# ICMP
Internet Control Message Protocol 

- Sends messages between two IP hosts
- Checks for errors along link
- Information about communication


| Code | ICMPv4                        | ICMPv6                                       |
| ---- | ----------------------------- | -------------------------------------------- |
| 0    | Net unrecheable               | No route to destination                      |
| 1    | Host unrecheable              | Communication is administratively prohibited |
| 2    | Protocol unrecheable          | Beyond scoupe of source address              |
| 3    | Port unrecheable              | Address unrecheable                          |
| 4    | Fragmentation Needed and D.F. | Port unrecheable                             |


# Time Exceeded
* ICMPv4: TTL = 0
* ICMPv6: Hop Limit = 0

# ICMPv6 messages

- Router Advertisement (RA)
	- Every 200s broadcasted to IPv6 hosts
	- Sends Prefix and default gateway link local
	- **Router:** *I am an IPv6 router, this is my prefix, use SLAAC/RID to generate a GUA, and this is my default gateway link-local address: ...*

- Router Solicitation (RS)
	- Sent if host needs RA message for IPv6 configuration
	- **Host:** *Hey! Is there any IPv6 router here? I need it to generate an IPv6 address.*

- Neighbour Solicitation (NS)
	- Sent to a host's address to check address uniqueness
	- If host receives NA message, it regenerates its IPv6 interface ID (randomly) and checks again
	- *"Hey you! Are you there? Do you have this address already?"*

- Neighbour Advertisement (NA)
	- Reply meant for NS messages
	- *"Hey, I'm using this address already! Use another one!"*