Internet Control Message Protocol (ICMP)

• It is used to send error & control information between ip network devices.
• There are many diffident messages that can be shared, including the following: 

1. Echo request: Determines whether on ip node ( a host or a router is available on network.)
2. Echo Reply: Replies to ICMP echo request.
3. Destination Unreachable: Informs the host that a datagram can't be delivered.
4. Source Quench: Informs the host to lower the rate at which it sends datagram because of congestion.

5. Redirect: Informs the host of a preferred rate.

6. Time Exceeded: Indicate that the TTL of IP datagram has expired.

What is Datagram?

A datagram is a basic transfer unit associated with a packet-switched network in which the delivery arrival time and order are not guaranteed. A datagram consists of header and data areas, where the header contains information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the equipment and the network. The source and destination addresses as well as a type field are found in the header of a datagram.

OSI v TCP/IP

OSI:

you can see it, HERE

TCP/IP:

• Rout able: it can pass through routers.
• Connect dissimilar systems: Win, Mac, Unix, etc...
• Method of access to the inventor. 

App Layer: using an app, it gains access to the network.

Telnet, FTP, TFTP , NFS, SMTP, LPD, DNS, DHCP.

Transportation: provides communication sessions between the two computers.  TCP, UDP

Internet Layer: routers the data from it's source to it's source to its destination.   ARP, RARP, IP, ICMP, IGMP.

Network Layer: it sends & receive data on the wire.  Ethernet, Token Ring, Frame, Relay, ATM 

(Well-Know Port Numbers:

FTP: 20 & 21, SMTP: 25,  DNS: 53, Telnet:23, HTTP: 80, HTTPS: 443)