3GPP TS 49.008 '4.3 Roles of MSC-A, MSC-I and MSC-T' defines distinct roles:
- MSC-A is responsible for managing subscribers,
- MSC-I is the gateway to the RAN.
- MSC-T is a second transitory gateway to another RAN during Handover.
After inter-MSC Handover, the MSC-I is handled by a remote MSC instance, while
the original MSC-A retains the responsibility of subscriber management.
MSC-T exists in this patch but is not yet used, since Handover is only prepared
for, not yet implemented.
Facilitate Inter-MSC and inter-BSC Handover by the same internal split of MSC
roles.
Compared to inter-MSC Handover, mere inter-BSC has the obvious simplifications:
- all of MSC-A, MSC-I and MSC-T roles will be served by the same osmo-msc
instance,
- messages between MSC-A and MSC-{I,T} don't need to be routed via E-interface
(GSUP),
- no call routing between MSC-A and -I via MNCC necessary.
This is the largest code bomb I have submitted, ever. Out of principle, I
apologize to everyone trying to read this as a whole. Unfortunately, I see no
sense in trying to split this patch into smaller bits. It would be a huge
amount of work to introduce these changes in separate chunks, especially if
each should in turn be useful and pass all test suites. So, unfortunately, we
are stuck with this code bomb.
The following are some details and rationale for this rather huge refactoring:
* separate MSC subscriber management from ran_conn
struct ran_conn is reduced from the pivotal subscriber management entity it has
been so far to a mere storage for an SCCP connection ID and an MSC subscriber
reference.
The new pivotal subscriber management entity is struct msc_a -- struct msub
lists the msc_a, msc_i, msc_t roles, the vast majority of code paths however
use msc_a, since MSC-A is where all the interesting stuff happens.
Before handover, msc_i is an FSM implementation that encodes to the local
ran_conn. After inter-MSC Handover, msc_i is a compatible but different FSM
implementation that instead forwards via/from GSUP. Same goes for the msc_a
struct: if osmo-msc is the MSC-I "RAN proxy" for a remote MSC-A role, the
msc_a->fi is an FSM implementation that merely forwards via/from GSUP.
* New SCCP implementation for RAN access
To be able to forward BSSAP and RANAP messages via the GSUP interface, the
individual message layers need to be cleanly separated. The IuCS implementation
used until now (iu_client from libosmo-ranap) did not provide this level of
separation, and needed a complete rewrite. It was trivial to implement this in
such a way that both BSSAP and RANAP can be handled by the same SCCP code,
hence the new SCCP-RAN layer also replaces BSSAP handling.
sccp_ran.h: struct sccp_ran_inst provides an abstract handler for incoming RAN
connections. A set of callback functions provides implementation specific
details.
* RAN Abstraction (BSSAP vs. RANAP)
The common SCCP implementation did set the theme for the remaining refactoring:
make all other MSC code paths entirely RAN-implementation-agnostic.
ran_infra.c provides data structures that list RAN implementation specifics,
from logging to RAN de-/encoding to SCCP callbacks and timers. A ran_infra
pointer hence allows complete abstraction of RAN implementations:
- managing connected RAN peers (BSC, RNC) in ran_peer.c,
- classifying and de-/encoding RAN PDUs,
- recording connected LACs and cell IDs and sending out Paging requests to
matching RAN peers.
* RAN RESET now also for RANAP
ran_peer.c absorbs the reset_fsm from a_reset.c; in consequence, RANAP also
supports proper RESET semantics now. Hence osmo-hnbgw now also needs to provide
proper RESET handling, which it so far duly ignores. (TODO)
* RAN de-/encoding abstraction
The RAN abstraction mentioned above serves not only to separate RANAP and BSSAP
implementations transparently, but also to be able to optionally handle RAN on
distinct levels. Before Handover, all RAN messages are handled by the MSC-A
role. However, after an inter-MSC Handover, a standalone MSC-I will need to
decode RAN PDUs, at least in order to manage Assignment of RTP streams between
BSS/RNC and MNCC call forwarding.
ran_msg.h provides a common API with abstraction for:
- receiving events from RAN, i.e. passing RAN decode from the BSC/RNC and
MS/UE: struct ran_dec_msg represents RAN messages decoded from either BSSMAP
or RANAP;
- sending RAN events: ran_enc_msg is the counterpart to compose RAN messages
that should be encoded to either BSSMAP or RANAP and passed down to the
BSC/RNC and MS/UE.
