commit 5e7b421b2e46f75d559abccc5980d4ea485e1b15 Author: Karsten Loesing karsten.loesing@gmx.net Date: Thu Jul 12 13:26:00 2012 +0200
Move #4255 report sources to tech-reports.git. --- task-4255/README | 4 +- task-4255/report.bib | 26 --- task-4255/report.tex | 415 -------------------------------------------------- 3 files changed, 1 insertions(+), 444 deletions(-)
diff --git a/task-4255/README b/task-4255/README index 7ff10c9..577f20f 100644 --- a/task-4255/README +++ b/task-4255/README @@ -23,7 +23,5 @@ Plot the results:
$ R --slave -f stability.R
-Compile the report: - - $ pdflatex report.tex +The report sources are in tech-reports.git/2011/bridge-stability/.
diff --git a/task-4255/report.bib b/task-4255/report.bib deleted file mode 100644 index 427e27c..0000000 --- a/task-4255/report.bib +++ /dev/null @@ -1,26 +0,0 @@ -@misc{dirspec, - author = {Roger Dingledine and Nick Mathewson}, - title = {Tor directory protocol, version 3}, - note = {\url{https://gitweb.torproject.org/tor.git/blob_plain/HEAD:/doc/spec/dir-spec.txt..., -} - -@techreport{loesing2011analysis, - title = {An Analysis of {Tor} Relay Stability}, - author = {Karsten Loesing}, - institution = {The Tor Project}, - year = {2011}, - month = {June}, - type = {Technical Report}, - note = {\url{https://metrics.torproject.org/papers/relay-stability-2011-06-30.pdf%7D%7D, -} - -@techreport{loesing2011overview, - title = {Overview of Statistical Data in the {Tor} Network}, - author = {Karsten Loesing}, - institution = {The Tor Project}, - year = {2011}, - month = {March}, - type = {Technical Report}, - note = {\url{https://metrics.torproject.org/papers/data-2011-03-14.pdf%7D%7D, -} - diff --git a/task-4255/report.tex b/task-4255/report.tex deleted file mode 100644 index d756449..0000000 --- a/task-4255/report.tex +++ /dev/null @@ -1,415 +0,0 @@ -\documentclass{article} -\usepackage{url} -\usepackage[pdftex]{graphicx} -\usepackage{graphics} -\usepackage{color} -\begin{document} -\title{An Analysis of Tor Bridge Stability\ -{\large --- Making BridgeDB give out at least one stable bridge per -user ---}} -\author{Karsten Loesing\{\tt karsten@torproject.org}} - -\maketitle - -\section{Introducing the unstable bridges problem} - -As of October 2011, the Tor network consists of a few hundred thousand -clients, 2,400 public relays, and about 600 non-public bridge relays. -Bridge relays (in the following: bridges) are entry points which are not -publicly listed to prevent censors from blocking access to the Tor -network. -Censored users request a small number of typically three bridge addresses -from the BridgeDB service via email or http and then connect to the Tor -network only via these bridges. -If all bridges that a user knows about suddenly stop working, the user -needs to request a new set of bridge addresses from BridgeDB. -However, BridgeDB memorizes the user's email or IP address and only gives -out new bridges every 24 hours to slow down enumeration attempts. -The result is that a user who is unlucky enough to receive only unreliable -bridges from BridgeDB won't be able to connect to the Tor network for up -to 24 hours before requesting a new set of bridges. - -In this report we propose that BridgeDB keeps bridge stability records, -similar to how the directory authorities keep relay stability records, and -includes at least one stable bridge in its responses to users. -In fact, BridgeDB currently attempts to do this by including at least one -bridge with the Stable flag assigned by the bridge authority in its -results. -This approach is broken for two reasons: -The first reason is that the algorithm that the bridge authority uses to -assign the Stable flag is broken to the extent that almost every bridge -has the Stable flag assigned. -The second reason is that the Stable flag was designed for clients to -select relays for long-running streams, not for BridgeDB to select -reliable entry points into the Tor network. -A better metric for stable bridges would be based on bridge uptime and on -the frequency of IP address changes. -We propose such a metric and evaluate its effectiveness for selecting -stable bridges based on archived bridge directories. - -\section{Defining a new bridge stability metric} -\label{sec:defining} - -The directory authorities implement a few relay stability metrics to -decide which of the relays to assign the Guard and Stable -flag~\cite{dirspec, loesing2011analysis}. -The requirements for stable bridges that we propose here are similar to -the entry guard requirements. -That is, stable bridges should have a higher fractional uptime than -non-stable ones. -Further, a stable bridge should be available under the same IP address and -TCP port. -Otherwise, bridge users who only know a bridge address won't be able to -connect to the bridge once it changes its address or port. -We propose the following requirements for a bridge to be considered -stable in the style of the Guard and Stable flag definition: - -\label{def:bridgestability} -\begin{quote} -A bridge is considered stable if its \emph{Weighted Mean Time Between -Address Change} is at least the median for known active bridges or at -least 30~days, if it is `familiar', and if its \emph{Weighted Fractional -Uptime} is at least the median for `familiar' active bridges or at least -98~%. -A bridge is `familiar' if 1/8 of all active bridges have appeared more -recently than it, or if it has been around for a \emph{Weighted