Internet Engineering Task Force (IETF) J. Chu Request for Comments: 6928 N. Dukkipati Category: Experimental Y. Cheng ISSN: 2070-1721 M. Mathis Google, Inc. April 2013 Increasing TCP's Initial Window
AbstractThis 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 purposes recommended by the IETF TCP Maintenance and Minor Extensions (TCPM) working group. Status of This Memo This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6928.
Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction ....................................................3 1.1. Terminology ................................................4 2. TCP Modification ................................................4 3. Implementation Issues ...........................................5 4. Background ......................................................6 5. Advantages of Larger Initial Windows ............................7 5.1. Reducing Latency ...........................................7 5.2. Keeping Up with the Growth of Web Object Size ..............8 5.3. Recovering Faster from Loss on Under-Utilized or Wireless Links .............................................8 6. Disadvantages of Larger Initial Windows for the Individual ......9 7. Disadvantages of Larger Initial Windows for the Network ........10 8. Mitigation of Negative Impact ..................................11 9. Interactions with the Retransmission Timer .....................11 10. Experimental Results From Large-Scale Cluster Tests ...........11 10.1. The Benefits .............................................11 10.2. The Cost .................................................12 11. Other Studies .................................................13 12. Usage and Deployment Recommendations ..........................14 13. Related Proposals .............................................15 14. Security Considerations .......................................16 15. Conclusion ....................................................16 16. Acknowledgments ...............................................16 17. References ....................................................16 17.1. Normative References .....................................16 17.2. Informative References ...................................17 Appendix A. List of Concerns and Corresponding Test Results .......21
RFC3390] and earlier work in this area. Due to lingering concerns about possible side effects to other flows sharing the same network bottleneck, some of the recommendations are conditional on additional monitoring and evaluation. The primary argument in favor of raising IW follows from the evolving scale of the Internet. Ten segments are likely to fit into queue space available at any broadband access link, even when there are a reasonable number of concurrent connections. Lower speed links can be treated with environment-specific configurations, such that they can be protected from being overwhelmed by large initial window bursts without imposing a suboptimal initial window on the rest of the Internet. This document reviews the advantages and disadvantages of using a larger initial window and includes summaries of several large-scale experiments showing that an initial window of 10 segments (IW10) provides benefits across the board for a variety of bandwidth (BW), round-trip time (RTT), and bandwidth-delay product (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 working group on the near-term usage and deployment of IW10 in the Internet.
A complementary set of slides for this proposal can be found at [CD10]. RFC2119]. 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 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 states (and RFC 5681 [RFC5681] has similar text): 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 retransmission timeout (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 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); and (3) to restart transmission after a retransmit timeout (the loss window). The change specified in this document affects the value of the initial window. Optionally, a TCP MAY set the restart window to the minimum of the value used for the initial window and the current value of cwnd (in other words, using a larger value for the restart window should never increase the size of cwnd). These changes do NOT change the loss window, which must remain 1 segment of MSS bytes (to permit the lowest possible window size in the case of severe congestion). Furthermore, to limit any negative effect that a larger initial window may have on links with limited bandwidth or buffer space, implementations SHOULD fall back to RFC 3390 for the restart window (RW) if any packet loss is detected during either the initial window or a restart window, and more than 4 KB of data is sent. Implementations must also follow RFC 6298 [RFC6298] in order to avoid spurious RTO as described in Section 9. Ste08], partly to circumvent the small initial window in order to speed up the loading of web pages as described above. When web browsers open simultaneous TCP connections to the same destination, they are working against TCP's congestion control mechanisms [FF99]. Combining this behavior with larger initial windows further increases the burstiness and unfairness to other traffic in the network. If a larger initial window causes harm to any other flows, then local application tuning will reveal that having fewer concurrent connections yields better performance for some users. Any content provider deploying IW10 in conjunction with content distributed across multiple domains is explicitly encouraged to perform measurement experiments to detect such problems, and to consider reducing the number of concurrent connections used to retrieve their content. Some implementations advertise a 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 Section 8 below.)
