Securing Protocol Layers   «Prev  Next»

Lesson 8 TCP/IP packet construction
Objective OSI model and how packets are sent across Internet.

TCP-IP Packet Construction

Describe the OSI model and how packets are sent across the Internet. Information sent across the Internet must be bundled into packets. The OSI model determines exactly how a network constructs packets to be sent across the Internet. This model also determines which portions of the TCP/IP model match which level of the operating system.


The fact that devices operate at different levels is especially important to remember when dealing with routers. A router is concerned only with directing packets from one computer to the appropriate location in a receiving computer. A router must understand the packet's structure only as far as the network layer is concerned.

IP datagrams

Because a router is restricted from other levels of the network, it is especially well suited for packet filtering. Packets are created using an encapsulation process, as shown in the SlideShow below.

IP Datagrams
The program request or response becomes the data section for a TCP or UDP packet. This entire packet, including the header information, in turn, becomes the data portion of an IP packet. Controlling this process through routers and firewalls yields a high degree of security and is important to both firewalls and Virtual Private Networks.

Hacker bypassing

The encapsulation process allows modification of specific layers of the TCP/IP protocol stack. For example, a hacker may route a virus through an unused router of your network directly bypassing a company's application gateways. The virus would reside in the stack's application layer and would not be detected by the router because the router looks only at the Internet layer.

Security for TCP/IP

In order to best protect a network, security issues should be individually addressed for all layers of the TCP/IP protocol stack.

(VPN) Virtual Private Network: An extended local area network (LAN) that enables an organization to conduct secure, real-time communication.

Packet sniffing, the activity in which a hacker can intercept and read datagram packets, is common.