draft-ietf-tcpm-initcwnd-00.txt   draft-ietf-tcpm-initcwnd-01.txt 
Internet Draft J. Chu Internet Draft J. Chu
draft-ietf-tcpm-initcwnd-00.txt N. Dukkipati draft-ietf-tcpm-initcwnd-01.txt N. Dukkipati
Intended status: TBD Y. Cheng Intended status: Standard Y. Cheng
Updates: 3390, 5681 M. Mathis Updates: 3390, 5681 M. Mathis
Creation date: October 6, 2010 Google, Inc. Creation date: April 15, 2011 Google, Inc.
Expiration date: April 2011 Expiration date: October 2011
Increasing TCP's Initial Window Increasing TCP's Initial Window
Status of this Memo Status of this Memo
Distribution of this memo is unlimited. Distribution of this memo is unlimited.
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 34 skipping to change at page 1, line 34
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This Internet-Draft will expire on April, 2011. This Internet-Draft will expire on October, 2011.
Copyright Notice Copyright Notice
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Abstract Abstract
This document proposes an increase in the permitted TCP initial This document proposes an increase in the permitted TCP initial
window (IW) from between 2 and 4 segments, as specified in RFC 3390, window (IW) from between 2 and 4 segments, as specified in RFC 3390,
to 10 segments. It discusses the motivation behind the increase, the to 10 segments. It discusses the motivation behind the increase, the
advantages and disadvantages of the higher initial window, and advantages and disadvantages of the higher initial window, and
presents results from several large scale experiments showing that presents results from several large scale experiments showing that
the higher initial window improves the overall performance of many the higher initial window improves the overall performance of many
web services without risking congestion collapse. Finally, it web services without risking congestion collapse. The document closes
outlines a list of concerns to be addressed in future tests. with a discussion of a list of concerns, and some results from recent
studies to address the concerns.
Terminology Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Editor's Note Table of Contents
This draft aims at updating RFC 3390, thus it follows RFC 3390's
layout closely. Much of the analysis from RFC 3390 remains valid.
Some non-critical details are intentionally excluded from this draft.
The intent is to have the draft published to solicit feedbacks early.
All the excluded pieces will be supplied in later revisions.
Note that the intended publication track is left as TBD for now as 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
there has not been a clear consensus within the TCP community on 2. TCP Modification . . . . . . . . . . . . . . . . . . . . . . . 3
whether the change suggested in this draft is appropriate as an 3. Implementation Issues . . . . . . . . . . . . . . . . . . . . . 4
Internet standard to be deployed universally. The plan is to 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
determine the right level through a TCPM working group consensus call 5. Advantages of Larger Initial Windows . . . . . . . . . . . . . 6
at a future time when the effect of the change is better understood. 5.1 Reducing Latency . . . . . . . . . . . . . . . . . . . . . . 6
5.2 Keeping up with the growth of web object size . . . . . . . 7
5.3 Recovering faster from loss on under-utilized or wireless
links . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Disadvantages of Larger Initial Windows for the Network . . . . 9
8. Mitigation of Negative Impact . . . . . . . . . . . . . . . . . 9
9. Interactions with the Retransmission Timer . . . . . . . . . . 9
10. Experimental Results From Large Scale Cluster Tests . . . . . 10
10.1 The benefits . . . . . . . . . . . . . . . . . . . . . . 10
10.2 The cost . . . . . . . . . . . . . . . . . . . . . . . . 11
11. List of Concerns and Corresponding Test Results . . . . . . . 12
12. Related Proposals . . . . . . . . . . . . . . . . . . . . . . 14
14. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 15
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . . . 16
Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
This document updates RFC 3390 to raise the upper bound on TCP's
initial window (IW) to 10 segments or roughly 15KB. It is patterned
after and borrows heavily from RFC 3390 [RFC3390] and earlier work in
this area.
TCP congestion window was introduced as part of the congestion The primary argument in favor of raising IW follows from the evolving
control algorithm by Van Jacobson in 1988 [Jac88]. The initial value scale of the Internet. Ten segments are likely to fit into queue
of one segment was used as the starting point for newly established space available at any broadband access link, even when there are a
connections to probe the available bandwidth on the network. reasonable number of concurrent connections.
The default value was increased to roughly 4KB more than a decade ago Lower speed links can be treated with environment specific
[RFC2414]. Since then, the Internet has continued to grow, both in configurations, such that they can be protected from being
speed and penetration [AKAM10]. Today's Internet is dominated by web overwhelmed by large initial window bursts without imposing a
traffic running on top of short-lived TCP connections [IOR2009]. The suboptimal initial window on the rest of the Internet.
relatively small initial window has become a limiting factor for the
performance of many web applications.
This document proposes an optional standard to allow TCP's initial This document reviews the advantages and disadvantages of using a
window to start at 10 segments or roughly 15KB, updating RFC 3390 larger initial window, and includes summaries of several large scale
experiments showing that an initial window of 10 segments provides
benefits across the board for a variety of BW, RTT, and BDP classes.
These results show significant benefits for increasing IW for users
at much smaller data rates than had been previously anticipated.
However, at initial windows larger than 10, the results are mixed. We
believe that these mixed results are not intrinsic, but are the
consequence of various implementation artifacts, including overly
aggressive applications employing many simultaneous connections.
[RFC3390]. It discusses the motivation, the advantages and We propose that all TCP implementations should have a settable TCP IW
disadvantages of the higher initial window, and includes test results parameter; the default setting may start at 10 segments and should be
from several large scale experiments showing improved latency across raised as we come to understand and and correct things that conflict.
the board for a variety of BW, RTT, and BDP classes.
It also discusses potential negative impacts and suggests mitigation. In addition, we introduce a minor revision to RFC 3390 and RFC 5681
A minor change to RFC 3390 and RFC 5681 [RFC5681] is proposed on [RFC5681] to eliminate resetting the initial window when the SYN or
resetting the initial window when the SYN or SYN/ACK is lost. SYN/ACK is lost.
The document closes with a discussion on remaining concerns, and The document closes with a discussion of a list of concerns that have
future tests to further validate the higher initial window. been brought up, and some recent test results showing most of the
concerns can not be validated.
A complementary set of slides for this proposal can be found at
[CD10].
2. TCP Modification 2. TCP Modification
This document proposes an increase in the permitted upper bound for This document proposes an increase in the permitted upper bound for
TCP's initial window (IW) to 10 segments. This increase is optional: TCP's initial window (IW) to 10 segments. This increase is optional:
a TCP MAY start with a larger initial window up to 10 segments. a TCP MAY start with a larger initial window up to 10 segments.
This upper bound for the initial window size represents a change from This upper bound for the initial window size represents a change from
RFC 3390 [RFC3390], which specified that the congestion window be RFC 3390 [RFC3390], which specified that the congestion window be
initialized between 2 and 4 segments depending on the MSS. initialized between 2 and 4 segments depending on the MSS.