The TCP Selective Acknowledgment (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]. Jac88]. The initial value of one segment was used as the starting point for newly established connections to probe the available bandwidth on the network. Today's Internet is dominated by web traffic running on top of short- lived TCP connections [IOR2009]. The relatively small initial window has become a limiting factor for the performance of many web applications. The global Internet has continued to grow, both in speed and penetration. According to the latest report from Akamai [AKAM10], the global broadband (> 2 Mbps) adoption has surpassed 50%, propelling the average connection speed to reach 1.7 Mbps, while the narrowband (< 256 Kbps) usage has dropped to 5%. In contrast, TCP's initial window has remained 4 KB for a decade [RFC2414], corresponding to a bandwidth utilization of less than 200 Kbps per connection, assuming an RTT of 200 ms. 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 (see [Sch08] and Section 3.4 of [RFC6077]). 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 subdomains [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 that 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 of these browsers, we argue that the browsers and other application
need to respect HTTP 1.1 regulation and stop increasing the 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 from the Chrome browser shows 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] and [CD10].
--------------------------------------- | 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 2 rather than 5 round trips to complete. RFC 3390 stated that the main motivation for increasing the initial window to 4 KB 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 4 KB and could be completed in a single RTT [All00]. Since RFC 3390 was published, web objects have gotten significantly larger [Chu09] [RJ10]. Today only a small percentage of web objects (e.g., 10% of Google's search responses) can fit in the 4 KB initial window. The average HTTP response size of gmail.com, a highly scripted web site, is 8 KB (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. 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. 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 130 ms or longer, sufficient to cover a burst of more than 10 packets at 1 Mbps 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.
RFC 3390. Although no severe unfairness has been detected on all the known tests so far, further study on this topic may be warranted. Some of the discussions from RFC 3390 are still valid for IW=10. 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 the TCP Selective Acknowledgment (SACK) option [RFC2018] in all major OSes today should keep the volume of duplicate segments in check. Recent measurements [Get11] provide evidence of extremely large queues (in the order of one second or more) at access networks of the Internet. While a significant part of the buffer bloat is contributed by large downloads/uploads such as video files, emails with large attachments, backups and download of movies to disk, some of the problem is also caused by web browsing of image-heavy sites [Get11]. This queuing delay is generally considered harmful for responsiveness of latency-sensitive traffic such as DNS queries, Address Resolution Protocol (ARP), DHCP, Voice over IP (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 Explicit Congestion Notification (ECN), Active Queue Management (AQM) schemes [CoDel], and traffic classification (QoS).
RFC3150]. 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 RFC 3390, Section 6. AKAM10], with a median connection speed of about 1.7 Mbps; SlowDC has a larger proportion of traffic from slow-bandwidth subnets with nearly 20% of traffic from connections below 100 Kbps; and a third was below 256 Kbps. 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?
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 -- about 5%. Contrary to what we expected, responses from low-bandwidth subnets experienced the best latency improvements (between 10-20%) in the 0-56 Kbps and 56-256 Kbps 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 less than or equal to 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 that is offered similar set of services by a common ISP). We find that, even at the subnet granularity, latency improved at all quantiles ranging from 5-11%.
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 the portion of traffic already experiencing retransmissions in the baseline with initial window of 3 segments. One of the worst-case scenarios where latency can be adversely impacted due to bottleneck buffer overflow is represented by traffic patterns from applications using multiple concurrent TCP connections, all operating with a large initial window. 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% (80 ms) 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 the performance of these applications. Further details on our experiments and analysis can be found in [Duk10] and [DCCM10]. Appendix A. A complete list of tests conducted, with their results and related studies, can be found at the [IW10] link. 1. [Sch08] described an earlier evaluation of various Fast Startup approaches, including the "Initial-Start" of 10 MSS. 2. [DCCM10] presented the result from Google's large-scale IW10 experiments, with a focus on areas with highly multiplexed links or limited broadband deployment such as Africa and South America.
