src/vnet/mtu.rst

.. _mtu_doc:

MTU in VPP
==========

Maximum Transmission Unit is a term used to describe the maximum sized
“thingy” that can be sent out an interface. It can refer to the maximum
frame size that a NIC can send. On Ethernet that would include the
Ethernet header but typically not the IGF. It can refer to the maximum
packet size, that is, on Ethernet an MTU of 1500, would allow an IPv4
packet of 1500 bytes, that would result in an Ethernet frame of 1518
bytes.


VPP allows setting of the physical payload MTU. I.e. not including L2
overhead. Setting the hardware MTU will program the NIC. This MTU will
be inherited by all software interfaces.

VPP also allows setting of the payload MTU for software interfaces.
Independently of the MTU set on the hardware. If the software payload
MTU is set higher than the capability of the NIC, the packet will be
dropped.

In addition VPP supports setting the MTU of individual network layer
protocols. IPv4, IPv6 or MPLS. For example an IPv4 MTU of 1500 (includes
the IPv4 header) will fit in a hardware payload MTU of 1500.

*Note we might consider changing the hardware payload MTU to hardware
MTU*. That is, the MTU includes all L2 framing. Then the payload MTU can
be calculated based on the interface’s configuration. E.g. 802.1q tags
etc.

There are currently no checks or warnings if e.g. the user configures a
per-protocol MTU larger than the underlying payload MTU. If that happens
packets will be fragmented or dropped.

Data structures
^^^^^^^^^^^^^^^

The hardware payload MTU is stored in the max_packet_bytes variable in
the vnet_hw_interface_t structure.

The software MTU (previously max_l3_packet_bytes) is in
vnet_sw_interface_t->in mtu[VNET_N_MTU].

MTU API
-------

Set physical MTU
^^^^^^^^^^^^^^^^

This API message is used to set the physical MTU. It is currently
limited to Ethernet interfaces. Note, this programs the NIC.

::

   autoreply define hw_interface_set_mtu
   {
    u32 client_index;
    u32 context;
    u32 sw_if_index;
    u16 mtu;
   };

Set the L2 payload MTU
^^^^^^^^^^^^^^^^^^^^^^

:: note
    (not including the L2 header) and per-protocol MTUs

This API message sets the L3 payload MTU. E.g. on Ethernet it is the
maximum size of the Ethernet payload. If a value is left as 0, then the
default is picked from VNET_MTU_L3.

::

   autoreply define sw_interface_set_mtu
   {
    u32 client_index;
    u32 context;
    u32 sw_if_index;
    /* $$$$ Replace with enum */
    u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
   };

Get interface MTU
^^^^^^^^^^^^^^^^^

The various MTUs on an interface can be queried with the
sw_interface_dump/sw_interface_details calls.

::

   define sw_interface_details
   {
     /* MTU */
     u16 link_mtu;

     /* Per protocol MTUs */
     u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
   };

MTU CLI
-------

::

   set interface mtu [packet|ip4|ip6|mpls] <value> <interface>