1. Address Resolution Protocol (ARP)
Relates to Lab 2.
This module is about the address resolution protocol.

2. Overview

3. IP and LAN addresses The Internet is based on 32bit IP addresses
Applications only deal with IP addresses
But all Internet devices connect to a physical link via hardware - Network Interface Card (NIC), that has an address.
Data link protocols (Ethernet, Frame Relay) have different addresses

4. Nature of MAC addresses
Hardare address allocation administered by IEEE
Manufacturer buys portion of hardware address space (to assure uniqueness)
Analogy of Internet Addresses:
Hardware address: like Social Security Number
IP address: like postal address
Hardware: flat address ➜ portable
assigned once, un-chageable, goes with you, move from one physical location to another it doesn’t change
IP: hierarchical address -> not portable
address assigned based on physical location, i.e., IP subnet to which device is attached, changes when you move from one network to another

5. IP and MAC Addresses LAN
adapter
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
LAN
71-65-F7-2B-08-53
IP addresses are “generally” known – i.e., application can find it in DNS database.
How do we find a device’s hardware address?
Use a ”dynamic binding” procedure - an address resolution process that finds hardware address for an IP address.

6. Address Resolution Protocol - ARP
Standard for dynamic address resolution in the Internet
The ARP protocol performs the translation between 32 bit IP addresses and link layer addresses to reach the physical location of a device on the same “single segment” network (i.e., all devices that have same IP network prefix)
Physical address burned in NIC ROM, sometimes software settable or firmware.
Example of an address on an Ethernet link (48bits):
1A-2F-BB AD
hexadecimal (base 16) notation (each “number” represents 4 bits)
Isolates hardware address at low level
Allows application programs to use IP addresses

7. ARP Description Allows device A to find device B’s hardware address
Technique: broadcast query and obtain unicast response
Query: sent as a “hardware” broadcast (link layer broadcast)
limited broadcast: ARP only used to map addresses within a single physical/segment network, never across multiple (IP) networks
Query contains A’s hardware address and B’s IP address
Response: sent as a unicast to A’s hardware address
contains B’s hardware address

8. ARP Operation A wants to send datagram to B
A starts with B’s IP address
A knows B is on the local network (resolved by A using network prefix)
A broadcasts ARP query packet, containing B's IP address
dest address in Ethernet frame = FF-FF-FF-FF-FF-FF
source address in Ethernet frame = A’s hardware address
all nodes on data link/single segment network, receive ARP query
Query (ARP packet) contains A’s hardware address and B’s IP address
B receives A’s ARP query packet, recognizes its IP address, replies to A with its (B's) hardware address
frame sent to A’s hardware address (unicast) containing B’s hardware address

9. ARP Packet Format 18

10. ARP Packet Format General: can be used with
Arbitrary hardware address (not just Ethernet)
Arbitrary protocol address (not just IP)
Variable length address fields (depends on type of datalink protocol)

11. Address Translation with ARP
ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of ?” ?

12. Address Translation with ARP
ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address ?

13. Example ARP Request from Argon:
Source hardware address: 00:a0:24:71:e4:44 Source protocol address: Target hardware address: 00:00:00:00:00:00 Target protocol address:
ARP Reply from Router137:
Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: Target hardware address: 00:a0:24:71:e4:44 Target protocol address:

14. Retention of Bindings
Sending an ARP request/reply for each IP datagram (to same IP address) is inefficient.
Solution -> maintain a table of bindings
devices maintain a cache of currently used IP addresses and their corresponding hardware addresses.
A device caches IP-to-hardware address pairs in a table until information becomes old (times out)
Soft state: information times out (goes away) unless refreshed (i.e., everytime an IP address is looked up in the table)
The entries expire after X minutes. Initially X=20, but because of mobility, it is now very short, sometimes just 30secs.
Effect
Use ARP one time, place results in table, then quick lookup many times (every packet sent to that same IP address)

15. Note on Bindings If A ARPs B, B keeps A’s information
B will probably send a packet to A soon
If A ARPs B, A keeps B’s information
A will probably send more packets to B
Other machines do not keep A’s information (even though they all see the broadcast with A’s information)
Avoids clogging ARP caches needlessly

16. ARP Cache (table) Contents of the ARP Cache: ARP is “plug-and-play”:
( ) at 00:10:4B:C5:D1:15 [ether] on eth0
( ) at 00:B0:D0:E1:17:D5 [ether] on eth0
( ) at 00:B0:D0:DE:70:E6 [ether] on eth0
( ) at 00:05:3C:06:27:35 [ether] on eth1
( ) at 00:B0:D0:E1:17:DB [ether] on eth0
( ) at 00:B0:D0:E1:17:DF [ether] on eth0
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator

17. Things to know about ARP
What happens if an ARP Request is made for a non-existing host?
Several ARP requests are made with increasing time intervals between requests. Eventually, ARP gives up.
What if a host sends an ARP request for its own IP address? Know as gratuitous ARP
No response hopefully
This is useful for detecting if an IP address has already been assigned (via DHCP).

18. ARP in our Example Router IP Address Router MAC address
Argon realizes that it needs to use router to reach Neon as the two hosts are on different IP networks.
Argon sends an ARP Request to the router for MAC address of NIC corresponding to IP address of router on the common network.
Router sends ARP Response to host Argon with MAC address corresponding to the IP address of its NIC on that connected network.
/24
/24
/24
/24 ?
Router IP Address is
Router MAC address

19. Proxy Arp
Allow devices on two different IP subnetworks to share a single IP network prefix
Source believes destination is on same IP network
Setup router to respond to ARP broadcast requests for destinations on different connected subnet
router masquerades as destination for ARP request sent by source on a subnet
the two devices are unaware that they are on different subnets, subnet mask indicates that they have the same network prefix.
Masquerades: router responds to broadcast ARP Request from source host that arrives on one of its connected networks for a destination host that is on one of its other connected networks.

20. Proxy ARP Operation in Our Example with Argon having /16 as its network prefix
Router responds to broadcast ARP Request from host Argon that arrives on one of its connected networks for host Neon that is on one of its other connected networks.
Host Argon believes Neon is on the same IP network because of its ”prefix” /16.
Argon sees it is on IP network When applying prefix /16 to Neon’s IP address > , which is the same as its own. so it sends a broadcast ARP request for Neon.
Router responds with its MAC address and then transfers the datagram to the next segment.
/24
/24
/24
Neon IP Address
Router MAC address

21. ARP Cache in two Examples
Non Proxy ARP:
A’s ARP cache
( ) at 00:e0:f9:23:a8:20 [ether] on eth0
Proxy ARP:
( ) at 00:e0:f9:23:a8:20 [ether] on eth0

22. ARP Transport
ARP message travels in data portion of data link layer frame
We say ARP message is encapsulated
Data area padded with zeroes if ARP message is shorter than minimum data link layer frame
For Ethernet: frame type field 0x0806 used for ARP