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
IA IB
PA PB
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 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 IA means 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

10. Why not just send the packet?
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

11. 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

12. Example Address Binding Table
IP Address
Hardware Address
0A:07:4B:12:82:36
0A:9C:28:71:32:8D
0A:11:C3:68:01:99
0A:74:59:32:CC:1F
0A:04:BC:00:03:28
0A:77:81:0E:52:FA
Example Address Binding Table

13. 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

14. 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

15. 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

16. 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

17. 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)

18. ARP Implementation Divided into two functional parts Mapping
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

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

20. 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

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

22. 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

23. Example ARP message format for Ethernet
Hardware Type
Protocol Type
HLEN
PLEN
Operation
Sender HA (octets 0-3)
Sender HA (octets 4-5)
Sender IP (octets 0-1)
Sender IP (octets 2-3)
Target HA (octets 0-1)
Target HA (octets 2-5)
Target IP (octets 0-3)

24. 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

25. 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)

26. 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

27. RARP 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