The string "z9hG4bK" is used in the Session Initiatation Protocol (SIP) as a magic cookie to identify compliance with some aspects of the specification. This website is a collection of information regarding SIP and other IETF protocols for voice over IP.
The Introduction contains some introductory articles on the matter at hand.
The SIP Section tries to summarize the three thousand odd pages of RFC published on the Session Initiation Protocol.
The RTP Section does the same for the Real-time Transport Protocol including information on media codecs.
The IETF Section contains information regarding the IETF standardization work.
A Glossary explains all those pesky terms.
Note Well
This site is currently under construction and still in its early stages. There is plenty if missing content and hence there are quite a few dead links.
Information is added RFC by RFC. Documents are processed in the order given on the page Relevant RFCs. Only those that have their own page in that section have been processed already (except for the last one, which is currently being processed).
Since the protocols have evolved over time, some of the information given on this site may be outdated, if not outright wrong, until all relevant RFCs have been processed. In any case, you are advised to double-check all information with the original text given in the standards documents.
Excerpts from RFCs and Internet Drafts:
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