Editing Tier 1 network (section)

==Routing through peering==
A bilateral private peering agreement typically involves a direct physical link between two partners. Traffic from one network to the other is then primarily routed through that direct link.

A Tier 1 network may have various such links to other Tier 1 networks.<ref>{{cite book | url=https://books.google.com/books?id=IM-Y2W0RIF0C&dq=tier+1+internet&pg=PA291 | title=Network Routing: Algorithms, Protocols, and Architectures | date=19 July 2010 | publisher=Elsevier | isbn=978-0-08-047497-7 }}</ref><ref>{{cite book | url=https://books.google.com/books?id=fTPmJytNeosC&dq=tier+1+network&pg=PA10 | title=Alcatel-Lucent Scalable IP Networks Self-Study Guide: Preparing for the Network Routing Specialist I (NRS 1) Certification Exam | isbn=978-0-470-52938-6 | last1=Hundley | first1=Kent | date=31 August 2009 | publisher=John Wiley & Sons }}</ref><ref>{{cite book | url=https://books.google.com/books?id=92ZYiSTXkWMC&dq=tier+1+peering&pg=PT113 | title=The Internet Peering Playbook: Connecting to the Core of the Internet | isbn=978-1-937451-02-8 | last1=Norton | first1=William B. | date=8 August 2011 | publisher=DrPeering Press }}</ref>  Peering is founded on the principle of equality of traffic between the partners and as such disagreements may arise between partners in which usually one of the partners unilaterally disconnects the link in order to force the other into a payment scheme. Such disruptive [[Peering#Depeering|de-peering]] has happened several times during the first decade of the 21st century. When this involves large-scale networks involving many millions of customers this may effectively partition a part of the Internet involving those carriers, especially if they decide to disallow routing through alternate routes. This is not largely a technical issue but a commercial matter in which a financial dispute is fought out using the other party's customers as hostages to obtain a better negotiating position. In the worst case, [[multihoming|single-homed]] customers of each network will not be able to reach the other network at all. The de-peering party then hopes that the other network's customers will be hurt more by the decision than its own customers which may eventually conclude the negotiations in its favor.<ref>{{cite web|url=http://www.renesys.com/2008/03/you-cant-get-there-from-here-1/|title=You can't get there from here|work=Renesys |date=2008-03-17|access-date=2014-05-11|quote=Cogent and Telia are having a lover’s quarrel and, as a result, the Internet is partitioned. That means customers of Cogent and Telia cannot necessarily reach one another.}}</ref><ref>{{cite web|url=http://www.internetnews.com/dev-news/article.php/3073411|title='Peering' Into AOL-MSN Outage|date=2003-09-05|access-date=2014-05-11|quote=Some industry watchers believe the problem shows signs of dispute over peering agreements—deals between Internet service providers to create a direct link to route each other's packets rather than pay a third-party network service provider for transport.}}</ref> Lower tier ISPs and other parties not involved in the dispute may be unaffected by such a partition as there exist typically multiple routes onto the same network. The disputes referenced have also typically involved transit-free peering in which one player only exchanged data with the other that involved each other's networks—there was no data transiting ''through'' the other's network destined for other parts of the Internet. By the strict definition of peering and the strict definition of a Tier 1 network, a Tier 1 network only peers with other Tier 1 networks and has no transit routes going anywhere. More practically speaking, Tier 1 networks ''serve'' as transit networks for lower tier networks and only peer with other Tier 1 networks that offer the same services on an adequate scale—effectively being "peers" in the truest sense of the word.<ref>{{cite web|url=http://www.level3.com/en/legal/ip-traffic-exchange-policy/|title=Level 3 IP traffic exchange policy|access-date=2014-05-11|quote=Must provide paid Internet transit services to at least 500 unique transit networks utilizing BGP on a global basis.}}</ref>

More appropriately then, peering means the exchange of an equitable and fair amount of data-miles between two networks, agreements of which do not preclude any pay-for-transit contracts to exist between the very same parties. On the subject of routing, settlement-free peering involves conditions disallowing the abuse of the other's network by sending it traffic not destined for that network (i.e. intended for transit). Transit agreements however would typically cater for just such outbound packets. Tier 1 providers are more central to the Internet backbone and would only purchase transit from other Tier 1 providers, while selling transit to providers of all tiers. Given their huge networks, Tier 1 providers often do not participate in public Internet Exchanges<ref>{{cite book | url=https://books.google.com/books?id=IM-Y2W0RIF0C&dq=tier+1+no+internet+exchange+points&pg=PA293 | isbn=978-0-08-047497-7 | title=Network Routing: Algorithms, Protocols, and Architectures | date=19 July 2010 | publisher=Elsevier }}</ref> but rather sell transit services to such participants and engage in private peering.<ref>{{cite book | url=https://books.google.com/books?id=Rjwr33pdogQC&dq=tier+1+no+internet+exchange+points&pg=PT237 | isbn=978-1-118-39457-1 | title=Global Networks: Engineering, Operations and Design | date=5 November 2012 | publisher=John Wiley & Sons }}</ref> 

In the most logical definition, a Tier 1 provider will never pay for transit because the set of all Tier 1 providers sells transit to all of the lower tier providers everywhere, and because{{olist|list-style-type=lower-alpha|all Tier 1 providers peer with every other Tier 1 provider globally and,|the peering agreement allows ''access'' to all of the transit customers, this means that|the Tier 1 network contains all hosts everywhere that are connected to the global Internet.}}As such, by the peering agreement, all the customers of any Tier 1 provider already have access to all the customers of all the other Tier 1 providers without the Tier 1 provider itself having to pay transit costs to the other networks. Effectively, the actual transit costs incurred by provider A on behalf of provider B are logically identical to the transit costs incurred by provider B on behalf of provider A—hence there not being any payment required.