Re: TCP one-by-one acking - RFC interpretation question

[Eric Dumazet <eric.dumazet@gmail.com>]

Cc Neal and Yuchung if they missed this thread.

On 04/06/2018 08:03 AM, Michal Kubecek wrote:
> On Fri, Apr 06, 2018 at 05:01:29AM -0700, Eric Dumazet wrote:
>>
>>
>> On 04/06/2018 03:05 AM, Michal Kubecek wrote:
>>> Hello
>>>
>>> I encountered a strange behaviour of some (non-linux) TCP stack which
>>> I believe is incorrect but support engineers from the company producing
>>> it claim is OK.
>>>
>>> Assume a client (sender, Linux 4.4 kernel) sends a stream of MSS sized
>>> segments but segments 2, 4 and 6 do not reach the server (receiver):
>>>
>>>          ACK             SAK             SAK             SAK
>>>       +-------+-------+-------+-------+-------+-------+-------+
>>>       |   1   |   2   |   3   |   4   |   5   |   6   |   7   |
>>>       +-------+-------+-------+-------+-------+-------+-------+
>>>     34273   35701   37129   38557   39985   41413   42841   44269
>>>
>>> When segment 2 is retransmitted after RTO timeout, normal response would
>>> be ACK-ing segment 3 (38557) with SACK for 5 and 7 (39985-41413 and
>>> 42841-44269).
>>>
>>> However, this server stack responds with two separate ACKs:
>>>
>>>   - ACK 37129, SACK 37129-38557 39985-41413 42841-44269
>>>   - ACK 38557, SACK 39985-41413 42841-44269
>>
>> Hmmm... Yes this seems very very wrong and lazy.
>>
>> Have you verified behavior of more recent linux kernel to such threats ?
> 
> No, unfortunately the problem was only encountered by our customer in
> production environment (they tried to reproduce in a test lab but no
> luck). They are running backups to NFS server and it happens from time
> to time (in the order of hours, IIUC). So it would be probably hard to
> let them try with more recent kernel.
> 
> On the other hand, they reported that SLE11 clients (kernel 3.0) do not
> run into this kind of problem. It was originally reported as a
> a regression on migration from SLE11-SP4 (3.0 kernel) to SLE12-SP2 (4.4
> kernel) and the problem was reported as "SLE12-SP2 is ignoring dupacks"
> (which seems to be mostly caused by the switch to RACK).
> 
> It also seems that part of the problem is specific packet loss pattern
> where at some point, many packets are lost in "every second" pattern.
> The customer finally started to investigate this problem and it seems it
> has something to do with their bonding setup (they provided no details,
> my guess is packets are divided over two paths and one of them fails).
> 
>> packetdrill test would be relatively easy to write.
> 
> I'll try but I have very little experience with writing packetdrill
> scripts so it will probably take some time.
> 
>> Regardless of this broken alien stack, we might be able to work around
>> this faster than the vendor is able to fix and deploy a new stack.
>>
>> ( https://en.wikipedia.org/wiki/Robustness_principle )
>> Be conservative in what you do, be liberal in what you accept from
>> others...
> 
> I was thinking about this a bit. "Fixing" the acknowledgment number
> could do the trick but it doesn't feel correct. We might use the fact
> that TSecr of both ACKs above matches TSval of the retransmission which
> triggered them so that RTT calculated from timestamp would be the right
> one. So perhaps something like "prefer timestamp RTT if measured RTT
> seems way too off". But I'm not sure if it couldn't break other use
> cases where (high) measured RTT is actually correct, rather than (low)
> timestamp RTT.
> 
> Michal Kubecek
>