Some examples of UDP packets.
************************************************************************

DNS query from 192.168.23.107 to DNS server at 192.168.23.1
asking about www.yahoo.com


Frame: 73 bytes
Ethernet:
    Destination: 20:4e:7f:6e:b9:74
    Source:      e8:39:df:68:b7:f3
    Type: IP (0x0800)
Internet Protocol:
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00
    Total Length: 59
    Identification: 0x68b0 (26800)
    Flags: 0x00
    Fragment offset: 0
    Time to live: 128
*   Protocol: UDP (17)
    Header checksum: 0x2245 [correct]
    Source:      192.168.23.107
    Destination: 192.168.23.1

User Datagram Protocol:   {in IP's Data field}
    Source port: 59186    {ephemeral port used on this client}
*   Destination port: domain (53)   {DNS server listens on port 53}
    Length: 39
    Checksum: 0xa52f [validation disabled]

Domain Name System (query)  {in UPD's Data field}
    Transaction ID: 0xfc3a     {various fields of DNS message}
    Flags: 0x0100 (Standard query)
    Questions: 1
    Answer RRs: 0
    Authority RRs: 0
    Additional RRs: 0
*   Queries
        www.yahoo.com: type A, class IN
            Name: www.yahoo.com
            Type: A (Host address)
            Class: IN (0x0001)

**************************
Response from that DNS server:

Frame: 203 bytes
Ethernet:
    Destination: e8:39:df:68:b7:f3
    Source:      20:4e:7f:6e:b9:74
    Type: IP (0x0800)
Internet Protocol:
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00
    Total Length: 189
    Identification: 0x0a35 (2613)
    Flags: 0x02 (Don't Fragment)
    Fragment offset: 0
    Time to live: 64
*   Protocol: UDP (17)
    Header checksum: 0x803e [correct]
    Source: 192.168.23.1 
    Destination: 192.168.23.107 

User Datagram Protocol:
*   Source port: domain (53)   {from the DNS server listening at port 53}
*   Destination port: 59186    {to the DNS client using this ephemeral port}
    Length: 169
    Checksum: 0xf698 [validation disabled]

Domain Name System (response)    {the various fields of DNS...}
    Transaction ID: 0xfc3a
    Flags: 0x8180 (Standard query response, No error)
    Questions: 1
    Answer RRs: 6
    Authority RRs: 0
    Additional RRs: 0
    Queries
        www.yahoo.com: type A, class IN
*   Answers
        www.yahoo.com: type CNAME, class IN, cname fd-fp3.wg1.b.yahoo.com
        fd-fp3.wg1.b.yahoo.com: type CNAME, class IN, cname ds-fp3.wg1.b.yahoo.com
        ds-fp3.wg1.b.yahoo.com: type CNAME, class IN, cname ds-fo-tw-sg.wg1.b.yahoo.com
        ds-fo-tw-sg.wg1.b.yahoo.com: type CNAME, class IN, cname ds-tw-fp3.wg1.b.yahoo.com
        ds-tw-fp3.wg1.b.yahoo.com: type A, class IN, addr 203.84.197.25
        ds-tw-fp3.wg1.b.yahoo.com: type A, class IN, addr 203.84.197.9


**********************************************************************************

NTP client request from 192.168.23.107 to NTP server at 65.55.21.15
(NTP is a protocol to set the client's clock)

Frame: 90 bytes
Ethernet:
    Destination: 20:4e:7f:6e:b9:74
    Source:      e8:39:df:68:b7:f3
    Type: IP (0x0800)
Internet Protocol:
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00
    Total Length: 76
    Identification: 0x67a0 (26528)
    Flags: 0x00
    Fragment offset: 0
    Time to live: 128
*   Protocol: UDP (17)
    Header checksum: 0xa4a7 [correct]
    Source:     192.168.23.107
    Destination: 65.55.21.15

User Datagram Protocol:  {in IP packet's Data field}
*   Source port: ntp (123)
*  Destination port: ntp (123)  {NTP client uses 123 as its ephemeral port}
    Length: 56
    Checksum: 0xb147 [validation disabled]

Network Time Protocol   {in UPD Data field}
    Flags: 0xdb
    Peer Clock Stratum: unspecified or unavailable (0)
    Peer Polling Interval: 17 (131072 sec)
    Peer Clock Precision: 0.015625 sec
    Root Delay:    0.7153 sec
    Root Dispersion:    8.8497 sec
    Reference Clock ID: NULL
    Reference Clock Update Time: Sep 13, 2012 13:19:27.357313 UTC
    Originate Time Stamp: NULL
    Receive Time Stamp: NULL
    Transmit Time Stamp: Sep 17, 2012 10:21:25.483313 UTC

**************************
NTP server response:

Frame: 90 bytes
Ethernet:
    Destination: e8:39:df:68:b7:f3
    Source:      20:4e:7f:6e:b9:74
    Type: IP (0x0800)
Internet Protocol
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00
    Total Length: 76
    Identification: 0x399d (14749)
    Flags: 0x00
    Fragment offset: 0
    Time to live: 44
*   Protocol: UDP (17)
    Header checksum: 0x26ab [correct]
    Source:       65.55.21.15 
    Destination: 192.168.23.107

User Datagram Protocol:
*   Source port: ntp (123)
*   Destination port: ntp (123)
    Length: 56
    Checksum: 0xb081 [validation disabled]

Network Time Protocol
    Flags: 0x1c
    Peer Clock Stratum: secondary reference (2)
    Peer Polling Interval: 17 (131072 sec)
    Peer Clock Precision: 0.015625 sec
    Root Delay:    0.0468 sec
    Root Dispersion:    0.0441 sec
    Reference Clock ID: 132.163.4.102
    Reference Clock Update Time: Sep 17, 2012 10:17:45.200395 UTC
    Originate Time Stamp: Sep 17, 2012 10:21:25.483313 UTC
    Receive Time Stamp: Sep 17, 2012 10:21:25.489395 UTC
    Transmit Time Stamp: Sep 17, 2012 10:21:25.489395 UTC