Default-free zone
Encyclopedia
In the context of Internet
routing
, the default-free zone (DFZ) refers to the collection of all Internet autonomous system
s that do not require a default route
to route a packet to any destination. Conceptually, DFZ routers have a "complete" BGP table, sometimes referred to as the Internet routing table, global routing table or global BGP table, but, realistically, the widespread use of route filtering
and the rapid rate of change in Internet routing ensure that no router anywhere has an absolutely complete view of all routes, and any such routing table would, in any case, look different from the perspective of different routers, even if a stable view could be achieved.
(APNIC) router in Tokyo
, which is a well-connected router that has as good a view of the Internet as any other single router. For serious routing research, however, routing information will be captured at multiple well-connected sites, including high-traffic ISPs (see the "skitter core") below.
As of September 17, 2011, there were 371690 routes seen by the APNIC router. These came from 38823 autonomous systems, of which only 134 were transit-only and 32200 were stub/origin-only. 5208 autonomous systems provided some level of transit.
, but there has been no true "core" since before the Border Gateway Protocol
(BGP) was introduced. In pre-BGP days, when the Exterior Gateway Protocol
(EGP) was the exterior routing protocol, it indeed could be assumed there was a single Internet core.
That concept, however, has been obsolete for a long time. At best, today's definition of the Internet core is statistical, with the "skitter core" being some number of AS with the greatest traffic according to the CAIDA
skitter measurements. The CAIDA measurements are constantly updated.
s (IXP)—in that they typically include full routes as seen by multiple ISPs, as well as customer routes, in their exchange fabric—are extremely good places to assess global Internet routing .
Before the current commercial Internet evolved, the NSFNET
, which interconnected five US government funded supercomputer centers, could have been considered the high-speed Internet core. Four IXPs supported NSFNET, but these IXPs evolved into a model where commercial traffic could meet there. While it is slightly difficult to point to a precise endpoint, NSF funding for transmission ceased by 1998.
but stub
(i.e., non-transit) autonomous system, for the BGP-speaking router(s) to take "full routes" from the various ISPs to which the AS is multihomed. Especially if there is more than one router connected to the same ISP, a common practice, it will receive more routes that are in the DFZ. Here's the reasoning: when you have two routers connected to a major ISP such as Sprint
, France Telecom
or Qwest
, that provider has a number of customer AS connected to it. The optimal route to those customer AS are important to the ISP itself, but also tells one customer AS which specific router has the best path to the other customer. The "full routes", or properly "full routes plus customer routes", coming to a customer router makes that customer router part of the DFZ, but certainly not part of the current concept of the "skitter core".
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
, the default-free zone (DFZ) refers to the collection of all Internet autonomous system
Autonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
s that do not require a default route
Default route
A default route, also known as the gateway of last resort, is the network route used by a router when no other known route exists for a given IP packet's destination address. All the packets for destinations not known by the router's routing table are sent to the default route...
to route a packet to any destination. Conceptually, DFZ routers have a "complete" BGP table, sometimes referred to as the Internet routing table, global routing table or global BGP table, but, realistically, the widespread use of route filtering
Route filtering
In the context of network routing, route filtering is the process by which certain routes are not considered for inclusion in the local route database, or not advertised to one's neighbours...
and the rapid rate of change in Internet routing ensure that no router anywhere has an absolutely complete view of all routes, and any such routing table would, in any case, look different from the perspective of different routers, even if a stable view could be achieved.
Highly Connected Autonomous Systems and Routers
The Weekly Routing Reports used by the ISP community come from the Asia-Pacific Network Information CentreAsia-Pacific Network Information Centre
The Asia Pacific Network Information Centre is the Regional Internet Registry for the Asia Pacific region.APNIC provides number resource allocation and registration services that support the global operation of the Internet...
(APNIC) router in Tokyo
Tokyo
, ; officially , is one of the 47 prefectures of Japan. Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the largest metropolitan area of Japan. It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family...
, which is a well-connected router that has as good a view of the Internet as any other single router. For serious routing research, however, routing information will be captured at multiple well-connected sites, including high-traffic ISPs (see the "skitter core") below.
