On 6/28/18, 1:48 PM, "netdev-ow...@vger.kernel.org on behalf of Neal Cardwell"
<netdev-ow...@vger.kernel.org on behalf of ncardw...@google.com> wrote:
On Thu, Jun 28, 2018 at 4:20 PM Lawrence Brakmo <bra...@fb.com> wrote:
>
> I just looked at 4.18 traces and the behavior is as follows:
>
> Host A sends the last packets of the request
>
> Host B receives them, and the last packet is marked with congestion
(CE)
>
> Host B sends ACKs for packets not marked with congestion
>
> Host B sends data packet with reply and ACK for packet marked with
congestion (TCP flag ECE)
>
> Host A receives ACKs with no ECE flag
>
> Host A receives data packet with ACK for the last packet of request
and has TCP ECE bit set
>
> Host A sends 1st data packet of the next request with TCP flag CWR
>
> Host B receives the packet (as seen in tcpdump at B), no CE flag
>
> Host B sends a dup ACK that also has the TCP ECE flag
>
> Host A RTO timer fires!
>
> Host A to send the next packet
>
> Host A receives an ACK for everything it has sent (i.e. Host B did
receive 1st packet of request)
>
> Host A send more packets…
Thanks, Larry! This is very interesting. I don't know the cause, but
this reminds me of an issue Steve Ibanez raised on the netdev list
last December, where he was seeing cases with DCTCP where a CWR packet
would be received and buffered by Host B but not ACKed by Host B. This
was the thread "Re: Linux ECN Handling", starting around December 5. I
have cc-ed Steve.
I wonder if this may somehow be related to the DCTCP logic to rewind
tp->rcv_nxt and call tcp_send_ack(), and then restore tp->rcv_nxt, if
DCTCP notices that the incoming CE bits have been changed while the
receiver thinks it is holding on to a delayed ACK (in
dctcp_ce_state_0_to_1() and dctcp_ce_state_1_to_0()). I wonder if the
"synthetic" call to tcp_send_ack() somehow has side effects in the
delayed ACK state machine that can cause the connection to forget that
it still needs to fire a delayed ACK, even though it just sent an ACK
just now.
neal
You were right Neal, one of the bugs is related to this and is caused by a lack
of state update to DCTCP. DCTCP is first informed that the ACK was delayed but
it is not updated when the ACK is sent with a data packet.
I am working on a patch to fix this which hopefully should be out today. Thanks
everyone for the great feedback!
