Header field              where      PRACK
__________________________________________
Priority                    R          -
Proxy-Authenticate         407         m
Proxy-Authenticate         401         o
Proxy-Authorization         R          o
Proxy-Require               R          o
Record-Route                R          o
Record-Route             2xx,18x       o
Reply-To                               -
Require                                c
Retry-After          404,413,480,486   o
                         500,503       o
                         600,603       o
Route                       R          c
Server                      r          o
Subject                     R          -
Supported                   R          o
Supported                  2xx         o
Timestamp                              o
To                          c          m
Unsupported                420         m
User-Agent                             o
Via                         c          m
Warning                     r          o
WWW-Authenticate           401         m