RFC: new type for NAS/EPS in gsmtap.h and question

Shinjo Park pshinjo at sec.t-labs.tu-berlin.de
Thu Nov 16 14:54:15 UTC 2017

Hi list,

While I was experimenting with osmo-qcdiag and other LTE stuff, I want to add 
NAS/EPS as a new payload type for gsmtap.h.

Unlike GSM and UMTS, LTE introduced separate layer for encryption of NAS and 
RRC. As a result, while NAS messages are piggybacked to LTE RRC, but after NAS 
security had been activated only encrypted NAS messages are available at RRC 
layer. This is reflected into the baseband diagnostics of various makers: 
Qualcomm provides separate diagnostic item for LTE NAS (both encrypted and 
plain) and RRC. Separate payload type for LTE RRC and LTE NAS will solve this 
issue. I can submit a patch if this looks positive.

Also, I have a question regarding ARFCN field. Currently (in version 2) ARFCN 
is a 16-bit integer, with 2-bit of flags (PCS band, uplink) therefore making 
14-bits available for raw value. This causes some problem in LTE:

1) EARFCNs for uplink are starting from 18000, which is larger than 2^14
2) There are EARFCNs even larger than 2^16 (Bands 65+, LTE-U frequencies)
3) No separate indicator for ARFCNs used by UMTS/LTE-TDD network

Also in UMTS, there are overlapping UARFCNs between bands, which necessitates 
a separate field for band indicator. Changes regarding these will break the 
GSMTAP header structure, therefore I want to discuss about how these could be 

Best Regards,
Shinjo Park <pshinjo at sec.t-labs.tu-berlin.de>
Security in Telecommunications <sec.t-labs.tu-berlin.de>
TU Berlin / Telekom Innovation Laboratories
Ernst-Reuter-Platz 7, Sekr TEL 17 / D - 10587 Berlin, Germany
Phone: +49 30 8353 58272

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