d4a70647e6
Type: improvement Change-Id: If453321785b04f9c16e8cea36fb1910efaeb2c59 Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
109 lines
2.9 KiB
ReStructuredText
109 lines
2.9 KiB
ReStructuredText
.. _mtu_doc:
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MTU in VPP
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==========
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Maximum Transmission Unit is a term used to describe the maximum sized
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“thingy” that can be sent out an interface. It can refer to the maximum
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frame size that a NIC can send. On Ethernet that would include the
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Ethernet header but typically not the IGF. It can refer to the maximum
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packet size, that is, on Ethernet an MTU of 1500, would allow an IPv4
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packet of 1500 bytes, that would result in an Ethernet frame of 1518
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bytes.
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VPP allows setting of the physical payload MTU. I.e. not including L2
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overhead. Setting the hardware MTU will program the NIC. This MTU will
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be inherited by all software interfaces.
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VPP also allows setting of the payload MTU for software interfaces.
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Independently of the MTU set on the hardware. If the software payload
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MTU is set higher than the capability of the NIC, the packet will be
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dropped.
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In addition VPP supports setting the MTU of individual network layer
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protocols. IPv4, IPv6 or MPLS. For example an IPv4 MTU of 1500 (includes
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the IPv4 header) will fit in a hardware payload MTU of 1500.
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*Note we might consider changing the hardware payload MTU to hardware
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MTU*. That is, the MTU includes all L2 framing. Then the payload MTU can
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be calculated based on the interface’s configuration. E.g. 802.1q tags
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etc.
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There are currently no checks or warnings if e.g. the user configures a
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per-protocol MTU larger than the underlying payload MTU. If that happens
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packets will be fragmented or dropped.
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Data structures
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^^^^^^^^^^^^^^^
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The hardware payload MTU is stored in the max_packet_bytes variable in
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the vnet_hw_interface_t structure.
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The software MTU (previously max_l3_packet_bytes) is in
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vnet_sw_interface_t->in mtu[VNET_N_MTU].
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MTU API
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-------
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Set physical MTU
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^^^^^^^^^^^^^^^^
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This API message is used to set the physical MTU. It is currently
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limited to Ethernet interfaces. Note, this programs the NIC.
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::
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autoreply define hw_interface_set_mtu
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{
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u32 client_index;
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u32 context;
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u32 sw_if_index;
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u16 mtu;
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};
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Set the L2 payload MTU
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^^^^^^^^^^^^^^^^^^^^^^
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:: note
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(not including the L2 header) and per-protocol MTUs
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This API message sets the L3 payload MTU. E.g. on Ethernet it is the
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maximum size of the Ethernet payload. If a value is left as 0, then the
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default is picked from VNET_MTU_L3.
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::
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autoreply define sw_interface_set_mtu
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{
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u32 client_index;
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u32 context;
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u32 sw_if_index;
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/* $$$$ Replace with enum */
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u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
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};
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Get interface MTU
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^^^^^^^^^^^^^^^^^
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The various MTUs on an interface can be queried with the
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sw_interface_dump/sw_interface_details calls.
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::
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define sw_interface_details
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{
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/* MTU */
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u16 link_mtu;
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/* Per protocol MTUs */
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u32 mtu[4]; /* 0 - L3, 1 - IP4, 2 - IP6, 3 - MPLS */
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};
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MTU CLI
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-------
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::
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set interface mtu [packet|ip4|ip6|mpls] <value> <interface>
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