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diff --git a/doc/manuals/chapters/dgsm.adoc b/doc/manuals/chapters/dgsm.adoc new file mode 100644 index 0000000..e506f5a --- /dev/null +++ b/doc/manuals/chapters/dgsm.adoc @@ -0,0 +1,491 @@ +== 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 |