Windows EC2 network loss

Last updated on Jun 5, 2026

Windows EC2 network loss is an AWS chaos fault that drops NETWORK_PACKET_LOSS_PERCENTAGE percent of network packets destined for DESTINATION_IPS and/or DESTINATION_HOSTS on one or more Windows EC2 instances for a configurable duration. Targets are selected by EC2_INSTANCE_ID or EC2_INSTANCE_TAG. The fault dispatches a Windows traffic-shaping command via AWS Systems Manager Run Command and removes the rule at the end of the fault.

Use this fault to test how a Windows-hosted workload behaves when the network becomes lossy on a specific path: whether retries recover requests, whether TCP backoff is acceptable, and whether monitoring detects the loss correctly.

Run your first experiment

If you have not configured the chaos infrastructure yet, go to Quickstart to install the chaos infrastructure and run an experiment end to end.

Use cases

Run this fault when you want to answer concrete questions like:

Retry behaviour: When NETWORK_PACKET_LOSS_PERCENTAGE percent of packets to a specific dependency is dropped, do retries recover requests?
TCP congestion control: Does the connection survive, or does it back off so far that it stalls?
Replica failover: For workloads with cross-AZ or cross-region replicas, does traffic shift away from the lossy path?
Monitoring fidelity: Do dashboards and alerts identify the packet loss quickly?
Recovery time: When the chaos ends, how quickly does throughput return to baseline?

Prerequisites

Kubernetes version: 1.21 or later for the chaos infrastructure cluster. Go to What's supported to confirm distribution support.
Target Windows EC2 instances: Each instance is in the running state and registered with AWS Systems Manager.
Selection criteria: Either EC2_INSTANCE_ID is set or EC2_INSTANCE_TAG is set.
Destinations: At least one of DESTINATION_IPS or DESTINATION_HOSTS is set.
AWS credentials available: Either an AWS credentials file uploaded as a File Secret in Harness Secret Manager (see Authentication below) or an IAM role for service accounts (IRSA) bound to the chaos infrastructure service account.
IAM permissions granted: The credentials or role include the permissions listed below.

Supported environments

Platform	Support status
Windows Server 2016 / 2019 / 2022 EC2	Supported
Linux EC2 instances	Not supported (use EC2 network loss)
AWS regions	Supported in every commercial region; pass the region in `REGION`

Permissions required

The IAM principal that the chaos pod uses (the credentials mounted from the Harness Secret Manager file secret, the IRSA role on the chaos service account, or the role assumed via ASSUME_ROLE_ARN) needs the following AWS actions.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:DescribeInstances",
        "ec2:DescribeInstanceStatus"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ssm:SendCommand",
        "ssm:CancelCommand",
        "ssm:GetCommandInvocation",
        "ssm:DescribeInstanceInformation"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ec2messages:AcknowledgeMessage",
        "ec2messages:DeleteMessage",
        "ec2messages:FailMessage",
        "ec2messages:GetEndpoint",
        "ec2messages:GetMessages",
        "ec2messages:SendReply"
      ],
      "Resource": "*"
    }
  ]
}

Go to common policy for all AWS faults to use a single superset IAM policy across every AWS fault.

Authentication

The fault supports three credential delivery models. Pick one based on how your chaos infrastructure is deployed.

Method	When to use it	How to configure
Harness Secret Manager file secret	Chaos infrastructure runs outside EKS, or you want explicit static credentials	Upload the AWS credentials file as a File Secret in Harness Secret Manager and reference its identifier via `AWS_AUTHENTICATION_SECRET`
IAM Roles for Service Accounts (IRSA)	Chaos infrastructure runs in EKS and uses an OIDC-bound service account	No tunable changes; the chaos pod inherits the role automatically. Go to AWS IAM integration to set it up
Assume role	The fault needs to act in a different account or with elevated permissions	Set `ASSUME_ROLE_ARN` to the role ARN; the chaos pod assumes the role on top of its base credentials

When using the Harness Secret Manager method, the contents of the File Secret should be the AWS credentials file in the standard ~/.aws/credentials format:

[default]
aws_access_key_id = REPLACE_WITH_ACCESS_KEY_ID
aws_secret_access_key = REPLACE_WITH_SECRET_ACCESS_KEY

Upload this file as a File Secret in Harness Secret Manager (Project Setup → Secrets → New File Secret), and pass the secret identifier in AWS_AUTHENTICATION_SECRET when configuring the fault.

Go to AWS named profile for chaos to switch between profiles inside a single credentials file.

Fault tunables

Configure the following fault parameters when you add Windows EC2 network loss to an experiment in Chaos Studio. Defaults are shown for reference.

