Increasing TCP's Initial Window
draft-ietf-tcpm-initcwnd-07
| The information below is for an old version of the document | |||
|---|---|---|---|
| Document | Type | Active Internet-Draft (tcpm WG) | |
| Authors | Jerry Chu , Nandita Dukkipati , Yuchung Cheng , Matt Mathis | ||
| Last updated | 2013-01-28 | ||
| Replaces | draft-hkchu-tcpm-initcwnd | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Responsible AD | Wesley Eddy | ||
| IESG note | Yoshifumi Nishida (nishida@sfc.wide.ad.jp) is the Document Shepherd for this document. | ||
| Send notices to | tcpm-chairs@tools.ietf.org, draft-ietf-tcpm-initcwnd@tools.ietf.org | ||
Internet Draft J. Chu
draft-ietf-tcpm-initcwnd-07.txt N. Dukkipati
Intended status: Experimental Y. Cheng
M. Mathis
Expiration date: July 2013 Google, Inc.
January 28, 2013
Increasing TCP's Initial Window
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Abstract
This document proposes an experiment to increase the permitted TCP
initial window (IW) from between 2 and 4 segments, as specified in
RFC 3390, to 10 segments, with a fallback to the existing
recommendation when performance issues are detected. It discusses the
motivation behind the increase, the advantages and disadvantages of
the higher initial window, and presents results from several large
scale experiments showing that the higher initial window improves the
overall performance of many web services without resulting in a
congestion collapse. The document closes with a discussion of usage
and deployment for further experimental purpose recommended by the
IETF TCP Maintenance and Minor Extensions (TCPM) working group.
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TCP Modification . . . . . . . . . . . . . . . . . . . . . . . 4
3. Implementation Issues . . . . . . . . . . . . . . . . . . . . 5
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Advantages of Larger Initial Windows . . . . . . . . . . . . . 7
5.1 Reducing Latency . . . . . . . . . . . . . . . . . . . . . . 7
5.2 Keeping up with the growth of web object size . . . . . . . 8
5.3 Recovering faster from loss on under-utilized or wireless
links . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Disadvantages of Larger Initial Windows for the Network . . . 9
8. Mitigation of Negative Impact . . . . . . . . . . . . . . . . 10
9. Interactions with the Retransmission Timer . . . . . . . . . . 10
10. Experimental Results From Large Scale Cluster Tests . . . . . 10
10.1 The benefits . . . . . . . . . . . . . . . . . . . . . . . 11
10.2 The cost . . . . . . . . . . . . . . . . . . . . . . . . . 11
11. Other Studies . . . . . . . . . . . . . . . . . . . . . . . . 12
12. Usage and Deployment Recommendations . . . . . . . . . . . . . 13
13. Related Proposals . . . . . . . . . . . . . . . . . . . . . . 14
14. Security Considerations . . . . . . . . . . . . . . . . . . . 14
15. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 15
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A - List of Concerns and Corresponding Test Results . . . 20
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Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
This document proposes to raise the upper bound on TCP's initial
window (IW) to 10 segments (maximum 14600B). It is patterned after
and borrows heavily from RFC 3390 [RFC3390] and earlier work in this
area. Due to lingering concerns about possible side effects to other
flows sharing the same network bottleneck, some of the
recommendations are conditional on additional monitoring and
evaluation.
The primary argument in favor of raising IW follows from the evolving
scale of the Internet. Ten segments are likely to fit into queue
space available at any broadband access link, even when there are a
reasonable number of concurrent connections.
Lower speed links can be treated with environment specific
configurations, such that they can be protected from being
overwhelmed by large initial window bursts without imposing a
suboptimal initial window on the rest of the Internet.
This document reviews the advantages and disadvantages of using a
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.
We recommend that all TCP implementations have a settable TCP IW
parameter as long as there is a reasonable effort to monitor for
possible interactions with other Internet applications and services
as described in Section 12. Furthermore, Section 10 details why 10
segments may be an appropriate value, and while that value may
continue to rise in the future, this document does not include any
supporting evidence for values of IW larger than 10.
In addition, we introduce a minor revision to RFC 3390 and RFC 5681
[RFC5681] to eliminate resetting the initial window when the SYN or
SYN/ACK is lost.
The document closes with a discussion of the consensus from the TCPM
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working group on the near-term usage and deployment of IW10 in the
Internet.
