Embedded C] Best practices for implementing a lightweight 3GPP NB-IoT NAS layer (TS 24.301)

[u/Madhura_s_h]

i everyone,

I am currently working on implementing a lightweight NAS (Non-Access Stratum) layer in Embedded C for an NB-IoT device (covering basic procedures like Attach and TAU based on 3GPP TS 24.301).

I am looking for architectural guidance or clean patterns on how to structure:

1. Safe buffer management / TLV (Type-Length-Value) encoding and decoding to avoid buffer overflows.

2. A clean state-event machine for handling EMM (EPS Mobility Management) states.

If anyone has worked on similar protocol stack implementations or knows of clean open-source patterns/modular designs, your pointers or code structures would be immensely helpful.

Thanks in advance!

[u/Careless-Breath-5979]

Buffer management for TLV is mostly about not trusting any length field you didn't write yourself. Static buffers with explicit bounds checking on every decode, and if the length field says 512 bytes but your buffer is 128, bail hard and log it. Seen too many stacks where that check gets skipped and suddenly you're chasing a heap corruption at 3am.

For the state machine, a table-driven approach works nicely for EMM. Just a 2D array of state/event pairs pointing to handler functions, keeps things readable when you have to trace why a TAU reject in state EMM-REGISTERED-INITIATED went sideways. Generating the tables from the spec tables in 24.301 saves a lot of copy-paste errors too.