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dilation-protocol.md: clarify a few things

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warner committed May 7, 2019
1 parent d9284cd commit c933c5cc06beedbe9a328bb95b358486d7f41e0b
Showing with 41 additions and 22 deletions.
  1. +41 −22 docs/dilation-protocol.md
@@ -105,11 +105,15 @@ resumed or reestablished.

Dilation is triggered by calling the `w.dilate()` API. This returns a
Deferred that will fire once the first L3 connection is established. It fires
with a 3-tuple of endpoints that can be used to establish subchannels.
with a 3-tuple of endpoints that can be used to establish subchannels, or an
error if dilation is not possible. If the other side's `versions` message
indicates that it does not support dilation, the Deferred will errback with
an `OldPeerCannotDilateError`.

For dilation to succeed, both sides must call `w.dilate()`, since the
resulting endpoints are the only way to access the subchannels. If the other
side never calls `w.dilate()`, the Deferred will never fire.
side is capable of dilation, but never calls `w.dilate()`, the Deferred will
never fire.

The L1 (mailbox) path is used to deliver dilation requests and connection
hints. The current mailbox protocol uses named "phases" to distinguish
@@ -260,7 +264,7 @@ trigger an immediate error for most non-magic-wormhole listeners (e.g. HTTP
servers that were contacted by accident). If the wrong handshake is received,
the connection will be dropped. For debugging purposes, the node might want
to keep looking at data beyond the first incorrect character and log
everything until the first newline.
a few hundred characters until the first newline.

Everything beyond that point is a Noise protocol message, which consists of a
4-byte big-endian length field, followed by the indicated number of bytes.
@@ -271,29 +275,44 @@ master PAKE key using HKDF. Each L2 connection uses the same dilation key,
but different ephemeral keys, so each gets a different session key.

The Leader sends the first message, which is a psk-encrypted ephemeral key.
The Follower sends the next message, its own psk-encrypted ephemeral key. The
Follower then sends an empty packet as the "key confirmation message", which
will be encrypted by the shared key.

The Leader sees the KCM and knows the connection is viable. It delivers the
protocol object to the L3 manager, which will decide which connection to
select. When the L2 connection is selected to be the new L3, it will send an
empty KCM of its own, to let the Follower know the connection being selected.
All other L2 connections (either viable or still in handshake) are dropped,
all other connection attempts are cancelled. All listening sockets may or may
not be shut down (TODO: think about it).

The Follower will wait for either an empty KCM (at which point the L2
connection is delivered to the Dilation manager as the new L3), a
disconnection, or an invalid message (which causes the connection to be
dropped). Other connections and/or listening sockets are stopped.
The Follower sends the next message, its own psk-encrypted ephemeral key.
These two messages are known as "handshake messages" in the Noise protocol,
and must be processed in a specific order (the Leader must not accept the
Follower's message until it has generated its own). Noise allows handshake
messages to include a payload, but we do not use this feature.

All subsequent messages as known as "Noise transport messages", and use
independent channels for each direction, so they no longer have ordering
dependencies. Transport messages are encrypted by the shared key, in a form
that evolves as more messages are sent.

The Follower's first transport message is an empty packet, which we use as a
"key confirmation message" (KCM).

The Leader doesn't send a transport message right away: it waits to see the
Follower's KCM, which indicates this connection is viable (i.e. the Follower
used the same dilation key as the Leader, which means they both used the same
wormhole code).

The Leader delivers the now-viable protocol object to the L3 manager, which
will decide which connection to select. When some L2 connection is selected
to be the new L3, the Leader finally sends an empty KCM of its own over that
L2, to let the Follower know which connection has been selected. All other L2
connections (either viable or still in handshake) are dropped, and all other
connection attempts are cancelled. All listening sockets may or may not be
shut down (TODO: think about it).

After sending their KCM, the Follower will wait for either an empty KCM (at
which point the L2 connection is delivered to the Dilation manager as the new
L3), a disconnection, or an invalid message (which causes the connection to
be dropped). Other connections and/or listening sockets are stopped.

Internally, the L2Protocol object manages the Noise session itself. It knows
(via a constructor argument) whether it is on the Leader or Follower side,
which affects both the role is plays in the Noise pattern, and the reaction
to receiving the ephemeral key (for which only the Follower sends an empty
KCM message). After that, the L2Protocol notifies the L3 object in three
situations:
to receiving the handshake message / ephemeral key (for which only the
Follower sends an empty KCM message). After that, the L2Protocol notifies the
L3 object in three situations:

* the Noise session produces a valid decrypted frame (for Leader, this
includes the Follower's KCM, and thus indicates a viable candidate for

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