Skip to main content

Network Ignition

This document describes the link bring-up procedure, or "ignition", at the E2E layer.

Controller Ignition Management

The controller's IgnitionApp is responsible for managing the ignition procedure. Ignition involves forming a link from an "initiator" node, which is already connected to the controller in this case, to a "responder" node.

Node Selection

IgnitionApp will automatically ignite links during network startup and whenever nodes or links subsequently fail. The app periodically runs a procedure to select nodes to ignite (every 5 seconds), using the current topology state to determine the ignition order; multiple links can be ignited in parallel. This algorithm is defined in IgnitionAppUtil, and is described below.

  1. Determine the set of "candidate" links to ignite, comprised of all offline links emanating from all currently-reachable nodes. Additionally, the initiator DN on a candidate link must be time-synchronized, i.e. in the ONLINE_INITIATOR state (refer to Topology Management for further details). Backup links to CNs are not tried until a minimum time has elapsed (5 minutes, via controller flag --linkup_backup_cn_link_interval) since any link to a particular CN could have been considered a candidate.
  2. An initiator node must wait for the current ignition attempt to finish, fail, or time out before accepting another request. On P2MP (point to multi-point) initiator nodes, wait until any response is received (up to 16 seconds, or controller config ignitionParams.bfTimeoutSec) before attempting ignition again for any link.
  3. Group candidates by link name. For links that can be ignited in both directions, pick the initiator node randomly and push the other candidate node to the end of the list (in case the first node gets filtered out in a subsequent step).
  4. Each node can only have one link ignited at a time, so filter out any candidate links to the same responder node.
  5. Ignition attempts across the same link are dampened (once per 10 seconds, or controller config ignitionParams.linkUpDampenInterval); filter out any candidate links that had ignition attempts within the dampening interval. Note that this timeout may get cancelled when the controller receives any link status event for the link (depending on controller flag --linkup_ignore_dampen_interval_after_resp). If the same link fails to ignite for an extended period (30 minutes, via controller flag --linkup_extended_dampen_failure_interval), the dampening interval is increased (to once per 5 minutes, via controller flag --linkup_extended_dampen_interval) in order to reduce possible interference to nearby nodes as well as loss of available link bandwidth caused by the association process.

The controller ignition algorithm has the following limitations:

  • While Terragraph firmware supports time propagation over two hops to a CN, the controller only allows ignition from time-synchronized DNs. For example, the following case is unsupported: DN1 (ONLINE_INITIATOR) -> DN2 (ONLINE) -> CN
  • Each initiator node can only be scheduled once per ignition cycle. On multi-sector DNs, ignition will not be performed in parallel on different sectors, even if this may be feasible depending on interference constraints.

Slot Exclusion

During ignition, the initiator node sweeps its transmit beam in all directions, which can create interference to nearby nodes. To avoid such interference potentially disrupting surrounding links, BF_SLOT_EXCLUSION_REQ commands may be sent to nodes in close proximity to an initiator, which instructs them to not use BF slots for the duration of the IBF procedure. This achieves greater link stability at the cost of reduced maximum throughput (due to reserved BF slots).

The IBF slot exclusion feature is disabled by default, and can be enabled via the controller flag --enable_linkup_slot_exclusion.

The slot exclusion set will include a node/radio if:

  1. It doesn't have any links being ignited in the current ignition cycle
  2. It has polarity opposite of the initiator
  3. It uses the same wireless channel as the initiator
  4. At least one of the following conditions are met:
    • Node is within a short distance of an initiator node (50 meters, or controller flag --linkup_exclusion_distance_short)
    • Node is within a longer distance of an initiator node (350 meters, or controller flag --linkup_exclusion_distance_long) and has one or more links with receive beam pointing towards the initiator (relative angle less than 5 degrees, or controller flag --linkup_exclusion_angle_rel)

For DNs, the BF_SLOT_EXCLUSION_REQ is sent to the node subject to interference from the initiator, whereas for CNs the command is sent to the parent DN.

Message Exchange

At this point, the controller has determined all links to ignite along with their initiator nodes and any nearby links that require slot exclusion to avoid interference. IgnitionApp issues a series of commands to minions, which are listed below.

  1. Send BF_RESP_SCAN to any DNs that should now become responders.
  2. Send BF_SLOT_EXCLUSION_REQ to any DN subject to interference that should avoid using BF slots during this ignition cycle.
  3. Wait for the previous messages to propagate (1 second).
  4. Send SET_LINK_STATUS to all initiator nodes, with any required firmware parameters (e.g. polarity, control superframe, channel, Golay index).

