Re: [RFC PATCH] doc: clarify VLAN and QinQ stripping behaviour

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Jul 28, 2025 at 04:51:30PM +0200, Morten Brørup wrote:
> > From: Dean Marx [mailto:dmarx@iol.unh.edu]
> > Sent: Friday, 18 July 2025 15.18
> > 
> > On Fri, Jul 18, 2025 at 4:23 AM Bruce Richardson
> > <bruce.richardson@intel.com> wrote:
> > >
> > > On Thu, Jul 17, 2025 at 05:03:13PM -0400, Dean Marx wrote:
> > > > I've created a v1 of a QinQ test suite around the set of test cases
> > > > discussed earlier (which is not set in stone, and I expect it to
> > > > change significantly across many future versions.) The PASS/FAIL
> > > > values can be mostly disregarded in the context of this conversation,
> > > > but I've added logging to explain which packets are sent, and what
> > > > happened upon reception, which I hope will be more informative. After
> > > > running on mlx5/i40e drivers, I got the following results:
> > > >
> > > > test_vlan_strip: QinQ strip OFF and VLAN strip ON
> > > > test_qinq_strip: QinQ strip ON and VLAN strip ON
> > > >
> > > > i40e:
> > > >     test_qinq_strip (sent packet: Single VLAN): FAIL
> > > >       reason: VLAN tags found in packet when should have been
> > > > stripped: Ether / Dot1Q / 802.1q (0x1c) vlan 1280 / LLC / Raw /
> > > > Padding
> > > >     test_qinq_strip (sent packet: Stacked VLAN): FAIL
> > > >       reason: Expected one VLAN tag but found 2: Ether / Dot1Q / Dot1Q
> > > > / 802.1q (0x1c) vlan 1280 / LLC / Raw / Padding
> > > >     test_qinq_strip (sent packet: Single S-VLAN): FAIL
> > > >       reason: VLAN tags found in packet when should have been
> > > > stripped: Ether / Dot1Q / 802.1q (??) vlan ?? / LLC / Raw / Padding
> > > >     test_qinq_strip (sent packet: QinQ): FAIL
> > > >       reason: VLAN tags found in packet when should have been
> > > > stripped: Ether / Dot1Q / Dot1AD / 802.1q (0x1c) vlan 1280 / LLC / Raw
> > > > / Padding
> > > >     test_vlan_strip (sent packet: Single VLAN): PASS
> > > >       reason: VLAN tag stripped from packet
> > > >     test_vlan_strip (sent packet: Stacked VLAN): PASS
> > > >       reason: Received packet had outer VLAN stripped, with inner VLAN
> > intact
> > > >     test_vlan_strip (sent packet: Single S-VLAN): PASS
> > > >       reason: S-VLAN tag stripped from packet
> > > >     test_vlan_strip (sent packet: QinQ): FAIL
> > > >       reason: Neither tag stripped
> > > >
> > >
> > > Can you confirm exactly what is being sent in each case for the ethertype
> > > of the VLAN tag? When you say single and stacked VLANs, that is VLANs with
> > > 0x8100 type, correct? Is single S-VLAN a tag with ethertype 0x88a8, and
> > > QinQ packet a packet with one 0x88a8 and one 0x8100? No other type options,
> > > e.g. 0x9100 were checked, right?
> > >
> > > /Bruce
> > 
> > That's correct, single VLAN is one 0x8100 tag, stacked is two, single
> > S-VLAN is one 0x88a8, and QinQ is 0x88a8 and 0x8100. No other types
> > were tested in the stripping case
> 
> 
> Bruce,
> 
> It seems the drivers have the ability to set the EtherType of the Outer (and sometimes Inner) tag:
> https://elixir.bootlin.com/dpdk/v25.07/source/lib/ethdev/rte_ethdev.h#L3752
> https://elixir.bootlin.com/dpdk/v25.07/source/drivers/net/intel/e1000/igb_ethdev.c#L2739
> https://elixir.bootlin.com/dpdk/v25.07/source/drivers/net/intel/i40e/i40e_ethdev.c#L4038
> 
> -Morten
>
Thanks. Question is, does this help us to clarify the behaviour for these
tags? Based on the fact that the comment for the tag_type says that the
outer is the same as single for vlan types, then should the behaviour be:

* VLAN strip - strip at most one tag as defined by the "outer/single" VLAN
  type.
* QinQ strip:
  - if outer/single VLAN tag matches the outer tag type, strip it
  - if outer tag has been stripped, and inner tag matches the tag type,
    strip that also.
  - if a single VLAN tag is present, it gets stripped only if it's tag type
    matches outer type - it is left alone if it matches the inner type
  - if two VLAN tags are present, and the inner tag matches, it is not
    stripped if the outer tag does not match/has not been stripped.

Also, should we specify for DPDK what the default tags should be for the
two cases. It seems for the Intel NICs that I tried, that both inner and
outer tags always start with 0x8100. That's probably a good default for
VLAN strip, but for QinQ strip, we probably want hardware to default to
0x88a8 and 0x8100. Alternatively, we could/should mandate that drivers
explicitly set the required tags before starting the port.

/Bruce