Hi, On 11/01/2020 05:07, Matt Corallo wrote: > Sadly, the large scale deployments of BBR are mostly not high-latency links > (as CDNs generally have a nearby datacenter for you to communicate with), and > the high retransmission rates may result in more “lag” for browsing when > absolute bandwidth isn’t the primary concern. On the flip side, Spotify’s > measurements seem to indicate that, at least in some cases, the jitter can > decrease enough to be noticeable for users.
BBR is good for Netflix, but is not so good for non-streaming traffic. You also get issues between competing flows which doesn't matter for Netflix (typically you only watch one video at a time) but would matter for Tor. We don't have good models of what Tor traffic looks like, but I strongly suspect it is different to the Neflix/YouTube typical workloads. > Is there a way we could do measurements of packet loss/latency profiles of > bridge users? This should enable simulation for things like this, but it > sounds like there’s no good existing work in this domain? We have two tools that build simulated/emulated Tor networks: chutney and shadow. Unfortunately, neither implements everything that would be required. We really want to see what happens when x% of the network switches congestion control algorithm and see how flows interact at large relays (either relay to relay, or guard connections). If you have a large openstack cluster available, you could set up with your favorite orchestration tool a number of VMs with emulated WAN links between them, and connect a bunch of Tor clients to that network in other VMs, and perform measurements. Last time I looked you could not switch TCP congestion control algorithm in Linux per-namespace (maybe you can now and you don't need to have multiple VMs). Generally I would recommend *not* changing from TCP cubic unless you really understand the interactions that are going on between flows. Thanks, Iain.
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