Re: about violation of standards

[神明達哉 <[e-mail]>]

On Thu, Apr 18, 2019 at 11:59 AM Alexandre Petrescu <
[e-mail]> wrote:

>    In private conversation this debate happened:

>    is an implementation that uses fe80:1::2 address on an interface a
>    violation of standards? (RFC 4291 does not allow for '1' to be there).

>    My point of view is that as long as that mplementation is widely used,
>    that is not a violation of standards.  Rather, the situation makes it
>    that that standard is not in agreement with implementations.

This logic doesn't make sense to me at all.  There was a very widely used
implementation of commercial router (I don't name it as it's not my
purpose to pick a particular vendor) that forwarded an IPv6 packet
whose source address is link-local from one link to another link,
instead of returning an ICMPv6 destination unreachable error, code 2,
as specified in RFC4443.  According to that logic, this implementation
would be considered not violating the RFC "because it's widely used";
most people call it an implementation bug.

Regarding Linux, I'd note that link-local addresses are automatically
generated by the system, and the generated address conforms to the
format specified in Section 2.5.6 of RFC4291.  More specifically, its
intermediate 54 bits are all set to 0.  Plus, as far as I know, the
vast majority of people never bother to change the auto-generated
link-local addresses.  In that sense the use of addresses like
"fe80:1::2" are not really widely used, even if the implementation
that allows its users to manually configure such addresses is widely
used.

Almost any implementation has some weapon that allows its user to
shoot their feet, often violating protocol standards.  An extreme case
is a tool like bpf, with which you can send out almost any broken
packets to the wire.  BPF is widely used tools, but as far as I know
no one uses the existence of that tool to justify the violation of the
standard.

Now, I'm open to the discussion of possibly updating RFC4291 to allow
non-0 value in the intermediate 54-bit field, starting from the fact
that it currently violates the standard.  But I don't buy an argument
that a behavior against the current standard is not a violation simply
because there's a system utility of a widely used OS that allows that
particular behavior.

--
JINMEI, Tatuya