a udp header:

        0                                             15 16                                            31
       --------------------------------------------------------------------------------------------------
       |                                                |                                               |
       |            source port number                  |         destination port number               |
       --------------------------------------------------------------------------------------------------
       |                                                |                                               |
       |               udp length                       |              udp checksum                     |
       --------------------------------------------------------------------------------------------------


           7 echo
          19 chargen
          37 time
          53 domain
          67 bootps (DHCP)
          68 bootpc (DHCP)
          69 tftp
         137 netbios-ns
         138 netbios-dgm
         161 snmp-trap
         500 isakmp
         514 syslog
         520 rip
       33434 traceroute