Re: on-link and off-link
Alexandre Petrescu <alexandre.petrescu@gmail.com> Mon, 12 July 2021 14:05 UTC
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Subject: Re: on-link and off-link
To: "Wes Beebee (wbeebee)" <wbeebee@cisco.com>, Jen Linkova <furry13@gmail.com>
Cc: 6man <ipv6@ietf.org>
References: <162512790860.6559.14490468072475126698@ietfa.amsl.com> <CAFU7BAT0O9nsuhs5FyNjvPfRKY+EM1fLYKaMYTwaPg2QjZAEpA@mail.gmail.com> <61e14cd7-ff37-5380-e547-8a9b6d3993da@gmail.com> <CAFU7BASF-vas+PP2dVNXuqScQArC+joB-fwRGzG3UZnsqq1QJg@mail.gmail.com> <6A22DCF6-5132-44D3-AB45-E9C151376D2C@cisco.com>
From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
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Date: Mon, 12 Jul 2021 16:04:58 +0200
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I think one question might be: do we agree that each IP address is assigned on an interface, and thus is on-link on a particular link? For my part, I think yes, each IP address is assigned to an interface and is on-link on a particular link. One might also wonder: are only IPv6 addresses _not_ associated automatically with an IPv6 'on-link' prefixes? For my part, I think that IPv4 addresses too are _not_ associated automatically with an IPv4 'on-link' prefix. For this reason, I dont think RFC5942 is right, right from the beginning. Because it says: > This document [RFC5942] spells out the most important difference: > that an IPv6 address isn't automatically associated with an IPv6 > on-link prefix. IPv4 addresses too can be _not_ associated with IPv4 on-link prefix. Further, an IPv6 address not derived by SLAAC (e.g. manually assigned) _is_ associated with an 'on-link' prefix (the /128 covering the address entirely) even though it is not the prefix present in the PIO in RA which has this 'on-link' flag set. Then, if I manually add this address in the Neighbour Cache of the router offering access, then certainly there is on-link determination for that address. Alex Le 12/07/2021 à 15:18, Wes Beebee (wbeebee) a écrit : > For a more complete discussion of on/off-link, please refer to "IPv6 > Subnet Model: The Relationship between Links and Subnet Prefixes", > RFC 5942. https://datatracker.ietf.org/doc/html/rfc5942 > > - Wes > > On 7/1/21, 6:47 PM, "ipv6 on behalf of Jen Linkova" > <ipv6-bounces@ietf.org on behalf of furry13@gmail.com> wrote: > > On Fri, Jul 2, 2021 at 12:05 AM Alexandre Petrescu > <alexandre.petrescu@gmail.com> wrote: >> Aside from the conversation above that I agree with, and aside of >> the fact that I agree that RFC 4861 does define the term >> "off-link" too by opposing it to a better defined "on-link" term, I >> must say that I never used or heard in practice people talking >> about off-link or on-link addresses. > > Well...I guess our experiences vary significantly. I hear those terms > a lot and I do use them all the time. > >> If one wants to talk about off-link or on-link addresses then one >> talks about neighbors or not neighbors. > > Well, strictly speaking, neighbor is a *node* attached to the same > link. Being on-link or off-link is a property (of an address, for > example). > >> We around me usually talk casually about link-local addresses >> (adresses 'lien', fr.), but we never talk about off-link or >> on-link addresses. > > I find the on-link/off-link terms very useful because in my work I > have to distinguish between "communication with neighbors" and > :communication with the rest of the network, via routers". Terms > like "intra-VLAN"/"inter-VLAN" are too topology-specific etc. > >> Then, there is the use of the term 'on-link' in RFC4861 which is at >> times glued to 'prefixes', even though it is formally defined to >> mean 'addresses'. For example, a full expansion of this RFC4861 >> text: "These options specify the prefixes that are on-link" would >> actually mean "These options specify the prefixes that are >> on-link, i.e. addresses assigned to an interface on a specified >> link" which is somehow difficult to understand. >> >> The most confusing is probably the expansion of this text: "L 1-bit >> on-link flag." which, when expanded, it would mean "L 1-bit on-link >> addresses flag" when it is, in fact, a flag about prefixes in PIOs, >> and not about addresses. These prefixes are often used for other >> operations than just forming addresses. It is thus difficult to >> grasp. > > 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. There are different ways to indicate that the address > is on-link and one of them is "the address is covered by an on-link > prefix, e,g, as indicated by the on-link flag in the Prefix > Information option". L bit just indicates that addresses covered by > the prefix shall be considered on-link. That's it. > >> This 'on-link' and 'off-link' discussion relates a lot to the >> difficulties we have in suggesting at IETF that a new extension is >> needed to tell that a prefix advertised on a link might not be >> for that link to be used for SLAAC, but for putting in a routing >> table entry. A little bit similar to RFC4191's RIOs. > > OK, disclaimer: I'm writing this before my first coffee...but...L > flag has nothing to do with SLAAC, A flag is used for that. L=1, A=0 > would just mean 'addresses on that prefix are on-link but do not use > the prefix for auto-configuration'. > > Smth like: > > 2001:db8:1::/64----node1-------node2 If node2 receives a PIO for > 2001:db8:1::/64 with L=1, A=0 it would assume that 2001:db8:1::f00, > for example, is on-link and would try to resolve its link-layer > address using ND. If node1 acts as an ND proxy, it would work. > >> But when told that the RIO of RFC4191 might be appropriate for the >> V2V case that I needed I always reply that what we need is an RIO >> that is always outside the link (I dont use the term 'off-link'), >> and always at least 2-hops away, never 1-hop away. SO there I >> dont use either the on/off-link terms. > > Sorry, I've not been following that discussion. Wouldn't "L=1, A=0 + > ND proxy" do what you want? > > -- SY, Jen Linkova aka Furry > > -------------------------------------------------------------------- > IETF IPv6 working group mailing list ipv6@ietf.org Administrative > Requests: https://www.ietf.org/mailman/listinfo/ipv6 > -------------------------------------------------------------------- >
- 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