BGP: What Is It and Why Does the Internet Depend on it?

BGP: What Is It and Why Does the Internet Depend on it?
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People scrambled to figure out what was causing Facebook’s six-hour outage on October 4, 2021. Part of the answer can be found in the Border Gateway Protocol, or BGP, which is an essential component of the internet.

So, What Exactly is BGP?

Several very appropriate metaphors have been used to explain BGP in recent articles. It’s been compared to everything from an air traffic controller to a constantly changing map of the internet. It’s even been dubbed “the internet’s duct tape.” And they are all fine.

BGP is the protocol that instructs data requests on the best path to take in order to reach the server. If you log in to Facebook or open the app to pull up your feed, BGP is what guides your data packet along the shortest route to Facebook’s servers.

Cloudflare refers to BGP as “the internet’s postal service,” because it selects the quickest and most efficient path for your requests to reach their intended server. BGP considers all possible routes for your data and selects the best one based on its analysis.

This frequently entails routing your data through the autonomous systems that comprise the internet as a whole. BGP determines which systems communicate with one another and then routes your data along the shortest path between them so that it arrives at its intended destination.

Continuing with the post office metaphor, each autonomous system on the internet is analogous to a post office branch. Even if your city has thousands of mailboxes, every piece of mail must still pass through the post office before it is delivered.

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On the internet, examples of autonomous systems include:

  • An internet service provider (ISP) such as Comcast, AT&T, Verizon, and others.
  • A company such as Facebook
  • Governments and universities are two examples of large organizations.

In an article for The Verge, Mitchell Clark compares BGP to a constantly updating map, and autonomous systems to islands on that map. Because there are far too many “islands” on the internet to build bridges between them all, BGP tells you where the bridges are already.

In fact, there are two kinds of BGP:

  • External BGP (eBGP): The protocol that the internet as a whole uses. This is analogous to international shipping in our post office metaphor.
  • Internal BGP (iBGP): A BGP protocol that autonomous systems can use to route data within their own networks. This is similar to how mail is delivered in different countries.

It is not required to have iBGP configured in order to access the wider internet’s eBGP, but some autonomous systems, such as large tech companies, use iBGP to route internal traffic.

How Do BGP and DNS Work Together?

BGP enables data routing on the internet, making it the glue—or duct tape—that holds the internet together. BGP advertises viable data routes as part of its operation. If BGP fails, those routes can’t be found and vanish from the internet, leaving the data with nowhere to go.

That is a component of what occurred at Facebook. In his blog post explaining the mechanics of the outage, Facebook’s VP of Infrastructure Santosh Janardhan put it this way:

“One of the jobs performed by our smaller facilities is to respond to DNS queries. DNS is the address book of the internet, enabling the simple web names we type into browsers to be translated into specific server IP addresses. Those translation queries are answered by our authoritative name servers that occupy well known IP addresses themselves, which in turn are advertised to the rest of the internet via another protocol called the border gateway protocol (BGP).”

In other words, the internet’s Domain Name System (DNS) protocol is a list of addresses, and BGP is the postal service that delivers mail to those addresses. Mail cannot be delivered if you have an address but no directions to your home.

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Janardhan goes on:

“…DNS servers disable those BGP advertisements if they themselves can not speak to our data centers, since this is an indication of an unhealthy network connection. In the recent outage the entire backbone was removed from operation, making these locations declare themselves unhealthy and withdraw those BGP advertisements. The end result was that our DNS servers became unreachable even though they were still operational. This made it impossible for the rest of the internet to find our servers.”

How BGP Can Ruin the Internet

A variety of factors can influence the path your data takes through the internet’s map. One consideration is cost, as some providers charge for access to their systems. Another factor is the changing nature of the internet itself.

Autonomous systems and websites can move or be completely removed from the internet’s map. They can also switch or add service providers, such as a college switching ISPs from Comcast to AT&T. BGP must update the routes that data can take on a regular basis to ensure that they remain current and that your request does not end up in a dead end, a la Wile E. Coyote.

Autonomous systems continuously perform BGP updates without incident. When they go wrong, however, they can go very wrong. Clark explains in their article that because BGP is designed to spread quickly from system to system, an error can have a ripple effect like the one we saw at Facebook.

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Fixing the Flaws in BGP

According to Cloudflare, a bad BGP update by Turkish ISP TTNet in 2004 temporarily advertised TTNet as the best destination for all internet traffic. This caused connectivity issues for an entire day until the problem was resolved.

Incidents like these highlight certain flaws in BGP, namely that the autonomous systems that comprise the internet as a whole will implicitly trust what BGP tells them is the best route for data. While glitches are rare, some have argued that BGP should be made more secure. A large-scale update, on the other hand, would necessitate the simultaneous updating of every autonomous system on the internet. That means making significant changes to the protocol would be difficult, to say the least.

BGP is only one of several components that enable the internet to function. Understanding its foundation can help you navigate and comprehend future outages and other issues.

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