COMERCIO DE VEHÍCULOS Y ACCESORIOS

IPv6 defines a new type of address, known as the anycast address. Although this form of address is deployed in a limited fashion in IPv4, IPv6 integrates this address type into its operations, which improves routing efficiency. In this section we will look into some of the characteristics of the anycast address in detail and discuss some of the interesting applications of the anycast address in the IPv6 Internet of the future.

An anycast address is an IPv6 address that is assigned to a group of one or more hosts, all of which serve a common purpose or function.When packets are sent to the IPv6 anycast address, routing will dictate which member of the group receives the packet via the machine closest to the source, as determined by the IGP of the network in question. (IGP is the Interior Gateway Protocol: the routing protocol you use in your routing domain; for example, RIP, EIGRP, or IS-IS,) In this way, it becomes possible to disperse functionality geographically across your network in a way that improves efficiency in two ways.This differs fundamentally from the multicast address. Although both the anycast and the multicast addresses are assigned to more than one host, the anycast address serves for data transmissions that are 1-to-1, whereas multicast addressing is used when a data transmission to multiple destinations is required. Let us look at the two pri- mary benefits of the anycast addressing scheme.

First, if you are going to the closest machine in a group and it does not matter which group member you exchange information with, you will usually save time by communicating with the closest (IGP-wise) group member. Second, communicating with the closest anycast group member saves bandwidth, because the distance a packet has to travel is, in most cases, minimized. So not only can anycast save you time, but it can also save you money (by using less bandwidth).

The anycast address does not have its own set of bits to define it; instead, any- cast addressing is derived from either scoped or Globally Routable Unicast

Introduction to IPv6 Architecture • Chapter 2 49 addresses. From the point of an IPv6-speaking machine, the anycast address is no

different than a unicast address.The only difference is that there may be other machines that are also numbered with the same scope of unicast address within the same region for which that scope is defined (for instance, you may have more than one machine with a site-local anycast address within a given site).

Now that we understand the differences between anycast and multicast addresses, let’s look into some possible uses of the anycast address. One applica- tion that anycast can help with is DNS (Domain Name Service). If we were to offer DNS to many people or customers, as in the case of most Tier 1 service providers today, we would need to build our DNS in a way that could handle a large number of queries from all parties for which we provide the service. Because of this, it is often more efficient to deploy multiple DNS servers and spread them out geographically.This will allow for fail-over if one DNS server becomes unreachable due to network failures, and it will also allow us to dis- tribute the load of our DNS service between these servers. However, we do not want to make our customers assign too many different IP addresses for DNS servers to point to for resolution, as most people only use one or two. Also, we want some way for one or two IP addresses to be used for all of our geographi- cally diverse service, for the fail-over reason just stated. One way to do this would be to assign each DNS server that has identical configuration and authoritative information the same IP address. If we then inject routes to each of these DNS servers into our backbone routing table, when someone queries our DNS the request will be sent to the DNS server that is geographically closest.This will allow us to split up the load among multiple DNS servers and to avoid too much backhauling of DNS queries across our backbone. So by this method of deploy- ment, we are saving both time for our customers (DNS servers are close, so their data transmission takes less time), and money for ourselves (bandwidth = money for service providers). Because DNS is User Datagram Protocol (UDP)-based rather than TCP-based, transactions between DNS servers and end-stations are quick and short, and don’t need to be kept track of with sequencing, error checking, etc.When we want to resolve a host name, a packet is sent to the DNS server requesting the address associated with a given Internet Domain Name and a response is sent back with the answer.This makes the anycast addressing model viable for this type of application. An illustration of anycast in use is provided in Figure 2.3.You can read more on this specific type of deploy- ment at www.globecom.net/ietf/draft/draft-catalone-rockell-hadns.00.txt.

50 Chapter 2 • Introduction to IPv6 Architecture