@Override
        public double sample() {
            return (ProxyOutputStream.amountTransferred.getAndSet(0) / 1000d);
        }
Creates the digest needed for a byte stream. It is the SHA1(sessionID +
initiator + target).

@param sessionID The sessionID of the stream negotiation
@param initiator The initiator of the stream negotiation
@param target The target of the stream negotiation
@return SHA-1 hash of the three parameters
xmppserver/src/main/java/org/jivesoftware/openfire/filetransfer/proxy/ProxyConnectionManager.java