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Today when I was doing tracert, in the 6th Hop I get RTO but my next hop seems to be reachable. How come it works? 

6         *       *       *           Request Timed Out
7         58ms    58ms    61ms        63.149.172.54

As per my knowledge:

Tracert will initiate ICMP echo request for 1st hop with TTL=1 with 3 packets.

When it reaches a router(1), the TTL becomes 0 and Time Exceeded Message is sent to the Source by the router(1).

Now source will set TTL=2 and sends it again. Now router(1) will decrement TTL to 1 and sends it to the next router(2).

Router(2) will send Time Exceeded Message to the source as TTL expires. Likewise Source increments TTL value and sends it towards destination.

If my 6th Hop gets RTO, the router is down or firewall blocking and hence it won't send ICMP reply to the source. Then, how come the source knows that next router is alive, without getting ICMP reply from the previous router, how source finds the next hop or router?

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2 Answers 2

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All traceroute does is send with increasing TTL. Your sixth hop could have ICMP messages for that hop disabled, or it could be too busy to reply in time (ICMP is a low priority for a router; the router wants to route first, and it will get around to ICMP when it has time). That doesn't mean it will not pass on ICMP from a different source, since the ICMP traffic from a different source is simply more data to your sixth hop.

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  • Correct me if I'm wrong. Since Router on the 6th hop is not responding to ICMP messages, Source will choose to send IMCP message to the router on different path. Am I right ?
    – KingsNeverDie
    Sep 15, 2017 at 2:27
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    No. The source has no control over the route taken by packets. That is completely up to the routers. If the sixth router doesn't respond in time, the source will add one to the TTL and try again. This time, the sixth hop will simply route the packet on as normal since it didn't expire at the sixth hop.
    – Ron Maupin ♦
    Sep 15, 2017 at 2:34
  • It's possible the six hop router is NOT configured to drop ICMP's, but ICMP pong process may have the lowest priority (it's usual practice). The only difference is the packet loss is close, but not equal to 100%.
    – user36844
    Sep 15, 2017 at 5:22
  • @Akina, that depends on how busy the router is. If it does eventually respond, but it is after the host has timed out, then you will get 100% loss.
    – Ron Maupin ♦
    Sep 15, 2017 at 5:23
  • @RonMaupin That's right. But when router is not overloaded with high-priority processes (night traffic minimum, for example), the pongs may restore.
    – user36844
    Sep 15, 2017 at 5:41
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As per my understanding, 6th hop will be used as layer 2 device and for firewall block use below test.

ICMP packets are disabled at firewall before 6 hop. from source, Operator couldn't check with ICMP testing tool. we can you TCP ports to test. like 6th hop IP address is 10.20.1.6, you can telnet that 6th hop. If 6th hop isn't a router/switch, it may be any device. it must be using tcp port for services. telnet has 23 tcp tool no. you can replace this no. with any TCP port no. It also helps to check whether any desired tcp port blocked or open in between the path.

you use telnet tool to test as

telnet 10.30.1.6 <tcp port no.>
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