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Network Working Group                                          M. AllmanRequest for Comments: 5681                                     V. PaxsonObsoletes:2581                                                     ICSICategory: Standards Track                                     E. Blanton                                                       Purdue University                                                          September 2009TCP Congestion ControlAbstract   This document defines TCP's four intertwined congestion control   algorithms: slow start, congestion avoidance, fast retransmit, and   fast recovery.  In addition, the document specifies how TCP should   begin transmission after a relatively long idle period, as well as   discussing various acknowledgment generation methods.  This document   obsoletesRFC 2581.Status 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) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it mayAllman, et al.              Standards Track                     [Page 1]

RFC 5681                 TCP Congestion Control           September 2009   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table Of Contents1. Introduction ....................................................22. Definitions .....................................................33. Congestion Control Algorithms ...................................43.1. Slow Start and Congestion Avoidance ........................43.2. Fast Retransmit/Fast Recovery ..............................84. Additional Considerations ......................................104.1. Restarting Idle Connections ...............................104.2. Generating Acknowledgments ................................114.3. Loss Recovery Mechanisms ..................................125. Security Considerations ........................................136. Changes betweenRFC 2001 andRFC 2581 ..........................137. Changes Relative toRFC 2581 ...................................148. Acknowledgments ................................................159. References .....................................................159.1. Normative References ......................................159.2. Informative References ....................................161.  Introduction   This document specifies four TCP [RFC793] congestion control   algorithms: slow start, congestion avoidance, fast retransmit and   fast recovery.  These algorithms were devised in [Jac88] and [Jac90].   Their use with TCP is standardized in [RFC1122].  Additional early   work in additive-increase, multiplicative-decrease congestion control   is given in [CJ89].   Note that [Ste94] provides examples of these algorithms in action and   [WS95] provides an explanation of the source code for the BSD   implementation of these algorithms.   In addition to specifying these congestion control algorithms, this   document specifies what TCP connections should do after a relatively   long idle period, as well as specifying and clarifying some of the   issues pertaining to TCP ACK generation.   This document obsoletes [RFC2581], which in turn obsoleted [RFC2001].   This document is organized as follows.Section 2 provides various   definitions that will be used throughout the document.Section 3   provides a specification of the congestion control algorithms.Section 4 outlines concerns related to the congestion control   algorithms and finally,section 5 outlines security considerations.Allman, et al.              Standards Track                     [Page 2]

RFC 5681                 TCP Congestion Control           September 2009   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 in [RFC2119].2.  Definitions   This section provides the definition of several terms that will be   used throughout the remainder of this document.   SEGMENT: A segment is ANY TCP/IP data or acknowledgment packet (or      both).   SENDER MAXIMUM SEGMENT SIZE (SMSS): The SMSS is the size of the      largest segment that the sender can transmit.  This value can be      based on the maximum transmission unit of the network, the path      MTU discovery [RFC1191,RFC4821] algorithm, RMSS (see next item),      or other factors.  The size does not include the TCP/IP headers      and options.   RECEIVER MAXIMUM SEGMENT SIZE (RMSS): The RMSS is the size of the      largest segment the receiver is willing to accept.  This is the      value specified in the MSS option sent by the receiver during      connection startup.  Or, if the MSS option is not used, it is 536      bytes [RFC1122].  The size does not include the TCP/IP headers and      options.   FULL-SIZED SEGMENT: A segment that contains the maximum number of      data bytes permitted (i.e., a segment containing SMSS bytes of      data).   RECEIVER WINDOW (rwnd): The most recently advertised receiver window.   CONGESTION WINDOW (cwnd): A TCP state variable that limits the amount      of data a TCP can send.  At any given time, a TCP MUST NOT send      data with a sequence number higher than the sum of the highest      acknowledged sequence number and the minimum of cwnd and rwnd.   INITIAL WINDOW (IW): The initial window is the size of the sender's      congestion window after the three-way handshake is completed.   LOSS WINDOW (LW): The loss window is the size of the congestion      window after a TCP sender detects loss using its retransmission      timer.   RESTART WINDOW (RW): The restart window is the size of the congestion      window after a TCP restarts transmission after an idle period (if      the slow start algorithm is used; seesection 4.1 for more      discussion).Allman, et al.              Standards Track                     [Page 3]

