7 ms·

Aren't network operators already adding artificial delay? I pay for X Mbps download speed, and my ISP artificially limits my download speed to that, even though

• tialaramex 9y ago
  No. Bandwidth limits and delay aren't the same thing. CPE (the WiFi router or similar in your home) often adds delay because a few megabytes of RAM is cheap and the people who built it don't understand what they're doing. This is called "buffer bloat". But inside the network core this rarely comes up.
  • ksec 9y ago
    Does anyone know how much buffer bloat or delay is added in the router, or CPE or Modem? I assume they are less then 1ms? I have been trying to figure this out for a while but information aren't really available.
    • tialaramex 9y ago
      Bufferbloat is potentially almost unlimited, assuming that the people who built it are idiots (a safe assumption for most consumer gear) it basically just depends how much they were willing to spend on RAM. For example, let's say we can move 10Mbps, and we've decided to use 10 megabytes of buffers to make our new WiFi router super-duper fast. Do a big download, the buffer fills with ten megabytes of data, that's eight whole seconds of transmission, now the latency of packets is eight seconds, so that's 8000 times larger than your "I assume less then 1ms". {Edited to correct numbers}
      • mikeash 9y ago
        Back around 2006, I had a RAZR that I figured out how to use to tether my computer onto Verizon’s 1xRTT service. I’m not sure which part of the system was to blame, but something had massive buffers and a strong aversion to dropping packets. If the connection got saturated I could easily see ping times of two minutes or more.
        • supertrope 9y ago
          To deal with the lossy RF environment, there are re-transmits at the PHY layer. Effectively creating a TCP on top of TCP situation.
      • floatboth 9y ago
        Why would a single big download fill the buffers? Router → LAN is typically an order of magnitude more bandwidth than internet → router, so shouldn't the buffer be emptied faster than filled?
        • supertrope 9y ago
          The bottleneck is at your ISP's CMTS or DSLAM and your modem. e.g. The DSLAM has 1 Gbps in and only 40 Mbps down the line to your VDSL modem. Or your cable modem has access to 600 Mbps of capacity but your plan is only 100 Mbps so the modem limits. So there's quick stepdown: 1 Gbps, 600 Mbps, 100 Mbps.
          • floatboth 9y ago
            Yeah, the stepdown is on the ISP side, how would it affect buffers of my consumer router?
            • supertrope 9y ago
              For downloads it's buffers in your ISP's hardware that matter. For uploads it's your router's egress buffer. e.g. You are syncing gigabytes to Dropbox. A poorly designed router will continue to accept packets far past upstream capacity. Now that's there's 2000 ms of bulk traffic in the router's queue, any real time traffic has to wait a minimum of 2 seconds before getting out.
              • floatboth 9y ago
                Yeah, exactly. I mean, in your original comment you talked about bufferbloat on crappy consumer hardware and the example was a big download :)
    • isostatic 9y ago
      Depends on the router, and the load. If you have a 100mbit uplink and 4x1 gig on the downlink side, you could easily have 10mbit of packets arrive in 3ms (70 packets per millisecond), but would take 30ms to send those packets. You can either 1) Drop -- despite total trafic being only 10mbit a second 2) Queue -- introducing a delay of 30ms. In reality you'd put latency critical applications (voip etc) at the top of the queue so the pcakets get transmitted without the delay, and your facebook packets get delayed by 31ms rather than 30ms.
    • supertrope 9y ago
      As much as 2000 ms when saturating the downlink with a long file transfer. Web browsing stops working and nothing loads in as every little resource request takes two seconds.
  • Buge 9y ago
    Isn't the only way to implement a bandwidth limit to delay or drop packets during specific periods?
    • tialaramex 9y ago
      The only _sensible_ thing you can do if the transmitter won't stop is to ignore them, and thus drop packets, yes. Queueing them up instead makes some artificial benchmark numbers look good but is a horrible end user experience, so you should never do this, but lots of crap home WiFi type gear does. So, as I said, bandwidth limits and delay are different. The canonical "station wagon full of tapes" is illustrative, it has _tremendous_ bandwidth but _enormous_ delays. In contrast a mid-century POTS telephone call from London to Glasgow has almost no delay (barely worse than the speed of light) but bandwidth is tightly constrained.
      • isostatic 9y ago
        I would far rather my udp packet is delayed by a millisecond than dropped. I have an RTP stream running from India to Europe at the moment, 3000 packets per millisecond, so 0.33ms between pacekts. Typical maximum interpacket delay is under 1.5ms, looking at the last 400,000 seconds of logs, 200k are 1-2ms, 170k are 0-1ms, and about 4-5k on the 2-3ms gap, 3-4ms gape, etc. Less than 1% does interpacket delay increase past 10ms.
        • tialaramex 9y ago
          You would rather all your UDP packets are delayed by a millisecond rather than have one dropped? Why?
          • isostatic 9y ago
            Because that UDP packet is important, it can lead to millions of people watching a TV program having a 3 second outage in their audio.
            • 9y ago
  • nvarsj 9y ago
    You need buffers to maintain high bandwidth. There is no way around that. The problem is lack of AQM in end user routers - where a single high bandwidth flow can fill up and hog those buffers, disrupting latency sensitive flows. We're slowly seeing some adoption of things like fq_codel, but it's not perfect since the user has to go and manually enter their upload/download speeds (which again are not easy to determine, especially for ISPs with "boost" limiting). Ideally home routers would dynamically adjust based on observed latencies.
    • tialaramex 9y ago
      Codel is knobless, there literally aren't parameters you _can_ set let alone ones you need to set according to your "upload/download speeds". So, I have no idea what you're tinkering with that you think needs to know "upload/download speeds" or why, but it's nothing to do with Codel.
      • nvarsj 9y ago
        fq_codel alone doesn't solve buffer bloat. There are still huge buffers upstream. You need to combine it with local rate limiting so you can control the buffer in your local home router. fq_codel/cake work by looking at time a packet spent in a local queue. If you don't rate limit locally, then your local queue is always empty and everything queues upstream. Modern routers with things like "Dynamic QOS" that use fq_codel all require providing downstream/upstream values. This is way simpler than more traditional qos, but it's still a barrier to wide spread adoption.