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

["Wes Beebee (wbeebee)" <[e-mail]>]

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" <[e-mail] on behalf of [e-mail]> wrote:

    On Fri, Jul 2, 2021 at 12:05 AM Alexandre Petrescu
    <[e-mail]> 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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