RFC 5681                 TCP Congestion Control           September 2009   FLIGHT SIZE: The amount of data that has been sent but not yet      cumulatively acknowledged.   DUPLICATE ACKNOWLEDGMENT: An acknowledgment is considered a      "duplicate" in the following algorithms when (a) the receiver of      the ACK has outstanding data, (b) the incoming acknowledgment      carries no data, (c) the SYN and FIN bits are both off, (d) the      acknowledgment number is equal to the greatest acknowledgment      received on the given connection (TCP.UNA from [RFC793]) and (e)      the advertised window in the incoming acknowledgment equals the      advertised window in the last incoming acknowledgment.      Alternatively, a TCP that utilizes selective acknowledgments      (SACKs) [RFC2018,RFC2883] can leverage the SACK information to      determine when an incoming ACK is a "duplicate" (e.g., if the ACK      contains previously unknown SACK information).3.  Congestion Control Algorithms   This section defines the four congestion control algorithms: slow   start, congestion avoidance, fast retransmit, and fast recovery,   developed in [Jac88] and [Jac90].  In some situations, it may be   beneficial for a TCP sender to be more conservative than the   algorithms allow; however, a TCP MUST NOT be more aggressive than the   following algorithms allow (that is, MUST NOT send data when the   value of cwnd computed by the following algorithms would not allow   the data to be sent).   Also, note that the algorithms specified in this document work in   terms of using loss as the signal of congestion.  Explicit Congestion   Notification (ECN) could also be used as specified in [RFC3168].3.1.  Slow Start and Congestion Avoidance   The slow start and congestion avoidance algorithms MUST be used by a   TCP sender to control the amount of outstanding data being injected   into the network.  To implement these algorithms, two variables are   added to the TCP per-connection state.  The congestion window (cwnd)   is a sender-side limit on the amount of data the sender can transmit   into the network before receiving an acknowledgment (ACK), while the   receiver's advertised window (rwnd) is a receiver-side limit on the   amount of outstanding data.  The minimum of cwnd and rwnd governs   data transmission.   Another state variable, the slow start threshold (ssthresh), is used   to determine whether the slow start or congestion avoidance algorithm   is used to control data transmission, as discussed below.Allman, et al.              Standards Track                     [Page 4]

RFC 5681                 TCP Congestion Control           September 2009   Beginning transmission into a network with unknown conditions   requires TCP to slowly probe the network to determine the available   capacity, in order to avoid congesting the network with an   inappropriately large burst of data.  The slow start algorithm is   used for this purpose at the beginning of a transfer, or after   repairing loss detected by the retransmission timer.  Slow start   additionally serves to start the "ACK clock" used by the TCP sender   to release data into the network in the slow start, congestion   avoidance, and loss recovery algorithms.   IW, the initial value of cwnd, MUST be set using the following   guidelines as an upper bound.   If SMSS > 2190 bytes:       IW = 2 * SMSS bytes and MUST NOT be more than 2 segments   If (SMSS > 1095 bytes) and (SMSS <= 2190 bytes):       IW = 3 * SMSS bytes and MUST NOT be more than 3 segments   if SMSS <= 1095 bytes:       IW = 4 * SMSS bytes and MUST NOT be more than 4 segments   As specified in [RFC3390], the SYN/ACK and the acknowledgment of the   SYN/ACK MUST NOT increase the size of the congestion window.   Further, 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 at most SMSS bytes.   A detailed rationale and discussion of the IW setting is provided in   [RFC3390].   When initial congestion windows of more than one segment 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.   The initial value of ssthresh SHOULD be set arbitrarily high (e.g.,   to the size of the largest possible advertised window), but ssthresh   MUST be reduced in response to congestion.  Setting ssthresh as high   as possible allows the network conditions, rather than some arbitrary   host limit, to dictate the sending rate.  In cases where the end   systems have a solid understanding of the network path, more   carefully setting the initial ssthresh value may have merit (e.g.,   such that the end host does not create congestion along the path).Allman, et al.              Standards Track                     [Page 5]

