On Tue, Dec 19, 2017 at 12:16 AM, Steve Ibanez <siba...@stanford.edu> wrote:
>
> Hi Neal,
>
> I started looking into this receiver ACKing issue today. Strangely,
> when I tried adding printk statements at the top of the
> tcp_v4_do_rcv(), tcp_rcv_established(), __tcp_ack_snd_check() and
> tcp_send_delayed_ack() functions they were never executed on the
> machine running the iperf3 server (i.e. the destination of the flows).
> Maybe the iperf3 server is using its own TCP stack?
>
> In any case, the ACKing problem is reproducible using just normal
> iperf for which I do see my printk statements being executed. I can
> now confirm that when the CWR marked packet (for which no ACK is sent)
> arrives at the receiver, the __tcp_ack_snd_check() function is never
> invoked; and hence neither is the tcp_send_delayed_ack() function.
> Hopefully this helps narrow down where the issue might be? I started
> adding some printk statements into the tcp_rcv_established() function,
> but I'm not sure where the best places to look would be so I wanted to
> ask your advice on this.
Thanks for the detailed report!
As a next step to narrow down why the CWR-marked packet is not acked,
I would suggest adding printk statements at the bottom of
tcp_rcv_established() in all the spots where we have a goto or return
that would cause us to short-circuit and not reach the
tcp_ack_snd_check() at the bottom of the function. This could be much
like your existing nice debugging printks, that log any time we get to
that spot and if (sk->sk_family == AF_INET && th->cwr). And these
could be in the following spots (marked "here"):
slow_path:
if (len < (th->doff << 2) || tcp_checksum_complete(skb))
goto csum_error; /* <=== here */
if (!th->ack && !th->rst && !th->syn)
goto discard; /* <=== here */
/*
* Standard slow path.
*/
if (!tcp_validate_incoming(sk, skb, th, 1))
return; /* <=== here */
step5:
if (tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT) < 0)
goto discard; /* <=== here */
thanks,
neal
