\chapter{\label{background}Background}
In this chapter we describe the key technologies involved.
\section{\label{background:P4}P4}
P4 is a programming language designed to program inside network
equipment. It's main features are protocol and target independence.
The \textit{protocol independence} refers to the separation of concerns in
terms of language and protocols: P4 generally speaking operates on
bits that are parsed and then accessible in the (self) defined
structures, also called headers. The general flow can be seen in
figure \ref{fig:p4fromnsg}: a parser parses the incoming packet and
prepares it for processing in the switching logic. Afterwards the
packets is output and deparsing of the parsed data might follow.
In the context of NAT64 this is a very important feature: while the
parser will read and parse in the ingress pipeline one protocol
(f.i. IPv6), the deparser will output a different protocol (f.i. IPv4).
\begin{figure}[h]
  \includegraphics[scale=0.9]{p4-from-nsg}
  \centering
  \caption{P4 protocol independence, \cite{vanbever:_progr_networ_data_planes}}
  \label{fig:p4fromnsg}
\end{figure}
The \textit{target independence} is the second very powerful feature
of P4: it allows code to be compiled to different targets. While in
theory the P4 code should be completely target independent, in reality
there are some modifications needed on a per-target basis and each
target faces different restrictions. The challenges arising from this
are discussed in section \ref{conclusion:P4}.

As opposed to general purpose programming languages, P4 lacks some
features, most notably loops. However within its constraints, P4 can guarantee
operation at line speed, which general purpose programming languages
cannot guarantee and also fail to achieve in reality
(see section \ref{results:softwarenat64} for details).

% ----------------------------------------------------------------------
\section{\label{background:ip}IPv6, IPv4 and Ethernet}
The first IPv6 RFC was published in 1998\cite{rfc2460}. Both IPv4 and
IPv4 operate on layer 3 of the OSI model. In this thesis we only
consider transmission via Ethernet, which operates at
layer 2. Inside the Ethernet frame a field named ``type'' specifies
the higher level protocol identifier (0x0800 for IPv4 \cite{rfc894}
and 0x86DD for IPv6 \cite{rfc2464}. This is important, because
Ethernet can only carry either of the two protocols.
The figures \ref{fig:ipv4header} and \ref{fig:ipv6header} show the
packet headers of IPv4 and IPv6. The most notable differences between
the two protocols for this thesis are:
\begin{itemize}
\item Different address lengths (32 vs 128 bit)
\item Lack of checksum in IPv6
\item Format of Pseudo headers (see section \ref{background:checksums})
\end{itemize}
\begin{figure}[h]
\begin{verbatim}

                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |Version| Traffic Class |           Flow Label                  |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |         Payload Length        |  Next Header  |   Hop Limit   |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +                         Source Address                        +
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +                      Destination Address                      +
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\end{verbatim}
  \centering
  \caption{IPv6 Header, \cite{rfc2460}}
  \label{fig:ipv6header}
\end{figure}

\begin{figure}[h]
\begin{verbatim}

                                  0                   1                   2                   3
                                  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |Version|  IHL  |Type of Service|          Total Length         |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |         Identification        |Flags|      Fragment Offset    |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |  Time to Live |    Protocol   |         Header Checksum       |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                       Source Address                          |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                    Destination Address                        |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                    Options                    |    Padding    |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

\end{verbatim}
  \caption{IPv4 Header, \cite{rfc791}}
  \label{fig:ipv4header}
\end{figure}
% ----------------------------------------------------------------------
\section{\label{background:transition}IPv6 Translation Mechanisms}
While in this thesis the focus was in NAT64 as a translation mechanism,
there are a variety of different approaches, some of which we would
like to portray here.
% ----------------------------------------------------------------------
\subsection{\label{background:transition:staticnat64}Static NAT64}
Static NAT64 describes static mappings between IPv6 and IPv4
addresses. This can be based on longest prefix matchings (LPM),
ranges, bitmasks or individual entries.

