Internet DRAFT - draft-mathis-tcpm-rfc6937bis
draft-mathis-tcpm-rfc6937bis
TCP Maintenance Working Group M. Mathis
Internet-Draft N. Dukkipati
Obsoletes: 6937 (if approved) Y. Cheng
Intended status: Standards Track Google, Inc.
Expires: 4 May 2021 31 October 2020
Proportional Rate Reduction for TCP
draft-mathis-tcpm-rfc6937bis-00
Abstract
This document updates the Proportional Rate Reduction (PRR) algorithm
described as experimental in RFC 6937 to standards track. PRR
potentially replaces the Fast Recovery and Rate-Halving algorithms.
All of these algorithms regulate the amount of data sent by TCP or
other transport protocol during loss recovery. PRR more accurately
regulates the actual flight size through recovery such that at the
end of recovery it will be as close as possible to the ssthresh, as
determined by the congestion control algorithm.
Status of This Memo
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This Internet-Draft will expire on 4 May 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Properties . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Measurements . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Conclusion and Recommendations . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. Normative References . . . . . . . . . . . . . . . . . . . . 12
10. Informative References . . . . . . . . . . . . . . . . . . . 13
Appendix A. Strong Packet Conservation Bound . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
This document updates the Proportional Rate Reduction (PRR) algorithm
described in [RFC6937]from experimental to standards track. PRR
accuracy regulates the amount of data sent during loss recovery, such
that at the end of recovery the flight size will be as close as
possible to the ssthresh, as determined by the congestion control
algorithm. PRR has been deployed in at least 3 major operating
systems covering the vast majority of today's web traffic. There
have been no changes to PRR as documented in the experimental RFC.
The descriptions here have been [will be] updated to normative
standards language. For a tutorial description of the algorithms and
the rationale behind them please see the original RFC.
The experimental RFC describes two different reduction bound
algorithms to limit the total window reduction due to all mechanisms,
including transient application stalls and the losses themselves:
Conservative Reduction Bound (CRB), which is strictly packet
conserving; and a Slow Start Reduction Bound (SSRB), which is more
aggressive than CRB by at most 1 segment per ACK. [RFC6937] left the
choice of Reduction Bound to the discretion of the implementer.
The paper "An Internet-Wide Analysis of Traffic Policing" [Flatch et
al] uncovered a crucial situation where the Reduction Bound mattered.
Under certain configurations, token bucket traffic policers
[token_bucket] can suddenly start discarding a large fraction of the
traffic. This happens without warning when policers run out of
tokens. The transport congestion control has no opportunity to
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measure the token rate, and sets ssthresh based on the recently
observed path performance. This value for ssthresh may be
substantially larger than can be sustained by the token rate,
potentially causing persistent high loss. Under these conditions,
both reduction bounds perform very poorly. PRR-CRB is too timid,
sometimes causing very long recovery times at smaller than necessary
windows, and PRR-SSRB is too aggressive, often causing many
retransmissions to be lost multiple times.
Investigating these environments led to the development of a
heuristic to dynamically switch between Reduction Bounds: use PRR-
SSRB only while snd.una is advancing without additional losses and
use PRR-CRB otherwise.
This heuristic is only invoked for what should be a rare corner case:
when losses or other events cause the flight size to fall below
ssthresh. The extreme loss rates that make the heuristic important
are only common in the presence of poorly configured token bucket
policers, which are pathologically wasteful and inefficient. In
these environments the heuristics serves to salvage a bad situation
and any reasonable implementation of the heuristic performs far
better than either bound by itself.
The algorithm below is identical to the algorithm presented in
[RFC6937]. The "conservative" parameter MAY be replaced by the
heuristic also described below.
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]
[All text below is copied from RFC 6937, it will be revised after
this document is adopted as a tcpm work item]
2. Definitions
The following terms, parameters, and state variables are used as they
are defined in earlier documents:
RFC 793: snd.una (send unacknowledged)
RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size
(SMSS)
RFC 6675: covered (as in "covered sequence numbers")
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Voluntary window reductions: choosing not to send data in response to
some ACKs, for the purpose of reducing the sending window size and
data rate
We define some additional variables:
SACKd: The total number of bytes that the scoreboard indicates have
been delivered to the receiver. This can be computed by scanning the
scoreboard and counting the total number of bytes covered by all sack
blocks. If SACK is not in use, SACKd is not defined.
