rfc2581.txt

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Network Working Group                                          M. Allman
Request for Comments: 2581                  NASA Glenn/Sterling Software
Obsoletes: 2001                                                V. Paxson
Category: Standards Track                                   ACIRI / ICSI
                                                              W. Stevens
                                                              Consultant
                                                              April 1999


                         TCP Congestion Control

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) The Internet Society (1999).  All Rights Reserved.

Abstract

   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.

1. Introduction

   This document specifies four TCP [Pos81] 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 [Bra89].

   This document is an update of [Ste97].  In addition to specifying the
   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.

   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.




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   This document is organized as follows.  Section 2 provides various
   definitions which 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.

   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 [Bra97].

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 [MD90] 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, 536 bytes
      [Bra89].  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.





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   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; see section 4.1 for
      more discussion).

   FLIGHT SIZE:  The amount of data that has been sent but not yet
      acknowledged.

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).

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.

   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.






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   IW, the initial value of cwnd, MUST be less than or equal to 2*SMSS
   bytes and MUST NOT be more than 2 segments.

   We note that a non-standard, experimental TCP extension allows that a
   TCP MAY use a larger initial window (IW), as defined in equation 1
   [AFP98]:

      IW = min (4*SMSS, max (2*SMSS, 4380 bytes))           (1)

   With this extension, a TCP sender MAY use a 3 or 4 segment initial
   window, provided the combined size of the segments does not exceed
   4380 bytes.  We do NOT allow this change as part of the standard
   defined by this document.  However, we include discussion of (1) in
   the remainder of this document as a guideline for those experimenting
   with the change, rather than conforming to the present standards for
   TCP congestion control.

   The initial value of ssthresh MAY be arbitrarily high (for example,
   some implementations use the size of the advertised window), but it
   may be reduced in response to congestion.  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 acknowledges new data.  Slow start ends when
   cwnd exceeds ssthresh (or, optionally, when it reaches it, as noted
   above) or when congestion is observed.

   During congestion avoidance, cwnd is incremented by 1 full-sized
   segment per round-trip time (RTT).  Congestion avoidance continues
   until congestion is detected.  One formula commonly used to update
   cwnd during congestion avoidance is given in equation 2:

      cwnd += SMSS*SMSS/cwnd                     (2)

   This adjustment is executed on every incoming non-duplicate ACK.
   Equation (2) 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 acknowledges every data
   segment, (2) proves slightly more aggressive than 1 segment per RTT,
   and for a receiver acknowledging every-other packet, (2) is less
   aggressive.)








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   Implementation Note: Since integer arithmetic is usually used in TCP
   implementations, the formula given in equation 2 can fail to increase
   cwnd when the congestion window is very large (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 (2).  This is
   incorrect and can actually lead to diminished performance [PAD+98].

   Another acceptable 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 be
   increased by more than the larger of either 1 full-sized segment per
   RTT, or the value computed using equation 2.

   Implementation Note: some implementations maintain cwnd in units of
   bytes, while others in units of full-sized segments.  The latter will
   find equation (2) 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, the value of ssthresh MUST be set to no more than the value
   given in equation 3:

      ssthresh = max (FlightSize / 2, 2*SMSS)            (3)

   As discussed above, FlightSize is the amount of outstanding data in
   the network.