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Re: IPv6 Link Local Addresses [was Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?]

Fred Baker <fredbaker.ietf@gmail.com> Mon, 10 June 2019 21:06 UTC

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From: Fred Baker <fredbaker.ietf@gmail.com>
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Subject: Re: IPv6 Link Local Addresses [was Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?]
Date: Mon, 10 Jun 2019 14:06:09 -0700
In-Reply-To: <B3D43A45-5E90-4D04-BA64-17150EE6D2AA@gmail.com>
Cc: Bob Hinden <bob.hinden@gmail.com>, IPv6 List <ipv6@ietf.org>
To: Dennis Ferguson <dennis.c.ferguson@gmail.com>
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I think I could argue it either way. Your option #1 assumes that the address is local (an interesting assumption in the context of RFC 4943), which would be consistent with a link-local address, which this address isn't - it's site-local if anything, and site-local is deprecated. Your option #2 assumes that the router knows where to find it, or will drop it if it doesn't, which seems as safe a safe bet as any. The only real question the operator might ask is "why in the world are you sending me these packets?"

If anything, I think I might suggest as a third option an "address whose usage is undefined", and discarding it because you don't know what else to do.

> On Jun 10, 2019, at 11:18 AM, Dennis Ferguson <dennis.c.ferguson@gmail.com> wrote:
> 
> 
>> On Jun 10, 2019, at 12:44, Bob Hinden <bob.hinden@gmail.com> wrote:
>> 
>> This means that link local addresses have a 10 bit prefix (1111111010) followed by 54 bits of zeros.  That is it, nothing more.   Address with different prefixes or with a 1111111010 prefix followed by non-zero 54 bits are not link local addresses.
>> 
>> This is not ambiguous.
> 
> So if an application asks my implementation to send a packet addressed to fe80:1::1 out a particular link what should the implementation do with it? It seems like there are only 3 choices:
> 
>   1. Run ND on that link to see if it can find a neighbor there with that address to send the packet to.
> 
>   2. Send the packet to the current default router.
> 
>   3. Something else (what?).
> 
> I had though 1. was the answer since that is what is done with packets addressed to link-local unicast destinations (which is what that address is according to RFC4191 section 2.4), but that now seems to be ambiguous.
> 
> Dennis Ferguson
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