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 (BGP) routing protocol, tacit agreement to norms of conduct and, in some extraordinarily rare cases (0.07%), 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 (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). 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,"