Packet Generation:
------------------
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.
ראשי
צ'אט
22-07-2008, 13:44
בעוד שמה ש ARP עושה זה מחפש כתובת של שכבה 2 עבור כתובת שכבה 3 ידועה וזאת באמצעות שידור לכל סגמנט ה Ethernet - שכבה 2)
מצב כלאיים
