Bootstrap and Autoconfiguration

Chapter 23

Introduction

• Each computer attached to a TCP/IP internet needs to know:– its IP address

– the address of a router

– which subnet mask to use

– the address of a name server

• These can be found at bootstrap time• The client and server communicate using UDP

Alternatives to RARP

• Drawbacks to RARP– It requires direct access to network hardware, thus difficult

for an application programmer to build a server

– The reply to a request is small (32 bit IP address) and smaller than minimum packet size on Ethernet

– The computer’s hardware address cannot be used on networks that dynamically assign hardware addresses

• Thus, BOOTP and later DHCP– BOOTP uses client-server, UDP and IP

– BOOTP provides other startup information as well

BOOTP Retransmission

• Responsibility for reliable communication is placed on the client

• BOOTP requires UDP to use checksums and does not allow fragmentation

• To handle datagram loss, BOOTP uses the conventional timeout and retransmission technique

BOOTP Message Format

• Replies and requests have same format shown in Figure 23.1

Bootstrap Procedure

• The client obtains an image using TFTP• A BOOTP server does not need to run on the same

machine that stores memory images– The server operates from a simple database that only

knows the names of memory images

– Configuration is kept separate from storage allows administrators to configure sets of machines alike or different

• If we have different hardware architectures, some could load one O.S., others could load another by BOOT FILE NAME

Bootstrap Procedure

• The BOOTP server checks in the configuration database to map the image for the specific BOOT file requested

Vendor-Specific Field

• The first four octets of this field are called a magic cookie and define the remaining items– Subnet mask

– Other items in Figure 23.3

– All use TLV encoding (type, length, value)

The Need for Dynamic Configuration• BOOTP was designed for a static environment in

which each host has a permanent network connection– The file did not change often because it was relatively

stable

• Then came wireless networking and portable computing– BOOTP does not adapt well to change

– A manager creates a file and stores the information in a BOOTP server configuration file

Dynamic Host Configuration

• DHCP extends BOOTP in two ways– Allows a computer to acquire all configuration needed

in a single message• IP address, subnet mask

– Allows a computer to obtain an IP address quickly and dynamically

• A DHCP server must have a set of IP addresses

• When a new computer connects to the network, the new computer contacts the server and requests an address

• The server chooses one and assigns it

Dynamic Host Configuration

• DHCP allows 3 types of address assignment– Manual configuration by manager

– Automatic configuration at attachment time

– Dynamic configuration where an address is loaned for a limited amount of time

• The server uses the identity of the client to decide which way to configure

Dynamic IP Address Assignment

• A DHCP server can be configured to permit an arbitrary computer to obtain an IP address and begin communicating– Thus, DHCP makes it possible to autoconfigure

– After a computer has been attached to a network, the computer uses DHCP to get the IP address, then configures its TCP/IP to use the address

– Autoconfiguration is subject to administrative constraints

Dynamic IP Address Assignment

• Dynamic addresses are assigned temporarily– We say that a DHCP server leases an address to a client

for a lease period

– At the end of the lease period, the client renews the lease or stops using the address

– How long should the lease last?• One hour, one day, one week?

• It depends on the needs of the network and host

Obtaining Multiple Addresses

• A multi-homed computer may need to obtain configuration information for each interface

• A relay-agent in BOOTP and DHCP relays a value (client identifier) to the server which can distinguish one interface from another

Address Acquisition States

• When using DHCP to obtain an address, a client is in one of six states as shown in Figure 23.4– INITIALIZE

– SELECT

– REQUEST

– BOUND

– REBIND

– RENEW

• When a client gets its IP address it starts 3 timers– After the first expires, the client may attempt to renew the lease

– If the second expires before renewal, it attempts to rebind

– If the third expires before renewal, the client gives up the IP @

DHCP Message Format

• DHCP uses a modified BOOTP message format• See Figure 23.5

– A DHCP server can answer BOOTP requests

– The 16-bit flags field replaces BOOTP’s unused field• Used for broadcasting (only the high order bit)

– OPTIONS are shown in Figure 23.7

DHCP and Domain Names

• DHCP does not interact with the domain name system– The binding between a host name and the IP address

assigned by DHCP must be managed independently• No name assigned

• A name is automatically assigned with the IP address

• A host can be assigned a permanent name– The host is known by the name independent of its location

– A DNS server must change the name-to-address binding when a host receives an IP address

• Currently there is no protocol to do this

Summary

• BOOTP provides an alternative to RARP– Using UDP it is possible to extend bootstrapping across

a router

– It allows a machine to determine a router address, a file server address and the name of a program it should run

– Small and simple, in ROM

• DHCP extends BOOTP by permitting a server to allocate IP addresses automatically or dynamically

For Next Time

• Read Chapters 24 and 25