Peering

In computer networking, peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the users of each network. The pure definition of peering is settlement-free, "bill-and-keep," or "sender keeps all," meaning that neither party pays the other in association with the exchange of traffic; instead, each derives and retains revenue from its own customers.

An agreement by two or more networks to peer is instantiated by a physical interconnection of the networks, an exchange of routing information through the (BGP) routing protocol and, in some special cases, a formalized contractual document.

Occasionally the word "peering" is used to describe situations where there is some settlement involved. In the face of such ambiguity, the phrase "settlement-free peering" is sometimes used to explicitly denote pure cost-free peering.

The Internet is a collection of separate and distinct networks referred to as autonomous systems, each one operating under a common framework of globally unique IP addressing and global BGP routing.

The relationships between these networks are generally described by one of the following three categories:

Furthermore, in order for a network to reach any specific other network on the Internet, it must either:

The Internet is based on the principle of global reachability (sometimes called end-to-end reachability), which means that any Internet user can reach any other Internet user as though they were on the same network. Therefore, any Internet connected network must by definition either pay another network for transit, or peer with every other network which also does not purchase transit.

Peering involves two networks coming together to exchange traffic with each other freely, and for mutual benefit. This 'mutual benefit' is most often the motivation behind peering, which is often described solely by "reduced costs for transit services". Other less tangible motivations can include:

The physical interconnections used for peering are categorized into two types:

Public peering is accomplished across a Layer 2 access technology, generally called a shared fabric. At these locations, multiple carriers interconnect with one or more other carriers across a single physical port. Historically, public peering locations were known as network access points (NAPs); today they are most often called exchange points or Internet exchanges ("IXP"). Many of the largest exchange points in the world can have hundreds of participants, and some span multiple buildings and colocation facilities across a city.

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