ICMP

ICMP

• Layer 4 protocol
• Diagnostics
• No Port numbers

Locating ICMP Header

1. Metadata
2. Layer 2
3. Layer 3
4. Layer 4 (ICMP)

• Every packet that you ever sent has an IP ID, is like a serial number for packets.
  • Not something that most people keep track off but can be handy in troubleshooting
• Time to Live main purpose is so that the packet does not live forever
  • Happens hop by hop by hop.
  • If there are no hops left, instead of forwarding, it throws this packet and returns an ICMP message that outputs ERROR.

ICMP's Protocol Number

• 1 is the protocol number for ICMP

ICMP

• Layer 4 header being carried by a Layer 3 header
• Type 8: ("Hey are you there?")
• Looking at the ICMP you can tell who is a Linux host, Windows host etc
  • There is all these atributes, different operative systems use different defaults and we can infere what type of host we are communicating with.
• If you do a PIN your computer is generating a Type 8 Echo request and if the pin is responded it returns a Type 0 Echo request.
• You can look for ICMP codes here