Hi Mikael,
> On Nov 29, 2017, at 13:49, Mikael Abrahamsson <swm...@swm.pp.se> wrote:
>
> On Wed, 29 Nov 2017, Sebastian Moeller wrote:
>
>> Well, ACK filtering/thinning is a simple trade-off: redundancy versus
>> bandwidth. Since the RFCs say a receiver should acknoledge every second full
>> MSS I think the decision whether to filter or not should be kept to
>
> Why does it say to do this?
According to RFC 2525:
"2.13.
Name of Problem
Stretch ACK violation
Paxson, et. al. Informational [Page 40]
RFC 2525 TCP Implementation Problems March 1999
Classification
Congestion Control/Performance
Description
To improve efficiency (both computer and network) a data receiver
may refrain from sending an ACK for each incoming segment,
according to [
RFC1122
]. However, an ACK should not be delayed an
inordinate amount of time. Specifically, ACKs SHOULD be sent for
every second full-sized segment that arrives. If a second full-
sized segment does not arrive within a given timeout (of no more
than 0.5 seconds), an ACK should be transmitted, according to
[
RFC1122
]. A TCP receiver which does not generate an ACK for
every second full-sized segment exhibits a "Stretch ACK
Violation".
Significance
TCP receivers exhibiting this behavior will cause TCP senders to
generate burstier traffic, which can degrade performance in
congested environments. In addition, generating fewer ACKs
increases the amount of time needed by the slow start algorithm to
open the congestion window to an appropriate point, which
diminishes performance in environments with large bandwidth-delay
products. Finally, generating fewer ACKs may cause needless
retransmission timeouts in lossy environments, as it increases the
possibility that an entire window of ACKs is lost, forcing a
retransmission timeout.
Implications
When not in loss recovery, every ACK received by a TCP sender
triggers the transmission of new data segments. The burst size is
determined by the number of previously unacknowledged segments
each ACK covers. Therefore, a TCP receiver ack'ing more than 2
segments at a time causes the sending TCP to generate a larger
burst of traffic upon receipt of the ACK. This large burst of
traffic can overwhelm an intervening gateway, leading to higher
drop rates for both the connection and other connections passing
through the congested gateway.
In addition, the TCP slow start algorithm increases the congestion
window by 1 segment for each ACK received. Therefore, increasing
the ACK interval (thus decreasing the rate at which ACKs are
transmitted) increases the amount of time it takes slow start to
increase the congestion window to an appropriate operating point,
and the connection consequently suffers from reduced performance.
This is especially true for connections using large windows.
Relevant RFCs
RFC 1122
outlines delayed ACKs as a recommended mechanism.
Paxson, et. al. Informational [Page 41]
RFC 2525 TCP Implementation Problems March 1999
Trace file demonstrating it
Trace file taken using tcpdump at host B, the data receiver (and
ACK originator). The advertised window (which never changed) and
timestamp options have been omitted for clarity, except for the
first packet sent by A:
12:09:24.820187 A.1174 > B.3999: . 2049:3497(1448) ack 1
win 33580 <nop,nop,timestamp 2249877 2249914> [tos 0x8]
12:09:24.824147 A.1174 > B.3999: . 3497:4945(1448) ack 1
12:09:24.832034 A.1174 > B.3999: . 4945:6393(1448) ack 1
12:09:24.832222 B.3999 > A.1174: . ack 6393
12:09:24.934837 A.1174 > B.3999: . 6393:7841(1448) ack 1
12:09:24.942721 A.1174 > B.3999: . 7841:9289(1448) ack 1
12:09:24.950605 A.1174 > B.3999: . 9289:10737(1448) ack 1
12:09:24.950797 B.3999 > A.1174: . ack 10737
12:09:24.958488 A.1174 > B.3999: . 10737:12185(1448) ack 1
12:09:25.052330 A.1174 > B.3999: . 12185:13633(1448) ack 1
12:09:25.060216 A.1174 > B.3999: . 13633:15081(1448) ack 1
12:09:25.060405 B.3999 > A.1174: . ack 15081
This portion of the trace clearly shows that the receiver (host B)
sends an ACK for every third full sized packet received. Further
investigation of this implementation found that the cause of the
increased ACK interval was the TCP options being used. The
implementation sent an ACK after it was holding 2*MSS worth of
unacknowledged data. In the above case, the MSS is 1460 bytes so
the receiver transmits an ACK after it is holding at least 2920
bytes of unacknowledged data. However, the length of the TCP
options being used [
RFC1323
] took 12 bytes away from the data
portion of each packet. This produced packets containing 1448
bytes of data. But the additional bytes used by the options in
the header were not taken into account when determining when to
trigger an ACK. Therefore, it took 3 data segments before the
data receiver was holding enough unacknowledged data (>= 2*MSS, or
2920 bytes in the above example) to transmit an ACK.
Trace file demonstrating correct behavior
Trace file taken using tcpdump at host B, the data receiver (and
ACK originator), again with window and timestamp information
omitted except for the first packet:
12:06:53.627320 A.1172 > B.3999: . 1449:2897(1448) ack 1
win 33580 <nop,nop,timestamp 2249575 2249612> [tos 0x8]
12:06:53.634773 A.1172 > B.3999: . 2897:4345(1448) ack 1
12:06:53.634961 B.3999 > A.1172: . ack 4345
12:06:53.737326 A.1172 > B.3999: . 4345:5793(1448) ack 1
12:06:53.744401 A.1172 > B.3999: . 5793:7241(1448) ack 1
12:06:53.744592 B.3999 > A.1172: . ack 7241
Paxson, et. al. Informational [Page 42]
RFC 2525 TCP Implementation Problems March 1999
12:06:53.752287 A.1172 > B.3999: . 7241:8689(1448) ack 1
12:06:53.847332 A.1172 > B.3999: . 8689:10137(1448) ack 1
12:06:53.847525 B.3999 > A.1172: . ack 10137
This trace shows the TCP receiver (host B) ack'ing every second
full-sized packet, according to [
RFC1122
]. This is the same
implementation shown above, with slight modifications that allow
the receiver to take the length of the options into account when
deciding when to transmit an ACK."
So I guess the point is that at the rates we are discussing (the the according
short periods between non-filtered ACKs the time-out issue will be moot). The
Slow start issue might also be moot if the sender does more than simple ACK
counting. This leaves redundancy... The fact that GRO/GSO effectively lead to
ack stretching already the disadvantages might not be as bad today (for high
bandwidth flows) than they were in the past...
> What benefit is there to either end system to send 35kPPS of ACKs in order to
> facilitate a 100 megabyte/s of TCP transfer?
>
> Sounds like a lot of useless interrupts and handling by the stack, apart from
> offloading it to the NIC to do a lot of handling of these mostly useless
> packets so the CPU doesn't have to do it.
>
> Why isn't 1kPPS of ACKs sufficient for most usecases?
This is not going to fly, as far as I can tell the ACK rate needs to be
high enough so that its inverse does not exceed the period that is equivalent
to the calculated RTO, so the ACK rate needs to scale with the RTT of a
connection.
But I do not claim to be an expert here, I just had a look at some RFCs that
might or might not be outdated already...
Best Regards
Sebastian
>
> --
> Mikael Abrahamsson email: swm...@swm.pp.se
_______________________________________________
Bloat mailing list
Bloat@lists.bufferbloat.net
https://lists.bufferbloat.net/listinfo/bloat
