What SIP is for

• Wikipedia says SIP initiates, maintains, modifies, and terminates communication sessions that involve voice, video, and messaging.
• It says SIP is text-based and incorporates elements of HTTP and SMTP, and that a call may have multiple media streams.
• It also says media type and setup are performed with the Session Description Protocol, and that SIP works with other protocols that carry the media.
• RFC 3261, opened as SIP: Session Initiation Protocol, is the standards-track specification. The page notes that it obsoletes RFC 2543.
• IANA's SIP parameters page says header field names are case-insensitive and points at RFC 3261. The page shows Last Updated 2026-08-17.

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