| == Distributed GSM / Multicast MS Lookup |
| |
| Distributed GSM (D-GSM) allows independent mobile core network stacks to provide voice, SMS and Roaming services to each |
| other, without the need for centralised entities or administration authority, and in a way that is resilient against |
| unstable network links between sites. |
| |
| D-GSM aims at communal networks, where several independent sites, let's call them villages, each have a full mobile core |
| network infrastructure. It elegantly provides ad-hoc service for subscribers moving across villages, and allows villages |
| to dynamically join or leave the cooperative network without the need for configuration changes at other sites. |
| |
| A challenge for linking separate sites is to find the current location of a subscriber. Typically, in mobile networks, a |
| centralized entity keeps track of where to Page for subscribers. Running several fully independent sites with unreliable |
| links between them makes it hard to provide such centralisation. |
| |
| D-GSM finds subscribers by mslookup, a service provided by OsmoHLR, typically using multicast DNS queries. This allows |
| routing Location Updating requests, calls, and SMS to the right site without administrative delay nor the need for a |
| reliable link to a central database. |
| |
| D-GSM is highly resilient against single sites or links becoming temporarily unavailable. Service between still |
| reachable sites simply continues; Service to a disconnected site resumes as soon as it becomes reachable again. |
| |
| This brings an entirely new paradigm to mobile core network infrastructure: as sites become reachable on the IP network |
| and join the common IP multicast group, services between them become available immediately. Basically, the only premise |
| is that IP routing and multicast works across sites, and that each site uses unique IPA names in the GSUP config. |
| |
| This chapter describes how D-GSM and mslookup work, and how to configure sites to use D-GSM, using Osmocom core network |
| infrastructure. |
| |
| === Finding Subscribers: mslookup Clients |
| |
| There are two fundamentally distinct subscriber lookups provided by the mslookup service. |
| |
| ==== Find the Current Location of an MSISDN |
| |
| [[fig_dgsm_connect]] |
| .mslookup for connecting subscribers: Alice is visiting village C; a phone call gets routed directly to her current location independently from her resident village infrastructure |
| [graphviz] |
| ---- |
| digraph G { |
| rankdir=LR |
| |
| subgraph cluster_village_b { |
| label="Village B" |
| ms_bob [label="Bob\n(from village B)",shape=box] |
| pbx_b [label="SIP B"] |
| } |
| |
| subgraph cluster_village_c { |
| label="Village C" |
| ms_alice [label="Alice\n(from village A)",shape=box] |
| msc_c [label="MSC C"] |
| hlr_c [label="HLR C"] |
| sip_c [label="SIP C"] |
| } |
| |
| ms_alice -> msc_c [style=dashed,arrowhead=none] |
| msc_c -> hlr_c [label="attached",style=dashed,arrowhead=none] |
| ms_bob -> pbx_b [label="call Alice"] |
| pbx_b -> hlr_c [label="mslookup by MSISDN",style=dotted,dir=both] |
| pbx_b -> sip_c -> msc_c -> ms_alice [label="call"] |
| } |
| ---- |
| |
| For example, if a subscriber is currently visiting another village, establish a phone call / send SMS towards that |
| village. |
| |
| - To deliver a phone call, a SIP agent integrates an mslookup client to request the SIP service of an MSISDN's current |
| location (example: <<dgsm_conf_dialplan>>). It receives an IP address and port to send the SIP Invite to. |
| |
| - To deliver an SMS, an ESME integrates an mslookup client to request the SMPP service of an MSISDN's current location |
| (example: <<dgsm_conf_esme_smpp>>). |
| |