RFC 5681                 TCP Congestion Control           September 2009   The slow start algorithm is used when cwnd < ssthresh, while the   congestion avoidance algorithm is used when cwnd > ssthresh.  When   cwnd and ssthresh are equal, the sender may use either slow start or   congestion avoidance.   During slow start, a TCP increments cwnd by at most SMSS bytes for   each ACK received that cumulatively acknowledges new data.  Slow   start ends when cwnd exceeds ssthresh (or, optionally, when it   reaches it, as noted above) or when congestion is observed.  While   traditionally TCP implementations have increased cwnd by precisely   SMSS bytes upon receipt of an ACK covering new data, we RECOMMEND   that TCP implementations increase cwnd, per:      cwnd += min (N, SMSS)                      (2)   where N is the number of previously unacknowledged bytes acknowledged   in the incoming ACK.  This adjustment is part of Appropriate Byte   Counting [RFC3465] and provides robustness against misbehaving   receivers that may attempt to induce a sender to artificially inflate   cwnd using a mechanism known as "ACK Division" [SCWA99].  ACK   Division consists of a receiver sending multiple ACKs for a single   TCP data segment, each acknowledging only a portion of its data.  A   TCP that increments cwnd by SMSS for each such ACK will   inappropriately inflate the amount of data injected into the network.   During congestion avoidance, cwnd is incremented by roughly 1 full-   sized segment per round-trip time (RTT).  Congestion avoidance   continues until congestion is detected.  The basic guidelines for   incrementing cwnd during congestion avoidance are:      * MAY increment cwnd by SMSS bytes      * SHOULD increment cwnd per equation (2) once per RTT      * MUST NOT increment cwnd by more than SMSS bytes   We note that [RFC3465] allows for cwnd increases of more than SMSS   bytes for incoming acknowledgments during slow start on an   experimental basis; however, such behavior is not allowed as part of   the standard.   The RECOMMENDED way to increase cwnd during congestion avoidance is   to count the number of bytes that have been acknowledged by ACKs for   new data.  (A drawback of this implementation is that it requires   maintaining an additional state variable.)  When the number of bytes   acknowledged reaches cwnd, then cwnd can be incremented by up to SMSS   bytes.  Note that during congestion avoidance, cwnd MUST NOT beAllman, et al.              Standards Track                     [Page 6]

RFC 5681                 TCP Congestion Control           September 2009   increased by more than SMSS bytes per RTT.  This method both allows   TCPs to increase cwnd by one segment per RTT in the face of delayed   ACKs and provides robustness against ACK Division attacks.   Another common formula that a TCP MAY use to update cwnd during   congestion avoidance is given in equation (3):      cwnd += SMSS*SMSS/cwnd                     (3)   This adjustment is executed on every incoming ACK that acknowledges   new data.  Equation (3) provides an acceptable approximation to the   underlying principle of increasing cwnd by 1 full-sized segment per   RTT.  (Note that for a connection in which the receiver is   acknowledging every-other packet, (3) is less aggressive than allowed   -- roughly increasing cwnd every second RTT.)   Implementation Note: Since integer arithmetic is usually used in TCP   implementations, the formula given in equation (3) can fail to   increase cwnd when the congestion window is larger than SMSS*SMSS.   If the above formula yields 0, the result SHOULD be rounded up to 1   byte.   Implementation Note: Older implementations have an additional   additive constant on the right-hand side of equation (3).  This is   incorrect and can actually lead to diminished performance [RFC2525].   Implementation Note: Some implementations maintain cwnd in units of   bytes, while others in units of full-sized segments.  The latter will   find equation (3) difficult to use, and may prefer to use the   counting approach discussed in the previous paragraph.   When a TCP sender detects segment loss using the retransmission timer   and the given segment has not yet been resent by way of the   retransmission timer, the value of ssthresh MUST be set to no more   than the value given in equation (4):      ssthresh = max (FlightSize / 2, 2*SMSS)            (4)   where, as discussed above, FlightSize is the amount of outstanding   data in the network.   On the other hand, when a TCP sender detects segment loss using the   retransmission timer and the given segment has already been   retransmitted by way of the retransmission timer at least once, the   value of ssthresh is held constant.Allman, et al.              Standards Track                     [Page 7]

