I have the link MTU (typically 1500 bytes), and a DTLS session is established.
I call DTLS_set_link_mtu() to set the link MTU. I need to know the DTLS data MTU — the maximum payload size, which depends on the cipher in use. For example for AES-128-GCM-SHA256 we'd start with 1500 and subtract: - 20 bytes for a Legacy IP header. - 8 bytes for UDP header. - 13 bytes for DTLS header - 16 bytes for the hash - 8 bytes for nonce ... and be left with 1435 bytes. In GnuTLS this is fairly trivial; I call gnutls_dtls_set_mtu() followed by gnutls_dtls_get_data_mtu(). How do I do it in OpenSSL? Do I need to build a big table of the overhead of all ciphers and calculate it for myself? -- dwmw2
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