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

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