draft-ietf-tcpm-proportional-rate-reduction-00.txt   draft-ietf-tcpm-proportional-rate-reduction-01.txt 
TCP Maintenance Working Group M. Mathis TCP Maintenance Working Group M. Mathis
Internet-Draft N. Dukkipati Internet-Draft N. Dukkipati
Intended status: Experimental Y. Cheng Intended status: Experimental Y. Cheng
Expires: April 26, 2012 Google, Inc Expires: August 27, 2012 Google, Inc
October 24, 2011 February 24, 2012
Proportional Rate Reduction for TCP Proportional Rate Reduction for TCP
draft-ietf-tcpm-proportional-rate-reduction-00.txt draft-ietf-tcpm-proportional-rate-reduction-01.txt
Abstract Abstract
This document describes an experimental algorithm, Proportional Rate This document describes an experimental algorithm, Proportional Rate
Reduction (PPR) to improve the accuracy of the amount of data sent by Reduction (PPR) to improve the accuracy of the amount of data sent by
TCP during loss recovery. Standard Congestion Control requires that TCP during loss recovery. Standard Congestion Control requires that
TCP and other protocols reduce their congestion window in response to TCP and other protocols reduce their congestion window in response to
losses. This window reduction naturally occurs in the same round losses. This window reduction naturally occurs in the same round
trip as the data retransmissions to repair the losses, and is trip as the data retransmissions to repair the losses, and is
implemented by choosing not to transmit any data in response to some implemented by choosing not to transmit any data in response to some
ACKs arriving from the receiver. Two widely deployed algorithms are ACKs arriving from the receiver. Two widely deployed algorithms are
used to implement this window reduction: Fast Recovery and Rate used to implement this window reduction: Fast Recovery and Rate
Halving. Both algorithms are needlessly fragile under a number of Halving. Both algorithms are needlessly fragile under a number of
conditions, particularly when there is a burst of losses that such conditions, particularly when there is a burst of losses that such
that the number of ACKs returning to the sender is small. that the number of ACKs returning to the sender is small.
Proportional Rate Reduction avoids these excess window reductions Proportional Rate Reduction minimizes these excess window reductions
such that at the end of recovery the actual window size will be as 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 close as possible to ssthresh, the window size determined by the
congestion control algorithm. It is patterned after Rate Halving, congestion control algorithm. It is patterned after Rate Halving,
but using the fraction that is appropriate for target window chosen but using the fraction that is appropriate for target window chosen
by the congestion control algorithm. by the congestion control algorithm.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
skipping to change at page 1, line 48 skipping to change at page 1, line 48
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 26, 2012. This Internet-Draft will expire on August 27, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
skipping to change at page 3, line 28 skipping to change at page 3, line 28
It is fragile because it can not compensate for the implicit window It is fragile because it can not compensate for the implicit window
reduction caused by the losses themselves, and is exposed to reduction caused by the losses themselves, and is exposed to
timeouts. For example if half of the data or ACKs are lost, Fast 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 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 pass and then not receive any ACKs for the recovery and suffer a
timeout. timeout.
The rate-halving algorithm improves this situation by sending data on The rate-halving algorithm improves this situation by sending data on
alternate ACKs during recovery, such that after one RTT the window alternate ACKs during recovery, such that after one RTT the window
has been halved. Rate-having is implemented in Linux after only has been halved. Rate-having is implemented in Linux after only
being informally published [RHweb], including from an uncompleted being informally published [RHweb], including an uncompleted
Internet-Draft [RHID]. Rate-halving does not adequately compensate Internet-Draft [RHID]. Rate-halving does not adequately compensate
for the implicit window reduction caused by the losses and also for the implicit window reduction caused by the losses and assumes a
assumes a 50% window reduction, which was completely standard at the 50% window reduction, which was completely standard at the time it
time it was written, but not appropriate for modern congestion was written, but not appropriate for modern congestion control
control algorithms such as Cubic [CUBIC], which can reduce the window algorithms such as Cubic [CUBIC], which can reduce the window by less
by less than 50%. As a consequence rate-halving often allows the than 50%. As a consequence rate-halving often allows the window to
window to fall further than necessary, reducing performance and fall further than necessary, reducing performance and increasing the
increasing the risk of timeouts if there are additional losses. risk of timeouts if there are additional losses.
