\chapter{\label{design}Design} %** Design.tex: How was the problem attacked, what was the design % the architecture In this chapter we describe the architecture of our solution and our design choices. We first introduce the general design of NAT64 in the P4 architecture. Afterwards we describe the design differences of the BMV2 and NetFPGA P4 architectures. Afterwards we discuss the design of stateless and stateful NAT64 in relation to P4 as well as two existing software NAT64 solutions. Lastly we discuss how we verify NAT64 functionality and present the network configurations that we use. % ---------------------------------------------------------------------- \section{\label{design:nat64}P4/NAT64} \begin{figure}[h] \includegraphics[scale=0.5]{switchdesign} \centering \caption{P4 Switch Architecture} \label{fig:switchdesign} \end{figure} In section \ref{background:transition} we discussed different translation mechanisms for IPv6 and IPv4. In this thesis we focus on the translation mechanisms ``stateless'' and ``stateful'' NAT64. While higher layer protocol dependent translations are more flexible, this topic has already been addressed in \cite{nico18:_implem_layer_ipv4_ipv6_rever_proxy} and the focus in this thesis is on the practicability of high speed NAT64 with P4. The high level design can be seen in figure \ref{fig:switchdesign}: a P4 capable switch is running our code to provide NAT64 functionality. A P4 switch cannot manage its tables on its own and needs support for this from a controller. The controller also has the role to handle unknown packets and can modify the runtime configuration of the switch. This is especially useful in the case of stateful NAT64. If only static table entries are required, they can usually be added at the start of a P4 switch and the controller can also be omitted. However, stateful NAT64 requires the use of a controller to create session entries in the switch tables. The P4 switch can use any protocol to communicate with the controller, as the connection to the controller is implemented as a separate Ethernet port. \begin{figure}[h] \includegraphics[scale=0.4]{v6-v4-standard} \centering \caption{Standard NAT64 translation} \label{fig:v6v4standard} \end{figure} Software NAT64 solutions typically require routing to be applied to transport the packet to the NAT64 translator as shown in figure \ref{fig:v6v4standard}. Our design differs here: while routing could be used like described above, NAT64 with P4 does not require any routing to be setup. Figure \ref{fig:v6v4mixed} shows the network design that we realise using P4. This design has multiple advantages: first it reduces the number of devices to pass and thus directly reduces the RTT, secondly it allows translation of IP addresses within the same logic network segment. \begin{figure}[h] \includegraphics[scale=0.4]{v6-v4-mixed} \centering \caption{In-network NAT64 translation} \label{fig:v6v4mixed} \end{figure} P4 switches in general look very similar to regular switches, however support executing logic while the packet passes through the switch. Figure \ref{fig:p4switch} illustrates how our solution is implemented and translates packets. \begin{figure}[h] \includegraphics[scale=0.5]{p4switch} \centering \caption{Our P4 Switch Architecture} \label{fig:p4switch} \end{figure} % ---------------------------------------------------------------------- \section{\label{design:bmv2}P4/BMV2} \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{P4/BMV2 checksumming} \label{fig:bmv2checksum} \end{figure} The software emulated switch that is implemented using Open vSwitch~\cite{openvswitch} and the behavioral model~\cite{_implem_your_switc_target_with_bmv2} offers the fastest and easiest way of P4 development. All NAT64 features are tested first on P4/BMV2 and in a second step ported to P4/NetFPGA and modified, where necessary. The development follows closely the general design shown in section \ref{design:nat64}. As outlined in section \ref{background:checksums}, checksums inside higher level protocols need to be adjusted after translation. 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 sophisticated and provides direct support for calculating the checksum over the 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. Sample code that calculates the required checksum for answering NDP queries is shown in figure \ref{fig:bmv2checksum}. The code shows how the field \texttt{hdr.icmp6.checksum} is updated with the \texttt{csum16} method depending on the IPv6 and ICMP6 headers as well as the payload. The second option of using the differences is described in section \ref{design:netpfga}. % ok % ---------------------------------------------------------------------- \section{\label{design:netpfga}P4/NetFPGA} \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} While the P4-NetFPGA project~\cite{netfpga:_p4_netpf_public_github} allows compiling P4 to the NetPFGA, the design slightly varies due to limitations in the available toolchain. In particular, the NetFPGA P4 compiler does not support reading the payload.