Time} of -8~days. -\end{quote} - -This bridge stability definition contains three main requirements: - -\begin{itemize} -\item The \emph{Weighted Mean Time Between Address Change (WMTBAC)} -metric is used to track the time that a bridge typically uses the same IP -address and TCP port. -The (unweighted) MTBAC measures the average time between last using -address and port $a_0$ to last using address and port $a_1$. -This metric is weighted to put more emphasis on recent events than on past -events. -Therefore, past address sessions are discounted by factor $0.95$ every -12~hours. -The current session is not discounted, so that a WMTBAC value of 30 days -can be reached after 30 days at the earliest. -\item The \emph{Weighted Fractional Uptime (WFU)} metric measures the -fraction of bridge uptime in the past. -Similar to WMTBAC, WFU values are discounted by factor $0.95$ every -12~hours, but in this case including the current uptime session. -\item The \emph{Weighted Time (WT)} metric is used to calculate a bridge's -WFU and to decide whether a bridge is around long enough to be considered -`familiar.' -WT is discounted similar to WMTBAC and WFU, so that a WT of 8 days can be -reached after around 16 days at the earliest. -\end{itemize} - -All three requirements consist of a dynamic part that depends on the -stability of other bridges (e.g., ``A bridge is familiar if 1/8 of all -active bridges have appeared more recently than it, \ldots'') and a static -part that is independent of other bridges (e.g., ``\ldots or if it has -been around for a Weighted Time of 8 days.''). -The dynamic parts ensure that a certain fraction of bridges is considered -stable even in a rather unstable network. -The static parts ensures that rather stable bridges are not excluded even -when most other bridges in the network are stable. - -\section{Extending BridgeDB to track bridge stability} - -There are at least two code bases that could be extended to track bridge -stability and include at least one stable bridge in BridgeDB results: the -bridge authority and BridgeDB. -The decision for extending either code base affects the available data -for tracking bridge stability and is therefore discussed here. - -The bridge authority maintains a list of all active bridges. -Bridges register at the bridge authority when joining the network, and the -bridge authority periodically performs reachability tests to confirm that -a bridge is still active. -The bridge authority takes snapshots of the list of active bridges every -30~minutes and copies these snapshots to BridgeDB. -BridgeDB parses these half-hourly snapshots and gives out bridges to users -based on the most recently known snapshot. - -The bridge stability history can be implemented either in the bridge -authority code or in BridgeDB. -On the one hand, an implementation in BridgeDB has the disadvantage that -bridge reachability data has a resolution of 30 minutes whereas the bridge -authority would learn about bridges joining or leaving the network -immediately. -On the other hand, the bridge stability information is not used by -anything in the Tor software, but only by BridgeDB. -Implementing this feature in BridgeDB makes more sense from a software -architecture point of view. -In the following we assume that BridgeDB will track bridge stability based -on half-hourly snapshots of active bridge lists, the bridge network -statuses. - -\section{Simulating bridge stability using archived data} - -We can analyze how BridgeDB would track bridge stability and give out -stable bridges by using archived bridge descriptors. -These archives contain the same descriptors that BridgeDB uses, but they -are public and don't contain any IP addresses or sensitive pieces of -information. -In Section~\ref{sec:missingdata} we look at the problem of missing data -due to either the bridge authority or BridgeDB failing and at the effect -on tracking bridge stability. -We then touch the topic of how bridge descriptors are sanitized and how we -can glue them back together for our analysis in -Section~\ref{sec:sanitizing}. -Next, we examine typical bridge stability values as requirements for -considering a bridge as stable in Section~\ref{sec:requirements}. -In Section~\ref{sec:fractions} we estimate what fraction of bridges would -be considered as stable depending on the chosen stability requirements. -Finally, in Section~\ref{sec:selectedstability} we evaluate how effective -different requirement combinations are for selecting stable bridges. -Result metrics are how soon selected bridges change their address or what -fractional uptime selected bridges have in the future. - -\subsection{Handling missing bridge status data} -\label{sec:missingdata} - -The bridge status data that we use in this analysis and that would also be -used by BridgeDB to track bridge stability is generated by the bridge -authority and copied over to BridgeDB every 30~minutes. -Figure~\ref{fig:runningbridge} shows the number of running bridges -contained in these snapshots from July 2010 to June 2011. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{runningbridge.pdf} -\caption{Median number of running bridges as reported by the bridge -authority} -\label{fig:runningbridge} -\end{figure} - -For most of the time the number of bridges is relatively stable. -But there are at least two irregularities, one in July 2010 and another -one in February 2011, resulting from problems with the bridge authority or -the data transfer to the BridgeDB host. -Figure~\ref{fig:runningbridge-detail} shows these two intervals in more -detail. