Date:	Mon, 26 Oct 2015 12:40:55 +0900
From:	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:	"David S . Miller" <davem@...emloft.net>
Cc:	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Patrick McHardy <kaber@...sh.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Vlad Yasevich <vyasevich@...il.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	intel-wired-lan@...ts.osuosl.org, toshiaki.makita1@...il.com,
	netdev@...r.kernel.org
Subject: [PATCH v2 net-next 0/4] Automatic adjustment of max frame size

This patch set tries to resolve packet drop by oversize error on receiving
double tagged packets and possibly other encapsulated packets.

Problem description:
Currently most NICs have 4 bytes room of receive buffer for vlan header and
can receive 1522 bytes frame at maximum.
This is, however, not sufficient once double tagged vlan is used.
As MEF [1] says, maximum frame size of double tagged packets need to be at
least 1526 to provide transparent ethernet VPN, and along the same line,
HW switches send 1526 bytes double tagged packets.
Thus, double tagged packets are dropped by default in most cases by
oversize error. NICs need to accept 1526 bytes packets in this situation.

Approaches:
To satisfy this requirement, this patch set introduces a way to indicate
needed extra buffer space to drivers.
This way can be re-used by other protocols than vlan, like mpls, vxlan, etc.

Other possible solutions:

- To adjust mtu automatically when stacked vlan device is created.
  This is suboptimal because lower device is not necessarily used for only
  vlan. Sometimes tagged and untagged traffic are both used at the same time.
  Also, there are devices that already reserve 8 bytes room, in which case mtu
  adjustment is unnecessary.

- To reserve more room by default.
  This is also suboptimal because there are devices that chages behavior
  when max acceptable frame size gets larger. For exapmle, e1000e enters
  jumbo frame mode which has some additional ristrictions than normal.
  Also, this is vlan-specific solution and not reusable by other encapsulation
  protocols.

This patch set introduces .ndo_enc_hdr_len() and I chose e1000e as the first
implementation. Patch 3 makes vlan driver utilize this API and automatically
expand max frame size of the real device. Patch 4 makes bridge use the API
in similar way as vlan.

Challenges:
- Restore/shrink extra header room after vlan devices are deleted.
  This will need some additional memory storage.
- Manual modification of extra buffer size (by iproute2).

Note:
- This problem was once discussed in Netdev 0.1 [2].
  This patch set is based on the conclusion of the discussion.

Changes:
 v2: Fixed chackpatch warnings

[1] https://wiki.mef.net/display/CESG/ENNI+Frame
[2] https://www.netdev01.org/docs/netdev01_bof_8021ad_makita_150212.pdf

Toshiaki Makita (4):
  net: Add ndo_enc_hdr_len to notify extra header room for encapsulated
    frames
  e1000e: Add ndo_enc_hdr_len
  vlan: Notify real device of encap header length
  bridge: Notify port device of encap header length

 drivers/net/ethernet/intel/e1000e/netdev.c | 82 ++++++++++++++++++++++--------
 include/linux/netdevice.h                  |  9 ++++
 net/8021q/vlan.c                           | 16 +++++-
 net/8021q/vlan_dev.c                       | 48 +++++++++++++++--
 net/bridge/br_vlan.c                       | 18 +++++++
 net/core/dev.c                             | 36 +++++++++++++
 6 files changed, 180 insertions(+), 29 deletions(-)

-- 
1.8.1.2


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