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7.10.1 Session resumption
Client side
To reduce time and roundtrips spent in a handshake the client can request session resumption from a server that previously shared a session with. For that the client has to retrieve and store the session parameters. Before establishing a new session to the same server the parameters must be re-associated with the GnuTLS session using gnutls_session_set_data.
Keep in mind that sessions will be expired after some time, depending on the server, and a server may choose not to resume a session even when requested to. The expiration is to prevent temporal session keys from becoming long-term keys. Also note that as a client you must enable, using the priority functions, at least the algorithms used in the last session.
It is highly recommended for clients to enable the session ticket extension using gnutls_session_ticket_enable_client in order to allow resumption with servers that do not store any state.
Server side
In order to support resumption a server can store the session security parameters in a local database or by using session tickets (see Session tickets) to delegate storage to the client. Because session tickets might not be supported by all clients, servers could combine the two methods.
A storing server needs to specify callback functions to store, retrieve and delete session data. These can be registered with the functions below. The stored sessions in the database can be checked using gnutls_db_check_entry for expiration.
- gnutls_db_set_retrieve_function
- gnutls_db_set_store_function
- gnutls_db_set_ptr
- gnutls_db_set_remove_function
A server utilizing tickets should generate ticket encryption and authentication keys using gnutls_session_ticket_key_generate. Those keys should be associated with the GnuTLS session using gnutls_session_ticket_enable_server.
A server enabling both session tickets and a storage for session data would use session tickets when clients support it and the storage otherwise.
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