1. Recall IPv4 Datagram Format

2. IP Errors Header checksum is used to detect transmission errors
When a host creates an IP datagram
the host includes a checksum that covers the entire header
Whenever a datagram is received
the checksum is verified to ensure that the header arrived intact
The IP header contains a TIME TO LIVE field used to prevent a datagram from circulating forever

3. IP Errors The response to a checksum error:
Datagram is discarded immediately without more processing
The receiver cannot trust any fields in the datagram header
The receiver cannot send an error message back to the sender

4. Internet Control Message Protocol (ICMP)
IP includes a companion protocol, ICMP
It is used to report errors back to the original source
IP and ICMP are co-dependent
IP depends on ICMP to report errors
and ICMP uses IP to carry error messages

5. Error Reporting with ICMP
When a datagram causes an error, ICMP can only report the error condition back to the original source of the datagram.
the source must relate the error to an individual application program or take other action to correct the problem.

6. ICMP Message Format Each ICMP message has its own format,
All begin with the same three fields:
TYPE field (8-bit)
identifies the message, an 8-bit
CODE field (8-bit)
provides further information about the message type
CHECKSUM field (16 bits).

7. ICMP Messages with the message number and purpose

8. Echo Request And Reply Message Format

9. Reports Of Unreachable Destinations

11. Internet Control Message Protocol (ICMP)
 ICMP contains two message types:
messages used to report errors
messages used to obtain information
example
Time Exceeded and Destination Unreachable are for reporting errors
Echo Request and Echo Reply do not correspond to an error
used by the ping application to test connectivity

12. ICMP Message Format and Encapsulation
ICMP uses IP to transport each error message:

13. ICMP Message Format and Encapsulation
ICMP messages do not have special priority
They are forwarded like any other datagram, with one minor exception
If an ICMP error message causes an error
no error message is sent