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Re: TCP behavior across WiFi pointers ?

Yuchung Cheng <ycheng@google.com> Wed, 29 November 2017 21:59 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Wed, 29 Nov 2017 13:58:28 -0800
Message-ID: <CAK6E8=dYVxrQDr_eMDO7xQ9k2KNK+s3pFm_AJU=7gp02HbTQ=Q@mail.gmail.com>
Subject: Re: TCP behavior across WiFi pointers ?
To: Mikael Abrahamsson <swmike@swm.pp.se>
Cc: Toerless Eckert <tte+ietf@cs.fau.de>, "tsv-area@ietf.org >> tsv-area@ietf.org" <tsv-area@ietf.org>
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On Tue, Nov 28, 2017 at 9:17 PM, Mikael Abrahamsson <swmike@swm.pp.se>
wrote:

> On Wed, 8 Nov 2017, Toerless Eckert wrote:
>
> I am primarily thinking that there could be a higher demand for
>> TCP (end-to-end) retransmissions when using WiFi because the L2/WiFi
>> local retransmissions are insufficient. And if so, what the
>> characteristics
>>
>
> I don't know of any work you're asking for, but from my own experience
> radio networks (wifi + cellular) has the following (general)
> characteristics:
>
> They deliver packets in-order per host.
>
My experience is different. It is more similar to this post
https://www.ietf.org/mail-archive/web/rmcat/current/msg00815.html



>
> The try very very hard to deliver all packets (true for unicast only on
> wifi), using L1/L2 retransmits.
>
> Because of this, they sometimes "stall" shorter or longer times, so you
> might get latency spikes of hundreds of milliseconds that are gone in the
> next second. My personal record is 180 SECONDS of RTT on a 2G network.
>
> These latency spikes might cause TCP to believe there was packet loss and
> cause retransmits, where there instead "only" was packet delay.
>
> Radio networks have airtime schedulers, so a single packet and a train of
> packets might have very different network experience. Some network types
> send multiple packets in a single transmit opportunity, and these transmit
> opportunities might be tens of milliseconds apart.
>
> --
> Mikael Abrahamsson    email: swmike@swm.pp.se
>
>

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