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
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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
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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
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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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5. 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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6. 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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7. 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
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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 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
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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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10. Dynamic Resolution Techniques
Table lookup (T)
Closed-form computation (C)
Dynamic message exchange (D)
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11. 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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12. 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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13. 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 AA V W X Y Z
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14. 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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15. 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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16. 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
H/W Adr Len
H/W Address Type
Protocol Address Type
Prot Adr Len
Operation
Sender’s H/W Address (6 bytes)
Sender’s Prot Address...
Target H/W Address….
Target Protocol Address (4 bytes)
.. (4 bytes) ..
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17. FRAME DATA AREA (What is the addr. of..)
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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18. 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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19. 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
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20. 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
Saving to local Cache is important:
Computer Communication involves 2-way traffic
Computer memory to store bindings is limited
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21. 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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22. 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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23. 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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24. 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
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