ttl...

the original intention for the time to live field in a packet was that it'd be decremented by the time in seconds the packet is queued by each router through which it is being forwarded...  that had limited practical appeal, and was never actually implemented... instead, routers decrement the packet's ttl field by one no matter the length of time it is queued... when it reaches zero it is dropped...

in  ipv6, the corresponding field known as hop limit (more aptly stated) works the same way

so in both ipv4(ttl) and ipv6(hop limit) this idea remains in effect...

ICMP is your lover...

ICMP

Part of the network layer; instrumental in controlling and managing, troubleshooting.  There is no transport header, the ICMP message resides inside an IP packet.  Rfc 792

Echo request/reply

Sent and received by ping.  Pinging means sending an echo request.  Any data sent in echo request is returned in echo reply.  See extended ping.

Destination unreachable

1) network unreachable-no match in routing table at destination- sent by router

2) host unreachable- routes to destination network but no host response- sent by router

3)  can’t fragment- don’t fragment bit is set and a router must fragment in order to forward the packet (packet too large)- sent by router

4) protocol unreachable(unlikely, TCP or UDP)- delivered to host but layer 4 (transport) protocol is not available- sent by host

5) port unreachable- delivered to host, but destination port is not open- sent by host

ping codes: ! = echo received

                   . = nothing received before ping time out

                 U = unknown host

                 N = unknown network

                 P = unreachable port

                Q = source quench

                M = can’t fragment code

                 ? = packet unknown

Time exceeded

TTL (time to live) field in IP header has expired, therefore discarded (decremented by a router before forwarding; at zero, see ya) TRACE uses time exceeded

Redirect 

Used to tell the host there is a better local router to ship the packet to