for reference https://github.com/rbit/pydtls/blob/41a71fccd990347d0de5f42418fea1e4e733359c/dtls/sslconnection.py#L308 https://github.com/stepheny/openssl-dtls-custom-bio/blob/6dccbb1c6d97f1f15d6cd9f835f425bf11ed73ec/server.c
