Internet Draft                                                    J. Chu
draft-ietf-tcpm-initcwnd-02.txt                             N. Dukkipati
Intended status: Standard                                       Y. Cheng
Updates: 3390, 5681                                            M. Mathis
Creation date: October 16, 2011                             Google, Inc.
Expiration date: April 2012

                    Increasing TCP's Initial Window

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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. The document closes
   with a discussion of a list of concerns, and some results from recent
   studies to address the concerns.


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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  TCP Modification  . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Implementation Issues . . . . . . . . . . . . . . . . . . . . . 4
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Advantages of Larger Initial Windows  . . . . . . . . . . . . . 6
      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

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   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.

   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 propose that all TCP implementations should have a settable TCP IW
   parameter; the default setting may start at 10 segments and should be
   raised as we come to understand and and correct things that conflict.

   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 a list of concerns that have
   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

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

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   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.

   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 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
   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.  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

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   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

   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

   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].

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   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
   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. E.g., data
   from the Chrome browser show that 35% of HTTP requests are made on
   new TCP connections. Our test data also 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, 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.

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   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

        | 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    |

   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

   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, 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.

5.3 Recovering faster from loss on under-utilized or wireless links

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   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-
   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.

   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.

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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.

   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

   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

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   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 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
   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,

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   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-

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 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)

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   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. List of Concerns and Corresponding Test Results

   Concerns have been raised since we first published our proposal 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, we, as well as people outside of Google have conducted
   numerous additional tests in the past year, using either Google's
   large scale clusters, simulations, or real testbeds. The following is
   a list of concerns and some of the findings.

   A complete list of tests conducted, their results and related studies
   can be found at [IW10].

   o 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.

     [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 known tests conducted on this subject so far [Duk10, Sch11,
     Sch11-1, CW10] show that, although bursts from the larger initial
     window tend to cause more packet drops, the increase tends to be
     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.

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   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 Google
     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 our testbed study

   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. A
     scheme to attempt to right size the initial window automatically
     over long timescales has been proposed in [Tou10].

   o Need more thorough analysis of the impact on slow links

     Although data from [Duk10] 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. We set up two testbeds for
     this purpose [CW10].

     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

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     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,

   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.

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   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
   TCP connections. Changing this value does not raise any known new
   security issues with TCP.

14. 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 near-term proposal while
   other proposals [All10, Tou10] may be best suited to grow IW beyond
   10 in the long run.

15. IANA Considerations


16. 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

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Normative References

   [RFC6298] Paxson, V., Allman, M., Chu, J. and M. Sargent, "Computing
             TCP's Retransmission Timer", RFC6298, June 2011.

   [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.

   [RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J. and P.
             Hurtig, "Early Retransmit for TCP and SCTP", RFC 5827,
             April 2010.

Informative References

   [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.

   [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
             Start", January, 2010. URL

   [CD10]    Chu, J. and N. Dukkipati, "Increasing TCP's Initial
             Window", Presented to 77th IRTF ICCRG & IETF TCPM working
             group meetings, March 2010. URL

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   [Chu09]   Chu, J., "Tuning TCP Parameters for the 21st Century",
             Presented to 75th IETF TCPM working group meeting, July
             2009. URL

   [CW10]    Chu, J. and Wang, Y., "A Testbed Study on IW10 vs IW3",
             Presented to 79th IETF TCPM working group meeting, Nov.
             2010. URL

   [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

   [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. URL

   [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

   [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

   [Jac88]   Jacobson, V., "Congestion Avoidance and Control", Computer

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Internet Draft      Increasing TCP's Initial Window         October 2011

             Communication Review, vol. 18, no. 4, pp. 314-329, Aug.

   [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

   [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

   [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
             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.

   [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.

   [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

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

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   [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

   [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

   [Sch11-1]  Scharf, M., "Comparison of end-to-end and network-
             supported fast startup congestion control schemes",
             Computer Networks, Feb. 2011. URL

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

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

   [Tou10]   Touch, J., "Automating the Initial Window in TCP",
             Internet-draft draft-touch-tcpm-automatic-iw-01.txt, work
             in progress.

   [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

   Jerry Chu
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043

   Nandita Dukkipati
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043

   Yuchung Cheng
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043

   Matt Mathis
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043


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

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