Module 1: Foundations

Network Diagnostics — Traceroute, MTR, and Path Analysis

Learn about network diagnostics — traceroute, mtr, and path analysis

Duration: 7 minutes
Level: Advanced
Lesson: 53

Why Path Analysis Matters for VoIP

In Lesson 50, you learned to isolate which call leg has the quality problem. Once you know the leg, you need to find where on the network path the degradation occurs. That's where traceroute and MTR come in.

VoIP is uniquely sensitive to network path issues. A web page tolerates 200ms of latency and 2% packet loss. A voice call becomes unusable under those same conditions. Path analysis tools help you pinpoint the exact hop where things go wrong.

Traceroute Fundamentals

How TTL-Based Tracing Works

Traceroute exploits the Time-To-Live (TTL) field in the IP header:

  1. Send a packet with TTL=1. The first router decrements TTL to 0, drops the packet, and sends back an ICMP "Time Exceeded" message.
  2. Send a packet with TTL=2. The second router sends back the ICMP reply.
  3. Repeat, incrementing TTL, until you reach the destination.
# Basic traceroute (ICMP on macOS/Windows, UDP on Linux)
traceroute 198.51.100.25

# Force ICMP mode (useful when UDP is filtered)
traceroute -I 198.51.100.25

# Use TCP SYN on port 5060 (SIP) — gets through most firewalls
traceroute -T -p 5060 198.51.100.25

Example output:

traceroute to 198.51.100.25, 30 hops max, 60 byte packets
 1  gw.telnyx.chi (10.0.0.1)      0.5 ms   0.4 ms   0.4 ms
 2  core-rtr1.chi (203.0.113.1)   1.2 ms   1.1 ms   1.2 ms
 3  transit-peer.example.net      5.8 ms   5.7 ms   5.9 ms
 4  * * *
 5  edge-rtr.carrier.co.uk       78.2 ms  78.1 ms  78.3 ms
 6  198.51.100.25                 79.0 ms  78.8 ms  79.1 ms

Reading Traceroute Output

Each line shows three RTT measurements. Look for:

  • Sudden RTT jumps — A jump from 5ms to 78ms between hops 3 and 5 likely indicates an ocean crossing (expected) or congestion (investigate).
  • Stars (* * *) — The router at hop 4 didn't respond to ICMP. This is NOT necessarily a problem (see below).
  • Consistent high RTT — If every hop after hop 3 shows high latency, the issue is at or before hop 3.

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  • ✅ Detailed technical explanations
  • ✅ Advanced configuration examples
  • ✅ Hands-on lab exercises
  • ✅ Real-world troubleshooting scenarios
  • ✅ Practice quizzes and assessments

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