RFC 5681                 TCP Congestion Control           September 2009   Implementation Note: An easy mistake to make is to simply use cwnd,   rather than FlightSize, which in some implementations may   incidentally increase well beyond rwnd.   Furthermore, upon a timeout (as specified in [RFC2988]) cwnd MUST be   set to no more than the loss window, LW, which equals 1 full-sized   segment (regardless of the value of IW).  Therefore, after   retransmitting the dropped segment the TCP sender uses the slow start   algorithm to increase the window from 1 full-sized segment to the new   value of ssthresh, at which point congestion avoidance again takes   over.   As shown in [FF96] and [RFC3782], slow-start-based loss recovery   after a timeout can cause spurious retransmissions that trigger   duplicate acknowledgments.  The reaction to the arrival of these   duplicate ACKs in TCP implementations varies widely.  This document   does not specify how to treat such acknowledgments, but does note   this as an area that may benefit from additional attention,   experimentation and specification.3.2.  Fast Retransmit/Fast Recovery   A TCP receiver SHOULD send an immediate duplicate ACK when an out-   of-order segment arrives.  The purpose of this ACK is to inform the   sender that a segment was received out-of-order and which sequence   number is expected.  From the sender's perspective, duplicate ACKs   can be caused by a number of network problems.  First, they can be   caused by dropped segments.  In this case, all segments after the   dropped segment will trigger duplicate ACKs until the loss is   repaired.  Second, duplicate ACKs can be caused by the re-ordering of   data segments by the network (not a rare event along some network   paths [Pax97]).  Finally, duplicate ACKs can be caused by replication   of ACK or data segments by the network.  In addition, a TCP receiver   SHOULD send an immediate ACK when the incoming segment fills in all   or part of a gap in the sequence space.  This will generate more   timely information for a sender recovering from a loss through a   retransmission timeout, a fast retransmit, or an advanced loss   recovery algorithm, as outlined insection 4.3.   The TCP sender SHOULD use the "fast retransmit" algorithm to detect   and repair loss, based on incoming duplicate ACKs.  The fast   retransmit algorithm uses the arrival of 3 duplicate ACKs (as defined   insection 2, without any intervening ACKs which move SND.UNA) as an   indication that a segment has been lost.  After receiving 3 duplicate   ACKs, TCP performs a retransmission of what appears to be the missing   segment, without waiting for the retransmission timer to expire.Allman, et al.              Standards Track                     [Page 8]

RFC 5681                 TCP Congestion Control           September 2009   After the fast retransmit algorithm sends what appears to be the   missing segment, the "fast recovery" algorithm governs the   transmission of new data until a non-duplicate ACK arrives.  The   reason for not performing slow start is that the receipt of the   duplicate ACKs not only indicates that a segment has been lost, but   also that segments are most likely leaving the network (although a   massive segment duplication by the network can invalidate this   conclusion).  In other words, since the receiver can only generate a   duplicate ACK when a segment has arrived, that segment has left the   network and is in the receiver's buffer, so we know it is no longer   consuming network resources.  Furthermore, since the ACK "clock"   [Jac88] is preserved, the TCP sender can continue to transmit new   segments (although transmission must continue using a reduced cwnd,   since loss is an indication of congestion).   The fast retransmit and fast recovery algorithms are implemented   together as follows.   1.  On the first and second duplicate ACKs received at a sender, a       TCP SHOULD send a segment of previously unsent data per [RFC3042]       provided that the receiver's advertised window allows, the total       FlightSize would remain less than or equal to cwnd plus 2*SMSS,       and that new data is available for transmission.  Further, the       TCP sender MUST NOT change cwnd to reflect these two segments       [RFC3042].  Note that a sender using SACK [RFC2018] MUST NOT send       new data unless the incoming duplicate acknowledgment contains       new SACK information.   2.  When the third duplicate ACK is received, a TCP MUST set ssthresh       to no more than the value given in equation (4).  When [RFC3042]       is in use, additional data sent in limited transmit MUST NOT be       included in this calculation.   3.  The lost segment starting at SND.UNA MUST be retransmitted and       cwnd set to ssthresh plus 3*SMSS.  This artificially "inflates"       the congestion window by the number of segments (three) that have       left the network and which the receiver has buffered.   4.  For each additional duplicate ACK received (after the third),       cwnd MUST be incremented by SMSS.  This artificially inflates the       congestion window in order to reflect the additional segment that       has left the network.       Note: [SCWA99] discusses a receiver-based attack whereby many       bogus duplicate ACKs are sent to the data sender in order to       artificially inflate cwnd and cause a higher than appropriateAllman, et al.              Standards Track                     [Page 9]

