1. CSC/ECE 573 Internet Protocols
ARP, RARP

2. Address Resolution Problem
Higher-level applications should work only with IP addresses
illusion of a single, virtual network
Communication is carried out by physical networks
IP datagrams are encapsulated in MAC frames  MAC (hardware) addresses are needed
Address resolution problem: mapping high-level (IP) addresses to physical (MAC) addresses
“Last net” problem

3. Solutions
Direct mapping: function f maps IP addresses to hardware addresses
computation of f must be efficient
works well when physical addresses are small and configurable
Configuration file provides the mapping
not scalable
not easy to update
Dynamic binding: ARP (RFC 826)
for broadcast networks such as Ethernet, Token Ring, etc.
(why not just broadcast all packets?)

4. ARP vs. RARP

5. ARP Operation

6. ARP Packet Format
(Ethernet frame type= 0x0806)

7. (Reminder: Ethernet Frame Format)

8. Using ARP: Case 1

9. Using ARP: Case 2

10. Using ARP: Case 3

11. Using ARP: Case 4

12. ARP Cache Sending n packets to destination x: n ARP Requests?
Inefficient; one ARP Request per data packet!
Better: cache ARP Replies
Also, upon receipt of an ARP Request, all machines update their cache entry for the sender of the request
Problems with caching
cache space may be limited
hosts move or change IP addresses
solution: drop (invalidate) cache entries after “a while” (20 minutes)

13. Gratuitous ARP
Every machine (should) broadcast its mapping when it boots
use ARP Request with own IP address as the target IP address
what should happen if an ARP Reply is received?

14. The arp Command Used to query or update the ARP cache Forms
arp hostname
arp -a
arp -d hostname
arp -f filename
arp -s hostname ether_address [ temp ] [ pub ] [ trail ] P
Flags
P = Publish; ARP will respond to ARP requests for this address
S = Static; not learned from the ARP protocol.
U= Unresolved; waiting for ARP response.
(M = Mapping; only used for multicast entries)

15. ARP Cache Contents Example
Net to Media Table
Device IP Address Mask Flags Phys Addr
le0 eos03ro.eos.ncsu.edu :d0:bc:ed:23:64
le0 perch.cacc.ncsu.edu :00:20:73:95:ab
le0 shrimp.cacc.ncsu.edu :00:20:90:a3:7f
le0 bass.cacc.ncsu.edu :00:20:73:98:c4
le0 multimedia :00:5a:8a:0c:79
le0 aphrodite :00:20:87:be:54
le0 artemis :00:20:87:bc:d4
le0 athena :00:20:21:87:4c
le0 BASE-ADDRESS.MCAST.NET SM 01:00:5e:00:00:00

16. ARP Protocol Design
(Simplified, bare-bones ARP only; no error-checking, error-handling, packet validation, etc.)
Three modules:
Output module
Input Module
Cache-control module
State of cache table entries: FREE, PENDING, RESOLVED

17. ARP Output Module (resolving an IP transmit request)
sleep until IP packet with destination IP address D is received
check cache table for entry corresponding to IP address D
if (not found)
create a queue, enqueue packet
create cache entry with state set to PENDING
send ARP Request, ATTEMPTS  1; return
if (found and state=PENDING)
enqueue packet; return
if (found and state=RESOLVED)
extract value of hardware address from entry
send packet; return

18. ARP Input Module (responding to ARP messages)
sleep until ARP packet (Request or Reply) arrives
check cache table for entry corresponding to this ARP packet
if (found and state=RESOLVED)
update the entry (why?)
if (found and state=PENDING)
update the entry
while (there are matching packets in the queue)
dequeue and send a matching packet
if (not found)
create an entry for this ARP packet
if (packet is an ARP request and I am the target)
send an ARP reply
return

19. ARP Cache-Control Module
sleep until periodic timer expires
for each entry in the cache table
if (state=FREE) continue
if (state=PENDING)
ATTEMPTS += 1
if (ATTEMPTS  MAX_ATTEMPTS)
send ARP Request
else
change state to FREE; destroy queue
if (state=RESOLVED)
decrement time-out value by value of elapsed time
if (time-out  0)
return

20. Proxy ARP

21. Proxy ARP
Maps single IP net prefix into two or more physical addresses
Router answers ARP requests for H3 or H4 from H1 or H2 with its own hardware address
Properties
Assumes trusting relationship
Simple: only requires changes to a single router
Only works for broadcast networks
More details: see

