H.323 Protocols Mapped to the OSI Reference Model
As shown in Figure 19-1:
■ H.225 Registration, Admission, and Status (RAS) messages define communications
between endpoints and gatekeepers.
■ H.225 administers security and authentication.
■ H.245 negotiates channel usage.
The content of the streams in H.323 is far more difficult for firewalls to understand than
existing protocols because H.323 encodes packets using Abstract Syntax Notation (ASN.1).
The H.323 control channel handles H.225 and H.245 and H.323 RAS. H.323 inspection
uses the following ports:
■ 1718—Gatekeeper discovery UDP port
■ 1719—RAS UDP port
■ 1720—TCP control port