ICMP

=Header=

=Messages=

= Troubleshooting =

Reply from 10.102.82.14: Destination host unreachable
 * IP address is unconfigured:

Request timed out
 * Ping disabled on host:

U.U.U ping response
 * Router performs ICMP rate limiting:

= Path MTU Discovery =

Don't Fragment (DF) bit of the IP header A subcode of the ICMP Destination Unreachable message - Fragmentation Needed
 * RFC 1191 defines path MTU discovery = process to detect smaller MTU to a destination
 * Two key components:


 * Setting DF bit in IP packet prevents a Router from fragmenting a packet larger than MTU
 * Packet is discarded and an ICMP Fragmentation Needed message is sent
 * RFC 1191 expands the Fragmentation Needed message to include the MTU of the link requiring fragmentation
 * Path MTU discovery is continuous process & can detect increase or decrease in MTU

tracepath -n 192.168.1.2 mturoute (Windows)

[[Media:Path_MTU_discovery.cap|Path MTU PCAP]]
 * A Fragmentation Needed message can be seen in packet #6 of this packet capture:

= Traceroute =

Source: Packetlife.net




 * Works by sequentially incrementing the TTL of UDP (or ICMP on Windows) packets.
 * All hops return a "TTL exceeded in transit" message
 * Last hop should return a "destination unreachable/port unreachable" message
 * UDP traceroute packets are sent to a pseudorandom high port on which the end host is not likely to be listening

[[Media:Tracert.pcap|Traceroute PCAP]]