What is TTL (Time to Live) in Networking?

Introduction

When users visit a website, everyone wants them to receive the most up-to-date information. However, they also need their site to load quickly. Otherwise, they risk a bounce, where the user gives up and leaves their site quickly. Fortunately, they can find a balance by adjusting their website’s Time to Live (TTL). But what is TTL in networking? In this blog, we will explain the TTL in detail and how its works. We will also discuss the benefits and drawbacks associated with it.

Before getting into more details, let’s first understand what TTL really is.

What is TTL in Networking?

TTL, or time to live, is a value included in each Internet Protocol data packet. It informs the network router if the packet should be kept in the network for an excessive amount of time or deleted. Typically, data packets may not be transferred within the allotted time to the intended location. The system default value, which is an 8-bit field of binary digits in the packet header, sets the TTL value. TTL establishes a specific time limit for broadcasting the packet header in seconds. The package would be thrown away when the allotted time had passed.

Every time a packet moves through a router, a counter called the TTL value decrements by 1. When the TTL hits 0, the router identifies the discarded packets and sends an ICMP message back to the source host.

TTL assists in keeping packets from circling the network repeatedly or clogging it. Without TTL, routing errors or a routing loop might cause packets to loop indefinitely between routers.

Time To Live is used in various environments. These are:

• IP packets
• DNS queries
• CDNs (Content delivery networks)

Now we have a basic understanding of TTL, let’s move on to the working of TTL for different use cases with examples.]]

How does Time to Live Works?

Let’s see how time to live works in various environments –

Working of TTL in Computer Networks/IP

Below, we have explained the working of TTL in computer networks/IP with the help of an example. The TTL mechanism can be seen in action as the packet passes through three routers, i.e., Router A, Router B, and Router C. The TTL value drops by one at each step until it reaches zero.

The process can be explained as:

• The source device has generated a packet with a TTL value of 255.
• The source device (Host I) sends the packet to the destination device (Host II).
• Upon receiving the packet, Router A reduces the TTL value to 254.
• After receiving the packet once again, Router B reduces its TTL value to 253.
• The TTL value is reduced to 252 by Router C as the packet continues its route.
• The packet eventually gets to the recipient device, which accepts it.

We now fully understand how TTL works in computer networks/IP; let’s move on to the working of Time To Live in DNS.

Working of Time to Live in DNS

The Time to Live (TTL) parameter controls how long a resolver will keep a DNS record in its cache. The domain owner or administrator may set it, which is specified in seconds. A shorter TTL indicates more frequent updates, which is advantageous in dynamic or changing contexts. Longer caching of the record is due to a greater TTL value, which can reduce the load on the DNS server and enhance speed. A longer propagation time for changes to the record is another drawback of a higher TTL, which might be problematic if the record is updated immediately.

Let’s understand working with an example now.

• When the client wants to access the website “pynetlabs.com“, it sends a DNS query to find its IP address.
• The website “pynetlabs.com” has an A record in its DNS record that specifies its IP address as 192.168.1.2 and a TTL of 900 seconds (15 minutes.)
• The DNS resolver keeps a copy of the DNS record for “pynetlabs.com” and its IP address in its cache. It checks its cache and finds the record for “pynetlabs.com”.
• If the TTL is still valid, the resolver gives the client the IP address 192.168.1.2 for the website for the requested domain.
• If the TTL has expired, the resolver asks the authoritative DNS server for a new record.

Let’s move on to how TTL functions in CDN (Content Delivery Networks).

Working of TTL in CDNs

A CDN determines if it has a cached copy of the content when a user requests it from it. If so, it gives the user access to it. Without that, it downloads the data from the original server and caches it for further requests. The TTL value instructs the CDN how long to hold the content in its cache before it must be reloaded from the origin server since it has expired. Less traffic to the origin server and less content freshness are two effects of a higher TTL value.

More traffic to the origin server and more recent material are both indicators of a shorter TTL value. In order to balance the performance and accuracy of the content supplied by a CDN, selecting an ideal TTL value is crucial.

Let’s understand with the help of an example.

• When a user requests specific content, the CDN’s edge server looks for it in its cache and checks if the TTL is still valid.
• The edge server can serve the content from its cache if it is there and the TTL has not expired. This is a “cache hit”. If the content is missing or the TTL has expired, the edge server has to get the content from the origin server. This is a “cache miss”.
• The edge server sends the content to the user, either from its cache (cache hit) or after getting it from the origin server (cache miss).
• The edge server may refresh the content from the origin server before the TTL expires to keep it fresh and update its cache.

We now have a detailed understanding of the working of TTL in different environments.

Why is TTL used?

TTL is used for many purposes, and some of the purposes are given below:

• TTL is used to prevent packets from being transmitted indefinitely in a network.
• TTL is used to avoid routing loops and network congestion.
• TTL is used to specify the maximum time a packet can remain on the network.
• If the TTL touches zero, the packet is rejected to prevent network problems.

