netdev - Re: [PATCH 0/4] Make tcp-metrics source-address aware

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Date:	Mon, 16 Dec 2013 11:53:31 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Christoph Paasch <christoph.paasch@...ouvain.be>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Julian Anastasov <ja@....bg>
Subject: Re: [PATCH 0/4] Make tcp-metrics source-address aware

On Mon, Dec 16, 2013 at 11:30 AM, Christoph Paasch
<christoph.paasch@...ouvain.be> wrote:
> On 16/12/13 - 10:45:13, Yuchung Cheng wrote:
>> On Sun, Dec 15, 2013 at 10:40 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Sun, 2013-12-15 at 13:10 +0100, Christoph Paasch wrote:
>> >> Currently tcp-metrics only stores per-destination addresses. This brings
>> >> problems, when a host has multiple interfaces (e.g., a smartphone having
>> >> WiFi/3G):
>> >>
>> >> For example, a host contacting a server over WiFi will store the tcp-metrics
>> >> per destination IP. If then the host contacts the same server over 3G, the
>> >> same tcp-metrics will be used, although the path-characteristics are completly
>> >> different (e.g., the ssthresh is probably not the same).
>> >
>> > ssthresh caching is very problematic anyway.
>> >
>> > hystart is way better to probe the actual capacity, as the real network
>> > conditions change every seconds or so.
>> >
>> >>
>> >> The same holds for the fast-open cookie. The server will generate a cookie
>> >> based on our source-address. So, if we contact the same server with another
>> >> source-IP we should request a new cookie.
>> >
>> > Yuchung, what do you think ? I think this should already be handled
>> > gracefully, as client be behind a NAT device using a pool of external IP
>> > addresses ?
>> Right. Today the Fast Open attempt will fall back to regular TCP
>> gracefully with the new cookie in SYN-ACK. So if the source ip changed
>> when the public IP/nat remain the same (common case?), the proposed
>> change will reduce Fast Open success rate. So it likely has negative
>> impact only.
>
> Agreed, in this case it would be negative. Although I doubt that it happens
> often that the source changes while the public IP remains the same.
>
> But for the WiFi/3G-case it is really bad that parameters like
> ssthresh/rtt/... are kept the same. We will exit slow-start too early when
> going from WiFi to 3G because ssthresh is too low.
>
> What if we copy the fast-open cookie across the different src/dst tcp_metrics-pairs
> to handle Yuchung's case?
> Or, maybe get rid of ssthresh/rtt/... in tcp-metrics? But that would probably be too
> agressive...
+1 to get rid of ssthresh caching. It often hurts than helps and
cubic-hystart already mitigates the SS overshoot. See sec 6.2.4 in a
recent research study
http://conferences.sigcomm.org/co-next/2013/program/p303.pdf

The only case caching RTT could help is the init RTO. But it's unclear
adding src-ip along will help like DM said.

Another metric that may have negative effect is reordering metrics.

>
>
> Cheers,
> Christoph
>
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