The SUBJECT header can also contain text displayed on the end device for a call.
The sender may wish to have the message ???Answer the phone!??? pop up when the call
begins ringing on the called party??™s phone (subject to call servers supporting such a
function). There are many other implementations for the SUBJECT header, depending
on operator implementation.
SUBJECT: Anyone Home?
Supported This header is used by either endpoint to communicate the SIP extensions
and capabilities supported by the sender. This is different than REQUIRED, which is
sent to communicate the SIP extensions that must be supported for a session.
The SUPPORTED header would be sent in response to REQUIRED or PROXYREQUIRE
to communicate the extensions supported, or it would be included in an
unsolicited request/response.
SUPPORTED: 100 REL
Timestamp While the use of this header is not fully defined in RFC 3261, it does allow
for the proxy or any other entity to enter a timestamp, which could then be used for determining
round-trip time (RTT). This would require each proxy to enter the timestamp
as the request/response was forwarded to the next hop, and the endpoint to have the
ability to process this information.
TIMESTAMP: 50
To The TO header serves as an address field for human consumption only. This means
that the information is not used by the network for routing purposes.
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