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Address Resolution Protocol (ARP)

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ARP is also used by routers (See next ) Address Resolution Some protocol suites adopt one of the following: Keep mapping tables in each machine Hardware ... – PowerPoint PPT presentation

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Title: Address Resolution Protocol (ARP)


1
Address Resolution Protocol (ARP)
2
Mapping IP Address to Data-Link Address
  • How does a machine map an IP address to its
    Data-Link layer (hardware or MAC) address?
  • This is needed by the source host A who needs to
    send an IP packet to another host B
  • if B is on same IP network as A, address frame
    to B. Thus, DL-Address of B is needed.
  • if B on a separate network, address frame to a
    router. Thus, DL-Address of router is needed.
  • ARP is also used by routers (See next slide)

3
Four cases using ARP
4
Address Resolution
  • Some protocol suites adopt one of the following
  • Keep mapping tables in each machine
  • Hardware (physical) addresses are encoded in the
    high level addresses
  • Both are ad-hoc, awkward solutions

5
Resolution Through Dynamic Binding
  • Ethernet uses 48-bit physical addresses
  • Addresses assigned by manufacturers
  • Replacing a faulty interface card meant a change
    to the machine physical address
  • Cant encode 48-bit long address into a 32-bit
    long IP address
  • TCP/IP solution Address Resolution Protocol (ARP)

6
ARP
  • Exploits broadcast capability of Ethernet
  • Allows a host to find the Ethernet address of a
    target host on the same network, given the
    targets IP address
  • Allows machines to be added or removed with no
    code recompilation
  • Builds and maintains dynamically a table to
    translate IP addresses into Ethernet physical
    addresses

7
ARP operation
8
ARP (contd.)
  • Hosts that use ARP maintain a small cache of
    recently acquired (IP,Hardware) address bindings
  • Cache is updated dynamically
  • Timer for each entry
  • Whenever a new binding is received, update the
    corresponding table entry and reset the
    associated timer
  • ARP is a low level protocol that hides the
    underlying network physical addressing,
    permitting us to assign IP addresses of our
    choosing to every machine

9
ARP (contd.)
  • We think of it as part of the physical network
    and not as part of the internet protocols
  • Functionally ARP is divided in two parts
  • One part that determines physical address of the
    destination of the outgoing packet
  • One part answers requests from other machines

10
ARP Implementation (contd.)
  • Address resolution of outgoing packet
  • IF dest. IP address is in cache THEN
  • Get corresponding physical address
  • Construct an Ethernet frame
  • Else
  • Broadcast an ARP request
  • Wait for an ARP reply

11
ARP Implementation (contd.)
  • Two types of incoming ARP packets
  • A Request
  • IF this machine is the target of ARP Request THEN
    Generate ARP Reply and send it to requester
  • ELSE Ignore the request
  • A Reply
  • Determine if we need to create a cache entry or
    update existing entry
  • Match Reply with initial Request

12
ARP Encapsulation in Ethernet Frame
  • Eth. DA (Ethernet Destination Address)
  • For ARP Request it is set to FF FF FF FF FF
    FF

13
ARP Packet Format (contd.)
8
0
16
24
31
Hardware Type
Protocol Type
(0x0001 Ethernet)
(0x0800 IP)
Operation (1/2 ARP-Req,/Rep, 3/4 RARP-Req/Rep)
HALEN
PALEN
SENDER HA (Bytes 0-3)
SENDER IP (Bytes 0-1)
SENDER HA (Bytes 4-5)
SENDER IP (Bytes 2-3)
TARGET HA (Bytes 0-1)
TARGET HA (Bytes 2-5)
TARGET IP (Bytes 0-3)
14
ARP Packet Format
  • To allow ARP to be used with a variety of network
    technologies, header of ARP messages does not
    have a fixed format
  • First fields in the header specify the length of
    succeeding fields
  • On Ethernet, 28-octet ARP message format is used

15
Example
A host with IP address 130.23.43.20 and physical
address B23455102210 has a packet to send to
another host with IP address 130.23.43.25 and
physical address A46EF45983AB. The two hosts
are on the same Ethernet network. Show the ARP
request and reply packets encapsulated in
Ethernet frames.
Solution Figure shows the ARP request and reply
packets. Note that the ARP data field in this
case is 28 bytes, and that the individual
addresses do not fit in the 4-byte boundary. That
is why we do not show the regular 4-byte
boundaries for these addresses.
16
an ARP request and reply
17
Additional Notes about ARP
  • Every host that receives the broadcast ARP
    request caches the Sender's IP Address/Hardware
    Address
  • When a station boots it sends a broadcast ARP
    request with target IP address matching its own
  • This enables other hosts to cache the sender's
    info.
  • If the station gets a reply, it knows that its IP
    address is used by some other host and thus we
    know of duplicate IP addresses

18
Determining an IP Address at Startup
  • Diskless machines use IP addresses to communicate
    with the file server
  • Also, many diskless machines use TCP/IP FTP
    protocols to obtain their initial boot image,
    thus requiring that they obtain and use IP
    addresses
  • Designers keep both the bootstrap code and
    initial OS images free from specific IP addresses
    for portability

19
Determining IP Address at Startup (contd.)
  • How does a diskless machine determine its IP
    address?
  • When bootstrap code starts execution on a
    diskless machine, it must use the network to
    contact a server to obtain the machines IP
    address
  • Usually, a machines IP address is kept on disk
    where OS finds it at startup

20
Reverse Address Resolution Protocol (RARP)
  • RARP is the protocol used to solve the reverse
    problem solved by ARP
  • Given a physical address, get the corresponding
    IP address
  • RARP uses the same message format as ARP
  • RARP messages are sent encapsulated in Ethernet
    frames
  • These days, DHCP (Dynamic Host Configuration
    Protocol) is a newer protocol that is used as a
    replacement for RARP. Thus, we can say that DHCP
    has rendered RARP (but not ARP) obsolete

21
RARP (contd.)
RARP_Request
RARP_Replies
Y
X
D
A
C
RARP Server
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