Movatterモバイル変換

Network Working Group                                          M. AllmanRequest for Comments: 2414                  NASA Lewis/Sterling SoftwareCategory: Experimental                                          S. Floyd                                                                    LBNL                                                            C. Partridge                                                        BBN Technologies                                                          September 1998Increasing TCP's Initial WindowStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.Abstract   This document specifies an increase in the permitted initial window   for TCP from one segment to roughly 4K bytes.  This document   discusses the advantages and disadvantages of such a change,   outlining experimental results that indicate the costs and benefits   of such a change to TCP.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 specifies an increase in the permitted upper bound for   TCP's initial window from one segment 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 could be   significantly larger than 4K bytes).  The upper bound for the initial   window is given more precisely in (1):          min (4*MSS, max (2*MSS, 4380 bytes))               (1)Allman, et. al.               Experimental                      [Page 1]

RFC 2414            Increasing TCP's Initial Window       September 1998   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: that a TCP MAY start with   a larger initial window, not that it SHOULD.   This upper bound for the initial window size represents a change fromRFC 2001 [S97], which specifies that the congestion window be   initialized to one segment.  If implementation experience proves   successful, then the intent is for this change to be incorporated   into a revision toRFC 2001.   This change applies to the initial window of the connection in the   first round trip time (RTT) of 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.   TCP implementations use slow start in as many as three different   ways: (1) to start a new connection (the initial window); (2) to   restart a transmission after a long idle period (the restart window);   and (3) to restart after a retransmit timeout (the loss window).  The   change proposed 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 (to 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 [MD90], 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.Allman, et. al.               Experimental                      [Page 2]

RFC 2414            Increasing TCP's Initial Window       September 1998   When larger initial windows are implemented along with Path MTU   Discovery [MD90], 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.  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 proposed in this document is not intended   as an 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.3.  Advantages of Larger Initial Windows   1.  When the initial window is one segment, a receiver employing       delayed ACKs [Bra89] 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).   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 [BLFN96,FJGFBL97]) 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.  ThisAllman, et. al.               Experimental                      [Page 3]

RFC 2414            Increasing TCP's Initial Window       September 1998       would 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 higher   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 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.Allman, et. al.               Experimental                      [Page 4]

RFC 2414            Increasing TCP's Initial Window       September 1998   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.   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.Allman, et. al.               Experimental                      [Page 5]

RFC 2414            Increasing TCP's Initial Window       September 1998   These potential dangers for the network are explored in simulations   and experiments described in the section below.  Our judgement would   be, 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.  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 of   queue management mechanisms such as RED, which is designed to be   tolerant of transient traffic bursts.7.  Simulations and Experimental Results7.1 Studies of TCP Connections using that Larger Initial Window   This section surveys simulations and experiments that have been used   to explore the effect of larger initial windows on the TCP connection   using that larger window.  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 simulationsAllman, et. al.               Experimental                      [Page 6]

RFC 2414            Increasing TCP's Initial Window       September 1998   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 [Nic97].   A second set of experiments has 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 particular area of concern has been TCP performance over low   speed tail circuits (e.g., dialup modem links) with routers with   small buffers.  A simulation study [SP97] 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.  Questions have been raised   concerning the effects of larger initial windows on the transfer time   for short transfers in this environment, but these effects have not   been quantified.  A question has also been raised concerning the   possible effect on existing TCP connections sharing the link.7.2 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.   One scenario of concern is heavily loaded links.  For instance, a   couple of years ago, one of the trans-Atlantic links was so heavily   loaded that the correct congestion window size for a connection was   about one segment.  In this environment, new connections using larger   initial windows would be starting with windows that were four times   too big.  What would the effects be?  Do connections thrash?   A simulation study in [PN98] 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 theAllman, et. al.               Experimental                      [Page 7]

