Re: [PATCH nft 1/2] src: add dscp support

[Patrick McHardy <kaber@trash.net>]

On 26.11, Pablo Neira Ayuso wrote:
> On Thu, Nov 26, 2015 at 09:54:45AM +0000, Patrick McHardy wrote:
> > On 26.11, Pablo Neira Ayuso wrote:
> > > > > ToS breaks useful things like ECN, and the more I keep reading docs on
> > > > > the Internet, the more problem I have to see how the user can benefit
> > > > > from this.
> > > > 
> > > > We *match* on ToS, that cannot possibly break anything. Also I'm unsure how
> > > > this could break ECN even otherwise, ToS does not even use the ECN bits.
> > > 
> > > ToS bits overlap with ECN bits, from that original ToS 8 bit-field now
> > > we use 6 bit for DSCP and 2 bits for ECN.
> > 
> > Sure, but our ToS definition is wrong anyway, the ToS-bits are actually
> > 3 + 3 + 2 unused bits (ECN).
> 
> That's right according to the original ToS definition, but looking a
> bit further on the RFCs, someone outthere extended this, so I found
> this:
> 
> See 22. Historical Definitions for the IPv4 TOS Octet from
> https://tools.ietf.org/html/rfc3168.
> 
> It refers to https://tools.ietf.org/html/rfc1349 that defines
> Minimize Monetary Cost TOS Value and it extends it to one extra bit
> (from 3 to 4, scratching one from the unused bits).
> 
> Bottom line says "Because of this unstable history, the definition of
> the ECN field in this document cannot be guaranteed to be backwards
> compatible with all past uses of these two bits."
> 
> Anyway, looking at the Linux header definitions, we have the
> 
> #define IPTOS_MINCOST           0x02
> 
> The mincost thing is there.
> 
> #define IPTOS_TOS_MASK          0x1E
> #define IPTOS_TOS(tos)          ((tos)&IPTOS_TOS_MASK)
> 
> So it seems our headers are considering ToS is 4 bits too.

Hmm Ok its really a mess. Let's get rid of it for now and we might reconsider
if a user turns up that can make a valid case.