From d4a70647e6b8de2cb81cbea3c53d08c299b65cc5 Mon Sep 17 00:00:00 2001 From: Nathan Skrzypczak Date: Fri, 8 Oct 2021 14:01:27 +0200 Subject: docs: convert vpp doc md->rst Type: improvement Change-Id: If453321785b04f9c16e8cea36fb1910efaeb2c59 Signed-off-by: Nathan Skrzypczak --- src/vnet/mtu.rst | 108 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 108 insertions(+) create mode 100644 src/vnet/mtu.rst (limited to 'src/vnet/mtu.rst') diff --git a/src/vnet/mtu.rst b/src/vnet/mtu.rst new file mode 100644 index 00000000000..c7e92523c7f --- /dev/null +++ b/src/vnet/mtu.rst @@ -0,0 +1,108 @@ +.. _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] -- cgit 1.2.3-korg