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I nternet C ontrol M essage P rotocol

Learn about the Internet Control Message Protocol (ICMP) and its role in supporting IP, error reporting, queries, and message formats in data communications.

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I nternet C ontrol M essage P rotocol

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  1. Internet Control Message Protocol • The IP (Internet Protocol) relies on several other protocols toperform necessary control and routing functions: • Control functions (ICMP) • Multicast signaling (IGMP) • Setting up routing tables (RIP, OSPF, BGP, PIM, …) CA528 - Advanced Data Communications

  2. Introduction • The Internet Control Message Protocol (ICMP) is ahelper protocol that supports IP with facility for • Error reporting • Simple queries • ICMP messages are encapsulated as IP datagrams: CA528 - Advanced Data Communications

  3. Message Format • 4 byte header: • Type (1 byte): type of ICMP message • Code (1 byte): subtype of ICMP message • Checksum (2 bytes): similar to IP header checksum. • Checksum is calculated over entire ICMP message • If there is no additional data, there are 4 bytes set to zero. • Each ICMP messages is at least 8 bytes long CA528 - Advanced Data Communications

  4. ICMP Query Message ICMP query: • ICMP Request sent by host to a router or host • ICMP Replysent back to querying host CA528 - Advanced Data Communications

  5. Example of ICMP Queries Type/Code: Description 8/0Echo Request 0/0Echo Reply 13/0 Timestamp Request 14/0 Timestamp Reply 10/0 Router Solicitation 9/0Router Advertisement The ping command uses Echo Request/ Echo Reply CA528 - Advanced Data Communications

  6. Example Query:Echo Request and Reply • Pings are handled directly by the kernel • Each Ping is translated into an ICMP Echo Request • The Pinged host responds with an ICMP Echo Reply CA528 - Advanced Data Communications

  7. Another Example:ICMP Timestamp • A system (host or router) asksanother system for the current time. • Time is measured in millisecondsafter midnight UTC (UniversalCoordinated Time) of the currentday • Sender sends a request, receiverresponds with reply CA528 - Advanced Data Communications

  8. ICMP Error Message • ICMP error messages report error conditions • Typically sent when a datagram is discarded • Error message is often passed from ICMP to theapplication program CA528 - Advanced Data Communications

  9. ICMP error messages include the complete IP header andthe first 8 bytes of the payload (typically: UDP, TCP) CA528 - Advanced Data Communications

  10. Common ICMP Error Messages CA528 - Advanced Data Communications

  11. Destination Unreachable CA528 - Advanced Data Communications

  12. Example: ICMP Port Unreachable • RFC 792: If, in the destination host, the IP module cannot deliver thedatagram because the indicated protocol module or processport is not active, the destination host may send a destinationunreachable message to the source host. CA528 - Advanced Data Communications

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