DeliveredData: The total number of bytes that the current ACK
indicates have been delivered to the receiver. When not in recovery,
DeliveredData is the change in snd.una. With SACK, DeliveredData can
be computed precisely as the change in snd.una, plus the (signed)
change in SACKd. In recovery without SACK, DeliveredData is
estimated to be 1 SMSS on duplicate acknowledgements, and on a
subsequent partial or full ACK, DeliveredData is estimated to be the
change in snd.una, minus 1 SMSS for each preceding duplicate ACK.
Note that DeliveredData is robust; for TCP using SACK, DeliveredData
can be precisely computed anywhere in the network just by inspecting
the returning ACKs. The consequence of missing ACKs is that later
ACKs will show a larger DeliveredData. Furthermore, for any TCP
(with or without SACK), the sum of DeliveredData must agree with the
forward progress over the same time interval.
We introduce a local variable "sndcnt", which indicates exactly how
many bytes should be sent in response to each ACK. Note that the
decision of which data to send (e.g., retransmit missing data or send
more new data) is out of scope for this document.
3. Algorithms
At the beginning of recovery, initialize PRR state. This assumes a
modern congestion control algorithm, CongCtrlAlg(), that might set
ssthresh to something other than FlightSize/2:
ssthresh = CongCtrlAlg() // Target cwnd after recovery
prr_delivered = 0 // Total bytes delivered during recovery
prr_out = 0 // Total bytes sent during recovery
RecoverFS = snd.nxt-snd.una // FlightSize at the start of recovery
Figure 1
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On every ACK during recovery compute:
DeliveredData = change_in(snd.una) + change_in(SACKd)
prr_delivered += DeliveredData
pipe = (RFC 6675 pipe algorithm)
if (pipe > ssthresh) {
// Proportional Rate Reduction
sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out
} else {
// Two versions of the Reduction Bound
if (conservative) { // PRR-CRB
limit = prr_delivered - prr_out
} else { // PRR-SSRB
limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
}
// Attempt to catch up, as permitted by limit
sndcnt = MIN(ssthresh - pipe, limit)
}
Figure 2
On any data transmission or retransmission:
prr_out += (data sent) // strictly less than or equal to sndcnt
Figure 3
3.1. Examples
We illustrate these algorithms by showing their different behaviors
for two scenarios: TCP experiencing either a single loss or a burst
of 15 consecutive losses. In all cases we assume bulk data (no
application pauses), standard Additive Increase Multiplicative
Decrease (AIMD) congestion control, and cwnd = FlightSize = pipe = 20
segments, so ssthresh will be set to 10 at the beginning of recovery.
We also assume standard Fast Retransmit and Limited Transmit
[RFC3042], so TCP will send 2 new segments followed by 1 retransmit
in response to the first 3 duplicate ACKs following the losses.
Each of the diagrams below shows the per ACK response to the first
round trip for the various recovery algorithms when the zeroth
segment is lost. The top line indicates the transmitted segment
number triggering the ACKs, with an X for the lost segment. "cwnd"
and "pipe" indicate the values of these algorithms after processing
each returning ACK. "Sent" indicates how much 'N'ew or
'R'etransmitted data would be sent. Note that the algorithms for
deciding which data to send are out of scope of this document.
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When there is a single loss, PRR with either of the Reduction Bound
algorithms has the same behavior. We show "RB", a flag indicating
which Reduction Bound subexpression ultimately determined the value
of sndcnt. When there are minimal losses, "limit" (both algorithms)
will always be larger than ssthresh - pipe, so the sndcnt will be
ssthresh - pipe, indicated by "s" in the "RB" row.
RFC 6675
ack# X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
cwnd: 20 20 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11
pipe: 19 19 18 18 17 16 15 14 13 12 11 10 10 10 10 10 10 10 10
sent: N N R N N N N N N N N
Rate-Halving (Linux)
ack# X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
cwnd: 20 20 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11
pipe: 19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11 10
sent: N N R N N N N N N N N
PRR
ack# X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
pipe: 19 19 18 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 10
sent: N N R N N N N N N N N
RB: s s
Cwnd is not shown because PRR does not use it.
