Ok, slight correction ICMP is assigned IP protocol 1.
plesae correct me if i am wrong i gone through this link (further i checked over IANA site too its not mentioned over there)
further please share link on behaf you says its using port 1
where as i checked about Port 1 used for
tcpmux 1/tcp TCP Port Service Multiplexer
tcpmux 1/udp TCP Port Service Multiplexer
I'm not saying that ICMP uses either TCP or UDP, what I am saying is it sits on top of IP and is assigned IP protocol number 1. As another example EIGRP doesn't use TCP or UDP, and is assigned IP protocol number 88.
Checkout the link below for a complete list of IP protocol numbers.
No worries, that part of the fun with CLN. Everyday I learn something new on here
Not exactly a "sequence number", but ICMP defines the messages by a type and code mechanism. Example, all the "unreachable" type messages are defined in the Type 3 type of message, and inside this ICMP Type multiple codes are defined to tell to the source the reason of the unreachable message, e.g. Type 3 with Code 13 it's "Administratively Prohibited" which means that the administrator placed some type of filter (an ACL) in the interface in order to block the communication to the destination.
For more information about ICMP visit the RFC 792.