Re: issue 11 -- window size

Hi Pasi,

>- Current path (A1,B1) stops working, and the peers switch 
>  to (A2,B2). A is behind a NAT in both cases.
>
>- After a while (A2,B2) still works (and continues working), but
>  the peers really prefer (A1,B1) because it's faster/cheaper/
>  whatever. So, A sends the last request using addresses (A1,B1) 
>  to see if it has become operational again.
>
>- Suppose that direction A1->B1 works, but B1->A1 does not
>  (e.g. because of a malfunctioning middlebox or something; the
>  cause does not really matter). B receives the packet, and if
>  it hasn't seen it yet, processes it, sends reply, and (if it
>  implements the NAT-T "SHOULD" from IKEv2) moves all traffic to 
>  path (A1,B1). Which does not work.
>  
>
OK, I see the problem now. It looks like there are
sevaral subtasks in what we are trying to do:

  (i) test
 (ii) commit to particular set of addresses
(iii) manage unidirectional connectivity (#19)

Tero's model, as I understand it, appears to combine
both (i) and (ii) as well as disallow unidirectional
connectivity. In a sense his model is less flexible, but
at the same time simpler. A good tradeoff? Lets think
about it.

Anyway, presumably you could modify Tero's proposal
to separate (i) and (ii). The drawback in that is that
if there's a change in connectivity between initiating
(ii) and completing it, you have no possibility to send a
test message -- only a commit message. This is because
the model requires sequenced delivery of all messages.

But what exactly is the problem that results from this?
Presumably you wanted to commit to a particular
set of addresses. So the only reason why you might
want to start testing is that you don't get a response.
If you have just one alternative left, then there's no
issue -- switch to it, with commit, if you can. But if there
are multiple alternatives, then you might wish to test
which one to choose. Question: could you do this by
ordering the retransmissions of the commit message
in a proper way? (Or am I now in the area of trying to
fix the special cases that you Pasi were worried about?)

--Jari