Internet DRAFT - draft-ietf-tsvwg-initwin


Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                              BBN/NASA GRC
File: draft-ietf-tsvwg-initwin-04.txt                        Sally Floyd
                                                         Craig Partridge
                                                        BBN Technologies
                                                              June, 2002
                                                 Expires: December, 2002

                    Increasing TCP's Initial Window

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
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    This document specifies an optional standard for TCP to increase the
    permitted initial window from one or two segment(s) to roughly 4K bytes,
    replacing RFC 2414.  This document discusses the advantages and
    disadvantages of the higher initial window.  The document includes
    discussion of experiments and simulations showing that the higher
    initial window does not lead to congestion collapse. Finally, the
    document provides guidance on implementation issues.


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

1.  TCP Modification

    This document updates [RFC2414] and specifies an increase in the
    permitted upper bound for TCP's initial window from one or two
    segment(s) to between two and four segments.  In most cases, this
    change results in an upper bound on the initial window of roughly 4K

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    bytes (although given a large segment size, the permitted initial
    window of two segments may be significantly larger than 4K bytes).
    The upper bound for the initial window is given more precisely in

          min (4*MSS, max (2*MSS, 4380 bytes))			    (1)

    Note: Sending a 1500 byte packet indicates an MSS of 1460 bytes
    (assuming no IP or TCP options).  Therefore, limiting the initial
    window's MSS to 4380 bytes allows the sender to transmit three
    segments initially in the common case when using 1500 byte packets.

    Equivalently, the upper bound for the initial window size is based
    on the maximum segment size (MSS), as follows:

        If (MSS <= 1095 bytes)
            then win <= 4 * MSS;
        If (1095 bytes < MSS < 2190 bytes)
            then win <= 4380;
        If (2190 bytes <= MSS)
            then win <= 2 * MSS;

    This increased initial window is optional: a TCP MAY start with a
    larger initial window.  However, we expect that most general-purpose
    TCP implementations would choose to use the larger initial
    congestion window given in equation (1) above.

    This upper bound for the initial window size represents a change
    from RFC 2581 [RFC2581], which specified that the congestion window
    be initialized to one or two segments.

    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 above that outlined in equation (1).  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.

    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

2.  Implementation Issues

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    When larger initial windows are implemented along with Path MTU
    Discovery [RFC1191], and the MSS being used is found to be too
    large, the congestion window `cwnd' SHOULD be reduced to prevent
    large bursts of smaller segments.  Specifically, `cwnd' SHOULD be
    reduced by the ratio of the old segment size to the new segment

    When larger initial windows are implemented along with Path MTU
    Discovery [RFC1191], alternatives are to set the "Don't Fragment"
    (DF) bit in all segments in the initial window, or to set the "Don't
    Fragment" (DF) bit in one of the segments.  It is an open question
    which of these two alternatives is best; we would hope that
    implementation experiences will shed light on this question.  In the
    first case of setting the DF bit in all segments, if the initial
    packets are too large, then all of the initial packets will be
    dropped in the network.  In the second case of setting the DF bit in
    only one segment, if the initial packets are too large, then all but
    one of the initial packets will be fragmented in the network.  When
    the second case is followed, setting the DF bit in the last segment
    in the initial window provides the least chance for needless
    retransmissions when the initial segment size is found to be too
    large, because it minimizes the chances of duplicate ACKs triggering
    a Fast Retransmit.  However, more attention needs to be paid to the
    interaction between larger initial windows and Path MTU Discovery.

    The larger initial window specified in this document is not intended
    as encouragement for web browsers to open multiple simultaneous TCP
    connections all with large initial windows.  When web browsers open
    simultaneous TCP connections to the same destination, this works
    against TCP's congestion control mechanisms [FF98], regardless of
    the size of the initial window.  Combining this behavior with larger
    initial windows further increases the unfairness to other traffic in
    the network.  We suggest the use of HTTP/1.1 [RFC2068] (persistent
    TCP connections and pipelining) as a way to achieve better
    performance of web transfers.

