SCTP

SCTP is the transport that sits under the control-plane signalling protocols — NGAP on N2 in 5G, S1AP in LTE, and others. It was chosen over TCP for good reasons specific to signalling. It's message-oriented, so it preserves the boundaries of each signalling message instead of handing up a byte stream you have to re-frame. It supports multi-streaming, so independent signalling exchanges don't block each other at the head of the line. And it supports multi-homing — an association can use more than one IP address per endpoint — so a transport path failure can be survived without dropping the signalling link.

Those last two are the ones that matter operationally. Multi-homing gives the resilient, redundant transport that carrier-grade signalling demands; multi-streaming keeps one stuck message from stalling unrelated procedures. You'll see SCTP associations being set up and monitored on the links between the RAN and the core, and an SCTP association going down takes the NGAP/S1AP signalling with it — so its health is something you watch directly when diagnosing control-plane outages.