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SubjectRe: Supporting 4 way connections in LKSCTP
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Hi Vlad

Thank for your reply. If it is based on the destination IP to find the
best route, why the problem didn't happen on single-homing sample?

In the single-homing sample that provided in the original email, both
of the interfaces (eth1 and eth2) are presented on NODE-B during the
test. However, the LKSCTP library know to use the interface eth1 to
respond to the SCTP request.

- PS

On Wed, Nov 27, 2013 at 7:09 AM, Sun Paul <paulrbk@gmail.com> wrote:
> Hi Vlad
>
> Thank for your reply. If it is based on the destination IP to find the
> best route, why the problem didn't happen on single-homing sample?
>
> In the single-homing sample that provided in the original email, both
> of the interfaces (eth1 and eth2) are presented on NODE-B during the
> test. However, the LKSCTP library know to use the interface eth1 to
> respond to the SCTP request.
>
> - PS
>
> On Tue, Nov 26, 2013 at 11:19 PM, Vlad Yasevich <vyasevich@gmail.com> wrote:
>> On 11/25/2013 08:03 PM, Sun Paul wrote:
>>> Hi
>>>
>>> we have a problem on using LKSCTP to form a 4 ways multi-homing network.
>>>
>>> Configuration
>>> - Node-A has 2 IP addresses in different subnets, known as IP-A (eth1),
>>> IP-B (eth2)
>>> - Node-B has 2 IP addresses in different subnets, known as IP-X (eth1),
>>> IP-Y (eth2)
>>>
>>
>> First of all, this is not a 4 way multi-homed network. As far as each
>> SCTP association is concerned, it has only 2 destinations to send to
>> so it has only 2 ways to get there. The fact that you have multiple
>> local addresses doesn't mean that every local address can and should
>> be used to connect to the remote.
>>
>>> the four way paths are shown below.
>>> 1. IP-A (11.1.1.1) to IP-X (11.1.1.11)
>>> 2. IP-B (12.1.1.1) to IP-Y (12.1.1.11)
>>> 3. IP-A (11.1.1.1) to IP-Y (12.1.1.11)
>>> 4. IP-B (12.1.1.1) to IP-X (11.1.1.11)
>>
>> No, actually you only have 2 paths: one to IPX and one to IP-Y.
>> Which source address you choose is based on routing policy
>> decisions and is outside the scope of SCTP.
>>
>>>
>>> the HB/HB_ACK is normal for the paths " IP-A to IP-X" and "IP-B to
>>> IP-Y", but it is not correct for the rest of two.
>>
>> Right, because linux is using a host addressing model, not an interface
>> addressing model. SCTP stack simply finds the best source address
>> that can be used to reach IP-X and it happens to be IP-A. So that
>> is what it is going to use.
>>
>> The above explains why you are seeing what you describe below.
>>
>> In the end, linux SCTP implementation determines paths solely based
>> on the destination address.
>>
>> -vlad
>>
>>>
>>> First of all, we are using iproute2 to form 2 table such that when
>>> IP-B arrives on IP-X, it will know how to route back to IP-B on the
>>> same interface, i.e (eth1). Same logic for the path "IP-A to IP-X".
>>>
>>> What we observed here is that when 12.1.1.1 sends INIT to 11.1.1.11,
>>> LKSCTP will send back the INIT_ACK to 12.1.1.1 using 12.1.1.11 but not
>>> using the IP 11.1.1.11.
>>>
>>> The above operation makes the subsequence HB/HB_ACK in using wrong IP address.
>>>
>>> TCP trace on eth1
>>> 18:02:41.058640 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [INIT]
>>> [init tag: 19933036] [rwnd: 102400] [OS: 16] [MIS: 16] [init TSN: 0]
>>> 18:02:41.061634 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [COOKIE ECHO]
>>> 18:02:41.062642 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:41.062846 IP 11.1.1.11.2905 > 11.1.1.1.2905: sctp (1) [HB ACK]
>>> 18:02:41.361811 IP 11.1.1.11.2905 > 11.1.1.1.2905: sctp (1) [HB ACK]
>>> 18:02:41.661791 IP 11.1.1.11.2905 > 11.1.1.1.2905: sctp (1) [HB ACK]
>>> 18:02:41.961791 IP 11.1.1.11.2905 > 11.1.1.1.2905: sctp (1) [HB ACK]
>>>
>>> TCP trace on eth2
>>> 18:02:41.058755 IP 12.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [INIT ACK]
>>> [init tag: 424726157] [rwnd: 131072] [OS: 5] [MIS: 5] [init TSN:
>>> 3340756356]
>>> 18:02:41.061696 IP 12.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [COOKIE ACK]
>>> 18:02:41.062663 IP 12.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [HB ACK]
>>> 18:02:41.062791 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:41.361777 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:41.661772 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:41.961772 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:42.161771 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:42.461770 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:02:42.675770 IP 11.1.1.1.2905 > 12.1.1.11.2905: sctp (1) [HB REQ]
>>>
>>>
>>> If we are using single homing, there is no problem on the SCTP
>>> communication. Below is the TCP trace on eth1 using sctp_test
>>>
>>> 18:09:55.356727 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [INIT]
>>> [init tag: 32516609] [rwnd: 102400] [OS: 16] [MIS: 16] [init TSN: 0]
>>> 18:09:55.356811 IP 11.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [INIT ACK]
>>> [init tag: 3168861995] [rwnd: 131072] [OS: 10] [MIS: 16] [init TSN:
>>> 1877695021]
>>> 18:09:55.357727 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [COOKIE ECHO]
>>> 18:09:55.357788 IP 11.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [COOKIE ACK]
>>> 18:09:55.358724 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:09:55.358740 IP 11.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [HB ACK]
>>> 18:09:55.379715 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [DATA]
>>> (B)(E) [TSN: 0] [SID: 0] [SSEQ 0] [PPID 0x3]
>>> 18:09:55.379735 IP 11.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [SACK]
>>> [cum ack 0] [a_rwnd 131064] [#gap acks 0] [#dup tsns 0]
>>> 18:09:55.657716 IP 12.1.1.1.2905 > 11.1.1.11.2905: sctp (1) [HB REQ]
>>> 18:09:55.657732 IP 11.1.1.11.2905 > 12.1.1.1.2905: sctp (1) [HB ACK]
>>>
>>> From the observations, it seems that the LKSCTP library is not able to
>>> use the original local address when multi-homing is being used. Is
>>> there anyway can be resolved it?
>>>
>>> Thanks
>>>
>>> PS
>>> --
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>>>
>>


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