The OpenBSC NAT is masquerading multiple BSCs behind one. It listens for incoming BSCs on port 5000 and connects to a specified MSC. It will forward Paging Requests and MGCP messgaes to the right BSC (if it is connected). Each incoming SCCP connection gets patched with a different Source Reference to avoid any collision between the different BSCs.
To the network the NAT looks like a real BSC, to the BSC the NAT looks like almost a real MSC. One difference is that MGCP messages are not send via UDP but inside the IPA protocol to avoid any issues with firewalls and such.
The bsc_nat is useful when the BSCs of a GSM network are not in the same IP network. The BSC might be behind a firewall, or the IP is changing, also the ports might be masqueraded. The bsc_nat is tunneling the MGCP protocol through the A/IP link to penetrate firewalls and UDP port discovery for RTP based audio. Being the central part to the GSM network it provides detailed statistics and other features.
IMSI based access control
The bsc_nat provides a per MSC and per BSC IMSI filter. The filter works during the Location Updating Procedure, the CM Service Request and other places. It allows to exclude IMSIs from certain locations.
The bsc_nat provides a way to rewrite destination numbers based on MNC and MCC of the subscriber.
The bsc_nat provides a way to forward USSD requests for certain number ranges for subscribers based on the MNC and MCC to an external process.
The bsc_nat provides detailed statistics about the traffic of each BSC, the reason for opening the connection, the number of active connections.
The bsc_nat tunnels MGCP through the A/IP link, the BSC will send a beacon from the source RTP port to make the external port visible to the NAT.
We want to implement certain USSD messages ourselves, the MSC does not provide an extension API so it must be done inside the NAT. On ther other hand we do not want to have complicated code inside the NAT, the development of new features should not require a restart of the NAT. This means we will have a simple bypass that detects USSD messages that we want to handle and then forwards them to an external process. To begin with we will have a static decision making, which will be turned into more dynamic pattern matching.
Implementation in the NAT
The difficulty is the management of the SCCP Connection. In case of a USSD operation as part of a SMS or as a voice call, we should hope that sending the response arrives before the MSC is sending the connection release. If the Connection is created with the intention of USSD we will have to take over the connection and close it on the MSC side. It might show that this approach is tricky and we might need better handling on the connection close.
Keep track of the IMSI of every SCCP Connection. The increase in storage size is acceptable right now. We have not started to tune anything in the NAT anyway.(done) Keep track of the Connection Type, identify SS requests in the CM Service Request.(done)
- Listen on a port for incoming USSD Bypass connections. We will use the IPA protocol, with one extra stream identifier for control messages.
- Identify USSD request to be forwarded to the MSC, send the IMSI and SCCP references to the bridge so it can start doing its job.