how does 'ping' command really work?

Question

How does the ping command really work? Specifically where does the ARP (Address Resolution Protocol) come into picture?

I was asked this question in an interview and I was not able to come up with a scenario when ARP could be used.

Answer 1

Ping is actually two different ICMP (Internet Control Message Protocol) packets.

To ping a host you first send a ICMP Echo Request Packet, the host will then reply with an ICMP Echo Reply.

For more information see: http://en.wikipedia.org/wiki/Ping

Answer 2

Ping and ARP are different things located at different layers in the network protocol stack.

Ping is at network layer (or Internet layer - Have a look to ICMP protocol like pointed out by @ServerMonkey).

Arp protocol is at link level (a lower level). Arp protocol is designed to allow physical connection between network hardware, that is directly connected.

In TCP/IP network stack, every layer uses the layer below to forward its data, encapsulating it inside the low level protocol. Each layer is independent from the other and possibly unaware of the other levels specific details and implementations (this is not always true: see cross-layer function).

Answer 3

This is a great trick question because you have to know that ping is an ICMP Echo Request; and that ICMP msgs are carried in IP datagrams; and therefore that any ARP activity has to arise from within the IP context. Really a good question.

Answer 4

If you really want to understand, there is an excellent (very well written) white paper here:

http://images.globalknowledge.com/wwwimages/whitepaperpdf/WP_Mays_Ping.pdf

Here is the summary ->

Ping (Program on the application layer) ------->
Opens a 'raw' socket to IP Layer ------>
IP layer (Layer 2 on OSI) packages ICMP packet and sends it

Since there is no TCP layer in between, the Ping (program) has to monitor all the incoming ICMP packets and filter only the one's from the destination.

Hope that helps.