Proportional Rate Reduction (PPR) avoids these excess window Proportional Rate Reduction (PPR) avoids these excess window
reductions such that at the end of recovery the actual window size reductions such that at the end of recovery the actual window size
will be as close as possible to, ssthresh, the window size determined will be as close as possible to, ssthresh, the window size determined
by the congestion control algorithm. It is patterned after Rate by the congestion control algorithm. It is patterned after Rate
Halving, but using the fraction that is appropriate for target window Halving, but using the fraction that is appropriate for target window
chosen by the congestion control algorithm. During PRR one of two chosen by the congestion control algorithm. During PRR one of two
additional reduction bound algorithms limits the total window additional reduction bound algorithms limits the total window
reduction due to all mechanisms, including application stalls and the reduction due to all mechanisms, including application stalls and the
losses themselves. losses themselves.
We describe two slightly different reduction bound algorithms: We describe two slightly different reduction bound algorithms:
conservative reduction bound (CRB), which is strictly packet conservative reduction bound (CRB), which is strictly packet
conserving; and a slow start reduction bound (SSRB), which is more conserving; and a slow start reduction bound (SSRB), which is more
aggressive than CRB by at most one segment per ACK. PRR-CRB meets aggressive than CRB by at most one segment per ACK. PRR-CRB meets
the strong conservative bound described in Appendix A, however in the strong conservative bound described in Appendix A, however in
real networks it does not perform as well as the algorithms described real networks it does not perform as well as the algorithms described
in RFC 3517, which prove to be non-conservative in a statistically in RFC 3517, which prove to be non-conservative in a significant
significant number of cases. SSRB offers a compromise by allowing number of cases. SSRB offers a compromise by allowing TCP to send
TCP to send one additional segment per ACK relative to CRB in some one additional segment per ACK relative to CRB in some situations.
situations. Although SSRB is less aggressive than RFC 3517 Although SSRB is less aggressive than RFC 3517 (transmitting fewer
(transmitting fewer segments or taking more time to transmit them) it segments or taking more time to transmit them) it outperforms it, due
outperforms it, due to the lower probability of additional losses to the lower probability of additional losses during recovery.
during recovery.
PRR and both reduction bounds are based on common design principles, PRR and both reduction bounds are based on common design principles,
derived from Van Jacobson's packet conservation principle: segments derived from Van Jacobson's packet conservation principle: segments
delivered to the receiver are used as the clock to trigger sending delivered to the receiver are used as the clock to trigger sending
the same number of segments back into the network. As much as the same number of segments back into the network. As much as
possible Proportional Rate Reduction and the reduction bound rely on possible Proportional Rate Reduction and the reduction bound rely on
this self clock process, and are only slightly affected by the this self clock process, and are only slightly affected by the
accuracy of other estimators, such as pipe [RFC3517] and cwnd. This accuracy of other estimators, such as pipe [RFC3517] and cwnd. This
is what gives the algorithms their precision in the presence of is what gives the algorithms their precision in the presence of
events that cause uncertainty in other estimators. events that cause uncertainty in other estimators.
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The following terms, parameters and state variables are used as they The following terms, parameters and state variables are used as they
are defined in earlier documents: are defined in earlier documents:
RFC 3517: covered (as in "covered sequence numbers") RFC 3517: covered (as in "covered sequence numbers")
RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size
(SMSS) (SMSS)
Voluntary window reductions: choosing not to send data in response to Voluntary window reductions: choosing not to send data in response to
some ACKs, for the purpose of reducing the sending window size or some ACKs, for the purpose of reducing the sending window size and
data rate. data rate.
We define some additional variables: We define some additional variables:
SACKd: The total number of bytes that the scoreboard indicates have SACKd: The total number of bytes that the scoreboard indicates have
been delivered to the receiver. This can be computed by scanning the been delivered to the receiver. This can be computed by scanning the
scoreboard and counting the total number of bytes covered by all sack scoreboard and counting the total number of bytes covered by all sack
blocks. If SACK is not in use, SACKd is not defined. blocks. If SACK is not in use, SACKd is not defined.
