Query regarding net.inet.tcp.nolocaltimewait

[suraj sandhu]

Hi,

I am using FreeBsd 8.2 and the behavior I am trying to understand is
why there is no final ACK being sent by the client if
net.inet.tcp.nolocaltimewait is set. Due to this server side
communication need to rely upon receiving the RST for clean-up.
Especially in the cases where client port is reused soon after the the
connection being closed, additional resets are triggered.

  198 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L198>
void
  199 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L199>
tcp_twstart <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tcp_twstart>(struct
tcpcb <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tcpcb>
*tp <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tp>)
  200 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L200>
{
  201 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L201>
        struct tcptw
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tcptw>
*tw;
  202 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L202>
        struct inpcb
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inpcb>
*inp <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inp>
= tp <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tp>->t_inpcb;
  203 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L203>
        int acknow;
  204 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L204>
        struct socket
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=socket>
*so <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=so>;
  205 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L205>
  206 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L206>
        INP_INFO_WLOCK_ASSERT
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=INP_INFO_WLOCK_ASSERT>(&V_tcbinfo
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=V_tcbinfo>);
     */* tcp_tw_2msl_reset(). */*
  207 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L207>
        INP_WLOCK_ASSERT
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=INP_WLOCK_ASSERT>(inp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inp>);
  208 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L208>
  209 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L209>
        if (V_nolocaltimewait
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=V_nolocaltimewait>
&& in_localip <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=in_localip>(inp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inp>->inp_faddr
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inp_faddr>))
{
  210 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L210>
                tp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tp> =
tcp_close <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tcp_close>(tp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tp>);<====
Closing the connection without sending Final ACK.
  211 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L211>
                if (tp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=tp> !=
NULL <http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=NULL>)
  212 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L212>
                        INP_WUNLOCK
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=INP_WUNLOCK>(inp
<http://fxr.watson.org/fxr/source/netinet/ident?v=FREEBSD82;i=inp>);
  213 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L213>
                return;
  214 <http://fxr.watson.org/fxr/source/netinet/source/netinet/tcp_timewait.c?v=FREEBSD82#L214>
        }

  ...

  ...


I will really appreciate any feedback on it.

-Suraj