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bgp: add Collision Checking and Prevention #126

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df0b4ba
bgp: add Collision Checking and Prevention
Paul-weqe Jun 21, 2026
1d3910d
ci: fix failing tests
Paul-weqe Jun 21, 2026
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53 changes: 44 additions & 9 deletions holo-bgp/src/events.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -44,10 +44,9 @@ pub(crate) fn process_tcp_accept(
return Ok(());
};

// Workaround to prevent connection collision until collision resolution
// is implemented.
if nbr.conn_info.is_some() {
return Ok(());
// Flags existence of a collision.
if let Some(local_conn_info) = &nbr.conn_info {
nbr.in_collision = collision_detection(local_conn_info, &conn_info);
}

// Initialize the accepted stream.
Expand Down Expand Up @@ -80,10 +79,9 @@ pub(crate) fn process_tcp_connect(
};
nbr.tasks.connect = None;

// Workaround to prevent connection collision until collision resolution
// is implemented.
if nbr.conn_info.is_some() {
return Ok(());
// Flags existence of a collision.
if let Some(peer_conn_info) = &nbr.conn_info {
nbr.in_collision = collision_detection(&conn_info, peer_conn_info);
}

// Invoke FSM event.
Expand All @@ -92,6 +90,17 @@ pub(crate) fn process_tcp_connect(
Ok(())
}

// ===== Flags for Collision existence ====

fn collision_detection(
local_conn_info: &TcpConnInfo,
peer_conn_info: &TcpConnInfo,
) -> bool {
// Returns true if a collision has happened.
(local_conn_info.local_addr == peer_conn_info.local_addr)
&& (local_conn_info.remote_addr == peer_conn_info.remote_addr)
}

// ===== neighbor message receipt =====

pub(crate) fn process_nbr_msg(
Expand All @@ -117,6 +126,28 @@ pub(crate) fn process_nbr_msg(

match msg {
Message::Open(msg) => {
// Collision Prevention.
let peer_id = nbr.identifier;
let _ = nbr;
let collision_resolved = if nbr.in_collision {
Neighbor::collision_prevention(
peer_id,
msg.identifier,
instance,
neighbors,
)
} else {
true
};
let Some(nbr) = neighbors.get_mut(&nbr_addr) else {
return Ok(());
};

if collision_resolved {
nbr.in_collision = false;
}

// Make sure to clarify we are no longer in collision.
nbr.fsm_event(instance, fsm::Event::RcvdOpen(msg));
}
Message::Update(msg) => {
Expand Down Expand Up @@ -286,10 +317,14 @@ fn process_nbr_reach_prefixes<A>(
}

// Initialize route origin and type.
let Some(identifier) = nbr.identifier else {
return;
};
let origin = RouteOrigin::Neighbor {
identifier: nbr.identifier.unwrap(),
identifier,
remote_addr: nbr.remote_addr,
};

let route_type = match nbr.peer_type {
PeerType::Internal => RouteType::Internal,
PeerType::External => RouteType::External,
Expand Down
67 changes: 67 additions & 0 deletions holo-bgp/src/neighbor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,7 @@ pub struct Neighbor {
pub tasks: NeighborTasks,
pub update_queues: NeighborUpdateQueues,
pub msg_txp: Option<UnboundedSender<NbrTxMsg>>,
pub in_collision: bool,
}

// BGP peer type.
Expand Down Expand Up @@ -214,9 +215,57 @@ impl Neighbor {
tasks: Default::default(),
update_queues: Default::default(),
msg_txp: None,
in_collision: false,
}
}

/// Carries out the collision prevention procedure.
///
/// # Arguments
/// * `current_peer_id` - peer ID that we have saved locally.
/// * `incoming_peer_id` - peer ID from the incoming OpenMsg.
/// * `instance`
/// * `neighbors` - list of neighbors. Needed when filtering through those with matching characteristics
///
/// * Returns
///
/// Returns true if the collision prevention procedure led to the closure of a connection
/// binded to a neighbor that is not the one sending the OPEN Message.
///
/// Returns false if the collision has not yet been handled, and the session to be
/// closed is the one that has just sent this OPEN message. This will be handled in
/// fsm_event()
pub(crate) fn collision_prevention(
current_peer_id: Option<Ipv4Addr>,
incoming_peer_id: Ipv4Addr,
instance: &mut InstanceUpView<'_>,
neighbors: &mut Neighbors,
) -> bool {
if let (Some(local_id), Some(current_peer_id)) =
(instance.config.identifier, current_peer_id)
&& current_peer_id == incoming_peer_id
&& local_id < current_peer_id
&& let Some((_, nbr)) = neighbors.iter_mut().find(|(_, n)| {
matches!(
n.state,
fsm::State::OpenConfirm | fsm::State::OpenSent
)
})
{
let msg = NotificationMsg::new(
ErrorCode::Cease,
CeaseSubcode::ConnectionCollisionResolution,
);