NAT64 translations as described in this thesis modify multiple layers
in the translation process:

\begin{itemize}
\item Ethernet (changing the type field)
\item IPv4 / IPv6 (changing the protocol, changing the fields)
\item TCP/UDP/ICMP/ICMP6 checksums
\end{itemize}

% ----------------------------------------------------------------------
\subsection{\label{background:transition:statefulnat64}Stateful NAT64}
Stateful NAT64 as defined in RFC6146\cite{rfc6146} defines how to
cretate 1:n mappings between IPv6 and IPv4 hosts. The motivation for
stateful NAT64 is similar to stateful NAT44\cite{/rfc3022}: it allows
translating many IPv6 addresses to one IPv4 address. While the
opposite translation is also technically possible, the differences in
address space don't justify its use in general.

Stateful NAT64 in particular uses information in higher level
protocols to multiplex connections: Given one IPv4 address and the tcp
protocol, outgoing connections from IPv6 hosts can dynamically mapped
to the range of possible tcp ports. After a session is closed, the
port can be reused again.
\begin{figure}[h]
  \includegraphics[scale=0.5]{statefulnat64}
  \centering
  \caption{Stateful NAT64}
  \label{fig:statefulnat64}
\end{figure}
The selection of mapped ports is usually based on the availability on
the IPv4 side and not related to the original port. To support
stateful NAT64, the translator needs to store the mapping in a table and
purge entries regularly.

Stateful usually NAT64 uses information found in protocols at layer 4
like TCP \cite{rfc793} or UDP \cite{rfc768}. However it can also
support ICMP \cite{rfc792} or ICMP6 \cite{rfc4443}.
% ----------------------------------------------------------------------
\subsection{\label{background:transition:Protocol dependent}Higher
  layer Protocol Dependent Translation}
Further translation can be achieved by using information in higher
level protocols like HTTP \cite{rfc2616} or TLS
\cite{rfc4366}. Application proxies like nginx
\cite{nginx:_nginx_high_perfor_load_balan} use layer 7 protocol
information to proxy towards backends. Within this proxying method,
the underlying IP protocol can be changed from IPv6 to IPv4 and vice
versa. However the requested hostname that is usually used for
selecting the backend is encrypted in TLS 1.3 \cite{rfc8446}, which
poses a challenge for implementations.

While protocol dependent translation has the highest amount of
information to choose from for translation, complex parsers or even
cryptographic methods are required for it. That reduces the
opportunities of protocol dependent translation
% ----------------------------------------------------------------------
\section{\label{background:checksums}Protocol Checksums}
One challenge for translating IPv6-IPv4 are checksums of higher level
protocols like TCP and UDP that incorporate information from the lower
level protocols. The pseudo header for upper layer protocols for
IPv6 is defined in RFC2460 \cite{rfc2460} and shown in figure
\ref{fig:ipv6pseudoheader}, the IPv4 pseudo header for TCP and UDP are
defined in RFC768 and RFC793 and are shown in \ref{fig:ipv4pseudoheader}.
\begin{figure}[h]
\begin{verbatim}
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +                         Source Address                        +
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +                      Destination Address                      +
                                 |                                                               |
                                 +                                                               +
                                 |                                                               |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                   Upper-Layer Packet Length                   |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |                      zero                     |  Next Header  |
                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\end{verbatim}
  \centering
  \caption{IPv6 Pseudo Header}
  \label{fig:ipv6pseudoheader}
\end{figure}
When translating, the checksum fields in the higher protocols need to be
adjusted. The checksums for TCP and UDP is calculated not only over the pseudo
headers, but also contain the payload of the packet. This is
important, because some targets (like the NetPFGA) do not allow to
access the payload.
\begin{figure}[h]
\begin{verbatim}
                                 0        7 8     15 16    23 24    31
                                 +--------+--------+--------+--------+
                                 |          source address           |
                                 +--------+--------+--------+--------+
                                 |        destination address        |
                                 +--------+--------+--------+--------+
                                 |  zero  |protocol|   UDP length    |
                                 +--------+--------+--------+--------+

