# What SIP is for

Study notes. Not vendor documentation and not legal advice.

- 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.

On a ladder, label the setup arrows SIP and the dashed media arrow RTP. If someone writes the codec on the SIP arrow, move the codec to SDP, which is the next lesson.