Subject: question on next-hop determination

From: "Qing Li" <[email-masked]>
Date: Thu, 10 Oct 2002 14:12:12 -0700

In RFC2461, section 5.2, paragraph 2, the last
	sentence is "If the Default Router List is empty,
	the sender assumes that the destination is on-link."

	Why should the destination be considered on-link ??

	-- Qing

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From: Thomas Narten <[email-masked]>
Date: Thu, 10 Oct 2002 21:13:56 -0400

> 	Why should the destination be considered on-link ??

Its a last resort. I.e., assuming on-link may be a better as the final
last step compared with just giving up.

Thomas

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From: "Qing Li" <[email-masked]>
Date: Thu, 10 Oct 2002 19:04:52 -0700

> last step compared with just giving up.

	I think the sending host should just drop the packet when the
	destination is not covered in its prefix list and there is
	no default route installed. Otherwise IMHO this last attempt 
	would lead to erroneous packet generation, and allows 
	misconfigured host or host running a buggy implementation to 
	continue with bad behavior. 

	Also the code put in to supporting this looks and feels 
	like hacks.

	Could you please describe a scenario in which such a situation 
	might occur, and it makes sense to make this final attempt ??

	-- Qing

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From: Erik Nordmark <[email-masked]>
Date: Fri, 11 Oct 2002 15:28:40 +0200 (CEST)

> 	continue with bad behavior. 

Imagine a case when host1 and host2 are on the same link, but
DHCPv6 has given them different /64 prefixes for some reason.
The routers have been configured to not advertise any on-link prefixes
i.e. the hosts send packets to the router and get redirected to
the on-link destination.

The communicate fine as long as the router is around.
When the router fails this communication local to the link would fail, unless
the fallback to treat all destinations as on-link is in place.

A less esoteric example is when the hosts are in the same prefix,
and either no on-link prefix is advertised (no 'O' bit in the prefix
advertisement) or the on-link prefix expires in relatively short time
i.e. the hosts quickly forget which destinations are on-link.

> 	Also the code put in to supporting this looks and feels 

That seems like a comment for the implementors of whatever
implementation you are looking at, and not a comment relating to
the RFC.

  Erik

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From: "Qing Li" <[email-masked]>
Date: Mon, 14 Oct 2002 15:37:30 -0700

> Imagine a case when host1 and host2 are on the same link, but

	In the case where host1 and host2 are on two different 
	links, and the default router had a short lifetime then went 
	down unexpectedly.

	What is the way to differentiating between these 2 cases?
	Or is it necessary to make such distinction?

	Because host1 will treat host2 as on-link so it will 
	generate unnecessary traffic on net1, similarly by 
	host2 on net2. This action applies to all those hosts residing 
	on net1 that want to communicate with hosts on net2, 
	and vice versa. 

	Is this an acceptable behavior? Considering the possible number
	of hosts and applications that may be running, could this be 
	significant enough to have an impact on bandwidth ?

	-- Qing