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What is Informational (1xx) Response?

Informational responses are response generally used to show a call progress. Generally the responses are end to end (except 100 Trying). The major aim of informational responses is to end retransmission of INVITE requests.

Informational responses include the following responses:

1. 100 Trying

a. This particular case response is only a hop-by-hop request.

b. It is never sent and may not contain a message body.

c. It is used to prevent the retransmission of INVITE requests.

2. 180 Ringing

a. This response is used to show that an INVITE has been received by the user agent and alerting is taking place.

3. 181 Call is being Forwarded

a. This response is used to show that the call has been sent to another endpoint.

b. It is sent when the information may be of use to the caller.

c. It gives the status of the caller, as a forwarding operation may result in the call taking longer to be answered.

4. 182 Call Queued

a. This response is used to show that the INVITE has been received and will be processed in a queue.

5. 183 Session Progress

a. It shows that information about the progress of a session may be available in a message body or media stream.

b. Unlike a 100 Trying response, a 183 is an end-to-end response and establishes a dialog.

c. A typical use of this response is to permit a UAC to hear a ringtone, busy tone, or recorded announcement in calls via a gateway into the PSTN.

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