TCP/IP consists of IP and TCP. TCP_IS_ENCAPSULATED_INSIDE_IP. Same goes for
udp. Here is IP packet header:

0                    7                  15                                       31
+----------------------------------------------------------------------------------+
| IP_ver  | IP_HD_LEN |   TOS (8 bit)     |   16bit total lenght in bytes          |
+---------------------+-------------------+-----+----------------------------------+
|        16bit identification             |flags|      13-bit fragemnt size        |
+---------------------+-------------------+-----+----------------------------------+
|  8 bit TTL          | 8 bit protocol    |  16bit header checksum                 |
+---------------------+-------------------+----------------------------------------+
|                                  32 bit source address                           |
+----------------------------------------------------------------------------------+
|                                  32 bit dest. address                            |
+----------------------------------------------------------------------------------+
|                                    options (if any)                              |
+----------------------------------------------------------------------------------+
|                                     data   (if any)                              |
+----------------------------------------------------------------------------------+

TOS=Type of service
TTL=Time To Live

Empty IP packet would be 20 bytes long ( 32*5=160 bits.. 160/8=20 bytes).
Also, you can note that there are maximum of 255 protocols IPv4 can
encapsulate. IP is connectionless as is UDP. TCP is connection oriented.

TCP header looks like this:


0   3
+-----------------------------------------+----------------------------------------+
|       16 bit source port                |  16 bit destination port               |
+-----------------------------------------+----------------------------------------+
|                              32 bit sequence number                              |
+----------------------------------------------------------------------------------+
|                              32 bit ACK number                                   |
+---+-------------------------+-+-+-+-+-+-+----------------------------------------+
|HL | 6bits reserved          |U|A|P|R|S|F|  16 bit window size                    |
|   |                         |R|C|S|S|Y|I|                                        |
|   |                         |G|K|H|T|N|N|                                        |
+---+-------------------------+-+-+-+-+-+-+----------------------------------------+
|  16 bit TCP cheksum                     |  16 bit urgent pointer                 |
+----------------------------------------------------------------------------------+
|                                   options (if any)                               |
+----------------------------------------------------------------------------------+
|                                     data (if any)                                |
+----------------------------------------------------------------------------------+

HL=header lenght.
sequencing ensures that sent packet is recived at the end - so that is main
catch how connection oriented protocol can be encapsulated inside
connectionless (IP) protocol.
Minimum tcp packet size is 5*32=160 bits or 20 bytes.  So, since TCP uses IP to 
encapsulate packets minimum tcp/ip packet size is 20+20=40 bytes (containing
only let's say SYN or RST flag).


UDP header looks like this:

+-----------------------------------------+----------------------------------------+
|       16 bit source port                |  16 bit destination port               |
+-----------------------------------------+----------------------------------------+
|       16 bit udp lenght                 |  16 bit udp cheksum                    |
+----------------------------------------------------------------------------------+
|                              data(if any)                                        |
+----------------------------------------------------------------------------------+


Overall it is 2*32=64 bits.. 64/8=8. So smallest UDP packet size is:
20+8=28 bytes.