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
Here is a packetdrill script that reproduces the problem:
// Repro bug that does not ack data, not even with delayed-ack
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
0.000 setsockopt(3, SOL_TCP, TCP_CONGESTION, "dctcp", 5) = 0
0.000 bind(3, ..., ...) = 0
0.000 listen(3, 1) = 0
0.100 < [ect0] SEW 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
0.100 > SE. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 5>
0.110 < [ect0] . 1:1(0) ack 1 win 257
0.200 accept(3, ..., ...) = 4
0.200 < [ect0] . 1:1001(1000) ack 1 win 257
0.200 > [ect0] . 1:1(0) ack 1001
0.200 write(4, ..., 1) = 1
0.200 > [ect0] P. 1:2(1) ack 1001
0.200 < [ect0] . 1001:2001(1000) ack 2 win 257
0.200 write(4, ..., 1) = 1
0.200 > [ect0] P. 2:3(1) ack 2001
0.200 < [ect0] . 2001:3001(1000) ack 3 win 257
0.200 < [ect0] . 3001:4001(1000) ack 3 win 257
0.200 > [ect0] . 3:3(0) ack 4001
0.210 < [ce] P. 4001:4501(500) ack 3 win 257
+0.001 read(4, ..., 4500) = 4500
+0 write(4, ..., 1) = 1
+0 > [ect0] PE. 3:4(1) ack 4501
+0.010 < [ect0] W. 4501:5501(1000) ack 4 win 257
+0 > [ect0] E. 4:4(0) ack 4501 // dup ack sent
+0.311 < [ect0] . 5501:6501(1000) ack 4 win 257 // Long RTO
+0 > [ect0] . 4:4(0) ack 6501 // now acks everything
+0.500 < F. 9501:9501(0) ack 4 win 257
