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Network Working Group                                          M. AllmanRequest for Comments: 3390                                  BBN/NASA GRCObsoletes:2414                                                 S. FloydUpdates:2581                                                       ICIRCategory: Standards Track                                   C. Partridge                                                        BBN Technologies                                                            October 2002Increasing TCP's Initial WindowStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2002).  All Rights Reserved.Abstract   This document specifies an optional standard for TCP to increase the   permitted initial window from one or two segment(s) to roughly 4K   bytes, replacingRFC 2414.  It discusses the advantages and   disadvantages of the higher initial window, and includes discussion   of experiments and simulations showing that the higher initial window   does not lead to congestion collapse.  Finally, this document   provides guidance on implementation issues.Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [RFC2119].1.  TCP Modification   This document obsoletes [RFC2414] and updates [RFC2581] 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 bytes (although given a large segment size, the permitted   initial window of two segments may be significantly larger than 4K   bytes).Allman, et. al.             Standards Track                     [Page 1]

RFC 3390            Increasing TCP's Initial Window         October 2002   The upper bound for the initial window is given more precisely in   (1):         min (4*MSS, max (2*MSS, 4380 bytes))                        (1)   Note: Sending a 1500 byte packet indicates a maximum segment size   (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 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 fromRFC 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 (toAllman, et. al.             Standards Track                     [Page 2]

RFC 3390            Increasing TCP's Initial Window         October 2002   permit the lowest possible window size in the case of severe   congestion).2.  Implementation Issues   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 size.   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 as   to 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, they are   working against TCP's congestion control mechanisms [FF99],   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]).Allman, et. al.             Standards Track                     [Page 3]

RFC 3390            Increasing TCP's Initial Window         October 2002   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       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       links.4.  Disadvantages of Larger Initial Windows for the Individual    Connection   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,RFC2309]).   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 linksAllman, et. al.             Standards Track                     [Page 4]

RFC 3390            Increasing TCP's Initial Window         October 2002   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 destination.   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       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.Allman, et. al.             Standards Track                     [Page 5]

RFC 3390            Increasing TCP's Initial Window         October 2002   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 inSection 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 [FF99] 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, 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 inRFC 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 inRFC 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.Allman, et. al.             Standards Track                     [Page 6]

RFC 3390            Increasing TCP's Initial Window         October 2002   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,   that involves a data packet, is received.   Another method would be to refrain from taking an 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 transmission.   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 an 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.   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 ofAllman, et. al.             Standards Track                     [Page 7]

RFC 3390            Increasing TCP's Initial Window         October 2002   queue management mechanisms such as RED, which is designed to be   tolerant of transient traffic bursts.8.  Simulations and Experimental Results8.1 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 the 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 [Nic98].   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   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.Allman, et. al.             Standards Track                     [Page 8]

RFC 3390            Increasing TCP's Initial Window         October 2002   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 20K in size.  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 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   timeouts.9.  Security Considerations   This document discusses the initial congestion window permitted for   TCP connections.  Changing this value does not raise any known new   security issues with TCP.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).Allman, et. al.             Standards Track                     [Page 9]

RFC 3390            Increasing TCP's Initial Window         October 200211. 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 and for feedback on this document.12. References   [AHO98]   Mark Allman, Chris Hayes, and Shawn Ostermann, An             Evaluation of TCP with Larger Initial Windows, March 1998.             ACM Computer Communication Review, 28(3), July 1998.  URL             "http://roland.lerc.nasa.gov/~mallman/papers/initwin.ps".   [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.   [FF99]    Sally Floyd, Kevin Fall.  Promoting the Use of End-to-End             Congestion Control in the Internet.  IEEE/ACM Transactions             on Networking, August 1999.  URL             "http://www.icir.org/floyd/end2end-paper.html".   [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 fromhttp://www-nrg.ee.lbl.gov/floyd/.   [Flo97]   Floyd, S., Increasing TCP's Initial Window.  Viewgraphs,             40th IETF Meeting - TCP Implementations WG. December, 1997.             URL "ftp://ftp.ee.lbl.gov/talks/sf-tcp-ietf97.ps".Allman, et. al.             Standards Track                    [Page 10]

RFC 3390            Increasing TCP's Initial Window         October 2002   [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             "http://roland.lerc.nasa.gov/~mallman/papers/nash98.ps".   [Mor97]   Robert Morris.  Private communication, 1997.  Cited for             acknowledgement purposes only.   [Nic98]   Kathleen Nichols. Improving Network Simulation With             Feedback, Proceedings of LCN 98, October 1998. URL             "http://www.computer.org/proceedings/lcn/8810/8810toc.htm".   [Pos82]   Postel, J., "Simple Mail Transfer Protocol", STD 10,RFC821, 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",RFC2616, 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",RFC2309, 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.Allman, et. al.             Standards Track                    [Page 11]

RFC 3390            Increasing TCP's Initial Window         October 2002   [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion             Control",RFC 2581, April 1999.   [RFC2821] Klensin, J., "Simple Mail Transfer Protocol",RFC 2821,             April 2001.   [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission             Timer",RFC 2988, November 2000.   [RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's             Loss Recovery Using Limited Transmit",RFC 3042, January             2001.   [RFC3168] Ramakrishnan, K.K., Floyd, S. and D. Black, "The Addition             of Explicit Congestion Notification (ECN) to IP",RFC 3168,             September 2001.Allman, et. al.             Standards Track                    [Page 12]

RFC 3390            Increasing TCP's Initial Window         October 2002Appendix A - 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 by using the   Fast Retransmit algorithm (unless the 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 inSection 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   segment.   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 theAllman, et. al.             Standards Track                    [Page 13]

RFC 3390            Increasing TCP's Initial Window         October 2002   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 in   which more than one duplicate segment will be transmitted.  Our   conclusion is than the number of duplicate segments transmitted as a   result of a larger initial window should be small.Author's Addresses   Mark Allman   BBN Technologies/NASA Glenn Research Center   21000 Brookpark Rd   MS 54-5   Cleveland, OH 44135   EMail: mallman@bbn.comhttp://roland.lerc.nasa.gov/~mallman/   Sally Floyd   ICSI Center for Internet Research   1947 Center St, Suite 600   Berkeley, CA 94704   Phone: +1 (510) 666-2989   EMail: floyd@icir.orghttp://www.icir.org/floyd/   Craig Partridge   BBN Technologies   10 Moulton St   Cambridge, MA 02138   EMail: craig@bbn.comAllman, et. al.             Standards Track                    [Page 14]

RFC 3390            Increasing TCP's Initial Window         October 2002Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Allman, et. al.             Standards Track                    [Page 15]