Cases12 cases

7 to 11 of 12

1. IL-0228 IPv4 routing across OVHcloud's global network broke after a router change put the full internet routing table into OSPF

   OVHcloud2021

   Symptoms

   At 09:20 CET on 13 October 2021, during a planned change on a router, IPv4 traffic across OVHcloud's whole network stopped being routed, while all IPv6 traffic stayed reachable. The team detected a problem with the router's performance at 09:21 CET and escalated at once.

   Network unreachableService outage

   Trigger

   A planned change to a router in the Vint Hill datacentre in the United States, meant to strengthen anti-DDoS protection, included a command to regulate the redistribution of BGP into OSPF, and the router did not interpret that command correctly.

   Root cause

   Configuration change

   Component

   Interior routing (IGP)BGP routingBackbone network (WAN)

   Contributing factor

   Ineffective rollbackExcessive blast radius

   Detection

   At 09:21 CET the team detected a problem with the router's performance and started the escalation process.

   Remediation

   • Disconnected the optical equipment to isolate the router's connectivity
   • Powered off the faulty router
   • Stabilised connectivity to restore the remaining services

   Action items

   • Re-evaluate the change procedure for this type of equipment, which applies the command line natively
   • Strengthen the related change process

   View case
2. IL-0221 A backbone maintenance command disconnected all Facebook data centers, and its DNS servers withdrew their BGP routes

   Meta2021

   Symptoms

   On 4 October 2021 Facebook's platforms went down worldwide: every connection in Facebook's global backbone network was lost, and the data centers were cut off from each other and from the internet. The rest of the internet could no longer find Facebook's servers, because Facebook's authoritative DNS servers had become unreachable.

   Service outageDNS resolution failuresNetwork unreachable

   Trigger

   During routine maintenance of the backbone, a command was issued to assess the availability of global backbone capacity, and it unintentionally took down all connections in the backbone network.

   Root cause

   Operator error

   Component

   Backbone network (WAN)BGP routingAuthoritative DNS servers and records

   Contributing factor

   Automation amplificationCircular dependencyLimited debug access

   Remediation

   • Sent engineers onsite to the data centers to debug the issue and restart systems
   • Brought the backbone network back online across the data center regions
   • Brought services back gradually, managing the rising load to avoid crashes from a traffic surge

   Action items

   • Find ways to simulate the global backbone being taken offline in storm drills
   • Strengthen testing, drills and overall resilience

   Lessons

   The hardening that protects Facebook's data centers against unauthorized access slowed recovery from an outage caused by Facebook's own error, and the author judges that tradeoff worth keeping. Storm drills prepared the teams to bring load back safely, although no drill had yet simulated the global backbone going offline.

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3. IL-0256 Basecamp was fully down twice in two days as hardware load balancers crashed and routers lost sync after failover

   37signals2020

   Symptoms

   On Tuesday 1 September 2020 Basecamp could not be loaded at all for 17 minutes, from 9:51am to 10:08am Central Time (14:51–15:08 UTC), and nothing seemed to work. On Wednesday 2 September it happened again for 15 minutes, from 10:50am to 11:05am Central Time (15:50–16:05 UTC).

   Service outageLink flappingHigh latency

   Trigger

   On 1 September the network links between the data centers went offline, and during the recovery a hardware load balancer crashed. On 2 September the primary load balancer in Virginia crashed again, and later its secondary crashed too.

   Root cause

   Software defect

   Component

   On-premises load balancerBackbone network (WAN)

   Contributing factor

   Overlapping failuresNo headroom

   Remediation

   • On 1 September, intervened by hand to get the network routers working with the standby load balancer
   • On 2 September, fixed the same router and load balancer desynchronization by hand

   Action items

   • Find the cause of the load balancer crash with the vendor and apply its bugfix
   • Replace the hardware load balancers
   • Reroute the redundant cross-datacenter network paths for proper circuit diversity
   • Evaluate moving from hardware to software load balancers, which can be deployed in minutes
   • Evaluate decentralizing the load balancer architecture to limit the impact of one failure
   • Assess the systems for resilience, fragility and risk, and review the assessment process itself

   Lessons

   Basecamp plans for failures like these and practises responses to catastrophic failures in its live systems, but three obscure issues in five days chained into overlapping failures. What caught the team by surprise were cascading failures that exposed unexpected fragility and hard paths to recovery, which Basecamp says it can prepare for.

