dhcp

PART XXII

BOOTSTRAP AND AUTOCONFIGURATION(DHCP)

Internetworking With TCP/IP vol 1 -- Part 22 1 2005

System Startup

d To keep protocol software general

IP stack designed with many parameters

Values lled in when system starts

d Two possible sources of information

Local storage device (e.g., disk)

Server on the network


Bootstrapping

d BOOTstrap Protocol (BOOTP)

Early alternative to RARP

Provided more than just an IP address

Obtained conguration parameters from a server

Used UDP

d Dynamic Host Conguration Protocol (DHCP)

Replaces and extends BOOTP

Provides dynamic address assignment


Apparent Contradiction

d DHCP used to obtain parameters for an IP stack

d DHCP uses IP and UDP to obtain the parameters

d Stack must be initialized before being initialized


Solving The Apparent Contradiction

d DHCP runs as application

d Only needs basic facilities

d In particular:

An application program can use the limited broadcast IPaddress to force IP to broadcast a datagram on the localnetwork before IP has discovered the IP address of the localnetwork or the machines IP address.

d Note: server cannot use ARP when replying to clientbecause client does not know its own IP address


DHCP Retransmission

d Client handles retransmission

d Initial timeout selected at random

d Timeout for successive retransmissions doubled


Two-Step Bootstrap

d DHCP provides information, not data

d Client receives

Name of le that contains boot image

Address of server

d Client must use another means to obtain the image to run(typically TFTP)


Dynamic Address Assignment

d Needed by ISPs

Client obtains an IP address and uses temporarily

When client nishes, address is available for anotherclient

d Also used on many corporate networks


DHCP Address Assignment

d Backward compatible with BOOTP

d Can assign addresses in three ways

Manual (manager species binding as in BOOTP)

Automatic (address assigned by server, and machineretains same address)

Dynamic (address assigned by server, but machine mayobtain new address for successive request)

d Manager chooses type of assignment for each address


DHCP Support For Autoconguration

Because it allows a host to obtain all the parameters needed forcommunication without manual intervention, DHCP permitsautoconfiguration. Autoconfiguration is, of course, subject toadministrative constraints.


Dynamic Address Assignment

d Client is granted a lease on an address

d Server species length of lease

d At end of lease, client must renew lease or stop usingaddress

d Actions controlled by nite state machine


Server Contact

To use DHCP, a host becomes a client by broadcasting amessage to all servers on the local network. The host thencollects offers from servers, selects one of the offers, andverifies acceptance with the server.


DHCP Finite State Machine

REBIND

INITIALIZE

BOUND

RENEW

SELECT

REQUEST

/ DHCPDISCOVER

Select Offer /DHCPREQUEST

DHCPOFFER

DHCPACK

DHCPACK

DHCPACK

DHCPNACKDHCPNACKor

Lease Expires

Lease Reaches50% Expiration /DHCPREQUEST

Lease Reaches87.5% Expiration /DHCPREQUEST

Host Boots

Cancel Lease / DHCPRELEASE


DHCP Message Format

0 8 16 24 31

OP HTYPE HLEN HOPS

TRANSACTION ID

SECONDS FLAGS

CLIENT IP ADDRESS

YOUR IP ADDRESS

SERVER IP ADDRESS

ROUTER IP ADDRESS

CLIENT HARDWARE ADDRESS (16 OCTETS)...

SERVER HOST NAME (64 OCTETS)...

BOOT FILE NAME (128 OCTETS)...

OPTIONS (VARIABLE)...


Message Type Field

0 8 16 23

CODE (53) LENGTH (1) TYPE (1 - 7)

TYPE FIELD Corresponding DHCP Message Type

1 DHCPDISCOVER2 DHCPOFFER3 DHCPREQUEST4 DHCPDECLINE5 DHCPACK6 DHCPNACK7 DHCPRELEASE8 DHCPINFORM


Questions For Discussion

d Explain the relationship between DHCP and DNS

d What basic facility is needed? Why?


Summary

d Two protocols available for bootstrapping

BOOTP (static binding of IP address to computer)

DHCP (extension of BOOTP that adds dynamic bindingof IP addresses)

d DHCP

Server grants lease for an address

Lease species length of time

Host must renew lease or stop using address when leaseexpires

Actions controlled by nite state machine


Questions?

dhcp

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