LinuxSA Mailing list archives

  From: Andrew Galdes <galdes1@iprimus.com.au>
  To  : Richard Russell <richardrussell@mail.com>
  Date: Tue, 9 Jan 2001 10:07:02 +1030

Re: ping

and now im awake. thanks for the help guys.
(a good book can't hurt)
andrew galdes

On Tuesday 09 January 2001 00:38, you wrote:
> > sorry its late.. am tired but..
> >
> > TCP is on the transport layer and ICMP is too. but surly TCP is needed 
> > to send a ping.  are you sure that ICMP is on the transport layer?
>
> yes, he is sure. Ports are implemented _within_ TCP (& UDP). TCP and UDP
> (and ICMP) packets are transported using IP.
>
> Think of it like a phone system. IP is like the number you dial. With TCP
> and UDP, it's like you are asking to speak to a particular person (port
> number). With ICMP, it's like you are talking to the phone, and hence,
> don't need to ask for anyone by name. But you still need the phone number
> to call to talk to that phone (you still need the IP transport packet).
>
> so: IP can contain TCP, UDP or ICMP (and others). TCP and UDP implement
> ports. ICMP doesn't. Note that all ports are is a mechanism for telling a
> particular program to deal with a particular packet. Send a packet to TCP
> port 80, and it goes to `httpd`. Send a packet to UDP port 69, and it goes
> to `tftpd`. If you have a telnet session set up, it will be listening on a
> random tcp port > 1024 -- that is how packets meant for it, as opposed to
> any other telnet session, are identified. There is no `pingd`. No program
> needs to be identified to run it. It's an _essential_ part of any TCP/IP
> stack (TCP/IP includes the entire suite of protocols -- IP, TCP, UDP, ICMP
> and so on). So, the networking software itself responds to the ICMP
> packets. There is no need for ports. It's just like talking to the
> telephone itself. BUT, just like with TCP and UDP, you still need the IP
> layer to get you to the correct computer in the first place.
>
> seriously, go get some sleep, then go and buy that book I mentioned. :)
>
> rr

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