1. ICMP : Internet Control Message Protocol

2. 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
Feedback of errors

3. Content depends on type and code
ICMP Packet Format
ICMP message IP
header
IP datagram
20 bytes 7 8 15 16 31
8-bit type
8-bit code
16-bit checksum
Content depends on type and code

4. ICMP Message types

5. Example: ICMP Port Unreachable Error
Subcode of destination unreachable
# tftp
tftp> connect svr 8888
tftp> get readme.txt
Tranfer timed out.
tftp> quit
svr
tftp
port 69 PC
Readme.txt

6. ICMP port unreachable generated by TFTP request
1 arp who-has svr tell pc
2 arp reply svr is-at 0:0:c0:c2:9b:26
3 pc.2924 > svr.8888: udp 20
4 svr > pc: icmp: svr udp port 8888 unreachable
5 pc.2924 > svr.8888: udp 20
6 svr > pc: icmp: svr udp port 8888 unreachable
IP datagram
ICMP message
ICMP data portion
14 bytes
20 bytes
8 bytes
20 bytes
8 bytes
Ethernet
header IP
header
ICMP
header
IP header of datagram
that generated error
UDP
header
UDP protocol
portnumber 8888