sufficiently regulate the late-comer's advantage.
6. Open Areas for Experimentation
We now outline some areas that need experimentation in the Internet
and under different network scenarios. These experiments should help
the community understand LEDBAT's dynamics and should help towards
further standardization of LEDBAT and LEDBAT-related documents.
6.1. Network Effects and Monitoring
Further study is required to fully understand the behavior and
convergence properties of LEDBAT in networks with non-tail-drop, non-
FIFO queues, in networks with frequent route changes, and in networks
with network-level load balancing. These studies should have two
broad goals: (i) to understand the effects of different network
mechanisms on LEDBAT, and (ii) to understand the impact of LEDBAT on
the network.
Network mechanisms and dynamics can influence LEDBAT flows in
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unintended ways. For instance, frequent route changes that result in
increasing base delays may, in the worst case, throttle a LEDBAT
flow's throughput significantly. The influence of different network
traffic management mechanisms on LEDBAT throughput should be studied.
An increasing number of LEDBAT flows in the network will likely
result in operator-visible network effects as well and should thus be
studied. For instance, as long as the bottleneck queue in a network
is larger than TARGET (in terms of delay), we expect that both the
average queueing delay and loss rate in the network should reduce as
LEDBAT traffic increasingly dominates the traffic mix in the network.
Note that for bottleneck queues that are smaller than TARGET, LEDBAT
will appear to behave very similar to standard TCP and it's flow-
level behavior may not be distinguishable from that of standard TCP.
We note that a network operator may be able to verify the operation
of a LEDBAT flow by monitoring per-flow behavior and queues in the
network---when the queueing delay at a bottleneck queue is above
TARGET as specified in this document, LEDBAT flows should be expected
to back off and reduce their sending rate.
6.2. Parameter Values
The throughput and response of LEDBAT to the proposed parameter
values of TARGET, decrease-GAIN, BASE_HISTORY, INIT_CWND, and
MIN_CWND should be evaluated with different types of competing
traffic in different network settings, including with different AQM
schemes at the bottleneck queue. TARGET controls LEDBAT's added
latency, while decrease-GAIN controls LEDBAT's response to competing
traffic. Since LEDBAT is intended to be minimally intrusive to
competing traffic, the impact of TARGET and decrease-GAIN on delay-
sensitive traffic should be studied. TARGET also impacts the growth
rate of the congestion window when off_target is smaller than 1.
This impact of TARGET on the rate of cwnd growth should be studied.
The amount of history maintained by the base delay estimator,
BASE_HISTORY, influences the responsiveness of LEDBAT to changing
network conditions. LEDBAT's responsiveness and throughput should be
evaluated in the wide area and under conditions where abrupt changes
in base delay might occur, such as with route changes and with
cellular handovers. The impact and efficacy of these parameters
should be carefully studied with tests over the Internet.
6.3. Filters
LEDBAT's effectiveness depends on a sender's ability to accurately
estimate end-to-end queueing delay from delay samples. Consequently,
the filtering algorithm used for this estimation, FILTER(), is an
important candidate for experiments. This document suggests the use
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of NULL, EWMA and MIN filters for estimating the current delay; the
efficacy of these and other possible filters for this estimate should
be investigated. FILTER() may also impact cwnd dynamics when delay
samples are bundled in ACKs, since cwnd adaption is done once per ACK
irrespective of the number of delay samples in the ACK. This impact
should be studied when the different filters are considered.
6.4. Framing
This document defines only a congestion control algorithm and assumes
that framing mechanisms for exchanging delay information exist within
the protocol in which LEDBAT is being implemented. If implemented in
a new protocol, both the sender and receiver may be LEDBAT-aware, but
if implemented in an existing protocol which is capable of providing
one-way delay information, LEDBAT may be implemented as a sender-
side-only modification. In either case, the parent protocol may
interact with LEDBAT's algorithms; for instance, the rate of ACK
feedback to the data sender may be dictated by other protocol
parameters, but will interact with the LEDBAT flow's dynamics.
