Re: [PATCHv2 net 2/2] vxlan: fix getting tos value from DSCP field

On Tue, Aug 04, 2020 at 12:47:56PM -0700, David Miller wrote:

From: Hangbin Liu <redacted>
Date: Tue,  4 Aug 2020 09:43:12 +0800

quoted

In commit 71130f29979c ("vxlan: fix tos value before xmit") we strict
the vxlan tos value before xmit. But as IP tos field has been obsoleted
by RFC2474, and updated by RFC3168 later. We should use new DSCP field,
or we will lost the first 3 bits value when xmit.

Fixes: 71130f29979c ("vxlan: fix tos value before xmit")
Signed-off-by: Hangbin Liu <redacted>

Looking at the Fixes: tag commit more closely, it doesn't make much
sense at all to me and I think the fix is that the Fixes: commit
should be reverted.

Hi David,

Both this patch and the Fixes: commit are not aim to fix the ECN bits.
ECN bits are handled by ip_tunnel_ecn_encap() correctly.

The Fixes: commit and this patch are aim to fix the TOS/DSCP field, just
as the commit subject said.

In my first patch "net: add IP_DSCP_MASK", as I said, the current
RT_TOS()/IPTOS_TOS_MASK will ignore the first 3 bits in IP header
based on RFC1349.

       0     1     2     3     4     5     6     7
    +-----+-----+-----+-----+-----+-----+-----+-----+
    |   PRECEDENCE    |          TOS          | MBZ |
    +-----+-----+-----+-----+-----+-----+-----+-----+

While in RFC3168 we defined DSCP field like

       0     1     2     3     4     5     6     7
    +-----+-----+-----+-----+-----+-----+-----+-----+
    |          DS FIELD, DSCP           | ECN FIELD |
    +-----+-----+-----+-----+-----+-----+-----+-----+

So if a user defined the IP DSCP/TOS field like 1111 1100, with
RT_TOS(tos) we will got tos 0001 1100, but based on RFC3168, we
should send the header with DSCP 1111 1100. That's why I add
RT_DSCP() in my first patch.

If you pass the raw TOS into ip_tunnel_ecn_encap(), then that has the
same exact effect as your patch series here.  The ECN encap routines
will clear the ECN bits before potentially incorporating the ECN value
from the inner header etc.  The clearing of the ECN bits done by your
RT_DSCP() helper is completely unnecessary, the ECN helpers do the
right thing.  So effectively the RT_DSCP() isn't changing the tos
value at all.

Yes, you are right. RT_DSCP() doesn't change the tos value in this case.
I will revert the Fixes: commit.

While RT_DSCP() is still needed in other place, I will re-post a new patch
set for that issue.

I also think that your commit messages are lacking, as you fail
(especially in the Fixes: commit) to show exactly where things go
wrong.  It's always good to give example code paths and show what
happens to the TOS and/or ECN values in these places, what part of
that transformation you feel is incorrect, and what exactly you
believe the correct transformation to be.

Thanks, I will try add more info in later patches.

Hangbin