This change applies to the initial window of the connection in the This change applies to the initial window of the connection in the
first round trip time (RTT) of data transmission following the TCP first round trip time (RTT) of data transmission following the TCP
three-way handshake. Neither the SYN/ACK nor its acknowledgment (ACK) three-way handshake. Neither the SYN/ACK nor its acknowledgment (ACK)
in the three-way handshake should increase the initial window size in the three-way handshake should increase the initial window size.
beyond 10 segments.
Furthermore, RFC 3390 and RFC 5681 [RFC5681] state that Furthermore, RFC 3390 and RFC 5681 [RFC5681] state that
"If the SYN or SYN/ACK is lost, the initial window used by a "If the SYN or SYN/ACK is lost, the initial window used by a
sender after a correctly transmitted SYN MUST be one segment sender after a correctly transmitted SYN MUST be one segment
consisting of MSS bytes." consisting of MSS bytes."
The proposed change to reduce the default RTO to 1 second [PAC10] The proposed change to reduce the default RTO to 1 second [PACS11]
increases the chance for spurious SYN or SYN/ACK retransmission, thus increases the chance for spurious SYN or SYN/ACK retransmission, thus
unnecessarily penalizing connections with RTT > 1 second if their unnecessarily penalizing connections with RTT > 1 second if their
initial window is reduced to 1 segment. For this reason, it is initial window is reduced to 1 segment. For this reason, it is
RECOMMENDED that implementations refrain from resetting the initial RECOMMENDED that implementations refrain from resetting the initial
window to 1 segment, unless either there have been multiple SYN or window to 1 segment, unless either there have been multiple SYN or
SYN/ACK retransmissions, or true loss detection has been made. SYN/ACK retransmissions, or true loss detection has been made.
TCP implementations use slow start in as many as three different TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window); restart transmission after a long idle period (the restart window);
skipping to change at page 4, line 17 skipping to change at page 4, line 45
change the loss window, which must remain 1 segment of MSS bytes (to change the loss window, which must remain 1 segment of MSS bytes (to
permit the lowest possible window size in the case of severe permit the lowest possible window size in the case of severe
congestion). congestion).
Furthermore, to limit any negative effect that a larger initial Furthermore, to limit any negative effect that a larger initial
window may have on links with limited bandwidth or buffer space, window may have on links with limited bandwidth or buffer space,
implementations SHOULD fall back to RFC 3390 for the restart window implementations SHOULD fall back to RFC 3390 for the restart window
(RW), if any packet loss is detected during either the initial (RW), if any packet loss is detected during either the initial
window, or a restart window, when more than 4KB of data is sent. window, or a restart window, when more than 4KB of data is sent.
3. Motivation 3. Implementation Issues
[Need to decide if a different formula is needed for PMTU != 1500.]
HTTP 1.1 specification allows only two simultaneous connections per
domain, while web browsers open more simultaneous TCP connections
[Ste08], partly to circumvent the small initial window in order to
speed up the loading of web pages as described above.
When web browsers open simultaneous TCP connections to the same
destination, they are working against TCP's congestion control
mechanisms [FF99]. Combining this behavior with larger initial
windows further increases the burstiness and unfairness to other
traffic in the network. A larger initial window will incentivize
applications to use fewer concurrent TCP connections.
Some implementations advertise small initial receive window (Table 2
in [Duk10]), effectively limiting how much window a remote host may
use. In order to realize the full benefit of the large initial
window, implementations are encouraged to advertise an initial
receive window of at least 10 segments, except for the circumstances
where a larger initial window is deemed harmful. (See the Mitigation
section below.)
TCP SACK option ([RFC2018]) was thought to be required in order for
the larger initial window to perform well. But measurements from both
a testbed and live tests showed that IW=10 without the SACK option
still beats the performance of IW=3 with the SACK option [CW10].
4. Background
TCP congestion window was introduced as part of the congestion
control algorithm by Van Jacobson in 1988 [Jac88]. The initial value
of one segment was used as the starting point for newly established
connections to probe the available bandwidth on the network.
Today's Internet is dominated by web traffic running on top of short-
lived TCP connections [IOR2009]. The relatively small initial window
has become a limiting factor for the performance of many web
applications.
The global Internet has continued to grow, both in speed and The global Internet has continued to grow, both in speed and
penetration. According to the latest report from Akamai [AKAM10], the penetration. According to the latest report from Akamai [AKAM10], the
global broadband (> 2Mbps) adoption has surpassed 50%, propelling the global broadband (> 2Mbps) adoption has surpassed 50%, propelling the
average connection speed to reach 1.7Mbps, while the narrowband (< average connection speed to reach 1.7Mbps, while the narrowband (<
256Kbps) usage has dropped to 5%. In contrast, TCP's initial window 256Kbps) usage has dropped to 5%. In contrast, TCP's initial window
has remained 4KB for a decade, corresponding to a bandwidth has remained 4KB for a decade [RFC2414], corresponding to a bandwidth
utilization of less than 200Kbps per connection, assuming an RTT of utilization of less than 200Kbps per connection, assuming an RTT of
200ms. 200ms.
A large proportion of flows on the Internet are short web A large proportion of flows on the Internet are short web
transactions over TCP, and complete before exiting TCP slow start. transactions over TCP, and complete before exiting TCP slow start.
Speeding up the TCP flow startup phase, including circumventing the Speeding up the TCP flow startup phase, including circumventing the
initial window limit, has been an area of active research [PWSB09, initial window limit, has been an area of active research [PWSB09,
Sch08]. Numerous proposals exist [LAJW07, RFC4782, PRAKS02, PK98]. Sch08]. Numerous proposals exist [LAJW07, RFC4782, PRAKS02, PK98].
Some require router support [RFC4782, PK98], hence are not practical Some require router support [RFC4782, PK98], hence are not practical
for the public Internet. Others suggested bold, but often radical for the public Internet. Others suggested bold, but often radical
ideas, likely requiring more years of research before standardization ideas, likely requiring more years of research before standardization
and deployment. and deployment.
In the mean time, applications have responded to TCP's "slow" start. In the mean time, applications have responded to TCP's "slow" start.
