Sunday, May 25, 2014

Fixing Bufferbloat on Comcast's "Blast" 50/10Mbps Service

At our new apartment here in the Bay Area, I ended up going with Comcast after realizing that it was about my only option for high speeds.

But I knew it was going to need to love to make it work.  It turned out to take very little effort to get pretty fantastic results.

First, latency under full load (upstream and downstream), to a server that's very close:

After a bit, it settles into 350ms of buffer-induced latency.  OTOH, this is pretty screaming fast on downlink (58Mbps!), but the uplink is suspicious looking.  It gets the advertised 10Mbps for about 10 seconds, and then falls apart.

I put in my WNDR3800 running CeroWRT (a bleeding-edge OpenWRT variant:  Then I set the qos parameters based on the above measurements:

ingress rate limit:  54.5Mbps
egress rate limit:  10Mbps

using simple.qos, and with ecn enabled (both directions).


I was amazed.  Ruler-flat throughput, at the advertised rates, good sharing between the various buckets (first time I've had enough bandwidth to see that actually work), and about 5ms of buffer-induced latency.  Unfortunately, the WNDR3800 is at it's CPU limit (load factor of 3-4).  OTOH, I don't push full bandwidth all that often (on the downstream side, I do on the upstream when I'm uploading photos).

This is what the Arris modem/router should have done out of the box.  Hopefully PIE will help with that in DOCSIS 3.1.  Maybe.  We'll see.