22. Diskless workstations (Web PCs):
Small startup program in ROM
startup program cannot contain machine's IP address
(why not?)
Startup program is used to obtain:
machine's IP address
IP address of file server and of nearest router
initial boot image to execute

23. Reverse ARP

24. RARP
One or more RARP servers store IP addresses for hosts on their network
A diskless client uses physical network addressing to obtain its IP address from the server(s)
RARP Request is broadcast
RARP uses the same message format as ARP, except
Ethernet frame type= 0x8035
Op field = 3 for RARP request, 4 for RARP reply
Client repeats request if no reply is received
how many times?
how much delay (time-out) between retransmissions?

25. RARP (cont'd) Primary RARP server Secondary RARP server(s)
provides mapping for many hosts (all diskless machines in the network)
changes message type to RARP Reply, sends directly to client
Secondary RARP server(s)
do not respond to first RARP Request
respond to second copy of RARP Request received within a short time
randomly delay the reply to avoid collisions with other non-primary servers

26. RARP Design and Implementation
ARP server
simple, part of TCP/IP implementation, in the kernel
RARP server
more complex, implemented as a user process
implementation of RARP server is system-dependent
RARP Requests are transmitted as Ethernet frames with a specific type field (0x8035)
the server must have a way of sending/receiving Ethernet frames of this specific type

27. Drawbacks of RARP
Low-level protocol, difficult for application programmers to build a server
Supplies only one small piece of information
Cannot be used for dynamic IP address assignment

28. Newer protocols: BOOTP, DHCP
Also implemented as user (application) processes
Operate according to the client-server paradigm
Use UDP for message transfer (i.e., above UDP)

29. BOOTstraP Protocol (RFC 951, RFC1542)
Single message exchange
but provides many items needed at startup, in addition to IP address
Application uses the limited broadcast IP address ( ) to broadcast the BOOTP request on the local net
Multi-homed hosts issue a BOOTP request for each interface
Source IP address = (valid, used only during bootstrapping)
BOOTP server must also use limited broadcast for sending the reply (why?)
BOOTP “Relay Agents” permit a machine to contact a server on a non-local net

30. BOOTP (cont’d)
Server uses well-known UDP port 67, client uses well-known port 68
“Don’t Fragment” bit in IP header set
To accommodate clients with limited memory which cannot reassemble datagrams
BOOTP reply is at least 300 bytes long
Client is responsible for ensuring reliable communication
Timeout and retransmission of request
random timeout and doubling after each retransmission (why?)
random delay before retransmission (why?)

31. BOOTP Message Format

32. Some BOOTP Details Why is server name needed? Encoding of options
“TLV” = tag (1 byte), length (1 byte), value (n bytes)
Client should set Broadcast flag to 1 if it does not have a valid IP address already
Otherwise, it should set client_address to it’s IP address and be prepared to received datagrams at that address

33. BOOTP Relays Clients ignore the “relay IP address” field
If a server decides to relay the request, it puts its incoming IP interface address in the “relay IP address” field and forwards to the BOOTP server
All BOOTREQUEST messages from a given client MUST be relayed to the same destination
BOOTP servers send BOOTREPLY messages directly to the relay agent identified in the ‘relay IP address' field.
All BOOTREPLY messages received by a relay agent are intended for BOOTP clients on its directly-connected networks
When a relay agent receives a BOOTREPLY message, it should examine the BOOTP ‘relay IP address', ‘your IP address', and ‘client hardware address' to deliver the BOOTREPLY message to the client

34. Dynamic Configuration
RARP, BOOTP designed for a relatively static environment
manual configuration appropriate
cannot dynamically assign values to individual machines
One problem: mobile clients
Another problem: need to reuse pool of IP addresses
IP address pool may be small, pool of potential clients may be large (e.g., dialup)
automated mechanism needed

35. DHCP (RFC 2131) Extends BOOTP to handle dynamic address assignment
“leases” an address for a limited time (1 sec to 100 years)
Backwards compatible with BOOTP clients, message format is the same
The DHCP server
Is given a set of addresses to manage
management algorithm is not standardized
leases addresses to clients (for how long?)
informs client of lease period, during which it will not lease same address to another client
at end of the lease period, the client must either renew or stop using the address

36. DHCP Client State Diagram
INPUT
OUTPUT
+ configuration parameters