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zephyrproject-rtos / zephyr

Primary Git Repository for the Zephyr Project. Zephyr is a new generation, scalable, optimized, secure RTOS for multiple hardware architectures.
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Networking: Dynamic interface MTU #76128

Open JordanYates opened 1 month ago

JordanYates commented 1 month ago

Is your enhancement proposal related to a problem? Please describe.

The Zephyr network interface APIs' expose an MTU (maximum transmission unit), but this is a compile-time hardware MTU, not a runtime network MTU.

https://github.com/zephyrproject-rtos/zephyr/blob/d06e95c49b75dfc911f2ab77c2a934abe624c751/include/zephyr/net/net_if.h#L657-L658

https://github.com/zephyrproject-rtos/zephyr/blob/d06e95c49b75dfc911f2ab77c2a934abe624c751/include/zephyr/net/net_if.h#L1234-L1243

The compile-time value in insufficient to avoid fragmentation because while the hardware may support an MTU of X, the network the hardware is currently connected to may only support a MTU of Y. The MTU of LTE networks in particular is highly variable between different providers, with devices jumping between networks as they travel.

Describe the solution you'd like

A common pattern where upon the transition into NET_IF_OPER_UP, the MTU of the current network is provided to the stack and is queryable by end users in the appropriate networking event callback (e.g. NET_EVENT_L4_CONNECTED, NET_EVENT_WIFI_CONNECT_RESULT)

rlubos commented 1 month ago

It looks that the MTU stored in iface->if_dev->mtu is already possible to override with net_if_set_mtu(), isn't that enough? Or do you propose that we store a separate value for the network MTU at the net_if level?