1. Bootstrap and Autoconfiguration (DHCP)
Chapter 22

2. Look at client-server for bootstrapping
Computer must know IP address before sending or receiving datagrams
Needs router address, subnet mask, address of name server
Look at protocol to allow host to determine information automatically at startup
Client & server communicate using UDP
Remember…UDP relies on IP to transfer messages
How use UDP to find IP address?
Will look at special IP addresses mentioned in Chap 3

3. History of Bootstrapping
Chap 5 mentioned RARP
Protocol to permit computer to get IP address
More general BOOTP replaced RARP
BOOTstrap Protocol
DHCP developed as successor to BOOTP
Dynamic Host Configuration Protocol
Most features of DCHP also apply to BOOTP
For simplicity, only talk about DHCP

4. DHCP uses UDP and IP Can be implemented with an application program
Operates in client-server paradigm
Requires a single packet exchange
IP address
Router address
Name server address
Option: field for vendor-specific info

5. Using IP to Determine an IP Address
DHCP uses UDP to carry messages
UDP messages encapsulated in IP datagrams
Recall in Chap 3 several special-case IP addrs
Specifically, all 1’s specifies limited broadcast
IP SW can do even before knowing its local IP addr
“An application program can use the limited broadcast IP address to force IP to broadcast a datagram on the local network before IP has discovered the IP address of the local network or the machine’s IP address”

6. Example Client A wants to use DHCP; B is server
(B is on same physical net)
A uses limited broadcast for its initial DHCP request
B probably cannot send a directed reply back
B sends reply datagram to network interface SW
Interface SW needs to map next hop IP addr to HW addr
Would use ARP, but
A does not know its address to answer the ARP request
B has two choices
Broadcast the reply
Manually add entry to its ARP cache
System may not allow application program to do

7. DHCP Retransmission Policy
UDP uses IP for delivery
We know it is an unreliable service
DHCP puts responsibility for reliability on client
DHCP also:
Requires that UDP use checksums
IP does not have checksum for the data
Requires that the do not fragment bit be set
To accommodate clients with little memory
Allows multiple replies (accepts & processes the 1st)

8. Timeout & retransmission used for reliability
Client sends request; starts timer
If no reply before expires, must retransmit
DHCP server(s) can get overrun with requests
After power failure & all machines booting
Machines use different retransmission timeout
DHCP specification recommends random delay
In addition, start with random timeout value (0-4 seconds)
Double the time after each retransmissions
After reach 60 seconds, not double but still use randomization

9. DHCP Message Format Fixed length fields Client & server used loosely
Clients sends request
Server sends reply
DHCP can be used even if client already knows its IP address
To get other info
Figure 22.1
The format of a DHCP message. To keep implementations small enough to fit in ROM, fields except for options have fixed length

10. Need for Dynamic Configuration
Early bootstrap protocols designed for static environment
Manager created configuration file
File changed infrequently
Then: wireless networking & portable computers
Static parameter assignment requires extensive involvement of managers
Enter parameters for each host & store in server config file
Assign each host an IP address
Configure server so it understands host ID to IP addr mapping

11. DHCP designed for automated address assignment
Gets all configuration information in one message
Allows dynamic address allocation to computers
Manager configures DHCP server with set of IP addresses
New computer connects & requests addr, server picks one

12. DHCP allows three types of assignments
Manual configuration
Manager configures specific address for specific computer
Automatic configuration
Server allowed to assign permanent address
Dynamic configuration
Server “loans” an address for a limited time
Assignment based on client’s identifier and the network to which it has connected
Server can allocate to different computers in different ways

13. DHCP Lease Concept Dynamic address assignment is temporary
DHCP server leases an address to a client
Server specifies the lease period at allocation
During the lease, server cannot lease that address to any other client
At end of lease, client must renew or stop using addr
How long should the lease be?
Depends on network and needs of host
DHCP does not specify a fixed constant
Client requests a certain period; server informs client of the period it grants

14. Multiple Addresses & Relays
DHCP provides info about one interface
Multi-homed host has multiple interfaces
Each interface handled independently
Relay agent
Lets computer contact server on nonlocal net
Complicates multi-homed host configuration
Get multiple requests from same computer
Assume multi-homed client can identify each interface uniquely so server can tell requests apart

15. Address Acquisition States
State diagram has six states
At boot, client enters INITIALIZE state
Broadcasts DHCPDISCOVER message
Moves to SELECT state
Gets zero or more DHCPOFFER responses
Each contains configuration info & IP address
Client must choose one of the offers (first to arrive)
Sends DHCPREQUEST message to negotiate

16. Client enters BOUND state upon receipt of ack
Enters REQUEST state
Server acks request and starts lease with DHCPACK
Client enters BOUND state upon receipt of ack
Proceeds to use the address
This is the normal state of operation
To terminate a lease early, send DHCPRELEASE
Cannot send any more datagrams using the address
Leaves BOUND state; must enter INITIALIZE to use IP
Client has three timers in this state
First: 50% of the lease time; must attempt to renew lease
Moves to RENEW state
Server can send DHCPACK or DHCPNACK

17. Second: 87.5%
If no response, server is either down or unreachable
When second timer expires, move to REBIND state
Broadcasts DHCPREQUEST to any server on net
If get positive response, move back to BOUND
If get negative response, move back to INITIALIZE
Third: full lease period
If get no response in REBIND state, move back to INITIALIZE when this timer expires

18. Figure 22.3

19. DHCP and Domain Names
Not all procedures for attaching permanent host to internet are automated
DHCP protocol does not specify interaction with the domain name system (DNS)
However, some implementations do interact with DNS

20. Summary DHCP allows a computer to obtain information at startup
Router address, DNS server address, IP address
Permits automatic and dynamic allocation of IP addresses
Dynamic allocation is necessary in environments where computers attach and detach quickly

21. Computer becomes client to use DHCP
Broadcasts request to DHCP servers
Selects one of the offers it receives
Exchanges messages to obtain lease on IP address
Relay agent can forward DHCP requests
Allows site to have one DHCP server for multiple subnets
Starts three timers
When first expires, must attempt to renew its lease
If second expires before renewal is done, attempts to rebind from any server
If third expires before renewal, client stops using IP address and returns to initial state to get new address