🧠 SIP Dialog vs Transaction: The Critical Distinction
In SIP-based communication systems, few things trip up developers and VoIP engineers more than the confusing difference between a SIP dialog and a SIP transaction.
Yet, mastering this distinction is crucial for effective debugging, call flow optimisation, and secure communications.
SIP Transactions: The Request–Response Engine
A SIP transaction is a single request–response cycle between two SIP endpoints.
For example:
- INVITE → 100 Trying → 180 Ringing → 200 OK → ACK
This entire exchange is considered one transaction.
Key characteristics:
- Transactions are short-lived.
- Managed by the SIP transaction layer.
- Can be of type INVITE or non-INVITE.
SIP Dialogs: The Call’s Lifespan
A dialog represents a relationship between two SIP endpoints established over time. It encompasses multiple transactions.
- It begins with an initial INVITE and lasts until the BYE completes.
- Identified using Call-ID, To/From tags, and CSeq.
Dialogs maintain state. This makes them essential for:
- Call routing
- Media negotiation (via SDP)
- Mid-call operations (hold, transfer)
Why It Matters
When building or debugging SIP softphones, understanding this difference helps:
- Troubleshoot re-INVITE or BYE issues
- Build features like call hold, transfer, or SIP forking
- Analyse SDP negotiation inside dialog state
- Avoid breaking dialog integrity with mismatched headers
Here’s a deeper look into SIP headers like Contact and CSeq, which help track these relationships.
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