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Network Working Group                                         W. StevensRequest for Comments: 2001                                          NOAOCategory: Standards Track                                   January 1997TCP Slow Start, Congestion Avoidance,Fast Retransmit, and Fast Recovery AlgorithmsStatus 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.Abstract   Modern implementations of TCP contain four intertwined algorithms   that have never been fully documented as Internet standards:  slow   start, congestion avoidance, fast retransmit, and fast recovery.  [2]   and [3] provide some details on these algorithms, [4] provides   examples of the algorithms in action, and [5] provides the source   code for the 4.4BSD implementation.RFC 1122 requires that a TCP   must implement slow start and congestion avoidance (Section 4.2.2.15   of [1]), citing [2] as the reference, but fast retransmit and fast   recovery were implemented afterRFC 1122.  The purpose of this   document is to document these four algorithms for the Internet.Acknowledgments   Much of this memo 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.  The four algorithms that are   described were developed by Van Jacobson.1.  Slow Start   Old TCPs would start a connection with the sender injecting multiple   segments into the network, up to the window size advertised by the   receiver.  While this is OK when the two hosts are on the same LAN,   if there are routers and slower links between the sender and the   receiver, problems can arise.  Some intermediate router must queue   the packets, and it's possible for that router to run out of space.   [2] shows how this naive approach can reduce the throughput of a TCP   connection drastically.Stevens                     Standards Track                     [Page 1]

RFC 2001                          TCP                       January 1997   The algorithm to avoid this is called slow start.  It operates by   observing that the rate at which new packets should be injected into   the network is the rate at which the acknowledgments are returned by   the other end.   Slow start adds another window to the sender's TCP:  the congestion   window, called "cwnd".  When a new connection is established with a   host on another network, the congestion window is initialized to one   segment (i.e., the segment size announced by the other end, or the   default, typically 536 or 512).  Each time an ACK is received, the   congestion window is increased by one segment.  The sender can   transmit up to the minimum of the congestion window and the   advertised window.  The congestion window is flow control imposed by   the sender, while the advertised window is flow control imposed by   the receiver.  The former is based on the sender's assessment of   perceived network congestion; the latter is related to the amount of   available buffer space at the receiver for this connection.   The sender starts by transmitting one segment and waiting for its   ACK.  When that ACK is received, the congestion window is incremented   from one to two, and two segments can be sent.  When each of those   two segments is acknowledged, the congestion window is increased to   four.  This provides an exponential growth, although it is not   exactly exponential because the receiver may delay its ACKs,   typically sending one ACK for every two segments that it receives.   At some point the capacity of the internet can be reached, and an   intermediate router will start discarding packets.  This tells the   sender that its congestion window has gotten too large.   Early implementations performed slow start only if the other end was   on a different network.  Current implementations always perform slow   start.2.  Congestion Avoidance   Congestion can occur when data arrives on a big pipe (a fast LAN) and   gets sent out a smaller pipe (a slower WAN).  Congestion can also   occur when multiple input streams arrive at a router whose output   capacity is less than the sum of the inputs.  Congestion avoidance is   a way to deal with lost packets.  It is described in [2].   The assumption of the algorithm is that packet loss caused by damage   is very small (much less than 1%), therefore the loss of a packet   signals congestion somewhere in the network between the source and   destination.  There are two indications of packet loss:  a timeout   occurring and the receipt of duplicate ACKs.Stevens                     Standards Track                     [Page 2]

RFC 2001                          TCP                       January 1997   Congestion avoidance and slow start are independent algorithms with   different objectives.  But when congestion occurs TCP must slow down   its transmission rate of packets into the network, and then invoke   slow start to get things going again.  In practice they are   implemented together.   Congestion avoidance and slow start require that two variables be   maintained for each connection: a congestion window, cwnd, and a slow   start threshold size, ssthresh.  The combined algorithm operates as   follows:   1.  Initialization for a given connection sets cwnd to one segment       and ssthresh to 65535 bytes.   2.  The TCP output routine never sends more than the minimum of cwnd       and the receiver's advertised window.   3.  When congestion occurs (indicated by a timeout or the reception       of duplicate ACKs), one-half of the current window size (the       minimum of cwnd and the receiver's advertised window, but at       least two segments) is saved in ssthresh.  Additionally, if the       congestion is indicated by a timeout, cwnd is set to one segment       (i.e., slow start).   4.  When new data is acknowledged by the other end, increase cwnd,       but the way it increases depends on whether TCP is performing       slow start or congestion avoidance.      If cwnd is less than or equal to ssthresh, TCP is in slow start;      otherwise TCP is performing congestion avoidance.  Slow start      continues until TCP is halfway to where it was when congestion      occurred (since it recorded half of the window size that caused      the problem in step 2), and then congestion avoidance takes over.      Slow start has cwnd begin at one segment, and be incremented by      one segment every time an ACK is received.  As mentioned earlier,      this opens the window exponentially:  send one segment, then two,      then four, and so on.  Congestion avoidance dictates that cwnd be      incremented by segsize*segsize/cwnd each time an ACK is received,      where segsize is the segment size and cwnd is maintained in bytes.      This is a linear growth of cwnd, compared to slow start's      exponential growth.  The increase in cwnd should be at most one      segment each round-trip time (regardless how many ACKs are      received in that RTT), whereas slow start increments cwnd by the      number of ACKs received in a round-trip time.Stevens                     Standards Track                     [Page 3]

