Default-free zone

From Wikipedia, the free encyclopedia

In the context of Internet routing, the default-free zone (DFZ) refers to both the collection of all Internet autonomous systems 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.

Contents

[edit] Highly Connected Autonomous Systems and Routers

The Weekly Routing Reports [1] used by the ISP community come from the Asia-Pacific Network Information Centre (APNIC) router in Tokyo, which is a well-connected router that has as good a view of the Internet as any 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 June 30, 2007, there were 224622 routes seen by the APNIC router. These came from 25577 autonomous systems, of which only 74 were transit-only and 22272 were stub/origin-only. 3305 autonomous systems provided some level of transit.

[edit] The obsolete idea of an "Internet core"

The term default-free zone (DFZ) is sometimes confused with an "Internet core" or internet backbone, but there has been no true "core" since before the Border Gateway Protocol (BGP) was introduced. Back in pre-Internet days, when the Exterior Gateway Protocol (EGP) was the exterior routing protocol, it indeed could be assumed there was a single Internet core.

[edit] Skitter Core

That concept, however, has been obsolete for a long time. At best, today's definition of the Internet core is statistical, with the "core" being some number of AS with the greatest traffic according to the CAIDA skitter measurements[2]. The CAIDA measurements are constantly updated.

[edit] Information at Internet Exchange Points

Large Internet Exchange Points (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 [3].

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.

[edit] Customer, non-ISP Participation in the DFZ

It's quite common practice, in a multihomed but stub (i.e., non-transit) autonomous system[4], 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".

[edit] See also

[edit] References

  1. ^ Weekly Routing Report,Routing Analysis Role Account
  2. ^ CAIDA "skitter core",Cooperative Association for Internet Data Analysis,April 2005
  3. ^ Resilience Characteristics of the Internet Backbone Routing Infrastructure,C. Labovitz et al.,1998
  4. ^ Guidelines for creation, selection, and registration of an Autonomous System (AS),RFC1930, J. Hawkinson & T. Bates,March 1996