Cases3 cases

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1. IL-0341 Traffic between Cloudflare and AWS us-east-1 had high latency, packet loss and origin failures for almost four hours

   Cloudflare2025

   Symptoms

   From 16:27 UTC on 21 August 2025, users connecting to Cloudflare from servers in AWS us-east-1, or receiving connections from Cloudflare there, saw high latency, packet loss and failures to reach origins. Cloudflare's network team was alerted to internal congestion in Ashburn (IAD) at 16:44 UTC.

   Packet lossHigh latencyIntermittent timeouts

   Trigger

   At about 16:27 UTC a single customer began sending many requests from AWS us-east-1 to Cloudflare for objects in Cloudflare's cache, doubling the total traffic from Cloudflare to AWS.

   Root cause

   Capacity exhaustion

   Component

   Transit gateway or network interconnectBGP routingCDN and edge network

   Contributing factor

   No headroomHarmful manual actionExcessive blast radius

   Detection

   Cloudflare's network team was alerted to internal congestion in Ashburn (IAD) at 16:44 UTC, and an incident was raised for customer impact at 17:45 UTC.

   Remediation

   • Rate limited the single customer whose traffic caused the surge
   • Took further traffic engineering actions that fully resolved the congestion
   • Asked AWS to revert its BGP prefix withdrawals, which AWS finished at 20:07 UTC

   Action items

   • Build a mechanism to deprioritise a customer's traffic when it congests the network enough to affect others
   • Speed up the Data Center Interconnect upgrades to add network capacity
   • Work with AWS so the two companies' BGP traffic engineering actions do not conflict
   • Build a traffic management system that gives each customer a network resource budget and automates the manual actions used here

   Lessons

   Cloudflare concludes that it must isolate customers better so that no single customer can take over shared network resources and degrade the platform for others, and that its network needs enough capacity to meet demand. It describes the outage as the result of insufficient network congestion management during an unusual high-traffic event.

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2. IL-0223 A connection surge congested the devices between AWS's internal and main networks in us-east-1, breaking EC2 APIs

   Amazon Web Services2021

   Symptoms

   From 7:30 AM PST on 7 December 2021, AWS services in us-east-1 that communicate between AWS's internal network and its main network saw rising latency and errors. The AWS operations teams lost real-time monitoring data at once, and the first thing found in the logs was elevated internal DNS errors.

   Control plane lagHigh latencyHigh error rateMonitoring outageDNS resolution failures

   Trigger

   At 7:30 AM PST an automated activity that scales the capacity of one AWS service hosted in the main AWS network triggered unexpected behaviour from a large number of clients inside the internal network.

   Root cause

   Software defect

   Component

   Transit gateway or network interconnectAutomated capacity manager

   Contributing factor

   Retry stormCircular dependency

   Detection

   The congestion immediately cut off real-time monitoring data for the AWS operations teams, who turned to logs and first identified elevated internal DNS errors.

   Remediation

   • Moved internal DNS traffic away from the congested network paths
   • Isolated the top sources of traffic onto dedicated network devices
   • Disabled some heavy network traffic services, including EventBridge event delivery
   • Brought additional networking capacity online
   • Disabled the automated scaling activities that triggered the event

   Action items

   • Keep the triggering scaling activities off until all remediations are deployed
   • Fix the latent issue that kept networking clients from backing off
   • Deploy network configuration that protects the affected networking devices in a similar congestion event
   • Keep API Gateway servers healthy when the internal network is unavailable and speed up their recycling
   • Release a new Service Health Dashboard that makes service impact easier to understand
   • Run the support system actively across multiple AWS regions

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3. IL-0362 PagerDuty could not dispatch notifications for 18 minutes when a peering point shared by two AWS regions degraded

   PagerDuty2013sev1

   Symptoms

   On Saturday 13 April 2013 at 8:11am Pacific Time, PagerDuty's on-call engineer was paged about one node of the notification dispatch system, and an attempt at 8:13 to bring the node back failed. Customers saw delayed notifications and 500 errors on PagerDuty's API endpoints.

   High latencyHigh error rateIntermittent connection failures

   Trigger

   At 7:57am Pacific Time, according to AWS, a peering point in Northern California began to degrade, causing connectivity problems for the two AWS regions that PagerDuty used as two of its three datacenters.

   Root cause

   Cloud provider outage

   Component

   Transit gateway or network interconnectCoordination and key-value store

   Contributing factor

   Single point of failureUnverified assumptionMonitoring blind spot

   Detection

   The PagerDuty on-call engineer was paged at 8:11am about an issue with one node of the notification dispatch system, and at 8:18 monitoring detected multiple-provider failures for notifications.

   Remediation

   • Restarted most of the failed coordinator processes by hand
   • The notification dispatch system reached quorum again and processed all queued notifications

   Action items

   • Add more logging and aggregate logs into a single searchable source
   • Add a process watcher that restarts failed coordinator processes automatically
   • Build a dashboard that shows inter-host connectivity
   • Train engineering staff on Cassandra and ZooKeeper
   • Investigate moving off one of the AWS regions to a new hosting provider and datacenter that avoids a single point of failure

   Lessons

   Datacenters chosen to be independent and physically separate can still share a network peering point, and a design built to survive the loss of one datacenter did not survive losing two at once.

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