Key for RB
s: sndcnt = ssthresh - pipe // from ssthresh
b: sndcnt = prr_delivered - prr_out + SMSS // from banked
d: sndcnt = DeliveredData + SMSS // from DeliveredData
(Sometimes, more than one applies.)
Figure 4
Note that all 3 algorithms send the same total amount of data. RFC
6675 experiences a "half window of silence", while the Rate-Halving
and PRR spread the voluntary window reduction across an entire RTT.
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Next, we consider the same initial conditions when the first 15
packets (0-14) are lost. During the remainder of the lossy RTT, only
5 ACKs are returned to the sender. We examine each of these
algorithms in succession.
RFC 6675
ack# X X X X X X X X X X X X X X X 15 16 17 18 19
cwnd: 20 20 11 11 11
pipe: 19 19 4 10 10
sent: N N 7R R R
Rate-Halving (Linux)
ack# X X X X X X X X X X X X X X X 15 16 17 18 19
cwnd: 20 20 5 5 5
pipe: 19 19 4 4 4
sent: N N R R R
PRR-CRB
ack# X X X X X X X X X X X X X X X 15 16 17 18 19
pipe: 19 19 4 4 4
sent: N N R R R
RB: b b b
PRR-SSRB
ack# X X X X X X X X X X X X X X X 15 16 17 18 19
pipe: 19 19 4 5 6
sent: N N 2R 2R 2R
RB: bd d d
Figure 5
In this specific situation, RFC 6675 is more aggressive because once
Fast Retransmit is triggered (on the ACK for segment 17), TCP
immediately retransmits sufficient data to bring pipe up to cwnd.
Our measurement data (see Section 5) indicates that RFC 6675
significantly outperforms Rate-Halving, PRR-CRB, and some other
similarly conservative algorithms that we tested, showing that it is
significantly common for the actual losses to exceed the window
reduction determined by the congestion control algorithm.
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The Linux implementation of Rate-Halving includes an early version of
the Conservative Reduction Bound [RHweb]. In this situation, the 5
ACKs trigger exactly 1 transmission each (2 new data, 3 old data),
and cwnd is set to 5. At a window size of 5, it takes 3 round trips
to retransmit all 15 lost segments. Rate-Halving does not raise the
window at all during recovery, so when recovery finally completes,
TCP will slow start cwnd from 5 up to 10. In this example, TCP
operates at half of the window chosen by the congestion control for
more than 3 RTTs, increasing the elapsed time and exposing it to
timeouts in the event that there are additional losses.
PRR-CRB implements a Conservative Reduction Bound. Since the total
losses bring pipe below ssthresh, data is sent such that the total
data transmitted, prr_out, follows the total data delivered to the
receiver as reported by returning ACKs. Transmission is controlled
by the sending limit, which is set to prr_delivered - prr_out. This
is indicated by the RB:b tagging in the figure. In this case, PRR-
CRB is exposed to exactly the same problems as Rate-Halving; the
excess window reduction causes it to take excessively long to recover
the losses and exposes it to additional timeouts.
PRR-SSRB increases the window by exactly 1 segment per ACK until pipe
rises to ssthresh during recovery. This is accomplished by setting
limit to one greater than the data reported to have been delivered to
the receiver on this ACK, implementing slow start during recovery,
and indicated by RB:d tagging in the figure. Although increasing the
window during recovery seems to be ill advised, it is important to
remember that this is actually less aggressive than permitted by RFC
5681, which sends the same quantity of additional data as a single
burst in response to the ACK that triggered Fast Retransmit.
For less extreme events, where the total losses are smaller than the
difference between FlightSize and ssthresh, PRR-CRB and PRR-SSRB have
identical behaviors.
4. Properties
The following properties are common to both PRR-CRB and PRR-SSRB,
except as noted:
PRR maintains TCP's ACK clocking across most recovery events,
including burst losses. RFC 6675 can send large unclocked bursts
following burst losses.
Normally, PRR will spread voluntary window reductions out evenly
across a full RTT. This has the potential to generally reduce the
burstiness of Internet traffic, and could be considered to be a type
of soft pacing. Hypothetically, any pacing increases the probability
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that different flows are interleaved, reducing the opportunity for
ACK compression and other phenomena that increase traffic burstiness.
