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Mapping Internet Addresses to Physical Addresses (ARP)

Mapping Internet Addresses to Physical Addresses (ARP). Chapter 5. Each host has 32-bit address (IP address) Internet uses only assigned addresses for packet sending and receiving Two machines can communicate only if they know each other’s physical address

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Mapping Internet Addresses to Physical Addresses (ARP)

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  1. Mapping Internet Addresses to Physical Addresses (ARP) Chapter 5

  2. Each host has 32-bit address (IP address) • Internet uses only assigned addresses for packet sending and receiving • Two machines can communicate only if they know each other’s physical address How does host (or router) map an IP address to a correct physical address??

  3. Address Resolution Problem • Refers to problem of mapping high-level addresses to physical addresses • Suppose two machines A and B are connected to the same physical network A------------------------------------B IAIB PAPB • What if A only has B’s IP address? How does A map this to B’s physical address?

  4. Address mapping must occur at each step along a path as well • In general, two cases: • Case 1: • Last step in delivering a packet • Last computer must map final destination’s internet address to the destination’s physical address • Case 2: • Any point along the path when sending to intermediate router • Sender must map intermediate router’s internet address to a physical address

  5. Two basic types of physical addresses: • proNET • Small, easily configurable physical addresses • Address resolution is easy • Ethernet • Large, fixed physical addresses • Address resolution is hard

  6. Direct Mapping Resolution • Consider a proNET token ring • Uses small integers for physical addresses • Allows user to choose hardware address • Make address resolution easy by making part of the IP & physical address the same • Assign IP addresses with hostid portion equal to 1, 2, 3, … • When installing interface card, select physical address of 1, 2, 3, … • IP 192.5.48.3 get physical address 3

  7. Mapping is easy • Just extract host portion of IP address • Conceptually, select a function f that maps IP addresses to physical addresses • Resolving IP address IAmeans computing PA = f (IA) • Want computation as efficient as possible

  8. Dynamic Binding Resolution • Consider an Ethernet • Each interface has a 48-bit physical address • Interface fails, address changes • 48-bits can’t be encoded into 32-bit IP addresses • Use Address Resolution Protocol (ARP) • Host sends packet requesting intended destination to respond with its physical address • All receive; destination recognizes its IP address and responds

  9. ARP: • allows a host to find the physical address • of a target on the same physical network • given only the target’s IP address • Why not just send the packet? • The request for address is broadcast • Too expensive to broadcast every packet • All machines must process the packet

  10. ARP Cache • Maintain a cache of recently acquired pairs • Keep IP, PhyAddr pairs to reduce comm costs • Looks in cache before sending packet • If have a binding, use it • If not, broadcast an ARP request • Since most communications involve multiple packets, even a small cache helps

  11. Example Address Binding Table

  12. Soft state • Information becomes stale without warning • Example: • Computer A gets binding for computer B • B then crashes • A does not get notified • A continues to send packets to B • Ethernet does not have guaranteed delivery! • No way of knowing ARP cache is wrong

  13. Responsibility for correctness lies with the owner of the information • Use timer to periodically delete old bindings • Typical timeout is 20 minutes (since set) • Information must be removed • Two cases then: • 1) If no more packets, nothing happens • 2) If have more packets, do another ARP broadcast • If destination reachable, will get new binding • If not, sender will know

  14. Advantage of soft state is autonomy • Computer knows to revalidate bindings independent of other computers • Sender does not need communication with anyone to invalidate a binding • No reliance on network hardware for reliable transfer • Disadvantage is delay • Time to detect a crash is potentially as large as the timeout window

  15. Refinements to ARP • Include own binding when send a request • Queried machine updates its ARP cache • All machines can update as well • After a crash, send out an ARP request as part of the booting process

  16. ARP is only one scheme for mapping • Some network technologies don’t need it • Really imposes new address scheme on to of lower-level hardware address mechanism • Think of ARP as part of physical network system (vs part of internet protocols)

  17. ARP Implementation • Divided into two functional parts • Mapping IP address to physical address • Answering requests • Mapping • If have binding in cache, use it • If not, send broadcast • May not get reply (lost or machine down) • Must store outgoing packets • If other programs run, must not generate multiple requests for the same address currently waiting on

  18. When ARP reply arrives • Update ARP cache • Remove packet(s) from queue • May have been packets from other applications • Send

  19. Answering requests • First extract sender’s binding • If cache entry exists, update it • Next, process the packet • If target of request, then answer • If not target, ignore rest of packet

  20. ARP Encapsulation • ARP messages travel in data portion of a frame • Sender assigns special type value in header

  21. ARP Protocol Format • ARP packets do not have fixed format header • Length of fields with addresses depend on the type of network • Header includes fixed fields near beginning • Format is general enough to be used with arbitrary physical addresses and arbitrary protocol addresses

  22. Example ARP message format for Ethernet

  23. Automatic Cache Revalidation • Jitter • Variance in packet transfer times • ARP timer expires; next datagram has extra delay • Automatic revalidation • Use a second (revalidation) timer for each entry • Goes off early; sends ARP request • Can still use ARP cache entry • If station replies, both timers reset • If no reply, act as normal when traditional timer expires

  24. Reverse ARP (RARP) • Operation field can specify Reverse ARP • Allows system to obtain its IP address at startup • Computer can know its hardware address, but perhaps not its IP address (ex.: diskless systems) • When booting, system broadcasts a RARP request • RARP server has to be configured and listening • RARP server sends reply giving requester’s IP address • RARP no longer used • At one time was essential for computers without stable storage (see DHCP, Chap. 22)

  25. Summary • IP addresses are independent of physical addresses • To send packets, must do mapping • Direct mapping • If physical addresses smaller than IP addresses • Dynamic mapping • ARP performs dynamic address resolution

  26. ARP broadcast finds physical addresses • All machines receive ARP broadcast • If IP address matches machine’s, answer request • Replies directed to one machine only (not broadcast) • For efficiency, bindings are cached • Cache helps eliminate many broadcast • Early revalidation can be used to avoid jitter • RARP • Could be used to obtain IP address at startup • Now obsolete

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