TPTEST Troubleshooting: Common Issues and FixesTPTEST is a diagnostic tool used to verify TCP performance, network path integrity, and application-layer connectivity. While it’s valuable for spotting problems quickly, users can encounter a range of issues that prevent accurate testing or produce misleading results. This article walks through common TPTEST problems, why they happen, and practical fixes so you can get reliable measurements.
1. Test fails to start or connection refused
Symptoms:
- TPTEST immediately returns “connection refused” or “no route to host.”
- The test terminates with a socket error before sending data.
Why it happens:
- Server or service not running on target port.
- Firewall blocking the test port (server-side or client-side).
- Wrong IP address, hostname, or port specified.
- Network ACLs or security groups blocking traffic.
Fixes:
- Verify the target host is reachable: ping the IP/hostname and check DNS resolution.
- Confirm the target service is listening on the intended port (use netstat, ss, or lsof on the server).
- Temporarily disable local firewall or add a rule to allow the test port; coordinate with ops/security to allow traffic on the server.
- Check cloud security groups / ACLs and allow inbound traffic for the test port.
- If the tool uses TCP vs UDP, ensure you selected the correct protocol.
2. Tests run but show very low throughput
Symptoms:
- Throughput much lower than expected (e.g., a 1 Gbps link showing 10–50 Mbps).
- Test shows many retransmissions or long transfer times.
Why it happens:
- Link congestion or bandwidth-saturated network.
- Poor TCP configuration (window size, congestion control, buffer sizes).
- Middleboxes (deep packet inspection, rate limiting, or shaping).
- Path MTU issues causing fragmentation.
- Single-stream TCP limitations over high-latency links.
Fixes:
- Run tests at different times to rule out transient congestion.
- Increase TCP window (receive/send buffer) on both client and server for high-bandwidth-delay product links.
- Use multiple parallel streams in TPTEST (if supported) to better utilize available bandwidth.
- Check for traffic shaping or QoS policies on routers/firewalls and adjust rules or schedule tests during low-priority windows.
- Diagnose MTU issues: run ping with DF flag and varying packet sizes, or use tracepath to find smallest MTU along the path.
- Test with an alternative route or from another network segment to isolate where the bottleneck is.
3. High packet loss or retransmissions reported
Symptoms:
- TPTEST shows packet loss, high retransmission rates, or frequent retransmits in TCP traces.
- Inconsistent or variable latency (jitter) reported.
Why it happens:
- Physical layer issues (bad cables, duplex mismatch, noisy wireless).
- Overloaded network devices or CPU-limited servers.
- Intermittent wireless interference.
- Faulty NICs or drivers.
- Misconfigured duplex/auto-negotiation on switches/hosts.
Fixes:
- Inspect physical connections: replace cables, test different ports, check SFP modules.
- Verify interface statistics (errors, drops, collisions) on switches and hosts.
- For wired links, ensure proper duplex/auto-negotiation settings and consistent configurations at both ends.
- Test from a wired client if using wireless to rule out RF interference.
- Update NIC drivers/firmware and ensure servers aren’t CPU-bound during tests.
- Run a longer-duration test to see if loss correlates with time-of-day or specific events.
4. Tests show correct speed but application still slow
Symptoms:
- TPTEST reports high throughput and low latency, but the actual application remains sluggish.
- Web pages, APIs, or file transfers using the application are slow despite good test metrics.
Why it happens:
- Application-layer problems (inefficient code, synchronous blocking, database slowness).
- Protocol or application-level throttling, authentication, or rate limiting.
- Slow DNS resolution or upstream service dependencies.
- Connection setup overhead (TLS handshakes, redirects) not captured by bulk throughput tests.
Fixes:
- Profile the application: check server logs, database query performance, and thread utilization.
- Test application flows end-to-end (use synthetic transactions or real user traces) rather than raw TCP throughput.
- Check DNS lookups and caching; measure DNS resolution times separately.
- Inspect TLS handshake times and certificate validation; consider TLS session resumption.
- Verify that the application isn’t serializing requests or waiting on external APIs.
5. Inconsistent or non-reproducible results
Symptoms:
- Re-running TPTEST shortly after yields widely different results.
- Results vary by time of day, client location, or test parameters.
Why it happens:
- Dynamic routing changes or CDN edge variability.
- Transient congestion on parts of the network or ISP-level shaping.
- Test environment differences (different client hardware, NIC offload settings).
