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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.

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Ch 17 - Binding Protocol Addresses

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  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

  5. 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.

  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)

  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

  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

  10. Dynamic Resolution Techniques • Table lookup (T) • Closed-form computation (C) • Dynamic message exchange (D)

  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

  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

  13. V W X Y Z V W X Y Z V W X Y Z 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

  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

  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

  16. 0 8 16 24 32 H/W Address Type Protocol Address Type H/W Adr Len Prot Adr Len Operation 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) ..

  17. 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

  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

  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

  20. 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

  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

  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

  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

  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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