netdev - Re: [PATCH net] selftests: forwarding: vxlan_bridge

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Message-Id: <20200217.190118.1525770684039829483.davem@davemloft.net>
Date:   Mon, 17 Feb 2020 19:01:18 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     liuhangbin@...il.com
Cc:     petrm@...lanox.com, pmachata@...il.com, netdev@...r.kernel.org,
        idosch@...lanox.com, petedaws@...il.com, daniel@...earbox.net
Subject: Re: [PATCH net] selftests: forwarding: vxlan_bridge_1d: fix tos
 value

From: Hangbin Liu <liuhangbin@...il.com>
Date: Tue, 18 Feb 2020 10:05:08 +0800

> On Mon, Feb 17, 2020 at 11:40:13AM +0100, Petr Machata wrote:
>> RFC2474 states that "DS field [...] is intended to supersede the
>> existing definitions of the IPv4 TOS octet [RFC791] and the IPv6 Traffic
>> Class octet [IPv6]". So the field should be assumed to contain DSCP from
>> that point on. In my opinion, that makes commit 71130f29979c incorrect.
>> 
>> (And other similar uses of RT_TOS in other tunneling devices likewise.)
> 
> Yes, that's also what I mean, should we update RT_TOS to match
> RFC2474?

The RT_TOS() value elides the two lowest bits so that we can store other
pieces of binary state into those two lower bits.

So you can't just blindly change the RT_TOS() definition without breaking
a bunch of things.