| Protocol | ims | Network | 5G and LTE |
|---|---|---|---|
| Spec | RFC 3261 / 3GPP TS 24.229 | Spec Section | SIP 100 Trying provisional response and IMS transaction handling |
| Direction | IMS network -> UE | Message Type | SIP provisional response |
| Full message name | IMS SIP 100 Trying |
|---|---|
| Protocol | IMS |
| Technology | 5G and LTE |
| Common deployment | VoNR and VoLTE |
| Direction | IMS network -> UE |
| Interface | Gm with onward IMS routing |
| Signaling bearer / channel | IMS SIP signaling / Provisional SIP transaction response |
| Typical trigger | The next hop accepted the request for processing and wants to show that the transaction is alive. |
| Main purpose | Stops unnecessary retransmission and shows that the request is being handled even though alerting or final acceptance has not happened yet. |
| Main specification | RFC 3261 / 3GPP TS 24.229, SIP 100 Trying provisional response and IMS transaction handling |
| Release added | See specification history |
| Procedures where used | IMS Session Setup, IMS Registration, VoNR Call Setup, VoLTE Call Setup |
100 Trying confirms that the request was received and transaction processing has started.
Stops unnecessary retransmission and shows that the request is being handled even though alerting or final acceptance has not happened yet.
100 Trying means the request is being processed, not that the service has already succeeded.
Previous message(s): IMS SIP INVITE, IMS SIP OPTIONS, IMS SIP REGISTER
Next message(s): 180 Ringing, 183 Session Progress, 200 OK
Security context: Uses the same protected IMS signaling path as the request it answers.
SIP/2.0 100 Trying
Via:
From:
To:
Call-ID:
CSeq:
Content-Length: 0
This is SIP response syntax rather than ASN.1.
SIP/2.0 100 Trying
Via: SIP/2.0/UDP ue.example.net;branch=z9hG4bK-inv1
From: <sip:alice@example.net>;tag=mo1
To: <sip:bob@example.net>
Call-ID: call-001@example.net
CSeq: 1 INVITE
Content-Length: 0
| IE | Required | Description |
|---|---|---|
Status-Line | Yes | Shows the 100 Trying response code and reason phrase. |
Via | Yes | Keeps the response on the correct transaction path. |
Call-ID | Yes | Correlates the provisional response with the original request. |
Status-LineShows the 100 Trying response code and reason phrase.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
ViaKeeps the response on the correct transaction path.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Call-IDCorrelates the provisional response with the original request.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Likely cause: The transaction started, but later routing, policy, or service handling stalled.
What to inspect: Check which network element generated 100 Trying and what happened immediately after it.
Next step: Follow the trace into later provisional or final responses.
It means the request was received and processing started, but there is no final service result yet.
Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.