SSL_COMP_add_compression_method - handle SSL/TLS integrated compression methods
int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
The TLS standard (or SSLv3) allows the integration of compression methods into the communication. The TLS RFC does however not specify compression methods or their corresponding identifiers, so there is currently no compatible way to integrate compression with unknown peers. It is therefore currently not recommended to integrate compression into applications. Applications for non-public use may agree on certain compression methods. Using different compression methods with the same identifier will lead to connection failure.
An OpenSSL client speaking a protocol that allows compression (SSLv3,
TLSv1) will unconditionally send the list of all compression methods
An OpenSSL server will match the identifiers listed by a client against its own compression methods and will unconditionally activate compression when a matching identifier is found. There is no way to restrict the list of compression methods supported on a per connection basis.
The OpenSSL library has the compression methods COMP_rle() and (when especially enabled during compilation) COMP_zlib() available.
Once the identities of the compression methods for the TLS protocol have been standardized, the compression API will most likely be changed. Using it in the current state is not recommended.