1. 14 Jun, 2019 33 commits
  2. 13 Jun, 2019 6 commits
  3. 12 Jun, 2019 1 commit
    • Eric Dumazet's avatar
      tcp: add optional per socket transmit delay · a842fe14
      Eric Dumazet authored
      Adding delays to TCP flows is crucial for studying behavior
      of TCP stacks, including congestion control modules.
      
      Linux offers netem module, but it has unpractical constraints :
      - Need root access to change qdisc
      - Hard to setup on egress if combined with non trivial qdisc like FQ
      - Single delay for all flows.
      
      EDT (Earliest Departure Time) adoption in TCP stack allows us
      to enable a per socket delay at a very small cost.
      
      Networking tools can now establish thousands of flows, each of them
      with a different delay, simulating real world conditions.
      
      This requires FQ packet scheduler or a EDT-enabled NIC.
      
      This patchs adds TCP_TX_DELAY socket option, to set a delay in
      usec units.
      
        unsigned int tx_delay = 10000; /* 10 msec */
      
        setsockopt(fd, SOL_TCP, TCP_TX_DELAY, &tx_delay, sizeof(tx_delay));
      
      Note that FQ packet scheduler limits might need some tweaking :
      
      man tc-fq
      
      PARAMETERS
         limit
             Hard  limit  on  the  real  queue  size. When this limit is
             reached, new packets are dropped. If the value is  lowered,
             packets  are  dropped so that the new limit is met. Default
             is 10000 packets.
      
         flow_limit
             Hard limit on the maximum  number  of  packets  queued  per
             flow.  Default value is 100.
      
      Use of TCP_TX_DELAY option will increase number of skbs in FQ qdisc,
      so packets would be dropped if any of the previous limit is hit.
      
      Use of a jump label makes this support runtime-free, for hosts
      never using the option.
      
      Also note that TSQ (TCP Small Queues) limits are slightly changed
      with this patch : we need to account that skbs artificially delayed
      wont stop us providind more skbs to feed the pipe (netem uses
      skb_orphan_partial() for this purpose, but FQ can not use this trick)
      
      Because of that, using big delays might very well trigger
      old bugs in TSO auto defer logic and/or sndbuf limited detection.
      Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      a842fe14