explain implementation details from #40805 fix

1 files changed, 19 insertions, 1 deletions

diff --git a/proposals/333-vanguards-lite.md b/proposals/333-vanguards-lite.md
index bdd4e21..8c1ccb9 100644
--- a/proposals/333-vanguards-lite.md
+++ b/proposals/333-vanguards-lite.md
@@ -69,7 +69,25 @@ Implemented-In: 0.4.7.1-alpha
   protected, and this proposal might provide those services with a false sense
   of security. Such services should still use the vanguards addon [VANGUARDS_REF].
 
-# 4. References
+# 4. Implementation nuances
+
+  Tor replaces an L2 vanguard whenever it is no longer listed in the most
+  recent consensus, with the goal that we will always have the right
+  number of vanguards ready to be used.
+
+  For implementation reasons, we also replace a vanguard if it loses
+  the Fast or Stable flag, because the path selection logic wants middle
+  nodes to have those flags when it's building preemptive vanguard-using
+  circuits.
+
+  The design doesn't have to be this way: we might instead have chosen
+  to keep vanguards in our list as long as possible, and continue to use
+  them even if they have lost some flags. This tradeoff is similar to
+  the one in https://bugs.torproject.org/17773 about whether to continue
+  using Entry Guards if they lose the Guard flag -- and Tor's current
+  choice is "no, rotate" for that case too.
+
+# 5. References
 
   [PROP292_REF]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/292-mesh-vanguards.txt
   [VANGUARDS_REF]: https://github.com/mikeperry-tor/vanguards