improve mycelium argument

Chapters/Background.tex

@@ -37,6 +37,83 @@ the 80\% success rate sets a baseline expectation, while the 55-second
 timeout informs analysis of each implementation's keep-alive behavior
 during source code review.
 
+\subsection{The Babel routing protocol}
+\label{sec:babel}
+
+Babel~\cite{chroboczek_babel_2021} is a distance-vector routing
+protocol designed for both wired and wireless mesh networks.  Each
+node periodically sends \emph{Hello} messages to discover neighbours
+and \emph{Update} messages to advertise reachable prefixes along with
+a numeric cost metric.  A node selects the route with the lowest
+cumulative metric for each destination, subject to a
+\emph{feasibility condition} that prevents routing loops.  Because
+Babel is distance-vector rather than link-state, nodes only know the
+cost of their own best path, not the full topology.
+
+Two properties of Babel matter for the benchmarks in
+Chapter~\ref{Results}.  First, route advertisements are periodic: a
+node will not learn about a new path until the next Update interval,
+which can be on the order of minutes depending on the implementation's
+timer settings.  Second, Babel intentionally resists frequent route
+changes to avoid flapping; a node may continue using a suboptimal path
+until a significantly better alternative is advertised.  Both
+properties can cause the selected route for a given destination to
+differ across consecutive benchmark runs, even when the physical
+topology has not changed.
+
+\subsection{TCP flow control and congestion control}
+\label{sec:tcp_windows}
+
+TCP uses two window mechanisms to regulate how much unacknowledged data
+a sender may have in flight.  The \emph{receive window}
+(\texttt{rwnd}), also called the \emph{send window} in
+\texttt{iperf3} output, is advertised by the receiver and reflects how
+much buffer space it has available.  The \emph{congestion window}
+(\texttt{cwnd}) is maintained locally by the sender and tracks the
+network's estimated capacity.  At any point, the sender may transmit
+up to $\min(\texttt{rwnd}, \texttt{cwnd})$ bytes beyond the last
+acknowledged byte \cite{rfc5681}.
+
+The congestion window starts small (typically a few segments) and
+grows during the \emph{slow-start} phase, doubling each round trip
+until it reaches a threshold or triggers a loss event.  After that,
+\emph{congestion avoidance} takes over and the window grows linearly.
+When the sender detects a loss (through duplicate ACKs or a
+retransmission timeout), it treats the loss as a signal of congestion:
+the window is reduced, often halved, and the sender enters a recovery
+phase before resuming growth.  Each retransmission therefore has a
+direct mechanical cost: it shrinks the congestion window and reduces
+the instantaneous sending rate.
+
+The \emph{bandwidth-delay product} (BDP) determines how large the
+window must be to fully utilize a link.  It is the product of the
+link's bandwidth and the round-trip time:
+\begin{equation}
+  \text{BDP} = \text{bandwidth} \times \text{RTT}
+  \label{eq:bdp}
+\end{equation}
+A 1\,Gbps link with a 1\,ms RTT has a BDP of 125\,KB: the sender
+must keep at least 125\,KB of unacknowledged data in flight to
+saturate the link.  If the congestion window is smaller than the BDP,
+the sender will finish transmitting its window and then wait idle for
+acknowledgements, leaving bandwidth unused.  High-latency paths make
+this problem worse because the BDP grows linearly with RTT.  A
+34\,ms RTT on the same 1\,Gbps link raises the BDP to 4.25\,MB, well
+beyond the default congestion window of most TCP stacks.  One common
+workaround is to run multiple TCP flows in parallel: each flow
+maintains its own congestion window, and their aggregate in-flight
+data can approach the BDP even when no single flow could.
+
+In VPN benchmarks these two windows appear as distinct bottlenecks.  A
+small receive window means the receiver (or the tunnel endpoint in
+front of it) cannot absorb data fast enough.  A small congestion
+window means the path between sender and receiver is experiencing
+loss, forcing TCP into repeated recovery cycles.  Comparing congestion
+windows across VPNs with different maximum segment sizes requires
+care, because the window is measured in bytes: a VPN with jumbo
+segments will report a larger byte-valued window for the same number
+of in-flight segments.
+
 \subsection{An Overview of Packet Reordering in TCP}
 TODO \cite{leung_overview_2007}
 

