Re: [Cake] Proposing COBALT

[Jonathan Morton <chromatix99@gmail.com>]

> On 20 May, 2016, at 16:41, David Lang <david@lang.hm> wrote:
> 
> On Fri, 20 May 2016, Jonathan Morton wrote:
> 
>> Normal traffic does not include large numbers of fragmented packets (I would expect a mere handful from certain one-shot request-response protocols which can produce large responses), so it is better to shunt them to a single queue per host-pair.
> 
> I don't agree with this.
> 
> Normal traffic on a well setup network should not include large numbers of fragmented packets. But I have seen too many networks that fragment almost everything as a result of there being a hop that goes through one or more tunneling layers that lower the effective MTU (and no, path mtu discovery does not always work)

One case of which would be the misconfigured PPPoE link Sebastian mentioned.

But I don’t think this is as big a problem as you do.  Most latency-sensitive protocols (and critical TCP phases such as handshake and teardown) use sub-MTU sized packets, so are less likely to be fragmented, so will still benefit from flow isolation.

And under normal circumstances, most MTU-sized packets are associated with congestion-responsive protocols, which can tolerate being shunted into a single AQM-managed subqueue per host-pair.  Flow isolation also still occurs between traffic to different hosts.

 - Jonathan Morton