2

I transmit a fairly large amount of google protocol buffer msgs over a VPN over wireless over internet via TCP, and I feel like I get a relatively high error rate (e.g. a boolean field switching from false to true or sth similar). Something between 1 in 10,000 and 1 in 50,000.

Is that possible? Wikipedia states that TCP has a weak checksum, but that this is typically fixed in underlying protocols:

The TCP checksum is a weak check by modern standards. Data Link Layers with high bit error rates may require additional link error correction/detection capabilities. The weak checksum is partially compensated for by the common use of a CRC or better integrity check at layer 2, below both TCP and IP, such as is used in PPP or the Ethernet frame.

Does anyone have any experience what expected error rates should be? If above rate is possible, what would be the recommended / easiest way of fixing it? Duplicating the fields? Sending the message twice? Or is there something else that can be done to increase reliability?

Thanks

Cookie
  • 12,004
  • 13
  • 54
  • 83
  • 2
    Should not happen. Like Wikipedia says, checksums in TCP and underlying transport protocols take care of that. You problem very likely lies elsewhere. – Thilo Dec 23 '11 at 00:25
  • 1
    You have something wrong. Single bit switch is detected by CRC. Also if VPN uses encryption it is 0% (in practice) that you get transmission error. – Luka Rahne Dec 23 '11 at 00:27
  • Bit errors do sometimes go undetected by TCP; this has resulted in some big outages in the past: http://status.aws.amazon.com/s3-20080720.html Incidentally, TCP does not use CRC, it uses ones-complement sums. – bdonlan Dec 23 '11 at 01:54

2 Answers2

4

No it is not (reasonably) possible. Assuming that you are not suffering a hardware failure (of memory, your network card, etc.), which should be easy to check--does it happen on more than one computer?

Much more likely is that you have an invalid memory access or the like within your application code, or that the data you are sending is simply not what you intended. Try running your code under valgrind or the like.

The idea of duplicating fields as part of normal operation seems absurd: basically nobody does that in the wild, and you shouldn't need to either. There are multiple layers of protection against silent data corruption in your system, so it's most likely a simple (or maybe not-so-simple) application error.

John Zwinck
  • 239,568
  • 38
  • 324
  • 436
1

John Zwink's answer is correct in that your results are not reasonable, and fixing at the application layer is a mistake.

The VPN (assuming SW layer) would go bonkers and lose connection if the network card were introducing that many errors.

Try Valgrind, as suggested, to find if your SW is corrupting the buffer.

Also, in case "easy to check" for the memory didn't seem so, a good bet is memtest86, http://www.memtest86.com, which you can stick on a USB drive or CD, and let run overnight or over the weekend.

EMM
  • 36
  • 2