SIP core: messages, transactions, dialogs

Rooms

1. Roles a trace is actually showing

   A trace shows a user agent, a proxy, a registrar, a redirect server, or a B2BUA, and products often combine those roles.

2. Methods and status codes

   The core methods are INVITE, ACK, OPTIONS, BYE, CANCEL, and REGISTER, and status codes fall into six classes.

3. Headers and compact forms

   Compact header names are aliases from RFC 3261, with a few more aliases defined in the extension RFCs.

4. Transactions and the branch cookie

   A transaction is identified by the branch, and a retransmission repeats that branch instead of starting a new one.

5. Dialog identity

   A dialog is identified by Call-ID, the From tag, and the To tag, and the initial request has no To tag.

6. T1, the transaction timeout, and Timer C

   T1 seeds retransmissions, 64 times T1 ends the transaction, and Timer C is the proxy's longer INVITE arm.

7. UDP, TCP, TLS, and WebSocket

   The transport is a property of the hop, written in Via, and it changes retransmission and framing rather than the SIP methods.

Ecosystem

SIP does not replace workforce management, quality monitoring, CRM, emergency policy, or a messaging registry. It is the session seam those systems hang from. Compliance notes on this page are educational and are not legal advice.

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