Download Chapter 22 Bootstrap and Auto configuration (DHCP)

Chapter 22 Bootstrap and Auto configuration
(DHCP)
History of Bootstrap
-Bootstrap is used to assign IP address to the
computer.
-Constant changes in the protocols for assignment of
IP address has made tremendous changes in network
administration.
-Changes are : RARP  BOOTP  DHCP
-As far as DHCP is concern, it uses UDP and IP both
as DHCP follows client server architecture.
In CS architecture, client starts communication by
sending “bootstrap request” to the server and server
will respond which includes IP address, router address
and name server address.
DHCP includes “options” field in reply to the request.
Using IP to determine an IP address
Any limited broadcast IP address (all 1’s) is used for
providing IP address to the computer.
UDP protocol is used to carry message from one
terminal to another terminal.
HOW?
DHCP
UDP
IP
Any terminal which doesn’t know its IP address, so for
the IP address it sends DHCP request to Broadcast
Address.
Any destination will not get the IP address on request
than it broadcast the request again within the network.
DHCP Retransmission Policy
In any client server architecture, DHCP has major
responsibilities regarding client.
For sending any request UDP will use IP. So to
remove delay of message, lost of message, remove
duplication, client includes checksum.
Checksum is useful to avoid any bit corruption in
message.
Request and Reply both sent with “do not fragment”
fields to reduce the occupancy of memory.
- Timeout and Retransmission are two major policies
which will take care of any datagram and avoid
datagram loss.
Timer starts when request is sent and if there will not
be any reply from destination the sender sends the
request again and starts the timer.
So timer and retransmission are two policies which are
used to send request from one sender to destination
and vice versa.
Resulting collisions are avoided with the help of
random delay of request.
The DHCP Message Format
Type of message will be known from OP field by 1 for
request and 2 for reply.
HTYPE for hardware type and HLEN hardware
address length.
HOPS field include 0, when client receives request
and pass it to another client.
DHCP increment the HOPS count.
Transaction ID, where client match the response with
request.
Whenever the client start, the SECOND field counts
the number of seconds.
Client IP address: - Gives client computer IP address.
Server IP address: This information is filled by the
client computer. If it is zero, any server will reply to the
client request. If non zero, than some specify server
will reply to the request.
Server’s reply to the client comes in YOUR IP
ADDRESS field.
FLAG field will control every request and reply. It is of
16 bits.
0
B
MUST BE ZERO
15
B is a broadcast request.
It is a broadcast request as the client doesn’t have any
IP address when it boots…
Anytime the IP address of destination doesn’t match
with the IP address which is broadcasted by any client,
than the datagram which was sent will be discarded.
DHCP is having features like BOOT FILE NAME.(it is
for diskless systems).
Here no need to encode any specific files as to do in
other diskless client machines..
OPTIONS : - uses Type – Length – Value (TLV)
Need for Dynamic Configuration
For address assignment, DHCP will allow any
computer to obtain the IP address dynamically.
Any request from the client to server for IP address,
the server will assign the IP address to that client.
Assignment of IP address is of three types: -
•Manual Configuration
•Auto Configuration
•Dynamic Configuration.
In Manual Configuration, the manager
configures a specific address for a computer.
In auto configuration, the IP address is
permanently assign to any computer when it
connects to the network.
In Dynamic Configuration, the assignment of an
IP address is for limited period of time.
DHCP Lease Concept
DHCP server will lease the address to the client for
finite period of time is called DHCP Lease.
The time of lease will be allocated by the server. This
allocation will be done at the time of assignment of
address by the server to the client.
When the lease period will finished, there are two
ways for client to proceed further.
1. Renew the lease period and
2. Stop to use the address.
Optimal time is depend on particular network and
needs of particular host.
Multiple Addresses And Relays
Relay agent is a router, it receives a broadcast request
from client and forward it to the server.
When a client sends multiple request to the server, the
server will identify the client by “client identifier”.
Server is able to differentiate the request from the client
and also from reply agent.
ADDRESS ACQUISITION STATES
There are six states, around which the client always
present…
•After booting client enters in to the INITIALIZE state,
and client connect to all available DHCP servers.
For connection with the DHCP server, client will
broadcast DHCPDISCOVER message and moves to
the SELECT state.
This DHCPDISCOVER message go in UDP datagram
and in response to this message particular server will
respond to the client by sending DHCPOFFER
response,
All the DHCPOFFER are collect by SELECT state.
The DHCPOFFER stores all the configuration
information of the client including the IP address which
was requested by the client.
Client will select the one of the response for a lease,
so for that client sends the server a DHCPREQUEST
message and enters in to the REQUEST state.
DHCPACK will send the acknowledgement of the
receipt of the request.
Once the acknowledgement arrives the client moves to
the BOUND state, where the client uses the address.
Early Lease Termination
Client is always in BOUND state.
Client stores its IP address (assignment by DHCP
server) in secondary storage device.
This storage of IP address is used at the next booting
process by the client (for sending the same IP address
request to the server)
When the address is not needed, DHCP permits the
client to terminate the lease without waiting for the
lease to terminate.
To terminate the lease the client
DHCPRELEASE message to the server.
sends
Once release message is send, other datagram will
not be sent further.
After sending DHCPRELEASE the client will leave the
BOUND state.
Lease Renewal State
Client in BOUND state will set three different timers.
Timer helps for renewal, rebinding and expiration.
The allotment of timer is done at the time of
assignment of IP address to the client.
Default timer time is one-half of the total lease time.
When ever the first timer expires the client will process
for renewal of the lease.
DHCPREQUEST message send from the server to the
client who needs the lease to be renewed.
Now client moves to RENEW state.
RENEWAL is accepted in two different ways, one is to
stop using address and second is it approved for
continuous use of it.
Once it is approved that client sends DHCPACK and
moves to BOUND state
&
If it is not approved than client will send DHCPNACK,
which will stop the client to use it and moves to the
INITIALIZE state.
Client always wait for the response in RENEW state.
When the client enters in the BOUND state the second
timers starts, this second timer will expire after 87.5%
timer period of lease time and moves the client from
RENEW state to REBIND state.
DHCP Options and Message Type
DHCP always kept the message in BOOTP format,
and use the OPTIONS field to identify the message.
(in DHCP)
CODE (53)
LENGTH (1)
TYPE (1-7)
Type field from 1 to 7 will describe DHCP message
type…