5 ms·

Latency != bandwidth. Also, one goal of DPI is to enable granular QoS policy. So the total bandwidth available would stay the same, but the bandwidth available

Latency != bandwidth. Also, one goal of DPI is to enable granular QoS policy. So the total bandwidth available would stay the same, but the bandwidth available to each app would vary depending on network conditions and the will of the network operator.

65b 14y ago
"In TCP connections, the large bandwidth-delay product of high latency connections, combined with relatively small TCP window sizes on many devices, effectively causes the throughput of a high latency connection to drop sharply with latency" --Wikipedia It does equal bandwidth until everything is jumbo frames UDP
- swalberg 14y ago
  If you would have pasted a larger part of the quote, you would have included the part of increasing the window size (e.g. window scaling, selective ACKs, like almost everything out there supports) and the mention of satellite links to refer to high latency conditions. And that notwithstanding that, we are talking about additional latency of microseconds against a typical delay of at least 30 milliseconds. "drop sharply" just doesn't apply here. If you want to argue against DPI that's fine but "it'll make the Internet slower" comes off as whining plus you're fighting against the exponential effects of Moore's law. A quick Google search tells me there are several products that'll do line rate DPI at 10Gbps. There are much better arguments to be made against DPI such as privacy, a slippery slope to a walled garden, or just plain unfairness.
politician 14y ago
Wikipedia says, "In computer networking and computer science, the words bandwidth, network bandwidth, data bandwidth, or digital bandwidth are terms used to refer to various bit-rate measures, representing the available or consumed data communication resources expressed in bits per second or multiples of it (bit/s, kbit/s, Mbit/s, Gbit/s, etc.)." All else being equal, increasing the denominator reduces the rate. It's just math.
- jholman 14y ago
  And it's the wrong math. You claim that increasing latency will necessarily decrease bandwidth, and that is simply not the case. If we assume that there are a fixed number of packets in flight at any given time, and we increase the flight time of all packets, then it will be true that the packets-processed-per-time will drop (and ceteris paribus that the bandwidth would drop). This assumption, however, is incorrect. One could (in theory, and within limits in practice) add extra pipeline steps to do extra processing, increasing the time taken to process each packet, without decreasing the rate at which packets traverse the processor (e.g. the router). This is hopefully what happens. Of course, as I and others said elsewhere, it is OFTEN the case that as processing-per-packet increases, this becomes a bottleneck on packets-per-time, and this does eventually limit the bandwidth. (Jumbo frames sometimes help, but not if the processing is being done on the entire application payload, which of course is what DPI does, so jumbo frames ain't gonna help there.) So in practice, more DPI might well decrease bandwidth. If you would like an analogy, imagine the hosts are cities, the packets are trucks carrying goods, and the route is a highway of fixed speed limit and width. (Yes, one weird thing about this analogy is that building new trucks is basically free and done on-demand.) Suppose we consider shipping an infinite amount of coal from city A to city B. The number of tons of coal shipped per day does not depend on the length of the highway. The amount of time it takes for each ton of coal to get there DOES depend on the length of the highway. Coal-per-day is like bandwidth, hours-per-truck-trip is latency. You can increase or decrease one without affecting the other, all else being equal. Note: really, I shoulda done that example as a series of tubes, oh well.
  - swalberg 14y ago
    I am reminded of the famous quote "Never underestimate the bandwidth of a station wagon loaded with backup tapes."
  - politician 14y ago
    You're right, I said something silly and invalid. What I was trying to say was simply that an in-series DPI implementation will increase round trip times and decrease bandwidth. My personal understanding of DPI is that they want to do it in-series so as to be able to reset TCP connections immediately upon detecting a hit, but that might not be how they do it.

65b 14y ago

"In TCP connections, the large bandwidth-delay product of high latency connections, combined with relatively small TCP window sizes on many devices, effectively causes the throughput of a high latency connection to drop sharply with latency" --Wikipedia It does equal bandwidth until everything is jumbo frames UDP

swalberg 14y ago
If you would have pasted a larger part of the quote, you would have included the part of increasing the window size (e.g. window scaling, selective ACKs, like almost everything out there supports) and the mention of satellite links to refer to high latency conditions. And that notwithstanding that, we are talking about additional latency of microseconds against a typical delay of at least 30 milliseconds. "drop sharply" just doesn't apply here. If you want to argue against DPI that's fine but "it'll make the Internet slower" comes off as whining plus you're fighting against the exponential effects of Moore's law. A quick Google search tells me there are several products that'll do line rate DPI at 10Gbps. There are much better arguments to be made against DPI such as privacy, a slippery slope to a walled garden, or just plain unfairness.

politician 14y ago

Wikipedia says, "In computer networking and computer science, the words bandwidth, network bandwidth, data bandwidth, or digital bandwidth are terms used to refer to various bit-rate measures, representing the available or consumed data communication resources expressed in bits per second or multiples of it (bit/s, kbit/s, Mbit/s, Gbit/s, etc.)." All else being equal, increasing the denominator reduces the rate. It's just math.

jholman 14y ago
And it's the wrong math. You claim that increasing latency will necessarily decrease bandwidth, and that is simply not the case. If we assume that there are a fixed number of packets in flight at any given time, and we increase the flight time of all packets, then it will be true that the packets-processed-per-time will drop (and ceteris paribus that the bandwidth would drop). This assumption, however, is incorrect. One could (in theory, and within limits in practice) add extra pipeline steps to do extra processing, increasing the time taken to process each packet, without decreasing the rate at which packets traverse the processor (e.g. the router). This is hopefully what happens. Of course, as I and others said elsewhere, it is OFTEN the case that as processing-per-packet increases, this becomes a bottleneck on packets-per-time, and this does eventually limit the bandwidth. (Jumbo frames sometimes help, but not if the processing is being done on the entire application payload, which of course is what DPI does, so jumbo frames ain't gonna help there.) So in practice, more DPI might well decrease bandwidth. If you would like an analogy, imagine the hosts are cities, the packets are trucks carrying goods, and the route is a highway of fixed speed limit and width. (Yes, one weird thing about this analogy is that building new trucks is basically free and done on-demand.) Suppose we consider shipping an infinite amount of coal from city A to city B. The number of tons of coal shipped per day does not depend on the length of the highway. The amount of time it takes for each ton of coal to get there DOES depend on the length of the highway. Coal-per-day is like bandwidth, hours-per-truck-trip is latency. You can increase or decrease one without affecting the other, all else being equal. Note: really, I shoulda done that example as a series of tubes, oh well.
- swalberg 14y ago
  I am reminded of the famous quote "Never underestimate the bandwidth of a station wagon loaded with backup tapes."
- politician 14y ago
  You're right, I said something silly and invalid. What I was trying to say was simply that an in-series DPI implementation will increase round trip times and decrease bandwidth. My personal understanding of DPI is that they want to do it in-series so as to be able to reset TCP connections immediately upon detecting a hit, but that might not be how they do it.