cos 420 day 22. agenda assignment 4 corrected 2 b’s assignment 5 posted chap 22-26 due may 4 final...
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COS 420
DAY 22
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Agenda Assignment 4 Corrected
2 B’s Assignment 5 posted
Chap 22-26 Due May 4
Final exam will be take home and handed out May 4 and Due May 10
Protocol Definition Needs some work Today we will discuss DHCP
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Project 2 Grading Meeting Timelines 10% Deliverables
Program requirements Due March 30 15%
late Protocol Definition Due April 13 15%
Needs work Working Network Application Due May 4 25% Final Paper Due May 1 25%
User Manual Protocol Program requirements Technical Specifications
Presentation Due May 4 10%
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PART XXII
BOOTSTRAP AND AUTOCONFIGURATION
(BOOTP, DHCP)
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System Startup
To keep protocol software general IP stack designed with many
parameters Values filled in when system starts
Two possible sources of information Local storage device (e.g., disk) Server on the network
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BOOTstrapping BOOTstrap Protocol (BOOTP)
Early alternative to RARP Provides more than just an IP address Obtains configuration parameters from a
server Uses UDP
Dynamic Host Configuration Protocol (DHCP) Replaces and extends BOOTP Provides dynamic address assignment
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Apparent Contradiction
DHCP used to obtain parameters for an IP stack
DHCP uses IP and UDP to obtain the parameters
Stack must be initialized before being initialized
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Solving The Apparent Contradiction DHCP runs as application Only needs basic facilities
In particular: 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 machine’s IP address.
Note: server cannot use ARP when replying to client because client does not know its own IP address
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DHCP Retransmission
Client handles retransmission Initial timeout selected at random Timeout for successive
retransmissions doubled
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Two-Step Bootstrap
DHCP provides information, not data
Client receives Name of file that contains boot image Address of server
Client must use another means to obtain the image to run (typically TFTP)
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Dynamic Address Assignment
Dynamic mapping needed by ISPs Client obtains an IP address and uses
temporarily When client finishes, address is
available for another client Also used on many corporate
networks
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DHCP Address Assignment DHCP is backward compatible with BOOTP Can assign addresses in three ways
Manual (manager specifies binding as in BOOTP)
Automatic (address assigned by server, and machine retains same address)
Dynamic (address assigned by server, but machine may obtain new address for successive request)
Manager chooses type of assignment for each address
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DHCP Support For autoconfiguration
Because it allows a host to obtain all the parameters needed for communication without manual intervention, DHCP permits autoconfiguration. Autoconfiguration is, of course, subject to administrative constraints.
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Dynamic Address Assignment
Client is granted a lease on an address
Server specifies length of lease At end of lease, client must renew
lease or stop using address Actions controlled by finite state
machine
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Server Contact
To use DHCP, a host becomes a client by broadcasting a message to all servers on the local network. The host then collects offers from servers, selects one of the offers, and verifies acceptance with the server.
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DHCP Finite State Machine
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DHCP Message Format
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Message Type Field
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More options
http://www.iana.org/assignments/bootp-dhcp-parameters
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Questions For Discussion
Explain the relationship between DHCP and DNS (Domain Name System)
What basic facility is needed? Why?
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Summary Two protocols available for bootstrapping
BOOTP (static binding of IP address to computer)
DHCP (extension of BOOTP that adds dynamic binding of IP addresses)
DHCP Server grants lease for an address Lease specifies length of time Host must renew lease or stop using address
when lease expires Actions controlled by finite state machine