OpenBSC ip.access Abis/IP proxy design
======================================================================

== the big picture ==

* behave as BSC 
 * listen for BTS OML connections (TCP port 3002, primary OML)
 * listen for BTS RSL connections (TCP port 3003)
 * actively connect to BTS (TCP port 3006, 2ndary OML)

* behave as BTS
 * actively connect OML to BSC (TCP port 3002)
 * actively connect RSL to BSC (TCP port 3003)

* provide UDP interface for packet injection
 * packet format inside UDP is exactly the same as ip.access
 * provide one port for each direction:
  * UDP port 5001: message to BTS
  * IDP port 5002: message to BSC

== use cases ==

* transparent proxy
 * simply pass all messages from BTS to BSC and vice-versa

* injection proxy
 * provide UDP (packet-based) interface for tools like scapy

== misc ==

* keep-alive is handled independently, i.e. connections to BTS / BSC are always
  kept open


== code architecture ==

=== incoming TCP connection ===
* connect TCP to BSC
* create 2 UDP sockets for packet injection

=== incoming TCP data from BTS ===
* determine if OML or RSL fd
* copy data to tx_queue of corresponding BSC fd

=== incoming TCP data from BSC ===
* determine if OML or RSL fd
* copy data to tx_queue of corresponding BTS fd

=== incoming UDP data ===
* determine OML or RSL and direction based on port number
* copy data to apropriate tx_queue

=== termination of TCP connection ===
* no matter if BSC or BTS disconnect, keep other TCP session open
* make sure upon re-connection, the correct pairs of connections
  are matched up