get details about the compression
associated with a session
If compression has been negotiated for an ssl session,
returns the id for the compression method, or 0 otherwise. The only built-in
supported compression method is zlib, which has an id of 1.
ssl(3), SSL_get_session(3), SSL_SESSION_get_id(3), SSL_SESSION_get_protocol_version(3), SSL_SESSION_new(3)
appeared in OpenSSL 1.0.1 and has been available since