To resume a TLS session, the server normally stores session parameters. This complicates deployment, and can be avoided by delegating the storage to the client. Because session parameters are sensitive they are encrypted and authenticated with a key only known to the server and then sent to the client. The Session Tickets extension is described in RFC 5077 [TLSTKT].
A disadvantage of session tickets is that they eliminate the effects of forward secrecy when a server uses the same key for long time. That is, the secrecy of all sessions on a server using tickets depends on the ticket key being kept secret. For that reason server keys should be rotated and discarded regularly.
Since version 3.1.3 GnuTLS clients transparently support session tickets, unless forward secrecy is explicitly requested (with the PFS priority string).