設置讀寫回調函數：
mbedtls_ssl_set_bio(&session->ssl, &session->server_fd, mbedtls_net_send, mbedtls_net_recv, NULL);
void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
void *p_bio,
mbedtls_ssl_send_t *f_send,
mbedtls_ssl_recv_t *f_recv,
mbedtls_ssl_recv_timeout_t *f_recv_timeout )
{
ssl->p_bio = p_bio;
ssl->f_send = f_send;
ssl->f_recv = f_recv;
ssl->f_recv_timeout = f_recv_timeout;
}
f_recv()->mbedtls_ssl_fetch_input()->mbedtls_ssl_read_record_layer()->mbedtls_ssl_read_record()->mbedtls_ssl_read()->mbedtls_client_read()
mbedtls_ssl_read_record_layer()函數內：
1、調用mbedtls_ssl_fetch_input()接收數據
2、ssl_parse_record_header()函數判斷一個 record 的數據是否超過緩存（MBEDTLS_SSL_MAX_CONTENT_LEN）大小
3、調用ssl_prepare_record_content()->ssl_decrypt_buf()對接收到的數據進行對稱解密