DeliveredData: The total number of bytes that the current ACK DeliveredData: The total number of bytes that the current ACK
skipping to change at page 6, line 16 skipping to change at page 6, line 16
prr_delivered += DeliveredData prr_delivered += DeliveredData
pipe = (RFC 3517 pipe algorithm) pipe = (RFC 3517 pipe algorithm)
if (pipe > ssthresh) { if (pipe > ssthresh) {
// Proportional Rate Reduction // Proportional Rate Reduction
sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out
} else { } else {
// Two version of the reduction bound // Two version of the reduction bound
if (conservative) { // PRR+CRB if (conservative) { // PRR+CRB
limit = prr_delivered - prr_out limit = prr_delivered - prr_out
} else { // PRR+SSRB } else { // PRR+SSRB
limit = MAX(prr_delivered - prr_out, DeliveredData) + 1 limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
} }
// Attempt to catch up, as permitted by limit // Attempt to catch up, as permitted by limit
sndcnt = MIN(ssthresh - pipe, limit) sndcnt = MIN(ssthresh - pipe, limit)
} }
On any data transmission or retransmission: On any data transmission or retransmission:
prr_out += (data sent) // strictly less than or equal to sndcnt prr_out += (data sent) // strictly less than or equal to sndcnt
The following examples will make these algorithms clearer.
3.1. Examples 3.1. Examples
We illustrate these algorithms by showing their different behaviors We illustrate these algorithms by showing their different behaviors
for two scenarios: TCP experiencing either a single loss or a burst for two scenarios: TCP experiencing either a single loss or a burst
of 15 consecutive losses. In all cases we assume bulk data, standard of 15 consecutive losses. In all cases we assume bulk data, standard
AIMD congestion control (the ssthresh is set to FlightSize/2) and AIMD congestion control and cwnd = FlightSize = pipe = 20 segments,
cwnd = FlightSize = pipe = 20 segments, so ssthresh will be set to 10 so ssthresh will be set to 10 at the beginning of recovery. We also
at the beginning of recovery. We also assume standard Fast assume standard Fast Retransmit and Limited Transmit, so we send two
Retransmit and Limited Transmit, so we send two new segments followed new segments followed by one retransmit on the first 3 duplicate ACKs
by one retransmit on the first 3 duplicate ACKs after the losses. after the losses.
Each of the diagrams below shows the per ACK response to the first Each of the diagrams below shows the per ACK response to the first
round trip for the various recovery algorithms when the zeroth round trip for the various recovery algorithms when the zeroth
segment is lost. The top line indicates the transmitted segment segment is lost. The top line indicates the transmitted segment
number triggering the ACKs, with an X for the lost segment. "cwnd" number triggering the ACKs, with an X for the lost segment. "cwnd"
and "pipe" indicate the values of these algorithms after processing and "pipe" indicate the values of these algorithms after processing
each returning ACK. "Sent" indicates how much 'N'ew or each returning ACK. "Sent" indicates how much 'N'ew or
'R'etransmitted data would be sent. Note that the algorithms for 'R'etransmitted data would be sent. Note that the algorithms for
deciding which data to send are out of scope of this document. deciding which data to send are out of scope of this document.
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window reduction causes it to take excessively long to recover the window reduction causes it to take excessively long to recover the
losses and exposes it to additional timeouts. losses and exposes it to additional timeouts.
