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What is Client Error (4xx) Response?

Client error responses in Telecommunication shows that the request cannot be achieved as some errors are recognized from the UAC side. The response codes are usually sent by UAS. After receiving an error message, the user agent should resend the request by changing it based on the response. The below are some of the discussed important client error responses.

1. 400 Bad Request

a. It shows that the request was not understood by the server.

b. The request might be missing required header fields such as To, From, Call-ID, or CSeq.

2. 401 Unauthorized
a. It shows that the request requires the user to carry out verification.

b. 401 Unauthorized is usually sent by a registrar server for REGISTER request.

c. The response contains WWW-Authenticate header field which requests for accurate credentials from the calling user agents.

d. A subsequent REGISTER will trigger from the User Agent with correct credentials.

3. 403 Forbidden

a. 403 Forbidden is sent when the server has understood the request, found the request to be appropriately formulated, but will not service the request.

b. This response is not used when permission is required.

4. 404 Not Found

a. 404 Not Found shows that the user recognized by the SIP URI in the Request-URI cannot be located by the server or that the user is not presently signed on with the user agent.

5. 405 Method Not Allowed

a. It shows that the server or user agent has received and understood a request but is not willing to perform the request.

b. Example: A REGISTER request might be sent to a user agent.

c. An Allow field must be available to inform the UAC as to what methods are acceptable.

6. 406 Not Acceptable

a. This response shows that the request cannot be processed due to a necessity in the request message.

b. The Accept header field in the request did not have any options supported by the UAS.

7. 407 Proxy Authentication Required

a. This request sent by a proxy shows that the UAC must first validate itself with the proxy before the request can be processed.

b. The response should have information about the type of credentials required by the proxy in a Proxy-Authenticate header field.

c. The request can be resubmitted with the appropriate credentials in a Proxy-Authorization header field.

8. 408 Request Timeout

a. This response is sent when an Expires header field is available in an INVITE request and the specified time period has passed.

b. It could be sent by a forking proxy or a user agent.

c. The request can be retried at any time by the UAC.

9. 422 Session Timer Interval Too Small

a. The response is used to decline a request having a Session-Expires header field.

b. The minimum allowed interval is specified in the required Min-SE header field.

c. The calling party may retry the request without the Session-Expires header field or with a value less than or equal to the specified minimum.

10. 423 Interval Too Brief

a. The response is returned by a registrar that is declining a registration request because the requested expiration time on one or more Contacts is too short.

b. The response must have a Min-Expires header field listing the minimum expiration interval that the registrar will accept.

11. 480 Temporarily Unavailable

a. This response shows that the request has reached the exact destination, but the called party is not reachable for some reason.

b. The response should contain a Retry-After header indicating when the request may be able to be fulfilled.

12. 481 Dialog/Transaction Does Not Exist

a. This response shows that a response referencing an existing calls or transaction has been received for which the server has no records or state information.

13. 483 Too Many Hops

a. This response shows that the request has been sent to the maximum number of times as set by the Max-Forwards header in the request.

b. This is specified by the receipt of a Max-Forward: 0 header in a request.

14. 486 Busy Here

a. This shows the user agent or client is busy and cannot accept the call.

15. 487 Request Terminated

a. This response can be forwarded by a UA that has received a CANCEL request for a pending INVITE request.

b. A 200 OK is sent to allow the CANCEL, and a 487 is sent to cancel the INVITE transaction.

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