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Network Working Group                                         M. HandleyRequest for Comments: 2861                                     J. PadhyeCategory: Experimental                                          S. Floyd                                                                   ACIRI                                                               June 2000TCP Congestion Window ValidationStatus 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 (2000).  All Rights Reserved.Abstract   TCP's congestion window controls the number of packets a TCP flow may   have in the network at any time.  However, long periods when the   sender is idle or application-limited can lead to the invalidation of   the congestion window, in that the congestion window no longer   reflects current information about the state of the network.  This   document describes a simple modification to TCP's congestion control   algorithms to decay the congestion window cwnd after the transition   from a sufficiently-long application-limited period, while using the   slow-start threshold ssthresh to save information about the previous   value of the congestion window.   An invalid congestion window also results when the congestion window   is increased (i.e., in TCP's slow-start or congestion avoidance   phases) during application-limited periods, when the previous value   of the congestion window might never have been fully utilized.  We   propose that the TCP sender should not increase the congestion window   when the TCP sender has been application-limited (and therefore has   not fully used the current congestion window).  We have explored   these algorithms both with simulations and with experiments from an   implementation in FreeBSD.1.  Conventions and Acronyms   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this   document, are to be interpreted as described in [B97].Handley, et al.               Experimental                      [Page 1]

RFC 2861            TCP Congestion Window Validation           June 20002. Introduction   TCP's congestion window controls the number of packets a TCP flow may   have in the network at any time.  The congestion window is set using   an Additive-Increase, Multiplicative-Decrease (AIMD) mechanism that   probes for available bandwidth, dynamically adapting to changing   network conditions.  This AIMD mechanism works well when the sender   continually has data to send, as is typically the case for TCP used   for bulk-data transfer.  In contrast, for TCP used with telnet   applications, the data sender often has little or no data to send,   and the sending rate is often determined by the rate at which data is   generated by the user.  With the advent of the web, including   developments such as TCP senders with dynamically-created data and   HTTP 1.1 with persistent-connection TCP, the interaction between   application-limited periods (when the sender sends less than is   allowed by the congestion or receiver windows) and network-limited   periods (when the sender is limited by the TCP window) becomes   increasingly important.  More precisely, we define a network-limited   period as any period when the sender is sending a full window of   data.   Long periods when the sender is application-limited can lead to the   invalidation of the congestion window.  During periods when the TCP   sender is network-limited, the value of the congestion window is   repeatedly "revalidated" by the successful transmission of a window   of data without loss.  When the TCP sender is network-limited, there   is an incoming stream of acknowledgements that "clocks out" new data,   giving concrete evidence of recent available bandwidth in the   network.  In contrast, during periods when the TCP sender is   application-limited, the estimate of available capacity represented   by the congestion window may become steadily less accurate over time.   In particular, capacity that had once been used by the network-   limited connection might now be used by other traffic.   Current TCP implementations have a range of behaviors for starting up   after an idle period.  Some current TCP implementations slow-start   after an idle period longer than the RTO estimate, as suggested in   [RFC2581] and in the appendix of [VJ88], while other implementations   don't reduce their congestion window after an idle period.RFC 2581   [RFC2581] recommends the following: "a TCP SHOULD set cwnd to no more   than RW [the initial window] before beginning transmission if the TCP   has not sent data in an interval exceeding the retransmission   timeout."  A proposal for TCP's slow-start after idle has also been   discussed in [HTH98].  The issue of validation of congestion   information during idle periods has also been addressed in contexts   other than TCP and IP, for example in "Use-it or Lose-it" mechanisms   for ATM networks [J96,J95].Handley, et al.               Experimental                      [Page 2]

