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BOOTP protocol and DHCP protocols explained.pptx
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- 2. Configuration Information ● Mostcomputers using TCP/IP need updated configuration information when they start up (boot). Examples of that type of information include: ● Default gateway ● Default file server and domain name server ● IP address and subnet mask. ● Etc.
- 3. Contd.. Two protocols arecommonly used to supply the client with this information: 1. BOOTP (Bootstrap Protocol) 2. DHCP (Dynamic Host Configuration Protocol)
- 4. Bootstrap Protocol (BOOTP) ●BOOTP (Bootstrap Protocol): Is a protocol that allows a diskless workstation to determine it’s IP address (and other information) without using RARP. ● BOOTP is a client / server program where a BOOTP agent requests bootstrap information from designated BOOTP servers. ● It uses UDP and IP, so it can be implemented in an application, thus client / server.
- 5. RARP has threedrawbacks: ● Lower-level, requiring direct access to hardware (difficult access for the average application programmer). ● RARP only supplies IP address and not other possible information. ● Cannot be used on networks that dynamically assign hardware addresses. ● BOOTP handles these inefficiencies.
- 6. BOOTP Efficiency Enhancements •BOOTP is more efficient because a single message contains many startup items including: – IP addresses. – Address of default gateway to use. – Address of bootstrap image server. – Vendor-specific hardware information and subnet masks.
- 7. BOOTP Efficiency Enhancements: HandlingService Problems • It places all responsibility for reliable communication of the BOOTP message on the client program. • Since the BOOTP configuration information must be transferred quickly, clients are often connectionless. • BOOTP handles connectionless service problems by: – Requiring UDP to use checksums.
- 8. BOOTP Efficiency Enhancements: HandlingService Problems – Requests and replies are sent with the ‘DON’T FRAGMENT’ bit set (for clients that have too little memory for reassembly). – Constructed to accept multiple replies and only processes the first one (handles duplicates). – Using time-out and retransmission techniques.
- 9. BOOTP: Message Format •The BOOTP Message Format contains fixed length fields, where bootstrap requests and replies use the same format. • BOOTP and DHCP protocols also share the exact same format with the exception of the OPTION FIELD (same as VENDOR-SPECIFIC AREA in BOOTP).
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- 11. • HOPS (8-bits): –Allows bootstrapping across multiple gateways. • TRANSACTION ID (32-bits): – A number the client uses to match requests with replies. • SECONDS (16-bits): – Number of seconds since the client started to boot. BOOTP: Message Format
- 12. • Client IPAddress (32-bits): – Client that knows it’s IP address, places it here. • Your IP Address (32-bits): – If client IP address is zero in request, it is placed here in the response. • Server IP Address and Router IP Address (32-bits each): – Filled in if the client already knows these addresses.
- 13. • All otherfields are filled by the client with as much information about the server as it knows. • A client pads the fields with zero if it does not know.
- 14. Bootstrap Procedure: • Two-stepBootstrap Procedure: – BOOTP provides the client with information needed to obtain a bootstrap configuration image. – The client uses the Trivial File Transfer Protocol (TFTP) to obtain the image from the specified server.
- 15. Bootstrap Procedure: • Separationof configuration and storage allows: – Memory images to be stored on machines that are not BOOTP servers – And for administrators to configure sets of machines to act independently or exactly the same.
- 16. Dynamic Host ConfigurationProtocol (DHCP): • Dynamic Host Configuration Protocol (DHCP) - Designed to: – Supply hosts with configuration parameters. – Lease dynamically allocated IP addresses. – Interoperate with BOOTP (acts as an enhancement to BOOTP). • The original definitions for DHCP can be found in RFC-1531, RFC-1533, and RFC-1534
- 17. Dynamic Host ConfigurationProtocol (DHCP): • DHCP supports three mechanisms for IP address allocation: – Automatic - assigns a permanent IP address. – Dynamic - the IP address is assigned for a limited time. – Manual - a network administrator assigns the address and DHCP conveys the address to the host.
- 18. DHCP Message Format: •The frame format is based on BOOTP’s to capture the BOOTP relay agent behavior. • This is also to promote the interoperability of existing BOOTP clients with DHCP servers. • The only two fields that differ are: – Flags (16-bits) = Not Used in BOOTP – Options (Variable) = Vendor-Specific area has been extended beyond 64 bytes to handle more options.
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- 20. Difference Between BOOTPand DHCP: • Two primary differences between BOOTP and DHCP: – A client can be assigned an IP addresses for a fixed time- lease and then that address can be reassigned again at a later time. – DHCP provides the mechanism for the client to acquire ALL IP configuration parameters it needs to be fully functional.
- 21. DHCP Functionality: • Howit works: – A client needing an IP address sends a DHCPDISCOVER broadcast message over the network which contains the client’s MAC address (hardware).
- 22. DHCP Functionality: • AnyDHCP server (may be several offers) can respond with a DHCPOFFER unicast message to the client’s MAC address offering an: • IP address • Subnet mask • IP address of the DHCP server • Expiration time of the lease.
- 23. DHCP Functionality: – Clientselects an offer by sending a DHCPREQUEST unicast message to the appropriate DHCP server and accepts the offered configuration information.
- 24. DHCP Functionality: – TheDHCP server responds with a DHCPACK unicast message to the client and officially assigns the address to the client node. – It then provides DNS and/or WINS (Windows Internet Name Service) server addresses, the default gateway IP address, etc.
- 25. Image source: Internetworkingwith TCP/IP vol I (1995 Prentice Hall, Douglas Comer)
- 26. Configuration and CompatibilityIssues: • DHCP currently: – Does not handle the registration of newly configured hosts with the DNS – Nor is it intended for use in configuring routers. • DHCP must provide service to existing BOOTP clients, to ensure
- 27. Configuration and CompatibilityIssues: • Administrators setup scopes (ranges) of IP addresses available on each subnet for dynamic allocation by DHCP. • Configuration options are defined either globally or per scope as appropriate.
- 28. Configuration and CompatibilityIssues: • When the DHCP service assigns IP addresses and configuration options to a client, it does so based on the scope of the subnet on which the client resides. • This can differ for each network in an autonomous system.
- 29. Address Leasing inDHCP: • Deciding efficient lease times is a key issue for DHCP: – Choosing times which are too short can cause unneeded overhead on a machine, the network, DHCP servers, and possibly administrators. – Choosing times which are too long can waste valuable IP addresses (to be used with other machines) and disrupt routers which try to send packets to an unused address.
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