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tcpretrans-perf(8)


NAME

   tcpretrans - show TCP retransmits, with address and other details. Uses
   Linux ftrace.

SYNOPSIS

   tcpretrans [-hsp]

DESCRIPTION

   This traces TCP retransmits that are sent by the system  tcpretrans  is
   executed  from,  showing  address, port, and TCP state information, and
   sometimes the PID (although usually not, since retransmits are  usually
   sent  by  the  kernel  on  timeout  events). To keep overhead low, only
   tcp_retransmit_skb() kernel calls are traced (this does not trace every
   packet).

   This  was  written  as  a  proof of concept for ftrace, for older Linux
   systems, and without kernel  debuginfo.  It  uses  dynamic  tracing  of
   tcp_retransmit_skb(),  and  reads /proc/net/tcp for socket details. Its
   use of dynamic tracing and  CPU  registers  is  an  unstable  platform-
   specific workaround, and may require modifications to work on different
   kernels and platforms. This would be better written using a tracer such
   as  SystemTap,  and will likely be rewritten in the future when certain
   tracing features are added to the Linux kernel.

   When   -l   is   used,   this   also   uses    dynamic    tracing    of
   tcp_send_loss_probe() and a register.

   Currently  only  IPv4  is  supported,  on  x86_64. If you try this on a
   different architecture, you'll  likely  need  to  adjust  the  register
   locations (search for %di).

   Since this uses ftrace, only the root user can use this tool.

REQUIREMENTS

   FTRACE  and  KPROBE  CONFIG, tcp_retransmit_skb() kernel function.  You
   may have these already have these on recent  kernels.  And  Perl.   TCP
   tail loss probes were added in Linux 3.10.

OPTIONS

   -h     Print usage message.

   -s     Include kernel stack traces.

   -l     Include TCP tail loss probes.

EXAMPLES

   Trace TCP retransmits
          # tcpretrans

   TIME   Time of retransmit (may be rounded up to the nearest second).

   PID    Process  ID  that  was on-CPU. This is less useful than it might
          sound, as it may usually be 0, for the kernel,  for  timer-based
          retransmits.

   LADDR  Local address.

   LPORT  Local port.

   --     Packet type: "R>" for retransmit, and "L>" for tail loss probe.

   RADDR  Remote address.

   RPORT  Remote port.

   STATE  TCP session state.

OVERHEAD

   The  CPU  overhead  is  relative to the rate of TCP retransmits, and is
   designed to be low as this does not  examine  every  packet.  Once  per
   second the /proc/net/tcp file is read, and a buffer of retransmit trace
   events is retrieved from the kernel and processed.

SOURCE

   This is from the perf-tools collection.

          https://github.com/brendangregg/perf-tools

   Also look under the examples  directory  for  a  text  file  containing
   example usage, output, and commentary for this tool.

OS

   Linux

STABILITY

   Unstable - in development.

AUTHOR

   Brendan Gregg

SEE ALSO

   tcpdump(1)



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