683  TTL and Loop Prevention

683.1 Learning Objectives

By the end of this section, you will be able to:

• Apply TTL Concept: Understand how Time-To-Live prevents routing loops
• Trace Packet Paths: Use traceroute to discover network paths
• Compare IPv4/IPv6: Understand TTL vs Hop Limit differences

683.2 Prerequisites

• Routing Basics: Understanding router operation and forwarding decisions

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683.3 Time-To-Live (TTL): Preventing Routing Loops

683.3.1 The Problem

What if routing tables have errors or loops?

Packets could circulate endlessly through routers, never reaching destination:

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graph LR
    A[Router A<br/>Send to B]
    B[Router B<br/>Send to C]
    C[Router C<br/>Send to A]

    A -->|Packet| B
    B -->|Packet| C
    C -->|Packet| A

    style A fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style B fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style C fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff

Figure 683.1: Routing loop problem: misconfigured routing tables cause packets to circulate endlessly between routers

683.3.2 The Solution: TTL Field

Every IP packet header contains a TTL (Time-To-Live) value:

• IPv4: TTL field (8 bits, max value 255)
• IPv6: Hop Limit field (8 bits, max value 255)

How it works: 1. Source host sets initial TTL (typically 64 or 128) 2. Each router decrements TTL by 1 3. If TTL reaches 0, router drops packet 4. Router may send ICMP “Time Exceeded” error back to source

683.3.3 TTL in Action

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flowchart LR
    Source[Source<br/>TTL=64]
    R1[Router 1<br/>TTL=63]
    R2[Router 2<br/>TTL=62]
    R3[Router 3<br/>TTL=61]
    Dest[Destination<br/>TTL=60]

    Source -->|Decrement TTL| R1
    R1 -->|Decrement TTL| R2
    R2 -->|Decrement TTL| R3
    R3 -->|Decrement TTL| Dest

    style Source fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style Dest fill:#16A085,stroke:#2C3E50,stroke-width:3px,color:#fff
    style R1 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style R2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style R3 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 683.2: TTL decrement in action: each router decrements TTL by 1, preventing infinite loops

TipAlternative View: TTL as a Countdown Timer

This variant shows the same TTL concept but emphasizes how TTL prevents routing loops by visualizing a loop scenario with countdown.

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sequenceDiagram
    participant S as Source<br/>TTL=3
    participant A as Router A
    participant B as Router B
    participant C as Router C

    Note over S,C: Misconfigured loop: A->B->C->A

    S->>A: Packet TTL=3
    Note over A: TTL=3->2
    A->>B: Packet TTL=2
    Note over B: TTL=2->1
    B->>C: Packet TTL=1
    Note over C: TTL=1->0
    C--xA: TTL=0 DROPPED!

    Note over C: ICMP Time Exceeded<br/>sent to Source
    C-->>S: ICMP Error

    Note over S,C: Without TTL, packet loops forever!<br/>With TTL, max 3 hops then dropped.

Figure 683.3: TTL Loop Prevention - Shows how TTL countdown stops packets from circulating forever in misconfigured networks

Key Insight: TTL is like a “self-destruct timer” for packets. Even if routing tables have errors, packets eventually die and don’t clog the network forever.

{
  const container = document.getElementById('kc-route-ttl-1');
  if (container && typeof InlineKnowledgeCheck !== 'undefined') {
    container.innerHTML = '';
    container.appendChild(InlineKnowledgeCheck.create({
      question: "An IoT mesh network has a misconfiguration causing a routing loop between three routers: A -> B -> C -> A. A sensor sends a packet with TTL=64. How many times will the packet traverse the loop before being dropped?",
      options: [
        {text: "The packet will loop forever until manually stopped", correct: false, feedback: "This is exactly what TTL prevents! Without TTL, routing loops would cause packets to circulate endlessly. TTL ensures packets are eventually dropped, protecting the network."},
        {text: "The packet will be dropped immediately when the loop is detected", correct: false, feedback: "Routers don't detect loops by examining packet history. They simply decrement TTL and forward. The loop is only 'detected' when TTL reaches 0 and the packet is dropped."},
        {text: "The packet will traverse the loop 21 times (64 hops / 3 routers per loop)", correct: true, feedback: "Correct! Each router in the loop decrements TTL by 1. With a 3-router loop and TTL=64, the packet traverses the loop 64/3 = 21.33 times. After 64 hops, TTL reaches 0 and the router drops the packet, sending an ICMP Time Exceeded error."},
        {text: "The packet will traverse the loop 64 times (once per TTL value)", correct: false, feedback: "Each hop decrements TTL by 1, not each complete loop. With 3 routers per loop, the packet makes 64 total hops across ~21 complete loops before TTL reaches 0."}
      ],
      difficulty: "medium",
      topic: "routing"
    }));
  }
}

