Re: on-link and off-link

Alexandre Petrescu <> Mon, 12 July 2021 14:05 UTC

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Subject: Re: on-link and off-link
To: "Wes Beebee (wbeebee)" <>, Jen Linkova <>
Cc: 6man <>
References: <> <> <> <> <>
From: Alexandre Petrescu <>
Message-ID: <>
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.


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.
> - Wes
> On 7/1/21, 6:47 PM, "ipv6 on behalf of Jen Linkova" 
> < on behalf of> wrote:
> On Fri, Jul 2, 2021 at 12:05 AM Alexandre Petrescu 
> <> 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
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