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Ch 17 - Binding Protocol Addresses. Address translation Address resolution Address resolution techniques: Table lookup Closed-form computation Dynamic resolution techniques ARP messages RARP messages. Protocol Addresses & Packet Delivery.

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ch 17 binding protocol addresses
Ch 17 - Binding Protocol Addresses
  • Address translation
  • Address resolution
  • Address resolution techniques:
    • Table lookup
    • Closed-form computation
    • Dynamic resolution techniques
  • ARP messages
  • RARP messages
protocol addresses packet delivery
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
address translation
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
address resolution techniques
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
address resolution techniques5
Address Resolution Techniques
  • 2. Closed-form computation: hardware address can be calculated from protocol address using basic logic & arithmetic operationsused in Configurable networks
  • 3. Message exchange: hostsexchange 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.
address resolution with table lookup
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)
table lookup cont d
Table Lookup (cont’d)
  • 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
closed form computation
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 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
message exchange arp
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
dynamic resolution techniques
Dynamic Resolution Techniques
  • Table lookup (T)
  • Closed-form computation (C)
  • Dynamic message exchange (D)
address resolution protocol arp
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
arp message exchange
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
arp message delivery example
















ARP Message Delivery Example
  • A computer broadcast a request:

“What is the HW address of”

  • The computer whose address is will reply with:

“the HW address for is 11.51.AA.33.55.66

arp message format
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
arp message format15
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
arp message format16

0 8 16 24 32

H/W Address Type

Protocol Address Type

H/W Adr Len

Prot Adr Len


Sender’s H/W Address (6 bytes)

Sender’s Prot Address...

Target H/W Address….

Target Protocol Address (4 bytes)

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)


sending an arp message
Sending an ARP Message
  • Sender constructs ARP message into HW frame
  • ARP message carried as data in hardware frame - encapsulation


FRAME DATA AREA (What is the addr. of..)




identifying arp frames
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
caching arp responses
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 (>20 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
processing arp messages
Processing ARP Messages
  • When ARP protocol arrives, the Receiver:
    • Extracts sender's hardware address and updates its local ARP table
    • Examines operationfield: 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
layering and address resolution
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


I P Addresses Used

Higher Layers of

Protocol Software

Protocol Address


Address Resolution

Device Driver


Addresses Used

Network Hardware

reverse address resolution protocol rarp
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
other arp
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
  • 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