1. Internet Control Message Protocol (ICMP)

2. ICMP • Internet Control Message Protocol • IP is the delivery standard at the TCP/IP internet layer • ICMP is the standard for supervisory messages

3. ICMP • Internet Control Message Protocol • IP and ICMP are designed to work together. Even have adjacent RFC (standard) numbers • ICMP PDU is carried in the information field of an IP packet IP Header ICMP PDU

4. ICMP • Several ICMP Message Types • Error messages warn of problems • Not error correction, because there is no transmission of lost or damaged packets B Error Message C A

5. ICMP • Several ICMP Message Types • Query asks host if it is active • Also called “Echo” and “Ping” B Query Request C A Query Response

6. ICMP • Several ICMP Message Types • Flow control: Source quench message asks other side to slow down B Source Quench C A

7. Source Quench • Weak form of flow control • When host sending packets gets source quench, it slows down • If another source quench message arrives, slows down even more • If source quench messages stop, slowly increases speed

8. ICMP • Redirection • Host uses a particular router (B) • Router B tells host to use a different router (A) for that destination host in the future Router A IP Packet Redirection: “Next time, use Router A for this destination host” Router B Host X