\end{verbatim}
  \centering
  \caption{IPv4 Pseudo Header}
  \label{fig:ipv4pseudoheader}
\end{figure}
% ----------------------------------------------------------------------
\section{\label{background:networkdesign}Network Designs}
\begin{figure}[h]
  \includegraphics[scale=0.5]{v4only}
  \centering
  \caption{IPv4 only network}
  \label{fig:v4onlynet}
\end{figure}
In relation to IPv6 and IPv4, there are in general three different
network designs possible:
The oldest form are IPv4 only networks (see figure
\ref{fig:v4onlynet}.
These networks consist of
hosts that are either not configured for IPv6 or are even technically
incapable of enabling the IPv6 protocol. These nodes are connected to
an IPv4 router that is connected to the Internet. That router might be
capable of translating IPv4 to IPv6 and vice versa.
\begin{figure}[h]
  \includegraphics[scale=0.5]{dualstack}
  \centering
  \caption{Dualstack network}
  \label{fig:dualstacknet}
\end{figure}
With the introduction of IPv6, hosts can have a separate IP stack
active and in that configuration hosts are called ``dualstack hosts''
(see figure \ref{fig:dualstacknet}).
Dualstack hosts are capabale of reaching both IPv6 and IPv4 hosts
directly without the need of any translation mechanism.

The last possible network design is based on IPv6 only hosts, as shown
in figure \ref{fig:v6onlynet}. While it is technically easy to disable IPv4, it
seems that completely removing the IPv4 stack in current operating
systems is not an easy task \cite{ungleich:_ipv4}.
\begin{figure}[h]
  \includegraphics[scale=0.5]{v6only}
  \centering
  \caption{IPv6 only network}
  \label{fig:v6onlynet}
\end{figure}
While the three network designs look similar, there are significant
differences in operating them and limitations that are not easy to
circumvent. In the following sections we describe the limitations and
reason how a translation mechanism like our NAT64 implementation
should be deployed.
% ----------------------------------------------------------------------
\subsection{\label{background:networkdesign:ipv4}IPv4 only network limitations}
As shown in figures \ref{fig:ipv4header} and \ref{fig:ipv6header}
the IPv4 address size is 32 bit, while the IPv6 address size is 128
bit.
Without an extension to the address space, there is no protocol independent
mapping of IPv4 address to IPv6 (see section
\ref{background:transition:nat64})
that can cover the whole IPv6 address space. Thus IPv4 only hosts can
never address every host in the IPv6 Internet. While protocol
dependent translations can try to minimise the impact, accessing all
IPv6 addresses independent of the protcol is not possible.
% ----------------------------------------------------------------------
\subsection{\label{background:networkdesign:dualstack}Dualstack network
  maintenance}
While dualstack hosts can address any host in either IPv6 or IPv4
networks, the deployment of dualstack hosts comes with a major
disadvantage: all network configuration double. The required routing
tables double, the firewall rules roughly double\footnote{The rulesets
  even for identical policies in IPv6 and IPv4 networks are not
  identical, but similar. For this reason we state that roughly double
  the amount of firewall rules are required for the same policy to be
  applied.} and the number of network supporting systems (like DHCPv4,
DHCPv6, router advertisement daemons, etc.) also roughly double.
Additionally services that run on either IPv6 or IPv4 might need to be
configured to run in dualstack mode as well and not every software
might be capable of that.
So while there is the  instant benefit of not requiring any transition mechanism
or translation method, we argue that the added complexity (and thus
operational cost) of running dual stack networks can be significant.
% ----------------------------------------------------------------------
\subsection{\label{background:networkdesign:v6only}IPv6 only networks}
IPv6 only networks are in our opinion the best choice for long term
deployments. The reasons for this are as follows: First of all hosts
eventually will need to support IPv6 and secondly
IPv6 hosts can address the whole 32 bit IPv4 Internet mapped in
a single /96 IPv6 network, whithout the need for address extension or
higher protocol dependent translations\footnote{Higher level protocols
  like TCP, UDP, ICMP6/ICMP.}
