Using the above example, this is how the same INVITE message might look within the trusted realm: As soon as any message leaves the trusted realm, the header is stripped and anonymity is restored. In other words, as a call is being processed by the SIP network, a P-Asserted-Identity header will be part of all SIP messages for that call (i.e. SIP Proxy) and preserved for the message’s entire time within the trusted realm. P-Asserted-Identity is inserted by a trusted SIP element (e.g. P-Asserted-Identity is used within the “trusted” realm of a SIP network to allow servers and services to process SIP messages for the known, authenticated user and not an anonymous caller. This is where another SIP header comes into play. A caller’s true identity is required for class of service processing, billing, call detail recording (CDR), call recording, or any other traditional network activity. Despite the fact that Andrew might want to keep his identity hidden from Linda, the network SIP elements might not want to deal with an unknown, anonymous caller. However, we have a bit of a problem with this. Now, when Linda receives the INVITE message, she is unaware of who is calling and her phone might display “Unknown” as the caller. Instead of putting his identity in the From header, Andrew’s SIP phone would do something similar to the following: However, what if Andrew didn’t want Linda to know who is calling? Similar to pressing *67 before dialing the land-line phone in his house, Andrew might want to suppress the sending of his identity. The point is that Andrew is the sender and Linda is the recipient. I don’t show you enough of the INVITE message to indicate the kind of media that will be used by this session, but that’s immaterial to the example. Here, Andrew is attempting to establish a session with Linda. For example, you might encounter an INVITE that looks similar to the following: As you would expect, the To header indicates with whom you wish to establish a session and the From header indicates who you are. SIP identity is established with the To and From headers. Today I want to write about how SIP headers can be used to protect your identity when making a multimedia call. Headers can be used for timeouts, loop detection, security, and scores of other activities. Headers can be used to replace one session with another. Headers can tell you whether this is a new session or an altered session. So, how is it possible to create all those features from only two basic messages? It all comes down to SIP headers. In fact, I could arguably knock that list down to two since ACK and BYE are only briefly involved in session creation and release. In fact, when it comes to session creation, management, and manipulation, there are really only four important ones – INVITE, ACK, BYE, and REFER. There are only a baker’s dozen request messages and yet you can create a multitude of different features using them. One of the things I love best about SIP is its simplicity.
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