Re: Thoughts about issues 10/19: changing address vs. paths, and so on...

 In your previous mail you wrote:

   During the meeting today, Francis pointed out another possibly
   option not really covered by 1-4.
   
=> thanks Pasi, you caught my idea.

   Option 5: "Peers decide the source addresses independently"
   
      Host A                           Host B
      ------                           -------
   
      List_A = (decided by A) -->
   
                                   <-- List_B = (decided by B)
   
      dst = List_B[1]                  dst = List_A[1]
      src = f_A(List_A, List_B,        src = f_B(List_B, List_A,
                local preferences,               local preferences,
                local info about                 local info about
                connectivity,                    connectivity,
                whatever)                        whatever)
   
=> note that f_A is the application of RFC 3484 so it is usually
predictable and always deterministic. Aniother point is this makes
IKE behavior "standard" in the common case.

   Pros and cons:
   + Reasonably simple
   - No guarantee A-to-B and B-to-A traffic choose the same traffic 
     pair (which complicates things with stateful packet filters)

=> no guaranteed but in usual cases easy to enforce.

   - Host A can't move the "upstream" traffic to some other interface
     without co-operation from B (if we have partial connectivity). 
     For instance, if Host A has one IPv4 interface and one IPv6 interface, 
     A can't move the traffic to the IPv6 interface unless it somehow 
     convinces B to reorder List_B: how does this happen? (And if the
     answer is that List_B depends on List_A which depends on List B,
     how does this work?)

=> I agree: as the idea is to control downstream traffic only nothing
is available directly about upstream traffic.

   - The same thing also applies to failures in the middle: even
     if both parties' addresses and preferences stay the same,
     somehow they must reorder either List_A, List_B, or both.
   
   (Hmm, I think that any option that is fully symmetric will have 
   these problems.. but there are certainly options that are 
   less asymmetric than option 3.)
   
=> I agree but we should find what is the minimal thing we need.
IMHO control on the destination of downstream traffic is mandatory
when an address can stop to work (i.e., for mobility)... and I can't
see something else.

Thanks!

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