AN2120: Connecting an M68HC08 Family Microcontroller to an ...

Application Note
Type Code Checksum
Type
Identification Sequence
8 - ECHO REQUEST
0 - ECHO REPLY
Optional Data
Code
Always 0 for ECHO messages
The ICMP message layout is very simple. Implementations of this
protocol should check the type and code fields to determine the nature
of the message. For instance, a type field set to 8 requires an echo reply
from the destination IP. The originator of this ICMP message can then
determine if the host is reachable or not. This is perhaps the most
popular ICMP application used today and is called ping (described next).
After the Code field, the checksum follows and is calculated over the
entire ICMP packet without taking the IP header into account.
This application note implements ICMP support to send and receive ping
messages. The format of a ping message (officially called echo request)
is shown in Figure 12.
Identification and Sequence
Two 16-bit fields to aid in matching echoes and replies
Data
This data is optional for the originator; however, for an upcoming ping
request, the data must be returned in the reply message.
Figure 12. Ping Message Format or Echo/Echo Reply Message Format
Once the sender sets the type field to 8 (echo request) and the code to 0,
it must initialize the identifier and sequence number prior to a ping
execution. Those fields are used when multiple echo requests are sent.
If desired, the ping originator can add optional data to the ICMP packet.
The maximum amount of data should be no more than 64 Kbytes long.
Since this amount also applies to incoming requests, this application
note silently discards such big packets.
AN2120
14 MOTOROLA