[Cake] SCE/CAKE coding curiosities

Toke Høiland-Jørgensen toke at redhat.com
Tue Apr 2 13:45:08 EDT 2019

Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> writes:

> Ahoy there team Bufferboat :-)
> I’ve been looking at the new SCE related code and a few questions have fallen out of the brain cell.
> 1)
> https://github.com/dtaht/sch_cake/blob/47d821f89f39c1b2216d6f65b014f609e46fc57c/sce.h#L50
> Curious as to why htons isn’t used here as per other instances in

No idea about this.

> 2) Immediately below that line we have a skb length guard before
> trying to set ECT ultimately via ipv?_change_dsfield - we have other
> places in CAKE where we play with dsfield bits that don’t have an
> (obvious) guard - should they?
> 3) And saving the most curious for last, in cake_update_flowkeys we check the protocol again (we want IPv4 packets only) but we use tc_skb_protocol(skb) https://github.com/dtaht/sch_cake/blob/47d821f89f39c1b2216d6f65b014f609e46fc57c/sch_cake.c#L628
> tc_skb_protocol says "/* We need to take extra care in case the skb came via
>          * vlan accelerated path. In that case, use skb->vlan_proto
>          * as the original vlan header was already stripped.
>          */
>         if (skb_vlan_tag_present(skb))
>                 return skb->vlan_proto;
>         return skb->protocol;
> "
> What???  Should cake_handle_diffserv use tc_skb_protocol too?  and
> INET_ECN_set_sce ??

Yup, I believe you are right about both of these; nice catch! I pushed
fixes for cake_handle_diffserv(); keeping my hands off the sce bits for
now ;)


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