RFC 2861            TCP Congestion Window Validation           June 2000   To address the revalidation of the congestion window after a   application-limited period, we propose a simple modification to TCP's   congestion control algorithms to decay the congestion window cwnd   after the transition from a sufficiently-long application-limited   period (i.e., at least one roundtrip time) to a network-limited   period.  In particular, we propose that after an idle period, the TCP   sender should reduce its congestion window by half for every RTT that   the flow has remained idle.   When the congestion window is reduced, the slow-start threshold   ssthresh remains as "memory" of the recent congestion window.   Specifically, ssthresh is never decreased when cwnd is reduced after   an application-limited period; before cwnd is reduced, ssthresh is   set to the maximum of its current value, and half-way between the old   and the new values of cwnd.  This use of ssthresh allows a TCP sender   increasing its sending rate after an application-limited period to   quickly slow-start to recover most of the previous value of the   congestion window.  To be more precise, if ssthresh is less than 3/4   cwnd when the congestion window is reduced after an application-   limited period, then ssthresh is increased to 3/4 cwnd before the   reduction of the congestion window.   An invalid congestion window also results when the congestion window   is increased (i.e., in TCP's slow-start or congestion avoidance   phases) during application-limited periods, when the previous value   of the congestion window might never have been fully utilized.  As   far as we know, all current TCP implementations increase the   congestion window when an acknowledgement arrives, if allowed by the   receiver's advertised window and the slow-start or congestion   avoidance window increase algorithm, without checking to see if the   previous value of the congestion window has in fact been used.  This   document proposes that the window increase algorithm not be invoked   during application-limited periods [MSML99].  In particular, the TCP   sender should not increase the congestion window when the TCP sender   has been application-limited (and therefore has not fully used the   current congestion window).  This restriction prevents the congestion   window from growing arbitrarily large, in the absence of evidence   that the congestion window can be supported by the network.  From   [MSML99,Section 5.2]: "This restriction assures that [cwnd] only   grows as long as TCP actually succeeds in injecting enough data into   the network to test the path."   A somewhat-orthogonal problem associated with maintaining a large   congestion window after an application-limited period is that the   sender, with a sudden large amount of data to send after a quiescent   period, might immediately send a full congestion window of back-to-   back packets.  This problem of sending large bursts of packets back-   to-back can be effectively handled using rate-based pacing (RBP,Handley, et al.               Experimental                      [Page 3]

RFC 2861            TCP Congestion Window Validation           June 2000   [VH97]), or using a maximum burst size control [FF96].  We would   contend that, even with mechanisms for limiting the sending of back-   to-back packets or pacing packets out over the period of a roundtrip   time, an old congestion window that has not been fully used for some   time can not be trusted as an indication of the bandwidth currently   available for that flow.  We would contend that the mechanisms to   pace out packets allowed by the congestion window are largely   orthogonal to the algorithms used to determine the appropriate size   of the congestion window.3. Description   When a TCP sender has sufficient data available to fill the available   network capacity for that flow, cwnd and ssthresh get set to   appropriate values for the network conditions.  When a TCP sender   stops sending, the flow stops sampling the network conditions, and so   the value of the congestion window may become inaccurate.  We believe   the correct conservative behavior under these circumstances is to   decay the congestion window by half for every RTT that the flow   remains inactive.  The value of half is a very conservative figure   based on how quickly multiplicative decrease would have decayed the   window in the presence of loss.   Another possibility is that the sender may not stop sending, but may   become application-limited rather than network-limited, and offer   less data to the network than the congestion window allows to be   sent.  In this case the TCP flow is still sampling network   conditions, but is not offering sufficient traffic to be sure that   there is still sufficient capacity in the network for that flow to   send a full congestion window.  Under these circumstances we believe   the correct conservative behavior is for the sender to keep track of   the maximum amount of the congestion window used during each RTT, and   to decay the congestion window each RTT to midway between the current   cwnd value and the maximum value used.   Before the congestion window is reduced, ssthresh is set to the   maximum of its current value and 3/4 cwnd.  If the sender then has   more data to send than the decayed cwnd allows, the TCP will slow-   start (perform exponential increase) at least half-way back up to the   old value of cwnd.   The justification for this value of "3/4 cwnd" is that 3/4 cwnd is a   conservative estimate of the recent average value of the congestion   window, and the TCP should safely be able to slow-start at least up   to this point.  For a TCP in steady-state that has been reducing its   congestion window each time the congestion window reached some   maximum value `maxwin', the average congestion window has been 3/4   maxwin.  On average, when the connection becomes application-limited,Handley, et al.               Experimental                      [Page 4]