3. [CW10] contained a testbed study on IW10 performance over slow links. It also studied how short flows with a larger initial window might affect the throughput performance of other coexisting, long-lived, bulk data transfers. 4. [Sch11] compared IW10 against a number of other fast startup schemes, and concluded that IW10 works rather well and is also quite fair. 5. [JNDK10] and later [JNDK10-1] studied the effect of IW10 over cellular networks. 6. [AERG11] studied the effect of larger sizes of initial congestion windows, among other things, on end users' page load time from Yahoo!'s Content Delivery Network. RFC 3390, and new connections SHOULD fall back to the initial window allowed by RFC 3390 if there is evidence of performance issues. Further experiments are needed on the design of such a cache and corresponding heuristics. Other relevant metrics that may indicate a need to reduce the IW include an increased overall percentage of packet loss or segment retransmissions as well as application-level metrics such as reduced data transfer completion times or impaired media quality. It is important also to take into account hosts that do not implement a larger initial window. Furthermore, any deployment of IW10 should be aware that there are potential side effects to real-time traffic
(such as VoIP). 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. By now, there are no adequate studies available that either prove or do not prove the impact of IW10 to real-time traffic. Further experimentation in this direction is encouraged. If no significant harm is reported, a follow-up document may revisit the question on whether a larger initial window can be safely used by default in all Internet hosts. Resolution of these experiments and tighter specifications of the suggestions here might be grounds for a future Standards Track document on the same topic. It is recognized that if IW10 is causing harm to other traffic, that this may not be readily apparent to the software on the hosts using IW10. In some cases, a local system or network administrator may be able to detect this and to selectively disable IW10. In the general case, however, since the harm may occur on a remote network to other cross-traffic, there may be no good way at all for this to be detected or corrected. Current experience and analysis does not indicate whether this is a real issue, beyond a hypothetical one. As use of IW10 becomes more prevalent, monitoring and analysis of flows throughout the network will be needed to assess the impact across the spectrum of scenarios found on the real Internet. 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 a long 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.
[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 Stream Control Transmission Protocol (SCTP)", RFC 5827, May 2010. [RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, June 2011. [AKAM10] Akamai Technologies, Inc., "The State of the Internet, 3rd Quarter 2009", January 2010, <http://www.akamai.com/html/ about/press/releases/2010/press_011310_1.html>. [AERG11] Al-Fares, M., Elmeleegy, K., Reed, B., and I. Gashinsky, "Overclocking the Yahoo! CDN for Faster Web Page Loads", Internet Measurement Conference, November 2011. [All00] Allman, M., "A Web Server's View of the Transport Layer", ACM Computer Communication Review, 30(5), October 2000. [All10] Allman, M., "Initial Congestion Window Specification", Work in Progress, November 2010. [Bel10] Belshe, M., "A Client-Side Argument For Changing TCP Slow Start", January 2010, <http://sites.google.com/a/chromium.org/dev/spdy/ An_Argument_For_Changing_TCP_Slow_Start.pdf>. [CD10] Chu, J. and N. Dukkipati, "Increasing TCP's Initial Window", presented to the IRTF ICCRG and IETF TCPM working group meetings, IETF 77, March 2010, <http://www.ietf.org/ proceedings/77/slides/tcpm-4.pdf>. [Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century", presented to TCPM working group meeting, IETF 75, July 2009. <http://www.ietf.org/proceedings/75/slides/tcpm-1>. [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 the TCPM working group meeting, IETF 79, November 2010, <http://www.ietf.org/proceedings/79/slides/tcpm-0>.
[DCCM10] Dukkipati, D., Cheng, Y., Chu, J., and M. Mathis, "Increasing TCP initial window", presented to the IRTF ICCRG meeting, IETF 78, July 2010, <http://www.ietf.org/proceedings/78/slides/iccrg-3.pdf>. [DGHS07] Dischinger, M., Gummadi, K., Haeberlen, A., and S. Saroiu, "Characterizing Residential Broadband Networks", Internet Measurement Conference, October 24-26, 2007. [Duk10] Dukkipati, N., Refice, T., Cheng, Y., Chu, J., Sutin, N., Agarwal, A., Herbert, T., and J. Arvind, "An Argument for Increasing TCP's Initial Congestion Window", ACM SIGCOMM Computer Communications Review, vol. 40 (2010), pp. 27-33. July 2010. [FF99] Floyd, S., and K. Fall, "Promoting the Use of End-to-End Congestion Control in the Internet", IEEE/ACM Transactions on Networking, August 1999. [FJ93] Floyd, S. and V. Jacobson, "Random Early Detection gateways for Congestion Avoidance", IEEE/ACM Transactions on Networking, V.1 N.4, August 1993, p. 397-413. [Get11] Gettys, J., "Bufferbloat: Dark buffers in the Internet", presented to the TSV Area meeting, IETF 80, March 2011, <http://www.ietf.org/proceedings/80/slides/tsvarea-1.pdf>. [Get11-1] Gettys, J., "IW10 Considered Harmful", Work in Progress, August 2011. [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", January 2012, <http://code.google.com/speed/protocols/tcpm-IW10.html>. [Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988. [JNDK10] Jarvinen, I., Nyrhinen. A., Ding, A., and M. Kojo, "A Simulation Study on Increasing TCP's IW", presented to the IRTF ICCRG meeting, IETF 78, July 2010, <http://www.ietf.org/proceedings/78/slides/iccrg-7.pdf>.
