From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [RFC] ethdev: add min/max MTU to device info
Date: Wed, 6 Feb 2019 08:11:07 -0800
Message-ID: <20190206081107.41c57bad@hermes.lan>
References: <20180905164157.844-1-stephen@networkplumber.org>
 <98CBD80474FA8B44BF855DF32C47DC35B4265B@smartserver.smartshare.dk>
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To: Morten =?UTF-8?B?QnLDuHJ1cA==?= <mb@smartsharesystems.com>
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On Wed, 6 Feb 2019 14:05:34 +0100
Morten Br=C3=B8rup <mb@smartsharesystems.com> wrote:

> Good work, Stephen.
>=20
> It should also be documented how PMDs should interpret this MTU.
>=20
> Obviously, a VLAN tagged Ethernet frame grows from 1518 to 1522 bytes inc=
l. header and CRC, and should be allowed with an Ethernet MTU of 1500 bytes=
. There's even a #define ETHER_MAX_VLAN_FRAME_LEN for this, but that's as f=
ar as it goes...
>=20
> But how about frames with even larger headers, e.g. 4 MPLS labels makes a=
 frame 16 bytes longer, i.e. it grows from 1518 to 1534 bytes... is such a =
frame acceptable with an MTU of 1500 bytes?

No. According to standard practice in Linux and FreeBSD, only the first VLA=
N tag is free.
After that any other headers count against MTU.

>=20
> According to Wikipedia (https://en.wikipedia.org/wiki/Maximum_transmissio=
n_unit), MTU describes the maximum payload size, and is not tied to the max=
imum frame size. However, the Linux kernel (at least the older versions I h=
ave been working with) incorrectly ties the MTU directly to the maximum fra=
me size, so the MTU has to increased to support larger headers (e.g. QinQ o=
r an MPLS stack). Do none, all or some DPDK PMDs suffer from the same error?
>=20

Maximum Transmission Unit and Maximum Receive Unit (MRU) are separate. On m=
ost hardware they are the same but
there is variation. Some hardware can receive larger sizes than MTU and som=
e have max receive length register.