Web sites use multiple sub-domains [Bel10] to circumvent HTTP 1.1 Web sites use multiple sub-domains [Bel10] to circumvent HTTP 1.1
regulation on two connections per physical host [RFC2616]. As of regulation on two connections per physical host [RFC2616]. As of
today, major web browsers open multiple connections to the same site today, major web browsers open multiple connections to the same site
(up to six connections per domain [Ste08] and the number is growing). (up to six connections per domain [Ste08] and the number is growing).
skipping to change at page 5, line 9 skipping to change at page 6, line 29
initial congestion window may cause congestion for certain users initial congestion window may cause congestion for certain users
using these browsers, we argue that the browsers and other using these browsers, we argue that the browsers and other
application need to respect HTTP 1.1 regulation and stop increasing application need to respect HTTP 1.1 regulation and stop increasing
number of simultaneous TCP connections. We believe a modest increase number of simultaneous TCP connections. We believe a modest increase
of the initial window will help to stop this trend, and provide the of the initial window will help to stop this trend, and provide the
best interim solution to improve overall user performance, and reduce best interim solution to improve overall user performance, and reduce
the server, client, and network load. the server, client, and network load.
Note that persistent connections and pipelining are designed to Note that persistent connections and pipelining are designed to
address some of the issues with HTTP above [RFC2616]. Their presence address some of the issues with HTTP above [RFC2616]. Their presence
does not diminish the need for a larger initial window, as the first does not diminish the need for a larger initial window. E.g., data
data chunk to respond is often the largest, and will easily hit the from the Chrome browser show that 35% of HTTP requests are made on
initial window limit. Our test data confirm significant latency new TCP connections. Our test data also confirm significant latency
reduction with the large initial window even with these two HTTP reduction with the large initial window even with these two HTTP
features ([Duk10]). features ([Duk10]).
Also note that packet pacing has been suggested as an effective Also note that packet pacing has been suggested as an effective
mechanism to avoid large bursts and their associated damage [VH97]. mechanism to avoid large bursts and their associated damage [VH97].
We do not require pacing in our proposal due to our strong preference We do not require pacing in our proposal due to our strong preference
for a simple solution. We suspect for packet bursts of a moderate for a simple solution. We suspect for packet bursts of a moderate
size, packet pacing will not be necessary. This seems to be confirmed size, packet pacing will not be necessary. This seems to be confirmed
by our test results. by our test results.
More discussion of the increase in initial window, including the More discussion of the increase in initial window, including the
choice of 10 segments can be found in [Duk10]. choice of 10 segments can be found in [Duk10, CD10].
4. Implementation Issues
[Need to decide if a different formula is needed for PMTU != 1500.]
HTTP 1.1 specification allows only two simultaneous connections per
domain, while web browsers open more simultaneous TCP connections
[Ste08], partly to circumvent the small initial window in order to
speed up the loading of web pages as described above.
When web browsers open simultaneous TCP connections to the same
destination, they are working against TCP's congestion control
mechanisms [FF99]. Combining this behavior with larger initial
windows further increases the burstiness and unfairness to other
traffic in the network. A larger initial window will incent
applications to use fewer concurrent TCP connections.
Some implementations advertise small initial receive window (Table 2
in [Duk10]), effectively limiting how much window a remote host may
use. In order to realize the full benefit of the large initial
window, implementations are encouraged to advertise an initial
receive window of at least 10 segments, except for the circumstances
where a larger initial window is deemed harmful. (See the Mitigation
section below.)
5. Advantages of Larger Initial Windows 5. Advantages of Larger Initial Windows
1. Reducing Latency 5.1 Reducing Latency
An increase of the initial window from 3 segments to 10 segments
reduces the total transfer time for data sets greater than 4KB by
up to 4 round trips.
The table below compares the number of round trips between IW=3
and IW=10 for different transfer sizes, assuming infinite
bandwidth, no packet loss, and the standard delayed acks with
large delay-ack timer.
--------------------------------------- An increase of the initial window from 3 segments to 10 segments
| total segments | IW=3 | IW=10 | reduces the total transfer time for data sets greater than 4KB by up
--------------------------------------- to 4 round trips.
| 3 | 1 | 1 |
| 6 | 2 | 1 |
| 10 | 3 | 1 |
| 12 | 3 | 2 |
| 21 | 4 | 2 |
| 25 | 5 | 2 |
| 33 | 5 | 3 |
| 46 | 6 | 3 |
| 51 | 6 | 4 |
| 78 | 7 | 4 |
| 79 | 8 | 4 |
| 120 | 8 | 5 |
| 127 | 9 | 5 |
---------------------------------------
For example, with the larger initial window, a transfer of 32 The table below compares the number of round trips between IW=3 and
segments of data will require only two rather than five round IW=10 for different transfer sizes, assuming infinite bandwidth, no
trips to complete. packet loss, and the standard delayed acks with large delayed-ack
timer.
2. Keeping up with the growth of web object size ---------------------------------------
| total segments | IW=3 | IW=10 |
---------------------------------------
| 3 | 1 | 1 |
| 6 | 2 | 1 |
| 10 | 3 | 1 |
| 12 | 3 | 2 |
| 21 | 4 | 2 |
| 25 | 5 | 2 |
| 33 | 5 | 3 |
| 46 | 6 | 3 |
| 51 | 6 | 4 |
| 78 | 7 | 4 |
| 79 | 8 | 4 |
| 120 | 8 | 5 |
| 127 | 9 | 5 |
---------------------------------------
RFC 3390 stated that the main motivation for increasing the For example, with the larger initial window, a transfer of 32
initial window to 4KB was to speed up connections that only segments of data will require only two rather than five round trips
transmit a small amount of data, e.g., email and web. The to complete.
majority of transfers back then were less than 4KB, and could be
completed in a single RTT [All00].
Since RFC 3390 was published, web objects have gotten 5.2 Keeping up with the growth of web object size
significantly larger [Chu09, RJ10]. A large percentage of web
objects today no longer fit in the 4KB initial window, and will
require more than one round trip to transfer. E.g., only 10% of
Google's search responses can fit in 4KB, while 90% can fit in 10
segments (15KB). The average HTTP response size of gmail.com, a
highly scripted web-site, is 8KB (Figure 1. in [Duk10]).
During the same period, the average web page, including all RFC 3390 stated that the main motivation for increasing the initial
static and dynamic scripted web objects on the page, has seen window to 4KB was to speed up connections that only transmit a small
even greater growth in size [RJ10]. HTTP pipelining [RFC2616] and amount of data, e.g., email and web. The majority of transfers back
new web transport protocols like SPDY [SPDY] allow multiple web then were less than 4KB, and could be completed in a single RTT
objects to be sent in a single transaction, potentially requiring [All00].
even larger initial window in order to transfer a whole web page
in one round trip.
3. Recovering faster from loss on under-utilized or wireless links Since RFC 3390 was published, web objects have gotten significantly
larger [Chu09, RJ10]. Today only a small percentage of web objects
(e.g., 10% of Google's search responses) can fit in the 4KB initial
window. The average HTTP response size of gmail.com, a highly
scripted web-site, is 8KB (Figure 1. in [Duk10]). The average web
page, including all static and dynamic scripted web objects on the
page, has seen even greater growth in size [RJ10]. HTTP pipelining
[RFC2616] and new web transport protocols like SPDY [SPDY] allow
multiple web objects to be sent in a single transaction, potentially
requiring even larger initial window in order to transfer a whole web
page in one round trip.
