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Versions: 00 01 draft-ietf-tcpm-initcwnd

Internet Draft                                                    J. Chu
draft-hkchu-tcpm-initcwnd-01.txt                            N. Dukkipati
Intended status: Standard                                       Y. Cheng
Updates: 3390, 5681                                            M. Mathis
Creation date: July 12, 2010                                Google, Inc.
Expiration date: January 2011

                    Increasing TCP's Initial Window

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   Distribution of this memo is unlimited.

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   This document proposes an increase in the permitted TCP initial
   window (IW) from between 2 and 4 segments, as specified in RFC 3390,
   to 10 segments. 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 risking congestion collapse. Finally, it
   outlines a list of concerns to be addressed in future tests.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Editor's Note

   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.

   The choice of 10 for initial window may not be the "optimal" one. Our
   most recent tests gave better performance with IW=16 compared to
   IW=10, while still showing little negative impact. We are still in
   the process of completing the latest large scale tests and analysis
   for IW=16, which might be a better, more future proofing choice.

1.  Introduction

   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.

   The default value was increased to roughly 4KB more than a decade ago
   [RFC2414]. Since then, the Internet has continued to grow, both in
   speed and penetration [AKAM10]. 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.

   This document proposes an optional standard to allow TCP's initial
   window to start at 10 segments or roughly 15KB, updating RFC 3390
   [RFC3390]. It discusses the motivation, the advantages and

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   disadvantages of the higher initial window, and includes test results
   from several large scale experiments showing improved latency across
   the board for a variety of BW, RTT, and BDP classes.

   It also discusses potential negative impacts and suggests mitigation.
   A minor change to RFC 3390 and RFC 5681 [RFC5681] is proposed on
   resetting the initial window when the SYN or SYN/ACK is lost.

   The document closes with a discussion on remaining concerns, and
   future tests to further validate the higher initial window.

2.  TCP Modification

   This document proposes an increase in the permitted upper bound for
   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.

   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 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
   beyond 10 segments.

   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 [PAC10]
   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 multiple 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);
   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

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   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

   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, when more than 4KB of data is sent.

3.  Motivation

   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, corresponding to a bandwidth
   utilization of less than 200Kbps per connection, assuming an RTT of

   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 [PWSB09,
   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
   best interim solution to improve overall user performance, and reduce

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   the server, client, and network load.

   Note that persistent connections and pipelining are designed to
   address some of the issues with HTTP above [RFC2616]. Their presence
   does not diminish the need for a larger initial window, as the first
   data chunk to respond is often the largest, and will easily hit the
   initial window limit. Our test data confirm significant latency
   reduction with the large initial window even with these two HTTP
   features ([Duk10]).

   Also note that packet pacing has been suggested as an effective
   mechanism to avoid large bursts and their associated damage [VH97].
   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
   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].

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

   1.  Reducing Latency

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       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.

            | total segments |   IW=3   |   IW=10   |
            |         3      |     1    |      1    |
            |         6      |     2    |      1    |
            |        10      |     3    |      1    |
            |        12      |     3    |      2    |
            |        21      |     4    |      2    |
            |        25      |     5    |      2    |
            |        32      |     5    |      3    |
            |        46      |     6    |      3    |
            |        51      |     6    |      4    |
            |        79      |     7    |      4    |
            |       121      |     8    |      5    |
            |       128      |     9    |      5    |

       For example, with the larger initial window, a transfer of 32KB
       data will require only two rather than five round trips to

   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]. 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
       static and dynamic scripted web objects on the page, has seen

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       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.

   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 [AAABH10] 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

   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-
   avoidance phase of the window increase algorithm. [Note: knowing the
   large initial window may cause premature segment drop, should one
   make an exception for it, i.e., by allowing ssthresh to remain
   unchanged if loss is from an enlarged initial window?]

   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.

   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

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   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 highly unlikely the increase will
   render a network in a persistent state of congestion, or even
   congestion collapse. This seems to have been confirmed by our large
   scale experiments described later.

   Some of the discussions from RFC 3390 are still valid for IW=10.
   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 [RFC3782], the wide support of TCP
   Selective Acknowledgment (SACK) option [RFC2018] in all major OSes
   today should keep the volume of duplicate segments in check.

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

   More suggestions to improve the end-to-end performance of slow links
   can be found in RFC 3150 [RFC3150].

   [Note: if packet loss is detected during IW through fast retransmit,
   should cwnd back down to 2 rather than FlightSize / 2?]

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 2988 [RFC2988] to restart the
   retransmission timer with the current value of RTO for each ack
   received that acknowledges new data.

10. Experimental Results

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   In this section we summarize our findings from large scale Internet
   experiments with an initial window of 10 segments, conducted via
   Google's front-end infrastructure serving a diverse set of
   applications. We present results from two datacenters, 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-

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10.2 The cost

   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 retransmissions 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

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11. List of Concerns and Future Tests

   Although we were a little hard pressed to find negative impact from
   the initial window increase in our large scale tests, we don't
   contend our test coverage is complete. The following is an attempt to
   compile a list of concerns and to suggest future tests. Ultimately we
   would like to enlist the help from the TCP community at IETF to study
   and address any concern that may come up.

