How Network Flow Monitor Works: TCP Observation with eBPF sock_ops

Published 2026-08-04·Updated 2026-09-17·16 min read

AI documentation guide

Overview

CloudWatch Network Flow Monitor (NFM) observes network performance from the workload's perspective. It collects flow-level retransmissions, round-trip time (RTT), and timeouts to help distinguish application problems from AWS network problems. The agent subscribes to socket-event callbacks (sock_ops) from the kernel TCP stack. It does not capture packet payloads, use packet mirroring, or attach an XDP program. Its observation scope is TCP.

The agent is open source (aws/network-flow-monitor-agent, Rust, Apache-2.0), so its collection and publishing paths can be inspected in code. This guide follows the path from kernel collection through userspace aggregation and Kubernetes enrichment to backend publishing, then covers EKS add-on deployment and troubleshooting.

Background: Backend Components

The backend uses the following concepts.

Component	Role
Scope	The set of accounts to observe. An AWS Organizations scope can include up to 100 accounts.
Workload insights	Aggregated flow metrics within a scope and top contributors for each metric, grouped into categories such as intra-AZ, inter-AZ, and inter-VPC traffic.
Monitor	Detailed tracking for selected local and remote resources, such as subnets, VPCs, AZs, or EKS clusters. Publishes end-to-end metrics and NHI.
Network Health Indicator (NHI)	A binary indicator. 100 means Degraded: at least one flow in the monitored segment experienced an AWS network issue.

NHI supplies AWS's assessment of network health during an incident. Its internal calculation is not public. The official documentation also notes that RTT is not available for every observation and can be sparse.

Architecture: The Data Path

Expand the diagram to read labels and follow connections.

Diagram

Kernel Collection

A Single sock_ops Program

The agent attaches one BPF_PROG_TYPE_SOCK_OPS program to cgroup v2 (nfm-bpf/src/main.rs). The kernel invokes it for events such as TCP state changes, RTT measurements, and retransmissions. The program accumulates per-socket statistics in BPF maps and returns BPF_OK so that observation does not change the connection's behavior.

Events and Collected Values

The following table summarizes handle_socket_event() in sock_ops_handler.rs.

sock_ops callback	Recorded values
TCP_CONNECT_CB	Registers a new client socket, increments connect_attempts, and records the connection start time.
PASSIVE_ESTABLISHED_CB	Registers a new server socket.
STATE_CB	Records connect_duration_us and connect_successes on ESTABLISHED; sets termination flags (TERMINATED_FROM_SYN / FROM_EST / CLOSED); records final byte and segment counts on CLOSE.
RTT_CB	Updates rtt_latest_us and rtt_smoothed_us; falls back to srtt_us and increments rtts_invalid when an argument is unavailable.
RETRANS_CB	Counts retransmitted segments by connection state: retrans_syn, retrans_est, and retrans_close.
RTO_CB	Counts retransmission timeouts by state: rtos_syn, rtos_est, and rtos_close.
PARSE_HDR_OPT_CB, HDR_OPT_LEN_CB	Updates sent and received byte and segment counts.

TIMEOUT_INIT, RWND_INIT, NEEDS_ECN, and ACTIVE_ESTABLISHED_CB return early; CONNECT and STATE events cover connection establishment. Separating retransmissions and RTOs into SYN, ESTABLISHED, and CLOSE phases helps distinguish connection-establishment failures from loss after a connection has formed.

Sampling and Capability Reduction

• Sampling is applied when a socket enters tracking, at CONNECT or PASSIVE_ESTABLISHED, using sampling_interval in the NFM_CONTROL map. Subsequent events for a tracked socket remain part of its statistics.
• The agent starts privileged, then drops CAP_SYS_ADMIN, CAP_PERFMON, and CAP_NET_ADMIN after loading eBPF. It retains CAP_BPF for BPF-map access; see drop_capabilities in lib.rs.

Userspace Aggregation, Enrichment, and Publishing

Timer-Driven Main Loop

Three timers drive the main loop. The principal options and defaults are:

Option	Default	Purpose
--aggregate-msecs	500	Interval for aggregating BPF-map data into userspace flows.
--publish-secs / --jitter-secs	30 / 5	Publishing interval and jitter, giving an effective interval of 25–35 seconds.
--top-k	500	Maximum flows per report; prioritizes flows with greater loss.
--notrack-secs	65	Stops tracking idle sockets; covers six TCP exponential-backoff intervals totaling up to 63 seconds.
--report-compression	gzip	Report compression.
--kubernetes-metadata	off	Enables Pod and EndpointSlice watchers. The EKS add-on entrypoint enables it.
--resolve-nat	off	Uses conntrack to recover addresses behind local NAT.

Kubernetes Enrichment: Adding Pod and Service Names

kubernetes_metadata_collector.rs supplies the Kubernetes context used by the EKS console's service map.

1. Pod and EndpointSlice watchers maintain an IP address → (TCP port → {pod, namespace, service}) map. The Service name comes from the EndpointSlice's kubernetes.io/service-name label, falling back to its owner reference. Pod events preserve entries populated by EndpointSlices because the latter contain richer metadata.
2. For each flow, the agent looks up the local and remote addresses. On a client flow, the remote port can identify the remote Pod. The local ephemeral port does not identify an application port, so the agent assigns a local Pod only when all ports associated with that IP map to the same Pod. The direction is reversed for a server flow.
3. IPv4-mapped IPv6 addresses such as ::ffff:10.0.0.1 are also looked up as IPv4 addresses. The map considers TCP ports only, ignoring UDP container ports.