A complementary set of slides for this proposal can be found at
[CD10].
2. TCP Modification
This document proposes an increase in the permitted upper bound for
TCP's initial window (IW) to 10 segments depending on the MSS. This
increase is optional: a TCP MAY start with an initial window that is
smaller than 10 segments.
More precisely, the upper bound for the initial window will be
min (10*MSS, max (2*MSS, 14600)) (1)
This upper bound for the initial window size represents a change from
RFC 3390 [RFC3390], which specified that the congestion window be
initialized between 2 and 4 segments depending on the MSS.
This change applies to the initial window of the connection in the
first round trip time (RTT) of data transmission during or following
the TCP three-way handshake. Neither the SYN/ACK nor its
acknowledgment (ACK) in the three-way handshake should increase the
initial window size.
Note that all the test results described in this document were based
on the regular Ethernet MTU of 1500 bytes. Future study of the effect
of a different MTU may be needed to fully validate (1) above.
Furthermore, RFC 3390 and RFC 5681 [RFC5681] state that
"If the SYN or SYN/ACK is lost, the initial window used by a
sender after a correctly transmitted SYN MUST be one segment
consisting of MSS bytes."
The proposed change to reduce the default RTO to 1 second [RFC6298]
increases the chance for spurious SYN or SYN/ACK retransmission, thus
unnecessarily penalizing connections with RTT > 1 second if their
initial window is reduced to 1 segment. For this reason, it is
RECOMMENDED that implementations refrain from resetting the initial
window to 1 segment, unless either there have been more than one SYN
or SYN/ACK retransmissions, or true loss detection has been made.
TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window);
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and (3) to restart transmission after a retransmit timeout (the loss
window). The change specified in this document affects the value of
the initial window. Optionally, a TCP MAY set the restart window to
the minimum of the value used for the initial window and the current
value of cwnd (in other words, using a larger value for the restart
window should never increase the size of cwnd). These changes do NOT
change the loss window, which must remain 1 segment of MSS bytes (to
permit the lowest possible window size in the case of severe
congestion).
Furthermore, to limit any negative effect that a larger initial
window may have on links with limited bandwidth or buffer space,
implementations SHOULD fall back to RFC 3390 for the restart window
(RW) if any packet loss is detected during either the initial window,
or a restart window, and more than 4KB of data is sent.
Implementations must also follow RFC6298 [RFC6298] in order to avoid
spurious RTO as described in section 9 later.
3. Implementation Issues
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. If a larger initial window causes harm to any
other flows then local application tuning will reveal that fewer
concurrent connections yields better performance for some users. Any
content provider deploying IW10 in conjunction with content
distributed across multiple domains is explicitly encouraged to
perform measurement experiments to detect such problems, and to
consider reducing the number of concurrent connections used to
retrieve their content.
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
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a testbed and live tests showed that IW=10 without the SACK option
outperforms 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
penetration. According to the latest report from Akamai [AKAM10], the
global broadband (> 2Mbps) adoption has surpassed 50%, propelling the
average connection speed to reach 1.7Mbps, while the narrowband (<
256Kbps) usage has dropped to 5%. In contrast, TCP's initial window
has remained 4KB for a decade [RFC2414], corresponding to a bandwidth
utilization of less than 200Kbps per connection, assuming an RTT of
200ms.
A large proportion of flows on the Internet are short web
transactions over TCP, and complete before exiting TCP slow start.
Speeding up the TCP flow startup phase, including circumventing the
initial window limit, has been an area of active research [RFC6077,
Sch08]. Numerous proposals exist [LAJW07, RFC4782, PRAKS02, PK98].
Some require router support [RFC4782, PK98], hence are not practical
for the public Internet. Others suggested bold, but often radical
ideas, likely requiring more years of research before standardization
and deployment.
In the mean time, applications have responded to TCP's "slow" start.
Web sites use multiple sub-domains [Bel10] to circumvent HTTP 1.1
regulation on two connections per physical host [RFC2616]. As of
today, major web browsers open multiple connections to the same site
(up to six connections per domain [Ste08] and the number is growing).
This trend is to remedy HTTP serialized download to achieve
parallelism and higher performance. But it also implies today most
access links are severely under-utilized, hence having multiple TCP
connections improves performance most of the time. While raising the
initial congestion window may cause congestion for certain users
using these browsers, we argue that the browsers and other
application need to respect HTTP 1.1 regulation and stop increasing
number of simultaneous TCP connections. We believe a modest increase
of the initial window will help to stop this trend, and provide the
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best interim solution to improve overall user performance, and reduce
the server, client, and network load.
