SIP Requests and Responses | SIP Architecture

Because SIP is a text-based protocol like HTTP, it is used to send information between clients and servers, and User Agent clients and User Agent servers, as a series of requests and responses. When requests are made, there are a number of possible signaling commands that might be used:

• REGISTER Used when a user agent first goes online and registers their SIP address and IP address with a Registrar server.
• INVITE Used to invite another User agent to communicate, and then establish a SIP session between them.
• ACK Used to accept a session and confirm reliable message exchanges.
• OPTIONS Used to obtain information on the capabilities of another user agent, so that a session can be established between them. When this information is provided a session isn’t automatically created as a result.
• SUBSCRIBE Used to request updated presence information on another user agent’s status. This is used to acquire updated information on whether a User agent is online, busy, offline, and so on.
• NOTIFY Used to send updated information on a User agent’s current status. This sends presence information on whether a User agent is online, busy, offline, and so on.
• CANCEL Used to cancel a pending request without terminating the session.
• BYE Used to terminate the session. Either the user agent who initiated the session, or the one being called can use the BYE command at any time to terminate the session.

When a request is made to a SIP server or another user agent, one of a number of possible responses may be sent back. These responses are grouped into six different categories, with a three-digit numerical response code that begins with a number relating to one of these categories. The various categories and their response code prefixes are as follows:

• Informational (1xx) The request has been received and is being processed.
• Success (2xx) The request was acknowledged and accepted.
• Redirection (3xx) The request can’t be completed and additional steps are required (such as redirecting the user agent to another IP address).
• Client error (4xx) The request contained errors, so the server can’t process the request
• Server error (5xx) The request was received, but the server can’t process it. Errors of this type refer to the server itself, and they don’t indicate that another server won’t be able to process the request.
• Global failure (6xx) The request was received and the server is unable to process it. Errors of this type refer to errors that would occur on any server, so the request wouldn’t be forwarded to another server for processing.

There are a wide variety of responses that apply to each of the categories. The different responses, their categories, and codes are shown in Table 1.

Table 1: Listing of Responses, Response Codes, and Their Meanings 

Response Code	Response Category	Response Description
100	Informational	Trying
180	Informational	Ringing
181	Informational	Call is being forwarded
182	Informational	Queued
200	Success	OK
300	Redirection	Multiple choices
301	Redirection	Moved permanently
302	Redirection	Moved temporarily
303	Redirection	See other
305	Redirection	Use proxy
380	Redirection	Alternative service
400	Client Error	Bad request
401	Client Error	Unauthorized
402	Client Error	Payment required
403	Client Error	Forbidden
404	Client Error	Not found
405	Client Error	Method not allowed
406	Client Error	Not acceptable
407	Client Error	Proxy authentication required
408	Client Error	Request timeout
409	Client Error	Conflict
410	Client Error	Gone
411	Client Error	Length required
413	Client Error	Request entity too large
414	Client Error	Request-URI too large
415	Client Error	Unsupported media type
420	Client Error	Bad extension
480	Client Error	Temporarily not available
481	Client Error	Call leg/transaction does not exist
482	Client Error	Loop detected
483	Client Error	Too many hops
484	Client Error	Address incomplete
485	Client Error	Ambiguous
486	Client Error	Busy here
500	Server Error	Internal server error
501	Server Error	Not implemented
502	Server Error	Bad gateway
503	Server Error	Service unavailable
504	Server Error	Gateway time-out
505	Server Error	SIP version not supported
600	Global Failures	Busy everywhere
603	Global Failures	Decline
604	Global Failures	Does not exist anywhere
606	Global Failures	Not acceptable