Chapter 19: Binding Protocol Addresses ARP

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Chapter 19 Binding Protocol Addresses (ARP)

• Jing Wang
• Towson University

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19.1. Introduction

• Before protocol software can send a packet across
  a physical network, it must translate IP address
  of the destination computer into hardware address
• Three mechanisms used to perform address mapping
• Table lookup
• Closed-form computation
• Message exchange

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19.2. Protocol Addresses And Packet Delivery

• Protocol addresses are abstractions provided by
  software physical network hardware does not know
  how to locate a computer from its protocol
  address.
• The protocol address of the next hop must be
  translated to an equivalent hardware address
  before a packet can be sent.

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19.3. Address Resolution

• Translation from a computers protocol address to
  an equivalent hardware address is known as
  address resolution, and a protocol address is
  said to be resolved to the correct hardware
  address
• Address resolution is local to a network.

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19.3. 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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19.3. Address Resolution

• 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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19.4. Address Resolution Techniques

• What algorithm does software use to translate a
  protocol address into a hardware address?
• The answer depends on the protocol and hardware
  addressing schemes
• The method used to resolve an IP address to an
  Ethernet address differs from the method used to
  resolve an IP address to an ATM address
• Because Ethernet addressing scheme differs from
  the ATM address scheme.
• A router or multi-homed host can connect to
  multiple types of physical networks, need more
  than one address translation module

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19.4. Address Resolution Techniques

• Table lookup
• Binding or mappings are stored in a table in
  memory, which the software searches when it needs
  to resolve an address
• Closed-form computation
• The protocol address assigned to a computer is
  chosen carefully so the computers hardware
  address can be computed from the protocol address
  using basic Boolean and arithmetic operation.
• Message exchange
• Computers exchange messages across a network to
  resolve an address. One computer sends a message
  that requests an address binding (i.e.,
  translation), and another computer sends a reply
  that contains the requested information.

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19.5. Address Resolution With Table Lookup

• Figure 19.2. An example address binding table.
  Each entry in the table contains a protocol
  address and the equivalent hardware address.

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19.5. Address Resolution With Table Lookup

• A separate address binding table is used for each
  physical network
• Chief advantage
• Generality
• Easiness
• For a network that contains less than a dozen
  hosts, a sequential search can suffice
• For large networks, to improve computational
  efficiency
• Hashing
• Direct indexing
• When protocol addresses are assigned from a
  compact range

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19.5. 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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19.6. 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.
• Suppose a configurable network 220.123.5.0/24
• IP address 220.123.5.1, hardware address 1
• Hardware_address ip_address 0xff

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19.7. Address Resolution With Message Exchange

• A distributed approach in which 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
  protocol address, and the reply carries the
  corresponding hardware address

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19.7. Address Resolution With Message Exchange

• Where should an address resolution request to
  sent?
• A network includes one or more servers that are
  assigned the task of answering address resolution
  requests.
• Centralization
• Easier to configure, manage, and control
• Servers expensive, bottleneck
• Each computer on the network participates in
  address resolution by agreeing to answer
  resolution requests for its address.
• Distributed computation
• Eliminates servers

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19.7. Address Resolution With Message Exchange

• 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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19.8. Address Resolution Protocol

• The method chosen depends on the addressing
  scheme used by hardware
• Table lookup WAN
• Closed-form computation configurable network
• Message exchange LAN hardware that has static
  addressing
• Address Resolution Protocol (ARP)
• Request an IP address
• Reply both IP address and hardware address

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19.9. ARP Message Delivery

• Figure 19.5. An ARP message exchange. (a)
  Computer W begins to broadcast an ARP request
  that contains computer Y's IP address. (b) All
  computers receive the request, and (c) computer Y
  sends a response directly to W.

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19.10. ARP Message Format

• Although the ARP message format is sufficiently
  general to allow arbitrary protocol and hardware
  addresses, ARP is almost always used to bind a
  32-bit IP address to a 48-bit Ethernet address.

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19.10. ARP Message Format

• Figure 19.6. The format for an ARP message when
  used to bind Internet protocol addresses to
  Ethernet hardware addresses.

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19.10. ARP Message Format

• HARDWARE ADDRESS TYPE
• 1 when ARP is used with Ethernet
• PROTOCOL ADDRESS TYPE
• 0x0800 when ARP is used with IP
• HADDR LEN
• Number of octets in a hardware address
• 6 octets of Ethernet hardware addresses
• PADDR LEN
• Number of octets in a protocol address
• 4 octets of IP protocol addresses
• OPERATION
• 1 request
• 2 - response

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19.11. 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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19.12. Identifying ARP Frames

• Figure 19.8. 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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19.13. Caching ARP Responses

• To reduce network traffic, ARP software extracts
  and saves the information from a response so it
  can be used for subsequent packets.
• ARP maintains a small table of bindings in
  memory, and manages the table as a cache.
• An entry is replaced whenever a response arrives,
  and the oldest entry is removed whenever the
  table runs out of space or after an entry had not
  been updated for a long period of time (e.g., 20
  min)
• Whenever ARP performs address binding, it
  searches the cache before using the network.

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19.14. Processing An Incoming ARP Message

• The receiver extracts the senders address
  binding and checks to see if the senders address
  is present in the cache. If so, update the
  previously stored binding.
• The receiver examines the OPERATION field to
  determine what to do
• After a computer replies to an ARP request, it
  extracts the senders address binding from the
  request and adds the binding to its cache for
  later use.

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19.15. Layering, Address Resolution, Protocol
Addresses

• 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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19.16. Summary

• Software must translate the protocol address to
  an equivalent hardware address before sending a
  packet the translation is called address
  resolution
• Three general methods are used for address
  resolution the method used depends on the
  underlying hardware
• TCP/IP suite contains a standard ARP, which
  defines the format of messages and rules for
  handling ARP messages.