Listings/mycelium_find_best_route.rs

@@ -0,0 +1,29 @@
+fn find_best_route<'a>(&self, routes: &'a RouteList)
+    -> Option<&'a RouteEntry>
+{
+    let source_table = self.source_table.read().unwrap();
+    let current = routes.selected();
+    let best = routes
+        .iter()
+        .filter(|re| !re.metric().is_infinite()
+                   && source_table.route_feasible(re))
+        .min_by_key(|re|
+            re.metric() + Metric::from(re.neighbour().link_cost()));
+
+    if let (Some(best), Some(current)) = (best, current) {
+        // Only switch if the metric is significantly better
+        // OR if the route is directly connected (metric 0).
+        if (best.source() != current.source()
+         || best.neighbour() != current.neighbour())
+        && !(best.metric()
+               + Metric::from(best.neighbour().link_cost())
+             < current.metric()
+               + Metric::from(current.neighbour().link_cost())
+               - SIGNIFICANT_METRIC_IMPROVEMENT
+            || best.metric().is_direct())
+        {
+            return Some(current); // keep existing route
+        }
+    }
+    best
+}

Listings/mycelium_route_constants.rs

@@ -0,0 +1,9 @@
+/// Time between HELLO messages, in seconds
+const HELLO_INTERVAL: u64 = 20;
+/// Max time used in UPDATE packets.
+const UPDATE_INTERVAL: Duration =
+    Duration::from_secs(HELLO_INTERVAL * 3 * 5); // 300 s
+
+/// The amount a metric of a route needs to improve
+/// before we will consider switching to it.
+const SIGNIFICANT_METRIC_IMPROVEMENT: Metric = Metric::new(10);

Listings/nebula_conntrack.go

@@ -0,0 +1,39 @@
+type FirewallConntrack struct {
+    sync.Mutex
+
+    Conns      map[firewall.Packet]*conn
+    TimerWheel *TimerWheel[firewall.Packet]
+}
+
+func (f *Firewall) inConns(
+    fp firewall.Packet, h *HostInfo,
+    caPool *cert.CAPool,
+    localCache firewall.ConntrackCache,
+) bool {
+    if localCache != nil {
+        if _, ok := localCache[fp]; ok {
+            return true
+        }
+    }
+    conntrack := f.Conntrack
+    conntrack.Lock()
+
+    // Purge every time we test
+    ep, has := conntrack.TimerWheel.Purge()
+    if has {
+        f.evict(ep)
+    }
+
+    c, ok := conntrack.Conns[fp]
+    if !ok {
+        conntrack.Unlock()
+        return false
+    }
+    // ... update expiry ...
+    conntrack.Unlock()
+
+    if localCache != nil {
+        localCache[fp] = struct{}{}
+    }
+    return true
+}

main.tex

@@ -98,6 +98,16 @@
   morestring=[b]",
   sensitive=true,
 }
+\lstdefinelanguage{Rust}{
+  morekeywords={as,break,const,continue,crate,else,enum,extern,false,fn,for,
+    if,impl,in,let,loop,match,mod,move,mut,pub,ref,return,self,Self,static,
+    struct,super,trait,true,type,unsafe,use,where,while,async,await,dyn,
+  Some,None,Option,Result,Ok,Err,Duration},
+  morecomment=[l]{//},
+  morecomment=[s]{/*}{*/},
+  morestring=[b]",
+  sensitive=true,
+}
 \lstdefinelanguage{Go}{
   morekeywords={break,case,chan,const,continue,default,defer,else,fallthrough,
     for,func,go,goto,if,import,interface,map,package,range,return,select,

master_citations.bib

@@ -617,3 +617,25 @@
     PDF:/home/lhebendanz/Zotero/storage/KM9D625Y/Whitner et al. - 2008
   - Improved Packet Reordering Metrics.pdf:application/pdf},
 }
+
+@misc{rfc5681,
+  title = {TCP Congestion Control},
+  author = {Allman, Mark and Paxson, Vern and Blanton, Ethan},
+  year = {2009},
+  month = sep,
+  howpublished = {RFC 5681},
+  doi = {10.17487/RFC5681},
+  url = {https://www.rfc-editor.org/rfc/rfc5681},
+  note = {Obsoletes RFC 2581},
+}
+
+@misc{chroboczek_babel_2021,
+  title = {The {Babel} Routing Protocol},
+  author = {Chroboczek, Juliusz and Schinazi, David},
+  year = {2021},
+  month = jun,
+  howpublished = {RFC 8966},
+  doi = {10.17487/RFC8966},
+  url = {https://www.rfc-editor.org/rfc/rfc8966},
+  note = {Obsoletes RFC 6126},
+}