External Memory

External Memory

Naming Prof. Leonardo Mostarda University of Camerino Prof. Leonardo Mostarda-- Camerino, 1 Last Lecture Berkeley Sockets Message-Passing Interface Message-queuing systems Message brokers IBMs WebSphere Message-Queuing System 2 Outline Flat naming

Forwarding Pointers Home-Based Approaches Distributed Hash Tables Hierarchical Approaches Name Spaces Domain Name Server Iterative Concurrent Caching 3 Learning objective Understand the basic terminology of naming

Discuss and understand the flat naming Forwarding Pointers Home-Based Approaches Distributed Hash Tables Hierarchical Approaches Name Spaces Discuss how the DNS works 4 Naming An entity has several access points. Each access

point has an address. IP, Telephone number, email Addresses are rarely used they are not simple, there is not access transparency and addresses can change. It is much better to let a service be known by a separate name independent of the address (location independence). Names are used to identify entities. Name resolution allows the access the named entity. To resolve names, it is necessary to implement a naming system. 5 FLAT NAMING Identifiers are special names used to uniquely represent entities We will take a look at how we can locate an entity when given only its identifier.

Broadcasting Address Resolution Protocol (ARP) Broadcasting becomes inefficient when the network grows. Multicasting only a restricted group of hosts receives the request 6 FLAT NAMING How can we locate a mobile entity? Forwarding Pointers The principle is simple: when an entity moves from A to B, it leaves behind in A a reference to its new location at B. Problem A chain for a highly mobile entity can become so long that locating that entity is prohibitively expensive.

All 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) The principle of forwarding pointers using (client stub, server stub) pairs. Chain can be too long Forwarding Pointers (2) Redirecting a forwarding pointer by storing a shortcut in a client stub. Scalability is still a problem Invocation request is sent to object

Server stub at object's current process returns the current location (a) Server stub is no longer referenced by any client stub Client stub sets a shortcut (b) Home-Based Approaches A popular approach to supporting mobile entities in large-scale networks is to introduce a home location Ex. 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 One of problems with Chord is that requests may be routed erratically across the Internet Network

proximity should be used Hierarchical Approaches Hierarchical 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 DNS DNS services Hostname to IP address translation Host aliasing Canonical and alias

names Mail server aliasing Load distribution Replicated Web servers: set of IP addresses for one canonical name Why not centralise DNS? single point of failure traffic volume distant centralised database maintenance doesnt scale! 13 13 Distributed, Hierarchical Database

Root DNS Servers com DNS servers yahoo.com amazon.com DNS servers DNS servers org DNS servers pbs.org DNS servers edu DNS servers ucl.ac.uk ucla.edu DNS serversDNS servers Client wants IP for www.amazon.com; 1st 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 www.amazon.com 14 14 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 a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) k RIPE London (also Amsterdam, d U Maryland College Park, MD Frankfurt) g US DoD Vienna, VA i Autonomica, Stockholm (plus 3 h ARL Aberdeen, MD

other locations) j Verisign, ( 11 locations) m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 13 root name servers worldwide 15 15 TLD and Authoritative Servers 16 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. Network solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers: organisations DNS servers, providing authoritative hostname to IP mappings for organisations servers (e.g., Web and mail). Can be maintained by organisation or service provider 16 Local Name Server 17 Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one.

Also called default name server When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into hierarchy. 17 Types of queries recursive query: iterative query: 2

3 7 local name server uni.mdx.ac.uk contacted server 1 replies with name of 8 server to contact I dont know this name, but ask this requesting host server leo.mdx.ac.uk 6

18 puts burden of name resolution on contacted name server heavy load? root name server 5 intermediate 4 name server (com server) authoritative name server ns1.google.com

www.google.com Example Host at mdx wants IP address for www.google.com root name server iterated query 2 3 4 5 local name server

uni.mdx.ac.uk 1 8 requesting host leo.mdx.ac.uk intermediate name server (com server) 7 6 authoritative name server ns1.google.com www.google.com19 19 Summary

Flat naming Forwarding Pointers Home-Based Approaches Distributed Hash Tables Hierarchical Approaches Name Spaces Domain Name Server Iterative Concurrent Caching 20

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