Careful experimentation is necessary to understand and integrate
LEDBAT into both new and existing protocols.
7. IANA Considerations
There are no IANA considerations for this document.
8. Security Considerations
LEDBAT's aggressiveness is contingent on the delay estimates and on
the TARGET delay value. If these parameter values at the sender are
compromised such that delay estimates are artificially set to zero
and the TARGET delay value is set to +INFINITY, the LEDBAT algorithm
deteriorates to TCP-like behavior. Thus, while LEDBAT is sensitive
to these parameters, the algorithm is fundamentally limited in the
worst case to be as aggressive as standard TCP.
An man-in-the-middle may be able to change queueing delay on a
network path, and/or modify the timestamps transmitted by a LEDBAT
sender and/or modify the delays reported by a LEDBAT receiver, thus
causing a LEDBAT flow to back off even when there's no congestion. A
protocol using LEDBAT ought to minimize the risk of such man-in-the-
middle attacks by at least authenticating the timestamp field in the
data packets and the delay field in the ACK packets.
LEDBAT is not known to introduce any new concerns with privacy,
integrity, or other security issues for flows that use it. LEDBAT is
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compatible with use of IPsec and TLS/DTLS.
9. Acknowledgements
We thank folks in the LEDBAT working group for their comments and
feedback. Special thanks to Murari Sridharan and Rolf Winter for
their patient and untiring shepherding.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.
10.2. Informative References
[Bra94] Brakmo, L., O'Malley, S., and L. Peterson, "TCP Vegas: New
techniques for congestion detection and avoidance",
Proceedings of SIGCOMM '94, pages 24-35, August 1994.
[Car10] Carofiglio, G., Muscariello, L., Rossi, D., Testa, C., and
S. Valenti, "Rethinking Low Extra Delay Background
Transport Protocols", arXiv:1010.5623v1, September 2010.
[Low02] Low, S., Peterson, L., and L. Wang, "Understanding TCP
Vegas: A Duality Model", JACM 49 (2), March 2002.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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[RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
Transport Protocols", RFC 6297, June 2011.
[Sch10] Schneider, J., Wagner, J., Winter, R., and H. Kolbe, "Out
of my Way -- Evaluating Low Extra Delay Background
Transport in an ADSL Access Network", Proceedings of 22nd
International Teletraffic Congress (ITC22), September
2010.
[g114] "SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS
AND NETWORKS; International telephone connections and
circuits - General; Recommendations on the transmission
quality for an entire international telephone connection;
One-way transmission time", ITU-T Recommendation G.114,
05/2003.
[uTorrent]
Hazel, G., "uTorrent Transport Protocol library",
http://github.com/bittorrent/libutp, July 2012.
Appendix A. Measurement Errors
LEDBAT measures and uses one-way delays, and we now consider
measurement errors in timestamp generation and use. In this Section,
we use the same locally-linear clock model and the same terminology
as Network Time Protocol (NTP) [RFC5905]. In particular, NTP uses
the terms "offset" to refer to the difference between measured time
and true time, and "skew" to refer to difference of clock rate from
the true rate. A clock thus has two time measurement errors: a fixed
offset from the true time, and a skew. We now consider these errors
in the context of LEDBAT.
A.1. Clock Offset
The offset of the clocks, both the sender's and the receiver's, shows
up as a fixed error in LEDBAT's one-way delay measurement. The
offset in the measured one-way delay is simply the difference in
offsets between the receiver's and the sender's clocks. LEDBAT
however does not use this estimate directly, but uses the difference
between the measured one-way delay and a measured base delay. Since
the offset error (difference of clock offsets) is the same for the
measured one-way delay and the base delay, the offsets cancel each
other out in the queuing delay estimate, which LEDBAT uses for its
window computations. Clock offset error thus has no impact on
LEDBAT.
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A.2. Clock Skew
Clock skew generally shows up as a linearly changing error in a time
estimate. Similar to the offset, the skew of LEDBAT's one-way delay
estimate is thus the difference between the two clocks' skews.