3.  Advantages of Larger Initial Windows

    1.  When the initial window is one segment, a receiver employing
        delayed ACKs [RFC1122] is forced to wait for a timeout before
        generating an ACK.  With an initial window of at least two
        segments, the receiver will generate an ACK after the second
        data segment arrives.  This eliminates the wait on the timeout
        (often up to 200 msec, and possibly up to 500 msec [RFC1122]).

    2.  For connections transmitting only a small amount of data, a
        larger initial window reduces the transmission time (assuming at
        most moderate segment drop rates).  For many email (SMTP
        [Pos82]) and web page (HTTP [RFC1945, RFC2068]) transfers that
        are less than 4K bytes, the larger initial window would reduce
        the data transfer time to a single RTT.

    3.  For connections that will be able to use large congestion
        windows, this modification eliminates up to three RTTs and a

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        delayed ACK timeout during the initial slow-start phase.  This
        will be of particular benefit for high-bandwidth large-
        propagation-delay TCP connections, such as those over satellite

4.  Disadvantages of Larger Initial Windows for the Individual

    In high-congestion environments, particularly for routers that have
    a bias against bursty traffic (as in the typical Drop Tail router
    queues), a TCP connection can sometimes be better off starting with
    an initial window of one segment.  There are scenarios where a TCP
    connection slow-starting from an initial window of one segment might
    not have segments dropped, while a TCP connection starting with an
    initial window of four segments might experience unnecessary
    retransmits due to the inability of the router to handle small
    bursts.  This could result in an unnecessary retransmit timeout.
    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.  These 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,

    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.

5.  Disadvantages of Larger Initial Windows for the Network

    In terms of the potential for congestion collapse, we consider two
    separate potential dangers for the network.  The first danger would
    be a scenario where a large number of segments on congested links
    were duplicate segments that had already been received at the
    receiver.  The second danger would be a scenario where a large
    number of segments on congested links were segments that would be
    dropped later in the network before reaching their final

    In terms of the negative effect on other traffic in the network, a
    potential disadvantage of larger initial windows would be that they
    increase the general packet drop rate in the network.  We discuss
    these three issues below.

    Duplicate segments:

        As described in the previous section, the larger initial window
        could occasionally result in a segment dropped from the initial

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        window, when that segment might not have been dropped if the
        sender had slow-started from an initial window of one segment.
        However, Appendix A shows that even in this case, the larger
        initial window would not result in the transmission of a large
        number of duplicate segments.

    Segments dropped later in the network:

        How much would the larger initial window for TCP increase the
        number of segments on congested links that would be dropped
        before reaching their final destination?  This is a problem that
        can only occur for connections with multiple congested links,
        where some segments might use scarce bandwidth on the first
        congested link along the path, only to be dropped later along
        the path.

        First, many of the TCP connections will have only one congested
        link along the path.  Segments dropped from these connections do
        not "waste" scarce bandwidth, and do not contribute to
        congestion collapse.

        However, some network paths will have multiple congested links,
        and segments dropped from the initial window could use scarce
        bandwidth along the earlier congested links before ultimately
        being dropped on subsequent congested links.  To the extent that
        the drop rate is independent of the initial window used by TCP
        segments, the problem of congested links carrying segments that
        will be dropped before reaching their destination will be
        similar for TCP connections that start by sending four segments
        or one segment.

    An increased packet drop rate:

        For a network with a high segment drop rate, increasing the TCP
        initial window could increase the segment drop rate even
        further.  This is in part because routers with Drop Tail queue
        management have difficulties with bursty traffic in times of
        congestion.  However, given uncorrelated arrivals for TCP
        connections, the larger TCP initial window should not
        significantly increase the segment drop rate.  Simulation-based
        explorations of these issues are discussed in Section 7.2.

