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Network Working Group                                          M. AllmanRequest for Comments: 3465                                  BBN/NASA GRCCategory: Experimental                                     February 2003TCP Congestion Control with Appropriate Byte Counting (ABC)Status 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 (2003).  All Rights Reserved.Abstract   This document proposes a small modification to the way TCP increases   its congestion window.  Rather than the traditional method of   increasing the congestion window by a constant amount for each   arriving acknowledgment, the document suggests basing the increase on   the number of previously unacknowledged bytes each ACK covers.  This   change improves the performance of TCP, as well as closes a security   hole TCP receivers can use to induce the sender into increasing the   sending rate too rapidly.Terminology   Much of the language in this document is taken from [RFC2581].   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].1   Introduction   This document proposes a modification to the algorithm for increasing   TCP's congestion window (cwnd) that improves both performance and   security.  Rather than increasing a TCP's congestion window based on   the number of acknowledgments (ACKs) that arrive at the data sender   (per the current specification [RFC2581]), the congestion window is   increased based on the number of bytes acknowledged by the arriving   ACKs.  The algorithm improves performance by mitigating the impact of   delayed ACKs on the growth of cwnd.  At the same time, the algorithm   provides cwnd growth in direct relation to the probed capacity of aAllman                        Experimental                      [Page 1]

RFC 3465            TCP Congestion Control with ABC        February 2003   network path, therefore providing a more measured response to ACKs   that cover only small amounts of data (less than a full segment size)   than ACK counting.  This more appropriate cwnd growth can improve   both performance and can prevent inappropriate cwnd growth in   response to a misbehaving receiver.  On the other hand, in some cases   the modified cwnd growth algorithm causes larger bursts of segments   to be sent into the network.  In some cases this can lead to a non-   negligible increase in the drop rate and reduced performance (seesection 4 for a larger discussion of the issues).   This document is organized as follows.Section 2 outlines the   modified algorithm for increasing TCP's congestion window.Section 3   discusses the advantages of using the modified algorithm.Section 4   discusses the disadvantages of the approach outlined in this   document.Section 5 outlines some of the fairness issues that must   be considered for the modified algorithm.Section 6 discusses   security considerations.   Statement of Intent      This specification contains an algorithm improving the performance      of TCP which is understood to be effective and safe, but which has      not been widely deployed.  One goal of publication as an      Experimental RFC is to be prudent, and encourage use and      deployment prior to publication in the standards track.  It is the      intent of the Transport Area to re-submit this specification as an      IETF Proposed Standard in the future, after more experience has      been gained.2   A Modified Algorithm for Increasing the Congestion Window   As originally outlined in [Jac88] and specified in [RFC2581], TCP   uses two algorithms for increasing the congestion window.  During   steady-state, TCP uses the Congestion Avoidance algorithm to linearly   increase the value of cwnd.  At the beginning of a transfer, after a   retransmission timeout or after a long idle period (in some   implementations), TCP uses the Slow Start algorithm to increase cwnd   exponentially.  According toRFC 2581, slow start bases the cwnd   increase on the number of incoming acknowledgments.  During   congestion avoidanceRFC 2581 allows more latitude in increasing   cwnd, but traditionally implementations have based the increase on   the number of arriving ACKs.  In the following two subsections, we   detail modifications to these algorithms to increase cwnd based on   the number of bytes being acknowledged by each arriving ACK, rather   than by the number of ACKs that arrive.  We call these changes   "Appropriate Byte Counting" (ABC) [All99].Allman                        Experimental                      [Page 2]