RFC 5681                 TCP Congestion Control           September 2009       sending rate to be used.  A TCP MAY therefore limit the number of       times cwnd is artificially inflated during loss recovery to the       number of outstanding segments (or, an approximation thereof).       Note: When an advanced loss recovery mechanism (such as outlined       insection 4.3) is not in use, this increase in FlightSize can       cause equation (4) to slightly inflate cwnd and ssthresh, as some       of the segments between SND.UNA and SND.NXT are assumed to have       left the network but are still reflected in FlightSize.   5.  When previously unsent data is available and the new value of       cwnd and the receiver's advertised window allow, a TCP SHOULD       send 1*SMSS bytes of previously unsent data.   6.  When the next ACK arrives that acknowledges previously       unacknowledged data, a TCP MUST set cwnd to ssthresh (the value       set in step 2).  This is termed "deflating" the window.       This ACK should be the acknowledgment elicited by the       retransmission from step 3, one RTT after the retransmission       (though it may arrive sooner in the presence of significant out-       of-order delivery of data segments at the receiver).       Additionally, this ACK should acknowledge all the intermediate       segments sent between the lost segment and the receipt of the       third duplicate ACK, if none of these were lost.   Note: This algorithm is known to generally not recover efficiently   from multiple losses in a single flight of packets [FF96].Section4.3 below addresses such cases.4.  Additional Considerations4.1.  Restarting Idle Connections   A known problem with the TCP congestion control algorithms described   above is that they allow a potentially inappropriate burst of traffic   to be transmitted after TCP has been idle for a relatively long   period of time.  After an idle period, TCP cannot use the ACK clock   to strobe new segments into the network, as all the ACKs have drained   from the network.  Therefore, as specified above, TCP can potentially   send a cwnd-size line-rate burst into the network after an idle   period.  In addition, changing network conditions may have rendered   TCP's notion of the available end-to-end network capacity between two   endpoints, as estimated by cwnd, inaccurate during the course of a   long idle period.Allman, et al.              Standards Track                    [Page 10]

RFC 5681                 TCP Congestion Control           September 2009   [Jac88] recommends that a TCP use slow start to restart transmission   after a relatively long idle period.  Slow start serves to restart   the ACK clock, just as it does at the beginning of a transfer.  This   mechanism has been widely deployed in the following manner.  When TCP   has not received a segment for more than one retransmission timeout,   cwnd is reduced to the value of the restart window (RW) before   transmission begins.   For the purposes of this standard, we define RW = min(IW,cwnd).   Using the last time a segment was received to determine whether or   not to decrease cwnd can fail to deflate cwnd in the common case of   persistent HTTP connections [HTH98].  In this case, a Web server   receives a request before transmitting data to the Web client.  The   reception of the request makes the test for an idle connection fail,   and allows the TCP to begin transmission with a possibly   inappropriately large cwnd.   Therefore, a TCP SHOULD set cwnd to no more than RW before beginning   transmission if the TCP has not sent data in an interval exceeding   the retransmission timeout.4.2.  Generating Acknowledgments   The delayed ACK algorithm specified in [RFC1122] SHOULD be used by a   TCP receiver.  When using delayed ACKs, a TCP receiver MUST NOT   excessively delay acknowledgments.  Specifically, an ACK SHOULD be   generated for at least every second full-sized segment, and MUST be   generated within 500 ms of the arrival of the first unacknowledged   packet.   The requirement that an ACK "SHOULD" be generated for at least every   second full-sized segment is listed in [RFC1122] in one place as a   SHOULD and another as a MUST.  Here we unambiguously state it is a   SHOULD.  We also emphasize that this is a SHOULD, meaning that an   implementor should indeed only deviate from this requirement after   careful consideration of the implications.  See the discussion of   "Stretch ACK violation" in [RFC2525] and the references therein for a   discussion of the possible performance problems with generating ACKs   less frequently than every second full-sized segment.   In some cases, the sender and receiver may not agree on what   constitutes a full-sized segment.  An implementation is deemed to   comply with this requirement if it sends at least one acknowledgment   every time it receives 2*RMSS bytes of new data from the sender,   where RMSS is the Maximum Segment Size specified by the receiver to   the sender (or the default value of 536 bytes, per [RFC1122], if the   receiver does not specify an MSS option during connectionAllman, et al.              Standards Track                    [Page 11]

