On Thu, Dec 04, 2003 at 07:39:27PM -0800, Joshua Banks wrote:
> Take ICMP for example (a ping). This is a Layer 3 protocol. Now, I
> thought that "windows" pc's used tcp/icmp to transport pings or
> tracert's and linux/unix (to include Cisco routers) pc's used udp/icmp
> to transport pings or traceroute's.
Nope. Take a look at /usr/include/netinet/in.h. This lists a lot of
the 'assigned numbers' of the general internet protocol. The first enum
in there is the list of IPPROTO_* values that define the various protocols
that operate on top of IP. These include ICMP(1), TCP(6), UDP(17), Multi-
cast Transport Protocol(92), IP over IP tunnelling(4), RSVP(46), and many
others, most of which you don't have to worry about unless you're building
high-end routers and gateways.
> After running Ethereal and watching a packet dump in real time, I only
> saw ICMP being used and NO UDP or TCP. Is ethereal missing something or
> am I?
>
> If some types of IP packets don't need Layer 4 transport helpers like
> Tcp or Udp then what are some examples (or types of apps) that utilize
> only IP, and still get from point A to point B without the help of Tcp
> or Udp.
Most of the other protocols are either very low-level (like ICMP),
restricted to communications between routers and gateways (like EGP),
or dedicated to IPv6 specialized features. In general, the actual sending
of packets from one machine to another is done at the IP level; UDP just
adds source and destination ports, TCP adds ports and stream functionality,
and so on. So an ICMP packet is machine-to-machine, not port-to-port.
Then, of course, you have the protocols such as ARP which operate _below_
the IP level right on top of the underlying physical network protocols.
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