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683.4 IPv4 vs IPv6 Packet Headers

683.4.1 IPv4 Header (TTL Field)

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL  |Type of Service|          Total Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Identification        |Flags|      Fragment Offset    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Time to Live |    Protocol   |         Header Checksum       |  <- TTL HERE
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Source Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Destination Address                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

683.4.2 IPv6 Header (Hop Limit Field)

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class |           Flow Label                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        |  Next Header  |   Hop Limit   |  <- Hop Limit HERE
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                         Source Address                        +
|                            (128 bits)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                      Destination Address                      +
|                            (128 bits)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

683.4.3 IPv4 vs IPv6 Comparison

Feature	IPv4	IPv6
Field Name	TTL (Time-To-Live)	Hop Limit
Size	8 bits	8 bits
Max Value	255	255
Typical Initial Value	64, 128, 255	64, 255
Decrement	-1 per router	-1 per router
When 0	Drop + ICMP Time Exceeded	Drop + ICMPv6 Time Exceeded

Both work identically for loop prevention!

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683.5 Using TTL: The traceroute Tool

The traceroute command uses TTL to map network paths:

traceroute google.com

How it works: 1. Send packet with TTL=1 - First router drops it, returns ICMP error (identifies router 1) 2. Send packet with TTL=2 - Second router drops it, returns ICMP error (identifies router 2) 3. Continue increasing TTL until destination reached

Output:

1  192.168.1.1 (Gateway)         1.234 ms
2  10.0.0.1 (ISP Router)         5.678 ms
3  203.0.113.1 (Regional Router) 12.345 ms
...
15 142.250.185.46 (google.com)   25.678 ms

Shows every router along the path!

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683.6 Quiz: TTL in Multi-Hop Networks

NoteQuiz: TTL Behavior Across Network Paths

Question: A LoRaWAN gateway forwards sensor data through 8 routers to reach a cloud server. The packet starts with TTL=64. On the return path, the cloud server’s response passes through 10 routers back to the gateway. What happens to the return packet?

Explanation: B. Each packet gets its own independent TTL. The response is a new packet created by the cloud server with a fresh TTL=64. After 10 hops, TTL = 64 - 10 = 54, so the packet is delivered successfully.

Key Insight: Each Direction is Independent

Request packet (sensor to cloud) has its own TTL. Response packet (cloud to sensor) is a completely different packet with its own fresh TTL.

Why Other Options Are Wrong:

• A: Packet would only drop if TTL reaches 0. After 10 hops from TTL=64, TTL=54, which is not 0.
• C: Misunderstands TTL as bidirectional. Each packet gets fresh TTL from its source.
• D: Correct concept (new TTL) but wrong final value. TTL decrements at each hop.

Question: Traceroute discovers the path to a destination by intentionally triggering which ICMP response from intermediate routers?

Explanation: C. Traceroute sends packets with small TTL values (1, 2, 3, …). Each router decrements the value; when it reaches 0, the router drops the packet and returns ICMP Time Exceeded, revealing that hop.

Question: In IPv6, what field performs the same role as IPv4’s TTL?

Explanation: B. IPv6 uses Hop Limit (decremented by each router) to prevent loops, analogous to IPv4’s TTL.

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683.7 IoT Implications of TTL

683.7.1 Long-Distance IoT Deployments

Example: Offshore oil rig sensors -> Cloud

Path might traverse:
- 5 hops (rig -> shore)
- 10 hops (shore -> internet backbone)
- 5 hops (backbone -> cloud provider)
Total: 20 hops

With TTL=64: Safe margin of 44 hops remaining
With TTL=32: Still safe (12 hops margin)

683.7.2 Mesh Networks

Example: Smart city sensors in mesh topology

Sensor -> 3 mesh hops -> Gateway -> 10 internet hops -> Cloud
Total: 13 hops

TTL concern: Mesh adds extra hops
Solution: Initial TTL=64 provides ample margin (51 hops remaining)

683.7.3 Checking TTL in Practice

# Linux: Check TTL of received ping
$ ping -c 1 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=117 time=10.2 ms
                                       ^^^
                                       TTL remaining

# Calculation:
# Google's server sent with TTL ~127 or 128
# We received with TTL=117
# Hops: 128 - 117 = 11 hops from Google to us

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683.8 Summary

• TTL (Time-To-Live) prevents packets from looping forever in misconfigured networks
• Each router decrements TTL by 1 when forwarding
• When TTL reaches 0, the packet is dropped and ICMP error is sent
• traceroute uses controlled TTL values to discover network paths
• IPv4 uses TTL, IPv6 uses Hop Limit - both work identically
• IoT networks: Plan for mesh hops by setting appropriate initial TTL values

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683.9 What’s Next

Continue to Routing Tables to understand how routers store and look up route information, and the different types of routes (connected, static, dynamic).