Note that persistent connections and pipelining are designed to
address some of the above issues with HTTP [RFC2616]. Their presence
does not diminish the need for a larger initial window. E.g., data
from the Chrome browser show that 35% of HTTP requests are made on
new TCP connections. Our test data also shows significant latency
reduction with the large initial window even in conjunction with
these two HTTP features ([Duk10]).
Also note that packet pacing has been suggested as a possible
mechanism to avoid large bursts and their associated harm [VH97].
Pacing is not required in this proposal due to a strong preference
for a simple solution. We suspect for packet bursts of a moderate
size, packet pacing will not be necessary. This seems to be confirmed
by our test results.
More discussion of the increase in initial window, including the
choice of 10 segments can be found in [Duk10, CD10].
5. Advantages of Larger Initial Windows
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 delayed-ACK
timer.
---------------------------------------
| 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 |
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| 120 | 8 | 5 |
| 127 | 9 | 5 |
---------------------------------------
For example, with the larger initial window, a transfer of 32
segments of data will require only two rather than five round trips
to complete.
5.2 Keeping up with the growth of web object size
RFC 3390 stated that the main motivation for increasing the initial
window to 4KB was to speed up connections that only transmit a small
amount of data, e.g., email and web. The 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 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 such as SPDY [SPDY] allow
multiple web objects to be sent in a single transaction, potentially
benefiting from an even larger initial window in order to transfer an
entire web page in a small number of round trips.
5.3 Recovering faster from loss on under-utilized or wireless links
A greater-than-3-segment initial window increases the chance to
recover packet loss through Fast Retransmit rather than the lengthy
initial RTO [RFC5681]. This is because the fast retransmit algorithm
requires three duplicate ACKs as an indication that a segment has
been lost rather than reordered. While newer loss recovery techniques
such as Limited Transmit [RFC3042] and Early Retransmit [RFC5827]
have been proposed to help speeding up loss recovery from a smaller
window, both algorithms can still benefit from the larger initial
window because of a better chance to receive more ACKs to react upon.
6. Disadvantages of Larger Initial Windows for the Individual Connection
The larger bursts from an increase in the initial window may cause
buffer overrun and packet drop in routers with small buffers, or
routers experiencing congestion. This could result in unnecessary
retransmit timeouts. For a large-window connection that is able to
recover without a retransmit timeout, this could result in an
unnecessarily-early transition from the slow-start to the congestion-
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avoidance phase of the window increase algorithm.
Premature segment drops are unlikely to occur in uncongested networks
with sufficient buffering, or in moderately-congested networks where
the congested router uses active queue management (such as Random
Early Detection [FJ93, RFC2309, RFC3150]).
Insufficient buffering is more likely to exist in the access routers
connecting slower links. A recent study of access router buffer size
[DGHS07] reveals the majority of access routers provision enough
buffer for 130ms or longer, sufficient to cover a burst of more than
10 packets at 1Mbps speed, but possibly not sufficient for browsers
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
initial window even if the burstiness of the initial window results
in premature segment drops. This will be true if (1) the TCP
connection recovers from the segment drop without a retransmit
timeout, and (2) the TCP connection is ultimately limited to a small
congestion window by either network congestion or by the receiver's
advertised window.
7. Disadvantages of Larger Initial Windows for the Network
An increase in the initial window may increase congestion in a
network. However, since the increase is one-time only (at the
beginning of a connection), and the rest of TCP's congestion backoff
mechanism remains in place, it's unlikely the increase by itself will
render a network in a persistent state of congestion, or even
congestion collapse. This seems to have been confirmed by the large
scale web experiments described later.
It should be noted that the above may not hold if applications open a
large number of simultaneous connections.
Until this proposal is widely deployed, a fairness issue may exist
between flows adopting a larger initial window vs flows that are
RFC3390-compliant. Although no severe unfairness has been detected on
all the known tests so far, further study on this topic may be
warranted.
Some of the discussions from RFC 3390 are still valid for IW=10.
