Re: Supporting 4 way connections in LKSCTP

[Michael Tuexen]

On Dec 4, 2013, at 7:23 PM, Vlad Yasevich <vyasevich@xxxxxxxxx> wrote:

> On 12/04/2013 11:25 AM, Michael Tuexen wrote:
>> On Dec 4, 2013, at 5:12 PM, Vlad Yasevich <vyasevich@xxxxxxxxx> wrote:
>> 
>>> On 12/04/2013 11:01 AM, Michael Tuexen wrote:
>>>> On Dec 4, 2013, at 4:41 PM, Vlad Yasevich <vyasevich@xxxxxxxxx> wrote:
>>>> 
>>>>> On 12/04/2013 09:50 AM, David Laight wrote:
>>>>>>>> In normal operation, IP-A sends INIT to IP-X, IP-X returns INIT_ACK to
>>>>>>>> IP-A. IP-A then sends HB to IP-X, IP-X then returns HB_ACK to IP-A. In
>>>>>>>> the meantime, IP-B sends HB to IP-Y and IPY returns HB_ACK.
>>>>>>>> 
>>>>>>>> In case of the path between IP-A and IP-X is broken, IP-B sends INIT
>>>>>>>> to IP-X, NODE-B uses IP-Y to return INIT_ACK to IP-B. Then IP-B sends
>>>>>>>> HB to IP-X, and IP-Y returns HB_ACK to IP-B. In the meantime, the HB
>>>>>>>> communication between IP-B and IP-Y follows the normal flow.
>>>>>>>> 
>>>>>>>> Can I confirm, is it really valid?
>>>>>>> 
>>>>>>> As long as NODE-B knows about both IP-A and IP-B, and NODE-A knows about
>>>>>>> both IP-X and IP-Y (meaning all the addresses were exchanged inside INIT
>>>>>>> and INIT-ACK), then this situation is perfectly valid.  In fact, this
>>>>>>> has been tested an multiple interops.
>>>>>> 
>>>>>> There are some network configurations that do cause problems.
>>>>>> Consider 4 systems with 3 LAN segments:
>>>>>> A) 10.10.10.1 on LAN X and 192.168.1.1 on LAN Y.
>>>>>> B) 10.10.10.2 on LAN X and 192.168.1.2 on LAN Y.
>>>>>> C) 10.10.10.3 on LAN X.
>>>>>> D) 10.10.10.4 on LAN X and 192.168.1.2 on LAN Z.
>>>>>> There are no routers between the networks (and none of the systems
>>>>>> are running IP forwarding).
>>>>>> 
>>>>>> If A connects to B everything is fine - traffic can use either LAN.
>>>>>> 
>>>>>> Connections from A to C are problematic if C tries to send anything
>>>>>> (except a HB) to 192.168.1.1 before receiving a HB response.
>>>>>> One of the SCTP stacks we've used did send messages to an
>>>>>> inappropriate address, but I've forgotten which one.
>>>>> 
>>>>> I guess that would be problematic if A can not receive traffic for
>>>>> 192.168.1.1 on the interface connected to LAN X.  I shouldn't
>>>>> technically be a problem for C as it should mark the path to 192.168.1.1
>>>>> as down.  For A, as long as it doesn't decide to ABORT the association,
>>>>> it shouldn't be a problem either.  It would be interesting to know more
>>>>> about what problems you've observed.
>>>>> 
>>>>>> 
>>>>>> Connections between A and D fail unless the HB errors A receives
>>>>>> for 192.168.1.2 are ignored.
>>>>> 
>>>>> Yes, this configuration is very error prone, especially if system B and
>>>>> system D are up at the same time.  Any attempts by system A to use
>>>>> LAN Y will result in an ABORT generated by system B.  I have seen
>>>>> this issue well in production and we had to renumber system D to solve
>>>>> it.
>>>> The point is that address scoping should be used. When sending an
>>>> INIT from 10.10.10.1 to 10.10.10.4 you should not list 192.168.1.1,
>>>> since you are transmitting an address to a node which might or might
>>>> not "be in the same scope". We had IDs for that in the past, but
>>>> they never made it to RFC state, because they were not progressed enough
>>>> by the authors. Maybe we should push them again...
>>> 
>>> But these 2 are technically in the same scope.  They are both private
>>> address types.  Also, this will not solve the problem either since
>> That is correct. But I think you should not transfer a private address
>> to another private address belonging to a different network.
>> I don't think this was specified in the older IDs...
>>> the configured addresses could be:
>>> System A) 10.0.0.1 on Lan X, 10.10.0.1 on Lan Y
>>> System B) 10.0.0.2 on Lan X, 10.10.0.2 on Lan Y
>>> System C) 10.0.0.3 on Lan X, 10.10.0.2 on Lan Z
>>> 
>>> Same problem will occur.
>> Depending on the subnet masks, it might work not not. Are you
>> configuring them with /8? 
> 
> No, /16 :).  With that, Sys A talking to Sys C will get an abort
> from Sys B when trying to talk to 10.10.0.2.  With /8, it'll be
> even worse since SysB and SysC will have duplicate addresses
> within the subnet. :)
> 
> The point is that you don't always know that the same private subnet
> is in reality 2 different subnets with duplicate addresses.
I agree, you can't do it perfectly right. But you can provide some
protection.
> 
> I've had to debug an actual production issue similar to this where
> customer had a very similar configuration to above, and their
> associations kept getting aborted.  When I tried accessing the
> system that kept sending aborts, I found it was some windows
> server and not a Diameter station they were expecting.
Interesting... Availability of SCTP on Windows is quite limited...
But people seem to use SCTP on Windows.

Best regards
Michael
> 
>>> 
>>> Btw, were there any IDs other then draft-stewart-tsvwg-sctp-ipv4?
>> Yes, one for IPv6.
>> http://tools.ietf.org/html/draft-stewart-tsvwg-sctpipv6-01
>> They need to be integrated and improved...
>> 
> 
> Ok.  I'll take a look.
> 
> Thanks
> -vlad
> 
>> Best regards
>> Michael
>>> 
>>> Thanks
>>> -vlad
>>> 
>>>> 
>>>> Best regards
>>>> Michael
>>>>> 
>>>>> -vlad
>>>>>> 
>>>>>> Of course the application could explicitly bind to only the 10.x address
>>>>>> but that requires the application know the exact network topology
>>>>>> and may be difficult for incoming calls.
>>>>>> 
>>>>>> 	David
>>>>>> 
>>>>> 
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>>>>> 
>>>> 
>>> 
>>> 
>> 
> 
> 

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