Subject: LLMNR Issue 69 (AD Review): On-Link

From: Bernard Aboba <[email-masked]>
Date: Thu, 2 Sep 2004 10:42:16 -0700 (PDT)

The list of current LLMNR issues and resolutions is found here:
http://www.drizzle.com/~aboba/DNSEXT/llmnrissues.html

The text of Issue 69 is enclosed below.

DISCUSSION

Other than in the definition in Section 1.2,
the term "reachable" is only used once in the document,
in Section 2.6 (Responder responsibilities):

[b] If an IPv4 address is returned, it MUST be reachable
through the link over which LLMNR is used.

Here the issue is whether a responder may return a particular
IPv4 address in a response. For that purpose it is not
relevant whether an ARP or NS/ND cache entry exists for
that address on the sender or responder. It is only
relevant whether the responder would respond to an
ARP or ND query for that address on the interface over
which the LLMNR query arrived.

PROPOSED RESOLUTION

The proposed resolution is as follows:

In Section 1.2, change:

Reachable
An address is considered reachable over a link if either an ARP or
neighbor discovery cache entry exists for the address on the link.

To:

Reachable
An LLMNR responder considers one of its addresses reachable
over a link if it will respond to an ARP or Neighbor Discovery
query for that address sent over the link.

In Section 2.5, change:

" For IPv4, an "on link" address is defined as a link-local address
[IPv4Link] or an address whose prefix belongs to a subnet on the
local link. For IPv6 [RFC2460] an "on link" address is either a
link-local address, defined in [RFC2373], or an address whose prefix
belongs to a subnet on the local link."

To:

" For IPv4, an "on link" address is defined as a link-local address
[IPv4Link] or an address whose prefix belongs to a subnet on the
local link. For IPv6 [RFC2460] an "on link" address is either a
link-local address, defined in [RFC2373], or one belonging to a prefix
that a Router Advertisement indicates is on-link [RFC2461]."

Add a normative reference to [RFC2461]:

Narten, T., Nordmark, E. and W. Simpson,
"Neighbor Discovery for IP Version 6 (IPv6), RFC 2461, December 1998.

------------------------------------------------------------------------
Issue 69: On-Link
Submitter: Thomas Narten
Submitter email address: [email-masked]
Date first submitted: August 27, 2004
Reference:
Document: LLMNR-34
Comment type: T
Priority: S
Section:  Various
Rationale/Explanation of issue:

> Reachable
> An address is considered reachable over a link if either an ARP or
> neighbor discovery cache entry exists for the address on the link.

funny definition, since an ARP/ND entry might exists but be very old.

> For IPv4, an "on link" address is defined as a link-local address
> [IPv4Link] or an address whose prefix belongs to a subnet on the
> local link. For IPv6 [RFC2460] an "on link" address is either a
> link-local address, defined in [RFC2373], or an address whose prefix
> belongs to a subnet on the local link.

Actually 2461 has a very specific definition of on-link. It's one
where the RA says the prefix is "on link". This may in some cases be
different than what is stated above. There maybe a subtle limitation
here. 2461 allows routers to say "nothing is on link, send it to me",
even if it then forwards stuff back onto the same link. This feature
is not being used today, but might be used in the future.

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