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Moreover, it is worth noting that although TCP NewReno increases the
chance of duplicate segments when trying to recover multiple packet
losses from a large window, the wide support of TCP Selective
Acknowledgment (SACK) option [RFC2018] in all major OSes 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 or more) 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 [Get11-1]. The mitigations proposed for the
broader problem of buffer bloating are also applicable in this case,
such as the use of ECN, AQM schemes [CoDel] and traffic
classification (QoS).
8. Mitigation of Negative Impact
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.
The negative impact can be mitigated by hosts directly connected to a
low-speed link advertising a smaller initial receive window than 10
segments. This can be achieved either through manual configuration by
the users, or through the host stack auto-detecting the low bandwidth
links.
Additional suggestions to improve the end-to-end performance of slow
links can be found in RFC 3150 [RFC3150].
9. Interactions with the Retransmission Timer
A large initial window increases the chance of spurious RTO on a low-
bandwidth path because the packet transmission time will dominate the
round-trip time. To minimize spurious retransmissions,
implementations MUST follow RFC 6298 [RFC6298] to restart the
retransmission timer with the current value of RTO for each ACK
received that acknowledges new data.
For a more detailed discussion see RFC3390, section 6.
10. Experimental Results From Large Scale Cluster Tests
In this section we summarize our findings from large scale Internet
experiments with an initial window of 10 segments, conducted via
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Google's front-end infrastructure serving a diverse set of
applications. We present results from two data centers, each chosen
because of the specific characteristics of subnets served: AvgDC has
connection bandwidths closer to the worldwide average reported in
[AKAM10], with a median connection speed of about 1.7Mbps; SlowDC has
a larger proportion of traffic from slow bandwidth subnets with
nearly 20% of traffic from connections below 100Kbps, and a third
below 256Kbps.
Guided by measurements data, we answer two key questions: what is the
latency benefit when TCP connections start with a higher initial
window, and on the flip side, what is the cost?
10.1 The benefits
The average web search latency improvement over all responses in
AvgDC is 11.7% (68 ms) and 8.7% (72 ms) in SlowDC. We further
analyzed the data based on traffic characteristics and subnet
properties such as bandwidth (BW), round-trip time (RTT), and
bandwidth-delay product (BDP). The average response latency improved
across the board for a variety of subnets with the largest benefits
of over 20% from high RTT and high BDP networks, wherein most
responses can fit within the pipe. Correspondingly, responses from
low RTT paths experienced the smallest improvements of about 5%.
Contrary to what we expected, responses from low bandwidth subnets
experienced the best latency improvements (between 10-20%) in the
buckets 0-56Kbps and 56-256Kbps buckets. We speculate low BW networks
observe improved latency for two plausible reasons: 1) fewer slow-
start rounds: unlike many large BW networks, low BW subnets with
dial-up modems have inherently large RTTs; and 2) faster loss
recovery: an initial window larger than 3 segments increases the
chances of a lost packet to be recovered through Fast Retransmit as
opposed to a lengthy RTO.
Responses of different sizes benefited to varying degrees; those
larger than 3 segments naturally demonstrated larger improvements,
because they finished in fewer rounds in slow start as compared to
the baseline. In our experiments, response sizes <= 3 segments also
demonstrated small latency benefits.
To find out how individual subnets performed, we analyzed average
latency at a /24 subnet level (an approximation to a user base
offered similar set of services by a common ISP). We find even at the
subnet granularity, latency improved at all quantiles ranging from 5-
11%.
10.2 The cost
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To quantify the cost of raising the initial window, we analyzed the
data specifically for subnets with low bandwidth and BDP,
retransmission rates for different kinds of applications, as well as
latency for applications operating with multiple concurrent TCP
connections. From our measurements we found no evidence of a negative
latency impacts that correlate to BW or BDP alone, but in fact both
kinds of subnets demonstrated latency improvements across averages
and quantiles.
As expected, the retransmission rate increased modestly when
operating with larger initial congestion window. The overall increase
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
application, the larger window resulted in a retransmission increase
of < 0.5% for services in the AvgDC. The exception is the Maps
application that operates with multiple concurrent TCP connections,
which increased its retransmission rate by 0.9% in AvgDC and 1.85% in
SlowDC (from 3.94% to 5.79%).
In our experiments, the percentage of traffic experiencing
retransmissions did not increase significantly. E.g. 90% of web
search and maps experienced zero retransmission in SlowDC
(percentages are higher for AvgDC); a break up of retransmissions by
percentiles indicate that most increases come from portion of traffic
already experiencing retransmissions in the baseline with initial
window of 3 segments.
