request a specific action to be taken by another user agent or server. The INVITE, REGISTER, BYE, ACK, CANCEL, and OPTIONS methods are the original six methods in SIP. The REFER, SUBSCRIBE, NOTIFY, MESSAGE, UPDATE, INFO, and PRACK methods are described in separate RFCs. Note that a proxy does not need to understand a request method in order to forward the request. A proxy treats an unknown method as if it were an OPTIONS; that is, it forwards the request to the destination if it can. This allows new features and methods useful for user agents to be introduced without requiring support from proxies that may be in the middle. A user agent receiving a method it does not support replies with a 501 Not Implemented response. Method names are case sensitive and conventionally use all uppercase for visual clarity to distinguish them from header fields, which use both upper and lower case. SIP: Understanding the Session Initiation Protocol 4.1.1 INVITE The INVITE method is used to establish media sessions between user agents. In telephony, it is similar to a Setup message in ISDN or an initial address mes- sage (IAM) in ISUP. (PSTN protocols are briefly introduced in Section 7.4.) Responses to INVITES are always acknowledged with the ACK method described in Section 4.1.4. Examples of the use of the INVITE method are described in Chapter 2. An INVITE usually has a message body containing the media informa- tion of the caller. The message body can also contain other session information such as quality of service (QoS) or security information. If an INVITE does not contain media information, the ACK contains the media information of the UAC. An example of this call flow is shown in Figure 4.1. If the media informa- tion contained in the ACK is not acceptable, then the called party must send a BYE to cancel the session-a CANCEL cannot be sent because the session is already established. A media session is considered established when the INVITE, 200 OK, and ACK messages have been exchanged between the UAC and the UAS. A success