\footnote{This feature could be implemented in theory, but isn't available at the moment, see~\cite{schottelius:_exter_p4_netpf}.} 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 is calculated using differences between the IPv6 and IPv4 headers. As the checksum calculation only depends on the 1-complement sums of headers and the payload (compare section \ref{background:checksums}) and only headers are modified during NAT64 translations, the higher level protocol checksums can be corrected based on the sum of differences of both headers. Thus our P4/NetFPGA implementation first calculates the sum of the relevant IPv4 headers (\texttt{v4sum()}), the sum of the relevant IPv6 headers (\texttt{v6sum()}) and then calculates the difference including a possible carry bit and adjusts the higher level protocol by this difference (\texttt{delta\_tcp\_from\_v6\_to\_v4()}). Figure \ref{fig:checksumbydiff} shows an excerpt of the code used for adjusting the checksum when translating TCP from IPv6 to IPv4. It is notable that not the full headers are used, but only a ``pseudo header'' is (compare figures \ref{fig:ipv6pseudoheader} and \ref{fig:ipv4pseudoheader}). % ok % ---------------------------------------------------------------------- \section{\label{design:statelessnat64}Stateless NAT64} As seen in section \ref{background:transition:stateless}, stateless NAT64 can be implemented using various factors. Our design for the stateless depends on the capabilities of the environment and is summarised in table \ref{tab:statelessnat64factors}. \begin{table}[htbp] \begin{center}\begin{minipage}{\textwidth} \begin{tabular}{| c | c |} \hline \textbf{Implementation} & \textbf{NAT64 match}\\ \hline P4/BMV2 & LPM (both directions)\\ & and individual entries (both directions)\\ \hline P4/NetPFGA & Individual entries\\ \hline Tayga & LPM (IPv6 to IPv4) and individual entries (IPv4 to IPv6)\\ \hline Jool & LPM (both directions)\\ \hline \end{tabular} \end{minipage} \caption{NAT64 match factors} \label{tab:statelessnat64factors} \end{center} \end{table} When using LPM for translating from IPv6 to IPv4, a /96 IPv6 network is configured for covering the whole IPv4 Internet and the individual IPv4 address is appended to the prefix (compare section \ref{design:configuration}). We also use LPM to match on an IPv4 sub network that translates to an IPv6 sub network. Individual entries are configured differently depending on the implementation: Limitations in the P4/NetFPGA environment require to use table entries. Jool supports individual entries as a special case of LPM, with a network mask matching only one IP address. Tayga supports LPM to translate from IPv6 to IPv4, but requires individual entries for translating from IPv4 to IPv6. Our P4/BMV2 offers the highest degree of flexibility, as it provides support for individual entries based on table entries and LPM table entries. % ---------------------------------------------------------------------- \section{\label{design:statefulnat64}Stateful NAT64} Similar to stateless NAT64, the design of stateful NAT64 depends on the features of the individual implementation. As pointed out in section \ref{background:transition:statefulnat64}, stateful NAT64 is very similar to stateless NAT64, with the main difference being an additional stateful table that helps to create 1:n mappings. We use different approaches within the implementations to solve this problem: \begin{itemize} \item For P4/BMV2 and P4/NetPFGA a python controller handles packets that don't have a table entry, sets the table entry in the P4 switch and inserts the original packet afterwards back into the switch. \item With tayga we rely on the Linux kernel NAT44 capabilities \item Jool implements its own stateful mechanism based on port ranges \end{itemize} All methods though operate in a very similar fashion: A ``controller'' inspects the IPv6 packet and depending on the source address, destination address, protocol (TCP, UDP, ICMP, ICMP6, etc.) and the protocol ID (source / destination TCP/UDP port, ICMP identifier) it selects an outgoing IPv4 address, and source port or ICMP identifier. In case of