Re: on-link and off-link addresses, side discussion
Simon Hobson <linux@thehobsons.co.uk> Fri, 09 July 2021 15:36 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: Fri, 09 Jul 2021 16:36:16 +0100
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Philip Homburg <pch-ipv6-ietf-7@u-1.phicoh.com> wrote: > >> Further, the definition says that "a node considers an address to >> be on-link it is covered by a particular prefix". That operation, >> 'being covered by', is not specified. >> >> One might think of the use of a longest prefiix match algorithm. >> >> A longest prefix match algorithm is defined in that RFC to be used >> to determine which prefix (from a set of prefixes) 'covers' an >> address. >> >> On another hand, the definition of 'on-link' says it wants to >> identify whether a _particular_ prefix 'covers' an address (not to >> try to identify whether an address matches better one prefix or >> another). That is a distinct algorithm from the longest prefix >> match algorithm. >> >> To give an example: by using a longest-prefix-match algorithm in >> a normal way (identify which prefix out of a set of prefixes matches >> better one particular address) one will always 'cover' an address >> with almost any prefix. All allocated prefixes start with this >> '000' triple bits. So they all cover any address; but in that >> 'cover' operation one specifically adds that it covers up to 3 >> bits. The 'cover' operation is successful, but just up to 3. > > I'm confused about what you are trying to say. Thanks, I was beginning to wonder if it was just me ! > The way I understand it is as follows: > An RA may contain a prefix that has the onlink bit set. The interface > is implicitly defined as the interface over which the RA arrives at the > host, so there is no need to make this explicit. Information in an RA only > applies to the interface over which the RA arrives. That was my interpretation as well. > Then 'covered by the prefix'. Covered by the prefix 2001:db8::/80 means that > if the first 80 bits of an address are the same as the first 80 bits of the > prefix, then the address is said to be covered by the prefix. For onlink, > there is no 64-bit boundary. Any prefix length should work. The prefix > length determines how many bits are compared to decide on a match. Agreed. > Then, in theory there could be overlapping prefixes, some might be onlink, > some might be not onlink. So in theory we could define a longest matching rule. > However, this leads to code complexity and there don't seem to be any > practical use cases for overlapping prefixes, so leaving this undefined > and assuming that routers do not generate RAs with overlapping prefixes > seems fine. I had assumed (yes I know, that's dangerous) that overlapping prefixes would be as allowed as overlapping IPv4 subnets - as in not allowed. It would at the very least be rather poor network design since it would create completely avoidable issues like you described. Simon
- Robert Wilton's No Objection on draft-ietf-6man-g… Robert Wilton via Datatracker
- Re: Robert Wilton's No Objection on draft-ietf-6m… Jen Linkova
- RE: Robert Wilton's No Objection on draft-ietf-6m… Rob Wilton (rwilton)
- Re: Robert Wilton's No Objection on draft-ietf-6m… Alexandre Petrescu
- Re: Robert Wilton's No Objection on draft-ietf-6m… Jen Linkova
- on-link and off-link addresses, side discussion Alexandre Petrescu
- Re: on-link and off-link addresses, side discussi… Simon Hobson
- Re: on-link and off-link addresses, side discussi… Alexandre Petrescu
- Re: on-link and off-link addresses, side discussi… Philip Homburg
- Re: on-link and off-link addresses, side discussi… Simon Hobson
- Re: Robert Wilton's No Objection on draft-ietf-6m… Wes Beebee (wbeebee)
- Re: why "is /64 [a] "default" prefix length when … Alexandre Petrescu
- Re: on-link and off-link Alexandre Petrescu
- Re: on-link and off-link Philip Homburg
- RE: on-link and off-link Vasilenko Eduard
- RE: on-link and off-link Pascal Thubert (pthubert)
- RE: on-link and off-link Vasilenko Eduard
- RE: on-link and off-link Pascal Thubert (pthubert)
- Re: why "is /64 [a] "default" prefix length when … Wes Beebee (wbeebee)
- Re: on-link and off-link Wes Beebee (wbeebee)
- RE: on-link and off-link Vasilenko Eduard