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Match a ladder before you change anything

A ladder is one dialog only after Call-ID, CSeq, the Via branch, and the To tag agree.

Learn in 5 minutes

  1. Match messages that belong to one dialog
  2. Separate a Via branch from a dialog
  3. Say where a 2xx ACK travels

Visual

Match this dialog, then the 2xx ACK · study drawing · cyphermonkeylabs.com AliceSBCBob INVITE 100 INVITE 180 180 200 200 ACK

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Read these before you rename a header or change a route.

Worked example

Find the end-to-end ACK Play the ladder. Which hop is the ACK for the 200 OK, and which branch does it use?

  1. Press Play.
  2. Step until the hop named in the question.
  3. Inspect that message and answer the check.

Fault

A 200 OK is on the screen and the phone still rings.

Call-ID a84b4c76e66710@example.com and Call-ID other-call@example.com are two dialogs.

Quiz

Which field identifies one transaction on one hop?


The ACK after 200 OK travels


The ACK after 487 travels


A To tag on the initial INVITE means


100 Trying tells you


RFC 3665 is useful here because it


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