New issue 19: Same addresses for both directions?

Hi,

The discussion about terminology in the design draft brought 
up a (somewhat) new issue about whether IPsec traffic in both
directions should use the same pair of addresses (in stable
situations), or whether each peer makes the selection for its
outbound traffic independently.

By "stable situations", I mean long periods of time when 
nothing MOBIKE-related is happening (it's clear that when 
traffic is being moved from one pair of addresses to another, 
the peers aren't necessarily synchronized all the time).

Some thoughts about this (more welcome, this is very
preliminary):

- Each peer can have some preferences about, e.g., on 
  which address it wants to receive traffic from the 
  other peer.

- Some information about these preferences can be sent to 
  the other peer in MOBIKE payloads. Current proposals have 
  an ordered list of addresses.

- Each peer thus has its own preferences, and some limited
  information about the other's preferences. But the choice
  is also constrained by what paths happen to work currently.

- It seems that this does not yet uniquely determine the pair of
  addresses that gets used. For instance, assume that peer A
  prefers address A1 to A2, and peer B prefers address B1 to B2.
  If, however, the combination A1,B1 does not work, but all
  others do, A could choose A1,B2 and B could choose B1,A2.

- One way to force the same addresses is that one of the peers 
  makes the decision, and tells the other (in MOPO-IKE, it's 
  always the initiator; presumably, some more complex protocol 
  could, e.g., try to negotiate it in the beginning, or choose 
  randomly.)

- Another way would be to specify the decision making 
  algorithm in the protocol specification. This would mean that 
  the decision is always based on whatever limited preferences 
  information can be encoded to the payloads (while if one party 
  always makes the decision, it can utilize more information 
  about its own preferences).

- More ways to ensure that the same address pair gets chosen 
  might be possible...

- But a good question is whether this is necessary at all. My  
  understanding of SCTP is that address selection is left as a 
  policy issue, and since both ends have their own policy, they 
  can end up using different addresses.

- In "mobile client connected to VPN gateway" situations, 
  I think it would make sense that the client's preferences
  (e.g. whether to use WLAN or GPRS) are followed whenever 
  possible.

Comments are welcome!

(Hannes: perhaps something about this could be added to the
design draft as well?)

Best regards,
Pasi