Traffic patterns from applications using multiple concurrent TCP
connections all operating with a large initial window represent one
of the worst case scenarios where latency can be adversely impacted
due to bottleneck buffer overflow. Our investigation shows that such
a traffic pattern has not been a problem in AvgDC, where all these
applications, specifically maps and image thumbnails, demonstrated
improved latencies varying from 2-20%. In the case of SlowDC, while
these applications continued showing a latency improvement in the
mean, their latencies in higher quantiles (96 and above for maps)
indicated instances where latency with larger window is worse than
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
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
conjecture that the amount of buffering at bottleneck links plays a
key role in performance of these applications.
Further details on our experiments and analysis can be found in
[Duk10, DCCM10].
11. Other Studies
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Besides the large scale Internet experiments described above, a
number of other studies have been conducted on the effects of IW10 in
various environments. These tests were summarized below, with more
discussion in Appendix A.
A complete list of tests conducted, with their results and related
studies can be found at the [IW10] link.
1. [Sch08] described an earlier evaluation of various Fast Startup
approaches, including the "Initial-Start" of 10 MSS.
2. [DCCM10] presented the result from Google's large scale IW10
experiments, with a focus on areas with highly multiplexed links or
limited broadband deployment such as Africa and South America.
3. [CW10] contained a testbed study on IW10 performance over slow
links. It also studied how short flows with a larger initial window
might affect the throughput performance of other co-existing, long
lived, bulk data transfers.
4. [Sch11] compared IW10 against a number of other fast startup
schemes, and concluded that IW10 works rather well and is also quite
fair.
5. [JNDK10] and later [JNDK10-1] studied the effect of IW10 over
cellular networks.
6. [AERG11] studied the effect of larger ICW sizes, among other
things, on end users' page load time from Yahoo!'s Content Delivery
Network.
12. Usage and Deployment Recommendations
Further experiments are required before a larger initial window shall
be enabled by default in the Internet. The existing measurement
results indicate that this does not cause significant harm to other
traffic. However, widespread use in the Internet could reveal issues
not known yet, e.g., regarding fairness or impact on latency-
sensitive traffic such as VoIP.
Therefore, special care is needed when using this experimental TCP
extension, in particular on large-scale systems originating a
significant amount of Internet traffic, or on large numbers of
individual consumer-level systems that have similar aggregate impact.
Anyone (stack vendors, network administrators, etc.) turning on a
larger initial window SHOULD ensure that the performance is monitored
before and after that change. A key metric to monitor is the rate of
packet losses, ECN marking, or segment retransmissions during the
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initial burst. The sender SHOULD cache such information about
connection setups using an initial window larger than allowed by RFC
3390, and new connections SHOULD fall back to the initial window
allowed by RFC 3390 if there is evidence of performance issues.
Further experiments are needed on the design of such a cache and
corresponding heuristics.
Other relevant metrics that may indicate a need to reduce the IW
include an increased overall percentage of packet loss or segment
retransmissions as well as application-level metrics such as reduced
data transfer completion times or impaired media quality.
It is important also to take into account hosts that do not implement
a larger initial window. Furthermore, non-TCP traffic (such as VoIP)
should be monitored as well. If users observe any significant
deterioration of performance, they SHOULD fall back to an initial
window as allowed by RFC 3390 for safety reasons. An increased
initial window MUST NOT be turned on by default on systems without
such monitoring capabilities.
The IETF TCPM working group is very much interested in further
reports from experiments with this specification and encourages the
publication of such measurement data. If no significant harm is
reported, a follow-up document may revisit the question on whether a
larger initial window can be safely used by default in all Internet
hosts. Resolution of these experiments and tighter specifications of
the suggestions here might be grounds for a future standards track
document on the same topic.
13. Related Proposals
Two other proposals [All10, Tou12] have been published to raise TCP's
initial window size over a large timescale. Both aim at reducing the
uncertain impact of a larger initial window at an Internet wide
scale. Moreover, [Tou12] seeks an algorithm to automate the
adjustment of IW safely over long haul period.
Although a modest, static increase of IW to 10 may address the near-
term need for better web performance, much work is needed from the
TCP research community to find a long term solution to the TCP flow
startup problem.