RFC 3465            TCP Congestion Control with ABC        February 20032.1 Congestion AvoidanceRFC 2581 specifies that cwnd should be increased by 1 segment per   round-trip time (RTT) during the congestion avoidance phase of a   transfer.  Traditionally, TCPs have approximated this increase by   increasing cwnd by 1/cwnd for each arriving ACK.  This algorithm   opens cwnd by roughly 1 segment per RTT if the receiver ACKs each   incoming segment and no ACK loss occurs.  However, if the receiver   implements delayed ACKs [Bra89], the receiver returns roughly half as   many ACKs, which causes the sender to open cwnd more conservatively   (by approximately 1 segment every second RTT).  The approach that   this document suggests is to store the number of bytes that have been   ACKed in a "bytes_acked" variable in the TCP control block.  When   bytes_acked becomes greater than or equal to the value of the   congestion window, bytes_acked is reduced by the value of cwnd.   Next, cwnd is incremented by a full-sized segment (SMSS).  The   algorithm suggested above is specifically allowed byRFC 2581 during   congestion avoidance because it opens the window by at most 1 segment   per RTT.2.2 Slow StartRFC 2581 states that the sender increments the congestion window by   at most, 1*SMSS bytes for each arriving acknowledgment during slow   start.  This document proposes that a TCP sender SHOULD increase cwnd   by the number of previously unacknowledged bytes ACKed by each   incoming acknowledgment, provided the increase is not more than L   bytes.  Choosing the limit on the increase, L, is discussed in the   next subsection.  When the number of previously unacknowledged bytes   ACKed is less than or equal to 1*SMSS bytes, or L is less than or   equal to 1*SMSS bytes, this proposal is no more aggressive (and   possibly less aggressive) than allowed byRFC 2581.  However,   increasing cwnd by more than 1*SMSS bytes in response to a single ACK   is more aggressive than allowed byRFC 2581.  The more aggressive   version of the slow start algorithm still falls within the spirit of   the principles outlined in [Jac88] (i.e., of no more than doubling   the cwnd per RTT), and this document proposes ABC for experimentation   in shared networks, provided an appropriate limit is applied (see   next section).2.3 Choosing the Limit   The limit, L, chosen for the cwnd increase during slow start,   controls the aggressiveness of the algorithm.  Choosing L=1*SMSS   bytes provides behavior that is no more aggressive than allowed byRFC 2581.  However, ABC with L=1*SMSS bytes is more conservative in aAllman                        Experimental                      [Page 3]

RFC 3465            TCP Congestion Control with ABC        February 2003   number of key ways (as discussed in the next section) and therefore,   this document suggests that even though with L=1*SMSS bytes TCP   stacks will see little performance change, ABC SHOULD be used.   A very large L could potentially lead to large line-rate bursts of   traffic in the face of a large amount of ACK loss or in the case when   the receiver sends "stretch ACKs" (ACKs for more than the two full-   sized segments allowed by the delayed ACK algorithm) [Pax97].   This document specifies that TCP implementations MAY use L=2*SMSS   bytes and MUST NOT use L > 2*SMSS bytes.  This choice balances   between being conservative (L=1*SMSS bytes) and being potentially   very aggressive.  In addition, L=2*SMSS bytes exactly balances the   negative impact of the delayed ACK algorithm (as discussed in more   detail insection 3.2).  Note that when L=2*SMSS bytes cwnd growth is   roughly the same as the case when the standard algorithms are used in   conjunction with a receiver that transmits an ACK for each incoming   segment [All98] (assuming no or small amounts of ACK loss in both   cases).   The exception to the above suggestion is during a slow start phase   that follows a retransmission timeout (RTO).  In this situation, a   TCP MUST use L=1*SMSS as specified inRFC 2581 since ACKs for large   amounts of previously unacknowledged data are common during this   phase of a transfer.  These ACKs do not necessarily indicate how much   data has left the network in the last RTT, and therefore ABC cannot   accurately determine how much to increase cwnd.  As an example, say   segment N is dropped by the network, and segments N+1 and N+2 arrive   successfully at the receiver.  The sender will receive only two   duplicate ACKs and therefore must rely on the retransmission timer   (RTO) to detect the loss.  When the RTO expires, segment N is   retransmitted.  The ACK sent in response to the retransmission will   be for segment N+2.  However, this ACK does not indicate that three   segments have left the network in the last RTT, but rather only a   single segment left the network.  Therefore, the appropriate cwnd   increment is at most 1*SMSS bytes.2.4 RTO Implications   [Jac88] shows that increases in cwnd of more than a factor of two in   succeeding RTTs can cause spurious retransmissions on slow links   where the bandwidth dominates the RTT, assuming the RTO estimator   given in [Jac88] and [RFC2988].  ABC stays within this limit of no   more than doubling cwnd in successive RTTs by capping the increase   (no matter what L is employed) by the number of previously   unacknowledged bytes covered by each incoming ACK.Allman                        Experimental                      [Page 4]

