Securing Protocol Layers   «Prev 

IP datagrams

Data transmitted over an internet using IP is carried in messages called IP datagrams. Similar to all network protocol messages, IP uses a specific format for its datagrams. We are looking at IP version 4 and we will examine the IPv4 datagram format, which was defined in RFC 791 along with IPv4.
The IPv4 datagram is conceptually divided into two pieces:
  1. header and
  2. the payload.
The header contains addressing and control fields, while the payload carries the actual data to be sent over the internetwork. Unlike some message formats, IP datagrams do not have a footer following the payload.
Even though IP is a relatively simple, connectionless, unreliable protocol, the IPv4 header carries certain information, which makes it somewhat large.

1) When you send an email message to someone at, your email program first transmits this message to a mail server

2) The SMTP mail program on your server issues a command to the stack to create a mail session to

3) The stack puts a header on this message, indicating that the data is meant for remote TCP port 25.

4) This entire TCP packet is then packaged as the data in an IP datagram. A datagram includes such information as your computer's IP address, and the IP address of

5) In turn, this datagram is put into a physical media packet, such as an ethernet packet, which includes the physical address of the first step on its way to dispersednet

6) At each step in its journey, the IP datagram is examined and then encapsulated into a new physical packet on the appropriate computer, or wire, that sends it to its true destination.

7) Eventually, the remote computer will receive your email message in the form of a packet, and will then pass the data to the destination server. When the destination server receives the packet, it reverses the encapsulation process, separating the pieces of the packet until the mail server has all the data.