Re: Robert Wilton's No Objection on draft-ietf-6man-grand-06: (with COMMENT)

Jen Linkova <> Thu, 01 July 2021 22:46 UTC

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From: Jen Linkova <>
Date: Fri, 02 Jul 2021 08:46:15 +1000
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Subject: Re: Robert Wilton's No Objection on draft-ietf-6man-grand-06: (with COMMENT)
To: Alexandre Petrescu <>
Cc: 6man <>
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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

> 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:

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