TCP Maintenance Working Group M. Mathis
Internet-Draft N. Dukkipati
Intended status: Experimental Y. Cheng
Expires: August 27, 2012 Google, Inc
February 24, 2012
Proportional Rate Reduction for TCP
draft-ietf-tcpm-proportional-rate-reduction-01.txt
Abstract
This document describes an experimental algorithm, Proportional Rate
Reduction (PPR) to improve the accuracy of the amount of data sent by
TCP during loss recovery. Standard Congestion Control requires that
TCP and other protocols reduce their congestion window in response to
losses. This window reduction naturally occurs in the same round
trip as the data retransmissions to repair the losses, and is
implemented by choosing not to transmit any data in response to some
ACKs arriving from the receiver. Two widely deployed algorithms are
used to implement this window reduction: Fast Recovery and Rate
Halving. Both algorithms are needlessly fragile under a number of
conditions, particularly when there is a burst of losses that such
that the number of ACKs returning to the sender is small.
Proportional Rate Reduction minimizes these excess window reductions
such that at the end of recovery the actual window size will be as
close as possible to ssthresh, the window size determined by the
congestion control algorithm. It is patterned after Rate Halving,
but using the fraction that is appropriate for target window chosen
by the congestion control algorithm.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 27, 2012.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Conclusion and Recommendations . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Packet Conservation Bound . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document describes an experimental algorithm, Proportional Rate
Reduction (PPR) to improve the accuracy of the amount of data sent by
TCP during loss recovery.
Standard Congestion Control [RFC5681] requires that TCP (and other
protocols) reduce their congestion window in response to losses.
Fast Recovery, described in the same document, is the reference
algorithm for making this adjustment. It's stated goal is to recover
TCP's self clock by relying on returning ACKs during recovery to
clock more data into the network. Fast Recovery adjusts the window
by waiting for one half RTT of ACKs to pass before sending any data.
It is fragile because it can not compensate for the implicit window
reduction caused by the losses themselves, and is exposed to
timeouts. For example if half of the data or ACKs are lost, Fast
Recovery's expected behavior would be wait for half window of ACKs to
pass and then not receive any ACKs for the recovery and suffer a
timeout.
The rate-halving algorithm improves this situation by sending data on
alternate ACKs during recovery, such that after one RTT the window
has been halved. Rate-having is implemented in Linux after only
being informally published [RHweb], including an uncompleted
Internet-Draft [RHID]. Rate-halving does not adequately compensate
for the implicit window reduction caused by the losses and assumes a
50% window reduction, which was completely standard at the time it
was written, but not appropriate for modern congestion control
algorithms such as Cubic [CUBIC], which can reduce the window by less
than 50%. As a consequence rate-halving often allows the window to
fall further than necessary, reducing performance and increasing the
risk of timeouts if there are additional losses.
Proportional Rate Reduction (PPR) avoids these excess window
reductions such that at the end of recovery the actual window size
will be as close as possible to, ssthresh, the window size determined
by the congestion control algorithm. It is patterned after Rate
Halving, but using the fraction that is appropriate for target window
chosen by the congestion control algorithm. During PRR one of two
additional reduction bound algorithms limits the total window
reduction due to all mechanisms, including application stalls and the
losses themselves.
We describe two slightly different reduction bound algorithms:
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 one segment per ACK. PRR-CRB meets
the strong conservative bound described in Appendix A, however in
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real networks it does not perform as well as the algorithms described
in RFC 3517, which prove to be non-conservative in a significant
number of cases. SSRB offers a compromise by allowing TCP to send
one additional segment per ACK relative to CRB in some situations.
Although SSRB is less aggressive than RFC 3517 (transmitting fewer
segments or taking more time to transmit them) it outperforms it, due
to the lower probability of additional losses during recovery.
PRR and both reduction bounds are based on common design principles,
derived from Van Jacobson's packet conservation principle: segments
delivered to the receiver are used as the clock to trigger sending
the same number of segments back into the network. As much as
possible Proportional Rate Reduction and the reduction bound rely on
this self clock process, and are only slightly affected by the
accuracy of other estimators, such as pipe [RFC3517] and cwnd. This
is what gives the algorithms their precision in the presence of
events that cause uncertainty in other estimators.