RFC 5681                 TCP Congestion Control           September 2009   establishment).  The sender may be forced to use a segment size less   than RMSS due to the maximum transmission unit (MTU), the path MTU   discovery algorithm or other factors.  For instance, consider the   case when the receiver announces an RMSS of X bytes but the sender   ends up using a segment size of Y bytes (Y < X) due to path MTU   discovery (or the sender's MTU size).  The receiver will generate   stretch ACKs if it waits for 2*X bytes to arrive before an ACK is   sent.  Clearly this will take more than 2 segments of size Y bytes.   Therefore, while a specific algorithm is not defined, it is desirable   for receivers to attempt to prevent this situation, for example, by   acknowledging at least every second segment, regardless of size.   Finally, we repeat that an ACK MUST NOT be delayed for more than 500   ms waiting on a second full-sized segment to arrive.   Out-of-order data segments SHOULD be acknowledged immediately, in   order to accelerate loss recovery.  To trigger the fast retransmit   algorithm, the receiver SHOULD send an immediate duplicate ACK when   it receives a data segment above a gap in the sequence space.  To   provide feedback to senders recovering from losses, the receiver   SHOULD send an immediate ACK when it receives a data segment that   fills in all or part of a gap in the sequence space.   A TCP receiver MUST NOT generate more than one ACK for every incoming   segment, other than to update the offered window as the receiving   application consumes new data (see [RFC813] and page 42 of [RFC793]).4.3.  Loss Recovery Mechanisms   A number of loss recovery algorithms that augment fast retransmit and   fast recovery have been suggested by TCP researchers and specified in   the RFC series.  While some of these algorithms are based on the TCP   selective acknowledgment (SACK) option [RFC2018], such as [FF96],   [MM96a], [MM96b], and [RFC3517], others do not require SACKs, such as   [Hoe96], [FF96], and [RFC3782].  The non-SACK algorithms use "partial   acknowledgments" (ACKs that cover previously unacknowledged data, but   not all the data outstanding when loss was detected) to trigger   retransmissions.  While this document does not standardize any of the   specific algorithms that may improve fast retransmit/fast recovery,   these enhanced algorithms are implicitly allowed, as long as they   follow the general principles of the basic four algorithms outlined   above.   That is, when the first loss in a window of data is detected,   ssthresh MUST be set to no more than the value given by equation (4).   Second, until all lost segments in the window of data in question are   repaired, the number of segments transmitted in each RTT MUST be no   more than half the number of outstanding segments when the loss was   detected.  Finally, after all loss in the given window of segmentsAllman, et al.              Standards Track                    [Page 12]

RFC 5681                 TCP Congestion Control           September 2009   has been successfully retransmitted, cwnd MUST be set to no more than   ssthresh and congestion avoidance MUST be used to further increase   cwnd.  Loss in two successive windows of data, or the loss of a   retransmission, should be taken as two indications of congestion and,   therefore, cwnd (and ssthresh) MUST be lowered twice in this case.   We RECOMMEND that TCP implementors employ some form of advanced loss   recovery that can cope with multiple losses in a window of data.  The   algorithms detailed in [RFC3782] and [RFC3517] conform to the general   principles outlined above.  We note that while these are not the only   two algorithms that conform to the above general principles these two   algorithms have been vetted by the community and are currently on the   Standards Track.5.  Security Considerations   This document requires a TCP to diminish its sending rate in the   presence of retransmission timeouts and the arrival of duplicate   acknowledgments.  An attacker can therefore impair the performance of   a TCP connection by either causing data packets or their   acknowledgments to be lost, or by forging excessive duplicate   acknowledgments.   In response to the ACK division attack outlined in [SCWA99], this   document RECOMMENDS increasing the congestion window based on the   number of bytes newly acknowledged in each arriving ACK rather than   by a particular constant on each arriving ACK (as outlined insection3.1).   The Internet, to a considerable degree, relies on the correct   implementation of these algorithms in order to preserve network   stability and avoid congestion collapse.  An attacker could cause TCP   endpoints to respond more aggressively in the face of congestion by   forging excessive duplicate acknowledgments or excessive   acknowledgments for new data.  Conceivably, such an attack could   drive a portion of the network into congestion collapse.6.  Changes betweenRFC 2001 andRFC 2581   [RFC2001] was extensively rewritten editorially and it is not   feasible to itemize the list of changes between [RFC2001] and   [RFC2581].  The intention of [RFC2581] was to not change any of the   recommendations given in [RFC2001], but to further clarify cases that   were not discussed in detail in [RFC2001].  Specifically, [RFC2581]   suggested what TCP connections should do after a relatively long idle   period, as well as specified and clarified some of the issuesAllman, et al.              Standards Track                    [Page 13]

