Re: [PATCH net-next 00/12] Unmask upper DSCP bits - part 2

[Guillaume Nault <gnault@redhat.com>]

On Thu, Aug 29, 2024 at 05:52:05PM +0300, Ido Schimmel wrote:
> On Thu, Aug 29, 2024 at 01:54:46PM +0200, Guillaume Nault wrote:
> > On Wed, Aug 28, 2024 at 03:09:19PM +0300, Ido Schimmel wrote:
> > > On Tue, Aug 27, 2024 at 06:45:53PM +0300, Ido Schimmel wrote:
> > > > On Tue, Aug 27, 2024 at 03:47:05PM +0200, Guillaume Nault wrote:
> > > > > On Tue, Aug 27, 2024 at 02:18:01PM +0300, Ido Schimmel wrote:
> > > > > > tl;dr - This patchset continues to unmask the upper DSCP bits in the
> > > > > > IPv4 flow key in preparation for allowing IPv4 FIB rules to match on
> > > > > > DSCP. No functional changes are expected. Part 1 was merged in commit
> > > > > > ("Merge branch 'unmask-upper-dscp-bits-part-1'").
> > > > > > 
> > > > > > The TOS field in the IPv4 flow key ('flowi4_tos') is used during FIB
> > > > > > lookup to match against the TOS selector in FIB rules and routes.
> > > > > > 
> > > > > > It is currently impossible for user space to configure FIB rules that
> > > > > > match on the DSCP value as the upper DSCP bits are either masked in the
> > > > > > various call sites that initialize the IPv4 flow key or along the path
> > > > > > to the FIB core.
> > > > > > 
> > > > > > In preparation for adding a DSCP selector to IPv4 and IPv6 FIB rules, we
> > > > > 
> > > > > Hum, do you plan to add a DSCP selector for IPv6? That shouldn't be
> > > > > necessary as IPv6 already takes all the DSCP bits into account. Also we
> > > > > don't need to keep any compatibility with the legacy TOS interpretation,
> > > > > as it has never been defined nor used in IPv6.
> > > > 
> > > > Yes. I want to add the DSCP selector for both families so that user
> > > > space would not need to use different selectors for different families.
> > > 
> > > Another approach could be to add a mask to the existing tos/dsfield. For
> > > example:
> > > 
> > > # ip -4 rule add dsfield 0x04/0xfc table 100
> > > # ip -6 rule add dsfield 0xf8/0xfc table 100
> > > 
> > > The default IPv4 mask (when user doesn't specify one) would be 0x1c and
> > > the default IPv6 mask would be 0xfc.
> > > 
> > > WDYT?
> > 
> > For internal implementation, I find the mask option elegant (to avoid
> > conditionals). But I don't really like the idea of letting user space
> > provide its own mask. This would let the user create non-standard
> > behaviours, likely by mistake (as nobody seem to ever have requested
> > that flexibility).
> > 
> > I think my favourite approach would be to have the new FRA_DSCP
> > attribute work identically on both IPv4 and IPv6 FIB rules and keep
> > the behaviour of the old "tos" field of struct fib_rule_hdr unchanged.
> > 
> > This "tos" field would still work differently for IPv4 and IPv6, as it
> > always did, but people wanting consistent behaviour could just use
> > FRA_DSCP instead. Also, FRA_DSCP accepts real DSCP values as defined in
> > RFCs, while "tos" requires the 2 bits shift. For all these reasons, I'm
> > tempted to just consider "tos" as a legacy option used only for
> > backward compatibility, while FRA_DSCP would be the "clean" interface.
> > 
> > Is that approach acceptable for you?
> 
> Yes. The patches I shared implement this approach :)

Thanks for confirming. And sorry for the misunderstanding in v1.