This section gives a brief overview of the \ac{ETSI} architecture for Intelligent Transport Systems. It isn't meant to be elaborate but has a focus on identifiers and other message contents allowing linkability of messages.
\acp{VANET} have some special requirements: Due to many nodes being constantly on the move at higher speeds, tolerance for quickly changing topologies and low-latency communication are important points. Multi-hop mesh-networking is an important ability to keep the network functional in areas without designated infrastructure.
A \ac{VANET} consists of different kinds of ITS stations: \\
\acfp{OBU} residing inside vehicles can be divided into the communication and \acl{CCU}, managing the \ac{ITS} specific network communication over the car's wireless interfaces, and \acfp{AU} utilising the network services provided by the \ac{CCU} to communicate transparently over a standard \acs{IPv6} stack. \\
On the stationary infrastructure side, \acfp{RSU} can either just provide some special local services or even be connected to a network operator's infrastructure and thus provide an uplink to the internet.
The protocol architecture of \ac{ITS} stations according to the \ac{ETSI} reference architecture \cite{europeantelecommunicationsstandardsinstituteetsiETSI3026652010} is mostly based on the well-known \ac{OSI} layer model.
\ac{OSI} layers 1 and 2 are combined into the \textit{Access} layer, \ac{OSI} layers 3 and 4 into the \textit{Networking \& Transport} layer and \ac{OSI} layers 5, 6 and 7 are put into the \textit{Facilities} layer (see Fig. \ref{fig:etsi-its-arch} ). \\
The two vertical \textit{Management} and \textit{Security} layers provide supporting functionality throughout the whole stack. \textit{Applications} make use of the \ac{ITS}-station services and thus sit on top of it all.
Designed for modularity, the \ac{ETSI}\ac{ITS} architecture allows for a big number of access protocols. Similarly, a great variety of applications can run on top of the stack. Because of that variety, access and application layer are considered out-of-scope of this survey.
The \textbf{Networking \& Transport} layer takes care of addressing and routing of messages within the ITS network and multiplexing them to higher-level services. Similarly to the \ac{OSI} model, the groundwork of this functionality is provided by various networking protocols: \\
\ac{ETSI} explicitly mentions the usage of \ac{IPv6} (possibly equipped with mobility support), the CALM FAST protocol \cite{TN_libero_mab2} and the \acf{GN} protocol, which can also be used to encapsulate \ac{IPv6} packets.
CALM FAST \cite{TN_libero_mab2} is a non-IP port-mapper protocol designed for single-hop communication between ITS stations and extensible with additional features. Due to a lack of proper access to the standard document, this protocol is considered out-of-scope of this survey.
\acf{GN} (\cite{europeantelecommunicationsstandardsinstituteetsiETSI30263612014} and following) is an \ac{ETSI}-standardized networking protocol for routing and forwarding packets through \acp{VANET} based on geographical Information. It sits between the link and network layer and provides its services to other networking and transport protocols. The background section of \cite{sandonisVehicleInternetCommunications2016} gives a good high-level overview of the \ac{GN} networking architecture and the rationale behind it.
For this to work, each node maintains a \ac{LT} with the positions of its direct neighbours. This \ac{LT} is populated with information from periodically-sent beaconing messages. These beacons advertise a node's position, \ac{GN} address, its speed, station type and heading (see \ref{GN-identifiers}. This information is also included in all other sent \ac{GN} packets. \ac{LT} entries have a lifetime attached, after which they expire if not refreshed periodically.
For allowing to retrieve the position of non-neighbour nodes, the \ac{LS}, a collaborative functionality of all nodes, forwards request packets, until the node with the destination \ac{GN} address is found and has replied via geo-unicast or a retransmission counter has expired.\todo{influence of frequent pseudonym change}
Security properties of \ac{GN} messages are ensured by signing (authenticity), encrypting (confidentiality) the messages and checking their plausability and consistency. The necessary information for that is given in a security header \cite{europeantelecommunicationsstandardsinstituteetsiETSI302636412017}.
\acsu{IPv6}\cite{RFC8200}\nocite{baeckerRFCE014IPv6} specifies the 6th version of the Internet Protocol, the routing protocol used in the Networking layer of the internet. Relevant details for \acp{VANET} are the addressing using 128 bit long IP addresses \cite{RFC4291} with the first up to 64 bits specifiying the network part and the last 64 bits specifying the interface ID (node ID) within that subnetwork. Additionally to the globally unique routable address, nodes are also addressable in the scope of the same \ac{OSI} layer 2 link using their link-local address automatically derived from lower-layer identifiers. Together with the huge number of globally unique \ac{IPv6} addresses, this new property makes it usable for vehicular ad-hoc networks. Another improvement in \ac{IPv6} is \textit{neighbour discovery}\cite{RFC4861} using link-local multicast. One application of that is the \textit{\acf{RA}}, where routers just periodically announce their parameters so clients are able to derive an address themselves without further negotiation.
Transparently exposing IP networking to higher layers allows re-using existing services based on the classical internet TCP/IP stack without modification. The \acf{GN6ASL}\cite{europeantelecommunicationsstandardsinstituteetsiETSI302636612014} specifies a mechanism for sending \ac{IPv6} packets over the GN protocol by using it as a sub-IP coupling layer. \ac{GN} takes care of encapsulating and routing the IP packet to its final destination node, so that the whole underlying \ac{VANET} looks like a flat layer 2 network to IP services.