Report Contents

NfmReport version 1.1 (report.rs) contains:

• network_stats[]: per-flow socket-state counts, sent and received bytes and segments, state-specific retransmissions and RTOs, and the connect_us, rtt_us, and rtt_smoothed_us histograms.
• host_stats.interface_stats[]: ENA allowance counters, including bw_in/out_allowance_exceeded, pps_allowance_exceeded, conntrack_allowance_exceeded/available, and linklocal_allowance_exceeded. Reporting these alongside flow metrics helps correlate loss with instance networking limits.
• process_stats: the agent's CPU use, memory use, and tracked-socket count.
• k8s_metadata: node_name and cluster_name.

Publishing Path

The agent converts NfmReport to an OpenTelemetry ExportMetricsServiceRequest protobuf, compresses it with gzip, signs it with SigV4 using the service name networkflowmonitor, and POSTs it to https://networkflowmonitorreports.<region>.api.aws/publish. See publisher_endpoint.rs. A response other than HTTP 200 increments failed_reports, which is included in a later report. A log entry containing HTTP request complete, status:200 ... publisher_endpoint confirms a successful publishing request.

The source also supports other destinations. --prometheus-workspace-id selects an Amazon Managed Service for Prometheus remote-write endpoint. Building with the open-metrics feature exposes a local Prometheus scrape server.

EKS Deployment and Operations

Installation

The EKS add-on name is aws-network-flow-monitoring-agent. The source guide describes Kubernetes 1.25+ and agent images in the v1.1.x series; select an add-on version compatible with the cluster when deploying. Pod Identity supplies the credentials used for SigV4 signing, so install eks-pod-identity-agent first and attach the managed policy CloudWatchNetworkFlowMonitorAgentPublishPolicy to the agent's IAM role.

aws eks create-addon --cluster-name <CLUSTER> \
  --addon-name aws-network-flow-monitoring-agent \
  --pod-identity-associations \
    serviceAccount=aws-network-flow-monitor-agent-service-account,roleArn=<ROLE_ARN>

Resource Names

The add-on, namespace, and DaemonSet have slightly different names.

Resource	Name
EKS add-on	aws-network-flow-monitoring-agent (monitoring)
Namespace	amazon-network-flow-monitor (monitor, without ing)
DaemonSet / Pod label	aws-network-flow-monitor-agent / name=aws-network-flow-monitor-agent
ServiceAccount	aws-network-flow-monitor-agent-service-account
Internal container-image name	aws-network-sonar-agent

# Check the agent using the exact namespace and label.
kubectl get pods -n amazon-network-flow-monitor -l name=aws-network-flow-monitor-agent
kubectl logs -n amazon-network-flow-monitor -l name=aws-network-flow-monitor-agent \
  --tail=50 | grep publisher_endpoint

Constraints

• TCP only: the sock_ops collection path does not observe UDP or ICMP flows.
• Kernel 5.8+ and cgroup v2 are required.
• Fargate is unsupported: the DaemonSet requires privileged access and a hostPath mount for cgroup.
• Distribution compatibility matters: the agent README identifies SUSE 15 SP5 and Ubuntu 20.04 as unsupported because their kernel configuration restricts required BPF helpers to GPL-licensed programs.

Enabled but No Data: Three Diagnostic Layers

Check the collection path from the agent upward.

1. Agent publishing: confirm an agent Pod is running on each expected node and inspect logs for status:200 ... publisher_endpoint. For HTTP 403, check the Pod Identity association and IAM policy. For timeouts, check the outbound path, including proxies and VPC endpoints.
2. Scope: confirm the account belongs to the intended NFM scope. A successful publishing request alone does not establish that the console is querying the appropriate scope.
3. Monitor: confirm that a monitor covers the local and remote resources for the flows being investigated. An EKS cluster can be selected as a local resource.

If the service map is empty despite --kubernetes-metadata being enabled, investigate enrichment. The agent's Flow enrichment completed. log message indicates that an enrichment pass has completed.

Summary

NFM uses a cgroup v2 sock_ops program to collect TCP socket events, aggregates them into flows in userspace, adds Kubernetes context through Pod and EndpointSlice watchers, and publishes signed OTLP reports. Workload insights provides scope-level contributors, while monitors provide end-to-end metrics and NHI. Use the publishing, scope, and monitor layers to locate missing-data problems, and account for the agent's TCP-only observation boundary.

References

Official Documentation

• Components and features of Network Flow Monitor — Scope, workload insights, monitors, and NHI.
• Using Network Flow Monitor — Service overview.
• Install the agent on EKS clusters — Add-on installation and Pod Identity.

Source Code

• nfm-bpf/src/main.rs — The sock_ops eBPF program.
• nfm-common/src/sock_ops_handler.rs — Callback handling.
• nfm-controller/src/lib.rs — Main loop, defaults, and capability reduction.
• kubernetes_metadata_collector.rs — Pod and EndpointSlice enrichment.
• reports/report.rs and publisher_endpoint.rs — Report schema and publishing.

Related Documents

• How VPC CNI Works — The datapath being observed.
• EKS Node Monitoring Agent — Node health monitoring.
• Networking Debugging — Network troubleshooting procedures.
• Nitro Architecture and Tuning — ENA allowance limits.