03 What is Transit in IT Terminology?
A Detailed Explanation of Its Definition and the Differences from Peering

The internet is a collection of tens of thousands of networks known as ASes ^*1. Only when these ASes connect with each other can users access content worldwide. There are two methods for connecting ASes: peering and transit. While these concepts might be unfamiliar to the general public, they are crucial for any business with an AS, as they are the key to handling traffic optimally. This article aims to explain transit and its differences from peering in a way that's easy for beginners to understand.

*1 AS (Autonomous System): A network managed by a single organization. The internet consists of numerous ASes interconnecting with each other.

What is Transit?

Transit refers to a type of connection in which an AS with a lot of route information on the internet provides this information to another AS. Transit is usually offered for a fee, typically by large-scale businesses known as Tier 1 providers. Connecting with such businesses and purchasing transit allows access to internet route information worldwide through a single connection, reducing the effort and management associated with interconnectivity. However, all the traffic that flows through this connection incurs costs, leading to higher expenses.

The Difference in Purpose Between Transit and Peering

Transit provides an AS with route information to the entire internet, whereas peering is a method where two ASes directly exchange their respective route information only.

The Cost Difference Between Transit and Peering

Transit is a paid service, requiring a fee to the AS providing the route information, while peering does not typically involve direct charges for the exchange of traffic. However, peering also incurs costs for connection lines and the use of an IX, so it's not entirely free. Unlike with transit, these costs are often fixed. These differences should be considered when choosing between the two.

To understand the difference between these two connection methods, let's compare them to interpersonal relationships.
Peering can be thought of as 'direct contact with friends.' Imagine peering as two friends, A and B, communicating directly. A and B know each other's contact information, allowing private interactions. However, becoming friends with each person can be time-consuming, and it's not always possible to be friends with everyone. That’s where transit comes in.

In contrast, the transit provider is a well-connected intermediary. Suppose A wants to contact C, D, E, and F but needs their contact information. A can consult T, the intermediary, who knows everyone's contacts and can connect A with them. This relationship resembles that of 'a friend of a friend.' But of course, unlike a friend of a friend, every time you contact this person, you would have to pay a service fee.

Conclusion

We have focused on transit, one of the methods for connecting ASes. Transit is a convenient but costly connection method. Therefore, peering through IXes (Internet Exchanges) can be beneficial when considering more economical options. In conclusion, a network operation by necessity utilizes both transit and peering, taking advantage of transit's benefits while compensating for its drawbacks with peering.