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