Google Talk uses XMPP (Extensible Messaging and Presence Protocol). You can use the XMPPFramework framework to implement it. This structure is listed on the XMPP website.
XMPPFramework provides the core implementation of RFC-3920 (xmpp standard), as well as the tools needed to read and write XML.
Check out the iOS Getting Started Guide . It contains an example iOS project.
Also, check out the Google Talk Developer Documentation . There is a section: I want to create a client that connects to the Google Talk service, which describes the steps that must be taken to implement the client.
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