Re: [homenet] DNCP/HNCP Revisited

Juliusz Chroboczek <jch@irif.fr> Sun, 22 September 2019 12:10 UTC

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Date: Sun, 22 Sep 2019 14:10:39 +0200
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From: Juliusz Chroboczek <jch@irif.fr>
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Subject: Re: [homenet] DNCP/HNCP Revisited
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> AFAICS there's certainly enough room for me to experiment with patches to
> i) reduce MTU to avoid problems arising from L2 MTU mismatches and
> ii) to reduce the amount of fragments (at the expense of more UDP packets)
> without any tweaking of the standards.

In case you're interested, here's the code that decides whether to
aggregate or to ship out in shncpd:

  https://github.com/jech/shncpd/blob/master/send.c#L99

In short -- we try to limit our payload to 1412 bytes whenever possible,
within the constraints of not being able to fragment large TLVs.  (This
should depend on the outgoing interface's MTU, as in Babel, but I decided
I'm lazy and just hardwired the constant.)

> My reason for investigating ii) is to potentially reduce the impact of
> the loss of an individual frame on a lossy link (we would lose 1 node
> status TLV from 1 device rather than multiple TLV's related to multiple
> devices).

Recall, however, that 802.11 has absolutely horrific per-frame overhead.
Recall further that HNCP does flooding, so that if there are parallel
lossy and lossless links, packet loss on the lossy link doesn't matter as
long as the data gets correctly flooded over the lossless link.

In short, there is a non-trivial tradeoff here.

-- Juliusz