    These potential dangers for the network are explored in simulations
    and experiments described in the section below.  Our judgment is
    that while there are dangers of congestion collapse in the current
    Internet (see [FF98] for a discussion of the dangers of congestion
    collapse from an increased deployment of UDP connections without
    end-to-end congestion control), there is no such danger to the
    network from increasing the TCP initial window to 4K bytes.

6.  Interactions with the Retransmission Timer
    Using a larger initial burst of data can exacerbate existing
    problems with spurious retransmit timeouts on low-bandwidth paths,

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    assuming the standard algorithm for determining the TCP
    retransmission timeout (RTO) [RFC2988].  The problem is that across
    low-bandwidth network paths on which the transmission time of a
    packet is a large portion of the round-trip time, the small packets
    used to establish a TCP connection do not seed the RTO estimator
    appropriately.  When the first window of data packets is
    transmitted, the sender's retransmit timer could expire before the
    acknowledgments for those packets are received.  As each
    acknowledgment arrives, the retransmit timer is generally reset.
    Thus, the retransmit timer will not expire as long as an
    acknowledgment arrives at least once a second, given the one-second
    minimum on the RTO recommended in RFC 2988.

    For instance, consider a 9.6 Kbps link.  The initial RTT measurement
    will be on the order of 67 msec, if we simply consider the
    transmission time of 2 packets (the SYN and SYN-ACK) each consisting
    of 40 bytes.  Using the RTO estimator given in [RFC2988], this
    yields an initial RTO of 201 msec (67 + 4*(67/2)).  However, we
    round the RTO to 1 second as specified in RFC 2988.  Then assume we
    send an initial window of one or more 1500-byte packets (1460 data
    bytes plus overhead).  Each packet will take on the order of 1.25
    seconds to transmit.  Therefore, the RTO will fire before the ACK
    for the first packet returns, causing a spurious timeout.  In this
    case, a larger initial window of three or four packets exacerbates
    the problems caused by this spurious timeout.

    One way to deal with this problem is to make the RTO algorithm more
    conservative.  During the initial window of data, for instance, the
    RTO could be updated for each acknowledgment received.  In addition,
    if the retransmit timer expires for some packet lost in the first
    window of data, we could leave the exponential-backoff of the
    retransmit timer engaged until at least one valid RTT measurement is
    received that involves a data packet.

    Another method would be to refrain from taking a RTT sample during
    connection establishment, leaving the default RTO in place until TCP
    takes a sample from a data segment and the corresponding ACK.  While
    this method likely helps prevent spurious retransmits it also may
    slow the data transfer down if loss occurs before the RTO is
    seeded.  The use of limited transmit [RFC3042] to aid a TCP
    connection in recovering from loss using fast retransmit rather than
    the RTO timer mitigates the performance degradation caused by using
    the high default RTO during the initial window of data

    This specification leaves the decision about what to do (if
    anything) with regards to the RTO when using a larger initial window
    to the implementer.  However, the RECOMMENDED approach is to refrain
    from sampling the RTT during the three-way handshake, keeping the
    default RTO in place until a RTT sample involving a data packet is
    taken.  In addition, it is RECOMMENDED that TCPs use limited
    transmit [RFC3042].

7.  Typical Levels of Burstiness for TCP Traffic.

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    Larger TCP initial windows would not dramatically increase the
    burstiness of TCP traffic in the Internet today, because such
    traffic is already fairly bursty.  Bursts of two and three segments
    are already typical of TCP [Flo97]; A delayed ACK (covering two
    previously unacknowledged segments) received during congestion
    avoidance causes the congestion window to slide and two segments to
    be sent.  The same delayed ACK received during slow start causes the
    window to slide by two segments and then be incremented by one
    segment, resulting in a three-segment burst.  While not necessarily
    typical, bursts of four and five segments for TCP are not rare.
    Assuming delayed ACKs, a single dropped ACK causes the subsequent
    ACK to cover four previously unacknowledged segments.  During
    congestion avoidance this leads to a four-segment burst and during
    slow start a five-segment burst is generated.

