Bootstrap and
Autoconfiguration
(DHCP)
Chapter 22
Look at client-server for bootstrapping
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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?
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Will look at special IP addresses mentioned in Chap 3
History of Bootstrapping
Chap 5 mentioned RARP
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Protocol to permit computer to get IP address
More general BOOTP replaced RARP
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BOOTstrap Protocol
DHCP developed as successor to BOOTP
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Dynamic Host Configuration Protocol
Most features of DCHP also apply to BOOTP
For simplicity, only talk about DHCP
DHCP uses UDP and IP
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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
Using IP to Determine an IP
Address
DHCP uses UDP to carry messages
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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”
Example
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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
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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
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Broadcast the reply
Manually add entry to its ARP cache
System may not allow application program to do
DHCP Retransmission Policy
UDP uses IP for delivery
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We know it is an unreliable service
DHCP puts responsibility for reliability on client
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DHCP also:
Requires that UDP use checksums
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IP does not have checksum for the data
Requires that the do not fragment bit be set
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To accommodate clients with little memory
Allows multiple replies (accepts & processes the 1st)
Timeout & retransmission used for reliability
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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
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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
DHCP Message Format
Fixed length
fields
Client & server
used loosely
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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
Clients sends
request
Server sends
reply
DHCP can be
used even if
client already
knows its IP
address
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To get other info
Need for Dynamic Configuration
Early bootstrap protocols designed for static
environment
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Manager created configuration file
File changed infrequently
Then: wireless networking & portable computers
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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
DHCP designed for automated address
assignment
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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
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DHCP allows three types of assignments
Manual configuration
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Manager configures specific address for specific computer
Automatic configuration
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Server allowed to assign permanent address
Dynamic configuration
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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
DHCP Lease Concept
Dynamic address assignment is temporary
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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
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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
Multiple Addresses & Relays
DHCP provides info about one interface
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Multi-homed host has multiple interfaces
Each interface handled independently
Relay agent
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Lets computer contact server on nonlocal net
Complicates multi-homed host configuration
Get multiple requests from same computer
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Assume multi-homed client can identify each
interface uniquely so server can tell requests
apart
Address Acquisition States
State diagram has six states
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At boot, client enters INITIALIZE state
Broadcasts DHCPDISCOVER message
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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
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Enters REQUEST state
Server acks request and starts lease with DHCPACK
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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
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Cannot send any more datagrams using the address
Leaves BOUND state; must enter INITIALIZE to use IP
Client has three timers in this state
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First: 50% of the lease time; must attempt to renew lease
Moves to RENEW state
Server can send DHCPACK or DHCPNACK
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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
Figure 22.3
DHCP and Domain Names
Not all procedures for attaching
permanent host to internet are automated
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DHCP protocol does not specify interaction with
the domain name system (DNS)
However, some implementations do interact with
DNS
Summary
DHCP allows a computer to obtain
information at startup
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Router address, DNS server address, IP address
Permits automatic and dynamic allocation
of IP addresses
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Dynamic allocation is necessary in environments
where computers attach and detach quickly
Computer becomes client to use DHCP
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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
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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