Web Networking recitation 2 .


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2. Space Name System - DNS. Gives mapping from ASCII area names to IP addressesHierarchical naming system.The set of names is isolated into fundamentally unrelated parts.Supplies instrument for worldwide information stockpiling and data recovery. Most essential principles:Distributed framework
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Web Networking recitation #2 DNS and DHCP Spring Semester 2009, Dept. of Computer Science, Technion

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Domain Name System - DNS Provides mapping from ASCII area names to IP addresses Hierarchical naming framework. The arrangement of names is isolated into totally unrelated parts. Supplies instrument for worldwide information stockpiling and data recovery. Most essential standards: Distributed framework – set of servers sharing data. Effectiveness - the greater part of the solicitations settled by nearby servers. Utilization of Caching. Dependability – works regardless of the possibility that a portion of the servers fall flat.

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DNS tree root com organization gov net il ibm cnn air conditioning co gov technion tau

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Reliability over UDP DNS is an application which keeps running over UDP Advantage: not as intricate as TCP. Disservice: requires unwavering quality execution at application level . Unwavering quality plan: Time out. Retransmissions - resends planned out question to an alternate server.

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DNS Terms Domain name : any name spoke to in the DNS arrange mail.yahoo.com \.name.example DNS name : each string between two "." ac.il – 2 names DNS zone : an arrangement of names that are under a similar expert cs.technion.ac.il , ee.technion.ac.il and www.technion.ac.il Delegation : exchange of specialist for an area example.org is a designation from organization.

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DNS Elements Resolver: stub: straightforward, just makes inquiries; recursive: takes basic question and makes every vital stride to find the full solution. Server (some perform both parts in the meantime): definitive: the servers that contain the zone petition for a zone, one Primary, at least one Secondaries; reserving: a recursive resolver that stores earlier outcomes and reuses them.

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How Does DNS Work? The resolver sends a DNS ask for message over UDP to a neighborhood space name server. DNS reserves store information for a brief span characterized by TTL on the Record. At the point when a name server does not have the asked for data, it begins at longest match on inquiry name it has when searching for information. The root server knows the IP address of each second-level space name server. Each server knows how to achieve servers that are specialists for names additionally down the progression. The resolver takes after appointments until it gets an answer.

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Root Server Ask organization NS Org Server Recursive Resolver Ask ietf.org NS Ietf.org Server www.ietf.org A 65.256.255.51 www.ietf.org A 65.256.255.51 DNS question case (from IETF slides) www.ietf.org Stub resolver

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Dynamic Host Configuration Protocol - DHCP RFC 2131 Runs over UDP too Provides programmed design of the host associated with system or booted Provides has with beginning setup data upon bootup: IP address with subnet veil, default portal, IP address of the DNS server .

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DHCP (cont.) (not chose) (chose) DHCP find DHCP find decide arrangement decide design DHCP offer DHCP offer select setup DHCP ask for DHCP ask for DHCP ack Initialization finishes utilizing the distributed arrangement elegant shutdown DHCP discharge dispose of rent

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DHCP Messages DHCPDISCOVER The customer communicates message looking for accessible DHCP servers. DHCPOFFER The server reaction to the customer DHCPDISCOVER with offer of arrangement parameters . DHCPREQUEST The customer communicates to the server, asking for offered parameters from one server particularly. Affirms rightness of beforehand designated address after, e.g., framework reboot. Develops the rent on a specific system address. DHCPRELEASE The customer to-server correspondence, giving up system address and wiping out residual rent.

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DHCP Messages (Cont.) DHCPACK The server-to-customer correspondence with setup parameters, including submitted arrange address. DHCPNAK Server to customer demonstrating customer\'s thought of system address is inaccurate (e.g., customer has moved to new subnet) or customer\'s rent as lapsed DHCPDECLINE The customer to-server correspondence, showing that the system address is as of now being used. DHCPINFORM The customer to-server correspondence, requesting just nearby design parameters that the customer as of now has remotely arranged as an address.

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DHCP message arrange xid Transaction ID. ciaddr Client IP address yiaddr Your IP address. siaddr Server IP address. chaddr Client MAC address

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DHCP message arrange (cont.) Transaction ID - an arbitrary number that is utilized by the customer to match approaching DHCP messages with pending solicitations. Customer IP address - utilized when customer knows its IP address. Your IP address - the DHCP server will put the offered customer IP address in this field, if the customer IP address is 0.0.0.0. Server IP address - the DHCP server address, in the event that it is known. Else, it is utilized as a part of DHCPOFFER and DHCPACK. choices - DHCP characterizes a \'customer identifier\' alternative that is utilized to pass an unequivocal customer identifier to a server.

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DHCP message sifting DHCPOFFER Only the customer sitting tight for this message will acknowledge it in UDP layer. Be that as it may, consider the possibility that two customers are sitting tight for the offer at the same time. The separating is conceivable just at the application layer. The exchange ID field is in charge of this sifting. DHCPREQUEST The message is communicated to every one of the servers. The server knows whether its offer was acknowledged just after perusing the server identifier field in the DHCP message.

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DHCP as UDP application DHCP server - port 67, customer - port 68 Reliability is not given by UDP. Customer is in charge of unwavering quality The customer executes clock to quantify timeout for the messages that were not reacted. The customer embraces a retransmission technique that joins a randomized exponential backoff calculation to decide the deferral between retransmissions. Each next message goes about as an affirmation for the past stride. For instance, DHCPREQUEST is an ACK for DHCPOFFER.

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