RFC 2861            TCP Congestion Window Validation           June 2000   cwnd will be 3/4 maxwin, and in this case cwnd itself represents the   average value of the congestion window.  However, if the connection   happens to become application-limited when cwnd equals maxwin, then   the average value of the congestion window is given by 3/4 cwnd.   An alternate possibility would be to set ssthresh to the maximum of   the current value of ssthresh, and the old value of cwnd, allowing   TCP to slow-start all of the way back up to the old value of cwnd.   Further experimentation can be used to evaluate these two options for   setting ssthresh.   For the separate issue of the increase of the congestion window in   response to an acknowledgement, we believe the correct behavior is   for the sender to increase the congestion window only if the window   was full when the acknowledgment arrived.   We term this set of modifications to TCP Congestion Window Validation   (CWV) because they are related to ensuring the congestion window is   always a valid reflection of the current network state as probed by   the connection.3.1. The basic algorithm for reducing the congestion window   A key issue in the CWV algorithm is to determine how to apply the   guideline of reducing the congestion window once for every roundtrip   time that the flow is application-limited.  We use TCP's   retransmission timer (RTO) as a reasonable upper bound on the   roundtrip time, and reduce the congestion window roughly once per   RTO.   This basic algorithm could be implemented in TCP as follows: When TCP   sends a new packet it checks to see if more than RTO seconds have   elapsed since the previous packet was sent.  If RTO has elapsed,   ssthresh is set to the maximum of 3/4 cwnd and the current value of   ssthresh, and then the congestion window is halved for every RTO that   elapsed since the previous packet was sent.  In addition, T_prev is   set to the current time, and W_used is reset to zero.  T_prev will be   used to determine the elapsed time since the sender last was network-   limited or had reduced cwnd after an idle period.  When the sender is   application-limited, W_used holds the maximum congestion window   actually used since the sender was last network-limited.   The mechanism for determining the number of RTOs in the most recent   idle period could also be implemented by using a timer that expires   every RTO after the last packet was sent instead of a check per   packet - efficiency constraints on different operating systems may   dictate which is more efficient to implement.Handley, et al.               Experimental                      [Page 5]

RFC 2861            TCP Congestion Window Validation           June 2000   After TCP sends a packet, it also checks to see if that packet filled   the congestion window.  If so, the sender is network-limited, and   sets the variable T_prev to the current TCP clock time, and the   variable W_used to zero.   When TCP sends a packet that does not fill the congestion window, and   the TCP send queue is empty, then the sender is application-limited.   The sender checks to see if the amount of unacknowledged data is   greater than W_used; if so, W_used is set to the amount of   unacknowledged data.  In addition TCP checks to see if the elapsed   time since T_prev is greater than RTO.  If so, then the TCP has not   just reduced its congestion window following an idle period.  The TCP   has been application-limited rather than network-limited for at least   an entire RTO interval, but for less than two RTO intervals.  In this   case, TCP sets ssthresh to the maximum of 3/4 cwnd and the current   value of ssthresh, and reduces its congestion window to   (cwnd+W_used)/2.  W_used is then set to zero, and T_prev is set to   the current time, so a further reduction will not take place until at   least another RTO period has elapsed.  Thus, during an application-   limited period the CWV algorithm reduces the congestion window once   per RTO.3.2.  Pseudo-code for reducing the congestion window   Initially:       T_last = tcpnow, T_prev = tcpnow, W_used = 0   After sending a data segment:       If tcpnow - T_last >= RTO           (The sender has been idle.)           ssthresh =  max(ssthresh, 3*cwnd/4)           For i=1  To (tcpnow - T_last)/RTO               win =  min(cwnd, receiver's declared max window)               cwnd =  max(win/2, MSS)           T_prev = tcpnow           W_used = 0       T_last = tcpnow       If window is full           T_prev = tcpnow           W_used = 0       Else           If no more data is available to send               W_used =  max(W_used, amount of unacknowledged data)               If tcpnow - T_prev >= RTO                   (The sender has been application-limited.)                   ssthresh =  max(ssthresh, 3*cwnd/4)Handley, et al.               Experimental                      [Page 6]