\ac{GN6ASL} specifies how to derive a \ac{GN} address from an \ac{IPv6} address and extends \ac{IPv6} with some \acl{GN} specific extensions like geographic multicast, Geographically
The transport layer protocol above \acl{GN} is the \acf{BTP}\cite{europeantelecommunicationsstandardsinstituteetsiETSI302636512017}. It provides a connectionless multiplexing/ demultiplexing of datagrams to the layers above, adding minimal overhead while providing an unreliable packet transport comparable to UDP.
If \ac{IPv6} over \ac{GN} is used at the network layer, transport protocols like TCP and UDP from the standard internet protocol suite can of course be used, too.
The \textbf{Facilities Layer} unifies the three upper \ac{OSI} layers (application, presentation, session layer) and provides different support tasks to services and applications like time management, position management, database management and session management. It is also responsible to manage service priorities when passing down data to the Network and Transport Layer.
The \textbf{Security Layer} is a vertical layer providing security functionality like identity, key and certificate management to all other layers. It also contains all cryptographic functions like encryption or verification of data.
The \textbf{Management Layer} takes care of software changes like updates and installation of additional components and is considered out-of-scope of this survey.
There are many different addresses, IDs or other identifying information scattered around the network layers. This sections gives a list of relevant identifiers and the information encoded in them. Media-dependent, that means bound to a certain physical or data link layer, additional identifiers are considered out-of-scope.
Each \ac{GN} node is identified by a 64bit GN\_ADDR address \cite{europeantelecommunicationsstandardsinstituteetsiETSI302636412017}, containing information about the \ac{ITS} station type (passenger car, cyclist, pedestrian, \ac{RSU}, …) and 48bit derived from the link-layer address. In case of a pseudonym change, only the latter part is supposed to change.
As shown in Fig. \ref{fig:GNstructure}, \ac{GN} packets have a basic, a common and an optional extended header. The \textit{basic header} contains information like the packet's maximum lifetime and the remaining hop limit. These information are non-critical for identification. The \textit{common header} also doesn't contain identifying, only the flag indicating a mobile or stationary \ac{ITS} station could slightly reduce the anonymity set. The \textit{extended header} fields depend on the actual \ac{GN} package type and can contain information like the sequence number (initialized with 0) and position vectors.
The \ac{LT} is populated with information from beaconing messages and all other messages received by the \ac{ITS} node. \acl{LT} entries also contain identifying data: Additionally to the GN\_ADDR, station type and link-layer address of the peer node it contains a timestamped geographical position (including accuracy), its current speed and its heading. \todo{update position/ reacquire it when changing pseudonym}
Parts of \ac{GN} packets can be secured by wrapping them into security headers as defined in \cite{europeantelecommunicationsstandardsinstituteetsiETSITS1032017} as shown in Fig. \ref{fig:GNstructure_secured}. This service is provided by the vertical security layer in the \ac{ETSI}\ac{ITS} architecture and secures all parts shown in Fig. \ref{fig:GNstructure_secured} between security header and trailer according to the chosen security profile. The standard defines security profiles for encrypted, signed, externally signed, and signed encrypted messages.
The certificates used contain information about signer subject (name, type, keys), validity restrictions and the actual certificate signature from the \ac{CA}.
The signer information can be given in form of a digest, certificate or certificate chain.
The \ac{BTP} header as defined in \cite{europeantelecommunicationsstandardsinstituteetsiETSI302636512017} is only 4 bytes long and has a quite simple structure. \\
There are 2 modes of operation for BTP: \textit{interactive packet transport} using the BTP-A header, meant for services requiring replies to their messages, and \textit{non-interactive packet transport} using the BTP-B header.
The BTP-A header consists out of 2 16bit numbers denoting the source and destination ports. The BTP-B header contains the 16bit long destination port and 16bit for optional destination port information (depending on the service).
Some of the facility layer services have well-known ports assigned in \cite{europeantelecommunicationsstandardsinstituteetsiETSITS1032016}, so the destination port might identify the service used.
While each IPv6-capable network interface can have multiple addresses, it has at least one link-local address with the interface ID (the lower 64bits) uniquely derived from its data-link layer address. The mapping of IPv6 link-local address and GN\_ADDR is straight-forward, as both addresses are deterministically derived from the same 48bit link layer address. Additionally to the IPv6 address, the IPv6 header can also contain a 20bit \textit{flow label}\cite{RFC6437} which could lead to partial linkability of packets even after an address change: Although a flow shall be identified by the triplet of flow label, source and destination address, an equal flow label could indicate the resumption of a connection even after an address change.
There exists a static mapping between IPv6 multicast groups and geographical areas (relative to the station). That means it is possible to contact IPv6-based services within a node's surrounding. But as this mapping is static and relative, it shouldn't help reidentifying hosts.
\acfp{GVL} are another important concept for understanding the visibility scope of IPv6 packets to other nodes. These virtual links are defined as non-overlapping, restricted geographical areas wherein all IPv6 multicasts within the same subnet are forwarded via \ac{GN} to all nodes of that \ac{GVL}. Usually this is a zone around a specific \ac{RSU} serving as an internet uplink and thus managing the whole subnet and its addresses. Globally routable IPv6 addresses are usually obtained via the stateless autoconfiguration with the help of \acp{RA}. So changing the \ac{GVL} means getting another IPv6 prefix announced via \ac{RA} and thus implies a change in the node's global IPv6 address.
There are no obvious identifiers specified in the Facilities layer, though some might be introduced in real-world implementations.
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