Ch 17 - Binding Protocol Addresses Address translation Address resolution Address resolution techniques: Table lookup Closed-form computation Dynamic resolution techniques ARP messages RARP messages 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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. 4/24/2019

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) 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

Dynamic Resolution Techniques Table lookup (T) Closed-form computation (C) Dynamic message exchange (D) 4/24/2019

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 4/24/2019

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 4/24/2019

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 V W X Y Z 4/24/2019

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 4/24/2019

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 4/24/2019

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) 0 8 16 24 32 .. (4 bytes) .. 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019

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 4/24/2019