A greater-than-3-segment initial window increases the chance to 5.3 Recovering faster from loss on under-utilized or wireless links
recover packet loss through Fast Retransmit rather than the A greater-than-3-segment initial window increases the chance to
lengthy initial RTO [RFC5681]. This is because the fast recover packet loss through Fast Retransmit rather than the lengthy
retransmit algorithm requires three duplicate acks as an initial RTO [RFC5681]. This is because the fast retransmit algorithm
indication that a segment has been lost rather than reordered. requires three duplicate acks as an indication that a segment has
While newer loss recovery techniques such as Limited Transmit been lost rather than reordered. While newer loss recovery techniques
[RFC3042] and Early Retransmit [AAABH10] have been proposed to such as Limited Transmit [RFC3042] and Early Retransmit [RFC5827]
help speeding up loss recovery from a smaller window, both have been proposed to help speeding up loss recovery from a smaller
algorithms can still benefit from the larger initial window window, both algorithms can still benefit from the larger initial
because of a better chance to receive more ACKs to react upon. window because of a better chance to receive more ACKs to react upon.
6. Disadvantages of Larger Initial Windows for the Individual 6. Disadvantages of Larger Initial Windows for the Individual Connection
Connection
The larger bursts from an increase in the initial window may cause The larger bursts from an increase in the initial window may cause
buffer overrun and packet drop in routers with small buffers, or buffer overrun and packet drop in routers with small buffers, or
routers experiencing congestion. This could result in unnecessary routers experiencing congestion. This could result in unnecessary
retransmit timeouts. For a large-window connection that is able to retransmit timeouts. For a large-window connection that is able to
recover without a retransmit timeout, this could result in an recover without a retransmit timeout, this could result in an
unnecessarily-early transition from the slow-start to the congestion- unnecessarily-early transition from the slow-start to the congestion-
avoidance phase of the window increase algorithm. [Note: knowing the avoidance phase of the window increase algorithm. [Note: knowing the
large initial window may cause premature segment drop, should one large initial window may cause premature segment drop, should one
make an exception for it, i.e., by allowing ssthresh to remain make an exception for it, i.e., by allowing ssthresh to remain
skipping to change at page 7, line 50 skipping to change at page 8, line 39
the congested router uses active queue management (such as Random the congested router uses active queue management (such as Random
Early Detection [FJ93, RFC2309, RFC3150]). Early Detection [FJ93, RFC2309, RFC3150]).
Insufficient buffering is more likely to exist in the access routers Insufficient buffering is more likely to exist in the access routers
connecting slower links. A recent study of access router buffer size connecting slower links. A recent study of access router buffer size
[DGHS07] reveals the majority of access routers provision enough [DGHS07] reveals the majority of access routers provision enough
buffer for 130ms or longer, sufficient to cover a burst of more than buffer for 130ms or longer, sufficient to cover a burst of more than
10 packets at 1Mbps speed, but possibly not sufficient for browsers 10 packets at 1Mbps speed, but possibly not sufficient for browsers
opening simultaneous connections. opening simultaneous connections.
A testbed study [CW10] on the effect of the larger initial window
with five simultaneously opened connections revealed that, even with
limited buffer size on slow links, IW=10 still reduced the total
latency of web transactions, although at the cost of higher packet
drop rates as compared to IW=3.
Some TCP connections will receive better performance with the larger Some TCP connections will receive better performance with the larger
initial window even if the burstiness of the initial window results initial window even if the burstiness of the initial window results
in premature segment drops. This will be true if (1) the TCP in premature segment drops. This will be true if (1) the TCP
connection recovers from the segment drop without a retransmit connection recovers from the segment drop without a retransmit
timeout, and (2) the TCP connection is ultimately limited to a small timeout, and (2) the TCP connection is ultimately limited to a small
congestion window by either network congestion or by the receiver's congestion window by either network congestion or by the receiver's
advertised window. advertised window.
7. Disadvantages of Larger Initial Windows for the Network 7. Disadvantages of Larger Initial Windows for the Network
skipping to change at page 8, line 27 skipping to change at page 9, line 22
congestion collapse. This seems to have been confirmed by our large congestion collapse. This seems to have been confirmed by our large
scale experiments described later. scale experiments described later.
Some of the discussions from RFC 3390 are still valid for IW=10. Some of the discussions from RFC 3390 are still valid for IW=10.
Moreover, it is worth noting that although TCP NewReno increases the Moreover, it is worth noting that although TCP NewReno increases the
chance of duplicate segments when trying to recover multiple packet chance of duplicate segments when trying to recover multiple packet
losses from a large window [RFC3782], the wide support of TCP losses from a large window [RFC3782], the wide support of TCP
Selective Acknowledgment (SACK) option [RFC2018] in all major OSes Selective Acknowledgment (SACK) option [RFC2018] in all major OSes
today should keep the volume of duplicate segments in check. today should keep the volume of duplicate segments in check.
Recent measurements [Get11] provide evidence of extremely large
queues (in the order of one second) at access networks of the
Internet. While a significant part of the buffer bloat is contributed
by large downloads/uploads such as video files, emails with large
attachments, backups and download of movies to disk, some of the
problem is also caused by Web browsing of image heavy sites [Get11].
This queuing delay is generally considered harmful for responsiveness
of latency sensitive traffic such as DNS queries, ARP, DHCP, VoIP and
Gaming. IW=10 can exacerbate this problem when doing short downloads
such as Web browsing. The mitigations proposed for the broader
problem of buffer bloating are also applicable in this case, such as
the use of ECN, AQM schemes and traffic classification (QoS).
8. Mitigation of Negative Impact 8. Mitigation of Negative Impact
Much of the negative impact from an increase in the initial window is Much of the negative impact from an increase in the initial window is
likely to be felt by users behind slow links with limited buffers. likely to be felt by users behind slow links with limited buffers.
The negative impact can be mitigated by hosts directly connected to a The negative impact can be mitigated by hosts directly connected to a
low-speed link advertising a smaller initial receive window than 10 low-speed link advertising a smaller initial receive window than 10
segments. This can be achieved either through manual configuration by segments. This can be achieved either through manual configuration by
the users, or through the host stack auto-detecting the low bandwidth the users, or through the host stack auto-detecting the low bandwidth
links. links.
skipping to change at page 8, line 44 skipping to change at page 10, line 4
the users, or through the host stack auto-detecting the low bandwidth the users, or through the host stack auto-detecting the low bandwidth
links. links.
More suggestions to improve the end-to-end performance of slow links More suggestions to improve the end-to-end performance of slow links
can be found in RFC 3150 [RFC3150]. can be found in RFC 3150 [RFC3150].
