A student's open lab: learning and sharing, free for everyone.

SIP UniverseFree University

Home / Foundations and how to read a call / What SIP is for

What SIP is for

SIP creates, changes, and ends sessions, while RTP carries the media those sessions describe.

Learn in 5 minutes

  1. Separate SIP signaling from RTP media
  2. Name user agent, proxy, registrar, and redirect server
  3. Treat 100 Trying as hop-by-hop quench, not as an answer

Visual

Signaling agrees the session · study drawing · cyphermonkeylabs.com AliceSBCBob INVITE 100 INVITE 180 180 200 200 ACK

SVG HTML PNG

SIP agrees the session. SDP names the media. RTP moves the samples.

Worked example

Find the offer Play the ladder and open the first INVITE. The audio offer is the SDP body, not a later response.

  1. Play the ladder through the first INVITE.
  2. Inspect that INVITE and find the m= line.
  3. Confirm the 100 Trying has no SDP body.

Fault

The ladder shows 200 OK with SDP, and the caller hears silence.

SDP names the RTP address and port. The samples are a separate flow.

Quiz

What does a SIP INVITE carry?


Which document is the base SIP specification?


A pure proxy is responsible for


A registrar stores


100 Trying means


An SBC most often behaves as


Ecosystem

Glossary, sister systems, and the integration seam for this room.

Workforce management WFM forecasts volume and builds schedules. It consumes contact-center events. It does not parse Via headers. When a forecast is wrong, the SIP symptom is usually queue delay and 480 or carrier congestion, not a broken dialog ID.

Glossary · Ecosystem map · Ports · Troubleshooting

Sources and go deeper