168112 5523 external ip port 4425803 1500 external

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Unformatted text preview: •  Each message is carried in an IP packet 77 ICMP Message Format (2) •  Each ICMP message has a Type, Code, and Checksum •  Olen carry the start of the offending packet as payload •  Each message is carried in an IP packet Por8on of offending packet, star8ng with its IP header Src=router, Dst=A Protocol = 1 Type=X, Code=Y Src=A, Dst=B XXXXXXXXXXXXXXX IP header ICMP header ICMP data 78 39 10/23/13 Example ICMP Messages Name Type / Code Usage Dest. Unreachable (Net or Host) 3 / 0 or 1 Lack of connectivity Dest. Unreachable (Fragment) 3/4 Path MTU Discovery Time Exceeded (Transit) 11 / 0 Traceroute Echo Request or Reply 8 or 0 / 0 Ping Tes8ng, not a forwarding error: Host sends Echo Request, and des8na8on responds with an Echo Reply 79 Traceroute •  IP header contains TTL (Time to live) field –  Decremented every router hop, with ICMP error if it hits zero –  Protects against forwarding loops 80 40 10/23/13 Traceroute (2) •  Traceroute repurposes TTL and ICMP func8onality –  Sends...
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