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ICMP : Internet Control Message Protocol

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Introduction - Problem

Error messages and conditions to draw attention

acted on IP - UDP - TCP

ICMP message can generate errors to user process

query-message <--> error message

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ICMP Packet Format

ICMP messageIP

headerIP datagram

20 bytes

8-bit type 8-bit code 16-bit checksum

Content depends on type and code

0 7 8 15 16 31

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ICMP Message types

TYPE CODE MEANING-----------------------4 0 SOURCE QUENCH5 REDIRECT

0 network1 host2 network & TOS3 host & TOS

8 0 ECHO REQUEST (ping request)9 0 ROUTER ADVERTISEMENT10 0 ROUTER SOLICITATION11 TIME EXCEEDED

0 TTL=0 during transmit1 TTL=0 during reassembly

12 PARAMETER PROBLEM13 0 TIMESTAMP REQUEST14 0 TIMESTAMP REPLY15 0 INFO REQUEST16 0 INFO REPLY17 0 ADDRESS MASK REQUEST18 0 ADDRESS MASK REPLY

TYPE CODE MEANING-----------------------0 0 ECHO REPLY (ping reply)3 DESTINATION UNREACHABLE

0 network unreachable1 host unreachable2 protocol unreachable3 port unreachable4 frag needed5 source route failed6 destination network unknown7 destination host unknown8 source host isolated9 destination network administratively prohibited10 destination host administratively prohibited11 network unreachable for TOS12 host unreachable for TOS13 prohibited by filtering14 host precedence violation15 precedence cutoff

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Example: ICMP Port Unreachable Error

Subcode of destination unreachable

# tftp

tftp> connect svr 8888

tftp> get readme.txt

Tranfer timed out.

tftp> quit

tftpport 69

tftpport 69

Readme.txt

PC

svr

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ICMP port unreachable generated by TFTP request

1 arp who-has svr tell pc2 arp reply svr is-at 0:0:c0:c2:9b:26

3 pc.2924 > svr.8888: udp 204 svr > pc: icmp: svr udp port 8888 unreachable

5 pc.2924 > svr.8888: udp 206 svr > pc: icmp: svr udp port 8888 unreachable

IP header

IP datagram

20 bytes

Ethernet header

14 bytes 8 bytes

ICMP headerIP header of datagram

that generated errorUDP

header

20 bytes 8 bytes

ICMP message

ICMP data portion

portnumber 8888UDP protocol