Iu / 3G testing of Osmo{MSC,SGSN,HNBGW} using TTCN-3
====================================================
:author:	Harald Welte <laforge@gnumonks.org>
:copyright:	2019 by Harald Welte (License: CC-BY-SA)
:backend:	slidy
:max-width:	45em


== Osmocom TTCN-3 Test Suites

* developed in 2017+2018
* compiled using Eclipse TITAN
** uses just a command-line compiler + Makefiles
** no IDE needed at all, don't let _Eclipse_ fool you
* containerized in Docker
* executed by Jenkins CI

== Test Suite Philosophy

* test one network element (our IUT)
* test external behavior (3GPP and non-3GPP)
* emulate entire environment from TTCN-3
* don't reuse Osmocom C-code protocol implementations in the tests
* test against independent TTCN-3 implementations!

== What to test?

* successful cases
* erroneous cases (no answer, NACK, ...)
** many difficult to reproduce with real phones/devices
* load / resource exhaustion
* spec compliance
* focus on functionality actually relevant to IUT

== Why TTCN-3 + TITAN

* TTCN-3 specifically designed for telecom protocol testing
* TITAN team released many telecom protocols in TTCN-3, such as
** BSSAP, L3 (RR/MM/CC), SMS (CP/RP/TP), SS, M3UA, SCCP, GTP, NS, BSSGP, ...
** shortens our test development cycle
** permits us to test against known working industry implementations

== SGSN_Tests.ttcn so far

* external interfaces
** Gb (emulates PCU side NS/BSSGP + MS)
** Gp (emulates GGSN)
** GSUP (emulates HLR)
** VTY

no 3G (IuCS) testing at all

[graphviz]
----
digraph G {
  rankdir=LR;
  SGSN [label="SGSN\nosmo-sgsn",shape="box"];
  ATS [label="ATS\nSGSN_Tests.ttcn"];

  ATS -> SGSN [label="Gb"];
  SGSN-> ATS [label="Gp (GTP)"];
  ATS -> SGSN [label="VTY"];
}
----

== A vs. Iu

* In theory, almost identical
** both use one SCCP connection per subscriber / radio channel
** both encapsulate the same Layer3 (DTAP) towards MS/UE
** more or less a 1:1: mapping between BSSMAP and RANAP
*** e.g. COMPLETE LAYER3 INFO -> InitialUeMessage
*** e.g. ASSIGNMENT -> RabAssignment
*** e.g. CLEAR -> IuRelease procedure
* In practise, _things are complicated..._

== A interface

image::gsm_control_stack_2g_cs.svg[width="100%"]

== Iu-CS interface

image::gsm_control_stack_3g_cs.svg[width="100%"]

== What's so special about Iu?

* not specified in human-readable text/tables, but ASN.1
* uses rather advanced features of ASN.1 syntax (information object classes)
* uses rather exotic ancoding APER (Aligned Packed Encoding Rules)
* Ericsson didn't release any titan.ProtocolModule.* for Iu

== TTCN-3 and ASN.1

* both TTCN-3 and ASN.1 originate at ITU
** TTCN-3 spec covers "native" use of ASN.1 within TTCN-3
*** no code generation (e.g. .asn1 -> .ttcn)
*** asn syntax can be directly used from TTCN-3 sources
**** all types are imported into the TTCN3 namespace

== Name mangling of ASN.1 types in TTCN-3

* replace all hyphen with underscore
** e.g. RANAP-PDU -> RANAP_PDU


== Eclipse TITAN and ASN.1

* Eclipse TITAN can parse the RANAP ASN.1 
** you simply add the *.asn files next to your *.ttcn files when compiling
* ASN.1 compiled directly to C++, no intermediate .ttcn code generated
** makes life a bit hard initially, while you still learn about the mapping of names/types/etc.
* TITAN only supports BER, no other encoding rules like APER