The RAN-specific implementations are completely contained by ran_msg_a.c and
ran_msg_iu.c.
In particular, Assignment and Ciphering have so far been distinct code paths
for BSSAP and RANAP, with switch(via_ran){...} statements all over the place.
Using RAN_DEC_* and RAN_ENC_* abstractions, these are now completely unified.
Note that SGs does not qualify for RAN abstraction: the SGs interface always
remains with the MSC-A role, and SGs messages follow quite distinct semantics
from the fairly similar GERAN and UTRAN.
* MGW and RTP stream management
So far, managing MGW endpoints via MGCP was tightly glued in-between
GSM-04.08-CC on the one and MNCC on the other side. Prepare for switching RTP
streams between different RAN peers by moving to object-oriented
implementations: implement struct call_leg and struct rtp_stream with distinct
FSMs each. For MGW communication, use the osmo_mgcpc_ep API that has originated
from osmo-bsc and recently moved to libosmo-mgcp-client for this purpose.
Instead of implementing a sequence of events with code duplication for the RAN
and CN sides, the idea is to manage each RTP stream separately by firing and
receiving events as soon as codecs and RTP ports are negotiated, and letting
the individual FSMs take care of the MGW management "asynchronously". The
caller provides event IDs and an FSM instance that should be notified of RTP
stream setup progress. Hence it becomes possible to reconnect RTP streams from
one GSM-04.08-CC to another (inter-BSC Handover) or between CC and MNCC RTP
peers (inter-MSC Handover) without duplicating the MGCP code for each
transition.
The number of FSM implementations used for MGCP handling may seem a bit of an
overkill. But in fact, the number of perspectives on RTP forwarding are far
from trivial:
- an MGW endpoint is an entity with N connections, and MGCP "sessions" for
configuring them by talking to the MGW;
- an RTP stream is a remote peer connected to one of the endpoint's
connections, which is asynchronously notified of codec and RTP port choices;
- a call leg is the higher level view on either an MT or MO side of a voice
call, a combination of two RTP streams to forward between two remote peers.
BSC MGW PBX
CI CI
[MGW-endpoint]
[--rtp_stream--] [--rtp_stream--]
[----------------call_leg----------------]
* Use counts
Introduce using the new osmo_use_count API added to libosmocore for this
purpose. Each use token has a distinct name in the logging, which can be a
globally constant name or ad-hoc, like the local __func__ string constant. Use
in the new struct msc_a, as well as change vlr_subscr to the new osmo_use_count
API.
* FSM Timeouts
Introduce using the new osmo_tdef API, which provides a common VTY
implementation for all timer numbers, and FSM state transitions with the
correct timeout. Originated in osmo-bsc, recently moved to libosmocore.
Depends: Ife31e6798b4e728a23913179e346552a7dd338c0 (libosmocore)
Ib9af67b100c4583342a2103669732dab2e577b04 (libosmocore)
Id617265337f09dfb6ddfe111ef5e578cd3dc9f63 (libosmocore)
Ie9e2add7bbfae651c04e230d62e37cebeb91b0f5 (libosmo-sccp)
I26be5c4b06a680f25f19797407ab56a5a4880ddc (osmo-mgw)
Ida0e59f9a1f2dd18efea0a51680a67b69f141efa (osmo-mgw)
I9a3effd38e72841529df6c135c077116981dea36 (osmo-mgw)
Change-Id: I27e4988e0371808b512c757d2b52ada1615067bd
In connection_for_subscriber(), do not return a ran_conn that is not yet
authenticated nor one that is already in release.
Using a ran_conn that is not yet authenticated may cause an auth/ciph
violation.
Using a ran_conn that is already in release may cause a use-after-free, see
OS#3842 for a description.