| The current location of a subscriber may change at any time, and, when moving across locations, a subscriber may |
| suddenly lose reception to the previous location without explicitly detaching. Hence an mslookup request for the current |
| location of an MSISDN may get numerous responses. To find the currently valid location, mslookup includes the age of the |
| subscriber record, i.e. how long ago the subscriber was last reached. The one response with the youngest age reflects |
| the current location. |
| |
| In order to evaluate several responses, mslookup always waits for a fixed amount of time (1 second), and then evaluates |
| the available responses. |
| |
| Services are not limited to SIP and SMPP, arbitrarily named services can be added to the mslookup configuration. |
| |
| .Message sequence for locating an MSISDN to deliver a voice call |
| ["mscgen"] |
| ---- |
| msc { |
| hscale="2"; |
| moms[label="MS,BSS\nvillage A"],momsc[label="MSC,MGW\nvillage A"],mosipcon[label="osmo-sip-connector\nvillage A"],mopbx[label="PBX\nvillage A"],mthlr[label="OsmoHLR\nvillage B"],mtsipcon[label="osmo-sip-connector\nvillage B"],mtmsc[label="MGW,MSC\nvillage B"],mtms[label="RAN,MS\nvillage B"]; |
| |
| moms =>> momsc [label="CC Setup"]; |
| momsc =>> mosipcon [label="MNCC_SETUP_IND"]; |
| mosipcon =>> mopbx [label="SIP INVITE"]; |
| mopbx rbox mopbx [label="dialplan: launch mslookup by MSISDN"]; |
| --- [label="multicast-DNS query to all connected sites"]; |
| ...; |
| mopbx <<= mthlr [label="mDNS response\n(age)"]; |
| mopbx rbox mopbx [label="wait ~ 1s for more mDNS responses"]; |
| ...; |
| mopbx =>> mtsipcon [label="SIP INVITE (MT)"]; |
| mtmsc <<= mtsipcon [label="MNCC_SETUP_REQ"]; |
| mtms <<= mtmsc [label="Paging (CC)"]; |
| moms rbox mtms [label="voice call commences"]; |
| |
| } |
| ---- |
| |
| ==== Find the Home HLR for an IMSI |
| |
| [[fig_dgsm_roaming]] |
| .mslookup for Roaming: Alice visits village B; she can attach to the local mobile network, which proxies HLR administration to her home village. |
| [graphviz] |
| ---- |
| digraph G { |
| rankdir=LR |
| |
| subgraph cluster_village_b { |
| label="Village B" |
| |
| ms_alice [label="Alice\n(from village A)",shape=box] |
| msc_b [label="MSC B"] |
| hlr_b [label="HLR B"] |
| } |
| |
| subgraph cluster_village_a { |
| label="Village A" |
| hlr_alice [label="Alice's home HLR"] |
| } |
| |
| ms_alice -> msc_b -> hlr_b [label="Location\nUpdating"] |
| hlr_b -> hlr_alice [label="mslookup by IMSI",style=dotted,dir=both] |
| hlr_b -> hlr_alice [label="GSUP proxy forwarding"] |
| } |
| ---- |
| |
| For example, when attaching to a local network, a local resident gets serviced directly by the local village's HLR, |
| while a visitor from another village gets serviced by the remote village's HLR (Roaming). |
| |
| A home HLR typically stays the same for a given IMSI. If the home site is reachable, there should be exactly one |
| response to an mslookup request asking for it. The age of such a home-HLR response is always sent as zero. |
| |
| If a response's age is zero, mslookup does not wait for further responses and immediately uses the result. |
| |
| If there were more than one HLR accepting service for an IMSI, the one with the shortest response latency is used. |
| |
| === mslookup Configuration |
| |
| OsmoHLR the main mslookup agent. It provides the responses for both current location services as well as for locating |
| the fixed home-HLR. But naturally, depending on the mslookup request's purpose, different OsmoHLR instances will respond |
| for a given subscriber. |
| |
| - When querying the home HLR, it is always the (typically single) home HLR instance that sends the mslookup response. As |
| soon as it finds the queried IMSI in the local HLR database, an OsmoHLR will respond to home-HLR requests. |
