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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
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