Subject: [homenet] homenet-babel-profile: determining link type

From: "STARK, BARBARA H" <[email-masked]>
Date: Mon, 20 Nov 2017 17:01:18 +0000

Hi Juliusz,
Not as chair....
I have a question about the following requirement:

   REQ6: a Homenet implementation of Babel SHOULD distinguish between
   wired and wireless links ; if it is unable to determine whether a link
   is wired or wireless, it SHOULD make the worst-case hypothesis that
   the link is wireless.  It SHOULD dynamically probe the quality of
   wireless links and derive a suitable metric from its quality
   estimation.  The algorithm described in Appendix A of RFC 6126 MAY be
   used.

Some older powerline technologies perform worse than Wi-Fi. But since powerline is "wired", this requirement suggests it would be preferred.
Also, it's not uncommon to use Wi-Fi to Ethernet or powerline bridges in home networks. A router attached to Ethernet that is subsequently bridged to Wi-Fi would look to the router like a wired link.

Should we really only suggest that the router dynamically probe the quality of wireless links? Or would it make sense to suggest dynamic probing of all links, because assuming the entire path between 2 routers uses a single physical layer technology may not be a good assumption?
Barbara

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From: Sander Steffann <[email-masked]>
Date: Mon, 20 Nov 2017 18:52:55 +0100

Hi Barbara,

> Should we really only suggest that the router dynamically probe the quality of wireless links? Or would it make sense to suggest dynamic probing of all links, because assuming the entire path betwe…

Good point. My gut feeling is that the percentage of cases where that assumption would be wrong is small but significant enough that we should consider probing all links.

Anybody with better (=any) data?

Cheers,
Sander

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From: Juliusz Chroboczek <[email-masked]>
Date: Mon, 20 Nov 2017 21:05:30 +0100

> Some older powerline technologies perform worse than Wi-Fi. But since
> powerline is "wired", this requirement suggests it would be preferred.

Do you have a suggestion for better wording?  I guess we could say
"lossless wired links" and "potentially lossy links".  I'll think about it.

> Also, it's not uncommon to use Wi-Fi to Ethernet or powerline bridges in
> home networks. A router attached to Ethernet that is subsequently
> bridged to Wi-Fi would look to the router like a wired link.

Yes, that's a problem for Babel in general, not just for Homenet.  It is
impossible to reliably determine the layer-2 topology.

There are two factors that mitigate the issue:

 1. usually, there is a wireless bridge on just one side of a link; if the
    link is being treated as wireless on the other side, we still end up
    with a reasonable metric;
 2. powerline links are not usually laid up in places where they are
    redundant; if there's a powerline, it's the only path, and so the
    metric doesn't matter in the first place.

> Should we really only suggest that the router dynamically probe the
> quality of wireless links?

We only have implementation experience with three categories of links --
lossless, wireless, and tunnels.  If we are to suggest a strategy for
powerline, we need to do more research.  We also need evidence that it
makes a difference.

> Or would it make sense to suggest dynamic probing of all links, because
> assuming the entire path between 2 routers uses a single physical layer
> technology may not be a good assumption?

Link-quality estimation slows down convergence, so I think we should only
suggest it where it makes a difference.  For it to make a difference, you
need to satisfy two conditions: (1) variable quality links that can be
measured and (2) sufficiently diverse paths so that the metric can make
you choose different paths.  It's easy to get the two to happen with
wireless links, much more difficult with powerline.

If you have evidence otherwise, I'm interested.

-- Juliusz

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From: Michael Richardson <[email-masked]>
Date: Mon, 20 Nov 2017 20:02:37 -0500

STARK, BARBARA H <[email-masked]> wrote:
    > quality of wireless links? Or would it make sense to suggest dynamic
    > probing of all links, because assuming the entire path between 2
    > routers uses a single physical layer technology may not be a good
    > assumption?

I agree that is should probe all interfaces!

Some wires might be better than others; assume use of random joiners between
cat3,cat5,cat6 and chicken wire in the home.

(a booth at N+I back in ~2000 or something had a very nice GbE over barbed wire demo)

--
Michael Richardson <[email-masked]>, Sandelman Software Works
 -= IPv6 IoT consulting =-

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From: Juliusz Chroboczek <[email-masked]>
Date: Tue, 21 Nov 2017 13:29:30 +0100

> I agree that is should probe all interfaces!

Then you need to define suitable algorithms for non-WiFi interfaces.

I'm open to collaboration, of course, since dynamically computed metrics
is something that's close to my heart.  (Somebody promised to send me some
G.hn hardware at some point, but they must have forgotten.)

> cat3,cat5,cat6 and chicken wire in the home.

Babeld's wireless link quality estimation triggers around 5% packet loss;
on WiFi, it only works because we're careful to send frames that are not
protected by ARQ -- after ARQ, the loss rate is way below that, even on
dodgy links.

You're not going to get loss rates that over chicken wire (unless you've
got really bad quality chicken wire).

> (a booth at N+I back in ~2000 or something had a very nice GbE over

Somewhat charged politically ;-)

-- Juliusz