[noPoll] Fixed -- Re: Websocket disconnect issues

[Kale, Rahul]

Hello Francis,

My test scenario just hit the condition. The connection however got broken with the following error message:

Failed to get next 2 bytes to read header from the wire, failed to received content, shutting down id=2 the connection, errno=0 (No error)

Since the number of bytes read was not printed, I am guessing it was 1.
The receive call did not actually fail but possibly delivered 1 byte.

Regards,

Rahul


From: Kale, Rahul
Sent: Friday, April 22, 2016 10:04 AM
To: 'francis.brosnan en aspl.es'
Cc: nopoll en lists.aspl.es
Subject: RE: Fixed -- Re: [noPoll] Websocket disconnect issues

Hello,

I am currently running the test scenario but the failure condition has not happened yet
in the first 12 hours. Sometimes it takes 2-3 days to reproduce. I will let you
know when it happens next.

Besides the SSL twist, even for plain TCP connection I believe this situation can occur.
Due to maximum transmission unit (MTU) limitations the sender can break up messages
at arbitrary byte boundaries. If the receiver is slow to read, or if the network
bandwidth is limited, a fast sender will accumulate data to send. It will send
piled up data (multiple websocket messages in our case) in chunks of MTU size
and end up breaking messages at arbitrary byte boundaries.

You are totally correct in saying that the OS will not deliver bytes collected
from the wire to user space in a broken fashion. But the OS on the sender can put the
bytes on the wire in arbitrary pieces (TCP segments) depending on transient network conditions,
acks received, TCP sliding window size, round trip time, MTU size etc.
I am no expert at this, but the few TCP protocols I have implemented I have always had to account for this
before the application achieved 24/7 reliability.  For non-blocking sockets this is not trivial to achieve.
For blocking sockets, it is much easier.

I am glad that you are considering fixing this properly. It will make the nopoll
library more robust for everyone :)


Thanks,

Rahul

From: Francis Brosnan Blázquez [mailto:francis.brosnan en aspl.es]
Sent: Friday, April 22, 2016 5:52 AM
To: Kale, Rahul
Cc: nopoll en lists.aspl.es<mailto:nopoll en lists.aspl.es>
Subject: Re: Fixed -- Re: [noPoll] Websocket disconnect issues

Hi Raul,

It is going to take us a bit to deeply resolve the issue and
more precisely, to support websockets peers sending byte
by byte important parts of the headers that should be received
at once...

In any case, I was wondering if you did have time to check
latest modifications... did them resolve the issue? Is it more
stable your test case?

Please, let us know how the latest changes performs in your
scenario,

Best Regards,

Hello,

Thanks very much for your excellent analysis. It is indeed surprising that such a small packet seems to be delivered in chunks occasionally. Perhaps the fact that this is via SSL is compounding the issue. The original websocket provided on the server side is proxied via Apache Webserver which adds an SSL layer.

I see that the noPoll code is already handling broken headers for the first two bytes and also for 4 bytes of mask. I was assuming that you have already faced similar issues in the past.

I really appreciate your efforts in chasing these issues and fixing them promptly.

Regards,

Rahul




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