However, these effects have not been quantified.
If there are minimal losses, PRR will converge to exactly the target
window chosen by the congestion control algorithm. Note that as TCP
approaches the end of recovery, prr_delivered will approach RecoverFS
and sndcnt will be computed such that prr_out approaches ssthresh.
Implicit window reductions, due to multiple isolated losses during
recovery, cause later voluntary reductions to be skipped. For small
numbers of losses, the window size ends at exactly the window chosen
by the congestion control algorithm.
For burst losses, earlier voluntary window reductions can be undone
by sending extra segments in response to ACKs arriving later during
recovery. Note that as long as some voluntary window reductions are
not undone, the final value for pipe will be the same as ssthresh,
the target cwnd value chosen by the congestion control algorithm.
PRR with either Reduction Bound improves the situation when there are
application stalls, e.g., when the sending application does not queue
data for transmission quickly enough or the receiver stops advancing
rwnd (receiver window). When there is an application stall early
during recovery, prr_out will fall behind the sum of the
transmissions permitted by sndcnt. The missed opportunities to send
due to stalls are treated like banked voluntary window reductions;
specifically, they cause prr_delivered - prr_out to be significantly
positive. If the application catches up while TCP is still in
recovery, TCP will send a partial window burst to catch up to exactly
where it would have been had the application never stalled. Although
this burst might be viewed as being hard on the network, this is
exactly what happens every time there is a partial RTT application
stall while not in recovery. We have made the partial RTT stall
behavior uniform in all states. Changing this behavior is out of
scope for this document.
PRR with Reduction Bound is less sensitive to errors in the pipe
estimator. While in recovery, pipe is intrinsically an estimator,
using incomplete information to estimate if un-SACKed segments are
actually lost or merely out of order in the network. Under some
conditions, pipe can have significant errors; for example, pipe is
underestimated when a burst of reordered data is prematurely assumed
to be lost and marked for retransmission. If the transmissions are
regulated directly by pipe as they are with RFC 6675, a step
discontinuity in the pipe estimator causes a burst of data, which
cannot be retracted once the pipe estimator is corrected a few ACKs
later. For PRR, pipe merely determines which algorithm, PRR or the
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Reduction Bound, is used to compute sndcnt from DeliveredData. While
pipe is underestimated, the algorithms are different by at most 1
segment per ACK. Once pipe is updated, they converge to the same
final window at the end of recovery.
Under all conditions and sequences of events during recovery, PRR-CRB
strictly bounds the data transmitted to be equal to or less than the
amount of data delivered to the receiver. We claim that this Strong
Packet Conservation Bound is the most aggressive algorithm that does
not lead to additional forced losses in some environments. It has
the property that if there is a standing queue at a bottleneck with
no cross traffic, the queue will maintain exactly constant length for
the duration of the recovery, except for +1/-1 fluctuation due to
differences in packet arrival and exit times. See Appendix A for a
detailed discussion of this property.
Although the Strong Packet Conservation Bound is very appealing for a
number of reasons, our measurements summarized in Section 5
demonstrate that it is less aggressive and does not perform as well
as RFC 6675, which permits bursts of data when there are bursts of
losses. PRR-SSRB is a compromise that permits TCP to send 1 extra
segment per ACK as compared to the Packet Conserving Bound. From the
perspective of a strict Packet Conserving Bound, PRR-SSRB does indeed
open the window during recovery; however, it is significantly less
aggressive than RFC 6675 in the presence of burst losses.
5. Measurements
In a companion IMC11 paper [IMC11], we describe some measurements
comparing the various strategies for reducing the window during
recovery. The experiments were performed on servers carrying Google
production traffic and are briefly summarized here.
The various window reduction algorithms and extensive instrumentation
were all implemented in Linux 2.6. We used the uniform set of
algorithms present in the base Linux implementation, including CUBIC
[CUBIC], Limited Transmit [RFC3042], threshold transmit (Section 3.1
in [FACK]) (this algorithm was not present in RFC 3517, but a similar
algorithm has been added to RFC 6675), and lost retransmission
detection algorithms. We confirmed that the behaviors of Rate-
Halving (the Linux default), RFC 3517, and PRR were authentic to
their respective specifications and that performance and features
were comparable to the kernels in production use. All of the
different window reduction algorithms were all present in a common
kernel and could be selected with a sysctl, such that we had an
absolutely uniform baseline for comparing them.
