RFC 2861 (rfc2861) - Page 2 of 11

TCP Congestion Window Validation

RFC 2861            TCP Congestion Window Validation           June 2000


2. 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
   [RFC 2581] and in the appendix of [VJ88], while other implementations
   don't reduce their congestion window after an idle period.  RFC 2581
   [RFC 2581] 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