fy a user, but identifies a node on a particular physical IP net- work. You have one IP address at your office, another at home, and still another when you log on remotely when you travel. Ideally, there would be one address 1. This string is needed because branch IDs generated by user agents prior to RFC 3261 may have constructed branch IDs which are not suitable as transaction identifiers. In this case, a client must construct its own transaction identifier using the To tag, From tag, Call-II and CSeq. SIP: Understanding the Session Initiation Protocol that would identify you wherever you are. In fact, there is an Internet protocol that does exactly that, with e-mail. SMTP uses a host or system independent name (an e-mail address) that does not correspond to a particular IP address. It allows e-mail messages to reach you regardless of what your IP address is and where you are logged on to the Internet. In addition, a request routed using only IP addresses will reach only one end point-only one device. Since communication is typically user-to-user instead of device-to-device, a more useful addressing scheme would allow a par- ticular user to call another particular user, which would result in the request reaching the target user regardless of which device they are currently using, or if they have multiple devices. SIP uses e-mail-like names for addresses. The addressing scheme is part of a family of Internet addresses known as URIs. SIP URIs can also handle tele- phone numbers, transport parameters, and a number of other items. A full description, including examples, can be found in Section 4.2. For now, the key point is that a SIP URI is a name that is resolved to an IP address by using SIP proxy server and DNS lookups at the time of the call, as will be seen in the next example. Figure 2.2 shows an example of a more typical SIP call with a type of SIP server called a "proxy server." In this example, the caller Schroedinger calls Heisenberg through a SIP proxy server. A SIP proxy operates in a similar way to a 