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Our experiments included an additional algorithm, PRR with an
unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe
falls below ssthresh. This behavior parallels RFC 3517.
An important detail of this configuration is that CUBIC only reduces
the window by 30%, as opposed to the 50% reduction used by
traditional congestion control algorithms. This accentuates the
tendency for RFC 3517 and PRR-UB to send a burst at the point when
Fast Retransmit gets triggered because pipe is likely to already be
below ssthresh. Precisely this condition was observed for 32% of the
recovery events: pipe fell below ssthresh before Fast Retransmit was
triggered, thus the various PRR algorithms started in the Reduction
Bound phase, and RFC 3517 sent bursts of segments with the Fast
Retransmit.
In the companion paper, we observe that PRR-SSRB spends the least
time in recovery of all the algorithms tested, largely because it
experiences fewer timeouts once it is already in recovery.
RFC 3517 experiences 29% more detected lost retransmissions and 2.6%
more timeouts (presumably due to undetected lost retransmissions)
than PRR-SSRB. These results are representative of PRR-UB and other
algorithms that send bursts when pipe falls below ssthresh.
Rate-Halving experiences 5% more timeouts and significantly smaller
final cwnd values at the end of recovery. The smaller cwnd sometimes
causes the recovery itself to take extra round trips. These results
are representative of PRR-CRB and other algorithms that implement
strict packet conservation during recovery.
6. Conclusion and Recommendations
Although the Strong Packet Conservation Bound used in PRR-CRB is very
appealing for a number of reasons, our measurements show that it is
less aggressive and does not perform as well as RFC 3517 (and by
implication RFC 6675), which permits bursts of data when there are
bursts of losses. RFC 3517 and RFC 6675 are conservative in the
original sense of Van Jacobson's packet conservation principle, which
included the assumption that presumed lost segments have indeed left
the network. PRR-CRB makes no such assumption, following instead a
Strong Packet Conservation Bound in which only packets that have
actually arrived at the receiver are considered to have left the
network. PRR-SSRB is a compromise that permits TCP to send 1 extra
segment per ACK relative to the Strong Packet Conservation Bound, to
partially compensate for excess losses.
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From the perspective of the Strong Packet Conservation Bound, PRR-
SSRB does indeed open the window during recovery; however, it is
significantly less aggressive than RFC 3517 (and RFC 6675) in the
presence of burst losses. Even so, it often outperforms RFC 3517
(and presumably RFC 6675) because it avoids some of the self-
inflicted losses caused by bursts.
At this time, we see no reason not to test and deploy PRR-SSRB on a
large scale. Implementers worried about any potential impact of
raising the window during recovery may want to optionally support
PRR-CRB (which is actually simpler to implement) for comparison
studies. Furthermore, there is one minor detail of PRR that can be
improved by replacing pipe by total_pipe, as defined by Laminar TCP
[Laminar].
One final comment about terminology: we expect that common usage will
drop "Slow Start Reduction Bound" from the algorithm name. This
document needed to be pedantic about having distinct names for PRR
and every variant of the Reduction Bound. However, we do not
anticipate any future exploration of the alternative Reduction
Bounds.
7. Acknowledgements
This document is based in part on previous incomplete work by Matt
Mathis, Jeff Semke, and Jamshid Mahdavi [RHID] and influenced by
several discussions with John Heffner.
Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the
experiments.
Ilpo Jarvinen reviewed the code.
Mark Allman improved the document through his insightful review.
Neal Cardwell for reviewing and testing the patch.
8. Security Considerations
PRR does not change the risk profile for TCP.
Implementers that change PRR from counting bytes to segments have to
be cautious about the effects of ACK splitting attacks [Savage99],
where the receiver acknowledges partial segments for the purpose of
confusing the sender's congestion accounting.
9. Normative References
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[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[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>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012,
<https://www.rfc-editor.org/info/rfc6675>.
10. Informative References
[CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed
TCP variant", PFLDnet 2005, February 2005.
