Re: why "is /64 [a] "default" prefix length when a length isn't specified."

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

Typically it's because EUI-64 has been widely implemented for use in SLAAC - not because having a default is the correct thing to do.

- Wes

On 7/12/21, 9:29 AM, "Alexandre Petrescu" <[e-mail]> wrote:

    Tell me, why 64 is a default in the eyes of an impplementer when a 
    length isn't specified?

    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" <[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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