\chapter{\label{design}Design}
Description of the theory/software/hardware that you designed.
%** Design.tex: How was the problem attacked, what was the design
%               the architecture
In this chapter we describe the architecture of our solution.

% ----------------------------------------------------------------------
\section{\label{Design:General}General}
The high level design can be seen in figure \ref{fig:switchdesign}: a
P4 capable switch is running our code to provide NAT64
functionality. The P4 switch cannot manage its tables on it own and
needs support for this from a controller. If only static table entries
are required, the controller can also be omitted. However stateful
NAT64 requires the use of a control to create session entries in the
switch tables.
\begin{figure}[h]
  \includegraphics[scale=0.5]{switchdesign}
  \centering
  \caption{General Design}
  \label{fig:switchdesign}
\end{figure}
The P4 switch can use any protocol to communicate with controller, as
the connection to the controller is implemented as a separate ethernet
port. The design allows our solution to be used as a standard NAT64
translation method or as an in network NAT64 translation (compare
figures \ref{fig:v6v4innetwork} and \ref{fig:v6v4standard}). The
controller is implemented in python, the NAT64 solution is implemented
in P4.
% ----------------------------------------------------------------------
\section{\label{Design:BMV2}BMV2}
Development of the thesis took place on a software emulated switch
that is implemented using Open vSwitch \cite{openvswitch}
and the behavioral model
\cite{_implem_your_switc_target_with_bmv2}. The development followed
closely the general design shown in section
\ref{Design:General}. Within the software emulation checksums can be
computed with two different methods:
\begin{itemize}
\item Recalculating the checksum by inspecting headers and payload
\item Calculating the difference between the translated headers
\end{itemize}
The BMV2 model is rather sophisticated and provides many standard
features including checksumming over payload. This allows the BMV2
model to operate as a full featured host, including advanced features
like responding to ICMP6 Neighbor discovery requests \cite{rfc4861}
that include payload checksums.
A typical code to create the checksum can be found in figure
\ref{fig:checksum}.
\begin{figure}[h]
\begin{verbatim}
/* checksumming for icmp6_na_ns_option */
update_checksum_with_payload(meta.chk_icmp6_na_ns == 1,
	{
        hdr.ipv6.src_addr,         /* 128 */
        hdr.ipv6.dst_addr,         /* 128 */
        meta.cast_length,          /* 32 */
        24w0,                      /* 24 0's */
        PROTO_ICMP6,               /* 8 */
        hdr.icmp6.type,            /* 8 */
        hdr.icmp6.code,            /* 8 */

        hdr.icmp6_na_ns.router,
        hdr.icmp6_na_ns.solicitated,
        hdr.icmp6_na_ns.override,
        hdr.icmp6_na_ns.reserved,
        hdr.icmp6_na_ns.target_addr,

        hdr.icmp6_option_link_layer_addr.type,
        hdr.icmp6_option_link_layer_addr.ll_length,
        hdr.icmp6_option_link_layer_addr.mac_addr
    },
    hdr.icmp6.checksum,
    HashAlgorithm.csum16
);
\end{verbatim}
  \centering
  \caption{IPv4 Pseudo Header}
  \label{fig:checksum}
\end{figure}

% ----------------------------------------------------------------------
\section{\label{Design:NetPFGA}NetFPGA}
While the P4-NetFPGA project \cite{netfpga:_p4_netpf_public_github}
allows compiling P4 to the NetPFGA, the design slightly varies.
In particular, the NetFPGA P4 compiler does not support reading
the payload. For this reason it also does not support
creating the checksum based on the payload.
To support checksum modifications in NAT64 on the NetFPGA, the
checksum was calculated on the netpfga using differences between
the IPv6 and IPv4 headers. Figure \ref{fig:checksumbydiff} shows an
excerpt of the code used for calculating checksums in the netpfga.
\begin{figure}[h]
\begin{verbatim}
action v4sum() {
    bit<16> tmp = 0;

    tmp = tmp + (bit<16>) hdr.ipv4.src_addr[15:0];              // 16 bit
    tmp = tmp + (bit<16>) hdr.ipv4.src_addr[31:16];             // 16 bit
    tmp = tmp + (bit<16>) hdr.ipv4.dst_addr[15:0];              // 16 bit
    tmp = tmp + (bit<16>) hdr.ipv4.dst_addr[31:16];             // 16 bit

    tmp = tmp + (bit<16>) hdr.ipv4.totalLen -20;                // 16 bit
    tmp = tmp + (bit<16>) hdr.ipv4.protocol;                    // 8 bit

    meta.v4sum = ~tmp;
}

/* analogue code for v6sum skipped */

action delta_tcp_from_v6_to_v4()
{
    v6sum();
    v4sum();

    bit<17> tmp = (bit<17>) hdr.tcp.checksum + (bit<17>) meta.v4sum;
    if (tmp[16:16] == 1) {
        tmp = tmp + 1;
        tmp[16:16] = 0;
    }
    tmp = tmp + (bit<17>) (0xffff - meta.v6sum);
    if (tmp[16:16] == 1) {
        tmp = tmp + 1;
        tmp[16:16] = 0;
    }

    hdr.tcp.checksum = (bit<16>) tmp;
}

\end{verbatim}
  \centering
  \caption{Calculating checksum based on header differences}
  \label{fig:checksumbydiff}
\end{figure}
The checksums for IPv4, TCP, UDP and ICMP6 are all based on the
``Internet Checksum'' (\cite{rfc791}, \cite{rfc1071}). Its calculation
can be summarised as follows:
\begin{quote}
    The checksum field is the 16-bit one's complement of the one's
    complement sum of all 16-bit words in the header. For purposes of
    computing the checksum, the value of the checksum field
    is zero.\footnote{Quote from Wikipedia\cite{wikipedia:_ipv4}.}.
\end{quote}
As the calculation mainly depends on on (1-complement) sums, the
checksums after translating the protocol can be corrected by
subtracting the differences of the relevant fields. It is notable that
not the full headers are used, but the pseudo headers (compare figures
\ref{fig:ipv6pseudoheader} and \ref{fig:ipv4pseudoheader}).
To compensate the carry bit, our code uses 17 bit integers for
correcting the carry.