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Posted by Aswin on July 18, 2007, 7:54 am
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How would a MAC layer look into the MAC header if it is the one to put
the MAC header onto the packet coming from upper layer (Network
layer)? I am talking about the packet that a node sends out to other
node. I am not talking about MAC layer's function after it "receives"
the packet from
other node. Please let me know how a MAC header is created by a node.
It has to some how look at the IP header of the frame coming from
above layer in order to append the MAC layer header to the packet.
More specifically, i would like to know that how a node that is
sending out a MAC packet would know the destination MAC address? Is it
communicated to it by the higher layer? If yes, how is it done for
higher layer protocols other than IP?
Thanks in advance.
Aswin
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Posted by Albert Manfredi on July 18, 2007, 10:11 am
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> How would a MAC layer look into the MAC header if it is the one to put
> the MAC header onto the packet coming from upper layer (Network
> layer)? I am talking about the packet that a node sends out to other
> node. I am not talking about MAC layer's function after it "receives"
> the packet from
> other node. Please let me know how a MAC header is created by a node.
> It has to some how look at the IP header of the frame coming from
> above layer in order to append the MAC layer header to the packet.
>
> More specifically, i would like to know that how a node that is
> sending out a MAC packet would know the destination MAC address? Is it
> communicated to it by the higher layer? If yes, how is it done for
> higher layer protocols other than IP?
You are looking for something called Address Resolution Protocol (ARP),
for IP Version 4, or Neighbor Discovery (ND), for IPv6. Respectively,
http://www.ietf.org/rfc/rfc0826.txt?number=826
and
http://www.ietf.org/rfc/rfc2461.txt?number=2461
These are used when the destination is on the same IP subnet as the
sending host. If the sending host sees that the destination IP address
is not on its own IP subnet, then the destination MAC address is the
address of the default router (typically).
ARP is not IP-specific, by the way. It can accept any Layer 3 addresses,
in principle.
Bert
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Posted by glen herrmannsfeldt on July 18, 2007, 2:47 pm
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Albert Manfredi wrote:
(snip)
>> More specifically, i would like to know that how a node that is
>> sending out a MAC packet would know the destination MAC address? Is it
>> communicated to it by the higher layer? If yes, how is it done for
>> higher layer protocols other than IP?
> You are looking for something called Address Resolution Protocol (ARP),
> for IP Version 4, or Neighbor Discovery (ND), for IPv6. Respectively,
> http://www.ietf.org/rfc/rfc0826.txt?number=826
(snip)
> ARP is not IP-specific, by the way. It can accept any Layer 3 addresses,
> in principle.
In principle, but do you know any other than IP that use it?
Apple's ethertalk uses AARP. I don't know where an online
reference is. See "Inside AppleTalk, second edition".
DECNet assigns the MAC address based on the DECNet area and node.
http://linux-decnet.sourceforge.net/faq-4.html#ss4.2
-- glen
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Posted by Albert Manfredi on July 19, 2007, 10:26 am
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>> ARP is not IP-specific, by the way. It can accept any Layer 3
>> addresses, in principle.
>
> In principle, but do you know any other than IP that use it?
No, but that's why I said "in principle." For example, even IPv6 *could*
use ARP. But IPv6 has its own unique mechanism.
Another point is that ARP gets a lot of bad press for using broadcasts
for its queries. But once virtually everything was migrated to IPv4, I
don't think it made a whole lot of difference anymore. There were not
many hosts out there that got interrupted unnecessarily. ND, with its
multicast queries, will eventually interrupt all hosts too, if IPv6
becomes the norm.
> Apple's ethertalk uses AARP. I don't know where an online
> reference is. See "Inside AppleTalk, second edition".
>
> DECNet assigns the MAC address based on the DECNet area and node.
DECnet had some sort of address resolution protocol. I forget the name.
You had to discover all the nodes on your network, but it was a one time
deal. Remember? It took a few minutes to populate the table.
>
> http://linux-decnet.sourceforge.net/faq-4.html#ss4.2
>
> -- glen
Bert
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Posted by Albert Manfredi on July 19, 2007, 10:49 am
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> DECnet had some sort of address resolution protocol. I forget the
> name. You had to discover all the nodes on your network, but it was a
> one time deal. Remember? It took a few minutes to populate the table.
Maybe I'm confusing that with LAT?
Bert
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