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

SIP UniverseFree University

Home / Foundations and how to read a call / Reading the first INVITE

Reading the first INVITE

The first INVITE has a magic-cookie branch, Max-Forwards 70, a From tag, and no To tag.

Learn in 5 minutes

  1. Read a request line and the required headers in order
  2. Check the branch cookie and the initial Max-Forwards
  3. Recognize documentation addresses from RFC 5737

Visual

Walk the first transaction · study drawing · cyphermonkeylabs.com AliceSBCBob INVITE 100 INVITE 180 180 200 200 ACK

SVG HTML PNG

Miss one row and the transaction or the dialog will not match later.

Worked example

Dissect a debug INVITE The capture bench strips the debug prefix. Read the branch and the missing To tag.

  1. Open the capture text.
  2. Find the Via branch.
  3. Confirm To has no tag.

Fault

A proxy never matches the response to the request. The Via branch is a random token with no cookie.

The cookie tells the peer that the branch was built as a unique transaction id.

Quiz

The branch magic cookie is


The initial Max-Forwards value SHOULD be


The To header on the initial INVITE


Dialog identity uses


203.0.113.10 is


CSeq on this INVITE contains


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.

Programmable communications APIs Twilio’s docs home is API reference, tutorials, and integration for Twilio products. Elastic SIP Trunking is the SIP-shaped product already cited in this lab. This page does not define the word CPaaS from an RFC. Read the vendor page before you copy a diagram that uses that word.

Glossary · Ecosystem map · Ports · Troubleshooting

Sources and go deeper