Re: [multipathtcp] MPTCP backup flag attack via MP_PRIO message

[Yoshifumi Nishida <[e-mail]>]

Hi Alan,

On Sun, Jul 23, 2017 at 6:04 AM, Alan Ford <[e-mail]> wrote:

> Hi Yoshi,
>
> On Fri, 21 Jul 2017 at 22:37, Yoshifumi Nishida <[e-mail]>
> wrote:
>
>> Hi Alan,
>>
>> On Fri, Jul 21, 2017 at 12:45 AM, Alan Ford <[e-mail]> wrote:
>>
>>> Hi Yoshi,
>>>
>>> On 20 Jul 2017, at 22:22, Yoshifumi Nishida <[e-mail]>
>>> wrote:
>>>
>>> On Thu, Jul 20, 2017 at 8:33 AM, Alan Ford <[e-mail]> wrote:
>>>>
>>>>
>>>> So the main reason for this was to permit the signalling of backup for
>>>> a subflow which was also signalled via ADD_ADDR. ADD_ADDR does not have a
>>>> ‘B’ bit in it, so the priority would be signalled separately.
>>>>
>>>
>>> I think adding 'B' bit in ADD_ADDR has been proposed to be added in the
>>> bis draft.
>>> I have seen a few supports while haven't seen any oppositions.
>>> Do we need more discussions on this?
>>>
>>>
>>> Actually on further reflection (i.e. Christoph and Olivier reminding me
>>> offline), this would be unnecessary, since MP_JOIN has a ‘B’ bit so it is
>>> not required in ADD_ADDR.
>>>
>>> Given this I can see no reason to have the Address ID in MP_PRIO and
>>> feel we can remove it.
>>>
>>> (The proposal for a bit in ADD_ADDR was, I believe, as a “do not even
>>> attempt to establish to this address unless all other subflows fail - which
>>> is different to the ‘B’ semantics in MP_PRIO and MP_JOIN)
>>>
>>
>> Yes, I thought it's a valid use case.
>> If we remove addr id from MP_PRIO, I think we will always need to
>> establish a subflow to make it backup.
>>
>
> That has always been the case - MP_PRIO only affects running subflows.
> With Address ID it could have affected policy on subflows that were later
> established, but would not affect subflow establishment policy.
>
> The proposal to signal a break-before-make backup flag has been suggested
> but has not received sufficient traction to be adopted.
>

Right. But, I am wondering if we might want to think about it again just in
case.

Let's say a server has two address S1, S2 while the client is behind FW. As
the client is behind firewall, the server uses ADD_ADDR to send the info
for S2, but it want it to be used as a backup.

In this case, if MP_PRIO has addr id field, the server is able to set the
addr as a backup.
But, if MP_PRIO doesn't have addr id, all the server can do is to wait
until the client establishes the connection with S2.
--
Yoshi