[openssl-dev] Calculating DTLS payload MTU

[David Woodhouse]

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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