PRR-SSRB increases the window by exactly 1 segment per ACK until pipe PRR-SSRB increases the window by exactly 1 segment per ACK until pipe
rises to sshthresh during recovery. This is accomplished by setting rises to sshthresh during recovery. This is accomplished by setting
limit to one greater than the data reported to have been delivered to limit to one greater than the data reported to have been delivered to
the receiver on this ACK, implementing slowstart during recovery, and the receiver on this ACK, implementing slowstart during recovery, and
indicated by RB:d tagging in the figure. Although increasing the indicated by RB:d tagging in the figure. Although increasing the
window during recovery seems to be ill advised, it is important to window during recovery seems to be ill advised, it is important to
remember that this actually less aggressive than permitted by RFC remember that this actually less aggressive than permitted by RFC
5681, which sends the same quantity of extra data as a single burst 5681, which sends the same quantity of additional data as a single
in response to the ACK that triggered Fast Retransmit burst in response to the ACK that triggered Fast Retransmit
Under less extreme conditions, when the total losses are smaller than For less extreme events, where the total losses are smaller than the
the difference between Flight Size and ssthresh, PRR-CRB and PRR-SSRB difference between Flight Size and ssthresh, PRR-CRB and PRR-SSRB
have identical behaviours. have identical behaviours.
4. Properties 4. Properties
The following properties are common to both PRR-CRB and PRR-SSRB: 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 Normally Proportional Rate Reduction will spread voluntary window
reductions out evenly across a full RTT. This has the potential to reductions out evenly across a full RTT. This has the potential to
generally reduce the burstiness of Internet traffic, and could be generally reduce the burstiness of Internet traffic, and could be
considered to be a type of soft pacing. Theoretically any pacing considered to be a type of soft pacing. Hypothetically, any pacing
increases the probability that different flows are interleaved, increases the probability that different flows are interleaved,
reducing the opportunity for ACK compression and other phenomena that reducing the opportunity for ACK compression and other phenomena that
increase traffic burstiness. However these effects have not been increase traffic burstiness. However these effects have not been
quantified. quantified.
If there are minimal losses, Proportional Rate Reduction will If there are minimal losses, Proportional Rate Reduction will
converge to exactly the target window chosen by the congestion converge to exactly the target window chosen by the congestion
control algorithm. Note that as TCP approaches the end of recovery control algorithm. Note that as TCP approaches the end of recovery
prr_delivered will approach RecoverFS and sndcnt will be computed prr_delivered will approach RecoverFS and sndcnt will be computed
such that prr_out approaches ssthresh. such that prr_out approaches ssthresh.
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recovery cause later voluntary reductions to be skipped. For small recovery cause later voluntary reductions to be skipped. For small
numbers of losses the window size ends at exactly the window chosen numbers of losses the window size ends at exactly the window chosen
by the congestion control algorithm. by the congestion control algorithm.
For burst losses, earlier voluntary window reductions can be undone For burst losses, earlier voluntary window reductions can be undone
by sending extra segments in response to ACKs arriving later during by sending extra segments in response to ACKs arriving later during
recovery. Note that as long as some voluntary window reductions are recovery. Note that as long as some voluntary window reductions are
not undone, the final value for pipe will be the same as ssthresh, not undone, the final value for pipe will be the same as ssthresh,
the target cwnd value chosen by the congestion control algorithm. the target cwnd value chosen by the congestion control algorithm.
Proportional Rate Reduction with either reduction round improves the Proportional Rate Reduction with either reduction bound improves the
situation when there are application stalls (e.g. when the sending situation when there are application stalls (e.g. when the sending
application does not queue data for transmission quickly enough or application does not queue data for transmission quickly enough or
the receiver stops advancing rwnd). When there is an application the receiver stops advancing rwnd). When there is an application
stall early during recovery prr_out will fall behind the sum of the stall early during recovery prr_out will fall behind the sum of the
transmissions permitted by sndcnt. The missed opportunities to send transmissions permitted by sndcnt. The missed opportunities to send
due to stalls are treated like banked voluntary window reductions: due to stalls are treated like banked voluntary window reductions:
specifically they cause prr_delivered-prr_out to be significantly specifically they cause prr_delivered-prr_out to be significantly
positive. If the application catches up while TCP is still in positive. If the application catches up while TCP is still in
recovery, TCP will send a partial window burst to catch up to exactly recovery, TCP will send a partial window burst to catch up to exactly
where it would have been, had the application never stalled. where it would have been, had the application never stalled.
