Bootstrap and Autoconfiguration DHCP

1
Bootstrap and Autoconfiguration (DHCP)

• Chapter 22

2

• Look at client-server for bootstrapping
• 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
• RememberUDP relies on IP to transfer messages
• How use UDP to find IP address?
• Will look at special IP addresses mentioned in
  Chap 3

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History of Bootstrapping

• Chap 5 mentioned RARP
• Protocol to permit computer to get IP address
• More general BOOTP replaced RARP
• BOOTstrap Protocol
• DHCP developed as successor to BOOTP
• Dynamic Host Configuration Protocol
• Most features of DCHP also apply to BOOTP
• For simplicity, only talk about DHCP

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• DHCP uses UDP and IP
• 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

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Using IP to Determine an IP Address

• DHCP uses UDP to carry messages
• UDP messages encapsulated in IP datagrams
• Recall in Chap 3 several special-case IP addrs
• Specifically, all 1s 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 machines IP address

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• Example
• 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
• 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
• Broadcast the reply
• Manually add entry to its ARP cache
• System may not allow application program to do

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DHCP Retransmission Policy

• UDP uses IP for delivery
• We know it is an unreliable service
• DHCP puts responsibility for reliability on
  client
• DHCP also
• Requires that UDP use checksums
• IP does not have checksum for the data
• Requires that the do not fragment bit be set
• To accommodate clients with little memory
• Allows multiple replies (accepts processes the
  1st)

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• Timeout retransmission used for reliability
• 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
• 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

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DHCP Message Format

• Fixed length fields
• Client server used loosely
• Clients sends request
• Server sends reply
• DHCP can be used even if client already knows its
  IP address
• To get other info

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
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Need for Dynamic Configuration

• Early bootstrap protocols designed for static
  environment
• Manager created configuration file
• File changed infrequently
• Then wireless networking portable computers
• 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

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• DHCP designed for automated address assignment
• 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
• Manager configures specific address for specific
  computer
• Automatic configuration
• Server allowed to assign permanent address
• Dynamic configuration
• Server loans an address for a limited time
• Assignment based on clients identifier and the
  network to which it has connected
• Server can allocate to different computers in
  different ways

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DHCP Lease Concept

• Dynamic address assignment is temporary
• 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
• 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

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Multiple Addresses Relays

• DHCP provides info about one interface
• Multi-homed host has multiple interfaces
• Each interface handled independently
• Relay agent
• Lets computer contact server on nonlocal net
• Complicates multi-homed host configuration
• Get multiple requests from same computer
• Assume multi-homed client can identify each
  interface uniquely so server can tell requests
  apart

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Address Acquisition States

• State diagram has six states
• At boot, client enters INITIALIZE state
• Broadcasts DHCPDISCOVER message
• 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
• 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
• Cannot send any more datagrams using the address
• Leaves BOUND state must enter INITIALIZE to use
  IP
• Client has three timers in this state
• 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

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Figure 22.3
19
DHCP and Domain Names

• Not all procedures for attaching permanent host
  to internet are automated
• DHCP protocol does not specify interaction with
  the domain name system (DNS)
• However, some implementations do interact with
  DNS

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Summary

• DHCP allows a computer to obtain information at
  startup
• Router address, DNS server address, IP address
• Permits automatic and dynamic allocation of IP
  addresses
• Dynamic allocation is necessary in environments
  where computers attach and detach quickly

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• Computer becomes client to use DHCP
• 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
• 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