owever, from the 600 Busy Everywhere response, which indicates that the request should not be tried elsewhere. In gen- eral, a 486 Busy Here is sent by a UAS unless it knows definitively that the user cannot be contacted. This response is equivalent to the busy tone in the PSTN. SIP: Understanding the Session Initiation Protocol 5.4.30 487 Request Terminated This response can be sent by a user agent that has received a CANCEL request for a pending INVITE request. A 200 OK is sent to acknowledge the CANCEL, and a 487 is sent in response to the INVITE. 5.4.31 488 Not Acceptable Here This response indicates that some aspect of the proposed session is not accept- able and may contain a Warning header field indicating the exact reason. This response has a similar meaning to 606 Not Acceptable, but only applies to one location and may not be true globally as the 606 response indicates. 5.4.32 489 Bad Event The 489 Bad Event response [3] is used to reject a subscription request or notification containing an Event package that is unknown or not supported by the UAS. The response code is also used to reject a subscription request that does not specify an Event package, assuming that the server does not support the PINT protocol (see Section 4.1.8). 5.4.33 491 Request Pending The 491 Request Pending response is used to resolve accidental simulta- neous re-INVITES by both parties in a dialog. Since both INVITES seek to change the state of the session, they cannot be processed at the same time. While a user agent is awaiting a final response to a re-INVITE, any re-INVITE request received must be replied to with this response code. This is analo- gous to the "glare" condition in telephony in which both ends seize a trunk at the same time. The reconsideration algorithm in SIP is for the user agent to gen- erate a delay (randomly selected within a range determined by if the user agent send the initial INVITE or not) then retry the re-INVITE, assuming that another re-INVITE has not been received in the meantime. In this way, one