RFC 5681                 TCP Congestion Control           September 2009   pertaining to TCP ACK generation.  Finally, the allowable upper bound   for the initial congestion window was raised from one to two   segments.7.  Changes Relative toRFC 2581   A specific definition for "duplicate acknowledgment" has been added,   based on the definition used by BSD TCP.   The document now notes that what to do with duplicate ACKs after the   retransmission timer has fired is future work and explicitly   unspecified in this document.   The initial window requirements were changed to allow Larger Initial   Windows as standardized in [RFC3390].  Additionally, the steps to   take when an initial window is discovered to be too large due to Path   MTU Discovery [RFC1191] are detailed.   The recommended initial value for ssthresh has been changed to say   that it SHOULD be arbitrarily high, where it was previously MAY.   This is to provide additional guidance to implementors on the matter.   During slow start, the usage of Appropriate Byte Counting [RFC3465]   with L=1*SMSS is explicitly recommended.  The method of increasing   cwnd given in [RFC2581] is still explicitly allowed.  Byte counting   during congestion avoidance is also recommended, while the method   from [RFC2581] and other safe methods are still allowed.   The treatment of ssthresh on retransmission timeout was clarified.   In particular, ssthresh must be set to half the FlightSize on the   first retransmission of a given segment and then is held constant on   subsequent retransmissions of the same segment.   The description of fast retransmit and fast recovery has been   clarified, and the use of Limited Transmit [RFC3042] is now   recommended.   TCPs now MAY limit the number of duplicate ACKs that artificially   inflate cwnd during loss recovery to the number of segments   outstanding to avoid the duplicate ACK spoofing attack described in   [SCWA99].   The restart window has been changed to min(IW,cwnd) from IW.  This   behavior was described as "experimental" in [RFC2581].   It is now recommended that TCP implementors implement an advanced   loss recovery algorithm conforming to the principles outlined in this   document.Allman, et al.              Standards Track                    [Page 14]

RFC 5681                 TCP Congestion Control           September 2009   The security considerations have been updated to discuss ACK division   and recommend byte counting as a counter to this attack.8.  Acknowledgments   The core algorithms we describe were developed by Van Jacobson   [Jac88,Jac90].  In addition, Limited Transmit [RFC3042] was   developed in conjunction with Hari Balakrishnan and Sally Floyd.  The   initial congestion window size specified in this document is a result   of work with Sally Floyd and Craig Partridge [RFC2414,RFC3390].   W. Richard ("Rich") Stevens wrote the first version of this document   [RFC2001] and co-authored the second version [RFC2581].  This present   version much benefits from his clarity and thoughtfulness of   description, and we are grateful for Rich's contributions in   elucidating TCP congestion control, as well as in more broadly   helping us understand numerous issues relating to networking.   We wish to emphasize that the shortcomings and mistakes of this   document are solely the responsibility of the current authors.   Some of the text from this document is taken from "TCP/IP   Illustrated, Volume 1: The Protocols" by W. Richard Stevens   (Addison-Wesley, 1994) and "TCP/IP Illustrated, Volume 2: The   Implementation" by Gary R. Wright and W. Richard Stevens (Addison-   Wesley, 1995).  This material is used with the permission of   Addison-Wesley.   Anil Agarwal, Steve Arden, Neal Cardwell, Noritoshi Demizu, Gorry   Fairhurst, Kevin Fall, John Heffner, Alfred Hoenes, Sally Floyd,   Reiner Ludwig, Matt Mathis, Craig Partridge, and Joe Touch   contributed a number of helpful suggestions.9.  References9.1.  Normative References   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7,RFC793, September 1981.   [RFC1122] Braden, R., Ed., "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.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.Allman, et al.              Standards Track                    [Page 15]