    There are also changes in progress that reduce the performance
    problems posed by moderate traffic bursts.  One such change is the
    deployment of higher-speed links in some parts of the network, where
    a burst of 4K bytes can represent a small quantity of data.  A
    second change, for routers with sufficient buffering, is the
    deployment of queue management mechanisms such as RED, which is
    designed to be tolerant of transient traffic bursts.

8.  Simulations and Experimental Results

8.1 Studies of TCP Connections using that Larger Initial Window

    This section surveys simulations and experiments that explore the
    effect of larger initial windows on TCP connections.  The first set
    of experiments explores performance over satellite links.  Larger
    initial windows have been shown to improve performance of TCP
    connections over satellite channels [All97b].  In this study, an
    initial window of four segments (512 byte MSS) resulted in
    throughput improvements of up to 30% (depending upon transfer size).
    [KAGT98] shows that the use of larger initial windows results in a
    decrease in transfer time in HTTP tests over the ACTS satellite
    system.  A study involving simulations of a large number of HTTP
    transactions over hybrid fiber coax (HFC) indicates that the use of
    larger initial windows decreases the time required to load WWW pages

    A second set of experiments explored TCP performance over dialup
    modem links.  In experiments over a 28.8 bps dialup channel [All97a,
    AHO98], a four-segment initial window decreased the transfer time of
    a 16KB file by roughly 10%, with no accompanying increase in the
    drop rate.  A simulation study [RFC2416] investigated the effects of
    using a larger initial window on a host connected by a slow modem
    link and a router with a 3 packet buffer.  The study concluded that
    for the scenario investigated, the use of larger initial windows was
    not harmful to TCP performance.

    Finally, [All00] illustrates that the percentage of connections at a
    particular web server that experience loss in the initial window of

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    data transmission increases with the size of the initial congestion
    window.  However, the increase is in line with what would be
    expected from sending a larger burst into the network.

8.2 Studies of Networks using Larger Initial Windows

    This section surveys simulations and experiments investigating the
    impact of the larger window on other TCP connections sharing the
    path.  Experiments in [All97a, AHO98] show that for 16 KB transfers
    to 100 Internet hosts, four-segment initial windows resulted in a
    small increase in the drop rate of 0.04 segments/transfer.  While
    the drop rate increased slightly, the transfer time was reduced by
    roughly 25% for transfers using the four-segment (512 byte MSS)
    initial window when compared to an initial window of one segment.

    A simulation study in [RFC2415] explores the impact of a larger
    initial window on competing network traffic.  In this investigation,
    HTTP and FTP flows share a single congested gateway (where the
    number of HTTP and FTP flows varies from one simulation set to
    another).  For each simulation set, the paper examines aggregate
    link utilization and packet drop rates, median web page delay, and
    network power for the FTP transfers.  The larger initial window
    generally resulted in increased throughput, slightly-increased
    packet drop rates, and an increase in overall network power.  With
    the exception of one scenario, the larger initial window resulted in
    an increase in the drop rate of less than 1% above the loss rate
    experienced when using a one-segment initial window; in this
    scenario, the drop rate increased from 3.5% with one-segment initial
    windows, to 4.5% with four-segment initial windows.  The overall
    conclusions were that increasing the TCP initial window to three
    packets (or 4380 bytes) helps to improve perceived performance.

