move openbsc/* to repos root

This is the first step in creating this repository from the legacy openbsc.git.

Like all other Osmocom repositories, keep the autoconf and automake files in
the repository root. openbsc.git has been the sole exception, which ends now.

Change-Id: I9c6f2a448d9cb1cc088cf1cf6918b69d7e69b4e7

1 files changed, 94 insertions, 0 deletions

diff --git a/doc/ipa-sccp.txt b/doc/ipa-sccp.txt
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+
+IPA SCCP message flow in the BSC
+
+February, 2013		Holger Hans Peter Freyther
+
+CONTENTS
+
+1. SCCP inside the IPA header
+2. Supported SCCP message types
+3. Receiving SCCP messages
+4. Sending SCCP messages
+
+
+1. SCCP inside the IPA header
+
+Many Soft-MSCs implement something that is called SCCP/lite. This means
+that SCCP messages are transported inside a small multiplexing protocol
+over TCP/IP. This is an alternative to a full SIGTRAN implementation.
+
+The multiplexing protocol is the same as used with the sysmoBTS and the
+ip.access nanoBTS. It is a three byte header with two bytes for the length
+in network byte order and one byte for the type. The type to be used for
+SCCP is 0xFD.
+
+	struct ipa_header {
+		uint16_t length_in_network_order;
+		uint8_t  type;
+	} __attribute__((packed));
+
+
+
+2. Supported SCCP message types
+
+To implement GSM 08.08 only a subset of SCCP messages need to be implemented.
+For transporting paging and reset messages SCCP UDT messages are used. For
+the connections with a Mobile Station (MS) a SCCP connection is opened. This
+means that the SCCP CR, SCCP CC, SCCP CREF, SCCP RLC, SCCP RLSD, SCCP DT1
+and SCCP IT messages are supported.
+
+
+3. Receiving SCCP UDT messages
+
+This is an illustration of the flow of messages. The IPA multiplexing protocol
+is used for various protocols. This means there is a central place where the
+multiplexing stream terminates. The stream is terminated in the osmo_bsc_msc.c
+file and the ipaccess_a_fd_cb method. For SCCP messages the SCCP dispatching
+sccp_system_incoming method is called. This function is implemented in the
+libosmo-sccp library.
+
+To receive UDT messages osmo_bsc_sccp.c:osmo_bsc_sccp_init is using the
+sccp_set_read function to register a callback for UDT messages. The callback
+is msc_sccp_read and it is calling bsc_handle_udt that is implemented in the
+osmo_bsc_bssap.c. This function will handle the GSM 08.08 BSSAP messages.
+Currently only the reset acknowledge and the paging messages are handled.
+
+The BSC currently does not accept incoming SCCP messages and is only opening
+SCCP connections to the MSC. When opening a connection the callbacks for state
+changes (connection confirmed, released, release complete) are set and a routine
+for handling incoming data. This registration is done in the osmo_bsc_sccp.c
+file and the bsc_create_new_connection method. The name of the callback is
+msc_outgoing_sccp_data and this will call bsc_handle_dt1 that is implemented
+in the osmo_bsc_bssap.c file. This will forward the messages to the right
+Mobile Station (MS).
+
+
+4. Sending SCCP messages
+
+There are three parts to sending that will be explained below. The first part
+is to send an entire SCCP frame (which includes the GSM 08.08 data) to the
+MSC. This is done by first registering the low level sending. sccp_system_init
+is called with the function that is responsible for sending a message. The
+msc_sccp_write_ipa will call the msc_queue_write function with the data and
+the right MSC connection. Below the msc_queue_write the IPA header will be
+prepended to the msg and then send to the MSC.
+
+The BSC supports multiple different A-link connections, the decision to pick
+the right MSC is done in this method. It is either done via the SCCP connection
+or the ctx pointer.
+
+When the BSC is starting a BSS RESET message will be sent to the MSC. The reset
+is created in osmo_bsc_msc.c:initialize_if_needed and sccp_write is called with
+the GSM 08.08 data and the connection to use. The libosmo-sccp library will
+embed it into a SCCP UDT message and call the msc_sccp_write_ipa method.
+
+When a new SCCP connection is to be created the bsc_create_new_connection
+in the osmo_bsc_sccp.c file. The sccp_connection_socket method will create
+the context for a SCCP connection. The state and data callback will be used
+to be notified about data and changes. Once the connection is configured the
+bsc_open_connection will be called that will ask the libosmo-sccp library to
+create a SCCP CR message using the sccp_connection_connect method. For active
+connections the sccp_connection_write method will be called.
+
+
+