Binding Protocol Addresses ARP

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Binding Protocol Addresses (ARP)

• P. Martins
• Department of Computer Science and Computer
  Information Systems

2
For Meditation.

• What is the purpose of ARP (Address Resolution
  Protocol) ?
• What are the three mechanisms used for address
  translation?

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Protocol Addresses

• When application program generates data to be
  sent across an internet, software places data in
  a packet
• The packet contains the protocol address (IP
  address) of the destination.
• Software works with IP addresses when forwarding
  packets.
• Hardware does not understand IP addressing.
• Protocol addresses cannot be used when
  transmitting frames across physical network
  hardware

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Protocol Addresses

• The software (protocol) address of a next hop
  must be translated to an equivalent hardware
  address before a frame can be sent.

ARP
HW address
SW address
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Protocol Addresses

• Protocol addresses are abstractions provided by
  software.
• Physical network hardware does not know how to
  locate a computer from its protocol address.

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Address Resolution

• Consider the simple internet that next slide
  illustrates.
• In the figure, hosts A and B attach to the same
  physical network.
• If an application on host A sends data to an
  application on host B, the application uses Bs
  IP address as the destination.
• Protocol software on A resolves Bs IP address to
  Bs hardware address, and uses the hardware
  address to send the frame directly.

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Address Resolution
Figure 19.1 A simple internet with routers R1 and
R2 connecting three physical networks each
network has two host computers attached. A
computer can only resolve the address of a
computer attached to the same physical network.
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Address Resolution

• Translation from a computers protocol address to
  an equivalent hardware address is known as
  address resolution.
• The protocol address is said to be resolved to
  the correct hardware address.
• Address resolution is local to a network.
• One computer can resolve the address of another
  computer only if both computers attach to the
  same physical network.

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Address Resolution

• If an application on host A sends a message to an
  application on host F, which lies on a remote
  network, software on host A does not resolve Fs
  address.
• Each computer that handles a packet resolves a
  next-hop address before sending.

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To summarize

• Mapping between a protocol address and a hardware
  address is called address resolution.
• A host or router uses address resolution when it
  needs to send a packet to another computer on the
  same physical network
• A computer never resolves the address of a
  computer that attaches to a remote network.

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Address Resolution Techniques

• What algorithm does software use to translate a
  protocol address into a hardware address?
• It depends on the protocol and hardware
  addressing schemes.
• The method used to resolve IP address to an
  Ethernet address differs from the method used to
  translate IP address to an ATM address.

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Address Resolution Techniques

• Address resolution algorithms can be
• Table lookup. Bindings or mappings are stored in
  a table in memory, which the software searches
  when it needs to resolve an address
• Closed-form computation. The hardware address is
  calculated from the IP address.
• Message exchange. Computers exchange messages
  across a network to resolve an address.

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Address Resolution with Table Lookup

• Requires a data structure data contains
  information about address bindings.
• The table consists of an array.
• Each entry in the table contains a pair (P,H),
  where P is the protocol address and H is the
  equivalent hardware address.

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Address Resolution with Table Lookup
Figure 19.2 An example of address binding
table. Each entry in the table contains a
protocol address and the equivalent hardware
address.
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Address Resolution with Table Lookup

• A separate address binding table is used for each
  physical network.
• Consequently, all IP address in a given table
  have the same prefix.
• Implementations can save space by omitting the
  prefix from table entries.

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Address Resolution with Table Lookup

• The chief advantage of the table lookup approach
  is generality
• A table can store the address bindings for an
  arbitrary set of computers on a given network.
• The algorithm is straightforward and the easiest
  to program

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Address Resolution with Table Lookup

• For a network that contains less than a dozen
  hosts, a sequential search can suffice
• For large networks, we use hashing or direct
  addressing.

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Address Resolution with Table Lookup
Figure 19.3 An example of direct lookup for a
class C network. The host portion of an address
is used as an array index.
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Address Resolution with Closed-Form Computation

• Many network technologies use static physical
  addresses
• Some technologies use configurable addressing in
  which a network interface can be assigned a
  specific hardware address.
• For such networks, it is possible to choose
  addresses that make closed-form address
  resolution possible.

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Address Resolution with Closed-Form Computation

• A resolver computes a mathematical function
  that maps an IP address to a hardware (MAC)
  address
• The computation requires only a few arithmetic
  operations.
• The addresses can be chosen to optimize the
  translation.

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Address Resolution with Closed-Form Computation

• When a computer connects to a network that uses
  configurable addressing, the local network
  administrator must choose a hardware address as
  well as an IP address.
• The two values can be chosen to make address
  resolution trivial.

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Address Resolution with Message Exchange

• A computer that needs to resolve an address sends
  a message across a network and receives a reply.
• The message carries a request that specifies the
  IP protocol address.
• The reply carries the corresponding MAC hardware
  address.

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Address Resolution with Message Exchange

• Two designs
• Centralized. In some protocol suites, there are
  address resolution servers. Requests are sent to
  the servers.
• Decentralized. Each computer participates in
  address resolution by agreeing to answer
  resolution requests for its address. No servers
  are needed. Requests are broadcast messages.

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Comparison
25
Summary of features
Figure 19.4 Comparison of address resolution
using a table lookup (T), closed form computation
C, and dynamic message exchange D
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Address Resolution Protocol

• TCP/IP can use any of the three resolution
  methods.
• Table lookup is usually employed to resolve IP
  addresses across a WAN
• Closed-form computation is used with configurable
  networks.
• Message exchange is used on LAN hardware that has
  static addressing.

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Address Resolution Protocol

• To guarantee that all computers agree on the
  exact format and meaning of messages used to
  resolve addresses, TCP/IP protocol suite includes
  an Address Resolution Protocol (ARP).
• The ARP defines two basic messages
• Request. contains the IP address and requests
  the corresponding hardware (MAC) address
• Response. Contains both the MAC address and the
  IP address.

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ARP Message Delivery
Figure 19.5 An ARP message exchange. A) Computer
W begins to broadcast an ARP request that
contains computer Ys IP address b) All computers
receive the request, and c) computer Y sends a
response directly to W.
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ARP Message Format
Figure 19.6 The format for an ARP message when
used to bind Internet protocol addresses to
Ethernet addresses.
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Sending an ARP Message
Figure 19.7 Illustration of an ARP message
encapsulated in an Ethernet frame. The entire
ARP message travels in the data area of the
frame the network hardware neither interprets
nor modifies contents of the ARP message.
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Identifying ARP frames
Figure 10.8 Illustration of the type field in
an Ethernet header used to specify the frame
contents. A value of 0x806 informs the receiver
that the frame contains an ARP message.
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Layering, Address Resolution, Protocol Addresses
Network interface physical layer
Figure 19.9 Layered protocol software in a
computer and the conceptual boundary between
the network interface layer and higher layers.
Software above the boundary uses protocol
addresses software below the boundary
translates each protocol address to an
equivalent hardware address.
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Layering, Address Resolution, Protocol Addresses

• The lowest layers of the TCP/IP model
• Physical layer (use MAC addresses)
• Network Interface (translate MAC/IP addresses)
• The higher layers use protocol addresses
• Internet Protocol
• Transport Protocol
• Application Protocol

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Definitions
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Direct Addressing

• Direct addressing is slightly more efficient, but
  less general technique.
• Direct addressing is possible only in cases where
  protocol addresses are assigned from a compact
  range.
• See figure 19.3. The addresses are sequential
  values that begin with 197.15.3.2.
• The host suffix is used as an index into the
  array. gtgt back