![]() ![]() Oftentimes tools like traceroute, PingPlotter, MTR are used not only to show the forward path but also to measure latency and loss to each hop. The first set of probes uses TTL 1, the second set uses TTL 2, the third set uses TTL 3 and so on until the last set of probes reach the destination.Īs we’ll see throughout this document traceroute’ s main use is to discover the forward path that traffic takes, that is the path from the host originating the traceroute command to the destination specified in the traceroute command itself. Traceroute takes advantage of this knowledge by sending out probes with the first set of probes starting with TTL 1 and each set of subsequent probes incrementing the TTL by 1. Once the TTL reaches 0 the packet is discarded at the device that notices the TTL reached 0. If a host sends a packet with a TTL of 255 at most the packet will transit 255 元 devices. Any time a packet traverses a 元 device (router, 元 switch, etc.) the TTL is decremented by one before being sent to the next device. The TTL field is eight bits which allows for a maximum value of 255 = (28-1). ![]() The TTL field is needed to prevent the packet from traversing the network forever via a routing loop. To show an example of this, if you forgot to put a stamp on the letter the postal service would send the letter back to your (source) address. This also allows the postal service to send the letter back to you if something goes wrong along the way. Typically, when you send a letter in the mail you also write down your address, analogous to a source IP address this way your friend knows where to send their response letter. The default behavior is to route traffic based on the destination IP address this is analogous to writing your friend’s address on an envelope to send a letter to them. The key to understanding traceroute is knowing the IP (Internet Protocol), specifically the TTL (Time to Live), source IP address, and destination IP address fields in the IP header and how they interact. Traceroute is used by networking professionals to discover the path network traffic takes. ![]() Unified Communications and Collaboration (UC&C)
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