To be paranoid, upon releasing a conn, go through the transaction freeing
motions again by calling trans_conn_closed(), just in case some odd code path
added another transaction while the conn was already in release.
Related: OS#3842
Change-Id: Id957032e0ae1ff8ba055a75c3523447d3d06cbc3
The connection ref-counting implementation is specific to RAN
connections, and is not applicable for anything else. Moreover,
the API of this code is declared in 'ran_conn.h', so let's
move the code to a more logical place.
Change-Id: I593675d9bf56eaef12afdaf596ee1337b9a44259
Add an SGs interface (3GPP TS 29.118) to osmo-msc in order to support
SMS tunneling and Circuit Switched Fallback (CSFB)
Change-Id: I73359925fc1ca72b33a1466e6ac41307f2f0b11d
Related: OS#3615
ran_conn_get_conn_id(): instead of a talloc allocated string, return a static
buffer in ran_conn_get_conn_id(). So far this function had no callers.
Refactor ran_conn_update_id() API: during early L3-Complete, when no subscriber
is associated yet, update the FSM Id by the MI type seen in the L3 Complete
message: ran_conn_update_id_from_mi(). Later on set the vsub and re-update.
Call vlr.ops->subscr_update when the TMSI is updated, so that log context
includes the TMSI from then on.
Enrich context for vlr_subscr_name and ran_conn fi name.
Include all available information in vlr_subscr_name(); instead of either IMSI
or MSISDN or TMSI, print all of them when present. Instead of a short log,
rather have more valuable context.
A context info would now look like:
Process_Access_Request_VLR(IMSI-901700000014706:MSISDN-2023:TMSI-0x08BDE4EC:GERAN-A-3:PAGING_RESP)
It does get quite long, but ensures easy correlation of any BSSAP / IuCS
messages with log output, especially if multiple subscribers are busy at the
same time.
Print TMSI and TMSInew in uppercase hexadecimal, which is the typical
representation in the telecom world.
When showing the RAN conn id
GERAN_A-00000017
becomes
GERAN-A-23
- We usually write the conn_id in decimal.
- Leading zeros are clutter and might suggest hexadecimal format.
- 'GERAN-A' and 'UTRAN-Iu' are the strings defined by osmo_rat_type_name().
Depends: I7798c3ef983c2e333b2b9cbffef6f366f370bd81 (libosmocore)
Depends: Ica25919758ef6cba8348da199b0ae7e0ba628798 (libosmocore)
Change-Id: I66a68ce2eb8957a35855a3743d91a86299900834
For each conn, set a default logging category, to distinguish categories for
BSSMAP and RANAP based conns.
LOG_RAN_CONN(): log with the conn's default category,
LOG_RAN_CONN_CAT(): log with a manually set category (mostly for keeping
previous DMM logging on the same category).
In some places, replace LOGP() using manual context with LOG_RAN_CONN(), and
remove the manual context info, now provided by the conn->fi->id.
This is loosely related to inter-BSC and inter-MSC handover: to speed up
refactoring, I want to avoid the need for manual logging context and just use
this LOG_RAN_CONN().
Change-Id: I0a7809840428b1e028df6eb683bc5ffcc8df474a
Replace locally defined enum ran_type with libosmocore's new enum
osmo_rat_type, and value_string ran_type_names with osmo_rat_type_names.
The string representations change, which has cosmetic effects on the test suite
expectations.
Depends: I659687aef7a4d67ca372a39fef31dee07aed7631 (libosmocore)
Change-Id: I2c78c265dc99df581e1b00e563d6912c7ffdb36b
With the dawn of inter-BSC,MSC handover, adopting the MSC-A,-I,-T roles from
3GPP TS 49.008, the RAN connection shall soon be a neatly separated corner of
osmo-msc, so gravitate ran_conn decarations to files of matching name.
Also, the current chaos of API defined in files with mismatching/meaningless
names drives me crazy.
Change-Id: Ice31e6c43e46678538c65261f150c67e1d0845e5
Neels Hofmeyr
Harald Welte