git: tcp: Correct sending idle detection and implement part of RFC2861

Sepherosa Ziehau sephe at
Wed Apr 18 02:32:31 PDT 2012

commit 3d127502478783869f3689073fdde11ca8bcad2f
Author: Sepherosa Ziehau <sephe at>
Date:   Wed Apr 18 15:31:26 2012 +0800

    tcp: Correct sending idle detection and implement part of RFC2861
    This commit mainly changes how cwnd is shinked after idle period on
    the send side.
    - Properly detect sending idle period according to RFC5681.  The problem
      of using reception time to detect sending idle period is described in
      RFC5681 as:
       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.
      This mainly affects HTTP/1.1 persistent connection performance after
      the connection is idled for a long time.  The impact probably should not
      be drastic, since 80% HTTP/1.1 persistent connection delay between two
      requests are less then minimum RTO (1 second) as discovered by:
      "Overclocking the Yahoo! CDN for Faster Web Page Loads"
      Sysctl node net.inet.tcp.idle_restart is added to disable the cwnd
      shinking after idle period.  It is on by default.  And you can set it
      to 0 to restore old behaviour against HTTP/1.1 persistent connection.
    - Implement part of RFC2861, which decays cwnd after idle period according
      to the length of sending idle period.  The main difference between our
      implementation and the RFC2861 is that we don't let cwnd go below the
      value allowed by RFC5861.
      Sysctl node net.inet.tcp.idle_cwv is added to disable CWV after sending
      idle period.  It is on by default.  Disable net.inet.tcp.idle_restart
      will also indirectly disable CWV after sending idle period.
      The CWV during the application-limited period is not implemented by this
      commit.  It is just too conservative, as discovered by:
      "Analysing TCP for Bursty Traffic, Int'l J. of Communications,
       Network and System Sciences, 7(3), July 2010."
    - Add statistics about how much sending idle happened

Summary of changes:
 sys/netinet/tcp_output.c |   95 +++++++++++++++++++++++++++++++++++++++-------
 sys/netinet/tcp_subr.c   |    1 +
 sys/netinet/tcp_var.h    |    4 +-
 usr.bin/netstat/inet.c   |    1 +
 4 files changed, 86 insertions(+), 15 deletions(-)

DragonFly BSD source repository

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