We evaluated these and other algorithms in a large scale measurement
study, summarized below. The most important results from that study
are presented in an companion paper [IMC11]. PRR+SSRB outperforms
both RFC 3517 and Linux Rate Halving under authentic network traffic,
even though it is less aggressive than RFC 3517.
The algorithms are described as modifications to RFC 5681 [RFC5681],
TCP Congestion Control, using concepts drawn from the pipe algorithm
[RFC3517]. They are most accurate and more easily implemented with
SACK [RFC2018], but they do not require SACK.
2. Definitions
The following terms, parameters and state variables are used as they
are defined in earlier documents:
RFC 3517: covered (as in "covered sequence numbers")
RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size
(SMSS)
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
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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 one 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 recov
prr_out = 0 // Total bytes sent during recovery
RecoverFS = snd.nxt-snd.una // FlightSize at the start of recov
On every ACK during recovery compute:
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DeliveredData = delta(snd.una) + delta(SACKd)
prr_delivered += DeliveredData
pipe = (RFC 3517 pipe algorithm)
if (pipe > ssthresh) {
// Proportional Rate Reduction
sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out
} else {
// Two version 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)
}
On any data transmission or retransmission:
prr_out += (data sent) // strictly less than or equal to sndcnt
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, standard
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, so we send two
new segments followed by one retransmit on the first 3 duplicate ACKs
after 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.
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 is 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. Since PRR does not
use cwnd during recovery it is not shown in the example.
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RFC 3517
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
Note that all three algorithms send same total amount of data. RFC
3517 experiences a "half-window of silence", while the Rate Halving
and PRR spread the voluntary window reduction across an entire RTT.
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.
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RFC 3517
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: f f f
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: d d d
In this specific situation, RFC 3517 is very non-conservative,
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 3517
significantly outperforms Rate Halving, PRR-CRB and some other
similarly conservative algorithms that we tested, suggesting that it
is significantly common for the actual losses to exceed the window
reduction determined by the congestion control algorithm.
The Linux implementation of Rate Halving includes an early version of
the conservative reduction bound [RHweb]. In this situation the five
ACKs trigger exactly one transmission each (2 new data, 3 old data),
and cwnd is set to 5. At a window size of 5, it takes three 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 slowstart cwnd from 5 up to 10. In this example,
TCP operates at half of the window chosen by the congestion control
for more than three RTTs, increasing the elapsed time and exposing it
to timeouts in the event that there are additional losses.
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PRR-CRB implements 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 was set to prr_delivered - prr_out. This
is indicated by the RB:f 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 sshthresh 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 slowstart 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 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 Flight Size and ssthresh, PRR-CRB and PRR-SSRB
have identical behaviours.
4. Properties
The following properties are common to both PRR-CRB and PRR-SSRB
except as noted:
Proportional Rate Reduction maintains TCPs ACK clocking across most
recovery events, including burst losses. RFC 3517 can send large
unclocked bursts following burst losses.
Normally Proportional Rate Reduction 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 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, Proportional Rate Reduction 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.
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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.
Proportional Rate Reduction 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). 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.
Proportional Rate Reduction with Reduction Bound is significantly
less sensitive to errors of the pipe estimator. While in recovery,
pipe is intrinsically an estimator, using incomplete information to
guess if un-SACKed segments are actually lost or out-of-order in the
network. Under some conditions pipe can have significant errors, for
example when a burst of reordered data is presumed to be lost and is
retransmitted, but then the original data arrives before the
retransmission. If the transmissions are regulated directly by pipe
as they are in RFC 3517, then errors and discontinuities in the value
of the pipe estimator can cause significant errors in the amount of
data sent. With Proportional Rate Reduction with Reduction Bound,
pipe merely determines how sndcnt is computed from DeliveredData.