The BF_RESP_SCAN message instructs nodes to send a FW_BF_RESP_SCAN (ON) message to the driver, setting the "BF responder mode" config which schedules the BF slot in the slotmap. Before the next ignition cycle, nodes will send a FW_BF_RESP_SCAN (OFF) message to the driver to disable responder mode.

Responder mode reserves the Rx BF slots, reducing maximum throughput by roughly 50%. Each radio starts in responder mode, and goes back into responder mode if all links have gone down or whenever a DN-to-DN link has gone down. Note that on P2MP DNs, this may cause the radio to go into responder mode regardless of whether there are other links currently up (and causing a throughput drop on those links). Nodes disable responder mode automatically once any link is formed. The controller does not need to send these commands to CNs because they can only form one link.


IgnitionApp normally performs network-wide ignition automatically, but accepts the following commands for manual management:

  • SET_LINK_STATUS_REQ - Bring any link up or down via the normal association and disassociation procedures, respectively.
  • FORCE_DISSOC_REQ - Bring down a link forcefully, used when the link may not exist in the current topology (e.g. deleted nodes/links).
  • SET_IGNITION_PARAMS - Set ignition parameters, including the time interval to run the ignition procedure, the dampening interval, and whether automatic ignition should be disabled network-wide or on any specific links. Note that these parameters are not persisted, and will be lost if the controller reboots.
  • GET_IGNITION_STATE - Retrieve the current ignition parameters, along with debug information about the node selection algorithm.

Note that IgnitionApp is always responsible for instructing nodes to report their link status to TopologyApp. If automatic ignition is enabled, this will happen as a result of the SET_LINK_STATUS commands. If disabled, then the app manually sends out GET_LINK_STATUS commands.

Minion Ignition Procedure

The minion's IgnitionApp accepts commands from the controller and exchanges ignition-related messages with the driver to associate or disassociate with a neighboring node.


The ignition procedure begins when the minion receives a SET_LINK_STATUS command from the controller's IgnitionApp, and normally ends with a LINK_STATUS notification to the controller's TopologyApp indicating the status of the link. If the link has not come up after a timeout interval (15 seconds, or FLAGS_linkup_resp_wait_timeout_s), the ignition attempt is abandoned and no LINK_STATUS message is sent.

Only one neighbor can be ignited at a time. The minion starts the process by sending a DR_SET_LINK_STATUS command to the driver, which attempts to form a link using the parameters received from the controller (e.g. polarity, control superframe, Golay index). Afterwards, the driver will pass a DR_LINK_STATUS message back to the minion indicating the link status.

When link-layer encryption ("wsec") is enabled on the minion, IgnitionApp also becomes responsible for managing two additional Linux processes for each wireless interface: an "authenticator" (hostapd) on the initiator node and a "supplicant" (wpa_supplicant) on the responder node. Logs for these processes are written to /tmp/hostapd_<iface> and /var/log/wpa_supplicant.log, respectively, with logging verbosity determined by the wsecParams node configuration field. These processes cannot both be enabled on the same interface. The current implementation uses a global wpa_supplicant process but spawns a separate hostapd process for each interface due to limitations in hostapd software.

Terragraph nodes support two wsec authentication mechanisms:

  1. WPA-PSK (wsecEnable = 1)
  2. WPA-Enterprise / 802.1X (wsecEnable = 2)

Each authentication mechanism uses a different configuration template for the authenticator and supplicant. Under 802.1X, some configurations are populated from the node configuration, and confidential fields (e.g. passwords) are pulled from device-specific secure storage and submitted via hostapd_cli and wpa_cli.

The minion assumes a responder role on startup and runs the supplicant for every interface. When a minion becomes the initiator for a link, it must remove the link's interface from the supplicant and start the authenticator. To do so, the minion first requests the interface name by sending a DR_DEV_ALLOC_REQ request to the driver; it receives a DR_DEV_ALLOC_RES response. If association is successful, then the authenticator will continue to run on this interface. If the ignition times out or the link goes down, then the minion returns to its initial state by killing the authenticator and re-adding the interface to the supplicant. The responder node should also reset its supplicant interface state as a safety measure. Between stopping one service and starting the other, the minion must wait for the interface to reset and become available for use again; this is a static duration which may need to be tuned based on the host hardware via the flag --wsec_stop_delay_ms (default 200ms).

After association, the minion must wait for the secure port to be set to the "authorized" state following successful key negotiation. This is signaled by the DR_WSEC_LINKUP_STATUS event from the driver, after which the minion will forward a corresponding LINK_AUTHORIZED notification to the controller. While waiting for key negotiation, the minion must not ignite other links. Additionally, no other authenticator or supplicant processes should be started or stopped; this means that actions for all LINK_DOWN messages on other interfaces should be queued for after the key negotiation (or failure/timeout).