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Since short term errors in pipe are smoothed out across multiple ACKs Since short term errors in pipe are smoothed out across multiple ACKs
and both Proportional Rate Reduction and the reduction bound converge and both Proportional Rate Reduction and the reduction bound converge
to the same final window, errors in the pipe estimator have less to the same final window, errors in the pipe estimator have less
impact on the final outcome. impact on the final outcome.
Under all conditions and sequences of events during recovery, PRR-CRB Under all conditions and sequences of events during recovery, PRR-CRB
strictly bounds the data transmitted to be equal to or less than the 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 amount of data delivered to the receiver. We claim that this packet
conservation bound is the most aggressive algorithm that does not conservation bound is the most aggressive algorithm that does not
lead to additional forced losses in some environments. It has the lead to additional forced losses in some environments. It has the
property that if there is a standing queue at a bottleneck that is property that if there is a standing queue at a bottleneck with no
carrying no other traffic, the queue will maintain exactly constant cross traffic, the queue will maintain exactly constant length for
length for the duration of the recovery (except for +1/-1 fluctuation the duration of the recovery, except for +1/-1 fluctuation due to
due to differences in packet arrival and exit times) . See differences in packet arrival and exit times. See Appendix A for a
Appendix A for a detailed discussion of this property. detailed discussion of this property.
Although the packet Packet Conserving Bound in very appealing for a Although the packet Packet Conserving Bound in very appealing for a
number of reasons, our measurements summarized in Section 5 number of reasons, our measurements summarized in Section 5
demonstrate that it is less aggressive and does not perform as well demonstrate that it is less aggressive and does not perform as well
as RFC3517, which permits large bursts of data when there are bursts 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 of losses. PRR-SSRB is a compromise that permits TCP to send one
extra segment per ACK as compared to the packet conserving bound. extra segment per ACK as compared to the packet conserving bound.
From the perspective of the packet conserving bound, PRR-SSRB does From the perspective of the packet conserving bound, PRR-SSRB does
indeed open the window during recovery, however it is significantly indeed open the window during recovery, however it is significantly
less aggressive than RFC3517 in the presence of burst losses. less aggressive than RFC3517 in the presence of burst losses.
5. Measurements 5. Measurements
In a companion IMC11 paper [IMC11] we describe some measurements In a companion IMC11 paper [IMC11] we describe some measurements
comparing the various strategies for reducing the window during comparing the various strategies for reducing the window during
recovery. The results are summarized here. recovery. The results are summarized here.
The various window reduction algorithms and extensive instrumentation The various window reduction algorithms and extensive instrumentation
were all implemented in Linux 2.6. We used the uniform set of were all implemented in Linux 2.6. We used the uniform set of
algorithms present in the base Linux implementation, including CUBIC algorithms present in the base Linux implementation, including CUBIC
[CUBIC], limited transmit [LT], threshold transmit from [FACK] and [CUBIC], limited transmit [RFC3742], threshold transmit from [FACK]
lost retransmission detection algorithms. We confirmed that the and lost retransmission detection algorithms. We confirmed that the
behaviors of Rate Halving (the Linux default), RFC 3517 and PRR were behaviors of Rate Halving (the Linux default), RFC 3517 and PRR were
authentic to their respective specifications and that performance and authentic to their respective specifications and that performance and
features were comparable to the kernels in production use. The features were comparable to the kernels in production use. The
different window reduction algorithms were all present in the same different window reduction algorithms were all present in the same
kernel and could be selected with a sysctl, such that we had an kernel and could be selected with a sysctl, such that we had an
absolutely uniform baseline for comparing them. absolutely uniform baseline for comparing them.
Our experiments included an additional algorithm, PRR with an Our experiments included an additional algorithm, PRR with an
unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe
falls below ssthresh. This behavior parallels RFC 3517. falls below ssthresh. This behavior parallels RFC 3517.