In addition to the following purposes, network managers supervise the progress of packets being transmitted through the network by using the TTL value. They can detect errors and other performance-related issues by examining the TTL value to see how long it takes for a packet to travel from one location to another.

Application of TTL

TTL value is allowed in the network efficiency like Ping, Traceroute, and Pathping.

The ping command allows to check the reachability of the destination computer. It means that it confirms whether communication occurs between the source and destination computers or network devices. It works by broadcasting ICMP echo request messages to the target computer and then waiting for a reply.

Implementations of the ping command share two essential pieces of information:

• how many replies are returned
• how much time they require to respond

The Tracert/Traceroute command enables two devices to trace a path, which means that multiple routers in the path do not know which connection is established. Therefore, in the path of two connecting devices, it will display the router name or IP address.

In IP multicast, TTL can handle packet forwarding scope or range.

• 0 is restricted to a similar host
• 1 is restricted to a similar subnet
• 32 is restricted to a similar site
• 64 is restricted to a similar region
• 128 is restricted to a similar continent
• 255 is unrestricted

TTL in Networking is also deployed in caching for Content Delivery Networks (CDNs) and allows identifying the time from the presentation of cached details until a new duplicate is downloaded from the origin server. If the period between pulls from the origin server is properly modified, a CDN can provide updated content with requests communicating back to the origin server. This progressive effect allows the CDN to efficiently provide details closer to the user while reducing the amount of bandwidth required at the origin location.

TTL is also deployed in caching for the Domain Name System (DNS) and is a numerical value (shows the time taken) used with DNS cache servers to serve DNS records before contacting the authoritative servers to obtain new duplicates.

Time to Live Values

The OS adjusts the initial TTL value or the application that created the IP packet. Below are some common default TTL values:

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

Adjusting higher TTL values enables the packet to traverse more hops before being rejected. However, higher values also indicate that more time is needed before issues can be identified. Lower TTL values allow routing errors to be detected more quickly but reduce the maximum distance a packet can travel.

According to the requirements of administrators, they can configure default OS TTL, and applications can also adjust custom TTL values for their traffic. For instance, DNS generally allows low TTL values for cache entries to increase quick expiration.

What is TTL Exceeded?

TTL exceeded refers to IP packets delivering web HTTP traffic over TCP that have travelled multiple network hops. Through this, each router reduces the Time To Live field of the shared IP on the network to 0. After that, the router rejects the IP packet and sends an ICMP packet to the sharing host via TTL, exceeding the error code.

Let’s now understand the benefits and drawbacks of TTL.

Benefits of Time to Live in Computer Network

Time to live offers various benefits. Some of these are:

• Avoids Looping Routes – TTL prevents packets from cycling in the network indefinitely. Routing loops may happen when routers use the wrong routes or when a loop is temporarily created by instability. TTL ensures packets are dropped after a maximum number of hops to prevent congestion. Congestion is a very common Network Issue.
• Boosts the Reliability of the Network – Dropping expired TTL packets alerts the source to possible problems. When it happens, the source may retransmit using a different route to avoid black holes. Overall transmission dependability is increased as a result.
• Limits Broadcast Traffic – A high TTL enables network-wide broadcasting of packets. Lowering broadcast TTLs enhances security and reduces resource depletion due to excessive broadcasting.
• Network performance is improved – In order to prevent excessively lengthy or short timeouts, an acceptable TTL value should be set high enough to enable packets to reach their destination. This offers the best compromise between consistency and speed.

Drawbacks of Time to Live in Computer Network

Apart from all the benefits TTL can provide, it has some drawbacks. Some of the drawbacks associated with TTL in computer networks are:

• Increased Latency – If the TTL value is set too long, the user experiences network latency. This is because a packet is transmitted multiple times by the router before it reaches the intended destination.
• Security Risks – Another drawback of TTL is when its value is not set correctly, it can lead to security risks. Long TTL values can lead to data being stored in the cache for a long time and hence can be affected by cyberattacks. Similarly, short TTL values can lead to DoS attacks.

These are the advantages and disadvantages of Time to Live in Computer Networks.

Frequently Asked Questions

Q1 – What is TTL in a computer network?

The time-to-live (TTL) value indicates how long a packet of data should remain on a computer or network before being deleted.

Q2 – What is TTL 64 and TTL 128?

TTL 64 is the default value for Linux/Unix, whereas TTL 128 is the default value for Windows.

Q3 – What does TTL 3600 mean?

A DNS record will be cached for 3600 seconds, or one hour, according to the TTL value, which is represented in seconds. After that time, it will need to be refreshed.

Q4 – What is TTL 64 in Ping?

The maximum number of hops a packet may travel before being dropped is 64.

Conclusion

Time to Live (TTL) is a crucial component of computer networking that promotes efficiency and dependability. TTL helps modern networks run smoothly by eliminating routing loops, regulating packet lifetimes, and managing network traffic. In this blog, we have explained what is TTL in networking, its different environments and its benefits and drawbacks.