| In <<fig_dgsm_roaming>>, Alice's home HLR responds to the Roaming request ("where is the home HLR?"). |
| |
| - When querying the location of an MSISDN, it is always the HLR proxy nearest to the servicing MSC that sends the |
| mslookup response. Even though the home HLR keeps the Location Updating record also for Roaming cases, it will only |
| respond to an mslookup service request if the subscriber has attached at a directly connected MSC. If attached at a |
| remote MSC, that MSC's remote HLR will be the GSUP proxy for the home HLR, and the remote HLR is responsible for |
| responding to service requests. |
| In <<fig_dgsm_roaming>>, HLR B is the nearest proxy and will answer all service requests ("where is this MSISDN?"). |
| Alice's home HLR will not answer service requests, because it detects that the servicing MSC is connected via another |
| HLR proxy. |
| |
| [[dgsm_example_config]] |
| ==== Example |
| |
| Here is an osmo-hlr.cfg mslookup configuration example for one site, which is explained in subsequent chapters. |
| |
| hlr |
| gsup |
| bind ip 10.9.8.7 |
| ipa-name hlr-23 |
| mslookup |
| mdns bind |
| server |
| service sip.voice at 10.9.8.7 5060 |
| service smpp.sms at 10.9.8.7 2775 |
| |
| OsmoHLR has both an mslookup server and a client. |
| |
| - The server responds to incoming service and home-HLR requests, when the local HLR is responsible. |
| - The client is used as GSUP proxy to a remote home HLR (found by mslookup upon a locally unknown IMSI). |
| - The client may also be used for forwarding SMS-over-GSUP. |
| |
| The mslookup service can be implemented by various methods. |
| At the time of writing, the only method implemented is mDNS. |
| |
| ==== mDNS |
| |
| The stock mslookup method is mDNS, multicast DNS. It consists of standard DNS encoding according to <<ietf-rfc1035>> and |
| <<ietf-rfc3596>>, but sent and received on IP multicast. In the response, standard A and AAAA records return the |
| service's IP address, while additional TXT records provide the service's port number and the MS attach age. |
| |
| TIP: To watch D-GSM mDNS conversations in wireshark, select "udp.port == 4266" (the default mslookup mDNS port |
| number), right click on the packet to "Decode as...", and select "DNS". |
| |
| In OsmoHLR, the mDNS server and client are typically both enabled at the same time: |
| |
| mslookup |
| mdns bind |
| |
| Server and client can also be enabled/disabled individually: |
| |
| mslookup |
| server |
| mdns bind |
| client |
| mdns bind |
| |
| These examples use the default mslookup multicast IP address and port. It is possible to configure custom IP address and |
| port, but beware that the IP address must be from a multicast range, see <<ietf-rfc5771>>: |
| |
| mslookup |
| mdns bind 239.192.23.42 4266 |
| |
| Domain names generated from mslookup queries (e.g. "sip.voice.123.msisdn") should not collide with IANA permitted |
| domains. Therefore we add the "mdns.osmocom.org" suffix. It can be overridden as follows: |
| |
| mslookup |
| mdns domain-suffix mdns.osmocom.org |
| |
| ==== Server: Site Services |
| |
| The mslookup server requires a list of service addresses provided at the local site, in order to respond to service |
| requests matching locally attached subscribers. |
| |
| mslookup |
| server |
| service sip.voice at 10.9.8.7 5060 |
| service smpp.sms at 10.9.8.7 2775 |
| |
| In this example: |
| |
| - "10.9.8.7 5060" are the IP address and port on which the local site's osmo-sip-connector is bound to receive SIP |
| Invite requests. |
| - "10.9.8.7 2775" are the local site's OsmoMSC SMPP bind address and port. |
| |
| Obviously, these IP addresses must be routable back to this site from all other sites. Using link-local or "ANY" |