- TPTEST configuration differences (single vs multiple streams, buffer sizes).
Fixes:
- Standardize test parameters — use the same number of streams, buffer sizes, and test duration.
- Run multiple tests and use median values instead of single runs.
- Test from multiple client locations to identify geographic or path-based variability.
- Disable NIC offloads (checksum offload, GRO, LRO) for consistent measurements when needed.
- Coordinate with your ISP or network provider to check for routing instabilities.
6. Timeouts or long connection setup delays
Symptoms:
- TPTEST spends long time establishing connections, or times out waiting.
- Large delays shown during TCP three-way handshake in packet captures.
Why it happens:
- Reverse DNS or ident lookups on the server delaying accept.
- High server load causing slow accept() processing.
- Intermediary devices performing deep inspection or TLS termination causing delays.
- Asymmetric routing causing ACK path problems.
Fixes:
- Disable reverse DNS / ident lookups in server services if enabled.
- Ensure accept queue sizes on servers are sufficient and server processes aren’t starved (increase backlog).
- Offload TLS termination properly or ensure the test uses raw TCP if TLS isn’t required.
- Capture packets on both ends to verify symmetric routing and confirm handshake timing.
7. Security/permission errors during testing
Symptoms:
- TPTEST cannot bind to a privileged port or lacks permissions to open raw sockets.
- Errors about insufficient privileges or capability denied.
Why it happens:
- Running without required privileges (binding to <1024 or using raw sockets).
- SELinux/AppArmor or OS-level policies block network operations.
- Missing capabilities on containers (e.g., CAP_NET_RAW).
Fixes:
- Run TPTEST with appropriate privileges or choose non-privileged ports.
- For containers, add needed capabilities (capabilities or NET_RAW) or run with elevated network permissions.
- Check and adjust SELinux/AppArmor policies or add exceptions for the testing tool.
8. False positives from monitoring or alerts
Symptoms:
- Monitoring systems flag TPTEST failures that don’t reflect real user impact.
- Alerts triggered by transient or expected deviations.
Why it happens:
- Thresholds set too tightly or not aligned with real-world behavior.
- Monitoring from a single location that doesn’t represent global users.
- Tests too short or scheduled during maintenance windows.
Fixes:
- Tune alert thresholds based on historical baselines and acceptable error budgets.
- Run multi-location tests or use synthetic transactions that mimic real user behavior.
- Increase test duration or run a series of tests before triggering alerts.
- Annotate maintenance windows and exclude them from alerting.
9. Incorrect test configuration or misuse
Symptoms:
- Results confusing or irrelevant (e.g., testing wrong port, protocol, or target).
- Users misinterpret what TPTEST measures vs what users experience.
Why it happens:
- Misunderstanding of TPTEST’s scope (network-layer vs application-layer).
- Default settings not suitable for the environment (single stream vs parallel).
- Wrong units interpreted (Mbps vs MB/s).
Fixes:
- Read the tool’s documentation and confirm which layer and metrics it measures.
- Use appropriate parameters: number of streams, test duration, buffer sizes, and protocol selection.
- Convert units correctly and present results in both Mbps and MB/s when sharing.
- Add contextual notes with results indicating what was tested (endpoints, ports, times).
10. Debugging methodology & useful commands
Best practices:
- Reproduce the issue with controlled, repeatable tests.
- Collect logs from both client and server, and note timestamps.
- Capture packet traces (tcpdump/wireshark) on both ends when possible.
- Compare results with alternative tools (iperf3, netcat, curl, traceroute).
Useful commands:
- Check listening ports: sudo ss -ltnp
- Interface stats: ip -s link; ethtool -S eth0
- TCP info: sudo ss -tin state established
- Packet capture: sudo tcpdump -i any host
and port -w capture.pcap - Path MTU: tracepath
or ping -M do -s - Disk and CPU: top, iostat, vmstat
Quick checklist before opening a support ticket
- Confirm target address/port and service are correct and listening.
- Reproduce test at different times and from multiple clients.
- Capture a short packet trace and include timestamps.
- Provide TPTEST command-line, version, and exact output.
- Include server-side logs and interface counters if possible.
Troubleshooting TPTEST issues is about isolating layers — physical, link, network, transport, and application — then verifying configuration and environmental factors. Following a consistent methodology and collecting packet captures and logs will usually reveal whether the problem is network-related or an application/configuration issue.
Leave a Reply