    Morris [Mor97] investigated larger initial windows in a highly
    congested network with transfers of size 20K.  The loss rate in
    networks where all TCP connections use an initial window of four
    segments is shown to be 1-2% greater than in a network where all
    connections use an initial window of one segment.  This relationship
    held in scenarios where the loss rates with one-segment initial
    windows ranged from 1% to 11%.  In addition, in networks where
    connections used an initial window of four segments, TCP connections
    spent more time waiting for the retransmit timer (RTO) to expire to
    resend a segment than was spent when using an initial window of one
    segment.  The time spent waiting for the RTO timer to expire
    represents idle time when no useful work was being accomplished for
    that connection.  These results show that in a very congested
    environment, where each connection's share of the bottleneck
    bandwidth is close to one segment, using a larger initial window can
    cause a perceptible increase in both loss rates and retransmit

9.  Security Considerations

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

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    security issues with TCP.

10. Conclusion

    This document specifies a small change to TCP that will likely be
    beneficial to short-lived TCP connections and those over links with
    long RTTs (saving several RTTs during the initial slow-start phase).

11. Acknowledgments

    We would like to acknowledge Vern Paxson, Tim Shepard, members of
    the End-to-End-Interest Mailing List, and members of the IETF TCP
    Implementation Working Group for continuing discussions of these
    issues for discussions and feedback on this document.

12. References

    [AHO98] Mark Allman, Chris Hayes, and Shawn Ostermann, An Evaluation
        of TCP with Larger Initial Windows, March 1998.  Submitted to
        ACM Computer Communication Review.  URL:

    [All97a] Mark Allman.  An Evaluation of TCP with Larger Initial
        Windows.  40th IETF Meeting -- TCP Implementations WG.
        December, 1997.  Washington, DC.

    [All97b] Mark Allman.  Improving TCP Performance Over Satellite
        Channels.  Master's thesis, Ohio University, June 1997.

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

    [FF96] Fall, K., and Floyd, S., Simulation-based Comparisons of
        Tahoe, Reno, and SACK TCP.  Computer Communication Review,
        26(3), July 1996.

    [FF98] Sally Floyd, Kevin Fall.  Promoting the Use of End-to-End
        Congestion Control in the Internet.  Submitted to IEEE
        Transactions on Networking.  URL "http://www-".

    [FJ93] Floyd, S., and Jacobson, V., Random Early Detection gateways
        for Congestion Avoidance. IEEE/ACM Transactions on Networking,
        V.1 N.4, August 1993, p. 397-413.

    [Flo94] Floyd, S., TCP and Explicit Congestion Notification.
        Computer Communication Review, 24(5):10-23, October 1994.

    [Flo96] Floyd, S., Issues of TCP with SACK. Technical report,
        January 1996.  Available from

    [Flo97] Floyd, S., Increasing TCP's Initial Window.  Viewgraphs,
        40th IETF Meeting - TCP Implementations WG. December, 1997.  URL

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    [KAGT98] Hans Kruse, Mark Allman, Jim Griner, Diepchi Tran.  HTTP
        Page Transfer Rates Over Geo-Stationary Satellite Links.  March
        1998.  Proceedings of the Sixth International Conference on
        Telecommunication Systems.  URL

    [Mor97] Robert Morris.  Private communication, 1997.  Cited for
        acknowledgement purposes only.

    [Nic97] Kathleen Nichols.  Improving Network Simulation with
        Feedback.  Com21, Inc. Technical Report.  Available from

    [Pos82] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
        821, August 1982.

    [RFC1122] Braden, R., "Requirements for Internet Hosts --
        Communication Layers", STD 3, RFC 1122, October 1989.

    [RFC1191] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
        November 1990.

    [RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
        Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

    [RFC2068] Fielding, R., Mogul, J., Gettys, J., Frystyk, H., and T.
        Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
        2068, January 1997.

    [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

    [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.

    [RFC2415] Poduri, K., and K. Nichols, "Simulation Studies of
        Increased Initial TCP Window Size", RFC 2415, September 1998.

    [RFC2416] Shepard, T., and C. Partridge, "When TCP Starts Up With
        Four Packets Into Only Three Buffers", RFC 2416, September 1998.
    [RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens. TCP
        Congestion Control, April 1999.  RFC 2581.

    [RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission
        Timer, November 2000.  RFC 2988.