| addresses, like 127.0.0.1 or 0.0.0.0, will not work here. Instead, each service config requires a public IP address that |
| all remote requestors are able to reach (not necessarily on the host that osmo-hlr is running on). |
| |
| If a site has more than one MSC, services can also be configured for each MSC individually, keyed by the IPA unit name |
| that each MSC sends on the GSUP link: |
| |
| mslookup |
| server |
| msc ipa-name msc-262-42-0 |
| service sip.voice at 10.11.12.13 5060 |
| service smpp.sms at 10.11.12.13 2775 |
| msc ipa-name msc-901-70-0 |
| service sip.voice at 10.9.8.7 5060 |
| service smpp.sms at 10.9.8.7 2775 |
| |
| Here, "msc-262-42-0" is the IPA name of a local OsmoMSC instance. To configure an OsmoMSC's IPA name on the GSUP link, |
| see osmo-msc.cfg, setting `hlr` / `ipa-name`. |
| |
| For mslookup service responses, only Location Updatings in the Circuit Switched domain are relevant. OsmoHLR does manage |
| IMSIs attaching in the Packet Switched domain (via an SGSN) similarly to Circuit Switched (via an MSC), but mslookup |
| completely ignores the Packet Switched attach status. |
| |
| ==== Server: Own GSUP Address |
| |
| When responding to home-HLR requests, OsmoHLR implicitly by default responds with its locally configured GSUP bind |
| address (setting `hlr` / `gsup` / `bind ip`). If required, an explicit local GSUP address and port can be configured, |
| for example: |
| |
| hlr |
| gsup |
| bind ip 0.0.0.0 |
| ipa-name hlr-23 |
| mslookup |
| server |
| # osmo-hlr's own GSUP address to send in mslookup responses: |
| service gsup.hlr at 10.9.8.7 4222 |
| |
| The gsup.hlr service can only be configured globally (because requests come from arbitrary mDNS clients, before a |
| Location Updating has associated the IMSI with the requesting MSC). |
| |
| ==== Client IPA Naming |
| |
| For reliable GSUP proxy routing to a remote HLR (Roaming), it is important that each GSUP client, i.e. each HLR, MSC and |
| SGSN instance, has a unique IPA name. |
| |
| Example for configuring an OsmoHLR instance's IPA name: |
| |
| hlr |
| gsup |
| ipa-name hlr-23 |
| |
| Here, "hlr-23" is the unique identification of this OsmoHLR instance across all potentially connected D-GSM sites. |
| |
| Furthermore, each MSC and SGSN must have a uniquely distinct IPA name across all sites (here "msc-262-42-0" and |
| "msc-901-70-0" are used as example IPA names for local MSCs). |
| |
| When this OsmoHLR connects to a remote HLR, be it for GSUP proxying or SMS-over-GSUP, it communicates its own IPA name |
| (on GSUP link-up) as well as the IPA name of the requesting client MSC/SGSN (as Source Name in each message) to the |
| remote OsmoHLR GSUP server. These names are used to route GSUP responses back to the respective requesting peer. |
| |
| If two MSCs were accidentally configured with identical names, a problem will occur as soon as both MSCs attempt to |
| attach to the same OsmoHLR (either directly or via GSUP proxying). The MSC that shows up first will work normally, but |
| any duplicate that shows up later will be rejected, since a route for its name already exists. |
| |
| === Queries |
| |
| In URL notation, typical mslookup queries look like: |
| |
| gsup.hlr.123456789.imsi |
| sip.voice.123.msisdn |
| smpp.sms.123.msisdn |
| |
| A query consists of |
| |
| - a service name ("gsup.hlr"), |
| - an id ("123456789"), |
| - the id type ("imsi"). |
| |
| The calling client also defines a timeout to wait for responses. |
| |
| The mslookup ID types are fixed, while service names can be chosen arbitrarily. |
| |
| .mslookup ID types, no other ID types are understood by mslookup |
| [options="header",width="100%",cols="20%,80%"] |
| |=== |
| |ID Type|Description |
