Subject: RA and Default routes?

From: Markku Savela <[email-masked]>
Date: Fri, 16 Feb 2001 16:20:40 +0200

While implement Host side of RA, I suddently noticed that my code
makes no difference for

   RA with Router Lifetime = 600
and
   RA with Prefix ::0/0, L=0, A=0, Lifetime = 600

Both create effectively a default route pointing to this router. Am I
doing something wrong? A default route could be deleted by having a
prefix option ::0/0 with lifetime = 0.

Additionally in RFC-2461, 6.3.5 says...

  Whenever the Lifetime of an entry in the Default Router List
  expires, that entry is discarded.  When removing a router from the
  Default Router list, the node MUST update the Destination Cache in
  such a way that all entries using the router perform next-hop
  determination again rather than continue sending traffic to the
  (deleted) router.

Suppose we had RA with

  Router Lifetime = 600
  Prefix X/48, L=0, A=0, Lifetime = 1200

The way 6.3.5 is written, seems to say that both ::0/0 and X/48 should
be removed. However, I think only default route should exipire, but
router continues to be available for X/48 still (another 600 secods).

The same question arises also, when router sends RA with Router
Lifetime=0. Should only default route disappear or all routes?

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---

From: Francis Dupont <[email-masked]>
Date: Fri, 16 Feb 2001 15:58:52 +0100

In your previous mail you wrote:

   
   doing something wrong?

=> perhaps, Prefix ::0/0 is an illegal prefix...
   
   Suppose we had RA with
   
     Router Lifetime = 600
   
   be removed.

=> you mix up router lifetime (which is in fact the lifetime as a default
router) and prefix lifetimes (which are the lifetimes of the prefix).
The router is removed from the default router list when *its* lifetime
is expired, this should have no effect on the prefix list.
   
   Lifetime=0.

=> this *only* means the router is not suitable as a default router,
nothing more.

Regards

[email-masked]

---

From: Markku Savela <[email-masked]>
Date: Fri, 16 Feb 2001 17:28:56 +0200

> is expired, this should have no effect on the prefix list.

Actually, I had the misunderstanding that RA prefix option with L=0
and A=0 was a useful way for router to inform, that it is willing to
route the matching addresses. I though this was a nice feature...

So, if I see prefix option with L=0 and A=0, it should be totally
ignored? (and not give the useful semantics to it?).

---

From: Erik Nordmark <[email-masked]>
Date: Fri, 16 Feb 2001 19:46:44 +0100 (CET)

> ignored? (and not give the useful semantics to it?).

Yes, such a prefix doesn't carry any semantics for Neighbor discovery
and stateless addrconf purposes.

But I believe Mobile IPv6 defines a 3rd flag for the prefix options (R)
thus one might see prefixes with L=0, A=0, R=1. And there might be
other future additions of flags,

  Erik