A Trusted Routing Point (TROP) generates a signaling message that includes an authorization token used to authorize a firewall to open a pinhole. The signaling message contains a first indicator that indicates whether a data field in the signaling message represents a source address of a media flow. The signaling message also includes a second indicator that indicates whether the firewall should derive the source address of the media flow from the data field. The authorization token is generated using a one-way hash function over information that may be included in the signaling message, including the first indicator and the second indicator.