Cases2 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-0361 Job searches on seek.com.au failed for 16 minutes as a Transit Gateway silently dropped 9001-byte packets

   SEEK2021

   Symptoms

   In May 2021 some users of seek.com.au got errors when they searched for jobs, for 16 minutes. SEEK was in the middle of moving the traffic between two VPCs, the one that carried searches, from a VPC peering connection to an AWS Transit Gateway, and the team noticed quickly that search was broken.

   High error ratePacket lossIntermittent timeouts

   Trigger

   In the second step of the migration, SEEK changed the routing table of the first VPC to use the transit gateway while the second VPC still routed over the peering connection, so the two directions of each connection took different paths.

   Root cause

   Configuration change

   Component

   Transit gateway or network interconnect

   Contributing factor

   Unknown limitUnreviewed defaultsMonitoring blind spot

   Detection

   The team noticed soon after the routing change that search on seek.com.au was not working.

   Remediation

   • Rolled back the migration of the search traffic's peering connection to the transit gateway

   Action items

   • Lower the MTU of EC2 instances to 8500, on Amazon Linux 2 with a script at /sbin/ifup-local that runs whenever a network interface comes up

   Lessons

   A transit gateway has a smaller MTU than a same-region peering connection, drops oversized packets silently and clamps the MSS only on packets that cross it. When a migration leaves the two directions of a TCP connection on different paths, the clamp lands on the wrong side of the handshake and large packets are lost; the fix is not to send packets larger than 8500 bytes.

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