prop340: Clarify RELAY_EARLY implications and add a parameter.

I think this reflects our discussions on tor#40840.

I had thought that this parameter existed, but it turns out that
it doesn't.

1 files changed, 32 insertions, 0 deletions

diff --git a/proposals/340-packed-and-fragmented.md b/proposals/340-packed-and-fragmented.md
index 65a8d03..a473c3f 100644
--- a/proposals/340-packed-and-fragmented.md
+++ b/proposals/340-packed-and-fragmented.md
@@ -192,6 +192,38 @@ rendezvous circuits that we currently need to fragment or pack.
 The `RELAY_EARLY` status for a command is determined based on the relay
 cell in which the command's _header_ appeared.
 
+Thus, a relay MUST close a circuit if the cell containing the _first_
+fragment of an `EXTEND` message is not `RELAY_EARLY`, and MUST allow
+but NOT require `RELAY_EARLY` to be set on other cells.
+
+This implies that a client only _needs_ to set `RELAY_EARLY` on the cell
+containing the _first_ fragment of an EXTEND message, but that it MAY
+set RELAY_EARLY on other cells, in order to prevent traffic fingerprinting.
+
+(Note: As now, relays and clients MUST destroy any circuit upon seeing a
+`RELAY_EARLY` message in the inbound direction.)
+
+> In our implementation, clients will continue to set `RELAY_EARLY` on
+> the first N cells of each circuit, as we do today.
+
+> Note that this description allows us to take two approaches when
+> we eventually _do_ support fragmented `EXTEND` messages.  We can
+> either set the `RELAY_EARLY` flag on the cell containing the first
+> fragment only, or we can continue to set it on the first N cells
+> sent on each circuit.  Both will work fine, assuming that the
+> limit of `RELAY_EARLY` cells is adjustable.  This brings us to:
+
+#### Making the `RELAY_EARLY` limit adjustable
+
+We add the following parameter, to support an eventual migration to
+longer extend cells, in case we decide to take the second approach in
+our note above.
+
+    "max_early_per_circ" -- Relays MUST destroy any circuit on
+    which they see more than this number of RELAY_EARLY cells.
+    Min: 5. Max: 65535. Default: 8.
+
+
 ### Handling `SENDME`s
 
 SENDME messages may not be fragmented; the body and the command must