2.2.7 From and To Header Field Extensions

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This protocol defines a new From and To header field parameter and its value. The original ABNF, as defined in [RFC5234], for the From and To header fields in [RFC3261] section 25 is extended as follows:

from-param = tag-param 
             / epid-param
             / generic-param
to-param = tag-param 
             / epid-param
             / generic-param
epid-param = "epid=" epid-param-value
epid-param-value = 1*16 tokenchar
tokenchar = (alphanum / "-" / "." / "!" / "%" / "*"
             / "_" / "+" / "`" / "'" / "~" )

alphanum is defined in [RFC3261] section 25.