Can lower layer tell there's connectivity? (Was: Re: New issue 16: No packets from other end?)

Vijay Devarapalli wrote:

>> - In general, lower-layer mechanisms cannot tell whether pair
>>   (Ai,Bj) provides connectivity between the IKEv2 endpoints:
>>   only an IKEv2 message exchange can tell that.
> 
> totally disagree with this particular statement. assuming the IP
> stack is the lower layer (with might include MIP6/MULTI6), why
> would MOBIKE be able to figure out something the IP stack couldnt?
> 
> UDP is a transport protocol on top of IP, right?

There appears to be several questions here. What Pasi is stating
need to made more specific before we can find agreement.
I think we are talking about:

(1) Difference wrt IP layer connectivity and the ability to
     talk between the IKEv2 peers.

     There is indeed a difference, the IKEv2 module could
     suddenly die... or the client could have been disconnected
     and the address recycled to someone else.

     Personally, I'm not sure this is worth covering. IP
     connectivity should be enough. But see (2) below.

(2) Ensuring that a given address really is the same
     node. When probing for the correctness of a claimed
     address, we may want to ensure that its indeed the
     same IKEv2 node at this address.

     It seems that the only way to do this is an IKEv2
     message. But note that we are talking about address
     update, not failure recovery here. So failure recovery
     might still work using only IP layer connectivity
     tests.

(3) What current lower layers are able to tell to MOBIKE.
     This is what I have been saying, that as far as I know,
     current lower layer support in IPv6, DNA, DHC, etc.
     only provides support for the discovery of available
     addresses and locally operational addresses. Not the
     discovery operational address pairs.

     I'm not saying that it wouldn't be possible to develop
     one, just that one doesn't exist now. But I could be
     missing something, I'm sure you guys will tell me what...

--Jari