Distributed Systems. 22. Naming Paul Krzyzanowski. Rutgers University. Fall 2013

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1 Distributed Systems 22. Naming Paul Krzyzanowski Rutgers University Fall 2013 November 21, Paul Krzyzanowski 1

2 My 15 MacBook Pro The rightmost computer on my desk Paul s aluminum laptop, but not the big or the small one hedwig hedwig.pk.org :14:51:ec:f2:5b 11/21/ Paul Krzyzanowski 2

3 Naming things User names Login, Machine names rlogin, , web Files Devices Variables in programs Network services 11/21/ Paul Krzyzanowski 3

4 Naming Service Allows you to look up names Often returns an address as a response Might be implemented as Search through file Client-server program Database query 11/21/ Paul Krzyzanowski 4

5 What s a name? Name: identifies what you want Address: identifies where it is Route: identifies how to get there Binding: associates a name with an address choose a lower-level-implementation for a higher-level semantic construct RFC 1498: Inter-network Naming, addresses, routing 11/21/ Paul Krzyzanowski 5

6 Names We may need names for: Services: e.g., time of day Nodes: computer that can run services Paths: route Objects within service: e.g. files on a file server Naming convention can take any format Ideally one that will suit application and user E.g., human readable names for humans, binary identifiers for machines 11/21/ Paul Krzyzanowski 6

7 Uniqueness of names Easy on a small scale Problematic on a large scale Globally unique names are hard to keep unique A hierarchy allows uniqueness to be maintained Globally unique components Example: Ethernet MAC address: 3 bytes: organization, 3 bytes: controller IP address: network & host (variable partition) compound name: set of atomic names connected with a separator Domain name (www.rutgers.edu) URLs File pathnames (/usr/share/dict/words) 11/21/ Paul Krzyzanowski 7

8 Terms: Naming convention Naming system determines syntax for names Unix file names: Parse components from left to right separated by / /home/paul/src/gps/gui.c Internet domain names: Ordered right to left and delimited by. LDAP names Attribute/value pairs ordered right to left, delimited by, cn=paul Krzyzanowski, o=rutgers, c=us 11/21/ Paul Krzyzanowski 8

9 Terms: Context A particular set of name object bindings Names are unique within the context E.g., /etc/postfix/main.cf on a specific computer Each context has an associated naming convention A name is always interpreted relative to some context E.g., directory /usr in a UNIX file system 11/21/ Paul Krzyzanowski 9

10 Terms: Naming System Connected set of contexts of the same type (same naming convention) along with a common set of operations For example: System that implements DNS (Internet domain names) System that implements LDAP (directory of people) 11/21/ Paul Krzyzanowski 10

11 Terms: Namespace A container for a set of names in the naming system A namespace may have a scope scope = region where the name exists & refers to the object For example, Names of all files in a directory All domain names within rutgers.edu E.g., Java package, local variables A namespace may be tree structured Fully-qualified or hierarchical names may be used to identify names outside the local namespace Global namespace = root of the tree 11/21/ Paul Krzyzanowski 11

12 Terms: Resolution Resolution = name lookup Return the underlying representation of the name Look up the binding of the name to its object For example, /21/ Paul Krzyzanowski 12

13 Terms: Name Service (or Naming Service) The service that performs name resolution A DNS (Domain Name System) Server will tell us /21/ Paul Krzyzanowski 13

14 Directory Service Extension of name service: Associates names with objects Allows objects to have attributes Can search based on attributes For example, LDAP (Lightweight Directory Access Protocol) Directory can be an object store: Look up printer object and send data stream to it 11/21/ Paul Krzyzanowski 14

15 Name resolution To send data to a service: 1. Find a node on which the service resides (service name resolution) 2. Find an address (or network attachment point) for that node (node name location) 3. Find a path from this location to the service (routing service) 11/21/ Paul Krzyzanowski 15

16 Name resolution E.g., access paul s service : File lookup: (service name to service address) paul s service aramis.rutgers.edu:1234 DNS lookup: (domain name to IP address) aramis.rutgers.edu IP routing: (no programmer involvement for IP) route: remus vlan13-lcsr-gw ( ) aramis ARP resolution: (IP name to MAC address) :21:56:ea:54: :00:20:90:9c:23 11/21/ Paul Krzyzanowski 16

