Proposed OpenBSC application interface

Hi! Collin Mulliner, Tobias Engel and myself have been meeting yesterday to discuss a generic application interface for OpenBSC. They are both doing security analysis and want to achieve a clean way how an external application can get access to a more or less transparent communication channel to the phone. The purpose of this is to be able to send intentionally malformed packets to the mobile phone GSM stack at various different levels within the stack. As of now, they have both hacked some custom code into openbsc that gets them half way where they want to be - but not quite all the way. The requirements can be summarized as follows: 1) Ability to establish a SDCCH or TCH channel by paging the phone As of now, the 'silent call' feature from the VTY already does this. 2) Ability to send arbitrary layer3 protocol messages to the phone Adding this is relatively easy (use rsl_sendmsg on the lchan from the silent call) 3) Ability to receive responses from the phone, as well as error conditions such as 'readio link failure'. We don't have a solution for this yet, and we also have no clean way to identify what might be a response from the phone to the external app, and what might be a message from the phone to the normal network code in OpenBSC 4) Ability to selectively disable partial protocol handling in OpenBSC. Let's say you want to play with the mobile phone call control implementation. In this case, you want to make sure all CC related messages go from/to the external program and not from the regular OpenBSC network code. So what I've been thinking of as a solution to the problem: * store a bypass_flags bitmask related to the subscriber structure, where we indicate values such as BYPASS_RR, BYPASS_MM, BYPASS_CC, BYPASS_SAPI3. * if we process an incoming message from the MS in gsm0408_rcvmsg(), we check if a bypass flag matching the message is found. If yes, forward the message to the external program * if we want to send a message from our own protocol stack to the MS, we check if a bypass flag matching the message is found. If yes, we drop the message that we were about to send. * any messages received from the application will be forwarded to the MS The application interface protocol will likely have a close resemblance to RSL RLL. We need to exchange the following primitives with the application, like: * ESTABLISH REQUEST -- app requests a channel be established to MS (by IMSI) * ESTABLISH CONFIRM -- network confirms a channel has been established * ESTABLISH INDICATION -- network tells app connection was made by MS * [UNIT] DATA REQUEST -- app requests data to be sent to MS * [UNIT] DATA INDICATION -- network indicates data was received from MS * ERROR INDICATION -- network tells app something went wrong * RELEASE REQUEST -- app asks network to release channel * RELEASE CONFIRM -- net tells app that channel was released (as rqd) * RELEASE INDICATION -- net tells app that channel was released (by MS) The channel_number of RSL (indicating on-air timeslot) doesn't make much sense in this context, of course. The link_identifier on the other hand is great as it allows the app to indicate SDCCH/FACCH or SACCH as well as the SAPI. The actual RSL-like protocol would be encapsulated by UDP and available on a socket of the MSC. What do you think? Regards, Harald -- - Harald Welte <laforge(a)gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)