B: Packets and the Internet
Posted by Laurel L. Russwurm on October 6, 2009
[The First Part of this series was <<A: Open Source. This is the second installment of my Stop Usage Based Billing alphabet series. After this one, the others should be a breeze. The third section will be C: Deep Packet Inspection>>]
what are packets and why should I care?
Before looking at DPI, we need to understand what packets are. In order to maximize efficiency, every thing that crosses the internet is first disaasembled into smaller chunks of information. A packet, in computerese, is made up of a clump of information that has been packed into a packet.
Everything that crosses the internet does so in packet form. Every bit of email, every bit of spam, every image, every word, every download from Maple Music is converted into packets. All of the elements on a web page, from the embedded YouTube video to the web page text crosses the internet in packets. A process known as packet switching then ensures that everything is transferred from source to destination as efficiently as possible.
Each internet user’s computer connects to the internet though their Internet Service Provider or ISP. Each ISP uses a protocol to communicate with the other components of the internet about the packets that go through their territory. The internet doesn’t go in straight lines. People who understand it usually represent the internet as a cloud.
Even though it appears that the email I send to Grandma is a single thing, it will actually be broken down into manageable packets for the journey, just like everything else that goes out over the internet. Since I’m a visual thinker I’ve created a diagram to clarify how this all works.
crossing the internet
In the top left corner you can see me@teksavvy where I’ve written an email on my computer. When I press “send”, my email program tells me the message has been “sent”. But before it actually goes anywhere it is disassembled into segments, and each segment is put inside a packet. In addition to the segment of data from my email, each packet also contains the addressing information, a verification system and everything necessary to correctly reassemble all the pieces of the email at the receiving end. Every time the packet encounters a new protocol, a new layer of wrapping is added to the packet.
We don’t usually see is how the email is broken down into smaller clumps, or packets. To differentiate between them, I’ve given each of the six resulting packets their own rainbow colors, so we can more easily see the routes they might follow through the internet cloud.
The DSL arrow shows my email packet crossing the DSL phone lines from my home to Bell Canada’s local switching office where my email packets will connect with the DSLAM and enter Bell Canada’s Gateway Access Service
After adding its wrapping to my packets, the GAS equipment forwards my email along the Bell Canada phone cable to the Central Office on Front Street in Toronto. This is a Bell CanadaColocation Center which houses the equipment used by the Independent Internet Service Providers, who can access the Bell Canada infrastructure here. On the diagram this is where we can see my email packets emerging from Teksavvy’s equipment, as this is where the Independent ISP takes over email transport.
For this visual representation of how the internet works, I’ve broken my email down into six packets, so you can follow each packet’s route by color. It is impossible to predict which route the packets will follow, so although I’ve pictured each packet travelling along a completely distinct route, in reality if it is possible the packets would most often travel together. If they can’t all fit along the optimum route, some of the packets will be routed differently. The idea is to get all of the packets to the destination at about the same time.
When my ISP sends out the packets into the internet cloud, the packets will go through one or more separate peering networks. These in turn may pass the packets through one of the major Internet Exchange Points or IXPs like MAE West which make up the backbone of the Internet.
No matter how widely spaced the packets may travel through the internet, as you can see on the diagram, they will all converge on their destination. In my example my email was sent to grandma@aol. So the various email packets arrive at the AOL destination IP address where the packets can now be opened and reassembled and checked before the email can be delivered intact to grandma’s computer.
The internet is made up of a lot of different components all connected together in one very large interconnected network. But there are many different parts, and they all use different protocols to communicate with each other.
As packets travel from system to system, each protocol adds another new layer of packaging to the packet before forwarding it on. So as it crosses the internet, each packet acquires more and more layers, like an onion.
When the packet you have sent is received by the ISP at the end of the line, the packets are opened, checked to make sure that nothing has been damaged, then the email will be reassembled and passed on to the recipient. If a packet has gone astray, the destination IP router will store the existing bits and request that the missing packet be resent.
Although I am using email as my example, everything on the internet is transfered in the same manner. Packets are the lifeblood of the internet. Packets are content.