151 lines
6.0 KiB
TeX
151 lines
6.0 KiB
TeX
\chapter{\label{design}Design}
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Description of the theory/software/hardware that you designed.
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%** Design.tex: How was the problem attacked, what was the design
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% the architecture
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In this chapter we describe the architecture of our solution.
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% ----------------------------------------------------------------------
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\section{\label{Design:General}General}
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The high level design can be seen in figure \ref{fig:switchdesign}: a
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P4 capable switch is running our code to provide NAT64
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functionality. The P4 switch cannot manage its tables on it own and
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needs support for this from a controller. If only static table entries
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are required, the controller can also be omitted. However stateful
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NAT64 requires the use of a control to create session entries in the
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switch tables.
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\begin{figure}[h]
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\includegraphics[scale=0.5]{switchdesign}
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\centering
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\caption{General Design}
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\label{fig:switchdesign}
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\end{figure}
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The P4 switch can use any protocol to communicate with controller, as
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the connection to the controller is implemented as a separate ethernet
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port. The design allows our solution to be used as a standard NAT64
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translation method or as an in network NAT64 translation (compare
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figures \ref{fig:v6v4innetwork} and \ref{fig:v6v4standard}). The
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controller is implemented in python, the NAT64 solution is implemented
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in P4.
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% ----------------------------------------------------------------------
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\section{\label{Design:BMV2}BMV2}
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Development of the thesis took place on a software emulated switch
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that is implemented using Open vSwitch \cite{openvswitch}
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and the behavioral model
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\cite{_implem_your_switc_target_with_bmv2}. The development followed
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closely the general design shown in section
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\ref{Design:General}. Within the software emulation checksums can be
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computed with two different methods:
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\begin{itemize}
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\item Recalculating the checksum by inspecting headers and payload
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\item Calculating the difference between the translated headers
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\end{itemize}
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The BMV2 model is rather sophisticated and provides many standard
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features including checksumming over payload. This allows the BMV2
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model to operate as a full featured host, including advanced features
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like responding to ICMP6 Neighbor discovery requests \cite{rfc4861}
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that include payload checksums.
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A typical code to create the checksum can be found in figure
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\ref{fig:checksum}.
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\begin{figure}[h]
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\begin{verbatim}
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/* checksumming for icmp6_na_ns_option */
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update_checksum_with_payload(meta.chk_icmp6_na_ns == 1,
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{
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hdr.ipv6.src_addr, /* 128 */
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hdr.ipv6.dst_addr, /* 128 */
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meta.cast_length, /* 32 */
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24w0, /* 24 0's */
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PROTO_ICMP6, /* 8 */
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hdr.icmp6.type, /* 8 */
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hdr.icmp6.code, /* 8 */
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hdr.icmp6_na_ns.router,
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hdr.icmp6_na_ns.solicitated,
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hdr.icmp6_na_ns.override,
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hdr.icmp6_na_ns.reserved,
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hdr.icmp6_na_ns.target_addr,
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hdr.icmp6_option_link_layer_addr.type,
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hdr.icmp6_option_link_layer_addr.ll_length,
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hdr.icmp6_option_link_layer_addr.mac_addr
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},
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hdr.icmp6.checksum,
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HashAlgorithm.csum16
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);
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\end{verbatim}
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\centering
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\caption{IPv4 Pseudo Header}
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\label{fig:checksum}
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\end{figure}
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% ----------------------------------------------------------------------
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\section{\label{Design:NetPFGA}NetFPGA}
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While the P4-NetFPGA project \cite{netfpga:_p4_netpf_public_github}
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allows compiling P4 to the NetPFGA, the design slightly varies.
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In particular, the NetFPGA P4 compiler does not support reading
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the payload. For this reason it also does not support
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creating the checksum based on the payload.
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To support checksum modifications in NAT64 on the NetFPGA, the
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checksum was calculated on the netpfga using differences between
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the IPv6 and IPv4 headers. Figure \ref{fig:checksumbydiff} shows an
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excerpt of the code used for calculating checksums in the netpfga.
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\begin{figure}[h]
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\begin{verbatim}
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action v4sum() {
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bit<16> tmp = 0;
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tmp = tmp + (bit<16>) hdr.ipv4.src_addr[15:0]; // 16 bit
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tmp = tmp + (bit<16>) hdr.ipv4.src_addr[31:16]; // 16 bit
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tmp = tmp + (bit<16>) hdr.ipv4.dst_addr[15:0]; // 16 bit
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tmp = tmp + (bit<16>) hdr.ipv4.dst_addr[31:16]; // 16 bit
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tmp = tmp + (bit<16>) hdr.ipv4.totalLen -20; // 16 bit
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tmp = tmp + (bit<16>) hdr.ipv4.protocol; // 8 bit
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meta.v4sum = ~tmp;
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}
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/* analogue code for v6sum skipped */
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action delta_tcp_from_v6_to_v4()
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{
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v6sum();
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v4sum();
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bit<17> tmp = (bit<17>) hdr.tcp.checksum + (bit<17>) meta.v4sum;
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if (tmp[16:16] == 1) {
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tmp = tmp + 1;
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tmp[16:16] = 0;
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}
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tmp = tmp + (bit<17>) (0xffff - meta.v6sum);
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if (tmp[16:16] == 1) {
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tmp = tmp + 1;
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tmp[16:16] = 0;
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}
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hdr.tcp.checksum = (bit<16>) tmp;
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}
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\end{verbatim}
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\centering
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\caption{Calculating checksum based on header differences}
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\label{fig:checksumbydiff}
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\end{figure}
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The checksums for IPv4, TCP, UDP and ICMP6 are all based on the
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``Internet Checksum'' (\cite{rfc791}, \cite{rfc1071}). Its calculation
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can be summarised as follows:
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\begin{quote}
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The checksum field is the 16-bit one's complement of the one's
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complement sum of all 16-bit words in the header. For purposes of
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computing the checksum, the value of the checksum field
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is zero.\footnote{Quote from Wikipedia\cite{wikipedia:_ipv4}.}.
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\end{quote}
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As the calculation mainly depends on on (1-complement) sums, the
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checksums after translating the protocol can be corrected by
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subtracting the differences of the relevant fields. It is notable that
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not the full headers are used, but the pseudo headers (compare figures
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\ref{fig:ipv6pseudoheader} and \ref{fig:ipv4pseudoheader}).
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To compensate the carry bit, our code uses 17 bit integers for
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correcting the carry.
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% FIXME: add note to python script / checksum diffing
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