| |imsi|An IMSI as existing in an OsmoHLR subscriber database |
| |msisdn|A phone number as configured in an OsmoHLR subscriber database |
| |=== |
| |
| .mslookup service name conventions, arbitrary service names can be added as required |
| [options="header",width="100%",cols="20%,20%,60%"] |
| |=== |
| |Service Name|Protocol|Description |
| |gsup.hlr | GSUP | Home HLR's GSUP server, to handle Location Updating related procedures |
| |sip.voice | SIP | OsmoSIPConnector, to receive a SIP Invite (MT side of a call) |
| |smpp.sms | SMPP | Destination OsmoMSC (or other SMPP server) to deliver an SMS to the recipient |
| |gsup.sms | GSUP | GSUP peer to deliver an SMS to the recipient using SMS-over-GSUP |
| |=== |
| |
| Arbitrarily named services can be added to the mslookup configuration and queried by mslookup clients; as soon as a |
| service name is present in osmo-hlr.cfg, it can be queried from any mslookup client. |
| |
| Service names should consist of a protocol name (like "sip", "gsup", "english") and an intended action/entity (like |
| "voice", "hlr", "greeting"). |
| |
| === Service Client Implementation |
| |
| In principle, arbitrary services could query target addresses via mslookup, leaving it up to any and all kinds of |
| clients to find their respective destination addresses. But of course, mslookup was designed with specific services in |
| mind, namely: |
| |
| - SIP call agents and |
| - SMS delivery (an ESME or SMSC) |
| |
| The following chapters describe examples of setting up a working distributed core network providing SIP voice calls and |
| SMS forwarding across sites. |
| |
| ==== mslookup Library |
| |
| The OsmoHLR provides an mslookup client C library, libosmo-mslookup. Service lookups can be integrated directly |
| in client programs using this library. However, its mDNS implementation requires the libosmocore select() loop, which |
| can be challenging to integrate in practice. An alternative solution is the osmo-mslookup-client tool. |
| |
| [[dgsm_osmo_mslookup_client]] |
| ==== osmo-mslookup-client |
| |
| The mslookup C library is available, but often, a simpler approach for client implementations is desirable: |
| |
| - When querying for a service address, the client is typically interested in the single final best result (youngest age |
| / first responding home HLR). |
| - Voice call and SMS clients typically would block until an mslookup result is known. For example, the FreeSwitch |
| dialplan integration expects a result synchronously, i.e. without waiting for mslookup responses via a select() loop. |
| - Integrating the libosmocore select() loop required for mDNS can break the already existing socket handling in the |
| client program. |
| |
| The osmo-mslookup-client cmdline tool provides a trivial way to synchronously acquire the single result for an mslookup |
| request. The service client can invoke an osmo-mslookup-client process per request and read the result from stdout. |
| |
| Each invocation obviously spawns a separate process and opens a multicast socket for mDNS. For better scalability, |
| osmo-mslookup-client can also be run as a daemon, providing results via a unix domain socket. Using synchronous write() |
| and recv() allows blocking until a result is received without interfering with the client program's select() setup. |
| |
| By itself, osmo-mslookup-client is also helpful as a diagnostic tool: |
| |
| ---- |
| $ osmo-mslookup-client sip.voice.1001.msisdn |
| sip.voice.1001.msisdn ok 10.9.8.7 5060 |
| |
| $ osmo-mslookup-client gsup.hlr.901700000014701.imsi |
| gsup.hlr.901700000014701.imsi ok 10.9.8.7 4222 |
| |
| $ osmo-mslookup-client gsup.hlr.111111.imsi |
| gsup.hlr.111111.imsi not-found |
| |
| $ osmo-mslookup-client gsup.hlr.1001.msisdn sip.voice.1001.msisdn smpp.sms.1001.msisdn foo.1001.msisdn |