Since short term errors in pipe are smoothed out across multiple ACKs
and both Proportional Rate Reduction and the reduction bound converge
to the same final window, errors in the pipe estimator have less
impact on the final outcome.
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 packet
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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 packet Packet Conserving Bound in 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 RFC3517, which permits large bursts of data when there are bursts
of losses. PRR-SSRB is a compromise that permits TCP to send one
extra segment per ACK as compared to the packet conserving bound.
From the perspective of the packet conserving bound, PRR-SSRB does
indeed open the window during recovery, however it is significantly
less aggressive than RFC3517 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 results are 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 [RFC3742], threshold transmit from [FACK]
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. The
different window reduction algorithms were all present in the same
kernel and could be selected with a sysctl, such that we had an
absolutely uniform baseline for comparing them.
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, in conjunction with
using only standard algorithms to trigger Fast Retransmit,
accentuates the tendency for RFC 3517 and PRR-UB to send a burst at
the point when Fast Retransmit gets triggered if pipe is already
below ssthresh.
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All experiments were performed on servers carrying production traffic
for multiple Google services.
In this configuration it is observed that for 32% of the recovery
events, pipe falls below ssthresh before Fast Retransmit is
triggered, thus the various PRR algorithms start in the reduction
bound phase, and RFC 3517 send 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 packet conserving bound is very appealing for a number
of reasons, our measurements demonstrate that it is less aggressive
and does not perform as well as RFC3517, which permits significant
bursts of data when there are large bursts of losses. PRR-SSRB is a
compromise that permits TCP to send one extra segment per ACK as
relative to the packet conserving bound. From the perspective of the
packet conserving bound, PRR-SSRB does indeed open the window during
recovery, however it is significantly less aggressive than RFC3517 in
the presence of burst losses. Even so, it often out performs
RFC3517, because it avoids some of the self inflicted losses caused
by bursts from RFC3517.
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.
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
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proportional rate reduction and every variant of the reduction bound.
However, once paired they become one.
7. Acknowledgements
This draft is based in part on previous incomplete work by Matt
Mathis, Jeff Semke and Jamshid Mahdavi [RHID] and influenced by
several discussion with John Heffner.
Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the
experiments.
Ilpo Jarvinen for reviewing the code.
8. Security Considerations
Proportional Rate Reduction 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. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
10. References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.
[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, March 2004.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
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[IMC11] Dukkipati, N., Mathis, M., and Y. Cheng, "Proportional
Rate Reduction for TCP", ACM Internet Measurement
Conference IMC11, December 2011.
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:
Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,
August 1996.
[RHID] Mathis, M., Semke, J., Mahdavi, J., and K. Lahey, "The
Rate-Halving Algorithm for TCP Congestion Control", draft-
ratehalving (work in progress), June 1999.
[RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding
Parameters", Web publication , December 1997.
[CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed
TCP variant", PFLDnet 2005, Feb 2005.
[Savage99]
Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP congestion control with a misbehaving receiver",
SIGCOMM Comput. Commun. Rev. 29(5), October 1999.
Appendix A. Packet Conservation Bound
PRR-CRB meets a conservative, philosophically pure and aesthetically
appealing notion of correct, described here. However, in real
networks it does not perform as well as the algorithms described in
RFC 3517, which proves to be non-conservative in a significant number
of cases.
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 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:
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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, I 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; which
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 the 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 which sends additional data will
cause a queue overflow and 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) may
increases 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 of 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
Matt Mathis
Google, Inc
1600 Amphitheater Parkway
Mountain View, California 93117
USA
Email: mattmathis@google.com
Mathis, et al. Expires August 27, 2012 [Page 15]
Internet-Draft Proportional Rate Reduction February 2012
Nandita Dukkipati
Google, Inc
1600 Amphitheater Parkway
Mountain View, California 93117
USA
Email: nanditad@google.com
Yuchung Cheng
Google, Inc
1600 Amphitheater Parkway
Mountain View, California 93117
USA
Email: ycheng@google.com
Mathis, et al. Expires August 27, 2012 [Page 16]