RFC 2001                          TCP                       January 1997   Many implementations incorrectly add a small fraction of the segment   size (typically the segment size divided by 8) during congestion   avoidance.  This is wrong and should not be emulated in future   releases.3.  Fast Retransmit   Modifications to the congestion avoidance algorithm were proposed in   1990 [3].  Before describing the change, realize that TCP may   generate an immediate acknowledgment (a duplicate ACK) when an out-   of-order segment is received (Section 4.2.2.21 of [1], with a note   that one reason for doing so was for the experimental fast-   retransmit algorithm).  This duplicate ACK should not be delayed.   The purpose of this duplicate ACK is to let the other end know that a   segment was received out of order, and to tell it what sequence   number is expected.   Since TCP does not know whether a duplicate ACK is caused by a lost   segment or just a reordering of segments, it waits for a small number   of duplicate ACKs to be received.  It is assumed that if there is   just a reordering of the segments, there will be only one or two   duplicate ACKs before the reordered segment is processed, which will   then generate a new ACK.  If three or more duplicate ACKs are   received in a row, it is a strong indication that a segment has been   lost.  TCP then performs a retransmission of what appears to be the   missing segment, without waiting for a retransmission timer to   expire.4.  Fast Recovery   After fast retransmit sends what appears to be the missing segment,   congestion avoidance, but not slow start is performed.  This is the   fast recovery algorithm.  It is an improvement that allows high   throughput under moderate congestion, especially for large windows.   The reason for not performing slow start in this case is that the   receipt of the duplicate ACKs tells TCP more than just a packet has   been lost.  Since the receiver can only generate the duplicate ACK   when another segment is received, that segment has left the network   and is in the receiver's buffer.  That is, there is still data   flowing between the two ends, and TCP does not want to reduce the   flow abruptly by going into slow start.   The fast retransmit and fast recovery algorithms are usually   implemented together as follows.Stevens                     Standards Track                     [Page 4]

RFC 2001                          TCP                       January 1997   1.  When the third duplicate ACK in a row is received, set ssthresh       to one-half the current congestion window, cwnd, but no less       than two segments.  Retransmit the missing segment.  Set cwnd to       ssthresh plus 3 times the segment size.  This inflates the       congestion window by the number of segments that have left the       network and which the other end has cached (3).   2.  Each time another duplicate ACK arrives, increment cwnd by the       segment size.  This inflates the congestion window for the       additional segment that has left the network.  Transmit a       packet, if allowed by the new value of cwnd.   3.  When the next ACK arrives that acknowledges new data, set cwnd       to ssthresh (the value set in step 1).  This ACK should be the       acknowledgment of the retransmission from step 1, one round-trip       time after the retransmission.  Additionally, this ACK should       acknowledge all the intermediate segments sent between the lost       packet and the receipt of the first duplicate ACK.  This step is       congestion avoidance, since TCP is down to one-half the rate it       was at when the packet was lost.   The fast retransmit algorithm first appeared in the 4.3BSD Tahoe   release, and it was followed by slow start.  The fast recovery   algorithm appeared in the 4.3BSD Reno release.5.  Security Considerations   Security considerations are not discussed in this memo.6.  References   [1]  B. Braden, ed., "Requirements for Internet Hosts --        Communication Layers,"RFC 1122, Oct. 1989.   [2]  V. Jacobson, "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.   [3]  V. Jacobson, "Modified TCP Congestion Avoidance Algorithm,"        end2end-interest mailing list, April 30, 1990.ftp://ftp.isi.edu/end2end/end2end-interest-1990.mail.   [4]  W. R. Stevens, "TCP/IP Illustrated, Volume 1: The Protocols",        Addison-Wesley, 1994.   [5]  G. R. Wright, W. R. Stevens, "TCP/IP Illustrated, Volume 2:        The Implementation", Addison-Wesley, 1995.Stevens                     Standards Track                     [Page 5]

RFC 2001                          TCP                       January 1997Author's  Address:    W. Richard Stevens    1202 E. Paseo del Zorro    Tucson, AZ  85718    Phone: 520-297-9416    EMail: rstevens@noao.edu    Home Page:http://www.noao.edu/~rstevensStevens                     Standards Track                     [Page 6]