netdev - Re: [Cake] [PATCH net-next 1/5] sch_cake: fix IP protocol handling in the presence of VLAN tags

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Message-ID: <20200626150059.785647cb@hermes.lan>
Date:   Fri, 26 Jun 2020 15:00:59 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Jonathan Morton <chromatix99@...il.com>
Cc:     Davide Caratti <dcaratti@...hat.com>, cake@...ts.bufferbloat.net,
        netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [Cake] [PATCH net-next 1/5] sch_cake: fix IP protocol handling
 in the presence of VLAN tags

On Fri, 26 Jun 2020 16:11:49 +0300
Jonathan Morton <chromatix99@...il.com> wrote:

> Toke has already replied, but:
> 
> > Sure, my proposal does not cover the problem of mangling the CE bit inside
> > VLAN-tagged packets, i.e. if we should understand if qdiscs should allow
> > it or not.  
> 
> This is clearly wrong-headed by itself.
> 
> Everything I've heard about VLAN tags thus far indicates that they should be *transparent* to nodes which don't care about them; they determine where the packet goes within the LAN, but not how it behaves.  In particular this means that AQM should be able to apply congestion control signals to them in the normal way, by modifying the ECN field of the IP header encapsulated within.
> 
> The most I would entertain is to incorporate a VLAN tag into the hashes that Cake uses to distinguish hosts and/or flows.  This would account for the case where two hosts on different VLANs of the same physical network have the same IP address.
> 
>  - Jonathan Morton
> 
> _______________________________________________
> Cake mailing list
> Cake@...ts.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cake

The implementation of VLAN's is awkward/flawed. The outer VLAN tag is transparent
but the inner VLAN is visible. Similarly the outer VLAN tag doesn't count towards
the MTU but inner one does.