What Happens When a Router Receives a Packet With a TTL of 0?


What Happens When a Router Receives a Packet with a TTL of 0 has come up a lot when discussing networking. Although multiple answers are available, many of them are difficult to comprehend. We will attempt to simplify it as much as we can in this article.

A router MUST NOT discard a datagram just because it was received
with TTL equal to zero or one; if it is to the router and otherwise
valid, the router MUST attempt to receive it.

Let’s first go through a few fundamentals to help you understand the subject before we respond to your inquiry.

What is a router?

Those who are new to networking should read this. To begin with, you must understand what a router is. Although routers are essential to our homes’ infrastructure, many of us don’t fully comprehend them. Many individuals even mistakenly thought they were modems.

What is its usage?

A router facilitates the connection of additional devices to a modem, such as a fiber, cable, or DSL modem, allowing for Internet communication. Most routers, particularly wireless routers, include numerous network ports that simultaneously connect several devices to the Internet.

A router functions similarly to a little computer, processing incoming and outgoing data using its CPU and memory. These data may be transmitted between devices or from those devices to the Internet.

How do packets move through a router?

When a packet arrives, a router checks its routing table to see if the target address belongs to a device on one of the networks it has connected to or if a different router is required to send the request.

The following system receives a message from the router. Before the message reaches its intended destination, this action is repeated on every router that receives it.

The packet is dropped if the TTL hits 0. When this happens, the router that lost the IP packet whose TTL reached 0 sends the IP packet’s originator an Internet Control Message Protocol (ICMP) message type 11 with the code 0 (time exceeded).

When TTL reaches a router, what happens?

To prevent loops, the time-to-live (TTL) field value in the packet header is ordinarily decremented by 1 for each hop the packet travels through in the LSP. Packets are dropped, and an Internet Control Message Protocol (ICMP) error packet is delivered to the originating router if the TTL field value exceeds 0.

When a router receives an IP packet with a TTL of 1, what does it do?

The first-hop router (presumably the originating host’s default gateway) will decrement the TTL by one after receiving a packet with a TTL of one, leaving a value of zero. Due to this, the router will drop the packet and notify the source that time has passed using ICMP (hopefully).

When a packet with a time to live TTL of 1 is received, what should a router do? A receiving LSR drops a tagged packet with a TTL of 1 and sends the IP packet’s originator an ICMP message with the message header “time exceeded” (type 11, code 0). This is the same behavior a router would display when an IP packet’s TTL was about to expire.


What occurs when a router gets a packet with a TTL of 0 should hopefully be clear to you. Additionally, you browsed the parts when the value was 1. With a basic understanding of computer networking, it is relatively simple to explore these topics. Verify that your router is functioning properly and that no data packets are lost.

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