The Expressway supports two protocols for traversing media across a firewall: H.460.18/19 and Assent. Both protocols use a TCP/TLS connection established from the client to the server for signaling, and UDP ports for media. The difference is that H.460.18/19 is a standard protocol for H.323 firewall traversal, while Assent is a Cisco proprietary protocol for SIP firewall traversal123

H.460.18/19 is an extension of the H.323 protocol that allows endpoints to communicate with each other through a firewall or a NAT device. H.460.18 defines the signaling procedures for establishing a TCP/TLS connection between the endpoint and the Expressway-E, which acts as the traversal server. H.460.19 defines the media procedures for sending and receiving RTP/RTCP packets through the same TCP/TLS connection orthrough UDP ports. The Expressway-E uses media demultiplexing to route the media packets to the correct destination124

Assent is a protocol that is similar to H.460.18/19, but it is used for SIP firewall traversal of media. The signaling is traversed through a TCP/TLS connection established from the client to the server. The media is traversed through UDP ports that are negotiated using the SDP offer/answer exchange. The Expressway-E uses media demultiplexing to route the media packets to the correct destination123

References := 1Cisco Expressway Administrator Guide (X14.0) - Firewall Traversal 2Cisco Expressway IP Port Usage for Firewall Traversal 3Firewall Traversal and NAT Traversal On Cisco Expressway Explained With Wireshark 4Firewall Traversal Mechanisms > CCIE Collaboration Quick Reference