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Could someone please advise on how iOS devices setup MTU on cellular (pdp_ip*) interfaces?

When iPhone is started in flight mode, all those interfaces have default MTU = 1500.

But after registering to LTE network, MTU on iPhone suddenly changes to 1450 - despite the fact that the LTE network is sending MTU=1500 in PCO during network attach.

This results in suboptimal (too low) MTU setting for upload direction, because this LTE network correctly delivers full 1500-byte user packets in the download direction (it uses larger MTU in the core to accomodate GTP and other overheads).

Seems 1450 is statically configured somewhere - does anyone know what parameter controls this?

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  • Static assignment is possible, but I would expect something more like negotiation and discovery based on the cellular profile the carrier provides iOS or en.wikipedia.org/wiki/Path_MTU_Discovery is there a problem you’re facing here or just “I want to know how this works”
    – bmike ♦
    Commented Oct 10 at 13:28
  • Added problem description to the original post. Negotiation via PCO is correctly performed, nevertheless all iOS devices always use MTU=1450 in this network.
    – phr
    Commented Oct 10 at 14:21

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