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Add dest_coords to SessionAck for return-path routing

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SessionAck previously only carried the responder's coordinates
(src_coords). When the return path diverged from the forward path
(e.g., after tree reconvergence), transit nodes on the return path
lacked the initiator's coordinates and couldn't route the SessionAck
back, causing handshake timeouts.

Add dest_coords (initiator's coordinates) to the SessionAck wire
format, mirroring SessionSetup's design. Transit nodes now cache both
endpoints' coordinates when forwarding a SessionAck, making the return
path self-sufficient regardless of path asymmetry.

Root cause confirmed by churn-20 sim log analysis: the n04-n14
handshake failure was caused by n15 (return-path transit) lacking
n04's coordinates, not by stale tree routes through a downed node.
This commit is contained in:
Johnathan Corgan
2026-02-21 14:18:48 +00:00
parent cfb087a95d
commit 19efe06622
8 changed files with 50 additions and 21 deletions
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docs/design/fips-session-layer.md
+5 -3
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@@ -116,7 +116,7 @@ The handshake is carried in SessionSetup and SessionAck messages:
1. **Initiator** sends SessionSetup containing Noise IK msg1 and both
parties' tree coordinates
2. **Responder** processes msg1, learns initiator identity, sends SessionAck
containing Noise IK msg2 and its own coordinates
containing Noise IK msg2 and both parties' tree coordinates
3. Both parties derive identical symmetric session keys
Packets that trigger session establishment are queued (with bounded buffer)
@@ -130,8 +130,10 @@ As the message transits intermediate nodes, each node caches these coordinates,
warming the path for subsequent data packets that carry only addresses (no
coordinates).
SessionAck carries the responder's coordinates back along the reverse path,
warming caches in the other direction.
SessionAck carries both the responder's and initiator's coordinates back
along the reverse path, warming caches in the other direction. This ensures
return-path transit nodes can route even when the reverse path diverges from
the forward path (e.g., after tree reconvergence).
### Simultaneous Initiation