RFC 2861            TCP Congestion Window Validation           June 2000                   win =  min(cwnd, receiver's declared max window)                   cwnd = (win + W_used)/2                   T_prev = tcpnow                   W_used = 04. Simulations   The CWV proposal has been implemented as an option in the network   simulator NS [NS].  The simulations in the validation test suite for   CWV can be run with the command "./test-all-tcp" in the directory   "tcl/test".  The simulations show the use of CWV to reduce the   congestion window after a period when the TCP connection was   application-limited, and to limit the increase in the congestion   window when a transfer is application-limited.  As the simulations   illustrate, the use of ssthresh to maintain connection history is a   critical part of the Congestion Window Validation algorithm.  [HPF99]   discusses these simulations in more detail.5. Experiments   We have implemented the CWV mechanism in the TCP implementation in   FreeBSD 3.2.  [HPF99] discusses these experiments in more detail.   The first experiment examines the effects of the Congestion Window   Validation mechanisms for limiting cwnd increases during   application-limited periods.  The experiment used a real ssh   connection through a modem link emulated using Dummynet [Dummynet].   The link speed is 30Kb/s and the link has five packet buffers   available.  Today most modem banks have more buffering available than   this, but the more buffer-limited situation sometimes occurs with   older modems.  In the first half of the transfer, the user is typing   away over the connection.  About half way through the time, the user   lists a moderately large file, which causes a large burst of traffic   to be transmitted.   For the unmodified TCP, every returning ACK during the first part of   the transfer results in an increase in cwnd.  As a result, the large   burst of data arriving from the application to the transport layer is   sent as many back-to-back packets, most of which get lost and   subsequently retransmitted.   For the modified TCP with Congestion Window Validation, the   congestion window is not increased when the window is not full, and   has been decreased during application-limited periods closer to what   the user actually used.  The burst of traffic is now constrained by   the congestion window, resulting in a better-behaved flow withHandley, et al.               Experimental                      [Page 7]

RFC 2861            TCP Congestion Window Validation           June 2000   minimal loss.  The end result is that the transfer happens   approximately 30% faster than the transfer without CWV, due to   avoiding retransmission timeouts.   The second experiment uses a real ssh connection over a real dialup   ppp connection, where the modem bank has much more buffering.  For   the unmodified TCP, the initial burst from the large file does not   cause loss, but does cause the RTT to increase to approximately 5   seconds, where the connection becomes bounded by the receiver's   window.   For the modified TCP with Congestion Window Validation, the flow is   much better behaved, and produces no large burst of traffic.  In this   case the linear increase for cwnd results in a slow increase in the   RTT as the buffer slowly fills.   For the second experiment, both the modified and the unmodified TCP   finish delivering the data at precisely the same time.  This is   because the link has been fully utilized in both cases due to the   modem buffer being larger than the receiver window.  Clearly a modem   buffer of this size is undesirable due to its effect on the RTT of   competing flows, but it is necessary with current TCP implementations   that produce bursts similar to those shown in the top graph.6. Conclusions   This document has presented several TCP algorithms for Congestion   Window Validation, to be employed after an idle period or a period in   which the sender was application-limited, and before an increase of   the congestion window.  The goal of these algorithms is for TCP's   congestion window to reflect recent knowledge of the TCP connection   about the state of the network path, while at the same time keeping   some memory (i.e., in ssthresh) about the earlier state of the path.   We believe that these modifications will be of benefit to both the   network and to the TCP flows themselves, by preventing unnecessary   packet drops due to the TCP sender's failure to update its   information (or lack of information) about current network   conditions.  Future work will document and investigate the benefit   provided by these algorithms, using both simulations and experiments.   Additional future work will describe a more complex version of the   CWV algorithm for TCP implementations where the sender does not have   an accurate estimate of the TCP roundtrip time.Handley, et al.               Experimental                      [Page 8]