17 Binding The association of a resolution Static binding Hard-coded Early binding Look up binding before use Cache previously used binding Late binding Look up just before use 11/21/ Paul Krzyzanowski 17

18 IP Domain Names Human readable names e.g., Hierarchical naming scheme No relation to IP address or network class 11/21/ Paul Krzyzanowski 18

19 Case Study: Internet Domain Name System (DNS) November 21, Paul Krzyzanowski 19

20 How are IP addresses assigned? IP addresses are distributed hierarchically Internet Assigned Numbers Authority (IANA) at the top IANA is currently run by ICANN Internet Corporation for Assigned Names and Numbers IANA Regional Internet Registries (RIR) AfriNIC APNIC ARIN LACNIC RIPE NCC Allocate blocks of addresses to ISPs RIR Map ISP ISP ISP ISP ISP ISP ISP ISP Your computer (or Internet gateway) - Permanent (static) or temporary (dynamic) 20

21 How are machine names assigned? Early ARPANET Globally unique names per machine (e.g., UCBVAX) Kept track at the Network Information Center at the Stanford Research Institute That doesn t scale! A domain hierarchy was created in 1984 (RFC 920) Domains are administrative entities: divide name management Tree-structured global name space Textual representation of domain names 21

22 Domain Name Hierarchy Root com edu gov info net org ac ae nl us zw rutgers generic TLDs country-code TLDs cs nb www 22

23 Top Level Domains (TLDs) cctld Country-code domains ISO 3166 codes e.g.,.us,.de,.ca,.es IDN cctld Internationalized country-code domains e.g.,,السعودية.. 中 國,.рф gtld Generic top-level domains e.g.,.biz,.com,.edu,.gov,.info,.net,.org There are currently 321 top-level domains Each top-level domain has an administrator assigned to it Assignment is delegated to various organizations by the Internet Assigned Numbers Authority (IANA) See for the latest count 23

24 Shared registration Domain name registry: this is the database Keeps track of all domain names registered in a top-level domain Domain name registry operator: this is the company that runs the db NIC = Network Information Center organization that keeps track of the registration of domain names under a top-level domain keeps the database of domain names Domain name registrar: this is the company you use to register Company that lets you register a domain name 24

25 Shared registration Until 1999: Network Solutions Inc. operated the.com,.org,.net registries Now Multiple domain registrars provide domain registration services Around 1,000 of these companies each is accredited by the ICANN The registrar you choose becomes the designated registrar for your domain Maximum period of registration for a domain name = 10 years The registry operator keeps the central registry database for the top-level domain Only the designated registrar can change information about domain names A domain name owner may invoke a domain transfer process Example Go Daddy is a domain registrar and is the designated registrar for poopybrain.com VeriSign is the registry operator for the.com gtld 25

26 The problem Every device connected to the internet has a unique Internet Protocol (IP) address How do you resolve user-friendly machine names to IP addresses?

27 Original solution Through the 1980s Search /etc/hosts file for machine name (see RFC 606) File periodically downloaded from Network Information Center (NIC) at the Stanford Research Institute (SRI) This was not sustainable with millions of hosts on the Internet A lot of data A lot of churn in the data new hosts added, deleted, addresses changed Maintenance Traffic volume Solution doesn t scale! 27

28 DNS: Domain Name System Distributed database: a hierarchy of name servers DNS is an application-layer protocol Name-address resolution is handled at the edge The network core is unaware of host names and does not care There is no special relationship between names and addresses I can make poopybrain.com resolve to cs.rutgers.edu 28

29 DNS provides Name to IP address translation Aliasing of names (called canonical names) Identification of name servers Mail server names Load distribution: Multiple name servers that can handle a query for a domain Caching Ability to provide a set of IP addresses for a name 29

30 DNS is a distributed, hierarchical database Root DNS Servers com DNS Servers edu DNS Servers org DNS Servers google.com DNS Servers rutgers.edu DNS Servers columbia.edu DNS Servers pk.org DNS Servers A collection of DNS servers 30