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bursts of data when there are large bursts of losses. PRR-SSRB is a 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 compromise that permits TCP to send one extra segment per ACK as
relative to the packet conserving bound. From the perspective of the relative to the packet conserving bound. From the perspective of the
packet conserving bound, PRR-SSRB does indeed open the window during packet conserving bound, PRR-SSRB does indeed open the window during
recovery, however it is significantly less aggressive than RFC3517 in recovery, however it is significantly less aggressive than RFC3517 in
the presence of burst losses. Even so, it often out performs the presence of burst losses. Even so, it often out performs
RFC3517, because it avoids some of the self inflicted losses caused RFC3517, because it avoids some of the self inflicted losses caused
by bursts from RFC3517. by bursts from RFC3517.
At this time we see no reason not to test and deploy PRR-SSRB on a At this time we see no reason not to test and deploy PRR-SSRB on a
large scale, indeed it already is. Implementers worried about any large scale. Implementers worried about any potential impact of
potential impact of raising the window during recovery may want to raising the window during recovery may want to optionally support
optionally support PRR-CRB (which is actually simpler to implement) PRR-CRB (which is actually simpler to implement) for comparison
for comparison studies. studies.
One final comment about terminology: we expect that common usage will One final comment about terminology: we expect that common usage will
drop "slow start reduction bound" from the algorithm name. This drop "slow start reduction bound" from the algorithm name. This
document needs to be pedantic about having distinct name for document needed to be pedantic about having distinct names for
proportional rate reduction and every variant of the reduction bound. proportional rate reduction and every variant of the reduction bound.
However, once paired they become one. However, once paired they become one.
7. Acknowledgements 7. Acknowledgements
This draft is based in part on previous incomplete work by Matt This draft is based in part on previous incomplete work by Matt
Mathis, Jeff Semke and Jamshid Mahdavi [RHID] and influenced by Mathis, Jeff Semke and Jamshid Mahdavi [RHID] and influenced by
several discussion with John Heffner. several discussion with John Heffner.
Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the
experiments. experiments.
Ilpo Jarvinen for reviewing the code.
8. Security Considerations 8. Security Considerations
Proportional Rate Reduction does not change the risk profile for TCP. Proportional Rate Reduction does not change the risk profile for TCP.
Implementers that change PRR from counting bytes to segments have to Implementers that change PRR from counting bytes to segments have to
be cautious about the effects of ACK splitting attacks[SPLIT], where be cautious about the effects of ACK splitting attacks [Savage99],
the receiver acknowledges partial segments for the purpose of where the receiver acknowledges partial segments for the purpose of
confusing the sender's congestion accounting. confusing the sender's congestion accounting.
9. IANA Considerations 9. IANA Considerations
This document makes no request of IANA. This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an Note to RFC Editor: this section may be removed on publication as an
RFC. RFC.
10. References 10. References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996. Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003. 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 [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, September 2009.
[IMC11] Dukkipati, N., Mathis, M., and Y. Cheng, "Proportional [IMC11] Dukkipati, N., Mathis, M., and Y. Cheng, "Proportional
Rate Reduction for TCP", ACM Internet Measurement Rate Reduction for TCP", ACM Internet Measurement
Conference IMC11, December 2011. Conference IMC11, December 2011.
[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment: [FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:
Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96, Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,
August 1996. August 1996.
skipping to change at page 14, line 15 skipping to change at page 14, line 23
[RHID] Mathis, M., Semke, J., Mahdavi, J., and K. Lahey, "The [RHID] Mathis, M., Semke, J., Mahdavi, J., and K. Lahey, "The
Rate-Halving Algorithm for TCP Congestion Control", draft- Rate-Halving Algorithm for TCP Congestion Control", draft-
ratehalving (work in progress), June 1999. ratehalving (work in progress), June 1999.
[RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding [RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding
Parameters", Web publication , December 1997. Parameters", Web publication , December 1997.
[CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed [CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed
TCP variant", PFLDnet 2005, Feb 2005. 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 Appendix A. Packet Conservation Bound
PRR-CRB meets a conservative, philosophically pure and aesthetically PRR-CRB meets a conservative, philosophically pure and aesthetically
appealing notion of correct, described here. However, in real appealing notion of correct, described here. However, in real
networks it does not perform as well as the algorithms described in networks it does not perform as well as the algorithms described in
RFC 3517, which prove to be non-conservative in a statistically RFC 3517, which proves to be non-conservative in a significant number
significant number of cases. of cases.
