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SubjectRe: Supporting 4 way connections in LKSCTP
On 12/04/2013 11:25 AM, Michael Tuexen wrote:
> On Dec 4, 2013, at 5:12 PM, Vlad Yasevich <> wrote:
>> On 12/04/2013 11:01 AM, Michael Tuexen wrote:
>>> On Dec 4, 2013, at 4:41 PM, Vlad Yasevich <> 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) on LAN X and on LAN Y.
>>>>> B) on LAN X and on LAN Y.
>>>>> C) on LAN X.
>>>>> D) on LAN X and 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 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
>>>> on the interface connected to LAN X. I shouldn't
>>>> technically be a problem for C as it should mark the path to
>>>> 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 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 to you should not list,
>>> 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) on Lan X, on Lan Y
>> System B) on Lan X, on Lan Y
>> System C) on Lan X, 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 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'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.

>> Btw, were there any IDs other then draft-stewart-tsvwg-sctp-ipv4?
> Yes, one for IPv6.
> They need to be integrated and improved...

Ok. I'll take a look.


> 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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