[Note: if packet loss is detected during IW through fast retransmit, [Note: if packet loss is detected during IW through fast retransmit,
should cwnd back down to 2 rather than FlightSize / 2?] should cwnd back down to 2 rather than FlightSize / 2?]
9. Interactions with the Retransmission Timer 9. Interactions with the Retransmission Timer
A large initial window increases the chance of spurious RTO on a low- A large initial window increases the chance of spurious RTO on a low-
bandwidth path because the packet transmission time will dominate the bandwidth path because the packet transmission time will dominate the
round-trip time. To minimize spurious retransmissions, round-trip time. To minimize spurious retransmissions,
implementations MUST follow RFC 2988 [RFC2988] to restart the implementations MUST follow RFC 2988 [RFC2988] to restart the
retransmission timer with the current value of RTO for each ack retransmission timer with the current value of RTO for each ack
received that acknowledges new data. received that acknowledges new data.
10. Experimental Results 10. Experimental Results From Large Scale Cluster Tests
In this section we summarize our findings from large scale Internet In this section we summarize our findings from large scale Internet
experiments with an initial window of 10 segments, conducted via experiments with an initial window of 10 segments, conducted via
Google's front-end infrastructure serving a diverse set of Google's front-end infrastructure serving a diverse set of
applications. We present results from two datacenters, each chosen applications. We present results from two data centers, each chosen
because of the specific characteristics of subnets served: AvgDC has because of the specific characteristics of subnets served: AvgDC has
connection bandwidths closer to the worldwide average reported in connection bandwidths closer to the worldwide average reported in
[AKAM10], with a median connection speed of about 1.7Mbps; SlowDC has [AKAM10], with a median connection speed of about 1.7Mbps; SlowDC has
a larger proportion of traffic from slow bandwidth subnets with a larger proportion of traffic from slow bandwidth subnets with
nearly 20% of traffic from connections below 100Kbps, and a third nearly 20% of traffic from connections below 100Kbps, and a third
below 256Kbps. below 256Kbps.
Guided by measurements data, we answer two key questions: what is the Guided by measurements data, we answer two key questions: what is the
latency benefit when TCP connections start with a higher initial latency benefit when TCP connections start with a higher initial
window, and on the flip side, what is the cost? window, and on the flip side, what is the cost?
skipping to change at page 10, line 30 skipping to change at page 11, line 37
in AvgDC is 0.3% (from 1.98% to 2.29%) and in SlowDC is 0.7% (from in AvgDC is 0.3% (from 1.98% to 2.29%) and in SlowDC is 0.7% (from
3.54% to 4.21%). In our investigation, with the exception of one 3.54% to 4.21%). In our investigation, with the exception of one
application, the larger window resulted in a retransmission increase application, the larger window resulted in a retransmission increase
of < 0.5% for services in the AvgDC. The exception is the Maps of < 0.5% for services in the AvgDC. The exception is the Maps
application that operates with multiple concurrent TCP connections, application that operates with multiple concurrent TCP connections,
which increased its retransmission rate by 0.9% in AvgDC and 1.85% in which increased its retransmission rate by 0.9% in AvgDC and 1.85% in
SlowDC (from 3.94% to 5.79%). SlowDC (from 3.94% to 5.79%).
In our experiments, the percentage of traffic experiencing In our experiments, the percentage of traffic experiencing
retransmissions did not increase significantly. E.g. 90% of web retransmissions did not increase significantly. E.g. 90% of web
search and maps experienced zero retransmissions in SlowDC search and maps experienced zero retransmission in SlowDC
(percentages are higher for AvgDC); a break up of retransmissions by (percentages are higher for AvgDC); a break up of retransmissions by
percentiles indicate that most increases come from portion of traffic percentiles indicate that most increases come from portion of traffic
already experiencing retransmissions in the baseline with initial already experiencing retransmissions in the baseline with initial
window of 3 segments. window of 3 segments.
Traffic patterns from applications using multiple concurrent TCP Traffic patterns from applications using multiple concurrent TCP
connections all operating with a large initial window represent one connections all operating with a large initial window represent one
of the worst case scenarios where latency can be adversely impacted of the worst case scenarios where latency can be adversely impacted
due to bottleneck buffer overflow. Our investigation shows that such due to bottleneck buffer overflow. Our investigation shows that such
a traffic pattern has not been a problem in AvgDC, where all these a traffic pattern has not been a problem in AvgDC, where all these
skipping to change at page 11, line 6 skipping to change at page 12, line 13
mean, their latencies in higher quantiles (96 and above for maps) mean, their latencies in higher quantiles (96 and above for maps)
indicated instances where latency with larger window is worse than indicated instances where latency with larger window is worse than
the baseline, e.g. the 99% latency for maps has increased by 2.3% the baseline, e.g. the 99% latency for maps has increased by 2.3%
(80ms) when compared to the baseline. There is no evidence from our (80ms) when compared to the baseline. There is no evidence from our
measurements that such a cost on latency is a result of subnet measurements that such a cost on latency is a result of subnet
bandwidth alone. Although we have no way of knowing from our data, we bandwidth alone. Although we have no way of knowing from our data, we
conjecture that the amount of buffering at bottleneck links plays a conjecture that the amount of buffering at bottleneck links plays a
key role in performance of these applications. key role in performance of these applications.
Further details on our experiments and analysis can be found in Further details on our experiments and analysis can be found in
[Duk10]. [Duk10, DCCM10].
11. List of Concerns and Future Tests 11. List of Concerns and Corresponding Test Results
Although we were a little hard pressed to find negative impact from Concerns have been raised since we first published our proposal based
the initial window increase in our large scale tests, we don't on a set of large scale experiments. To better understand the impact
contend our test coverage is complete. The following is an attempt to of a larger initial window in order to confirm or dismiss these
compile a list of concerns and to suggest future tests. Ultimately we concerns, we, as well as people outside of Google have conducted
would like to enlist the help from the TCP community at IETF to study numerous additional tests in the past year, using either Google's
and address any concern that may come up. large scale clusters, simulations, or real testbeds. The following is
a list of concerns and some of the findings.
1. How complete are our tests in traffic pattern coverage? A complete list of tests conducted, their results and related studies
can be found at [IW10].
Google today offers a large portfolio of services beyond web o How complete are our tests in traffic pattern coverage?
search. The list includes Gmail, Google Maps, Photos, News,
Sites, Images, Videos,..., etc. Our tests included most of
Google's services, covering a wide variety of traffic sizes and
patterns. One notable exception is YouTube because we don't think
the large initial window will have much material impact, either
positive or negative, on bulk data services.
