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Re: on-link and off-link addresses, side discussion

Simon Hobson <linux@thehobsons.co.uk> Tue, 06 July 2021 16:32 UTC

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Subject: Re: on-link and off-link addresses, side discussion
From: Simon Hobson <linux@thehobsons.co.uk>
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Date: Tue, 06 Jul 2021 17:32:06 +0100
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Alexandre Petrescu <alexandre.petrescu@gmail.com> wrote:

>> I suggest you look at it from a different angle. "on-link address" is
>> defined as an address that is assigned to an interface on a specified
>> link.
> 
> But, each address is on a link.
> 
> On-link means the address is on this link that we are assuming commonly.
> The term off-link means that the address is not on this link we assume
> commonly, but is on-link on another link.

Isn't it simpler than that ?
On link == hosts can directly contact other hosts in the prefix via this link
Not on link == hosts can only contact other hosts in the prefix via one or more intermediaries

Intermediary in this case is quite flexible and could include routing packets or mechanisms like proxy ND

I think any more is just making things complicated. If the link is of a type (e.g. unfiltered ethernet) where neighbours can find each other (e.g. using ND), then a prefix can be on-link. Otherwise (non-broadcast link technology, security filtering, etc), prefixes will need to be off-link.


OK, so what have I missed here ?

Simon

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