Troubleshooting Network Latency with tcprouteNetwork latency — the delay between a client sending a request and receiving a response — can degrade user experience, slow applications, and complicate operations. tcproute is a useful diagnostic tool that helps network engineers identify latency sources by tracing the path TCP packets take to a destination. This article explains how tcproute works, when to use it, how to interpret its output, common latency causes it can reveal, troubleshooting workflows, and practical examples.
What is tcproute?
tcproute is a traceroute-like utility that traces the network path to a destination using TCP packets instead of ICMP or UDP. Because many firewalls and routers handle TCP traffic differently (and often allow TCP to common ports such as 80 or 443), tcproute can succeed where traditional traceroute may be blocked or filtered. It reveals the sequence of IP hops between your host and the destination while also providing round-trip time (RTT) measurements for each hop.
Key fact: tcproute sends TCP probes with progressively higher TTL values to discover each hop along the path and reports per-hop RTTs.
When to use tcproute
- When classic traceroute (ICMP/UDP) results are incomplete or blocked.
- When you suspect selective filtering that affects non-TCP probes.
- When diagnosing latency to servers that expect TCP connections (web servers, application servers).
- When you need to test connectivity using the same transport protocol and port as the service in question (e.g., TCP/443 for HTTPS).
How tcproute works (brief technical overview)
tcproute sends TCP SYN packets to the target port with increasing Time-To-Live (TTL) values starting at 1. Each intermediate router that decrements the TTL to zero should return an ICMP Time Exceeded message, revealing its IP address and allowing tcproute to record the hop. When a probe reaches the destination, the host typically responds with either a SYN/ACK (if the port is open) or a RST (if closed), indicating the trace is complete. Measuring the time between sending the probe and receiving the response yields the RTT for that hop.
Installing tcproute
tcproute may not be installed by default on all systems. On many Linux distributions you can install it from package repositories; on macOS or BSD it may be available via ports or package managers.
Example (Debian/Ubuntu):
sudo apt update sudo apt install tcproute Example (macOS with Homebrew, if available as a formula):
brew install tcproute If tcproute isn’t packaged for your platform, you can compile from source following the project’s README.
Running tcproute — common options
Typical basic usage:
tcproute <destination> <port> Useful options (may vary by implementation):
- Specify destination port (e.g., 80, 443) to match the service.
- Increase probe timeout and number of retries to account for transient packet loss.
- Adjust maximum TTL to explore longer paths.
Always run tcproute with appropriate privileges if required (raw socket access may need root).
Interpreting tcproute output
A tcproute output typically lists hop numbers, IP addresses (and optionally hostnames), and RTTs. Example simplified output:
- 10.0.0.1 1.23 ms
- 192.0.2.1 5.12 ms
- 198.51.100.5 45.67 ms
- destination.example.com (203.0.113.10) 46.01 ms [SYN/ACK]
How to read it:
- Increasing RTTs along the path are normal; spikes can indicate where latency accumulates.
- Large RTT jump between two adjacent hops suggests a problematic segment.
- If an intermediate hop shows high RTT but subsequent hops return to lower RTTs, the device may deprioritize TTL-exceeded replies or rate-limit ICMP — not necessarily the cause of actual path latency.
- Frequent timeouts (*) at a hop mean probes got no response; continuing hops may still display responses if the device silently drops ICMP but forwards packets.
Important: TCP-based probes often reveal a more accurate picture for TCP services but still rely on intermediate ICMP responses for TTL-exceeded messages — those can be treated differently by network equipment.
Common latency causes tcproute can help identify
- Congested link between two hops (high RTT jump).
- Routing asymmetry (different return path causing unexpected latency not visible on forward path).
- Firewall or router deprioritization/filtering of probe replies (intermediate hop shows high RTT or timeouts but overall service is fine).
- Long-distance hops (expected higher RTT due to physical distance).
- Misconfigured MTU or fragmentation issues (may cause retransmissions visible in application-level performance rather than tcproute traces).
- Middlebox inspection or traffic shaping that treats probe TTL-exceeded messages differently.
Troubleshooting workflow using tcproute
- Confirm the problem: reproduce latency with application-level tests (curl, ping, or application logs).
- Run tcproute to the service port (e.g., 443 for HTTPS) from the affected client location.
- Note hop where RTT jumps significantly or where timeouts begin.
- Cross-check with:
- traceroute (ICMP/UDP) to compare behavior.
- mtr for long-term jitter and packet loss statistics.
- ping to suspect hops if they respond to ICMP.
- If the problematic hop is within your administrative domain, examine interface counters, QoS, and device CPU/load.
- If outside your domain, contact upstream provider or CDN with hop details, timestamps, and sample traces.
- Repeat tests at different times to identify intermittent problems and correlate with scheduled tasks/backschedules.
Practical examples
Example 1 — Diagnosing a big RTT jump:
- Run tcproute to example.com:443. If hops 4→5 show RTT from 10 ms to 120 ms, that link likely has congestion or queuing. Check interface utilization and queue stats on hop 4 if you control it.
Example 2 — ICMP rate-limiting confusing results:
- If hop 3 shows * or very high RTT but hops 4+ show normal RTT, the device at hop 3 likely rate-limits TTL-exceeded messages. Verify by testing the actual service (TCP connect). If TCP connect latency is low, you can treat the intermediate hop’s high RTT as misleading.
Limitations and gotchas
- Some routers/firewalls block or rate-limit TTL-exceeded messages; tcproute may show incomplete paths or misleading high RTTs.
- Asymmetric routing may hide return-path latency. tcproute measures forward-path RTT to the point where TTL expires or the destination responds — it does not directly measure return-path routing differences.
- Middleboxes that terminate TCP sessions or use proxying can change responses; use service-appropriate ports.
- Root/privileged access may be required to send raw TCP packets on some platforms.
Additional tools to use with tcproute
- traceroute (ICMP/UDP) — compare behaviors.
- mtr — combined traceroute+ping for ongoing statistics.
- tcpdump/wireshark — capture packets to confirm probe behavior and retransmissions.
- ping — quick reachability and basic RTT checks.
- netstat/ss, application logs — check server-side connection handling.
Summary
tcproute is a valuable addition to your networking toolbox when diagnosing latency to TCP services, especially where ICMP/UDP probes are blocked or filtered. Use it alongside traceroute, mtr, and packet captures, interpret hop RTTs carefully (watch for rate-limiting and asymmetry), and gather timestamps and repeated traces when escalating issues to upstream providers.
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