2. Larger bursts from the increase in the initial window cause Google today offers a large portfolio of services beyond web
significantly more packet drops search. The list includes Gmail, Google Maps, Photos, News, Sites,
Images, Videos,..., etc. Our tests included most of Google's
services, covering a wide variety of traffic sizes and patterns.
One notable exception is YouTube because we don't think the large
initial window will have much material impact, either positive or
negative, on bulk data services.
Let the max burst capacity of an end-to-end path be the largest [CW10] contains some result from a testbed study on how short flows
burst of packets a given path can absorb before packet is with a larger initial window might affect the throughput
dropped. To analyze the impact from the larger initial window, it performance of other co-existing, long lived, bulk data transfers.
helps to study the distribution of the max burst capacity of the
current Internet.
In the past similar studies were conducted by actively probing, o Larger bursts from the increase in the initial window cause
e.g., through the TCP echo/discard ports from a large set of significantly more packet drops
endhosts. However, most endhosts today are behind firewall
enabled NAT boxes, making active probing infeasible.
Our plan is to monitor TCP connections used to carry Google's All the known tests conducted on this subject so far [Duk10, Sch11,
bulk data services like YouTube, and infer the max burst capacity Sch11-1, CW10] show that, although bursts from the larger initial
on a per-client basis from TCP internal connection parameters window tend to cause more packet drops, the increase tends to be
such as ssthresh, max cwnd, and packet drop pattern. very modest. The only exception is from our own testbed study
[CW10] when under extremely high load and/or simultaneous opens.
But both IW=3 and IW=10 suffered very high packet loss rates under
those conditions.
3. Need more thorough analysis of the impact on slow links o A large initial window may severely impact TCP performance over
highly multiplexed links still common in developing regions
Although our data showed the large initial window reduced the Our large scale experiments described in section 10 above also
average latency even for the dialup link class of only 56Kbps in covered Africa and South America. Measurement data from those
bandwidth, it is only prudent to perform more microscopic regions [DCCM10] revealed improved latency even for those Google
analysis on its effect on slow links. Moreover, data from the services that employ multiple simultaneous connections, at the cost
YouTube study above will likely be biased toward broadband users, of small increase in the retransmission rate. It seems that the
leaving out users behind slow links. round trip savings from a larger initial window more than make up
the time spent on recovering more lost packets.
The narrowband classes here should include 56Kbps dialup modem, Similar phenomenon have also been observed from our testbed study
2.5G and GPRS mobile network. [CW10].
4. How will the larger initial window affect flows with initial o Why 10 segments?
windows 4KB or less?
Flows with the larger initial window will likely grab more Questions have been raised on how the number 10 was picked. We have
bandwidth from a bottleneck link when competing against flows tried different sizes in our large scale experiments, and found
with smaller initial window, at least initially. How long will that 10 segments seem to give most of the benefits for the services
this "unfairness" last? Will there be any "capture effect" where we tested while not causing significant increase in the
flows with larger initial window possess a disproportional share retransmission rates. Going forward 10 segments may turn out to be
of bandwidth beyond just a few round trips? too small when the average of web object sizes continue to grow. A
scheme to attempt to right size the initial window automatically
over long timescales has been proposed in [Tou10].
If there is any "unfairness" issue from flows with different o Need more thorough analysis of the impact on slow links
initial windows, it did not show up in our large scale
experiments, as the average latency for the bucket of all
responses < 4KB did not seem to be affected by the presence of
many other larger responses employing large initial window. As a
matter of fact they seemed to benefit from the large initial
window too, as shown in Figure 7 of [Duk10].
More study can be done through simulation, similar to the set Although data from [Duk10] showed the large initial window reduced
described in RFC 2415 [RFC2415]. the average latency even for the dialup link class of only 56Kbps
in bandwidth, it is only prudent to perform more microscopic
analysis on its effect on slow links. We set up two testbeds for
this purpose [CW10].
12. Security Considerations Both testbeds were used to emulate a 300ms RTT, bottleneck link
bandwidth as low as 64Kbps, and route queue size as low as 40
packets. Although we've tried a large combination of test
parameters, almost all tests we ran managed to show some latency
improvement from IW=10, with only a modest increase in the packet
drop rate until a very high load was injected. The testbed result
was consistent with both our own large scale data center
experiments [CD10, DCCM10] and a separate study using NSC
simulations [Sch11, Sch11-1].
o How will the larger initial window affect flows with initial
windows 4KB or less?
Flows with the larger initial window will likely grab more
bandwidth from a bottleneck link when competing against flows with
smaller initial window, at least initially. How long will this
"unfairness" last? Will there be any "capture effect" where flows
with larger initial window possess a disproportional share of
bandwidth beyond just a few round trips?
If there is any "unfairness" issue from flows with different
initial windows, it did not show up in our large scale experiments,
as the average latency for the bucket of all responses < 4KB did
not seem to be affected by the presence of many other larger
responses employing large initial window. As a matter of fact they
seemed to benefit from the large initial window too, as shown in
Figure 7 of [Duk10].
The same phenomenon seems to exist in our testbed experiments.
Flows with IW=3 only suffered slightly when competing against flows
with IW=10 in light to median loads. Under high load both flows'
latency improved when mixed together. Also long-lived, background
bulk-data flows seemed to enjoy higher throughput when running
against many foreground short flows of IW=10 than against short
flows of IW=3. One plausible explanation was IW=10 enabled short
flows to complete sooner, leaving more room for the long-lived,
background flows.
An independent study using NSC simulator has also concluded that
IW=10 works rather well and is quite fair against IW=3 [Sch11,
Sch11-1].
o How will a larger initial window perform over cellular networks?
Some simulation studies [JNDK10, JNDK10-1] have been conducted to
study the effect of a larger initial window on wireless links from
2G to 4G networks (EGDE/HSPA/LTE). The overall result seems mixed
in both raw performance and the fairness index.
There has been on-going studies by people from Nokia on the effect
of a larger initial window on GPRS and HSDPA networks. Initial test
results seem to show no or little improvement from flows with a
larger initial window. More studies are needed to understand why.
12. Related Proposals
Two other proposals [All10, Tou10] have been made with the goal to
raise TCP's initial window size over a large timescale. Both aim at
addressing the concern about the uncertain impact from raising the
initial window size at an Internet wide scale. Moreover, [Tou10]
seeks an algorithm to automate the adjustment of IW safely over long
haul period.
Based on our test results from the past couple of years, we believe
our proposal - a modest, static increase of IW to 10, to be the best
near-term solution that is both simple and effective. The other
proposals, with their added complexity and much longer deployment
cycles, seem best suited for growing IW beyond 10 in the long run.
13. Security Considerations
This document discusses the initial congestion window permitted for This document discusses the initial congestion window permitted for
TCP connections. Changing this value does not raise any known new TCP connections. Changing this value does not raise any known new
security issues with TCP. security issues with TCP.