RFC 2414            Increasing TCP's Initial Window       September 1998   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 very   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   timeouts.8.  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.9.  Conclusion   This document proposes a small change to TCP that may be beneficial   to short-lived TCP connections and those over links with long RTTs   (saving several RTTs during the initial slow-start phase).10.  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.Allman, et. al.               Experimental                      [Page 8]

RFC 2414            Increasing TCP's Initial Window       September 199811.  References   [All97a]    Mark Allman.  An Evaluation of TCP with Larger Initial               Windows.  40th IETF Meeting -- TCP Implementations WG.               December, 1997.  Washington, DC.   [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: "http://gigahertz.lerc.nasa.gov/~mallman/papers/initwin.ps".   [All97b]    Mark Allman.  Improving TCP Performance Over Satellite               Channels.  Master's thesis, Ohio University, June 1997.   [BLFN96]    Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext               Transfer Protocol -- HTTP/1.0",RFC 1945, May 1996.   [Bra89]     Braden, R., "Requirements for Internet Hosts --               Communication Layers", STD 3,RFC 1122, October 1989.   [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-nrg.ee.lbl.gov/floyd/end2end-paper.html".   [FJGFBL97]  Fielding, R., Mogul, J., Gettys, J., Frystyk, H., and T.               Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",RFC 2068, January 1997.   [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.               Experimental                      [Page 9]

RFC 2414            Increasing TCP's Initial Window       September 1998   [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://gigahertz.lerc.nasa.gov/~mallman/papers/nash98.ps".   [MD90]      Mogul, J., and S. Deering, "Path MTU Discovery",RFC1191, November 1990.   [MMFR96]    Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP               Selective Acknowledgment Options",RFC 2018, October               1996.   [Mor97]     Robert Morris.  Private communication, 1997.  Cited for               acknowledgement purposes only.   [Nic97]     Kathleen Nichols.  Improving Network Simulation with               Feedback.  Com21, Inc. Technical Report.  Available fromhttp://www.com21.com/pages/papers/068.pdf.   [PN98]      Poduri, K., and K. Nichols, "Simulation Studies of               Increased Initial TCP Window Size",RFC 2415, September               1998.   [Pos82]     Postel, J., "Simple Mail Transfer Protocol", STD 10,RFC821, August 1982.   [RF97]      Ramakrishnan, K., and S. Floyd, "A Proposal to Add               Explicit Congestion Notification (ECN) to IPv6 and to               TCP", Work in Progress.   [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.   [S97]       Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast               Retransmit, and Fast Recovery Algorithms",RFC 2001,               January 1997.   [SP97]      Shepard, T., and C. Partridge, "When TCP Starts Up With               Four Packets Into Only Three Buffers",RFC 2416,               September 1998.Allman, et. al.               Experimental                     [Page 10]

RFC 2414            Increasing TCP's Initial Window       September 199812.  Author's Addresses   Mark Allman   NASA Lewis Research Center/Sterling Software   21000 Brookpark Road   MS 54-2   Cleveland, OH 44135   EMail: mallman@lerc.nasa.govhttp://gigahertz.lerc.nasa.gov/~mallman/   Sally Floyd   Lawrence Berkeley National Laboratory   One Cyclotron Road   Berkeley, CA 94720   EMail: floyd@ee.lbl.gov   Craig Partridge   BBN Technologies   10 Moulton Street   Cambridge, MA 02138   EMail: craig@bbn.comAllman, et. al.               Experimental                     [Page 11]

RFC 2414            Increasing TCP's Initial Window       September 199813.  Appendix - Duplicate Segments   In the current environment (without Explicit Congestion Notification   [Flo94] [RF97]), 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   been received 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 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.  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.               Experimental                     [Page 12]

RFC 2414            Increasing TCP's Initial Window       September 1998   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.Allman, et. al.               Experimental                     [Page 13]

RFC 2414            Increasing TCP's Initial Window       September 199814.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  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.Allman, et. al.               Experimental                     [Page 14]