SDP for BTstack

General:
 * Protocol: request/response, half-duplex
 * Endianess = BIG/Network! not little endian as usual
 * Server maintains a list of service records
 * service (record) is instance of a service class
 * a service record = list of service attributes
 * service attribute = { attribute ID, attribute value }
 * service record handle: 32 bit number (used internally)
 * service record handle 
   * 0x00000000 used for SDP server, 
   * 0x00000001-0x0000FFFF reserved
 * attribute ID: 16 bit (pre-defined), data element
 * attribute value: var length field, data element
 * service class: UUID (128 bit) stored as ServiceClassIDList attribute
 * service classes form an inheritance tree, which each subclass specifying more attributes
 * Bluetooth Base UUID: 00000000-0000-1000-8000- 00805F9B34FB
 * 16 bit UUID => Bluetooth Base UUID + 16_bit_value * 2^96
 * 32 bit UUID => Bluetooth Base UUID + 32_bit_value * 2^96
 * Service Search Pattern: list of UUIDs required in a service record to be present for a match
 * Browse Group UUID: all top-level services contain browse group UUID in the BrowseGroupList attribute
 * Service Browsing Hierarchy is possible - not supported by BTstack for now
 * Data representation: like XML
 * PDU: PDU ID(1), Transaction ID(2), Param length(2), params*
   * ServiceSearchRequest/ServiceSearchResponse: list of UUIDs to be matched by service record -> list of service record handles
   * ServiceAttributeRequest/ServiceAttributeResponse: get list of attributes for a given service record handle
   * Service SearchAttributeRequest/SearchAtrributeResponse: for all service records that match ServiceSearchPattern: return list of attributes that match AttributeIDList
   * Transaction ID of response = ID of request
   * param lengh in bytes
 * Continuation state parameter: send to client to denote further data (max 16 bytes)
 * To get RFCOMM channel, we use SDP_ServiceSearchAttributeRequest(RFCOMM service, RFCOMM type)

Implementation:
 * DataElement creation API - no extra memory
 * helper functions:
   * attribute ID in AttributeIDList
   * get attributes specified by AttributeIDList
   * service record contains UUID
   * does service search pattern matches record
   * get attribute value (DE) for attribute ID
   * create service record handle (incl. ServiceRecordHandle management)
 * visitor pattern for DES traversal
   * call handler on every element: int handle_element(uint8_t element, void *context) - done?
 * Dispatch packets for protocols implemented by BTdaemon
   * add packet_handler to l2cap_service_t and l2cap_channel_t
     * pass acl/event handler to l2cap_register_service
     * copy acl/event handler to l2cap_channel_t
     * if specified, call custom packet_handler instead of general one 
   * acl -> l2cap -> l2cap_channel -> acl/event handler OR daemon
 * LinkedList of service records
   * alloc { btstack_linked_list_item_t ; ServiceRecord }
   * add service record: service record -> service record handle or 0
   * remove service record: service record handle (32bit)
 * SDP as part of stack itself
   * Register SDP PSM=0x0001
   * Accept incoming connections
   * Define all SDP requests/response PDU-IDs
   * Handle all three SDP requests by denying
     * ServiceSearchRequest
     * ServiceAttributeRequest
     * ServiceSearchAttributeRequest
   * Dynamically create denial
   * Handle ServiceSearchAttributeRequest
   * Handle ServiceSearchRequest
   * Handle ServiceAttributeRequest
   * Extract sdp_create_error_response
   * call sdp_create_error_response when service record handle is invalid
 * Define and implement client commands & events for SDP registry
 * Store client connection in service record item
 * Unregister service when client disconnects 
 * Send SDP_ErrorResponse for invalid records
 * Implement Continuation: limited nr of ServiceRecordHandles or AttributeList(s)ByteCount
   * ServiceSearchRequest: Iterate over all records - use record index as cont.
   * ServiceAttributeRequest: Iterate over all attributes - use attribute index as cont.
   * ServiceSearchAttributeRequest: Iterate over all records and attributes. Use {record, attribute} index
 * Test continuation for all three commands
   * add 2 records, get all data back with limited result size
 * sdp_handle_service_search_attribute_request
* implement SDP_sequence_traversal -> simplify traversing
 
 * keep track of remote incoming MTU during l2cap establishment
 * segment l2cap packets according to MTU size

 * Packet for HID is shortened - bluez uses 0x0096 as MTU - segment packets
 * String Handling in Linux SDP/sdptool probably buggy