13. Conclusion 14. Conclusion
This document suggests a change to TCP that will likely be beneficial This document suggests a simple change to TCP that will reduce the
to short-lived TCP connections and those over links with long RTTs application latency over short-lived TCP connections or links with
(saving several RTTs during the initial slow-start phase). However, long RTTs (saving several RTTs during the initial slow-start phase)
more tests are likely needed to fully understand its impact to the with little or no negative impact over other flows. Extensive tests
Internet. We welcome any help from the TCP community at IETF in have been conducted through both testbeds and large data centers with
moving this proposal forward. most results showing improved latency with only a small increase in
the packet retransmission rate. Based on these results we believe a
modest increase of IW to 10 is the best near-term proposal while
other proposals [All10, Tou10] may be best suited to grow IW beyond
10 in the long run.
14. IANA Considerations 15. IANA Considerations
None None
Acknowledgments 16. Acknowledgments
Many people at Google have helped to make the set of large scale Many people at Google have helped to make the set of large scale
tests possible. We would especially like to acknowledge Amit Agarwal, tests possible. We would especially like to acknowledge Amit Agarwal,
Tom Herbert, Arvind Jain and Tiziana Refice for their major Tom Herbert, Arvind Jain and Tiziana Refice for their major
contributions. contributions.
Normative References Normative References
[PAC10] Paxson, V., Allman, M., and J. Chu, "Computing TCP's [PACS11] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing
Retransmission Timer", Internet-draft draft-paxson-tcpm- TCP's Retransmission Timer", Internet-draft draft-paxson-
rfc2988bis-00, work in progress, February, 2010. tcpm-rfc2988bis-02, work in progress.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgement Options", RFC 2018, October 1996. Selective Acknowledgement Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P. and T. Berners-Lee, "Hypertext Transfer L., Leach, P. and T. Berners-Lee, "Hypertext Transfer
Protocol -- HTTP/1.1", RFC 2616, June 1999. Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000. Timer", RFC 2988, November 2000.
[RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's [RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002. Initial Window", RFC 3390, October 2002.
[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.
Informative References [RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J. and P.
Hurtig, "Early Retransmit for TCP and SCTP", RFC 5827,
April 2010.
[AAABH10] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J. and P. Informative References
Hurtig, "Early Retransmit for TCP and SCTP", Internet-draft
draft-ietf-tcpm-early-rexmt-04.txt, work in progress.
[AKAM10] "The State of the Internet, 3rd Quarter 2009", Akamai [AKAM10] "The State of the Internet, 3rd Quarter 2009", Akamai
Technologies, Inc., January 2010. Technologies, Inc., January 2010.
[All00] Allman, M., "A Web Server's View of the Transport Layer", [All00] Allman, M., "A Web Server's View of the Transport Layer",
ACM Computer Communication Review, 30(5), October 2000. ACM Computer Communication Review, 30(5), October 2000.
[All10] Allman, M., "Initial Congestion Window Specification",
Internet-draft draft-allman-tcpm-bump-initcwnd-00.txt work
in progress.
[Bel10] Belshe, M., "A Client-Side Argument For Changing TCP Slow [Bel10] Belshe, M., "A Client-Side Argument For Changing TCP Slow
Start", January, 2010. URL Start", January, 2010. URL
http://sites.google.com/a/chromium.org/dev/spdy/ http://sites.google.com/a/chromium.org/dev/spdy/
An_Argument_For_Changing_TCP_Slow_Start.pdf An_Argument_For_Changing_TCP_Slow_Start.pdf
[CD10] Chu, J. and N. Dukkipati, "Increasing TCP's Initial
Window", Presented to 77th IRTF ICCRG & IETF TCPM working
group meetings, March 2010. URL
http://www.ietf.org/proceedings/77/slides/tcpm-4.pdf
[Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century", [Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century",
Presented to 75th IETF TCPM working group meeting, July Presented to 75th IETF TCPM working group meeting, July
2009. http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf. 2009. URL http://www.ietf.org/proceedings/75/slides/tcpm-
1.pdf.
[CW10] Chu, J. and Wang, Y., "A Testbed Study on IW10 vs IW3",
Presented to 79th IETF TCPM working group meeting, Nov.
2010. URL http://www.ietf.org/proceedings/79/slides/tcpm-
0.pdf.
[DCCM10] Dukkipati, D., Cheng, Y., Chu, J. and M. Mathis,
"Increasing TCP initial window", Presented to 78th IRTF
ICCRG working group meeting, July 2010. URL
http://www.ietf.org/proceedings/78/slides/iccrg-3.pdf
[DGHS07] Dischinger, M., Gummadi, K., Haeberlen, A. and S. Saroiu, [DGHS07] Dischinger, M., Gummadi, K., Haeberlen, A. and S. Saroiu,
"Characterizing Residential Broadband Networks", Internet "Characterizing Residential Broadband Networks", Internet
Measurement Conference, October 24-26, 2007. Measurement Conference, October 24-26, 2007.
[Duk10] Dukkipati, N., Refice, T., Cheng, Y., Chu, J., Sutin, N., [Duk10] Dukkipati, N., Refice, T., Cheng, Y., Chu, J., Sutin, N.,
Agarwal, A., Herbert, T. and J. Arvind, "An Argument for Agarwal, A., Herbert, T. and J. Arvind, "An Argument for
Increasing TCP's Initial Congestion Window", ACM SIGCOMM Increasing TCP's Initial Congestion Window", ACM SIGCOMM
Computer Communications Review, vol. 40 (2010), pp. 27-33. Computer Communications Review, vol. 40 (2010), pp. 27-33.
July 2010. URL July 2010. URL
http://www.google.com/research/pubs/pub36640.html http://www.google.com/research/pubs/pub36640.html
[FF99] Floyd, S., and K. Fall, "Promoting the Use of End-to-End [FF99] Floyd, S., and K. Fall, "Promoting the Use of End-to-End
Congestion Control in the Internet", IEEE/ACM Transactions Congestion Control in the Internet", IEEE/ACM Transactions
on Networking, August 1999. on Networking, August 1999.
[FJ93] Floyd, S. and V. Jacobson, "Random Early Detection gateways [FJ93] Floyd, S. and V. Jacobson, "Random Early Detection gateways
for Congestion Avoidance", IEEE/ACM Transactions on for Congestion Avoidance", IEEE/ACM Transactions on
Networking, V.1 N.4, August 1993, p. 397-413. Networking, V.1 N.4, August 1993, p. 397-413.
