Internet Control Message Protocol (ICMP)

ICMP

• Internet Control Message Protocol

– IP is the delivery standard at the TCP/IP internet layer

– ICMP is the standard for supervisory messages

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

ICMP

• Several ICMP Message Types– Error messages warn of problems– Not error correction, because there is no

transmission of lost or damaged packets

Error MessageA

B

C

ICMP

• Several ICMP Message Types– Query asks host if it is active – Also called “Echo” and “Ping”

QueryRequest

QueryResponse

A

B

C

ICMP

• Several ICMP Message Types– Flow control: Source quench message asks

other side to slow down

Source Quench

A

B

C

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

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

Redirection:“Next time, use Router A for this destination host”

Router A

Router B

IP Packet

Host X