Time To Live (TTL)

Time to Live (TTL) is a computer networking term that refers to the lifespan of data on the network. TTL determines how long a packet stays alive before the router discards it. Setting the right TTL value is crucial for optimizing network performance and reliability.

What is Time to Live?

TTL refers to a value set in the header of an Internet Protocol (IP) packet that tells network devices the maximum number of router hops the packet can make before it is discarded.

The TTL value is a counter that is decremented by 1 every time the packet passes through a router. Once the TTL reaches 0, the router no longer forwards the packet and then drops it.

TTL helps prevent packets from endlessly circulating the network or clogging the network. Without TTL, packets could loop infinitely between routers because of routing misconfigurations or a routing loop.

How Does TTL Work?

When a device originates an IP packet, it assigns an initial TTL value. The common default TTL values are:

• 64 – Linux/MAC OSX systems
• 128 – Windows systems
• 255 – Network devices like routers

As this packet travels from the source to the destination, each router along the path decrements the TTL by 1 before forwarding it to the next hop. Once the TTL reaches 0, the router drops the packet and may send an Internet Control Message Protocol (ICMP) message back to the source that the packet’s time is exceeded.

For example, if the initial TTL is 64 and it passes through 6 routers, the TTL is decremented as follows:

• 64 (source)
• 63 (router 1)
• 62 (router 2)
• 61 (router 3)

When the packet’s TTL reaches 0, the router drops the packet. It may also send an ICMP time exceeded message to the source indicating the packet TTL expired in transit.

How TTL Works in Different Environments

TTL in CDNs

Content Delivery Networks (CDNs) use TTL to control how long content is cached. For example, a TTL of 3600 seconds would indicate the content should be cached for 1 hour before revalidating. Shorter TTLs make content expire faster.

CDNs customize TTL based on the ability to cache the content. Dynamic content has lower TTL while static assets use higher TTLs. Using appropriate cache lifetime improves performance.

TTL in Computer Networks

The TTL field limits packet lifetime in computer networks and IP packets to prevent routing issues. The TTL should be set high enough to allow packets to reach their destination while avoiding too many hops.

Network engineers optimize TTL based on topology, reliability requirements, and application types. Interactive apps need low TTL, while bulk transfers can use higher TTL.

TTL in DNS

DNS records have a TTL that controls caching duration. Typical DNS TTLs range from a few minutes for frequently changing records to days for stable records.

Using low TTLs ensures changes propagate faster but leads to more frequent queries. Higher TTL reduces DNS traffic but adds latency after changes. Tuning the DNS TTL improves resolution performance.

TTL Best Practices

Here are some TTL best practices for different environments:

CDNs

• Set lower TTL for dynamic content like API data
• Use longer TTL (days) for static assets like images, CSS
• Adjust TTL based on actual content change frequency
• Leverage extensions like stale-while-revalidate for cache resilience

Computer Networks

• Default OS TTL values (64 or 128) are reasonable starting points
• For the public Internet, 64-128 is commonly used
• Consider topology when setting TTL – set it higher for larger networks
• Verify end-to-end TTL behavior with trace-routes
• Change TTL if issues like excessive ICMP timeouts occur

DNS

• Use shorter TTL for frequently updated records and longer for stable records
• Match TTL to the expected change frequency of each record
• Use 60s or 300s TTL for most internal DNS records
• For public DNS, higher values like 86400 (1 day) are common
• Adjust TTL gradually and monitor for issues

TTL Use Cases and Examples

The optimal TTL value depends on the specific use case and requirements. Applications that involve rapidly changing data need lower TTLs to ensure low latency updates. Stable content can use higher TTLs to improve caching efficiency. Monitoring actual change patterns and fine-tuning based on experience helps select the best TTL for each scenario.

TTL Values

The operating system sets the initial TTL value or application creating the IP packet. Here are some common default TTL values:

• Linux/MAC OS – 64
• Windows – 128
• Cisco Routers – 255
• DNS – depends on the DNS resolver (can range from 128 to 86400)

Setting higher TTL values allows packets to traverse more hops before it’s discarded. However, higher values also mean more time is needed before detecting errors. Lower TTL values help detect routing issues faster but reduce the maximum distance packets can travel.

Administrators can configure the default OS TTL based on their network requirements. Applications may also set custom TTL values for their traffic. For example, DNS typically uses low TTL values for cache entries to promote faster expiration.

Why is TTL Important?

TTL serves several crucial functions:

Prevents Routing Loops

TTL stops packets from continuously looping in the network. Routing loops can occur if routers have incorrect routes, or if a temporary instability causes a loop. TTL ensures packets are discarded after maximum hops to avoid congestion.

Improves Network Reliability

Dropping expired TTL packets signals potential issues to the source. The source can then retransmit using an alternate path or avoid blackholes. This improves overall transmission reliability.

Limits Broadcast Traffic

A high TTL allows packets to be broadcasted network-wide. Setting lower TTLs for broadcasts improves security and prevents resource exhaustion from excessive broadcasts.

Optimizes Network Performance

An appropriate TTL value should be set high enough to allow packets to reach their destination while avoiding overly long or short timeouts. This provides an optimal balance between reliability and speed.

Facilitates Geolocation

Decrementing TTL values can indicate the approximate number of hops a packet has traversed. This helps trace the origin of traffic and geolocate hosts.

Configuring TTL

The default TTL value can be configured on an OS and network device:

Linux

Use the sysctl command to view and set the default TTL:

# View current TTL

sysctl net.ipv4.ip_default_ttl

 

# Set default TTL to 64

sysctl -w net.ipv4.ip_default_ttl=64

Windows

The registry key that controls the default TTL is:

`HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\DefaultTTL`

Set this to the desired decimal TTL value like **128**. A reboot may be required for changes to take effect.

Cisco IOS

Use the `ip ttl-expires` command to configure the TTL on Cisco routers:

router(config)# ip ttl-expires 30

This sets the TTL expiry value to 30 hops. The default is 64.

Application Layer TTL

Some application layer protocols like DNS and MQTT allow configuring per-message TTL values. Consult the application’s documentation to adjust TTL based on use-case.

Checking TTL

There are several ways to inspect the TTL value of packets:

• Trace-route – Shows TTL decrement along the path
• Wireshark – Inspect TTL in packet capture
• Firewall rules – Match on TTL value
• Middleware – APIs for adjusting TTL

Checking end-to-end TTL helps diagnose network issues and verifies configuration.

Conclusion

TTL is a fundamental network parameter that controls the lifespan of packets. It prevents congestion and improves reliability by eliminating infinite packet loops. Setting the right default TTL value and leveraging it effectively allows optimizing network performance. As networks grow larger and more complex, configuring TTL becomes increasingly important for administrators.