Re: [multipathtcp] 6824bis -11 review

[Alan Ford <[e-mail]>]

Addendum...

> On 28 Jul 2018, at 18:19, Alan Ford <[e-mail]> wrote:
> 
>> 3. Section 3.4.1. Address Advertisement 
>>  "A host can send an ADD_ADDR message with an already assigned Address
>>  ID, but the Address MUST be the same as previously assigned to this
>>  Address ID, and the Port MUST be different from one already in use
>>  for this Address ID."
>> [RJ]: Didn't understand the rationale for allowing reuse of Address ID only for port change and not for IP change.
> 
> I cannot recall the rationale here either.
> 
> Furthermore - as written - this would seem to prevent retransmission. Which is crazy.
> 
> If nobody can recall the reason for this I would suggest we remove it.

Having looked at the text again here, I think I can recall the rationale. But we need to consider whether this is actually the intended behaviour.

Address ID was originally designed to provide a mechanism whereby a client behind a NAT could refer to its address without using the IP address, which would be altered by the NAT. This would allow a client, whose wifi interface had disappeared, to tell the far end over its cellular interface that it had gone, by referring to the Address ID it had used in the MP_JOIN, in the REMOVE_ADDR.

There is a secondary use here which I realise we haven’t explicitly stated. If a client declared its private address in ADD_ADDR, but also sent a MP_JOIN with the same address ID, but naturally a different address, the far end would say “ah, these addresses don’t match, so I should not bother attempting to connect to the Address in the ADD_ADDR”. This is an optimisation but one that should probably be mentioned.

So the point is that Address ID refers purely to the address, and not to the address + port pair. Therefore a new port can be advertised in a new ADD_ADDR using the same Address ID.

So we should clarify this:

a) retransmitting an ADD_ADDR with same ID, address, and port is fine
b) this could permit the far end to trigger another connection attempt
c) sending a new ADD_ADDR with same ID + address and different port is fine
d) to reassign an address ID you must first REMOVE_ADDR

Regards,
Alan