Unlike the offset however, skew does not cancel out when the queuing
delay estimate is computed, since it causes the two clocks' offsets
to change over time.
While the offset could be large, with some clocks off by minutes or
even hours or more, skew is typically small. Typical skews of
untrained clocks seem to be around 100-200 PPM [RFC5905], where a
skew of 100 PPM translates to an error accumulation of 6 milliseconds
per minute. This accumulation is limited in LEDBAT, since any error
accumulation is limited to the amount of history maintained by the
base delay estimator, as dictated by the BASE_HISTORY parameter. The
effects of clock skew error on LEDBAT should generally be
insignificant unless the skew is unusually high, or unless extreme
values have been chosen for TARGET (extremely low) and BASE_HISTORY
(extremely large). Nevertheless, we now consider the possible impact
of skew on LEDBAT behavior.
Clock skew can manifest in two ways: the sender's clock can be faster
than the receiver's clock, or the receiver's clock can be faster than
the sender's clock. In the first case, the measured one-way delay
will decrease as the sender's clock drifts forward. While this drift
can lead to an artificially low estimate of the queueing delay, the
drift should also lead to a lower base delay measurement, which
consequently absorbs the erroneous reduction in the one-way delay
estimates.
In the second case, the one-way delay estimate will artifically
increase with time. This increase can reduce a LEDBAT flow's
throughput unnecessarily. In this case, a skew correction mechanism
can be beneficial.
We now discuss an example clock skew correction mechanism. In this
example, the receiver sends back raw (sending and receiving)
timestamps. Using this information, the sender can estimate one-way
delays in both directions, and the sender can also compute and
maintain an estimate of the base delay as would be observed by the
receiver. If the sender detects the receiver reducing its estimate
of the base delay, it may infer that this reduction is due to clock
drift. The sender then compensates by increasing its base delay
estimate by the same amount. To apply this mechanism, timestamps
need to be transmitted in both directions.
We now outline a few other ideas that can be used for skew
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correction.
o Skew correction with faster virtual clock:
Since having a faster clock on the sender will result in
continuous updates of the base delay, a faster virtual clock can
be used for sender timestamping. This virtual clock can be
computed from the default machine clock through a linear
transformation. For instance, with a 500 PPM speed-up the
sender's clock is very likely to be faster than a receiver's
clock. Consequently, LEDBAT will benefit from the implicit
correction when updating the base delay.
o Skew correction with estimating drift:
A LEDBAT sender maintains a history of base delay minima. This
history can provide a base to compute the clock skew difference
between the two hosts. The slope of a linear function fitted to
the set of minima base delays gives an estimate of the clock skew.
This estimation can be used to correct the clocks. If the other
endpoint is doing the same, the clock should be corrected by half
of the estimated skew amount.
o Byzantine skew correction:
When it is known that each host maintains long-lived connections
to a number of different other hosts, a byzantine scheme can be
used to estimate the skew with respect to the true time. Namely,
a host calculates the skew difference for each of the peer hosts
as described with the previous approach, then take the median of
the skew differences. While this scheme is not universally
applicable, it combines well with other schemes, since it is
essentially a clock training mechanism. The scheme also corrects
fast, since state is preserved between connections.
Authors' Addresses
Stanislav Shalunov
BitTorrent Inc
612 Howard St, Suite 400
San Francisco, CA 94105
USA
Email: shalunov@shlang.com
URI: http://shlang.com
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Greg Hazel
BitTorrent Inc
612 Howard St, Suite 400
San Francisco, CA 94105
USA
Email: greg@bittorrent.com
Janardhan Iyengar
Franklin and Marshall College
415 Harrisburg Ave.
Lancaster, PA 17603
USA
Email: jiyengar@fandm.edu
Mirja Kuehlewind
University of Stuttgart
Stuttgart
DE
Email: mirja.kuehlewind@ikr.uni-stuttgart.de
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