Tracer Route Messages for Network Analysis: Example “tcpdump” program

This program, which exists as a standard program on Linux or Windows machines, is useful to test an internet connection.   

Possible messages after executing “tcpdump [-n]”

!H   [means host unreachable]

!N   [means net unreachable]

!P   [means protocol unreachable]

!S   [means source route failed]

!X   [means communication administration prohibited]

!V   [means host precedence violation]

!C   [mean precedence cut off]

!<num>   [means ICMP unreachable]

Note.  “tcpdump” program can also capture TCP packets with particular flag combinations.    There are eight flags (CWR, ECE, URG, ACK, PSH, RST, SYN, FIN).    The common flags used to establish a TCP connections are SYN and ACK.    The procedure for establishing an TCP connection is called “3 Way Handshake” protocol: (a) Caller sends a SYN, (b) Recipient responds with SYN, ACK, and (c) the caller sends an ACK.