For CNs, because only one link can be formed, the minion will remove all other interfaces from the supplicant after a link successfully associates (i.e. reaches the "authorized" state). On a LINK_DOWN event, the minion must then restart supplicants on every interface.

After a LINK_UP event, the driver sends DR_WSEC_STATUS to the minion when wsec is disabled (in place of DR_WSEC_LINKUP_STATUS). This event is ignored by the minion. Previously, the message was part of an "open mode fallback" feature to allow an open connection even when operating in secure mode, where the minion would kill any supplicant or authenticator on the given interface to allow the connection to complete. This fallback feature is no longer supported in firmware.


The default WPA-PSK passphrase (i.e. 8-63 ASCII characters) can be changed via the node configuration field radioParamsBase.wsecParams.wpaPskParams.wpa_passphrase. In addition to the default passphrase, link-specific passphrases can also be set via the field radioParamsBase.wsecParams.wpaPskParams.wpa_passphrase_override, which maps from neighbor radio MAC addresses to passphrases.


802.1X requires a RADIUS server to be present in the same network to authenticate requests from the authenticator and supplicant. It also requires certificates to be provisioned to the nodes and to the RADIUS server by the same certificate authority (CA). Nodes support the EAP-TLS protocol.

The authentication process is as follows:

  1. The supplicant initiates the process with the authenticator using EAPoL.
  2. The authenticator creates a UDP-based TLS tunnel with the RADIUS server using a shared secret, and forwards EAPoL messages from the supplicant to the RADIUS server in this tunnel.
  3. The supplicant now creates a secure session with the RADIUS server using a shared client username and password.
  4. Once the communication is secured, the supplicant sends the pre-provisioned device certificate to the RADIUS server.
  5. The RADIUS server verifies the certificate and sends an "Accept" or "Reject" indication (in the UDP datagram header).
  6. Upon seeing "Accept", the authenticator proceeds with the creation of an encrypted link between the nodes.

Parameters associated with 802.1X reside in the node configuration structure eapolParams.

Message Exchange

The order of the ignition-related messages during association is shown below. Steps marked with (*) are only applicable in secure mode.


  1. SET_LINK_STATUS (LINK_UP) - Request from the controller's IgnitionApp to begin igniting a link.
  2. (*) DR_DEV_ALLOC_REQ - Request to the driver for the link's interface name.
  3. (*) DR_DEV_ALLOC_RES - Response from the driver with the link's interface name, at which point the minion removes the interface from the supplicant and starts the authenticator.
  4. DR_SET_LINK_STATUS - Request to the driver to begin igniting a link with the given parameters.
  5. DR_LINK_STATUS (LINK_UP) - Event from the driver that the association completed and the link is up.
  6. (*) DR_WSEC_LINKUP_STATUS - Event from the driver that the secure port is authorized. If operating in open mode, DR_WSEC_STATUS will be received instead.


  1. DR_LINK_STATUS (LINK_UP) - Event from the driver that the association completed and the link is up.
  2. (*) DR_WSEC_LINKUP_STATUS - Event from the driver that the secure port is authorized. If operating in open mode, DR_WSEC_STATUS will be received instead.

Other Messages

Some other common ignition-related messages are described below.

  • DR_LINK_STATUS (LINK_DOWN) - Event from the driver that a link went down, including the cause (see thrift::LinkDownCause).
  • DR_LINK_STATUS (LINK_PAUSE) - Event from the driver that either end of a link is impaired (i.e. LSM state LINK_UP_DATADOWN). In this transient state, the link is unusable for data but is still able to pass link-level heartbeats. The "fast link impairment" algorithm will send another LINK_DOWN event, this time with cause HB_KA_LOSS_DETECTED, if this condition persists, or transition to LINK_UP if it resolves. The minion treats impaired links in the LINK_PAUSE state as alive, but also brings down links that have been impaired for too long (FLAGS_link_pause_dissoc_delay_s).
  • SET_LINK_STATUS (LINK_DOWN) - Request from the controller's IgnitionApp to dissociate a link (user-triggered).
  • DR_DEV_UPDOWN_STATUS - Event from the driver (forwarded from StatusApp) that a backhaul baseband device went offline (so all associated links are now also down) or has come back online ("up" notification may be emitted twice for unknown reasons).

Distributed Ignition

"Distributed ignition" is a best-effort algorithm for node-initiated link ignition, where the minion attempts to bring up links to pre-configured neighbors until connected to the controller.