   1.  How complete are our tests in traffic pattern coverage?

       Google today offers a large portfolio of services beyond web
       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
       significantly more packet drops

       Let the max burst capacity of an end-to-end path be the largest
       burst of packets a given path can absorb before packet is
       dropped. To analyze the impact from the larger initial window, it
       helps to study the distribution of the max burst capacity of the
       current Internet.

       In the past similar studies were conducted by actively probing,
       e.g., through the TCP echo/discard ports from a large set of
       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
       bulk data services like YouTube, and infer the max burst capacity
       on a per-client basis from TCP internal connection parameters
       such as ssthresh, max cwnd, and packet drop pattern.

   3.  Need more thorough analysis of the impact on slow links

       Although our data showed the large initial window reduced 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. Moreover, data from the
       YouTube study above will likely be biased toward broadband users,
       leaving out users behind slow links.

       The narrowband classes here should include 56Kbps dialup modem,

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       2.5G and GPRS mobile network.

   4.  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].

       More study can be done through simulation, similar to the set
       described in RFC 2415 [RFC2415].

12. Security Considerations

   This document discusses the initial congestion window permitted for
   TCP connections. Changing this value does not raise any known new
   security issues with TCP.

13. Conclusion

   This document suggests a change to TCP that will likely be beneficial
   to short-lived TCP connections and those over links with long RTTs
   (saving several RTTs during the initial slow-start phase). However,
   more tests are likely needed to fully understand its impact to the
   Internet. We welcome any help from the TCP community at IETF in
   moving this proposal forward.

14. IANA Considerations



   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

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Normative References

   [PAC10]   Paxson, V., Allman, M., and J. Chu, "Computing TCP's
             Retransmission Timer", Internet-draft draft-paxson-tcpm-
             rfc2988bis-00, work in progress, February, 2010.

   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
             Selective Acknowledgement 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.

   [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
             Timer", RFC 2988, November 2000.

   [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.

Informative References

   [AAABH10] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J. and P.
             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
             Technologies, Inc., January 2010.

   [All00]   Allman, M., "A Web Server's View of the Transport Layer",
             ACM Computer Communication Review, 30(5), October 2000.

   [Bel10]   Belshe, M., "A Client-Side Argument For Changing TCP Slow
             Start", January, 2010. URL

   [Chu09]   Chu, J., "Tuning TCP Parameters for the 21st Century",
             Presented to 75th IETF TCPM working group meeting, July
             2009. http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf.

   [DGHS07]  Dischinger, M., Gummadi, K., Haeberlen, A. and S. Saroiu,
             "Characterizing Residential Broadband Networks", Internet

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             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", March, 2010.
             URL http://code.google.com/speed/articles/

   [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.

   [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.

   [Jac88]   Jacobson, V., "Congestion Avoidance and Control", Computer
             Communication Review, vol. 18, no. 4, pp. 314-329, Aug.

   [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

   [PK98]    Padmanabhan V.N. and R. Katz, "TCP Fast Start: A technique
             tbr speeding up web transfers", in Proceedings of IEEE
             Globecorn '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.

   [PWSB09]  Papadimitriou, D., Welzl, M., Scharf, M. and B. Briscoe,
             "Open Research Issues in Internet Congestion Control",
             section 3.4, Internet-draft draft-irtf-iccrg-welzl-
             congestion-control-open-research-05.txt, work in progress.

   [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.

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   [RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
             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
             Loss Recovery Using Limited Transmit", RFC 3042, January

   [RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
             to-end Performance Implications of Slow Links", RFC 3150,
             July 2001.

   [RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
             Modification to TCP's Fast Recovery Algorithm", RFC 3782,
             April 2004.

   [RFC4782] Floyd, S., Allman, M., Jain, A. and P. Sarolahti, "Quick-
             Start for TCP and IP", RFC 4782, January 2007.

   [RJ10]    Ramachandran, S. and A. Jain, "Aggregate Statistics of Size
             Related Metrics of Web Pages metrics", 2010. URL

   [Sch08]   Scharf, M., "Quick-Start, Jump-Start, and Other Fast
             Startup Approaches", November 17, 2008. URL

   [SPDY]    "SPDY: An experimental protocol for a faster web", URL

   [Ste08]   Sounders S., "Roundup on Parallel Connections", High
             Performance Web Sites blog. URL

   [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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Author's Addresses

   H.K. Jerry Chu
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   EMail: hkchu@google.com

   Nandita Dukkipati
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   EMail: nanditad@google.com

   Yuchung Cheng
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   EMail: ycheng@google.com

   Matt Mathis
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   EMail: mattmathis@google.com


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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