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4. IL-0255 Basecamp's two redundant links between data centers dropped together and a Redis replica resync loop stalled chat

   37signals2020

   Symptoms

   At 11:59AM CDT on Friday 28 August 2020, Basecamp's Ops team received a flood of alerts from PagerDuty, Nagios and Prometheus. Monitoring showed that the newer of the two metro links between the data centers in Ashburn, VA and Chicago, IL had been lost for about 20-30 seconds, and BC3 kept responding slowly after the link came back.

   Link flappingHigh latencyReplication lag

   Trigger

   Both redundant metro links between Basecamp's Ashburn and Chicago data centers dropped for short periods; monitoring showed the newer link down for about 20-30 seconds.

   Root cause

   Dependency failure

   Component

   Backbone network (WAN)BGP routingIn-memory cacheDatabase replication

   Contributing factor

   Single point of failureRetry stormSignal buried in noise

   Detection

   A flood of alerts from PagerDuty, Nagios and Prometheus, with monitoring showing the lost metro link. A lingering alert for one Redis DB instance later pointed to the replication problem.

   Remediation

   • Turned chat off at the load balancer layer so the rest of BC3 could stabilise
   • Restarted Redis on the replica that was stuck in the resynchronization loop

   Action items

   • Make the documented Redis configuration changes for the known replication issue
   • Make concrete plans to remove the shared physical path under the two metro links
   • Prove the failover systems in production at peak load
   • Make explicit room for post-incident follow-up work and push other work out

   Lessons

   Planned redundancy and live testing had given Basecamp a false sense of security, especially at peak load: moving between redundant network links or shifting load across data centers should work without issue, and Basecamp would rather find out in a controlled test with the full team ready. The follow-up from an outage needs urgency and explicitly cleared time, not just a place on a list.

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5. IL-0222 Google Cloud US regions had heavy packet loss and some went unreachable after the network control plane was descheduled

   Google Cloud2019

   Symptoms

   On Sunday 2 June 2019, with user impact beginning between 11:47 and 11:49 US/Pacific, Google Cloud projects in several US regions saw elevated packet loss from network congestion, and some regions became unreachable. Customers saw higher latency, intermittent errors and lost connectivity to instances in us-central1, us-east1, us-east4, us-west2, northamerica-northeast1 and southamerica-east1, while us-west1, Europe and Asia had no regional congestion.

   Packet lossNetwork unreachableHigh latencyHigh error rateMonitoring outage

   Trigger

   At 11:45 US/Pacific a relatively rare type of datacenter maintenance event started in one physical location, and the automation that runs it descheduled logical clusters that also held network control plane jobs in other physical locations.

   Root cause

   Software defect

   Component

   Network control plane (SDN)BGP routingBackbone network (WAN)

   Contributing factor

   Excessive blast radiusCircular dependencyLimited debug accessIneffective rollback

   Detection

   Google engineers were alerted to the failure two minutes after it began and started the incident process used for the most serious production incidents. The first customer notice appeared on the status page at 12:53 US/Pacific.

   Remediation

   • Lowered the priority of less critical network traffic classes to reduce congestion
   • Halted the datacenter automation software responsible for the maintenance event
   • Rescheduled the network control plane and its supporting infrastructure in each location
   • Rebuilt and redistributed the lost network control plane configuration, rolling it out from 14:03
   • Directed service traffic away from the affected regions
   • Moved the Cloud VPN control plane in three regions out of its fail-safe state

   Action items

   • Keep the maintenance automation off until safeguards stop it descheduling jobs in several physical locations at once
   • Harden the cluster management software to reject such requests whatever their origin
   • Reconfigure the network control plane to reject maintenance requests of the type involved
   • Make the network control plane in each location persist its configuration
   • Let the network stay in fail static mode longer when the control plane is lost
   • Review, update and test emergency response tooling and procedures, including customer communication tooling, for such network failures
   • Extend continuous disaster recovery testing to this and similarly catastrophic failures

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