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A question about the command tracert and pathping for the Networking Fundamentals Exam 98-366

    Question

  • I am using the book from Wiley, Microsoft Official Academic Course, Networking Fundamentals, Exam 98-366.

    tracert

    On page 110, it states the following: "The time to live (TTL) for the pings increases with each hop to another network". I can relate to this because the pings sent from my computer is moving towards the target through different connections. Now, I always thought of this as the hops increase, but the TTL decreases. Each router that receives the packet subtracts at least 1 from the count and if the count remains greater than 0, the router forwards the packet, otherwise it discards it. Does the tracert do this differently, starting with one hop and one TTL and then 2 hops and 2 TTL's?

    pathping

    I am doing a pathping -n to google.com. [The book does the same on page 111-112]. I was just curious to the the % lost and sent. As it goes through 4 of the connections at my ISP, towards google, 3 of the connections at my ISP is showing 100% loss. If the loss is 100%, how is it possible to continue towards google? I did a ping at google.com which came out with 4 packets sent and received. I did a ping at one of the connections that showed 100% loss in pathping, and it showed 4 packets lost. Now, I know that these pings done afterwards might have taken a different route towards google, but the question remains. If the pathping shows 100% loss, how can it then continue to travel?

    Hmm?

    Trying to attach a jpg image, showing this, but the site tells me: [Body text cannot contain images or links until we are able to verify your account] -which has been active since 2013. I guess that means use your imagination.

    [Attaching a jpg image of the pathping -n command with the 2 pings.In the jpg image, there is a column for "This Node/Link" in the computing statistics, which could indicate a try number 2 at the connections which showed a loss?]

    Saturday, July 16, 2016 6:01 PM

Answers

  • As to the tracert, it begins by sending a packet with the TTL set to 1, so the first router replies back that the packet expired in transit and by looking at who replied you know who the first router is. Then the TTL is increased to 2, and the second router replies, and so on. Therefore, tracert works by increasing progressively the initial TTL of each packet (and as the packet flows through the network, its TTL decreases on each hop).

    Both tracert or pathping might fail to receive an error back from one of the nodes, when such node is configured to not send it. In the case of pathping, this is reported as a 100% packet loss. This doesn't mean that the node will not forward the packet to the next node, it only means that it doesn't report back to you when the packet expired at that node. The "100% packet loss" is misleading; the packets are not actually lost at that node, what is lost is the response informing you that a packet expired in transit.

    To attach images, you have to be "validated" meaning that you need to have some points in the forum, which you gain when you respond to a message and your response is marked as an answer, or is voted as helpful.

    Sunday, July 17, 2016 3:24 PM