Internet Control Message Protocol (ICMP)

Overview

• ICMP is primarily used for signaling error conditions between hosts and routers.
• Users typically interact with ICMP through tools like ping and traceroute.
• This section covers the basics of ICMP, consisting of two slides.

Functionality

• ICMP is used by hosts and routers to communicate network-level information, particularly for error reporting.
  • Examples of errors: Network unreachable, host unreachable, port unreachable, protocol unreachable.
• ICMP messages are carried as payloads inside IP datagrams, similar to UDP and TCP segments.
  • This positions ICMP as a sibling protocol to UDP and TCP, but it is not considered a transport layer protocol.

Protocol Details

• Protocol Number: ICMP has a protocol number of 1.
  • Used for demultiplexing from IP to ICMP, UDP, or TCP.
• ICMP Message Structure:
  • 1 byte for Type
  • 1 byte for Code
  • 2 bytes for Checksum
  • Header and up to 8 bytes of the IP datagram that triggered the ICMP message.
  • Example: Type 11, Code 0 indicates "TTL expired" message.

Traceroute Functionality

• Traceroute sends sets of UDP datagrams towards a destination with incrementing TTL values:
  • First set: TTL = 1
  • Second set: TTL = 2
  • Third set: TTL = 3
• Routers decrement the TTL field; if it reaches zero, the packet is dropped and an ICMP TTL expired message may be sent back to the source.
  • The source can determine which router dropped the packet based on the IP address in the ICMP message.
• The time from sending the IP datagram to receiving the ICMP message is recorded to calculate Round Trip Time (RTT).

Final Notes

• If a UDP segment reaches the destination, the destination may respond with an ICMP Port Unreachable message (Type 3, Code 3).
• This indicates that the source has reached the end of the path.
• ICMP serves as a tool for network management, commonly used in tools like ping and traceroute.
• More advanced network management techniques will be discussed in the next section.