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

[Alexandre Petrescu <[e-mail]>]

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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