For this, it sends a packet with a TTL value of 2. The source has discovered the first router! So let’s go to the next one.The router drops the packet due to this TTL value and sends an ICMP error message back to the source.
The packet hits the first router in the network path.The source sends a first IP packet with a TTL header field value of 1.When performing a traceroute, this is what happens:
So traceroute relies on the fact that routers will send this ICMP error message back to the source in case of a TTL value reaching 1. It also informs the source about this by sending a specific ICMP error message back to the source (ICMP error message “TTL Exceeded In Transit”). So it drops the packet (to avoid potential looping problem). When a packet with a TTL value of 1 hits a router, this latter cannot route the packet anymore (it would mean a TTL=0). Each time the packet hits a router and must be routed, the TTL value decreases by 1. When a host sends a packet on a network, its initial TTL value is between generally 32 and 255, depending on the operating system used. This field is mainly used to avoid loops in networks, where packets could be routed indefinitely in a loop under certain circumstances. The whole traceroute principle is based on the TTL (Time To Live) field of the IP packet header. The main principle of traceroute is shown hereunder: It can also detect any network path variation that may occur through BGP peering/routing configuration changes. It is extremely useful to measure the network quality (congestions, …).
In this article, we explain the main concepts of Traceroute and how it can be used to measure network latency and packet loss.
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