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ARP Message Format
Address resolution using ARP
is accomplished through the exchange
of messages between the source device
seeking to perform the resolution, and the destination device that responds
to it. As with other protocols, a special message format is used
containing the information required for each step of the resolution
process.
ARP messages use a relatively simple
format. It includes a field describing the type of message (its operational
code or opcode) and information on both layer two and layer
three addresses. In order to support addresses that may be of varying
length, the format specifies the type of protocol used at both layer
two and layer three and the length of addresses used at each of these
layers. It then includes space for all four of the address combinations
we saw in the
previous topic.
The format used for ARP messages
is described fully in Table 42,
and illustrated in Figure 49.
Table 42: Address Resolution Protocol (ARP) Message Format
Field
Name
|
Size (bytes)
|
Description
|
HRD
|
2
|
|
PRO
|
2
|
Protocol
Type: This field is the complement of the Hardware Type
field, specifying the type of layer three addresses used in the message.
For IPv4 addresses, this value is 2048 (0800 hex), which corresponds
to the EtherType code for the Internet Protocol.
|
HLN
|
1
|
Hardware Address Length:
Specifies how long hardware addresses are in this message. For Ethernet
or other networks using IEEE 802 MAC addresses, the value is 6.
|
PLN
|
1
|
Protocol
Address Length: Again, the complement of the preceding field;
specifies how long protocol (layer three) addresses are in this message.
For IP(v4) addresses this value is of course 4.
|
OP
|
2
|
|
SHA
|
(Variable,
equals value in HLN field)
|
Sender
Hardware Address: The hardware (layer two) address of the device
sending this message (which is the IP datagram source device on a request,
and the IP datagram destination on a reply, as discussed in the topic
on ARP operation).
|
SPA
|
(Variable, equals
value in PLN field)
|
Sender Protocol Address:
The IP address of the device sending this message.
|
THA
|
(Variable,
equals value in HLN field)
|
Target
Hardware Address: The hardware (layer two) address of the device
this message is being sent to. This is the IP datagram destination device
on a request, and the IP datagram source on a reply)
|
TPA
|
(Variable, equals
value in PLN field)
|
Target Protocol Address:
The IP address of the device this message is being sent to.
|
Figure 49: Address Resolution Protocol (ARP) Message Format The ARP message format is designed to accommodate layer two and layer three addresses of various sizes. This diagram shows the most common implementation, which uses 32 bits for the layer three (Protocol) addresses, and 48 bits for the layer two hardware addresses. These numbers of course correspond to the address sizes of the Internet Protocol version 4 and IEEE 802 MAC addresses, used by Ethernet.
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After the ARP message has
been composed it is passed down to the data link layer for transmission.
The entire contents of the message becomes the payload for the message
actually sent on the network, such as an Ethernet frame. Note that the
total size of the ARP message is variable, since the address fields
are of variable length. Normally, though, these messages are quite small:
for example, they are only 28 bytes for a network carrying IPv4 datagrams
in IEEE 802 MAC addresses.
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Version 3.0 - Version Date: September 20, 2005
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