Questions:
I'd like to know how ICMP packets travel in a network? Are they encapsulated within IP datagram?
The RFC 792 doesn't explicitly states the layer of ICMP. Are ICMP messages sent using a header of their own? Which layer does ICMP belong to in TCP/IP?
It is indeed specified in RFC 792, on the top of page 2:
Message Formats
ICMP messages are sent using the basic IP header.
You will also found in this RFC :
ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP,
AS for the layer, sees this related quesiton: Why is the ICMP protocol considered to run at layer 3 on the Wikipedia page?
which explain why ICMP is in fact a layer 3 protocol.