== BER - APER transcoding

* we need a transcoder that
** parses RANAP in APER and converts it to BER (Rx)
** parses RANAP in BER and converts it to APER (Tx)
* use which tool to do the job?
** asn1c (APER still not upstream; we don't want to run asn1c on ATS + IUT)
** erlang (we don't want to run an Erlang VM next to every TTCN-3 test)
** ffasn1c (proprietary, but sysmocom has a license and can share generated binary code)

== libfftranscode

* binary-only shared library generated by sysmocom, available from http://ftp.osmocom.org/binaries/libfftranscode/
** contact me if you have a valid use case for running tests on non-x86_64 / non-debian systems
* contains libffasn1c runtime code and ffasn1c-generated code for RANAP, RUA, HNBAP
* can be extended for other APER based protcols like S1AP (LTE) as needed
** contact me if you want to write TTCN3 tests for other protocols

== Writing RANAP templates

How to write RANAP templates without a ttcn source file?

Theory:

* use binary RANAP packets from pcap files
* feed them into the TITAN decoder
* look at the log output
* copy + paste + customize

== Problems

Problems:

 RANAP_Selftests.ttcn:49: Dynamic test case error: While BER-decoding type '@RANAPMIN.RANAP-PDU': While decoding '@RANAPMIN.RANAP-PDU' type: Alternative 'initiatingMessage': While decoding '@RANAPMIN.InitiatingMessage' type: Component 'value_': While checking tags (expecting [2]): Tag mismatch: Received: [UNIVERSAL 16].

* somehow TITAN cannot parse the BER generate by ffasn1c :(
** this is where I gave up for ~6 months after the first attempt
** I re-visited this in early April 2018
** one direction (BER encode) could be fixed by hacking runtime library source
** other direction required fix in compiler
** new version of ffasn1c has been released -> works

== Integrating with existing test suites

* existing tests are written without planning for 3G
* a lot of what they test is in L3/DTAP and should work over A + IuCS
* how to reuse them with 3G?

== BSSAP_Emulation -> RAN_Emulation

image::bssap_ranap_rename.png[width="100%"]


== BSSAP_Emulation -> RAN_Emulation

* copy + paste + edit `BSSAP_CodecPort` -> `RANAP_CodecPort`
* modularize protocol support (build without MGCP/BSSAP)
* rename `f_bssap_*` to `f_ran_adapter_*`
* add config option `cfg.transport == RANAP_TRANSPORT_IuCS`
* connect lower end of `RAN_Emulation` to `RANAP_CodecPort`, not `BSSAP_CodecPort`

== How does this look to the test case

* whenever L3 PDUs (DTA) are sent/received: no change
* whenever BSSMAP is transceived: distinguish BSSMAP vs RANAP
* good abstraction using functions can help

image::bssap_ranap_conditional_altsteps.png[width="100%"]

== Abstracting the send side

image::cl3_or_initialue.png[width="100%"]

image::cl3_or_initialue2.png[width="100%"]

== Extending a 2G test case to 3G

image::msc_iu_testcase.png[width="100%"]

== OsmoSGSN test suite

* MSC was easy: IuCS is very much like A.
* as opposed to the A interface (MSC) Gb doesn't use SCCP transport
* no similarity between 2G (Gb) and 3G (IuPS) protocol stack

== Gb interface

image::gsm_control_stack_2g_ps.svg[width="100%"]

== Iu-PS interface

image::gsm_control_stack_3g_ps.svg[width="100%"]

== OsmoSGSN test suite

* no concept of "subscriber connection" in 2G / Gb
* 3G IuPS very much like IuCS: SCCP connection per subscriber
* less common infrastructure that can be shared between 2G and 3G tests

== Further Reading

* http://git.osmocom.org/osmo-ttcn3-hacks/
* http://git.osmocom.org/docker-playground/
* http://osmocom.org/projects/cellular-infrastructure/wiki/Titan_TTCN3_Notes

== EOF

End of File