Under all conditions and sequences of events during recovery, PRR-CRB Under all conditions and sequences of events during recovery, PRR-CRB
strictly bounds the data transmitted to be equal to or less than the 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 amount of data delivered to the receiver. We claim that this packet
conservation bound is the most aggressive algorithm that does not conservation bound is the most aggressive algorithm that does not
lead to additional forced losses in some environments. It has the lead to additional forced losses in some environments. It has the
property that if there is a standing queue at a bottleneck that is property that if there is a standing queue at a bottleneck that is
carrying no other traffic, the queue will maintain exactly constant carrying no other traffic, the queue will maintain exactly constant
length for the entire recovery duration (except for +1/-1 fluctuation length for the entire duration of the recovery, except for +1/-1
due to differences in packet arrival and exit times) . Any less fluctuation due to differences in packet arrival and exit times. Any
aggressive algorithm will result in a declining queue at the less aggressive algorithm will result in a declining queue at the
bottleneck. Any more aggressive algorithm will result in an bottleneck. Any more aggressive algorithm will result in an
increasing queue or additional losses at the bottleneck. increasing queue or additional losses if it is a full drop tail
queue.
We demonstrate this property with a little thought experiment: We demonstrate this property with a little thought experiment:
Imagine a network path that has insignificant delays in both Imagine a network path that has insignificant delays in both
directions, except the processing time and queue at a single directions, except for the processing time and queue at a single
bottleneck in the forward path. By insignificant delay, I mean when bottleneck in the forward path. By insignificant delay, I mean when
a packet is "served" at the head of the bottleneck queue, the a packet is "served" at the head of the bottleneck queue, the
following events happen in much less than one bottleneck packet time: following events happen in much less than one bottleneck packet time:
the packet arrives at the receiver; the receiver sends an ACK; which the packet arrives at the receiver; the receiver sends an ACK; which
arrives at the sender; the sender processes the ACK and sends some arrives at the sender; the sender processes the ACK and sends some
data; the data is queued at the bottleneck. data; the data is queued at the bottleneck.
If sndcnt is set to DeliveredData and nothing else is inhibiting If sndcnt is set to DeliveredData and nothing else is inhibiting
sending data, then clearly the data arriving at the bottleneck queue sending data, then clearly the data arriving at the bottleneck queue
will exactly replace the data that was served at the head of the will exactly replace the data that was served at the head of the
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reordering on the ACK path only cause wider fluctuations in the queue 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 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 is in order or out-of-order (including loss recovery from an earlier
RTT). Any more aggressive algorithm which sends additional data will RTT). Any more aggressive algorithm which sends additional data will
cause a queue overflow and loss. Any less aggressive algorithm will cause a queue overflow and loss. Any less aggressive algorithm will
under fill the queue. Therefore setting sndcnt to DeliveredData is under fill the queue. Therefore setting sndcnt to DeliveredData is
the most aggressive algorithm that does not cause forced losses in the most aggressive algorithm that does not cause forced losses in
this simple network. Relaxing the assumptions (e.g. making delays this simple network. Relaxing the assumptions (e.g. making delays
more authentic and adding more flows, delayed ACKs, etc) may more authentic and adding more flows, delayed ACKs, etc) may
increases the fine grained fluctuations in queue size but does not increases the fine grained fluctuations in queue size but does not
change it's basic behavior. change its basic behavior.
Note that the congestion control algorithm implements a broader Note that the congestion control algorithm implements a broader
notion of optimal that includes appropriately sharing of the network. notion of optimal that includes appropriately sharing of the network.
Typical congestion control algorithms are likely to reduce the data Typical congestion control algorithms are likely to reduce the data
sent relative to the packet conserving bound implemented by PRR sent relative to the packet conserving bound implemented by PRR
bringing TCP's actual window down to ssthresh. bringing TCP's actual window down to ssthresh.
Authors' Addresses Authors' Addresses
Matt Mathis Matt Mathis
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