12.1 Introduction
Over the coming years, there will probably be a
gradual transition of the Internet from IPv4 to IPv6. During this
transition phase, it is important that existing IPv4 applications
continue to work with newer IPv6 applications. For example, a
vendor cannot provide a telnet client that works only with
IPv6 telnet servers but must provide one that works with
IPv4 servers and one that works with IPv6 servers. Better yet would
be one IPv6 telnet client that can work with both IPv4 and
IPv6 servers, along with one telnet server that can work
with both IPv4 and IPv6 clients. We will see how this is done in
this chapter.
We assume throughout this chapter that the hosts
are running dual stacks, that is,
both an IPv4 protocol stack and an IPv6 protocol stack. Our example
in Figure 2.1 is a
dual-stack host. Hosts and routers will probably run like this for
many years into the transition to IPv6. At some point, many systems
will be able to turn off their IPv4 stack, but only time will tell
when (and if) that will occur.
In this chapter, we will discuss how IPv4
applications and IPv6 applications can communicate with each other.
There are four combinations of clients and servers using either
IPv4 or IPv6 and we show these in Figure 12.1.
We will not say much more about the two
scenarios where the client and server use the same protocol. The
interesting cases are when the client and server use different
protocols.
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