Header Formats

Ethernet Header (DIX a.k.a. II)

   0       4       8       12      16      20      24      28     32      36      40      44     47 
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |                                      destination address                                     |
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |                                         source address                                       |
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |            protocol           |
   +-------+-------+-------+-------+


Ethernet Header (802.3 w/ 802.2 SNAP)

   0       4       8       12      16      20      24      28     32      36      40      44     47 
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |                                      destination address                                     |
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |                                         source address                                       |
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |             length            |    --   c  o  n  t  i  n  u  e  d      b  e  l  o  w   --
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |      0xAA     |      0xAA     |      0x03     |              0x000000                        |
   +-------+-------+-------+-------+-------+-------+-------+------+-------+-------+-------+-------+
   |            protocol           |
   +-------+-------+-------+-------+


IP Header

   0       4       8       12      16      20      24      28    31
   +-------+-------+-------+-------+-------+-------+-------+------+
   |  ver  | hdrlen|     T O S     |      total length (bytes)    |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |         identification        | |D|M|  frag offset (13bits)  |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |     T T L     |    protocol   |      header checksum         |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                       source IP address                      |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                     destination IP address                   |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                       options (optional)                     |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                                                              |
   |                           D  A  T  A                         |
   |                                                              |
   +-------+-------+-------+-------+-------+-------+-------+------+

   Flags:
     D, DF: Don't fragment
     M, MF: More fragments


UDP Header


   0       4       8       12      16      20      24      28    31
   +-------+-------+-------+-------+-------+-------+-------+------+
   |          source port          |        destination port      |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |           UDP length          |   UDP checksum (covers all)  |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                                                              |
   |                           D  A  T  A                         |
   |                                                              |
   +-------+-------+-------+-------+-------+-------+-------+------+


TCP Header

   0       4       8       12      16      20      24      28    31
   +-------+-------+-------+-------+-------+-------+-------+------+
   |          source port          |        destination port      |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                          sequence number                     |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                      acknowledgement number                  |
   +-------+-------+-------+-------+-------+-------+-------+------+
   | hdrlen|           |U|A|P|R|S|F|         window size          |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |        TCP checksum           |         urgent pointer       |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                       options (optional)                     |
   +-------+-------+-------+-------+-------+-------+-------+------+
   |                                                              |
   |                           D  A  T  A                         |
   |                                                              |
   +-------+-------+-------+-------+-------+-------+-------+------+

   Flags: 
     U, URG: urgent pointer valid
     A, ACK: acknowledgement number valid
     P, PSH: push data
     R, RST: reset connection
     S, SYN: synchronize sequence numbers, initiate connection
     F, FIN: finish connection