Ch 17 - Binding Protocol Addresses

1
Ch 17 - Binding Protocol Addresses

• Address translation
• Address resolution
• Address resolution techniques
• Table lookup
• Closed-form computation
• Dynamic resolution techniques
• ARP messages
• RARP messages

2
Protocol Addresses Packet Delivery

• Application layers use protocol addresses (IP)
  for hosts routers
• IP addresses are maintained by software (virtual)
• "Virtual network" addressing scheme hides
  hardware details
• Network hardware uses Physical (hardware)
  addresses for eventual delivery
• Therefore, protocol address must be translated
  into hardware address for delivery

3
Address Translation

• Upper layers use only protocol addresses (IP)
• Upper layer hands down protocol address of
  destination to data link layer
• Data link layer translates it into hardware
  address for use by hardware layer
• Finding a hardware address (MAC) for any protocol
  address (IP)
• Data link layer resolves protocol address to
  hardware address
• Resolution is local to a network
• Network component resolves address for other
  components on the same network only

4
Address Resolution Techniques

• Binding is the association between the IP address
  and the hardware (Physical) address
• Address resolution is local to a network
• TCP/IP uses three techniques to find a Physical
  address from IP Address depending on the hardware
  technologies
• 1. Table lookup bindings stored in table with
  protocol address as key used in WAN
• Data link layer looks up protocol address to find
  hardware address

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Address Resolution Techniques

• 2. Closed-form computation hardware address can
  be calculated from protocol address using basic
  logic arithmetic operations used in
  Configurable networks
• 3. Message exchange hosts exchange messages
  across a network to resolve address when needed
  used in LAN
• Data link layer sends message requesting hardware
  address destination responds with its hardware
  address
• Which algorithm should be used? Depends on the
  protocol and hardware addressing schemes.

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Address Resolution with Table Lookup

• Use an array of 2 entries IP addresses and
  hardware addresses (P, H) for each host on the
  net
• Use protocol address (IP) to extract
  corresponding hardware address (H)

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Table Lookup (contd)

• Sequential (Direct) search is suitable for a
  small net (O(n2))
• Advantages easy to implement and can be used for
  an arbitrary set of computers
• Disadvantage large computation time for large
  networks
• Indexing or hashing is efficient for large nets
  (O(n))
• use host-id part of IP address as an index

8
Closed-form Computation

• Used for small configurable hardware addresses
• Network administrator can choose hardware
  address based on the IP address
• Example hardware uses one octet address that can
  be configured
• For IP address 198.109.25.23 use 23 as hardware
  address
• Simply choose hardware address to be hostid to
  make translation trivial
• Now, any host can determine hardware address as
• hardware_address ip_address AND 000f
• It is often used with configurable networks
• Easy to program
• Computational efficient
• There is no need to maintain a table

9
Message Exchange (ARP)

• Any computer that needs to find HW address sends
  a message across the network and waits for a
  reply
• Two approaches
• Server-based (centralized) computer sends
  message to ARP server to resolve address
  serveranswers all address resolution inquires
• Advantages centralization, easy to configure,
  manage and maintain on non-broadcast media (e.g.,
  ATM)
• Disadvantages availability and reliability
• Distributed all computers participate
  destination provides hardware address to host
• Advantages no ARP servers required, no
  administration
• Disadvantages could increase network traffic

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Dynamic Resolution Techniques

• Table lookup (T)
• Closed-form computation (C)
• Dynamic message exchange (D)

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Address Resolution Protocol (ARP)

• IP uses distributed resolution technique
• ARP is part of TCP/IP protocol suite
• Two types of messages
• Request contains an IP address and requests a
  hardware address for this IP Address
• An ARP request is broadcast to all computers
• Reply from destination carrying both the IP
  address and the requested hardware address.
• An ARP response is sent as a reply only to the
  requesting computer

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ARP Message Exchange

• ARP request message dropped into hardware frame
  and broadcast
• Uses separate protocol type in hardware frame
  (Ethernet 806)
• Sender inserts IP address into message and
  broadcast
• Every computer examines the request
• Computer whose IP address is in request responds
• Puts hardware address in response
• Unicasts to sender
• Original requester can then extract hardware
  address and send IP packet to destination

