NAME

       tcpretrans - Trace or count TCP retransmits and TLPs. Uses Linux eBPF/bcc.

SYNOPSIS

       tcpretrans [-h] [-s] [-l] [-c] [-4 | -6]

DESCRIPTION

       This  traces  TCP  retransmits,  showing  address,  port,  and  TCP state information, and
       sometimes the PID (although usually not, since retransmits are usually sent by the  kernel
       on  timeouts).  To  keep  overhead very low, only the TCP retransmit functions are traced.
       This does not trace every packet (like tcpdump(8) or a packet sniffer). Optionally, it can
       count  retransmits  over  a  user  signalled interval to spot potentially dropping network
       paths the flows are traversing.

       This uses dynamic tracing of the  kernel  tcp_retransmit_skb()  and  tcp_send_loss_probe()
       functions, and will need to be updated to match kernel changes to these functions.

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

REQUIREMENTS

       CONFIG_BPF and bcc.

OPTIONS

       -h     Print usage message.

       -s     Display TCP sequence numbers.

       -l     Include tail loss probe attempts (in some cases the kernel may not complete the TLP
              send).

       -c     Count occurring retransmits per flow.

       -4     Trace IPv4 family only.

       -6     Trace IPv6 family only.

EXAMPLES

       Trace TCP retransmits:
              # tcpretrans

       Trace TCP retransmits and TLP attempts:
              # tcpretrans -l

       Trace IPv4 family only:
              # tcpretrans -4

       Trace IPv6 family only:
              # tcpretrans -6

FIELDS

       TIME   Time of the retransmit.

       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.

       IP     IP address family (4 or 6).

       LADDR  Local IP address.

       LPORT  Local port.

       T>     Type of event: R> == retransmit, L> == tail loss probe.

       RADDR  Remote IP address.

       RPORT  Remote port.

       STATE  TCP session state.

       SEQ    TCP sequence.

       RETRANSMITS
              Accumulated occurred retransmits since start.

OVERHEAD

       Should  be negligible: TCP retransmit events should be low (<1000/s), and the low overhead
       this tool adds to each event should make the cost negligible.

SOURCE

       This is from bcc.

              https://github.com/iovisor/bcc

       Also look in the bcc distribution for a companion _examples.txt  file  containing  example
       usage, output, and commentary for this tool.

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg

SEE ALSO

       tcpconnect(8), tcpaccept(8)