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

Juliusz Chroboczek <jch@irif.fr> Mon, 20 November 2017 20:05 UTC

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Date: Mon, 20 Nov 2017 21:05:30 +0100
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From: Juliusz Chroboczek <jch@irif.fr>
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Subject: Re: [homenet] homenet-babel-profile: determining link type
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>    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.

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