Via MIT Information Policy Project – Internet scientist David Clark – “The goal of the research described here is to determine the location and extent of congestion in the core of the Internet. In particular, as a first focus, we are interested in whether the interconnections among ISPs, both customer-provider (transit) and ISP-ISP (peering) links, are subject to widespread congestion. This topic is relevant to the current debates about network neutrality because congestion is one of the reasons given for network management. Our goal is to bring a focus on the more important issues in this space. Definition: Congestion occurs when a part of the Internet (a link, a server, and so on) does not have  the capacity to service all the demand that is presented to it. Congestion is not in itself a bad thing: if the network has enough capacity so that congestion never occurs, this might well imply unjustified investment. The transport protocols of the Internet (e.g., TCP) are designed to detect and respond to congestion by slowing down so that the offered demand matches the capacity. In this way, the design of the Internet allows it to adapt gracefully to instantaneous fluctuations in demand. Economic causes of congestion: However, long-lived and recurring congestion can be taken as a signal of a real mismatch between capacity and demand. The underlying causes of such a mismatch are usually economic—either the very high cost of a specific link (e.g., a trans-oceanic link), inability to obtain new capacity in a cost-effective, timely manner (which may happen in cellular networks due to issues of cell siting, etc.), or a business disagreement about the terms under which shared  capacity is to be paid for. The recent dispute between major access ISPs and Netflix over the provisioning of direct connections is a highly visible example of such a disagreement. Results: For several of the major U.S. broadband providers, including Comcast, Time Warner, Verizon and Cox, we have measured all the interconnection links visible from our vantage points. The links we probe include interconnection points with providers of high volume content such as Netflix and Google (YouTube), and content delivery networks (CDNs) such as Akamai and Limelight. We have also looked at ISPs in other parts of the world, including BT (UK) and Free (France). Most of the congestion we find over the last several months can be traced to specific circumstances, in particular the adjustments and negotiations related to the delivery of Netflix content into the access networks. We see peering links carrying Netflix traffic that appear to be congested for 18 hours a day, and we also see all of this apparent congestion vanish essentially overnight as new interconnection links are put in place, presumptively as a result of the new business arrangement between Comcast and Netflix.”