[Codel] Using fq_codel with a WiFi uplink to the Internet

Phineas Gage phineas919 at gmail.com
Wed Sep 21 05:59:04 EDT 2016

I have two questions about using fq_codel on an edge router when the Internet uplink is through point-to-point WiFi:

Question #1: Is it still effective to run fq_codel on our edge router when I have a WiFi uplink to the Internet, instead of a cabled connection like ADSL? And related to that, is a high quality point-to-point WiFi connection indistinguishable from a cabled connection as far as fq_codel is concerned?

Question #2: Assuming the answer to Question #1 is an overall "yes", is it then better to have a guaranteed speed from the ISP, instead of having a variable but potentially higher speed, so that I can control the queue and have fq_codel and HTB prioritization work effectively?

Background: I manage the network for a camp / conference center that supports up to about 120 users (5-10 simultaneous, at times). For Internet, we've been using ADSL with 5 Mbit download, 0.4 Mbit upload (remote area, so it’s slow). The only thing that keeps it usable is running fq_codel on a transparent Linux bridge that sits between the LAN and ADSL modem. On this bridge, I restrict the upload and download rate to about 85-90% of maximum, and use fq_codel, plus some HTB prioritization rules. It’s extremely effective at providing usable latency, so kudos to the fq_codel algorithm and implementation. It looks like this:

LAN <=> Linux bridge with fq_codel <=> ADSL Modem 0.4 / 5.0 Mbps <=> DSLAM …

But now, we have a chance to improve our throughput problem by switching to a point-to-point WiFi uplink that could hit speeds of 30-40 Mbit symmetric (more on the speeds later). We have to decide on starting a contract with them. At the same time, I’ll be switching the bridge to a Ubiquiti EdgeRouter X, which has fq_codel in its kernel, but should have the same effect. It would look something like:

LAN <=> Ubiquiti EdgeRouter X <=> WiFi Client (Mikrotik 802.11n MIMO 2x2) <=> WiFi AP from ISP …

We already have a test setup in place. The link rate to the ISP's AP, as reported by Mikrotik's admin console, is currently 86.6 Mbps transmit and 144.4 Mbps receive, with CCQ (connection quality) at 64% transmit / 99% receive. First of all, I'll try to have them get that transmit CCQ up to 99% like the receive, to make sure it's a stable link. But I also know that the actual Internet throughput will be less than the link rates, and speedtest.net results are currently around 30-40 Mbps symmetric.

Moreover, it's WiFi, and that led to my Question #1. I know that latency and throughput can vary, and that there are more queues and more things happening with packet aggregation, etc that I don’t understand. Can this aspect of the variable latency, throughput and packet transmission characteristics of WiFi make fq_codel less effective when used in this way on an intermediate bridge or edge router, or does it more depend on the quality of the WiFi link, where a high quality point-to-point WiFi uplink to a good upstream network (there’s another unknown) is indistinguishable from a cabled connection?

My Question #2 came from the fact that I have two options from the ISP:

Option A: We can choose a maximum of 40/40 Mbit with 1:5 aggregation, meaning we could get 40 Mbit, but we could also get a lot less at times (8 Mbit I assume), depending on other network load.

Option B: We can get a guaranteed bandwidth, but it costs more, so the maximum throughput we can pay for would be less. We would probably choose around 6/6 Mbit off-season, and 20/20 Mbit on-season, as the camp is a seasonal business.

My feeling, assuming that the answer to Question #1 is "yes" and I can effectively use fq_codel with WiFi at all, is to go with Option B, the guaranteed bandwidth. That way, I could set fq_codel to a little less than this bandwidth, and hopefully manage buffer bloat and do HTB prioritization in the same way I do now. But it depends on the answers to my two questions, is fq_codel still effective when using a WiFi uplink in general, and if so, is it better to go with a guaranteed bandwidth.

Thanks for any thoughts on this.
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