[tls] Support stateful session resumption

Record the session ID (if any) provided by the server and attempt to
reuse it for any concurrent connections to the same server.

If multiple connections are initiated concurrently (e.g. when using
PeerDist) then defer sending the ClientHello for all but the first
connection, to allow time for the first connection to potentially
obtain a session ID (and thereby speed up the negotiation for all
remaining connections).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tls.h

@@ -242,13 +242,40 @@ struct md5_sha1_digest {
 /** MD5+SHA1 digest size */
 #define MD5_SHA1_DIGEST_SIZE sizeof ( struct md5_sha1_digest )
 
+/** A TLS session */
+struct tls_session {
+	/** Reference counter */
+	struct refcnt refcnt;
+	/** List of sessions */
+	struct list_head list;
+
+	/** Server name */
+	const char *name;
+	/** Session ID */
+	uint8_t id[32];
+	/** Length of session ID */
+	size_t id_len;
+	/** Master secret */
+	uint8_t master_secret[48];
+
+	/** List of connections */
+	struct list_head conn;
+};
+
 /** A TLS connection */
 struct tls_connection {
 	/** Reference counter */
 	struct refcnt refcnt;
 
-	/** Server name */
-	const char *name;
+	/** Session */
+	struct tls_session *session;
+	/** List of connections within the same session */
+	struct list_head list;
+	/** Session ID */
+	uint8_t session_id[32];
+	/** Length of session ID */
+	size_t session_id_len;
+
 	/** Plaintext stream */
 	struct interface plainstream;
 	/** Ciphertext stream */