When a session is created by Session Initiation Protocol (SIP) between networks across a Network Address Translator (NAT), IP addresses in SIP payload have to be translated. We proposed the SIP Application Level Gateway (SIP-ALG) adopted "Add-On method" which translates IP addresses in SIP payload by associating with NAT. The SIP-ALG requests binding-address entries for NAT and caches them inside the SIP-ALG in an initiation of SIP dialog/transaction, and deletes the entries/cached entries in a termination of SIP dialog/transaction subsequently. The SIP-ALG has to manage SIP call state of "initiation", "continuance", "termination" for the purpose of controlling the entries/cached entries appropriately. In this paper we discuss the call state model in the SIP-ALG.
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