Peering
Peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the customers of each network. The pure definition of peering is settlement-free or "sender keeps all," meaning that neither party pays the other for the exchanged traffic, instead, each derives revenue from its own customers. Marketing and commercial pressures have led to the word peering routinely being used when there is some settlement involved, even though that is not the accurate technical use of the word. The phrase "settlement-free peering" is sometimes used to reflect this reality and unambiguously describe the pure cost-free peering situation.
Peering requires physical interconnection of the networks, an exchange of routing information through the Border Gateway Protocol (BGP) routing protocol and is often accompanied by peering agreements of varying formality, from "handshake" to thick contracts.
How peering works
The Internet is a collection of separate and distinct networks, 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:
- Transit (or pay) - You pay money (or settlement) to another network for Internet access (or transit).
- Peer (or swap) - Two networks exchange traffic between each other's customers freely, and for mutual benefit.
- Customer (or sell) - Another network pays you money to provide them with Internet access.
Furthermore, in order for a network to reach any specific other network on the Internet, it must either:
- Sell transit (or Internet access) service to that network (making them a 'customer'),
- Peer directly with that network, or with a network who sells transit service to that network, or
- Pay another network for transit service, where that other network must in turn also sell, peer, or pay for access.
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 who also does not purchase transit.
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 (by reducing dependence on one or more transit providers).
- Increased capacity for extremely large amounts of traffic (distributing traffic across many networks).
- Increased routing control over your traffic.
- Improved performance (attempting to bypass potential bottlenecks with a "direct" path).
- Improved perception of your network (being able to claim a "higher tier").
- Ease of requesting for emergency aid (from friendly peers).
Physical interconnections for peering
Scheme of interconnection and peering of autonomous systems
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 (NAPs); today they are most often called exchange points or Internet exchanges ("IXP" or "IX"). 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 who are just beginning to peer, find that public peering exchange points provide an excellent way to meet and interconnect with other networks who 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. These operators typically go to great lengths to promote communication and encourage new peering, and will often arrange social events for these purposes.
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 interconnections occur at carrier hotels 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.
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 who have not yet proven that they will provide a mutual benefit.
Peering agreements/contracts
Throughout the history of the Internet, there have been a spectrum of kinds of agreements between peers, ranging from handshake deals to peering contracts which may be required by one or both sides. Such a contract sets 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, and the ones operating in the most heavily-regulated economies, accounting for about 1-2% of peering relationships overall.
History of peering
The first Internet exchange point was the Commercial Internet Exchange (CIX), formed by Alternet/UUNET (now Verizon Business), PSI, and CERFNET to exchange traffic without regard for whether the traffic complied with the acceptable use policy (AUP) 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 (DEC).
Another early exchange point was Metropolitan Area Ethernet, or MAE, in Tysons Corner, Virginia. When the United States government decided to de-fund the NSFNET backbone, Internet exchange points were needed to replace its function, and initial governmental funding was used to aid the 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:
- MAE-East - Located in Tysons Corner, VA, and later relocated to Ashburn, Virginia
- Chicago NAP - Operated by Ameritech and located in Chicago, Illinois
- New York NAP - Operated by Sprint and located in Pennsauken, New Jersey
- San Francisco NAP - Operated by PacBell and located in the Bay Area
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 (155 Mbit/s) and OC-12 (622 Mbit/s) of capacity.
Other prospective exchange point operators moved directly into offering Ethernet technology, such as gigabit Ethernet (1000 Mbit/s), 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 ten gigabit Ethernet (10,000 Mbit/s) 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.
Depeering
By definition, peering is the voluntary and free exchange of traffic between two networks, for mutual benefit. If one or both networks believes that there is no longer a mutual benefit, they may decide to cease peering: this is kn
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