Internet Control Message Protocol ICMP

1
Internet Control Message Protocol(ICMP)

• CPSC 441 Computer Communications
• Tutorial

2
ICMP

• Internet Control Message Protocol
• ICMP messages are IP packets
• Used by network hosts to announce
• Network errors
• Network congestion
• Network timeouts
• Not used directly by user except
• ICMP Echo Request/Reply messages used in Ping
• Traceroute

3
ICMP Header

• ICMP Header starts after IP Header

4
ICMP Messages

• Destination network/host/port unreachable
• Source Quench Message
• Echo Request/Reply (ping)
• Time Exceeded/TTL Expiry (traceroute)

5
Ping

• Check if target host is alive
• Troubleshoot network connectivity problems
• Check RTT
• ICMP Echo Request
• 64 byte packet
• Host replies with ICMP Echo Reply

6
Ping - Example

• gtping www.ucalgary.ca
• Pinging www.ucalgary.ca 136.159.x.x with 32
  bytes of data
• Reply from 136.159.34.17 bytes32 time1ms
  TTL62
• Reply from 136.159.34.17 bytes32 time43ms
  TTL62
• Reply from 136.159.34.17 bytes32 time1ms
  TTL62
• Reply from 136.159.34.17 bytes32 time33ms
  TTL62
• Ping statistics for 136.159.34.17
• Packets Sent 4, Received 4, Lost 0 (0
  loss),
• Approximate round trip times in milli-seconds
• Minimum 1ms, Maximum 43ms, Average 19ms

7
Traceroute

• Trace route of packets across the network
• Command line tools
• Traceroute
• Tracepath
• Tracert (Windows)
• Pathping (Win NT based systems)

8
Traceroute

• Time to Live (TTL)
• 8 bit field in IP header
• Specifies the time a packet is allowed to live
  in the network
• At each hop, router or host decrements TTL value
  of packet by 1
• When TTL 1
• Packet discarded
• ICMP Time Exceeded error datagram sent back to
  source host

9
Traceroute

• Sends out a batch of packets
• First three packets have TTL 1
• Second three packets have TTL 2
• and so on.
• Each host along the way sees packet with TTL 1
• Sends ICMP Time Exceeded packet
• Source host uses these messages to build list of
  all hosts in the route

10
References

• http//www.ping127001.com/pingpage.htm
• http//en.wikipedia.org/wiki/Internet_Control_Mess
  age_Protocol
• http//www.erg.abdn.ac.uk/users/gorry/course/inet-
  pages/icmp.html

11
The end

• Questions?