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Posted by Aswin on July 19, 2007, 12:06 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
(e.g. Destination MAC address) to the packet.
If someone can help me with this.... pls........
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Posted by Albert Manfredi on July 19, 2007, 10:07 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.
When the ARP query initially goes out, the sender is asking "What is
that MAC address of the destination with this IP address?" Since the
sending host has no idea of the destination MAC address, in ARP, that
host sends the query with a broadcast MAC address. All 1s. So every host
in the IP subnet is interrupted, and has to check whether it needs to
respond to this query.
ND, instead, uses a multicast address for the query.
RFC 826 is not written in the clearest language, but here's the text you
need to parse through. I broke it up in paragraphs to make it a little
clearer (hopefully):
As a packet is sent down through the network layers, routing
determines the protocol address of the next hop for the packet
and on which piece of hardware it expects to find the station
with the immediate target protocol address. In the case of the
10Mbit Ethernet, address resolution is needed and some lower
layer (probably the hardware driver) must consult the Address
Resolution module (perhaps implemented in the Ethernet support
module) to convert the <protocol type, target protocol address>
pair to a 48.bit Ethernet address.
"The Address Resolution module
tries to find this pair in a table. If it finds the pair, it
gives the corresponding 48.bit Ethernet address back to the
caller (hardware driver) which then transmits the packet.
"If it does not, it probably informs the caller that it is throwing the
packet away (on the assumption the packet will be retransmitted
by a higher network layer), and generates an Ethernet packet with
a type field of ether_type$ADDRESS_RESOLUTION. The Address
Resolution module then sets the ar$hrd field to
ares_hrd$Ethernet, ar$pro to the protocol type that is being
resolved, ar$hln to 6 (the number of bytes in a 48.bit Ethernet
address), ar$pln to the length of an address in that protocol,
ar$op to ares_op$REQUEST, ar$sha with the 48.bit ethernet address
of itself, ar$spa with the protocol address of itself, and ar$tpa
with the protocol address of the machine that is trying to be
accessed. It does not set ar$tha to anything in particular,
because it is this value that it is trying to determine.
"It could set ar$tha to the broadcast address for the hardware (all
ones in the case of the 10Mbit Ethernet) if that makes it
convenient for some aspect of the implementation. It then causes
this packet to be broadcast to all stations on the Ethernet cable
originally determined by the routing mechanism."
RFC 2461 is much clearer. I guess the RFC wrirters learned to write more
clearly over the years:
"Nodes accomplish address resolution by multicasting a Neighbor
Solicitation that asks the target node to return its link-layer
address. Neighbor Solicitation messages are multicast to the
solicited-node multicast address of the target address. The target
returns its link-layer address in a unicast Neighbor Advertisement
message. A single request-response pair of packets is sufficient for
both the initiator and the target to resolve each other's link-layer
addresses; the initiator includes its link-layer address in the
Neighbor Solicitation."
Bert
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