RFC 3465            TCP Congestion Control with ABC        February 20033   Advantages   This section outlines several advantages of using the ABC algorithm   to increase cwnd, rather than the standard ACK counting algorithm   given in [RFC2581].3.1 More Appropriate Congestion Window Increase   The ABC algorithm outlined insection 2 increases TCP's cwnd in   proportion to the amount of data actually sent into the network.  ACK   counting, on the other hand, increments cwnd by a constant upon the   arrival of each ACK.  For instance, consider an interactive telnet   connection (e.g., ssh or telnet) in which ACKs generally cover only a   few bytes of data, but cwnd is increased by 1*SMSS bytes for each ACK   received.  When a large amount of data needs to be transmitted (e.g.,   displaying a large file) the data is sent in one large burst because   the cwnd grows by 1*SMSS bytes per ACK rather than based on the   actual amount of capacity used.  Such a line-rate burst of data can   potentially cause a large amount of segment loss.   Congestion Window Validation (CWV) [RFC2861] addresses the above   problem as well.  CWV limits the amount of unused cwnd a TCP   connection can accumulate.  ABC can be used in conjunction with CWV   to obtain an accurate measure of the network path.3.2 Mitigate the Impact of Delayed ACKs and Lost ACKs   Delayed ACKs [RFC1122,RFC2581] allow a TCP receiver to refrain from   sending an ACK for each incoming segment.  However, a receiver SHOULD   send an ACK for every second full-sized segment that arrives.   Furthermore, a receiver MUST NOT withhold an ACK for more than 500   ms.  By reducing the number of ACKs sent to the data originator the   receiver is slowing the growth of the congestion window under an ACK   counting system.  Using ABC with L=2*SMSS bytes can roughly negate   the negative impact imposed by delayed ACKs by allowing cwnd to be   increased for ACKs that are withheld by the receiver.  This allows   the congestion window to grow in a manner similar to the case when   the receiver ACKs each incoming segment, but without adding extra   traffic to the network.  Simulation studies have shown increased   throughput when a TCP sender uses ABC when compared to the standard   ACK counting algorithm [All99], especially for short transfers that   never leave the initial slow start period.   Note that delayed ACKs should not be an issue during slow start-based   loss recovery, asRFC 2581 recommends that receivers should not delay   ACKs that cover out-of-order segments.  Therefore, as discussed   above, ABC with L > 1*SMSS bytes is inappropriate for such slow start   based loss recovery and MUST NOT be used.Allman                        Experimental                      [Page 5]

RFC 3465            TCP Congestion Control with ABC        February 2003   Note: In the case when an entire window of data is lost, a TCP   receiver will likely generate delayed ACKs and an L > 1*SMSS bytes   would be safe.  However, detecting this scenario is difficult.   Therefore to keep ABC conservative, this document mandates that L   MUST NOT be > 1*SMSS bytes in any slow start-based loss recovery.   ACK loss can also retard the growth of a congestion window that   increases based on the number of ACKs that arrive.  When counting   ACKs, dropped ACKs represent forever-missed opportunities to increase   cwnd.  Using ABC with L > 1*SMSS bytes allows the sender to mitigate   the effect of lost ACKs.3.3 Prevents Attacks from Misbehaving Receivers   [SCWA99] outlines several methods for a receiver to induce a TCP   sender into violating congestion control and transmitting data at a   potentially inappropriate rate.  One of the outlined attacks is "ACK   Division".  This scheme involves the receiver sending multiple ACKs   for each incoming data segment, each ACKing only a small portion of   the original TCP data segment.  Since TCP senders have traditionally   used ACK counting to increase cwnd, ACK division causes   inappropriately rapid cwnd growth and, in turn, a potentially   inappropriate sending rate.  A TCP sender that uses ABC can prevent   this attack from being used to undermine standard congestion control   because the cwnd increase is based on the number of bytes ACKed,   rather than the number of ACKs received.   To prevent misbehaving receivers from inducing inappropriate sender   behavior, this document suggests TCP implementations use ABC, even if   L=1*SMSS bytes (i.e., not allowing ABC to provide more aggressive   cwnd growth than allowed byRFC 2581).4   Disadvantages   The main disadvantages of using ABC with L=2*SMSS bytes are an   increase in the burstiness of TCP and a small increase in the overall   loss rate.  [All98] discusses the two ways that ABC increases the   burstiness of the TCP sender.  First, the "micro burstiness" of the   connection is increased.  In other words, the number of segments sent   in response to each incoming ACK is increased by at most 1 segment   when using ABC with L=2*SMSS bytes in conjunction with a receiver   that is sending delayed ACKs.  During slow start this translates into   an increase from sending 2 back-to-back segments to sending 3 back-   to-back packets in response to an ACK for a single packet.  Or, an   increase from 3 packets to 4 packets when receiving a delayed ACK for   two outstanding packets.  Note that ACK loss can cause larger bursts.   However, ABC only increases the burst size by at most 1*SMSS bytes   per ACK received when compared to the standard behavior.  This slightAllman                        Experimental                      [Page 6]