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:
Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,
August 1996.
[IMC11] Dukkipati, N., Mathis, M., Cheng, Y., and M. Ghobadi,
"Proportional Rate Reduction for TCP", Proceedings of the
11th ACM SIGCOMM Conference on Internet Measurement
2011, Berlin, Germany, November 2011.
[Jacobson88]
Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
Comput. Commun. Rev. 18(4), August 1988.
[Laminar] Mathis, M., "Laminar TCP and the case for refactoring TCP
congestion control", Work in Progress, 16 July 2012.
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[RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
DOI 10.17487/RFC3042, January 2001,
<https://www.rfc-editor.org/info/rfc3042>.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517,
DOI 10.17487/RFC3517, April 2003,
<https://www.rfc-editor.org/info/rfc3517>.
[RFC6937] Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937,
May 2013, <https://www.rfc-editor.org/info/rfc6937>.
[RHID] Mathis, M., Semke, J., and J. Mahdavi, "The Rate-Halving
Algorithm for TCP Congestion Control", Work in Progress,
August 1999.
[RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding
Parameters", Web publication, December 1997,
<http://www.psc.edu/networking/papers/FACKnotes/current/>.
[Savage99] Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
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SIGCOMM Comput. Commun. Rev. 29(5), October 1999.
Appendix A. Strong Packet Conservation Bound
PRR-CRB is based on a conservative, philosophically pure, and
aesthetically appealing Strong Packet Conservation Bound, described
here. Although inspired by Van Jacobson's packet conservation
principle [Jacobson88], it differs in how it treats segments that are
missing and presumed lost. Under all conditions and sequences of
events during recovery, PRR-CRB strictly bounds the data transmitted
to be equal to or less than the amount of data delivered to the
receiver. Note that the effects of presumed losses are included in
the pipe calculation, but do not affect the outcome of PRR-CRB, once
pipe has fallen below ssthresh.
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We claim that this Strong Packet Conservation Bound is the most
aggressive algorithm that does not lead to additional forced losses
in some environments. It has the property that if there is a
standing queue at a bottleneck that is carrying no other traffic, the
queue will maintain exactly constant length for the entire duration
of the recovery, except for +1/-1 fluctuation due to differences in
packet arrival and exit times. Any less aggressive algorithm will
result in a declining queue at the bottleneck. Any more aggressive
algorithm will result in an increasing queue or additional losses if
it is a full drop tail queue.
We demonstrate this property with a little thought experiment:
Imagine a network path that has insignificant delays in both
directions, except for the processing time and queue at a single
bottleneck in the forward path. By insignificant delay, we mean when
a packet is "served" at the head of the bottleneck queue, the
following events happen in much less than one bottleneck packet time:
the packet arrives at the receiver; the receiver sends an ACK that
arrives at the sender; the sender processes the ACK and sends some
data; the data is queued at the bottleneck.
If sndcnt is set to DeliveredData and nothing else is inhibiting
sending data, then clearly the data arriving at the bottleneck queue
will exactly replace the data that was served at the head of the
queue, so the queue will have a constant length. If queue is drop
tail and full, then the queue will stay exactly full. Losses or
reordering on the ACK path only cause wider fluctuations in the queue
size, but do not raise its peak size, independent of whether the data
is in order or out of order (including loss recovery from an earlier
RTT). Any more aggressive algorithm that sends additional data will
overflow the drop tail queue and cause loss. Any less aggressive
algorithm will under-fill the queue. Therefore, setting sndcnt to
DeliveredData is the most aggressive algorithm that does not cause
forced losses in this simple network. Relaxing the assumptions
(e.g., making delays more authentic and adding more flows, delayed
ACKs, etc.) is likely to increase the fine grained fluctuations in
queue size but does not change its basic behavior.
Note that the congestion control algorithm implements a broader
notion of optimal that includes appropriately sharing the network.
Typical congestion control algorithms are likely to reduce the data
sent relative to the Packet Conserving Bound implemented by PRR,
bringing TCP's actual window down to ssthresh.
Authors' Addresses
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Matt Mathis
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America
Email: mattmathis@google.com
Nandita Dukkipati
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America
Email: nanditad@google.com
Yuchung Cheng
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America
Email: ycheng@google.com
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