When both ends dial each other, exactly one TCP connection must survive
and both ends must pick the same one. Previously ferrix relied on the
other router to sort this out and could briefly hold two sessions.
- collision: a shared map keyed by the peer's BGP Identifier holds the one
connection currently preferred for that peer plus a kill signal. Each
connection calls admit() on receiving the peer's OPEN. The connection
initiated by the higher BGP Identifier wins; the loser gets Cease /
Connection Collision Resolution, whether it is the new connection or an
existing one in OpenConfirm running in another task.
- A collision with an Established session closes the new connection (RFC
default). The optional CollisionDetectEstablishedState is available as
collision_detect_established_state: a restarted peer's new connection
then replaces the stale session at once instead of being refused until
the hold timer expires. Enabled in the lab config.
- Two connections opened by the same side: the newer wins.
- OPEN validation rejects a peer using our own BGP Identifier or 0.0.0.0
with Bad BGP Identifier; collision resolution needs distinct identifiers.
- OpenConfirm no longer times out instantly when the negotiated hold time
is zero.
- Sessions record which side opened them; shown as "opened by" on the
status page and as `direction` in /api/peers. New counter
bgp_connection_collisions_total{closed}.
- Tests: unit tests for the rule (including that both ends agree), and
integration tests for simultaneous open in both identifier orders and
both arrival orders, Established default, restart replacement with route
flush, distinct identifiers, slot release, and Bad BGP Identifier.
Existing fan-out test now gives its two peers distinct identifiers.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Phase 5 of the teaching demo plus two fixes found while exercising the
dialer on the status page.
- Adj-RIB-In: routes learned from a peer are removed when its session ends
(RFC 4271 §8.2.2), emitting a Withdrawn event per prefix before the
session Down event. Previously stale routes from dead sessions stayed in
the received table.
- statusz: poll the peers table every 5 s so dial state (Connecting /
Backoff and its retry countdown) stays current; those changes carry no
event.
- docs/lab: docker-compose topology with ferrix (AS65000) and two FRR
routers. r2 accepts nothing directly from ferrix so student prefixes
only reach it via r1, then come back with AS path 65002 65001 65000. r1
applies community-driven policy: NO_EXPORT stops forwarding, 65001:100
prepends, BLACKHOLE drops. Dockerfile for the daemon.
- docs/lab/HANDOUT.md: student exercises walking through announcement,
reading the bytes, prepending, NO_EXPORT, action communities,
blackholing and withdrawal.
The container files have not been executed (no runtime available here);
docs/lab/README.md lists what is most likely to need adjusting.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
