TTL and traceroute and ICMP

what is TTL:
en.wikipedia.org/wiki/Time_to_live

how traceroute works:

http://www.cs.cf.ac.uk/Dave/Internet/node77.html


icmp error message and its reason:

pages 56 and 57:

If a router cannot forward a packet because it has no routes at all (including no default route) to the destination specified in the packet, then the router MUST generate a Destination Unreachable, Code 0 (Network Unreachable) ICMP message. If the router does have routes to the destination network specified in the packet but the (Type of Service) TOS specified for the routes is neither the default TOS (0000) nor the TOS of the packet that the router is attempting to route, then the router MUST generate a Destination Unreachable, Code 11 (Network Unreachable for TOS) ICMP message.
If a packet is to be forwarded to a host on a network that is directly connected to the router (i.e., the router is the last-hop router) and the router has ascertained that there is no path to the destination host then the router MUST generate a Destination Unreachable, Code 1 (Host Unreachable) ICMP message. If a packet is to be forwarded to a host that is on a network that is directly connected to the router and the router cannot forward the packet because no route to the destination has a TOS that is either equal to the TOS requested in the packet or is the default TOS (0000) then the router MUST generate a Destination Unreachable, Code 12 (Host Unreachable for TOS) ICMP message.

 URL:

http://www.networksorcery.com/enp/protocol/icmp/msg3.htm


other reason for time exceeded icmp msg:

http://www.tcpipguide.com/free/t_ICMPv4TimeExceededMessages.htm


perfect example for how tra ceroute sends and gets the path:

http://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-121-mainline/12778-ping-traceroute.html
https://learningnetwork.cisco.com/thread/31415


ICMP positve way and negative hacking way:

http://www.techrepublic.com/article/prevent-hacker-probing-block-bad-icmp-messages/


path mtu and vpn:
http://www.enterprisenetworkingplanet.com/netsp/article.php/3584166/Networking-101-Understanding-and-Using-ICMP.htm