1 Prof. Leonardo Mostarda University of Camerino Naming Prof. Leonardo Mostarda-- Camerino,

Last Lecture zBerkeley Sockets zMessage-Passing Interface zMessage-queuing systems zMessage brokers  IBM ’ s WebSphere Message-Queuing System 2

Outline zFlat naming yForwarding Pointers yHome-Based Approaches yDistributed Hash Tables yHierarchical Approaches zName Spaces zDomain Name Server yIterative yConcurrent yCaching 3

Learning objective zUnderstand the basic terminology of naming zDiscuss and understand the flat naming yForwarding Pointers yHome-Based Approaches yDistributed Hash Tables yHierarchical Approaches yName Spaces zDiscuss how the DNS works 4

Naming zAn entity has several access points. Each access point has an address. yIP, Telephone number, zAddresses are rarely used they are not simple, there is not access transparency and addresses can change. zIt is much better to let a service be known by a separate name independent of the address (location independence). zNames are used to identify entities. zName resolution allows the access the named entity. zTo resolve names, it is necessary to implement a naming system. 5

FLAT NAMING zIdentifiers are special names used to uniquely represent entities zWe will take a look at how we can locate an entity when given only its identifier. zBroadcasting yAddress Resolution Protocol (ARP) yBroadcasting becomes inefficient when the network grows. zMulticasting yonly a restricted group of hosts receives the request 6

FLAT NAMING zHow can we locate a mobile entity? zForwarding Pointers yThe principle is simple: when an entity moves from A to B, it leaves behind in A a reference to its new location at B. zProblem yA chain for a highly mobile entity can become so long that locating that entity is prohibitively expensive. yAll intermediate locations in a chain will have to maintain their part of the chain of forwarding pointers as long as needed. 7

Forwarding Pointers (1) zThe principle of forwarding pointers using (client stub, server stub) pairs. zChain can be too long

Forwarding Pointers (2) zRedirecting a forwarding pointer by storing a shortcut in a client stub. zScalability is still a problem

Home-Based Approaches zA popular approach to supporting mobile entities in large- scale networks is to introduce a home location yEx. Mobile IP Problem? 1 can be time consuming Node can move forever Or can move in another Stable location Solution? Name server

Distributed Hash Tables General Mechanism zOne of problems with Chord is that requests may be routed erratically across the Internet zNetwork proximity should be used

Hierarchical Approaches zHierarchical organization of a location service into domains, each having an associated directory node. o A location record for entity E in the directory node N for a leaf domain D contains the entity's current address in that domain. o The directory node N' for the next higher-level domain D' that contains D, will have a location record for E containing only a pointer to N

13 DNS Why not centralise DNS? single point of failure traffic volume distant centralised database maintenance doesn’t scale! DNS services Hostname to IP address translation Host aliasing yCanonical and alias names Mail server aliasing Load distribution yReplicated Web servers: set of IP addresses for one canonical name

14 Root DNS Servers com DNS servers org DNS serversedu DNS servers ucl.ac.uk DNS servers ucla.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers Distributed, Hierarchical Database Client wants IP for 1 st approx: Client queries a root server to find com DNS server Client queries com DNS server to get amazon.com DNS server Client queries amazon.com DNS server to get IP address for

15 DNS: Root name servers contacted by TLD that can not resolve name root name server: There are currently 13 root name servers specified, with names in the form letter.root-servers.net 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations)

16 TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, gov, int, etc, and all top-level country domains uk, fr, ca, jp, ly. yNetwork solutions maintains servers for com TLD yEducause for edu TLD Authoritative DNS servers: organisation’s DNS servers, providing authoritative hostname to IP mappings for organisation’s servers (e.g., Web and mail). yCan be maintained by organisation or service provider

17 Local Name Server Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one. yAlso called “default name server” When a host makes a DNS query, query is sent to its local DNS server yActs as a proxy, forwards query into hierarchy.

18 Types of queries recursive query: puts burden of name resolution on contacted name server heavy load? iterative query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” requesting host leo.mdx.ac.uk root name server local name server uni.mdx.ac.uk authoritative name server ns1.google.com intermediate name server (com server) 5 8

19 Example Host at mdx wants IP address for requesting host leo.mdx.ac.uk root name server local name server uni.mdx.ac.uk authoritative name server ns1.google.com intermediate name server (com server) 5 8 iterated query

Summary zFlat naming yForwarding Pointers yHome-Based Approaches yDistributed Hash Tables yHierarchical Approaches zName Spaces zDomain Name Server yIterative yConcurrent yCaching 20