RFC 5681                 TCP Congestion Control           September 20099.2.  Informative References   [CJ89]    Chiu, D. and R. Jain, "Analysis of the Increase/Decrease             Algorithms for Congestion Avoidance in Computer Networks",             Journal of Computer Networks and ISDN Systems, vol. 17, no.             1, pp. 1-14, June 1989.   [FF96]    Fall, K. and S. Floyd, "Simulation-based Comparisons of             Tahoe, Reno and SACK TCP", Computer Communication Review,             July 1996,ftp://ftp.ee.lbl.gov/papers/sacks.ps.Z.   [Hoe96]   Hoe, J., "Improving the Start-up Behavior of a Congestion             Control Scheme for TCP", In ACM SIGCOMM, August 1996.   [HTH98]   Hughes, A., Touch, J., and J. Heidemann, "Issues in TCP             Slow-Start Restart After Idle", Work in Progress, March             1998.   [Jac88]   Jacobson, V., "Congestion Avoidance and Control", Computer             Communication Review, vol. 18, no. 4, pp. 314-329, Aug.             1988.ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.   [Jac90]   Jacobson, V., "Modified TCP Congestion Avoidance             Algorithm", end2end-interest mailing list, April 30, 1990.ftp://ftp.isi.edu/end2end/end2end-interest-1990.mail.   [MM96a]   Mathis, M. and J. Mahdavi, "Forward Acknowledgment:             Refining TCP Congestion Control", Proceedings of             SIGCOMM'96, August, 1996, Stanford, CA.  Available fromhttp://www.psc.edu/networking/papers/papers.html   [MM96b]   Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding             Parameters", Technical report.  Available fromhttp://www.psc.edu/networking/papers/FACKnotes/current.   [Pax97]   Paxson, V., "End-to-End Internet Packet Dynamics",             Proceedings of SIGCOMM '97, Cannes, France, Sep. 1997.   [RFC813]  Clark, D., "Window and Acknowledgement Strategy in TCP",RFC 813, July 1982.   [RFC2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast             Retransmit, and Fast Recovery Algorithms",RFC 2001,             January 1997.   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP             Selective Acknowledgment Options",RFC 2018, October 1996.Allman, et al.              Standards Track                    [Page 16]

RFC 5681                 TCP Congestion Control           September 2009   [RFC2414] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's             Initial Window",RFC 2414, September 1998.   [RFC2525] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J.,             Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known TCP             Implementation Problems",RFC 2525, March 1999.   [RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion             Control",RFC 2581, April 1999.   [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An             Extension to the Selective Acknowledgement (SACK) Option             for TCP",RFC 2883, July 2000.   [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., Floyd, S., and D. Black, "The Addition of             Explicit Congestion Notification (ECN) to IP",RFC 3168,             September 2001.   [RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's             Initial Window",RFC 3390, October 2002.   [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte             Counting (ABC)",RFC 3465, February 2003.   [RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A             Conservative Selective Acknowledgment (SACK)-based Loss             Recovery Algorithm for TCP",RFC 3517, April 2003.   [RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno             Modification to TCP's Fast Recovery Algorithm",RFC 3782,             April 2004.   [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU             Discovery",RFC 4821, March 2007.   [SCWA99]  Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,             "TCP Congestion Control With a Misbehaving Receiver", ACM             Computer Communication Review, 29(5), October 1999.   [Ste94]   Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols",             Addison-Wesley, 1994.Allman, et al.              Standards Track                    [Page 17]

RFC 5681                 TCP Congestion Control           September 2009   [WS95]    Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:             The Implementation", Addison-Wesley, 1995.Authors' Addresses   Mark Allman   International Computer Science Institute (ICSI)   1947 Center Street   Suite 600   Berkeley, CA 94704-1198   Phone: +1 440 235 1792   EMail: mallman@icir.orghttp://www.icir.org/mallman/   Vern Paxson   International Computer Science Institute (ICSI)   1947 Center Street   Suite 600   Berkeley, CA 94704-1198   Phone: +1 510/642-4274 x302   EMail: vern@icir.orghttp://www.icir.org/vern/   Ethan Blanton   Purdue University Computer Sciences   305 North University Street   West Lafayette, IN  47907   EMail: eblanton@cs.purdue.eduhttp://www.cs.purdue.edu/homes/eblanton/Allman, et al.              Standards Track                    [Page 18]