def toString(self):
        """
        Serialize this header for use in a request or response.
        """
        s = "SIP/2.0/%s %s:%s" % (self.transport, self.host, self.port)
        if self.hidden:
            s += ";hidden"
        for n in "ttl", "branch", "maddr", "received":
            value = getattr(self, n)
            if value is not None:
                s += ";%s=%s" % (n, value)
        if self.rportRequested:
            s += ";rport"
        elif self.rportValue is not None:
            s += ";rport=%s" % (self.rport,)

        etc = sorted(self.otherParams.items())
        for k, v in etc:
            if v is None:
                s += ";" + k
            else:
                s += ";%s=%s" % (k, v)
        return s

        Serialize this header for use in a request or response.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/sip.py