14. Security Considerations
This document discusses the initial congestion window permitted for
TCP connections. Although changing this value may cause more packet
loss, it is highly unlikely to lead to a persistent state of network
congestion or even a congestion collapse. Hence it does not raise any
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known new security issues with TCP.
15. Conclusion
This document suggests a simple change to TCP that will reduce the
application latency over short-lived TCP connections or links with
long RTTs (saving several RTTs during the initial slow-start phase)
with little or no negative impact over other flows. Extensive tests
have been conducted through both testbeds and large data centers with
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 solution for the near-term
deployment, while scaling IW over the long run remains a challenge
for the TCP research community.
16. IANA Considerations
None
17. Acknowledgments
Many people at Google have helped to make the set of large scale
tests possible. We would especially like to acknowledge Amit Agarwal,
Tom Herbert, Arvind Jain and Tiziana Refice for their major
contributions.
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Normative References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P. and T. Berners-Lee, "Hypertext Transfer
Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J. and P.
Hurtig, "Early Retransmit for TCP and SCTP", RFC 5827, May
2010.
[RFC6298] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing
TCP's Retransmission Timer", RFC 6298, June 2011.
Informative References
[AKAM10] "The State of the Internet, 3rd Quarter 2009", Akamai
Technologies, Inc., January 2010.
URL=http://www.akamai.com/html/about/press/releases/2010/
press_011310_1.html
[AERG11] Al-Fares, M., Elmeleegy, K., Reed, B. and I. Gashinsky,
"Overclocking the Yahoo! CDN for Faster Web Page Loads",
Internet Measurement Conference, November 2011.
[All00] Allman, M., "A Web Server's View of the Transport Layer",
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, last updated November 2010.
[Bel10] Belshe, M., "A Client-Side Argument For Changing TCP Slow
Start", January, 2010. URL
http://sites.google.com/a/chromium.org/dev/spdy/
An_Argument_For_Changing_TCP_Slow_Start.pdf
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[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",
Presented to 75th IETF TCPM working group meeting, July
2009. URL http://www.ietf.org/proceedings/75/slides/tcpm-
1.pdf.
[CoDel] Nichols, K. and V. Jacobson, "Controlling Queue Delay", ACM
QUEUE, May 6, 2012.
[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,
"Characterizing Residential Broadband Networks", Internet
Measurement Conference, October 24-26, 2007.
[Duk10] Dukkipati, N., Refice, T., Cheng, Y., Chu, J., Sutin, N.,
Agarwal, A., Herbert, T. and J. Arvind, "An Argument for
Increasing TCP's Initial Congestion Window", ACM SIGCOMM
Computer Communications Review, vol. 40 (2010), pp. 27-33.
July 2010.
[FF99] Floyd, S., and K. Fall, "Promoting the Use of End-to-End
Congestion Control in the Internet", IEEE/ACM Transactions
on Networking, August 1999.
[FJ93] Floyd, S. and V. Jacobson, "Random Early Detection gateways
for Congestion Avoidance", IEEE/ACM Transactions on
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
[Get11-1] Gettys, J., "IW10 Considered Harmful", Internet-draft
draft-gettys-iw10-considered-harmful-00, work in progress,
August 2011.
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[IOR2009] Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide,
J. Jahanian, F. and M. Karir, "Atlas Internet Observatory
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
Communication Review, vol. 18, no. 4, pp. 314-329, Aug.
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
Control Without a Startup Phase", Protocols for Fast, Long
Distance Networks (PFLDnet) Workshop, February 2007. URL
http://www.icir.org/mallman/papers/jumpstart-pfldnet07.pdf
[PK98] Padmanabhan V.N. and R. Katz, "TCP Fast Start: A technique
for speeding up web transfers", in Proceedings of IEEE
Globecom '98 Internet Mini-Conference, 1998.
[PRAKS02] Partridge, C., Rockwell, D., Allman, M., Krishnan, R. and
J. Sterbenz, "A Swifter Start for TCP", Technical Report
No. 8339, BBN Technologies, March 2002.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S.,
Wroclawski, J. and L. Zhang, "Recommendations on Queue
Management and Congestion Avoidance in the Internet", RFC
2309, April 1998.
[RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998.
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January
2001.
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[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
to-end Performance Implications of Slow Links", BCP 0048,
July 2001.
[RFC4782] Floyd, S., Allman, M., Jain, A. and P. Sarolahti, "Quick-
Start for TCP and IP", RFC 4782, January 2007.
