1. Distributed Web Systems Name Services Lecturer Department University

2. Outline Naming in distributed systems Name services Case study: DNS

3. Naming Naming is fundamental to DS: –any process requiring access to a specific resource must poses a name or identifier for that resource –Examples: file names, URLs, remote object reference Name vs address: –address identifies location of an object Binding = associating name with an object Resolving = translating name into data about the object (e.g. its address etc)

4. Example: access to a resource from a URL file Web server Socket http://www.cdk3.net:8888/WebExamples/earth.html URL Resource ID (IP number, port number, pathname) 138.37.88.61WebExamples/earth.html8888 DNS lookup (Ethernet) Network address 2:60:8c:2:b0:5a ARP lookup © Pearson Education 2005

5. Name services Name service: –stores a collection of sets (aka naming contexts) of bindings between names and attributes for objects –resolve names, also create/delete/edit bindings Name space: –the collection of all valid names recognised by a particular name service (e.g. all valid DNS names, all valid file names) Naming domain: –a name space for which there exists a single overall administrative authority for assigning names within it

6. Requirements for name spaces Allow simple but meaningful names to be used Potentially infinite number of names Structured –to allow similar subnames without clashes –to group related names Allow re-structuring of name trees –for some types of change, old programs should continue to work Management of trust

7. Name resolution Large-scale name services do not store all bindings on a single server –hence, may need to locate the correct server –the process is called navigation Multicast navigation Iterative navigation: Client 1 2 3 A client iteratively contacts name servers NS1–NS3 in order to resolve a name NS2 NS1 NS3 Name servers

8. Name resolution (contd.) Iterative server-controlled resolution: 1 2 3 4 client NS2 NS1 NS3 Non-recursive server-controlled Recursive server-controlled 1 2 3 5 4 client NS2 NS1 NS3 A name server NS1 communicates with other name servers on behalf of a client © Pearson Education 2005

9. Domain Name System (DNS) A distributed naming database Name structure reflects administrative structure of the Internet Rapidly resolves domain names to IP addresses –exploits caching heavily –typical query time ~100 milliseconds Scales to millions of computers –partitioned database –caching Resilient to failure of a server –replication

10. DNS name resolution a.root-servers.net (root) ns0.ja.net (ac.uk) dns0.dcs.qmw.ac.uk (dcs.qmw.ac.uk) alpha.qmw.ac.uk (qmw.ac.uk) dns0-doc.ic.ac.uk (ic.ac.uk) ns.purdue.edu (purdue.edu) uk purdue.edu ic.ac.uk qmw.ac.uk... dcs.qmw.ac.uk *.qmw.ac.uk *.ic.ac.uk *.dcs.qmw.ac.uk *.purdue.edu ns1.nic.uk (uk) ac.uk... co.uk yahoo.com.... Basic DNS algorithm for name resolution: Look for the name in the local cache Try a superior DNS server, which responds with: another recommended DNS server the IP address (which may not be entirely up to date) resolver © Pearson Education 2005

11. DNS name resolution (contd.) DNS naming data are subdivided into zones: –data for names in a domain less any sub-domains administered by lower-level authorities –names and addresses of at least two servers providing authoritative data for the zone –names and addresses of servers that hold authoritative data for delegated sub-domains Primary and secondary servers Non-authoritative servers –caching –time to leave (specified by the zone data)

12. Issues with DNS Name tables change infrequently, but when they do, caching can result in the delivery of stale data. –Clients are responsible for detecting this and recovering Its design makes changes to the structure of the name space difficult. For example: –merging previously separate domain trees under a new root –moving subtrees to a different part of the structure (e.g. if Scotland became a separate country, its domains should all be moved to a new country-level domain. Some systems try to resolve this: –GNS, a research system that solves the above issues (see the textbook)

13. Directory services DNS stores collections of pairs that can be looked up by name What if we want to lookup by the attribute value? –Yellow pages vs white pages Directory services let you do that Examples –X.500, LDAP –UDDI

14. Summary Naming is fundamental to distributed systems: naming computers, resources, objects, etc. Name service – a component of a distributed system storing mappings between names and object attributes (such as addresses) Case studies: –Naming: DNS –Directories: X.500, LDAP, UDDI Questions