Required parameters

Tunable	Description	Default
`REGION`	AWS region that hosts the target instances (for example `us-east-1`).	(required)
`NETWORK_PACKET_LOSS_PERCENTAGE`	Percentage of packets to drop on the affected path (0-100).	`100`

Targeting parameters

Tunable	Description	Default
`EC2_INSTANCE_ID`	Comma-separated list of EC2 instance IDs to affect.	`""`
`EC2_INSTANCE_TAG`	Tag in `Key:Value` form matching candidate instances when `EC2_INSTANCE_ID` is empty.	`""`
`INSTANCE_AFFECTED_PERC`	Percentage of matching instances (by tag) to affect.	`0`
`DESTINATION_IPS`	Comma-separated list of destination IPs/CIDRs to drop traffic for.	`""`
`DESTINATION_HOSTS`	Comma-separated list of destination hostnames to drop traffic for.	`""`

Chaos parameters

Tunable	Description	Default
`TOTAL_CHAOS_DURATION`	Duration of the fault in seconds.	`60`
`INSTALL_DEPENDENCIES`	Install the traffic-shaping tooling on the Windows host if missing.	`true`
`SEQUENCE`	Order in which multiple instances are affected: `parallel` applies the rule on all selected instances at once; `serial` does so one at a time.	`parallel`
`RAMP_TIME`	Wait period in seconds before and after the fault. Go to ramp time to read how it is applied.	`0`

Authentication

Tunable	Description	Default
`ASSUME_ROLE_ARN`	ARN of an IAM role to assume on top of the base credentials. Leave empty to use the base credentials directly.	`""`
`AWS_AUTHENTICATION_SECRET`	Identifier of the File Secret in Harness Secret Manager that contains the AWS credentials file. Not required when using IRSA.	`""`

Tunables that apply to every fault are documented in common tunables for all faults. AWS-specific shared tunables are documented in common AWS fault tunables.

Fault execution in brief

Resolves the target instance list from EC2_INSTANCE_ID (or by EC2_INSTANCE_TAG), dispatches a Windows traffic-shaping command via AWS Systems Manager Run Command to each selected instance, installs a packet-drop rule for traffic destined for DESTINATION_IPS and DESTINATION_HOSTS, holds the rule for TOTAL_CHAOS_DURATION seconds, then removes the rule.

Expected behavior during fault execution

New packets leaving the target instances destined for the configured IPs or hosts are dropped at the configured percentage.
TCP connections experience retransmissions; congestion windows shrink. Throughput drops sharply.
UDP-based workloads see lost datagrams; application-level error counters rise.
Application timeouts may fire if retransmissions exceed the configured timeout.

When the fault ends

The chaos pod removes the loss rule on each host. New packets are not dropped; TCP congestion windows recover over the next several round-trip times.

Signals to watch

Attach resilience probes to assert each layer:

Loss observation: Use a command probe that runs ping <dest> and asserts packet-loss percentage.
Application errors: Use a Prometheus probe on application-level retry and error counters.
Throughput: Use a Prometheus probe on network throughput metrics to see the drop in throughput.
Application availability: Use an HTTP probe and assert success rate stays above SLO.

Verify the fault execution effect

While the experiment is running, confirm packets are dropped and then recover:

Ping from the affected host (via SSM Session Manager).
```
Test-Connection -ComputerName <destination> -Count 20
```
Reply success ratio should drop to approximately 100 - NETWORK_PACKET_LOSS_PERCENTAGE during the chaos window.

Inspect SSM command status.

aws ssm list-command-invocations --region <region> --details --filters "key=Status,value=InProgress"

Compare to a control host.

Run Test-Connection from a peer Windows instance that is not part of the experiment; loss should remain at baseline.

Recovery and cleanup

End of duration: The chaos pod removes the loss rule on each host.
Abort the experiment: Stopping the experiment from Chaos Studio cancels the SSM command and removes the rule.
Manual recovery: If the fault exits before the rule is removed, send an SSM command (PowerShell) to clear the rule, or reboot the host.
Workload recovery: TCP throughput may take a few seconds to recover as congestion windows grow.

Limitations

Windows-only.
SSM-managed hosts only.
One direction only: Loss is applied on egress; return-path losses are unaffected.
At 100% loss the destination is effectively isolated.
Cross-region targeting: A single experiment targets one region (the value of REGION).

Troubleshooting

Windows EC2 network loss fails with AccessDeniedException in Harness Chaos Engineering

The credentials supplied to the chaos pod do not have the required EC2 or SSM permissions. Confirm the IAM policy attached to the user, role, or IRSA service account includes ec2:DescribeInstances, ssm:SendCommand, ssm:GetCommandInvocation, and the ec2messages:* actions used by SSM.

Loss is applied but application keeps working without errors

The most common causes are: NETWORK_PACKET_LOSS_PERCENTAGE is low enough that TCP retransmissions absorb the loss invisibly; the dependency resolves to an IP not in DESTINATION_IPS; or a long-lived TCP connection bypasses the rule. Add the resolved IPs to DESTINATION_IPS explicitly, raise NETWORK_PACKET_LOSS_PERCENTAGE, or force a fresh connection.

Loss rule persists after the chaos window

If the cleanup SSM command failed, the rule may persist. Use SSM Session Manager to log in to the host and remove the traffic-shaping rule using the PowerShell command shown in the chaos pod logs, or reboot the host.

Tags match more instances than expected

EC2_INSTANCE_TAG matches any instance carrying that tag in the target REGION. Use a more specific tag or set INSTANCE_AFFECTED_PERC to bound the blast radius. Verify with 'aws ec2 describe-instances --filters Name=tag:Key,Values=Value'.

Windows EC2 network latency: Add latency instead of dropping packets.
Windows EC2 blackhole chaos: Blackhole specific IPs/hosts entirely.
EC2 network loss: Network loss on Linux EC2 instances.
Common AWS fault tunables: Shared environment variables for AWS faults.

Use cases​

Prerequisites​

Supported environments​

Permissions required​

Authentication​

Fault tunables​

Fault execution in brief​

Expected behavior during fault execution​

Signals to watch​

Verify the fault execution effect​

Recovery and cleanup​

Limitations​

Troubleshooting​

Related faults​