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    [RFC3042] M. Allman, H. Balakrishnan, and S. Floyd, Enhancing TCP's
        Loss Recovery Using Limited Transmit, RFC 3042, January 2001.
    [RFC3168] Ramakrishnan, K.K., Floyd, S., and Black, D., "The
        Addition of Explicit Congestion Notification (ECN) to IP", RFC
        3168, September 2001.

13. Author's Addresses

    Mark Allman
    BBN Technologies/NASA Glenn Research Center
    21000 Brookpark Road
    MS 54-5
    Cleveland, OH 44135

    Sally Floyd
    ICSI Center for Internet Research
    1947 Center St, Suite 600
    Berkeley, CA 94704
    Phone: +1 (510) 666-2989

    Craig Partridge
    BBN Technologies
    10 Moulton Street
    Cambridge, MA 02138

14.  Appendix - Duplicate Segments

    In the current environment (without Explicit Congestion Notification
    [Flo94] [RFC2481]), all TCPs use segment drops as indications from
    the network about the limits of available bandwidth.  We argue here
    that the change to a larger initial window should not result in the
    sender retransmitting a large number of duplicate segments that have
    already arrived at the receiver.

    If one segment is dropped from the initial window, there are three
    different ways for TCP to recover: (1) Slow-starting from a window
    of one segment, as is done after a retransmit timeout, or after Fast
    Retransmit in Tahoe TCP; (2) Fast Recovery without selective
    acknowledgments (SACK), as is done after three duplicate ACKs in
    Reno TCP; and (3) Fast Recovery with SACK, for TCP where both the
    sender and the receiver support the SACK option [MMFR96].  In all
    three cases, if a single segment is dropped from the initial window,
    no duplicate segments (i.e., segments that have already been
    received at the receiver) are transmitted.  Note that for a TCP
    sending four 512-byte segments in the initial window, a single
    segment drop will not require a retransmit timeout, but can be
    recovered from using the Fast Retransmit algorithm (unless the

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    retransmit timer expires prematurely).  In addition, a single
    segment dropped from an initial window of three segments might be
    repaired using the fast retransmit algorithm, depending on which
    segment is dropped and whether or not delayed ACKs are used.  For
    example, dropping the first segment of a three segment initial
    window will always require waiting for a timeout, in the absence of
    Limited Transmit [RFC3042].  However, dropping the third segment
    will always allow recovery via the fast retransmit algorithm, as
    long as no ACKs are lost.

    Next we consider scenarios where the initial window contains two to
    four segments, and at least two of those segments are dropped.  If
    all segments in the initial window are dropped, then clearly no
    duplicate segments are retransmitted, as the receiver has not yet
    received any segments.  (It is still a possibility that these
    dropped segments used scarce bandwidth on the way to their drop
    point; this issue was discussed in Section 5.)

    When two segments are dropped from an initial window of three
    segments, the sender will only send a duplicate segment if the first
    two of the three segments were dropped, and the sender does not
    receive a packet with the SACK option acknowledging the third

    When two segments are dropped from an initial window of four
    segments, an examination of the six possible scenarios (which we
    don't go through here) shows that, depending on the position of the
    dropped packets, in the absence of SACK the sender might send one
    duplicate segment.  There are no scenarios in which the sender sends
    two duplicate segments.

    When three segments are dropped from an initial window of four
    segments, then, in the absence of SACK, it is possible that one
    duplicate segment will be sent, depending on the position of the
    dropped segments.

    The summary is that in the absence of SACK, there are some scenarios
    with multiple segment drops from the initial window where one
    duplicate segment will be transmitted.  There are no scenarios where
    more that one duplicate segment will be transmitted.  Our conclusion
    is that the number of duplicate segments transmitted as a result of
    a larger initial window should be small.

15. Full Copyright Statement

    Copyright (C) The Internet Society (2001). All Rights Reserved.

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