| gsup.hlr.1001.msisdn ok 10.9.8.7 4222 |
| foo.1001.msisdn not-found |
| smpp.sms.1001.msisdn ok 10.9.8.7 2775 |
| sip.voice.1001.msisdn ok 10.9.8.7 5060 |
| |
| $ osmo-mslookup-client --csv-headers gsup.hlr.901700000014701.imsi |
| QUERY RESULT V4_IP V4_PORT V6_IP V6_PORT |
| gsup.hlr.901700000014701.imsi ok 10.9.8.7 4222 |
| |
| $ osmo-mslookup-client -f json gsup.hlr.901700000014701.imsi |
| {"query": "gsup.hlr.901700000014701.imsi", "result": "ok", "v4": ["10.9.8.7", "4222"]} |
| ---- |
| |
| For full help including example client invocations in Python, see the output of: |
| |
| osmo-mslookup-client -h |
| |
| ==== SIP Service Client |
| |
| [[dgsm_conf_dialplan]] |
| ===== FreeSwitch dialplan.py |
| |
| The FreeSWITCH PBX software <<freeswitch_pbx>> offers a Python integration to determine a SIP call recipient by a custom |
| dialplan implementation. An example dialplan implementation for FreeSWITCH that uses D-GSM mslookup is provided in the |
| osmo-hlr source tree under `contrib`, called `freeswitch_dialplan_dgsm.py`. |
| |
| To integrate it with your FREESWITCH setup, add a new `extension` block to your `dialplan/public.xml`: |
| |
| ---- |
| <extension name="outbound"> |
| <condition field="destination_number" expression=".*"> |
| <action application="set" data="hangup_after_bridge=true"/> |
| <action application="set" data="session_in_hangup_hook=true"/> |
| <action application="set" data="ringback=%(2000, 4000, 440.0, 480.0)"/> |
| <action application="python" data="freeswitch_dialplan_dgsm"/> |
| </condition> |
| </extension> |
| ---- |
| |
| Make sure that the dir containing `freeswitch_dialplan_dgsm.py` is in your `PYTHONPATH` environment variable, and start |
| the server: |
| |
| ---- |
| $ export PYTHONPATH="$PYTHONPATH:/home/user/code/osmo-hlr/contrib/dgsm" |
| $ freeswitch -nf -nonat -nonatmap -nocal -nort -c |
| ---- |
| |
| ==== SMS Service Client |
| |
| [[dgsm_conf_esme_smpp]] |
| ===== SMS via SMPP Port |
| |
| An example ESME using D-GSM mslookup, `esme_dgsm.py`, is provided in the osmo-hlr source tree under `contrib`. It |
| attaches to OsmoMSC's SMPP port to send SMS to recipients determined by mslookup. |
| |
| OsmoMSC should be configured as "smpp-first", so that all SMS routing is determined by mslookup. If configured without |
| smpp-first, OsmoMSC may try to deliver an SMS locally, even though the recipient has recently moved to a different site. |
| |
| An example OsmoMSC configuration to work with esme_dgsm.py: |
| |
| ---- |
| smpp |
| local-tcp-ip 127.0.0.1 2775 |
| system-id test-msc |
| policy closed |
| smpp-first |
| # outgoing to esme_dgsm.py |
| esme OSMPP |
| no alert-notifications |
| password foo |
| default-route |
| # incoming from esme_dgsm.py |
| esme ISMPP |
| no alert-notifications |
| password foo |
| ---- |
| |
| Launch esme_dgsm.py alongside OsmoMSC: |
| |
| ---- |
| ./esme_dgsm.py --src-host 127.0.0.1 |
| ---- |
| |
| esme_dgsm.py will be notified via SMPP for each SMS to be delivered, and will forward them either to a remote |
| recipient, or back to the same OsmoMSC, depending on the mslookup result. If the MSISDN is not reachable (or |
| esme_dgsm.py can't handle the message for other reasons), it returns the RSYSERR code back to OsmoMSC. |
| |
| Note that the esme_dgsm.py is a proof of concept and should not be used in production. It has several limitations, such |
| as not supporting multipart SMS messages. |
| |
| ===== SMS-Over-GSUP |
| |
| The GSUP protocol defines SMS delivery messages. When OsmoMSC is configured to deliver SMS via GSUP, MO SMS are directly |
| forwarded to the HLR, which will determine where to forward the SMS-over-GSUP messages using its mslookup client. |
| |
| FIXME implement this |