[JNDK10-1] Jarvinen, I., Nyrhinen. A., Ding, A., and M. Kojo, "Effect of IW and Initial RTO changes", presented to the TCPM working group meeting, IETF 79, November 2010, <http://www.ietf.org/proceedings/79/slides/tcpm-1.pdf>. [LAJW07] Liu, D., Allman, M., Jin, S., and L. Wang, "Congestion Control Without a Startup Phase", Protocols for Fast, Long Distance Networks (PFLDnet) Workshop, February 2007, <http://www.icir.org/mallman/papers/ jumpstart-pfldnet07.pdf>. [PK98] Padmanabhan V.N. and R. Katz, "TCP Fast Start: A technique for speeding up web transfers", in Proceedings of IEEE Globecom '98 Internet Mini-Conference, 1998. [PRAKS02] Partridge, C., Rockwell, D., Allman, M., Krishnan, R., and J. Sterbenz, "A Swifter Start for TCP", Technical Report No. 8339, BBN Technologies, March 2002. [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J., and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998. [RFC2414] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's Initial Window", RFC 2414, September 1998. [RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing TCP's Loss Recovery Using Limited Transmit", RFC 3042, January 2001. [RFC3150] Dawkins, S., Montenegro, G., Kojo, M., and V. Magret, "End-to-end Performance Implications of Slow Links", BCP 48, RFC 3150, 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., Ed., Welzl, M., Scharf, M., and B. Briscoe, "Open Research Issues in Internet Congestion Control", RFC 6077, February 2011. [RJ10] Ramachandran, S. and A. Jain, "Aggregate Statistics of Size Related Metrics of Web Pages metrics", May 2010, <http://code.google.com/speed/articles/web-metrics.html>.
[Sch08] Scharf, M., "Quick-Start, Jump-Start, and Other Fast Startup Approaches", presented to the IRTF ICCRG meeting, IETF 73, November 2008, <http://www.ietf.org/proceedings/73/slides/iccrg-2.pdf>. [Sch11] Scharf, M., "Performance and Fairness Evaluation of IW10 and Other Fast Startup Schemes", presented to the IRTF ICCRG meeting, IETF 80, March 2011, <http://www.ietf.org/proceedings/80/slides/iccrg-1.pdf>. [Sch11-1] Scharf, M., "Comparison of end-to-end and network- supported fast startup congestion control schemes", Computer Networks, Feb. 2011, <http://dx.doi.org/10.1016/j.comnet.2011.02.002>. [SPDY] "SPDY: An experimental protocol for a faster web", <http://dev.chromium.org/spdy>. [Ste08] Sounders S., "Roundup on Parallel Connections", High Performance Web Sites blog, March 2008, <http://www.stevesouders.com/blog/2008/03/20/ roundup-on-parallel-connections>. [Tou12] Touch, J., "Automating the Initial Window in TCP", Work in Progress, July 2012. [VH97] Visweswaraiah, V. and J. Heidemann, "Improving Restart of Idle TCP Connections", Technical Report 97-661, University of Southern California, November 1997.
CW10] contains some results from a testbed study on how short flows with a larger initial window might affect the throughput performance of other coexisting, 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 a modest increase of packet drops. The only exception is from the testbed study [CW10] 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 a 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 phenomena have also been observed from the testbed study [CW10].
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 continues to grow. But a scheme to "right size" the initial window automatically over long timescales has yet to be developed. o More thorough analysis of the impact on slow links is needed. Although [Duk10] showed the large initial window reduced the average latency even for the dialup link class of only 56 Kbps in bandwidth, more studies 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 300 ms RTT, bottleneck link bandwidth as low as 64 Kbps, and route queue size as low as 40 packets. A large combination of test parameters were used. Almost all tests showed varying degrees 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 the Network Simulation Cradle (NSC) framework [Sch11] [Sch11-1]. o How will the larger initial window affect flows with initial windows of 4 KB or less? Flows with the larger initial window will likely grab more bandwidth from a bottleneck link when competing against flows with smaller initial windows, 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 less than 4 KB 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 against flows with IW=10 in light to medium 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 that IW=10 enabled short flows to complete sooner, leaving more room for the long-lived, background flows. A study using an 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.