As of September 17, 2011, there were 371690 routes seen by the APNIC router. These came from 38823 autonomous systems, of which only 134 were transit-only and 32200 were stub/origin-only. 5208 autonomous systems provided some level of transit.
The obsolete idea of an "Internet core"
The term "default-free zone" is sometimes confused with an "Internet core" or internet backboneInternet backbone
The Internet backbone refers to the principal data routes between large, strategically interconnected networks and core routers in the Internet...
, but there has been no true "core" since before the Border Gateway Protocol
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
(BGP) was introduced. In pre-BGP days, when the Exterior Gateway Protocol
Exterior Gateway Protocol
The Exterior Gateway Protocol is a now obsolete routing protocol for the Internet originally specified in 1982 by Eric C. Rosen of Bolt, Beranek and Newman, and David L. Mills. It was first described in RFC 827 and formally specified in RFC 904...
(EGP) was the exterior routing protocol, it indeed could be assumed there was a single Internet core.
That concept, however, has been obsolete for a long time. At best, today's definition of the Internet core is statistical, with the "skitter core" being some number of AS with the greatest traffic according to the CAIDA
Cooperative Association for Internet Data Analysis
The Cooperative Association for Internet Data Analysis is a collaborative undertaking among organizations in the commercial, government, and research sectors aimed at promoting greater cooperation in the engineering and maintenance of a robust, scalable global Internet infrastructure. CAIDA...
skitter measurements. The CAIDA measurements are constantly updated.
Information at Internet Exchange Points
Large Internet Exchange PointInternet Exchange Point
An Internet exchange point is a physical infrastructure through which Internet service providers exchange Internet traffic between their networks . IXPs reduce the portion of an ISP's traffic which must be delivered via their upstream transit providers, thereby reducing the average per-bit...
s (IXP)—in that they typically include full routes as seen by multiple ISPs, as well as customer routes, in their exchange fabric—are extremely good places to assess global Internet routing .
Before the current commercial Internet evolved, the NSFNET
National Science Foundation Network
The National Science Foundation Network was a program of coordinated, evolving projects sponsored by the National Science Foundation beginning in 1985 to promote advanced research and education networking in the United States...
, which interconnected five US government funded supercomputer centers, could have been considered the high-speed Internet core. Four IXPs supported NSFNET, but these IXPs evolved into a model where commercial traffic could meet there. While it is slightly difficult to point to a precise endpoint, NSF funding for transmission ceased by 1998.
Customer, non-ISP Participation in the DFZ
It's quite common practice, in a multihomedMultihoming
Multihoming is a technique used to increase the reliability of the Internet connection for an IP network. As an adjective, it is typically used to describe a customer, rather than an Internet service provider network...
but stub
Stub network
A stub network is a somewhat casual term describing a computer network, or part of an internetwork, with no knowledge of other networks, that will typically send much or all of its non-local traffic out via a single path, with the network aware only of a default route to non-local destinations...
(i.e., non-transit) autonomous system, for the BGP-speaking router(s) to take "full routes" from the various ISPs to which the AS is multihomed. Especially if there is more than one router connected to the same ISP, a common practice, it will receive more routes that are in the DFZ. Here's the reasoning: when you have two routers connected to a major ISP such as Sprint
Sprint Nextel
Sprint Nextel Corporation is an American telecommunications company based in Overland Park, Kansas. The company owns and operates Sprint, the third largest wireless telecommunications network in the United States, with 53.4 million customers, behind Verizon Wireless and AT&T Mobility...
, France Telecom
France Télécom
France Telecom S.A. is the main telecommunications company in France, the third-largest in Europe and one of the largest in the world. It currently employs about 180,000 people and has 192.7 million customers worldwide . In 2010 the group had revenue of €45.5 billion...
or Qwest
Qwest
Qwest Communications International, Inc. was a large United States telecommunications carrier. Qwest provided local service in 14 western U.S. states: Arizona, Colorado, Idaho, Iowa, Minnesota, Montana, Nebraska, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.On April...
, that provider has a number of customer AS connected to it. The optimal route to those customer AS are important to the ISP itself, but also tells one customer AS which specific router has the best path to the other customer. The "full routes", or properly "full routes plus customer routes", coming to a customer router makes that customer router part of the DFZ, but certainly not part of the current concept of the "skitter core".