[tor-bugs] #12890 [Tor]: Design and implement optimizations for socket write limits

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#12890: Design and implement optimizations for socket write limits
-----------------------------+--------------------------------
     Reporter:  robgjansen   |      Owner:  robgjansen
         Type:  enhancement  |     Status:  new
     Priority:  normal       |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor          |    Version:
   Resolution:               |   Keywords:  tor-relay
Actual Points:               |  Parent ID:  #12541
       Points:               |
-----------------------------+--------------------------------
Changes (by robgjansen):

 * milestone:  Tor: 0.2.7.x-final => Tor: 0.2.6.x-final


Comment:

 Currently talking with Nick about the design of this component. We decided
 that this could completely be implemented in a separate thread with no
 feedback to the master thread.

 '''Get the buffer lengths:'''

 {{{
 ioctl(fd, SIOCINQ, &inqlen);
 ioctl(fd, SIOCOUTQ, &outqlen);
 }}}

 This gives us the length (ie, data waiting for io) of the send and receive
 buffer.

 '''Get the TCP info:'''

 {{{
 getsockopt(fd, SOL_TCP, TCP_INFO, (void *)&tcp_info, &tcp_info_length);
 }}}

 That gives us this struct (or something like it):

 {{{
 struct tcp_info {
         __u8    tcpi_state;
         __u8    tcpi_ca_state;
         __u8    tcpi_retransmits;
         __u8    tcpi_probes;
         __u8    tcpi_backoff;
         __u8    tcpi_options;
         __u8    tcpi_snd_wscale : 4, tcpi_rcv_wscale : 4;

         __u32   tcpi_rto;
         __u32   tcpi_ato;
         __u32   tcpi_snd_mss;
         __u32   tcpi_rcv_mss;

         __u32   tcpi_unacked;
         __u32   tcpi_sacked;
         __u32   tcpi_lost;
         __u32   tcpi_retrans;
         __u32   tcpi_fackets;

         /* Times. */
         __u32   tcpi_last_data_sent;
         __u32   tcpi_last_ack_sent;     /* Not remembered, sorry. */
         __u32   tcpi_last_data_recv;
         __u32   tcpi_last_ack_recv;

         /* Metrics. */
         __u32   tcpi_pmtu;
         __u32   tcpi_rcv_ssthresh;
         __u32   tcpi_rtt;
         __u32   tcpi_rttvar;
         __u32   tcpi_snd_ssthresh;
         __u32   tcpi_snd_cwnd;
         __u32   tcpi_advmss;
         __u32   tcpi_reordering;

         __u32   tcpi_rcv_rtt;
         __u32   tcpi_rcv_space;

         __u32   tcpi_total_retrans;
 };
 }}}

 [See here - http://linuxgazette.net/136/pfeiffer.html More info here.]

 '''Then we want setsockopt to set the buffer sizes based on the current
 length and the tcp window:'''

 {{{
 setsockopt(fd, SOL_SOCKET, SO_SNDBUF, (void *)&snd_size, snd_size_len);
 setsockopt(fd, SOL_SOCKET, SO_RCVBUF, (void *)&rcv_size, rcv_size_len);
 }}}

 '''NOTE:''' When setting `SO_SNDBUF` and `SO_RCVBUF` with `setsockopt`,
 the value applied is double the valued passed (it is automatically doubled
 by the kernel).

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12890#comment:5>
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