Peering: The Invisible Engine of Global Internet Connectivity
At the heart of the global internet lies a complex web of interconnections that allow data to travel from one side of the world to the other. This process is driven by peering—the voluntary interconnection of administratively separate internet networks. Through peering, networks exchange traffic between their respective "down-stream" users, ensuring seamless connectivity across the digital landscape.
Most peering is settlement-free, also known as "bill-and-keep." In these arrangements, neither party pays the other for the exchange of traffic; instead, each network derives revenue from its own customers. While a tiny fraction of cases (0.02%) involve financial settlements, the vast majority of the internet relies on this cost-free exchange to function efficiently.
Key Facts
- Settlement-free peering is the standard, where no money is exchanged between networks.
- Peering is typically facilitated via the Border Gateway Protocol (BGP).
- Most peering agreements are informal "handshake" agreements rather than written contracts.
- Internet Exchange Points (IXPs) are central hubs for public peering.
- Modern exchange points primarily utilize Ethernet technology for high capacity.
How Network Interconnection Works
To understand peering, one must understand how Autonomous Systems (AS)—the individual networks that make up the internet—interact. There are two primary types of interconnection relationships between these systems:
- Transit: One network sells service to another, acting as a gateway to the rest of the internet.
- Peering: Two networks connect directly to exchange traffic without charging one another.
For a network to reach a specific destination on the internet, it must either purchase transit service from a provider or peer with that destination (or with a network that provides transit to that destination).
Types of Peering Connections
Networks can establish connections through different physical and logical methods, categorized into public and private arrangements.
Public Peering
Public peering occurs at Internet Exchange Points (IXPs). These are physical locations where many different networks connect to a shared switching fabric, allowing them to exchange traffic with many peers simultaneously. This is highly efficient for reaching a wide variety of networks through a single point of presence.
Private Peering
Private peering involves a direct interconnection between only two networks. This is typically done across a dedicated Layer 1 or Layer 2 medium that provides capacity not shared by any other parties. While early private peering often used SONET circuits, modern connections frequently occur in carrier hotels or colocation facilities via a Private Network Interconnect (PNI). This method is often more cost-effective than traditional telecommunications circuits.
The Evolution of Exchange Technology
The history of internet exchange points is a story of rapid technological scaling. Early Network Access Points (NAPs), such as MAE-East in Virginia or the Chicago NAP, often faced bottlenecks because they utilized FDDI technology, which provided only 100 Mbit/s of capacity. As traffic grew, some transitioned to ATM technology (offering up to 622 Mbit/s).
Today, the industry has largely moved to Ethernet technology. Gigabit Ethernet quickly became the standard due to its lower cost and higher capacity. Modern, significant exchange points now offer services ranging from 10, 40, and even 100 gigabits per second.
| Technology Type | Typical Capacity | Status in Modern IXPs |
|---|---|---|
| FDDI | 100 Mbit/s | Legacy / Obsolete |
| ATM (OC-12) | 622 Mbit/s | Legacy |
| Ethernet (Gigabit) | 1,000 Mbit/s | Predominant Standard |
| High-Speed Ethernet | 10 - 100 Gbps | Current Standard for Large IXPs |
Multilateral Peering and Route Servers
While many connections are point-to-point, the majority of BGP adjacencies are the result of Multilateral Peering Arrangements (MLPAs). In this model, an unlimited number of parties agree to exchange traffic on common terms using a single agreement.
Technically, this is often implemented via a route server or route reflector. Instead of every network needing a direct connection to every other network (a partial-mesh topology), they connect to a central hub (a hub-and-spoke topology). This minimizes configuration complexity and allows new peers to contribute routes quickly. However, critics note that this can break the "shared fate" of the routing and forwarding planes, as a participant's connection to the route server might remain active even if their direct Layer 2 connection to another peer fails.
Global Peering Hubs
As of 2021, certain locations have emerged as the world's most critical interconnection hubs. The largest exchange points by number of peering networks include:
- Ponto de Troca de Tráfego Metro São Paulo (Brazil): 2,289 networks
- OpenIXP (Jakarta, Indonesia): 1,097 networks
- DE-CIX (Frankfurt, Germany): 1,050 networks
Other major players include AMS-IX in Amsterdam, LINX and LONAP in London, and NYIIX in New York. While the United States has a massive amount of traffic, it relies more heavily on private peering and commercial public peering compared to other regions that favor non-profit membership exchange points.
Frequently Asked Questions
What is the difference between peering and transit?
Peering is a voluntary, usually settlement-free interconnection where two networks exchange traffic directly. Transit is a service where one network pays another to access the rest of the internet.
Why do networks engage in "depeering"?
Depeering occurs when networks terminate their interconnection agreement. This can happen due to commercial disputes, technical issues, or changes in business strategy, as seen in historical disputes between various major ISPs.
How common are formal written peering contracts?
Formal written contracts are actually quite rare. Research indicates that as of 2011, they accounted for less than 0.5% of all peering agreements; the vast majority are informal "handshake" agreements.
What is a route server in an IXP?
A route server is a technical component used in multilateral peering to redistribute routing information via a hub-and-spoke topology, making it easier for many networks to connect to each other without needing individual connections to every peer.
Is internet interconnection regulated like telephone networks?
No, internet interconnection is not regulated in the same way as public telephone networks. However, it has faced government scrutiny, such as the Department of Justice blocking the MCI Worldcom/Sprint merger due to its potential impact on the internet backbone market.