[Get11] Gettys, J., "Bufferbloat: Dark buffers in the Internet",
Presented to 80th IETF TSV Area meeting, March 2011. URL
http://www.ietf.org/proceedings/80/slides/tsvarea-1.pdf
[IOR2009] Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide, [IOR2009] Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide,
J. Jahanian, F. and M. Karir, "Atlas Internet Observatory J. Jahanian, F. and M. Karir, "Atlas Internet Observatory
2009 Annual Report", 47th NANOG Conference, October 2009. 2009 Annual Report", 47th NANOG Conference, October 2009.
[IW10] "TCP IW10 links", URL
http://code.google.com/speed/protocols/tcpm-IW10.html
[Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer [Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329, Aug. Communication Review, vol. 18, no. 4, pp. 314-329, Aug.
1988. 1988.
[JNDK10] Jarvinen, I., Nyrhinen. A., Ding, A. and M. Kojo, "A
Simulation Study on Increasing TCP's IW", Presented to 78th
IRTF ICCRG working group meeting, July 2010. URL
http://www.ietf.org/proceedings/78/slides/iccrg-7.pdf
[JNDK10-1] Jarvinen, I., Nyrhinen. A., Ding, A. and M. Kojo, "Effect
of IW and Initial RTO changes", Presented to 79th IETF TCPM
working group meeting, Nov. 2010. URL
http://www.ietf.org/proceedings/79/slides/tcpm-1.pdf
[LAJW07] Liu, D., Allman, M., Jin, S. and L. Wang, "Congestion [LAJW07] Liu, D., Allman, M., Jin, S. and L. Wang, "Congestion
Control Without a Startup Phase", Protocols for Fast, Long Control Without a Startup Phase", Protocols for Fast, Long
Distance Networks (PFLDnet) Workshop, February 2007. URL Distance Networks (PFLDnet) Workshop, February 2007. URL
http://www.icir.org/mallman/papers/jumpstart-pfldnet07.pdf http://www.icir.org/mallman/papers/jumpstart-pfldnet07.pdf
[PK98] Padmanabhan V.N. and R. Katz, "TCP Fast Start: A technique [PK98] Padmanabhan V.N. and R. Katz, "TCP Fast Start: A technique
tbr speeding up web transfers", in Proceedings of IEEE for speeding up web transfers", in Proceedings of IEEE
Globecorn '98 Internet Mini-Conference, 1998. Globecom '98 Internet Mini-Conference, 1998.
[PRAKS02] Partridge, C., Rockwell, D., Allman, M., Krishnan, R. and [PRAKS02] Partridge, C., Rockwell, D., Allman, M., Krishnan, R. and
J. Sterbenz, "A Swifter Start for TCP", Technical Report J. Sterbenz, "A Swifter Start for TCP", Technical Report
No. 8339, BBN Technologies, March 2002. No. 8339, BBN Technologies, March 2002.
[PWSB09] Papadimitriou, D., Welzl, M., Scharf, M. and B. Briscoe, [PWSB09] Papadimitriou, D., Welzl, M., Scharf, M. and B. Briscoe,
"Open Research Issues in Internet Congestion Control", "Open Research Issues in Internet Congestion Control",
section 3.4, Internet-draft draft-irtf-iccrg-welzl- section 3.4, Internet-draft draft-irtf-iccrg-welzl-
congestion-control-open-research-05.txt, work in progress. congestion-control-open-research-05.txt, work in progress.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S.,
Wroclawski, J. and L. Zhang, "Recommendations on Queue Wroclawski, J. and L. Zhang, "Recommendations on Queue
Management and Congestion Avoidance in the Internet", RFC Management and Congestion Avoidance in the Internet", RFC
2309, April 1998. 2309, April 1998.
[RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's [RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998. Initial Window", RFC 2414, September 1998.
[RFC2415] Poduri, K. and K. Nichols, "Simulation Studies of Increased
Initial TCP Window Size", RFC 2415, September 1998.
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January Loss Recovery Using Limited Transmit", RFC 3042, January
2001. 2001.
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End- [RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
to-end Performance Implications of Slow Links", RFC 3150, to-end Performance Implications of Slow Links", RFC 3150,
July 2001. July 2001.
[RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno [RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782, Modification to TCP's Fast Recovery Algorithm", RFC 3782,
skipping to change at page 16, line 36 skipping to change at page 19, line 22
[RJ10] Ramachandran, S. and A. Jain, "Aggregate Statistics of Size [RJ10] Ramachandran, S. and A. Jain, "Aggregate Statistics of Size
Related Metrics of Web Pages metrics", 2010. URL Related Metrics of Web Pages metrics", 2010. URL
http://code.google.com/speed/articles/web-metrics.html http://code.google.com/speed/articles/web-metrics.html
[Sch08] Scharf, M., "Quick-Start, Jump-Start, and Other Fast [Sch08] Scharf, M., "Quick-Start, Jump-Start, and Other Fast
Startup Approaches", November 17, 2008. URL Startup Approaches", November 17, 2008. URL
http://www.ietf.org/old/2009/proceedings/08nov/slides/ http://www.ietf.org/old/2009/proceedings/08nov/slides/
iccrg-2.pdf iccrg-2.pdf
[Sch11] Scharf, M., "Performance and Fairness Evaluation of IW10
and Other Fast Startup Schemes", Presented to 80th IRTF
ICCRG working group meeting, Nov. 2010. URL
http://www.ietf.org/proceedings/80/slides/iccrg-1.pdf
[Sch11-1] Scharf, M., "Comparison of end-to-end and network-
supported fast startup congestion control schemes",
Computer Networks, Feb. 2011. URL
http://dx.doi.org/10.1016/j.comnet.2011.02.002
[SPDY] "SPDY: An experimental protocol for a faster web", URL [SPDY] "SPDY: An experimental protocol for a faster web", URL
http://dev.chromium.org/spdy http://dev.chromium.org/spdy
[Ste08] Sounders S., "Roundup on Parallel Connections", High [Ste08] Sounders S., "Roundup on Parallel Connections", High
Performance Web Sites blog. URL Performance Web Sites blog. URL
http://www.stevesouders.com/blog/2008/03/20/roundup-on- http://www.stevesouders.com/blog/2008/03/20/roundup-on-
parallel-connections parallel-connections
[Tou10] Touch, J., "Automating the Initial Window in TCP",
Internet-draft draft-touch-tcpm-automatic-iw-00.txt, work
in progress.
[VH97] Visweswaraiah, V. and J. Heidemann, "Improving Restart of [VH97] Visweswaraiah, V. and J. Heidemann, "Improving Restart of
Idle TCP Connections", Technical Report 97-661, University Idle TCP Connections", Technical Report 97-661, University
of Southern California, November 1997. of Southern California, November 1997.
Author's Addresses Author's Addresses
H.K. Jerry Chu H.K. Jerry Chu
Google, Inc. Google, Inc.
1600 Amphitheatre Parkway 1600 Amphitheatre Parkway
Mountain View, CA 94043 Mountain View, CA 94043
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