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ARP Message Delivery Example

• A computer broadcast a request
• What is the HW address of 198.109.25.23?
• The computer whose address is 198.109.25.23 will
  reply with
• the HW address for 198.109.25.23 is
  11.51.AA.33.55.66

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ARP Message Format

• Maps IP address to hardware address
• Both protocol address and hardware address sizes
  are variable depending on the technology
• Ethernet 6 octets (48 bits).
• IP 4 octets (32 bits)
• Can be used for other protocols and hardware
  types

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ARP Message Format

• ARP messages are sent directly to MAC layer
• The ARP standard describes a general form for ARP
  messages
• H/W ADDRESS TYPE 1 for Ethernet
• PROTOCOL ADDRESS TYPE 0x0800 for IP
• OPERATION 1 for request, 2 for response
• Contains both target and sender mappings from
  protocol address to hardware address
• Request sets hardware address of target to 0
• Target can extract hardware address of sender
  (saving an ARP request)
• Target exchanges sender/target in response

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ARP Message Format

• (6bytes)
• Properties
• Hardware address fields are not fixed
• A fixed field specifies the size of the hardware
  addresses
• Most often used to bind a 32-bit IP address to a
  48-bit hardware address

.. (4 bytes)
..
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Sending an ARP Message

• Sender constructs ARP message into HW frame
• ARP message carried as data in hardware frame -
  encapsulation

ARP MESSAGE
FRAME DATA AREA (What is the addr. of..)
FRAME HEADER
CRC
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Identifying ARP Frames

• How does a computer know if the frame is ARP?
• Uses separate frame type for Ex. Ethernet uses
  type 0x0806 to indicate
• Operation field used to indicate incoming message
  as Request or Response

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Caching ARP Responses

• Problem sending multiple ARP requests is not
  efficient for networks i.e. use ARP for each IP
  packet which adds two packets of overhead for
  each IP packet
• Solution uses locality of reference concept and
  caches ARP responses in a ARP table in memory
• New entry replaces old entry when the table is
  out of space
• Oldest entry is removed if it has not been
  updated for a "long" time (gt20 Min)
• Cache searched prior to sending ARP request
• If binding is in cache, ARP will use it
• Otherwise
• ARP broadcasts a request
• waits for a response
• Updates the cache and then proceeds to use the
  binding

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Processing ARP Messages

• When ARP protocol arrives, the Receiver
• Extracts sender's hardware address and updates
  its local ARP table
• Examines operation field request or response ?
• Response
• Adds sender's address to local cache
• Sends pending IP packet(s)
• Request
• If receiver is target, forms response
• Unicasts to sender
• Adds sender's address to local cache
• Saving to local Cache is important
• Computer Communication involves 2-way traffic
• Computer memory to store bindings is limited

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Layering and Address Resolution

• IP addresses and ARP used to hide the details of
  physical addressing and allows generality in
  upper layers
• ARP is accomplished in the network interface
  layer

Applications
I P Addresses Used
Higher Layers of Protocol Software
Protocol Address Boundary
Address Resolution Device Driver
Hardware Addresses Used
Network Hardware
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Reverse Address Resolution Protocol (RARP)

• RARP-request is used by diskless computer to
  find its own IP address (broadcast Dst. is all
  1s)
• What is the IP address for this HW address?
• RARP-SERVER will process this request and reply
  with
• The IP address for this HW address is ..
• RARP-SERVERS have Cache table which includes both
  addresses

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Other ARP

• Proxy RARP A router may act as a proxy for many
  IP addresses
• Inverse ARP
• find an IP address of a computer on the opposite
  end of a hardware connection
• Used for ATM and Frame Relay

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Summary

• Address resolution translates IP address to
  hardware address which can be done by
• Table lookup (static) which is often used for WAN
• Closed form computation which is used with
  config. net to extract HW address from IP address
  
• Message exchange (Dynamic) uses network messages
  to resolve protocol address
• TCP/IP protocol uses ARP for address resolution
  to find the physical address of an IP address
• RARP-request is used by diskless computer to
  find its own IP address