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LinkMonitor.md

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LinkMonitor

This module is responsible for learning link information from the underlying system and managing neighbor sessions. At a high level it:

  • Discovers the links on the system and enables/disables neighbor discovery on them
  • Maintains the local node's link-state in KvStore
  • Manages peering sessions of KvStore (one per neighbor)

APIs

For more information about message formats, check out

Link/Address Discovery

LinkMonitor relies on the external Platform service to provide interface and address information. By default, OpenR comes with NetlinkPlatform which learns about interface and addresses via the netlink library and can be used on most platforms.

PUB Channel

Any link activity (address or status) learned via netlink is published to PUB channel which can be consumed in real-time by other applications. For now, FIB and Spark listen to these messages and take appropriate actions (e.g. shrinking ECMP group or start/stop neighbor discovery on link).

ROUTER Command Socket

LinkMonitor accepts various commands for operation of the link-state protocol, an important one being overloading link/node commands to alter link-state in the network. The commands it accepts are:

  • SET/UNSET OVERLOAD => Toggles transit traffic through node
  • SET/UNSET LINK_OVERLOAD => Toggles transit traffic through a specific link
  • SET/UNSET LINK_METRIC => Customize metric value on a link for soft drains
  • DUMP_LINKS => Retrieves the link information of a node

LinkState Management

LinkMonitor listens to Spark events (described below) (e.g. NEIGHBOR_UP, NEIGHBOR_DOWN) and maintains the link-state of the local node. From there, it gathers any custom link metrics, and overload bits for the node or any link and prepares the AdjacencyDatabse object for the node and keeps it up to date in the KvStore (so that everyone else in the network can see this LinkDatabase).

On link down, neighbor discovery is immediately stopped on the link, link state is updated and KvStore peering sessions are torn down.

NOTE that Link-Up has a backoff but Link-Down doesn't. This is because we want to be as fast as possible to react to down events to avoid potential packet drops.

Link Events Dampening

Sometimes link status can flap badly. This can happen for various reasons, namely, bad optics, or poor wireless reach. In such cases, we would like to avoid control plane churn across the whole network, as nodes will try to route through/around when the link is up/down. To avoid such scenario we have added dampening support in LinkMonitor. If a link flaps, then back off is applied which gets doubled if the link flaps again within backoff period and so on. If the link shows itself stable within a period, it is enabled for neighbor discovery.

You can configure backoffs for link event dampening with following flags

  • --link_flap_initial_backoff_ms=1000 (default=1s)
  • --link_flap_max_backoff_ms=60000 (default=60s)

Link Metric

LinkMonitor is responsible for computing the metric value for each Adjacency to neighbors which are then used to compute the cost of a path in Decision's SPF computation. For now, we support two kinds of metrics (configured via flags)

  • hop_count => Use 1 (constant) metric value for each Adjacency
  • rtt_metric => rtt_us / 10 where rtt_us is measured rtt in microseconds

OpenR is pretty flexible and using other parameters like loss, jitter, signal strength is potentially doable (via Platform abstraction)