Peering


In computer networking, peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the "down-stream" users of each network. Peering is settlement-free, also known as "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 Border Gateway Protocol, tacit agreement to norms of conduct and, in some extraordinarily rare cases, a formalized contractual document.
In 0.02% of cases the word "peering" is used to describe situations where there is some settlement involved. Because these outliers can be viewed as creating ambiguity, the phrase "settlement-free peering" is sometimes used to explicitly denote normal cost-free peering.

History

The first Internet exchange point was the Commercial Internet eXchange, formed by Alternet/UUNET, PSI, and CERFNET to exchange traffic without regard for whether the traffic complied with the acceptable use policy of the NSFNet or ANS' interconnection policy. The CIX infrastructure consisted of a single router, managed by PSI, and was initially located in Santa Clara, California. Paying CIX members were allowed to attach to the router directly or via leased lines. After some time, the router was also attached to the Pacific Bell SMDS cloud. The router was later moved to the Palo Alto Internet Exchange, or PAIX, which was developed and operated by Digital Equipment Corporation. Because the CIX operated at OSI layer 3, rather than OSI layer 2, and because it was not neutral, in the sense that it was operated by one of its participants rather than by all of them collectively, and it conducted lobbying activities supported by some of its participants and not by others, it would not today be considered an Internet exchange point. Nonetheless, it was the first thing to bear that name.
The first exchange point to resemble modern, neutral, Ethernet-based exchanges was the Metropolitan Area Ethernet, or MAE, in Tysons Corner, Virginia. When the United States government de-funded the NSFNET backbone, Internet exchange points were needed to replace its function, and initial governmental funding was used to aid the preexisting MAE and bootstrap three other exchanges, which they dubbed NAPs, or "Network Access Points," in accordance with the terminology of the National Information Infrastructure document. All four are now defunct or no longer functioning as Internet exchange points:
As the Internet grew, and traffic levels increased, these NAPs became a network bottleneck. Most of the early NAPs utilized FDDI technology, which provided only 100 Mbit/s of capacity to each participant. Some of these exchanges upgraded to ATM technology, which provided OC-3 and OC-12 of capacity.
Other prospective exchange point operators moved directly into offering Ethernet technology, such as gigabit Ethernet, which quickly became the predominant choice for Internet exchange points due to the reduced cost and increased capacity offered. Today, almost all significant exchange points operate solely over Ethernet, and most of the largest exchange points offer 10, 40, and even 100 gigabit service.
During the dot-com boom, many exchange point and carrier-neutral colocation providers had plans to build as many as 50 locations to promote carrier interconnection in the United States alone. Essentially all of these plans were abandoned following the dot-com bust, and today it is considered both economically and technically infeasible to support this level of interconnection among even the largest of networks.

How peering works

The Internet is a collection of separate and distinct networks referred to as autonomous systems, each one consisting of a set of globally unique IP addresses and a unique global BGP routing policy.
The interconnection relationships between Autonomous Systems are of exactly two types:
  • Peering - Two networks exchange traffic between their users freely, and for mutual benefit.
  • Transit – One network pays another network for access to the Internet.
Therefore, in order for a network to reach any specific other network on the Internet, it must either:
  • Sell transit service to that network or a chain of resellers ending at that network,
  • Peer with that network or with a network which sells transit service to that network, or
  • Buy transit service from any other network.
The Internet is based on the principle of global or end-to-end reachability, which means that any Internet user can transparently exchange traffic with any other Internet user. Therefore, a network is connected to the Internet if and only if it buys transit, or peers with every other network which also does not purchase transit.
Public peering is done at Internet exchange points, while private peering can be done with direct links between networks.

Motivations for peering

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:
  • Increased redundancy.
  • Increased capacity for extremely large amounts of traffic.
  • Increased routing control over one's traffic.
  • Improved performance.
  • Improved perception of one's network.
  • Ease of requesting for emergency aid.

    Physical interconnections for peering

The physical interconnections used for peering are categorized into two types:
  • Public peering – Interconnection utilizing a multi-party shared switch fabric such as an Ethernet switch.
  • Private peering – Interconnection utilizing a point-to-point link between two parties.

    Public peering

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. Today they are most often called exchange points or Internet exchanges. 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.
Since public peering allows networks interested in peering to interconnect with many other networks through a single port, it is often considered to offer "less capacity" than private peering, but to a larger number of networks. Many smaller networks, or networks which are just beginning to peer, find that public peering exchange points provide an excellent way to meet and interconnect with other networks which may be open to peering with them. Some larger networks utilize public peering as a way to aggregate a large number of "smaller peers", or as a location for conducting low-cost "trial peering" without the expense of provisioning private peering on a temporary basis, while other larger networks are not willing to participate at public exchanges at all.
A few exchange points, particularly in the United States, are operated by commercial carrier-neutral third parties which often are data centers, which are critical for achieving cost-effective data center connectivity.

Private peering

Private peering is the direct interconnection between only two networks, across a Layer 1 or 2 medium that offers dedicated capacity that is not shared by any other parties. Early in the history of the Internet, many private peers occurred across "telco" provisioned SONET circuits between individual carrier-owned facilities. Today, most private peering interconnections occur at carrier hotels data centers or carrier neutral colocation facilities, where a direct crossconnect can be provisioned between participants within the same building, usually for a much lower cost than telco circuits. Colocation centers often host private peering connections between their customers, internet transit providers and cloud providers, meet-me rooms for connecting customers together, Internet exchange points, and landing points and terminal equipment for fiber optic submarine communication cables, connecting the internet.
Most of the traffic on the Internet, especially traffic between the largest networks, occurs via private peering. However, because of the resources required to provision each private peer, many networks are unwilling to provide private peering to "small" networks, or to "new" networks which have not yet proven that they will provide a mutual benefit.
Tier 1 networks often do not participate in public Internet Exchanges but rather sell transit services to their participants and engage in private peering. Colocation centers often host private peering connections between their customers, internet transit providers and cloud providers.

Peering agreement

Throughout the history of the Internet, there have been a spectrum of kinds of agreements between peers, ranging from handshake agreements to written contracts as required by one or more parties. Such agreements set forth the details of how traffic is to be exchanged, along with a list of expected activities which may be necessary to maintain the peering relationship, a list of activities which may be considered abusive and result in termination of the relationship, and details concerning how the relationship can be terminated. Detailed contracts of this type are typically used between the largest ISPs, as well as the ones operating in the most heavily regulated economies. As of 2011, such contracts account for less than 0.5% of all peering agreements.