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Old 12-21-2007, 11:12 AM
John Summerfield
 
Default arp who-has? tell?

Dave Burns wrote:

On Dec 20, 2007 8:06 AM, John Cornelius <jc@lht.com> wrote:

If you want to ignore all of those ARP packets run tcpdump as tcpdump not
arp and you'll see all of the actual Internet traffic to and from your
neighbors' homes.


You might want to check you ISP's policy on that first, some people
would consider that intrusive.


If he can, the ISP has a problem as soon as a client recognises the fact
and goes public.





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Cheers
John

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Old 12-21-2007, 11:59 AM
John Summerfield
 
Default arp who-has? tell?

John Cornelius wrote:
I had not intended to start a religious argument here when I said the


It's hardly a religious matter when the truth of the matter can be
ascertained.


DHCP server ARPs to find out if the address it's going to assign is
already in use.


John Summerfield is correct that the specification calls for an ICMP
echo request to be sent.


Mogens is correct that the ICMP echo is the mechanism used to generate
the ARP request and subsequently verify the existence or absence of the
address on the segment.


ARP is the mechanism for providing an association between the physical
link layer and the logical link layer entities. IP addresses are logical
link layer entities and ethernet (MAC) addresses are physical link layer
entities.


ARP is the mechanism used on ethernet networks, but it's not the only
medium capable of transporting IP traffic. PPP, for example, carries IP
traffic (though DHCP doesn't support address management over it). I can
imagine a wired local network though, that does not use ethernet frames,
but is capable of transporting encapsulated IP traffic. For a simple
case, imagine something like ethernet but with 64-bit addresses.


I'm trying hard to remember my HDLC/SDLC packets....

Here's a short summary:
http://www.pulsewan.com/data101/sdlc_basics.htm

here's the course notes to a CS course.
http://www.cs.mtu.edu/~soner/courses/cs4461/shared/index.htm
The ARP packets contain ethernet addresses:
http://www.cs.mtu.edu/~soner/courses/cs4461/ip/sld010.htm
they would not be usable on a network that's like ethernet but with a
different-sized address field.


the point is that, as the DHCP server itself doesn't create the ARP
packets, it doesn't care whether ARP or some other mechanism is used to
help transport ICMP packets.


Also, using ICMP and not ARP protects (or confuses) things if there are
two networks on different wire with the same IP addresses. This could
(and does) happen if two LANs are joined by a VPN.





ICMP may not depend on knowing how the traffic is transported but it
does depend on knowing that it can be transported. That's what address
resolution is all about. If the logical address (IP) cannot be resolved
to a MAC address it cannot be transported.


Merry Christmas, Happy Hanukah, and a prosperous New Year to you all.

--jc

Mogens Kjaer wrote:


John Summerfield wrote:
..


ICMP doesn't depend on knowing how IP traffic is transported.



If I ping (send ICMP echo request) a non-existing, local IP
address, the first thing my machine does is sending an
arp who-has on the wire. No ICMP package is sent before
my machine receives the is-at answer.

Mogens






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Cheers
John

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