datapath: Fix handling of 802.1Q and SNAP headers.

datapath: Fix handling of 802.1Q and SNAP headers.

The kernel and user datapaths have code that assumes that 802.1Q headers
are used only inside Ethernet II frames, not inside SNAP-encapsulated
frames.  But the kernel and user flow_extract() implementations would
interpret 802.1Q headers inside SNAP headers as being valid VLANs.  This
would cause packet corruption if any VLAN-related actions were to be taken,
so change the two flow_extract() implementations only to accept 802.1Q as
an Ethernet II frame type, not as a SNAP-encoded frame type.

802.1Q-2005 says that this is correct anyhow:

    Where the ISS instance used to transmit and receive tagged frames is
    provided by a media access control method that can support Ethernet
    Type encoding directly (e.g., is an IEEE 802.3 or IEEE 802.11 MAC) or
    is media access method independent (e.g., 6.6), the TPID is Ethernet
    Type encoded, i.e., is two octets in length and comprises solely the
    assigned Ethernet Type value.

    Where the ISS instance is provided by a media access method that
    cannot directly support Ethernet Type encoding (e.g., is an IEEE
    802.5 or FDDI MAC), the TPID is encoded according to the rule for
    a Subnetwork Access Protocol (Clause 10 of IEEE Std 802) that
    encapsulates Ethernet frames over LLC, and comprises the SNAP
    header (AA-AA-03) followed by the SNAP PID (00-00-00) followed by
    the two octets of the assigned Ethernet Type value.

All of the media that OVS handles supports Ethernet Type fields, so to me
that means that we don't have to handle 802.1Q-inside-SNAP.

On the other hand, we *do* have to handle SNAP-inside-802.1Q, because this
is actually allowed by the standards.  So this commit also adds that
support.

I verified that, with this change, both SNAP and Ethernet packets are
properly recognized both with and without 802.1Q encapsulation.

I was a bit surprised to find out that Linux does not accept
SNAP-encapsulated IP frames on Ethernet.

Here's a summary of how frames are handled before and after this commit:

Common cases
------------

       Ethernet
    +------------+
1.  |dst|src|TYPE|
    +------------+

       Ethernet       LLC         SNAP
    +------------+ +--------+ +-----------+
2.  |dst|src| len| |aa|aa|03| |000000|TYPE|
    +------------+ +--------+ +-----------+

       Ethernet       802.1Q
    +------------+ +---------+
3.  |dst|src|8100| |VLAN|TYPE|
    +------------+ +---------+

       Ethernet       802.1Q      LLC         SNAP
    +------------+ +---------+ +--------+ +-----------+
4.  |dst|src|8100| |VLAN| LEN| |aa|aa|03| |000000|TYPE|
    +------------+ +---------+ +--------+ +-----------+

Unusual cases
-------------

       Ethernet       LLC         SNAP         802.1Q
    +------------+ +--------+ +-----------+ +---------+
5.  |dst|src| len| |aa|aa|03| |000000|8100| |VLAN|TYPE|
    +------------+ +--------+ +-----------+ +---------+

       Ethernet       LLC
    +------------+ +--------+
6.  |dst|src| len| |xx|xx|xx|
    +------------+ +--------+

       Ethernet       LLC         SNAP
    +------------+ +--------+ +-----------+
7.  |dst|src| len| |aa|aa|03| |xxxxxx|xxxx|
    +------------+ +--------+ +-----------+

       Ethernet       802.1Q      LLC
    +------------+ +---------+ +--------+
8.  |dst|src|8100| |VLAN| LEN| |xx|xx|xx|
    +------------+ +---------+ +--------+

       Ethernet       802.1Q      LLC         SNAP
    +------------+ +---------+ +--------+ +-----------+
9.  |dst|src|8100| |VLAN| LEN| |aa|aa|03| |xxxxxx|xxxx|
    +------------+ +---------+ +--------+ +-----------+

Behavior
--------

   ---------------  ---------------  -------------------------------------
       Before           After
     this commit      this commit
   dl_type dl_vlan  dl_type dl_vlan  Notes
   ------- -------  ------- -------  -------------------------------------
1.   TYPE    ffff     TYPE    ffff   no change
2.   TYPE    ffff     TYPE    ffff   no change
3.   TYPE    VLAN     TYPE    VLAN   no change
4.    LEN    VLAN     TYPE    VLAN   proposal fixes behavior
5.   TYPE    VLAN     8100    ffff   802.1Q says this is invalid framing
6.   05ff    ffff     05ff    ffff   no change
7.   05ff    ffff     05ff    ffff   no change
8.    LEN    VLAN     05ff    VLAN   proposal fixes behavior
9.    LEN    VLAN     05ff    VLAN   proposal fixes behavior

Signed-off-by: Ben Pfaff <blp@nicira.com>