an attacker probing a UA for vulnerabilities could learn the particular vendor and software load that is suscep- tible to a particular attack and reuse that attack against other UAs that have the same software as identified by the User-Agent header field. Examples include: Table 6.9 Examples of Timestamp Header Field Header Field Meaning Timestamp 235.15 Client has stamped a start time for the request. Timestamp: 235.15 This header field from the response has the delay time added by the server. SIP: Understanding the Session Initiation Protocol Table 6.10 Examples of To Header Field Header Field Meaning To: sip :babage@engine.org;tag=2443a8f7 A single SIP URI with a tag and without a display name To: Thomas Edison A display name is used, so the sips URI is :edison@electric.com> enclosed in < > t: "Jim B. <brattain@bell.org> A display name in quotes along with a SIP URI enclosed within < To: <+1-314-555-1212@carrier.com Both a URI parameter and tag are ; user=phone>;tag=8f7f7ad6675 used, so URI is enclosed in <> User-Agent: Acme SIP Phone v2.2 User-Agent: IP Carrier Gateway Av6.4 6.1.17 Via The required Via header field is used to record the SIP route taken by a request and is used to route a response back to the originator. A user agent generating a request records its own address in a Via header field in the request. While the ordering of most SIP header fields is not significant, the Via header fields order is significant because it is used to route responses. A proxy forwarding the request adds a Via header field containing its own address to the top of the list of Via header fields. A proxy adding a Via header field always includes a branch tag containing a cryptographic hash of the To, From, Call-ID header fields and the Request-URI. A proxy or user agent generating a response to a request copies all the Via header fields from the request in order into the response, then sends the response to the address specified in the top Via header field. A proxy receiving a response checks the top Via header field to en