Data structure for sending outgoing message. Application normally creates this buffer by calling pjsip_endpt_create_tdata.
The lifetime of this buffer is controlled by the reference counter in this structure, which is manipulated by calling pjsip_tx_data_add_ref and pjsip_tx_data_dec_ref. When the reference counter has reached zero, then this buffer will be destroyed.
A transaction object normally will add reference counter to this buffer when application calls pjsip_tsx_send_msg, because it needs to keep the message for retransmission. The transaction will release the reference counter once its state has reached final state.
Forward declaration for transmit data/buffer (sip_transport.h).