Jool and Tayga this decision is based on a session table inside the Linux kernel, in case of P4 this decision is based on a session table inside the python controller. While the Jool and Tayga both support cleaning up old session entries, our P4 based solution does not support this feature at the moment. % ---------------------------------------------------------------------- \section{\label{design:tests}NAT64 Verification} We use socat~\cite{rieger:_multip} to verify basic operation of the NAT64 gateway and iperf~\cite{dugan:_tcp_udp_sctp} to test stability of the implementation and measure bandwidth. In particular we use the commands listed in table \ref{tab:nat64verification}. The socat commands allow interactive testing on TCP and UDP connections, while the iperf commands fully utilise the available bandwidth with test data. The socat and iperf commands are used to verify all three NAT64 implementations (p4, tayga, jool). \begin{table}[htbp] \begin{center}\begin{minipage}{\textwidth} \begin{tabular}{| c | c | c |} \hline \textbf{Command} & \textbf{Example} & \textbf{Description} \\ \hline \texttt{socat - TCP6:HOST:PORT} & socat - TCP6:[2001:db8:42::a00:2a]:2345 & Connect via IPv6/TCP\\ & & to IPv4 host\\ %\hline \texttt{socat - UDP6:HOST:PORT} & socat - UDP6:[2001:db8:42::a00:2a]:2345 & Connect via IPv6/UDP \\ & & to IPv4 host\\ %\hline \texttt{socat - TCP:HOST:PORT} & socat - TCP:10.0.1.42:2345 & Connect via IPv4/TCP \\ & & to IPv6 host \\ %\hline \texttt{socat - UDP:HOST:PORT} & socat - UDP:10.0.1.42:2345 & Connect via IPv4/UDP \\ & & to IPv6 host \\ \hline \texttt{socat - UDP6-LISTEN:PORT} & socat - UDP6-LISTEN:2345 & Listen on IPv6/UDP \\ %\hline \texttt{socat - TCP6-LISTEN:PORT} & socat - TCP6-LISTEN:2345 & Listen on IPv6/TCP \\ %\hline \texttt{socat - UDP-LISTEN:PORT} & socat - UDP-LISTEN:2345 & Listen on IPv4/UDP \\ %\hline \texttt{socat - TCP-LISTEN:PORT} & socat - TCP-LISTEN:2345 & Listen on IPv4/TCP \\ \hline \texttt{iperf3 -PROTO -p PORT} & iperf3 -4 -p 2345 & IPv4 iperf server\\ \texttt{-B IP -s} & -B 10.0.0.42 -s &\\ & iperf3 -6 -p 2345 & IPv6 iperf server\\ & -B 2001:db8:42::42 -s & \\ \hline \texttt{iperf3 -PROTO -p PORT } & iperf3 -6 -p 2345& Connect to iperf server\\ \texttt{-O IGNORETIME -t RUNTIME} & -O 10 -t 190 & Run for 190 seconds, \\ & & skip first 10 seconds\\ \texttt{-P PARALLEL -c IP} & -P20 -c 2001:db8:23::2a & with 20 sessions\\ & & connecting to\\ & & 2001:db8:23::2a\\ \texttt{iperf3 -PROTO -p PORT} & & Same as above,\\ \texttt{-O IGNORETIME -t RUNTIME} & & but connect via UDP\\ \texttt{-P PARALLEL -c IP} & & \\ \texttt{-u -b0} & & \\ \hline \end{tabular} \end{minipage} \caption{NAT64 verification commands} \label{tab:nat64verification} \end{center} \end{table} % ---------------------------------------------------------------------- \section{\label{design:configuration}IPv6 and IPv4 configuration} The following sections refer to host and network configurations. In this section we describe the IPv6 and IPv4 configurations as a basis for the discussion. All IPv6 addresses are from the documentation block \textit{2001:DB8::/32}~\cite{rfc3849}. In particular the following sub networks and IPv6 addresses are used: \begin{table}[htbp] \begin{center}\begin{minipage}{\textwidth} \begin{tabular}{| c | c |} \hline \textbf{Address} & \textbf{Description} \\ \hline 2001:db8:42::/64 & IPv6 host network \\ \hline 2001:db8:23::/96 & IPv6 mapping to the IPv4 Internet \\ \hline 2001:db8:42::42 & IPv6 host address \\ \hline 2001:db8:42::77 & IPv6 router address \\ \hline 2001:db8:42::a00:2a & In-network IPv6 address mapped to 10.0.0.42 (p4)\\ \hline 2001:db8:23::a00:2a & IPv6 address mapped to 10.0.0.42 (tayga) \\ \hline 2001:db8:23::2a & IPv6 address mapped to 10.0.0.42 (jool)\\ \hline \end{tabular} \end{minipage} \caption{IPv6 address and network overview} \label{tab:ipv6address} \end{center} \end{table} We use private IPv4 addresses as specified by RFC1918~\cite{rfc1918} from the 10.0.0.0/8 range as follows: \begin{table}[htbp] \begin{center}\begin{minipage}{\textwidth} \begin{tabular}{| c | c |} \hline \textbf{Address} & \textbf{Description} \\ \hline 10.0.0.0/24 & IPv4 host network \\ \hline 10.0.1.0/24 & IPv4 network mapping to IPv6\\ \hline 10.0.0.77 & IPv4 router address\\ \hline 10.0.0.66 & In-network IPv4 address mapped to 2001:db8:42::42 (p4)\\ \hline 10.0.1.42 & IPv4 address mapped to 2001:db8:42::42 (tayga)\\ \hline 10.0.1.66 & IPv4 address mapped to 2001:db8:42::42 (jool)\\ \hline \end{tabular} \end{minipage} \caption{IPv4 address and network overview} \label{tab:ipv4address} \end{center} \end{table} % ok