Node Configuration

The minion's IgnitionApp attempts distributed ignition using a structure of one or more (in the case of point-to-multi-point) wireless neighbors defined in the node configuration (topologyInfo.neighborInfo). This structure is kept up-to-date by the controller, usually immediately but sometimes delayed by up to 5 minutes (FLAGS_topology_info_sync_interval).

To improve the likelihood of success and reduce interference, the controller assigns a coloring to each radio, and buckets ignition attempts into different time slots based on colors. The total number of colors is defined by the even-number constant ConfigHelper::kNumColors. Radios at the same site should avoid igniting simultaneously, and radios on opposite ends of a link should also avoid igniting simultaneously. These constraints match the polarity assignment constraints, so color assignments are based on radio polarity. The color assignment algorithm is defined in ConfigHelper, and is described below.

  1. Divide kNumColors by 2 to determine the number of colors available for each polarity (with software-hybrid polarity treated as non-hybrid). Radios with different polarities will always receive different colors, but radios with the same polarity may receive the same color for small kNumColors.
  2. For each site in the topology, assign colors as follows:
    1. Compute the approximate angle of every link originating from each radio at the site.
    2. Sort the radios based on link angle.
    3. Assign colors to each radio based on its polarity and an offset based on the iteration index (i.e. alternating colors in the order of link angle). This aims to reduce same-site interference by picking different colors for adjacent radios. For example, on a site with 4 evenly-spaced odd-polarity radios and kNumColors = 4, radios 90 degrees apart will have different colors and radios 180 degrees apart will have the same color.

Distributed Ignition Procedure

The minion's IgnitionApp manages the distributed ignition procedure via a 5-second loop (FLAGS_distributed_ignition_cooldown_duration_ms). The feature is disabled if neighbor information is missing, or if switched off in the node configuration (sysParams.distributedIgnitionEnabled).


  1. GPS_ENABLED - Event from the driver (forwarded by the minion's StatusApp) that the GPS module is initialized. Ignition requires GPS-based time synchronization, so distributed ignition is only enabled after receiving this message.
  2. CONTROLLER_CONNECTED - Event from the minion's StatusApp that the minion is now connected to the controller, which disables the distributed ignition feature until the minion process restarts.

Note that while distributed ignition is active, the minion will automatically enable responder mode on a radio after receiving any LINK_UP event, ensuring that links can still be initiated to it if it serves as a P2MP DN in the topology. After connecting to the controller, the minion will switch responder mode off on all radios.

Time Synchronization

Ignition attempts are bucketed into time slots based on each node's GPS clock, ensuring that all ignition attempts are properly time-synchronized to avoid collisions. The ignition algorithm contains configurable parameters to modify the length of ignition slots and the acceptable time bounds where ignition can still occur before or after a bucket boundary.

For example, consider a cooldown duration of 5 seconds, a maximum offset of 1 second (i.e. highest permitted timing error in either direction), and kNumColors = 4. The 4 buckets are distributed into a 20-second time range as follows:

  • Bucket #1 starts at t seconds (+/- 1 second)
  • Bucket #2 starts at t+5 seconds (+/- 1 second)
  • Bucket #3 starts at t+10 seconds (+/- 1 second)
  • Bucket #4 starts at t+15 seconds (+/- 1 second)

This pattern repeats every 20-second interval on all nodes, based on the current GPS time modulo 20 seconds.


To avoid repeated ignition failures for any given link, attempts are subject to exponential backoff as follows:

  • On the initial ignition failure, set the cooldown time to the total bucket time range (as described above).
  • On each subsequent ignition attempt for the same link, double the cooldown duration.
  • Randomly delay ignition attempts by an additional cycle (25% of the time). This adds some jitter to prevent repeated synchronized collisions.

Backoff is stored per-link. To achieve quicker ignition, 3 link ignition attempts are allowed before beginning the backoff procedure (FLAGS_distributed_ignition_attempts_before_backoff). After 18 ignition attempts, link ignition stops (FLAGS_distributed_ignition_max_attempts).

Ignition Algorithm

The minion's distributed ignition algorithm is described below.

  1. Determine the time until the next ignition bucket, and schedule a timer to fire at that time.
  2. When the timer fires, if the current time does not fall within the maximum allowed offset for a synchronized attempt slot (i.e. too far from a bucket boundary), skip this interval.
  3. Calculate the bucket index based on the current time and find a viable candidate link. Ignore any links already alive, any links for which the backoff timer has not expired, as well as any links with a color not matching the current bucket.
  4. Increment the link iteration index. Links from a site are considered in a round robin fashion. For example, P2MP radios will attempt to ignite each link once before revisiting previously attempted links.