[RFC6077] Papadimitriou, D., Welzl, M., Scharf, M. and B. Briscoe,
"Open Research Issues in Internet Congestion Control",
section 3.4, RFC 6077, February 2011.
[RJ10] Ramachandran, S. and A. Jain, "Aggregate Statistics of Size
Related Metrics of Web Pages metrics", May 2010. URL
http://code.google.com/speed/articles/web-metrics.html
[Sch08] Scharf, M., "Quick-Start, Jump-Start, and Other Fast
Startup Approaches", Internet Research Task Force ICCRG,
November 17, 2008. URL
http://www.ietf.org/proceedings/73/slides/iccrg-2.pdf
[Sch11] Scharf, M., "Performance and Fairness Evaluation of IW10
and Other Fast Startup Schemes", Internet Research Task
Force ICCRG, March 2011. 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
http://dev.chromium.org/spdy
[Ste08] Sounders S., "Roundup on Parallel Connections", High
Performance Web Sites blog. March 2008. URL
http://www.stevesouders.com/blog/2008/03/20/roundup-on-
parallel-connections
[Tou12] Touch, J., "Automating the Initial Window in TCP",
Internet-draft draft-touch-tcpm-automatic-iw-03.txt, work
in progress, July 16, 2012.
[VH97] Visweswaraiah, V. and J. Heidemann, "Improving Restart of
Idle TCP Connections", Technical Report 97-661, University
of Southern California, November 1997.
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Appendix A - List of Concerns and Corresponding Test Results
Concerns have been raised since this proposal was first published
based on a set of large scale experiments. To better understand the
impact of a larger initial window in order to confirm or dismiss
these concerns, additional tests have been conducted using either
large scale clusters, simulations, or real testbeds. The following
attempts to compile the list of concerns and summarize findings from
relevant tests.
o How complete are various tests in covering many different traffic
patterns?
The large scale Internet experiments conducted at Google front-end
infrastructure covered a large portfolio of services beyond web
search. It includes Gmail, Google Maps, Photos, News, Sites,
Images,..., etc, 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.
[CW10] contains some result from a testbed study on how short flows
with a larger initial window might affect the throughput
performance of other co-existing, long lived, bulk data transfers.
o Larger bursts from the increase in the initial window cause
significantly more packet drops
All the tests conducted on this subject [Duk10, Sch11, Sch11-1,
CW10] so far have shown only modest increase on packet drops. The
only exception is from the testbed study [CW10] when under
extremely high load and/or simultaneous opens. But under those
conditions both IW=3 and IW=10 suffered very high packet loss rates
though.
o A large initial window may severely impact TCP performance over
highly multiplexed links still common in developing regions
Our large scale experiments described in section 10 above also
covered Africa and South America. Measurement data from those
regions [DCCM10] revealed improved latency even for those services
that employ multiple simultaneous connections, at the cost of small
increase in the retransmission rate. It seems that the round trip
savings from a larger initial window more than make up the time
spent on recovering more lost packets.
Similar phenomenon have also been observed from testbed study
[CW10].
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o Why 10 segments?
Questions have been raised on how the number 10 was picked. We have
tried different sizes in our large scale experiments, and found
that 10 segments seem to give most of the benefits for the services
we tested while not causing significant increase in the
retransmission rates. Going forward 10 segments may turn out to be
too small when the average of web object sizes continue to grow.
But a scheme to right size the initial window automatically over
long timescales has yet to be developed.
o Need more thorough analysis of the impact on slow links
Although [Duk10] showed the large initial window reduced the
average latency even for the dialup link class of only 56Kbps in
bandwidth, more studied were needed in order to understand the
effect of IW10 on slow links at the microscopic level. [CW10] was
conducted for this purpose.
Testbeds in [CW10] emulated a 300ms RTT, bottleneck link bandwidth
as low as 64Kbps, and route queue size as low as 40 packets. A
large combination of test parameters were used. Almost all tests
showed varying degree of 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 the 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 the 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 the testbed experiments
[CW10]. Flows with IW=3 only suffered slightly when competing
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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.
A 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.
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Author's Addresses
Jerry Chu
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
EMail: hkchu@google.com
Nandita Dukkipati
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
EMail: nanditad@google.com
Yuchung Cheng
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
EMail: ycheng@google.com
Matt Mathis
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
EMail: mattmathis@google.com
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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