nbr.in_collision = false;
nbr.session_close(&mut instance.state.rib, instance.tx, Some(msg));
nbr.state = fsm::State::Idle;
return true;
}

false
}

// Injects an event into the neighbor's FSM.
pub(crate) fn fsm_event(
&mut self,
Expand Down Expand Up @@ -381,6 +430,14 @@ impl Neighbor {
Some(fsm::State::Idle)
}
fsm::Event::RcvdOpen(_msg) => {
if self.in_collision {
let error_code = ErrorCode::Cease;
let error_subcode =
CeaseSubcode::ConnectionCollisionResolution;
let msg =
NotificationMsg::new(error_code, error_subcode);
self.session_close(rib, instance.tx, Some(msg));
}
// TODO: collision detection
Some(fsm::State::Idle)
}
Expand Down Expand Up @@ -716,6 +773,16 @@ impl Neighbor {
instance: &mut InstanceUpView<'_>,
msg: OpenMsg,
) -> fsm::State {
// Check for a collision.
if self.in_collision {
let error_code = ErrorCode::Cease;
let error_subcode = CeaseSubcode::ConnectionCollisionResolution;
let msg = NotificationMsg::new(error_code, error_subcode);
let rib = &mut instance.state.rib;
self.session_close(rib, instance.tx, Some(msg));
return fsm::State::Idle;
}

use crate::packet::iana::OpenMessageErrorSubcode as ErrorSubcode;

// Validate the received message.
Expand Down
47 changes: 40 additions & 7 deletions holo-bgp/tests/conformance/topologies/topo1-1/rt1/config.json
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2,19 +2,22 @@
"ietf-interfaces:interfaces": {
"interface": [
{
"name": "lo",
"name": "eth1",
"type": "iana-if-type:ethernetCsmacd",
"ietf-ip:ipv4": {}
"ietf-ip:ipv4": {},
"ietf-ip:ipv6": {}
},
{
"name": "eth-rt2",
"name": "eth2",
"type": "iana-if-type:ethernetCsmacd",
"ietf-ip:ipv4": {}
"ietf-ip:ipv4": {},
"ietf-ip:ipv6": {}
},
{
"name": "eth-rt3",
"name": "lo",
"type": "iana-if-type:ethernetCsmacd",
"ietf-ip:ipv4": {}
"ietf-ip:ipv4": {},
"ietf-ip:ipv6": {}
}
]
},
Expand All @@ -23,7 +26,7 @@
"control-plane-protocol": [
{
"type": "ietf-bgp:bgp",
"name": "test",
"name": "main",
"ietf-bgp:bgp": {
"global": {
"as": 65100,
Expand All @@ -44,6 +47,20 @@
}
]
}
},
{
"name": "iana-bgp-types:ipv6-unicast",
"enabled": true,
"apply-policy": {
"default-import-policy": "accept-route"
},
"ipv6-unicast": {
"holo-bgp:redistribution": [
{
"type": "ietf-routing:direct"
}
]
}
}
]
}
Expand All @@ -62,6 +79,14 @@
"default-import-policy": "accept-route",
"default-export-policy": "accept-route"
}
},
{
"name": "iana-bgp-types:ipv6-unicast",
"enabled": true,
"apply-policy": {
"default-import-policy": "accept-route",
"default-export-policy": "accept-route"
}
}
]
}
Expand All @@ -78,6 +103,14 @@
"default-import-policy": "accept-route",
"default-export-policy": "accept-route"
}
},
{
"name": "iana-bgp-types:ipv6-unicast",
"enabled": true,
"apply-policy": {
"default-import-policy": "accept-route",
"default-export-policy": "accept-route"
}
}
]
}
Expand Down
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