RFC 2861            TCP Congestion Window Validation           June 20007. References   [FF96]     Fall, K., and Floyd, S., Simulation-based Comparisons of              Tahoe, Reno, and SACK TCP, Computer Communication Review,              V. 26 N. 3, July 1996, pp. 5-21.  URL              "http://www.aciri.org/floyd/papers.html".   [HPF99]    Mark Handley, Jitendra Padhye, Sally Floyd, TCP Congestion              Window Validation, UMass CMPSCI Technical Report 99-77,              September 1999.  URL "ftp://www-net.cs.umass.edu/pub/Handley99-tcpq-tr-99-77.ps.gz".   [HTH98]    Amy Hughes, Joe Touch, John Heidemann, "Issues in TCP              Slow-Start Restart After Idle", Work in Progress.   [J88]      Jacobson, V., Congestion Avoidance and Control, Originally              from Proceedings of SIGCOMM '88 (Palo Alto, CA, Aug.              1988), and revised in 1992.  URL "http://www-nrg.ee.lbl.gov/nrg-papers.html".   [JKBFL96]  Raj Jain, Shiv Kalyanaraman, Rohit Goyal, Sonia Fahmy, and              Fang Lu, Comments on "Use-it or Lose-it", ATM Forum              Document Number:  ATM Forum/96-0178, URL              "http://www.netlab.ohio-state.edu/~jain/atmf/af_rl5b2.htm".   [JKGFL95]  R. Jain, S. Kalyanaraman, R. Goyal, S. Fahmy, and F. Lu, A              Fix for Source End System Rule 5, AF-TM 95-1660, December              1995, URL "http://www.netlab.ohio-state.edu/~jain/atmf/af_rl52.htm".   [MSML99]   Matt Mathis, Jeff Semke, Jamshid Mahdavi, and Kevin Lahey,              The Rate-Halving Algorithm for TCP Congestion Control,              June 1999.  URL              "http://www.psc.edu/networking/ftp/papers/draft-ratehalving.txt".   [NS]       NS, the UCB/LBNL/VINT Network Simulator.  URL              "http://www-mash.cs.berkeley.edu/ns/".   [RFC2581]  Allman, M., Paxson, V. and W. Stevens, TCP Congestion              Control,RFC 2581, April 1999.   [VH97]     Vikram Visweswaraiah and John Heidemann. Improving Restart              of Idle TCP Connections, Technical Report 97-661,              University of Southern California, November, 1997.Handley, et al.               Experimental                      [Page 9]

RFC 2861            TCP Congestion Window Validation           June 2000   [Dummynet] Luigi Rizzo, "Dummynet and Forward Error Correction",              Freenix 98, June 1998, New Orleans.  URL              "http://info.iet.unipi.it/~luigi/ip_dummynet/".8. Security Considerations   General security considerations concerning TCP congestion control are   discussed inRFC 2581.  This document describes a algorithm for one   aspect of those congestion control procedures, and so the   considerations described inRFC 2581 apply to this algorithm also.   There are no known additional security concerns for this specific   algorithm.9. Authors' Addresses   Mark Handley   AT&T Center for Internet Research at ICSI (ACIRI)   Phone: +1 510 666 2946   EMail: mjh@aciri.org   URL:http://www.aciri.org/mjh/   Jitendra Padhye   AT&T Center for Internet Research at ICSI (ACIRI)   Phone: +1 510 666 2887   EMail: padhye@aciri.org   URL:http://www-net.cs.umass.edu/~jitu/   Sally Floyd   AT&T Center for Internet Research at ICSI (ACIRI)   Phone: +1 510 666 2989   EMail: floyd@aciri.org   URL:http://www.aciri.org/floyd/Handley, et al.               Experimental                     [Page 10]

RFC 2861            TCP Congestion Window Validation           June 200010. Full Copyright Statement   Copyright (C) The Internet Society (2000).  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.Handley, et al.               Experimental                     [Page 11]