RFC 3465            TCP Congestion Control with ABC        February 2003   increase in the burstiness should only cause problems for devices   that have very small buffers.  In addition, ABC increases the "macro   burstiness" of the TCP sender in response to delayed ACKs in slow   start.  Rather than increasing cwnd by roughly 1.5 times per RTT, ABC   roughly doubles the congestion window every RTT.  However, doubling   cwnd every RTT fits within the spirit of slow start, as originally   outlined [Jac88].   With the increased burstiness comes a modest increase in the loss   rate for a TCP connection employing ABC (see the next section for a   short discussion on the fairness of ABC to non-ABC flows).  The   additional loss can be directly attributable to the increased   aggressiveness of ABC.  During slow start cwnd is increased more   rapidly.  Therefore when loss occurs cwnd is larger and more drops   are likely.  Similarly, a congestion avoidance cycle takes roughly   half, as long when using ABC and delayed ACKs when compared to an ACK   counting implementation.  In other words, a TCP sender reaches the   capacity of the network path, drops a packet and reduces the   congestion window by half roughly twice as often when using ABC.   However, as discussed above, in spite of the additional loss an ABC   TCP sender generally obtains better overall performance than a non-   ABC TCP [All99].   Due to the increase in the packet drop rate we suggest ABC be   implemented in conjunction with selective acknowledgments [RFC2018].5   Fairness Considerations   [All99] presents several simple simulations conducted to measure the   impact of ABC on competing traffic (both ABC and non-ABC).  The   experiments show that while ABC increases the drop rate for the   connection using ABC, competing traffic is not greatly effected.  The   experiments show that standard TCP and ABC both obtain roughly the   same throughput, regardless of the variant of the competing traffic.   The simulations also reaffirm that ABC outperforms non-ABC TCP in an   environment with varying types of TCP connections.  On the other   hand, the simulations presented in [All99] are not necessarily   realistic.  Therefore we are encouraging more experimentation in the   Internet.6   Security Considerations   As discussed insection 3.3, ABC protects a TCP sender from a   misbehaving receiver that induces the sender into transmitting at an   inappropriate rate with an "ACK division" attack.  This, in turn,   protects the network from an overly aggressive sender.Allman                        Experimental                      [Page 7]

RFC 3465            TCP Congestion Control with ABC        February 20037   Conclusions   This document RECOMMENDS that all TCP stacks be modified to use ABC   with L=1*SMSS bytes.  This change does not increase the   aggressiveness of TCP.  Furthermore, simulations of ABC with L=2*SMSS   bytes show a promising performance improvement that we encourage   researchers to experiment with in the Internet.Acknowledgments   This document has benefited from discussions with and encouragement   from Sally Floyd.  Van Jacobson and Reiner Ludwig provided valuable   input on the implications of byte counting on the RTO.  Reiner Ludwig   and Kostas Pentikousis provided valuable feedback on a draft of this   document.Normative References   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --             Communication Layers", STD 3,RFC 1122, October 1989.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion             Control",RFC 2581, April 1999.Informative References   [All98]   Mark Allman.  On the Generation and Use of TCP             Acknowledgments. ACM Computer Communication Review, 29(3),             July 1998.   [All99]   Mark Allman.  TCP Byte Counting Refinements. ACM Computer             Communication Review, 29(3), July 1999.   [Jac88]   Van Jacobson.  Congestion Avoidance and Control.  ACM             SIGCOMM 1988.   [Pax97]   Vern Paxson.  Automated Packet Trace Analysis of TCP             Implementations.  ACM SIGCOMM, September 1997.   [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP             Selective Acknowledgment Options",RFC 2018, October 1996.   [RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion             Window Validation",RFC 2861, June 2000.Allman                        Experimental                      [Page 8]

RFC 3465            TCP Congestion Control with ABC        February 2003   [SCWA99]  Stefan Savage, Neal Cardwell, David Wetherall, Tom             Anderson.  TCP Congestion Control with a Misbehaving             Receiver.  ACM Computer Communication Review, 29(5),             October 1999.Author's Address   Mark Allman   BBN Technologies/NASA Glenn Research Center   Lewis Field   21000 Brookpark Rd.  MS 54-5   Cleveland, OH  44135   Fax: 216-433-8705   Phone: 216-433-6586   EMail: mallman@bbn.comhttp://roland.grc.nasa.gov/~mallmanAllman                        Experimental                      [Page 9]

RFC 3465            TCP Congestion Control with ABC        February 2003Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Allman                        Experimental                     [Page 10]