May 14, 2008

Networks have layers. Onions have layers. Ogres have layers.

1. Part 1 — Networks have layers (this article)
2. Part 2 — The Physical Layer
3. Part 3 — Layers 2, 3, 4, 5

OSI Reference Model

If you’ve ever read the first book of the CCNA course load, or sat through a computer science course you’ve been exposed to the layering model in networking.

The most famous is the OSI reference model. 7 layers of networking, from the electrical signals on the wire, encapsulated, re-encapsulated, fragmented and re-ordered again and again until the data packet gets to your desktop. The layers and uses of layers come from the long history of Ethernet, as a common standard that has been changed and molded over time from 1972 into what we use today.

IETF 5 Layer Internet Model

But nobody really pays much attention to the OSI model anymore, except as historical reference as to where we’ve come from. What really matters is the IETF 5 layer Internet model.

http://en.wikipedia.org/wiki/Internet_protocol_suite

http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Internet-Protocols.html

As networking purists, we will focus on the first three layers of networking. There are networking professionals who specialize in optimizing Layer 5 protocols — but that is a discussion for another article.

Pull The Layers Off and Wrap It Up Again

The one critical thing to remember is that the upper layers depend on the lower layers. For example, an application in Layer 5 (DNS for example) makes requests of Layer 4 (UDP), Layer 4 makes requests of Layer 3 (IP), then Layer 3 makes requests to Layer 2 (Ethernet), and so on until Layer 1 (your NIC).

All the way down from Layer 5 to Layer 1, these layers wrap themselves around the data payload. Once the payload gets to Layer 1 these layers are peeled off, as they are handed back up step by step to Layer 4 where the final payload is processed by the application.

Confused? Here is a diagram:

The key thing to remember here is that as a data payload is transmitted from one host to another, the transportation of this payload is managed by many different protocols with many different approaches and objectives for transmitting data.

At each step along the way, a protocol wraps itself around the payload and the management header of the preceding protocol. Once the data gets to the physical layer this process is reversed, and each protocol is called upon to remove its own headers and pass the payload on to the next protocol.

One Response to “Networks have layers. Onions have layers. Ogres have layers.”

1. Networks have layers. Onions have layers. Ogres have layers « WOZNEY enterprises ltd says:

   April 21, 2010 at 10:45 am

   [...] And here is a link to the original article that you were looking for! http://wozney.ca/2008/05/14/layers/ [...]