5 ms·

With 1500 Byte packets at 1Gbps you're pushing 83,333 packets per second. If 1% of those (833) are corrupted and 1 in every 2^16 corrupted packets has a valid C

by clinta 11y ago

With 1500 Byte packets at 1Gbps you're pushing 83,333 packets per second. If 1% of those (833) are corrupted and 1 in every 2^16 corrupted packets has a valid CRC then you have 1 corrupt packet with a valid CRC every 78 seconds. Still not something to ignore, but not nearly as bad as the author indicates.

cperciva 11y ago
Right, I think 1% PER is around the worst case in terms of having lots of errors while not causing the data rate to scale back dramatically in common internet-facing applications. I was estimating around one undetected error per minute; as you say, not something to ignore, but definitely not as bad as the author suggested either.
- jsnell 11y ago
  Actually, I don't think that's the case. While it's true that the theoretical maximum speed of an individual connection would decrease rapidly as packet loss increases, the aggregate data rate of the traffic going through a single network element would not necessarily be affected all that much. It's totally routine to see much higher network wide packet loss rates higher than 1%. The most I can remember was >15% sustained for weeks, for a few Gbps of real life traffic in a mobile network. ("Real life" as in hundreds of thousands of normal users, with the traffic coming directly from whatever servers they were actually accessing).
  - cperciva 11y ago
    Right, mobile networks are unusual. My point about "common internet-facing appliances" is that most systems maintain cwnd values of at least 10 segments, or else people notice and complain about poor performance.
ryacko 11y ago
ftp://ftp.cis.upenn.edu/pub/mbgreen/papers/ton98.pdf A study was done on the Performance of CRCs on Real Data. CRC16 as used for TCP has real biases. "In one dramatic case, 0.01% of the check values appeared nearly 15% of the time"

5 ms·

With 1500 Byte packets at 1Gbps you're pushing 83,333 packets per second. If 1% of those (833) are corrupted and 1 in every 2^16 corrupted packets has a valid C

by clinta 11y ago

cperciva 11y ago
Right, I think 1% PER is around the worst case in terms of having lots of errors while not causing the data rate to scale back dramatically in common internet-facing applications. I was estimating around one undetected error per minute; as you say, not something to ignore, but definitely not as bad as the author suggested either.
- jsnell 11y ago
  Actually, I don't think that's the case. While it's true that the theoretical maximum speed of an individual connection would decrease rapidly as packet loss increases, the aggregate data rate of the traffic going through a single network element would not necessarily be affected all that much. It's totally routine to see much higher network wide packet loss rates higher than 1%. The most I can remember was >15% sustained for weeks, for a few Gbps of real life traffic in a mobile network. ("Real life" as in hundreds of thousands of normal users, with the traffic coming directly from whatever servers they were actually accessing).
  - cperciva 11y ago
    Right, mobile networks are unusual. My point about "common internet-facing appliances" is that most systems maintain cwnd values of at least 10 segments, or else people notice and complain about poor performance.
ryacko 11y ago
ftp://ftp.cis.upenn.edu/pub/mbgreen/papers/ton98.pdf A study was done on the Performance of CRCs on Real Data. CRC16 as used for TCP has real biases. "In one dramatic case, 0.01% of the check values appeared nearly 15% of the time"