Hidden Headers for HMR and LRT lookup

When an LRT lookup key is more complex than just the URI’s userinfo or a Tel-URI, HMR can be used to extract the data and build a special header.

By using a header name that begins with the at-sign ”@” (e.g. @lrt-key), the header can be hidden and not included in outgoing SIP message, thus eliminating the need for an extra HMR rule to remove it.

Since '@' is not a valid character in a header name as defined by RFC 3261, there is no possibility of a collision between a header name defined in the future and a hidden header name beginning with @.