Datagram Details

• The datagram header contains both the source and destination IP addresses as well as various other administrative data. This means that every datagram can be identified, in terms of "where it came from" and "where it's going".
   
  
• Virtually all "real world" datagrams have a minimal length (20 byte) header. Although various optional (extra) header fields are permitted, these are rarely used in practice.
   
  
• Each network in the Internet is characterised by a Maximum Transmission Unit (MTU), which defines the largest datagram which can be sent on that network. Whilst the total length of a datagram can (in theory) be up to 64 KB, "real world" MTU limits usually mean datagrams are < 1500 bytes in total length.
   
  
• The data field contains "higher layer" protocol data -- usually a TCP segment^[1] which IP is delivering across the Internet. We say that the TCP segment is encapsulated in an IP datagram. The TCP segment, in turn, usually contains application protocol data in its data field.