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Increasing TCP's Initial Window

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6928.
Authors Jerry Chu , Nandita Dukkipati , Yuchung Cheng , Matt Mathis
Last updated 2012-12-13 (Latest revision 2012-11-16)
Replaces draft-hkchu-tcpm-initcwnd
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
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IESG IESG state Became RFC 6928 (Experimental)
Consensus boilerplate Unknown
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Needs a YES.
Responsible AD Wesley Eddy
IESG note
Send notices to,
Internet Draft                                                    J. Chu
draft-ietf-tcpm-initcwnd-06.txt                             N. Dukkipati
Intended status: Experimental                                   Y. Cheng
Updates: 3390, 5681                                            M. Mathis
Expiration date: May 2013                                   Google, Inc.
                                                       November 16, 2012

                    Increasing TCP's Initial Window

Status of this Memo

   Distribution of this memo is unlimited.

   This Internet-Draft is submitted in full conformance with the
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   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.


   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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  TCP Modification  . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Implementation Issues . . . . . . . . . . . . . . . . . . . . . 5
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   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  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   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

   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

   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 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

   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. 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
   outperforms IW=3 with the SACK option [CW10].

4.  Background

   TCP congestion window was introduced as part of the congestion

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   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 [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
   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

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   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

        | 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

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   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 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-
   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]).

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   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

   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, 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

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   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

   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
   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

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   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-

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

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   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

   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.


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   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

   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

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
   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

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   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

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
   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)

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   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


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


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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

   [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.

   [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


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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

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

   [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

   [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.

   [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

   [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

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

   [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


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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

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

   [Sch11]   Scharf, M., "Performance and Fairness Evaluation of IW10
             and Other Fast Startup Schemes", Internet Research Task
             Force ICCRG, March 2011. 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. March 2008. URL

   [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

     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

   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

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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

   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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