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{runningbridge-detail.pdf} -\caption{Number of Running bridges during phases when either the bridge -authority or the BridgeDB host were broken} -\label{fig:runningbridge-detail} -\end{figure} - -The missing data from July 14 to 27, 2010 comes from BridgeDB host not -accepting new descriptors from the bridge authority because of an -operating system upgrade of the BridgeDB host. -During this time, the bridge authority continued to work, but BridgeDB was -unable to learn about new bridge descriptors from it. - -During the time from January 27 to February 16, 2011, the \verb+tor+ -process running the bridge authority silently died twice after a Tor -version upgrade, but the script to copy descriptors to BridgeDB kept -running. -In this case, BridgeDB received fresh tarballs containing stale -descriptors with a constant number of 687 relays, visualized in light -gray. -These stale descriptors have been excluded from the sanitized descriptors -and the subsequent analysis. -The bridge authority was restarted on February 16, 2011, resulting in the -number of running bridges slowly stabilizing throughout the day. - -Both this analysis and a later implementation in BridgeDB need to take -extended phases of missing or stale data into account. - -\subsection{Detecting address changes in sanitized descriptors} -\label{sec:sanitizing} - -The bridge descriptor archives that we use in this analysis have been -sanitized to remove all addresses and otherwise sensitive -parts~\cite{loesing2011overview}. -Part of this sanitizing process is that bridge IP addresses are replaced -with keyed hashes using a fresh key every month. -More precisely, every bridge IP address is replaced with the private IP -address \verb+10.x.x.x+ with \verb+x.x.x+ being the 3 most significant -bytes of \verb+SHA-256(IP address | bridge identity | secret)+. - -A side-effect of this sanitizing step is that a bridge's sanitized IP -address changes at least once per month, even if the bridge's real IP -address stays the same. -We need to detect these artificial address changes and distinguish them -from real IP address changes. - -In this analysis we use a simple heuristic to distinguish between real IP -address changes and artifacts from the sanitizing process: -Whenever we find that a bridge has changed its IP address from one month -to the next, we look up how long both IP addresses were in use in either -month. -If both addresses were contained in bridge descriptors that were published -at least 36~hours apart, we consider them stable IP addresses and -attribute the apparent IP address change to the sanitizing process. -Otherwise, we assume the bridge has really changed its IP address. -Obviously, this simple heuristic might lead us to false conclusions in -some cases. -But it helps us handle cases when bridges rarely or never change their IP -address which would otherwise suffer from monthly address changes in this -analysis. - -\subsection{Examining typical stability metric values} -\label{sec:requirements} - -The definition of bridge stability on page~\pageref{sec:defining} contains -three different metrics, each of which having a dynamic and a static part. -The dynamic parts compares the value of a bridge's stability metric to the -whole set of running bridges. -Only those bridges are considered as stable that exceed the median value -(or the 12.5th percentile) of all running bridges. -The static requirement parts are fixed values for all stability metrics -that don't rely on the stability of other bridges. - -Figure~\ref{fig:requirements} visualizes the dynamic (solid lines) and -static parts (dashed lines) of all three requirements. -The dynamic WMTBAC requirements are higher than previously expected. -A value of 60 means that, on average, bridges keep their IP address and -port for 60 days. -The dynamic values are cut off at 30 days by the static requirement which -should be a high enough value. -The goal here is to give blocked users a stable enough set of bridges so -that they don't have to wait another 24~hours before receiving new ones. - -We can further see that the dynamic requirements are relatively stable -over time except for the two phases of missing bridge status data. -The first phase in July 2010 mostly affects WT, but neither WMTBAC nor -WFU. -The second phase in February 2011 affects all three metrics. -We can expect the selection of stable bridges during February 2010 to be -more random than at other times. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{requirements.pdf} -\caption{Dynamic requirements for considering a bridge as stable} -\label{fig:requirements} -\end{figure} - -\subsection{Estimating fractions of bridges considered as stable} -\label{sec:fractions} - -Requiring a bridge to meet or exceed either or both WMTBF or WFU metric -results in considering only a subset of all bridges as stable. -The first result of this analysis is to outline what fraction of bridges -would be considered as stable if BridgeDB used either or both -requirements. -In theory, all parameters in the bridge stability definition on -page~\pageref{def:bridgestability} could be adjusted to change the set of -stable bridges or focus more on address changes or on fractional uptime. -We're leaving the fine-tuning for future work when specifying and -implementing the BridgeDB extension. - -Figure~\ref{fig:stablebridge} shows the fraction of stable bridges over -time. -If we only require bridges to meet or exceed the median WMTBAC or the -fixed value of 30 days, roughly 55~% of the bridges are considered as -stable. -If bridges are only required to meet or exceed the WT and WFU values, -about $7/8 \times 1/2 = 43.75~%$ of bridges are considered as stable. -Requiring both WFU and WMTBAC leads to a fraction of roughly 35~% stable -bridges. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{stablebridge.pdf} -\caption{Impact of requiring stable bridges to meet or exceed the median -WFU and/or WMTBAC on the fraction of running bridges considered as stable} -\label{fig:stablebridge} -\end{figure} - -The fraction of 33~% stable bridges seems appropriate if 1 out of -3~bridges in the BridgeDB results is supposed to be a stable bridge. -If more than 1~bridge should be a stable bridge, the requirements need to -be lowered, so that a higher fraction of bridges is considered stable. -Otherwise, the load on stable bridges might become too high. - -\subsection{Evaluating different requirements on stable bridges} -\label{sec:selectedstability} - -The main purpose of this analysis is to compare the quality of certain -requirements and requirement combinations on the stability of selected -bridges. -Similar to the previous section, we only compare whether or not the WMTBAC -or WFU requirement is used, but don't change their parameters. - -The first result is the future uptime that we can expect from a bridge -that we consider stable. -We calculate future uptime similar to past uptime by weighting events in -the near future more than those happening later. -We are particularly interested in the almost worst-case scenario here, -which is why we're looking at the 10th percentile weighted fractional -uptime in the future. -This number means that 10~% of bridges have a weighted fractional uptime -at most this high and 90~% of bridges have a value at least this high. - -Figure~\ref{fig:fwfu-sim} visualizes the four possible combinations of -using or not using the WMTBAC and WFU requirements. -In this plot, the ``WFU & WMTBAC'' and ``WFU'' lines almost entirely -overlap, meaning that the WMTBAC requirement doesn't add anything to -future uptime of selected bridges. -If the WFU requirement is not used, requiring bridges to meet the WMTBAC -requirement increases future uptime from roughly 35~% to maybe 55~%. -That means that there is a slight correlation between the two metrics, -which is plausible. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{fwfu-sim.pdf} -\caption{Impact of requiring stable bridges to meet or exceed the median -WFU and/or WMTBAC on the 10th percentile weighted fractional uptime in the -future} -\label{fig:fwfu-sim} -\end{figure} - -The second result is the time that a selected bridge stays on the same -address and port. -We simply measure the time that the bridge will keep using its current -address in days. -Again, we look at the 10th percentile. -90~% of selected bridges keep their address longer than this time. - -Figure~\ref{fig:tosa-sim} shows for how long bridges keep their address -and port. -Bridges meeting both WFU and WTMBAC requirements keep their address for 2 -to 5~weeks. -This value decreases to 1 to 3~weeks when taking away the WFU requirement, -which is also a result of the two metrics beeing correlated. -The bridges that only meet the WFU requirement and not the WMTBAC -requirement change their address within the first week. -If we don't use any requirement at all, which is what BridgeDB does today, -10~% of all bridges change their address within a single day. - -\begin{figure}[t] -\includegraphics[width=\textwidth]{tosa-sim.pdf} -\caption{Impact of requiring stable bridges to meet or exceed the median -WFU and/or WMTBAC on the 10th percentile time on the same address} -\label{fig:tosa-sim} -\end{figure} - -\section{Concluding the bridge stability analysis} - -In this report we propose to extend BridgeDB to make it give out at least -one stable bridge per user. -Bridge stability can be calculated based on bridge status information over -time, similar to how the directory authorities calculate relay stability. -The bridge stability metric proposed here is based on a bridge's past -uptime and the frequency of changing its address and/or port. -Requiring at least 1 bridge of the 3 to be given out to users greatly -reduces the worst case probability of all bridges being offline or -changing their addresses or ports. -The price for this increase in stability is that stable bridges will be -given out more often than non-stable bridges and will therefore see more -usage. - -We suggest to implement the described bridge stability metric in BridgeDB -and make it configurable to tweak the requirement parameters if needed. -Maybe it turns out to be more useful to lower the requirements for a -bridge to become stable and give out two stable bridges per response. -It's also possible that the requirement for a bridge to keep its address -becomes less important in the future when bridge clients can request a -bridge's current address from the bridge authority. -All these scenarios can be analyzed before deploying them using archived -data as done in this report. - -\bibliography{report} -\bibliographystyle{plain} - -\end{document} -