arp who-has? tell?

John Summerfield debian at herakles.homelinux.org
Fri Dec 21 12:59:00 UTC 2007


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
>>
> 


-- 

Cheers
John

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