# Ephemeral Node Identifiers for Enhanced Flow Privacy
## 5 Discussion
### Ephemeral Identifiers Improve Inter-Flow Privacy  
Ephemeral NIDs are superior to TA-SLAAC in terms of the privacy they provide, and the way in which they are managed. TA-SLAAC improves privacy by reducing address reuse between flows; eNIDs eliminate it entirely. The summary of the key characteristics and the improvement with the use of eNIDs is shown in Table 6.  
eNIDs provide the logical conclusion to the development of temporary addresses:  
* the use of an eNID for a single flow gives temporal privacy;
* the pseudo-random generation on a node independently gives spatial privacy;
* the generation of the values on the node itself confines the scope of trust to that node. These benefits apply even if the remote host does not support ILNP: ILNP is defined to fallback to use IPv6, using the NID and L64 as the IID and routing prefix, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mechanism** & **IID Reused** & **IID Expired** & **Linkable Flows** \\ \hline SLAAC & Yes & Never & All \\ TA-SLAAC & Yes & After 24 h & Some \\ eNIDs & No & End of flow & None \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Features of different IID management mechanisms under their default configurations. \begin{table}
\begin{tabular}{l l l l} \hline \hline pcap **Filename** & **Network** & **Flows** & **Application** \\ \hline testbed-data/slaac.pcap & Testbed 1  & 864 & curl, nginx \\ testbed-data/ta-slaac.pcap & Testbed 1  & 864 & curl, nginx \\ testbed-data/enid.pcap & Testbed 1  & 864 & curl, nginx \\ testbed-data/enid.pcap & Testbed 1  & 864 & curl, nginx \\ ietf118-data/rsync.pcap & Global 2  & 1 & rsync \\ ietf118-data/ping.pcap & Global 2  & 3 & ping6 \\ ietf118-data/ssh.pcap & Global 2  & 1 & ssh \\ \hline \hline \end{tabular}
\end{table} 
Table 5: List of the pcap files for the experiments carried out on the Testbed network and the Global network. All communication was initiated from a client system so that the eNID values are generated on-demand when a communication starts. The mechanism for managing eNIDs would continue to function, preventing reuse, even though the NID was being used as an IID for IPv6. Similarly, the logic used to manage eNIDs could, in principle, be used to manage IIDs for IPv6. However, this would require changes to the IPv6-layer and transport-layer code to correctly deal with the use of eNIDs as IIDs, with new data structures and additional coding for correct behaviour to associate and manage IIDs with transport state. Of course, this behaviour is already implemented as part of ILNP and has a greater semantic cohesion within the use of the ILNP addressing architecture compared to IPv6. Other mechanisms that hide or rewrite addresses do not provide the same protection. NAT-like mechanisms for edge networks potentially increase the number of users behind a single IP address, however this represents only a small change in the granularity of the attack, and individuals within that group may still be identifiable [20]. CGNAT potentially hides users in larger crowds, but is beyond the view of the end system, so users cannot verify that it is in place or check how large the crowd is. For IPv6, the limited privacy protection of NAT-like approaches is especially pronounced, as NPT leaves most of the IID bytes unmodified, exposing identity information about the nodes behind the NPT gateway. Address rewriting does not reliably or effectively protect identity privacy, but eNIDs do--even when used in conjunction with a mechanism like NPT. VPNs and Tor both hide IP addresses by tunnelling connections through extra servers in order to protect location privacy. As the management of these tunnels is costly, they are often shared by all connections from a device. As all connections use the same tunnel, they can be correlated to a common IID or IP address: the address corresponding to the end of the tunnel. While this value will not relate to the device itself, it is still sufficient to perform an identity privacy attack.