[Cake] diffserv based on firewall mark

Jonathan Morton chromatix99 at gmail.com
Wed Oct 12 11:36:32 EDT 2016

> On 12 Oct, 2016, at 15:40, ching lu <lsching17 at gmail.com> wrote:
> DSCP -> unreliable, easily spoofed by attacker

I’d like to address the “easily spoofed by attacker” point specifically.

Cake’s interpretation of Diffserv is as a three-way tradeoff between throughput priority, latency priority, and altruism.  If you choose a DSCP meaning “low latency” such as CS6 or EF, Cake gives it higher weight than average, but *only* if the aggregate bandwidth of supposedly low-latency traffic is below a reasonable fraction of the link bandwidth.  Beyond that point, it gets *lower* weight than average, but is still able to use spare bandwidth that happens to be available.

There is no way to get “absolute priority”, which this type of attacker would presumably want, just by setting a particular DSCP.  The default “best effort” DSCP is in fact interpreted as “throughput priority”, which is what most bulk traffic wants.  In this respect, Cake differs from the original IP Precedence specification (which is long obsolete) and most other naive Diffserv implementations.

In short, Cake does not unreasonably *trust* the DSCP field, but instead offers explicit incentives for traffic to set it correctly.  This adheres to the relevant PHB specifications, which are published as RFCs and thus can be used as a standard.

The CS1 or “Background” DSCP is the one with an altruistic meaning.  It has low priority whichever side of the bandwidth threshold it lies, so it always mostly yields to other traffic.  Clients are of course permitted to not set it, but that’s what your firewall rules are for.  The Diffserv spec explicitly allows you to change the DSCP on entry to your own network, which is what I suggested in my first reply.

Setting the DSCP with iptables rules should work just as well and in the same way as using the “firewall mark” functionality as you already do.  Set it up that way in the first instance, directly replacing each HTB+fq_codel combination with a Cake instance, and see how it works.

 - Jonathan Morton

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