Traceroute and Request time out

Answered Question
Sep 24th, 2009

Why do i receive request time out in the middle of a traceroute.. what does that means

I have this problem too.
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Correct Answer by Richard Burts about 7 years 2 months ago

Meenakshi

I believe that a different explanation is more likely the cause of your issue. If your source IP address is not permitted at certain hops as suggested by Jong, then how does the traffic get past those hops without being dropped?

To explain what is happening we should start by reviewing how traceroute works. Traceroute sends packets to find the path through the network to the destination. Traceroute manipulates the TTL of the packet. The first set of probe packets has TTL set to 1. And so at the first hop the TTL expires, the first hop router discards the packet and generates an ICMP error message. This ICMP error message is what identifies the address of the first hop. Traceroute receives the ICMP error or if it does not receive the ICMP error message then Traceroute has a timeout. Then traceroute increments the TTL and sends packets with TTL of 2. The packet gets to the second hop where it expires and the second hop generates an ICMP error message. The error message identifies the second hop in the path. Traceroute continues to increment the TTL and to look for the ICMP error messages to identify the hop until the packet reaches the destination or until the traceroute gets to the max number of hops.

If some hops along the path do not send the ICMP error message then this is what generates the timeout in traceroute. Some providers configure their routers to not generate the ICMP error message because of a security concern and the desire to not provide identifying information about their public routers. I believe that this is a much more likely reason for some timeouts along the way in your traceroute.

HTH

Rick

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jong_r0602 Thu, 09/24/2009 - 10:22

If the traceroute's completed. It could be your source ip address is not permitted on that particular hops to send/received ICMP packets.

Regards,

Jong

Correct Answer
Richard Burts Thu, 09/24/2009 - 13:29

Meenakshi

I believe that a different explanation is more likely the cause of your issue. If your source IP address is not permitted at certain hops as suggested by Jong, then how does the traffic get past those hops without being dropped?

To explain what is happening we should start by reviewing how traceroute works. Traceroute sends packets to find the path through the network to the destination. Traceroute manipulates the TTL of the packet. The first set of probe packets has TTL set to 1. And so at the first hop the TTL expires, the first hop router discards the packet and generates an ICMP error message. This ICMP error message is what identifies the address of the first hop. Traceroute receives the ICMP error or if it does not receive the ICMP error message then Traceroute has a timeout. Then traceroute increments the TTL and sends packets with TTL of 2. The packet gets to the second hop where it expires and the second hop generates an ICMP error message. The error message identifies the second hop in the path. Traceroute continues to increment the TTL and to look for the ICMP error messages to identify the hop until the packet reaches the destination or until the traceroute gets to the max number of hops.

If some hops along the path do not send the ICMP error message then this is what generates the timeout in traceroute. Some providers configure their routers to not generate the ICMP error message because of a security concern and the desire to not provide identifying information about their public routers. I believe that this is a much more likely reason for some timeouts along the way in your traceroute.

HTH

Rick

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