31 Authoritative DNS server An authoritative name server is responsible for answering queries about its zone Configured by the administrator Zone = group of machines under a node in the tree E.g., rutgers.edu 31

32 A DNS server returns answers to queries Key data that a DNS server maintains (partial list) Information Abbreviation Description Host A Host address (name to address) Includes name, IP address, time-to-live (TTL) Canonical name CNAME Name for an alias Mail exchanger MX Host that handles for the domain Name server NS Identifies the name server for the zone: tell other servers that yours is the authority for info within the domain Start of Zone Authority SOA Specifies authoritative server for the zone. Identifies the zone, time-to-live, and primary name server for the zone 32

33 Finding your way How do you find the DNS Server for rutgers.edu? That s what the domain registry keeps track of When you register a domain, you supply the addresses of at least two DNS servers that can answer queries for your zone So how do you find it? Start at the root 33

34 Root name servers The root name server answers can return a list of authoritative name servers for top-level domains 13 root name servers A.ROOT-SERVERS.NET, B.ROOT-SERVERS.NET, Each has redundancy (via anycast routing or load balancing) Download the latest list at 34

35 Name Resolution Approaches Iterative (non-recursive) name resolution Send query to root name server Send query to edu name server Send query to rutgers name server Advantage: stateless Recursive name resolution (true recursion): Send query to root name server Root name server sends query to edu name server Edu name server sends query to rutgers name server Advantage: increased caching opportunities, reduced communication For DNS Servers Recursive usually means that a name server will take on the responsibility of fully resolving the name, even if it s through an iterative process A DNS server does not need to support recursion 35

36 DNS Resolvers: local name server DNS Resolver Not really a part of the DNS hierarchy Acts as an intermediary between programs that need to resolve names and the name servers A resolver is responsible for performing the full resolution of the query Where are they? Local system has one: that s what applications contact Local cache; may be a process or a library On Linux & Windows, these are limited DNS servers (called stub resolvers): they are not capable of handling referrals and expect to talk with a name server that can handle recursion (full resolution) ISPs (and organizations) run them on behalf of their customers Including a bunch of free ones (OpenDNS, Google Public DNS) Resolvers cache past lookups they are not responsible for zones 36

37 Using a DNS resolver To look up a name: Send a DNS query to the local resolver (recursion requested) Local resolver If the local resolver has cached results, it can return the answer Otherwise, consult a local hosts file (e.g., /etc/hosts) to return locallyconfigured name address mappings Otherwise contact a DNS server that the client knows about this is typically another resolver that is provided by the ISP The local system is configured with one or more addresses of external name servers ISP Resolver Check cache Check a locally-configured zone file (if any). If the desired data is there, return an authoritative answer Otherwise, do an iterative set of queries to traverse the hierarchy to find the desired name server and get results 37

38 DNS Resolvers in action Local server ISP app DNS resolver DNS resolver Interative lookup app cache cache DNS hierarchy /etc/hosts zone info Local stub resolver: - check local cache - check local hosts file - send request to external resolver External resolver - DNS server that accepts recursion - Running at ISP, Google Public DNS, OpenDNS, etc. E.g., on Linux: resolver is configured via the /etc/resolv.conf file 38

39 Sample query Rutgers registered rutgers.edu with the.edu domain educause.net is the domain registry for the.edu gtld The root name server contains addresses for the name servers of all the top-level domains The local name server is provided the list of addresses of root name servers 39

40 Sample Query Submit query to a local DNS resolver: 1. query(cs.rutgers.edu) root name server root name servers identify authoritative servers for top-level domains send query to A.ROOT_SERVERS.NET: referral to edu name server returns list of DNS servers for.edu: L3.NSTLD.COM: query(cs.rutgers.edu) edu name server send query to referral to rutgers.edu name servers: - DNS1.rutgers.edu DNS2.rutgers.edu DNS3.rutgers.edu query(cs.rutgers.edu) rutgers name server send query to The rutgers name server returns A: address MX: dragon.rutgers.edu domain name for 40

41 Caching Starting every query at the root would place a huge load on root name servers A name server can be configured to cache results of previous queries Save query results for a time-to-live amount of time The time-to-live value is specified in the domain name record by an authoritative name server Caching name servers are also recursive name servers 41

42 The End November 21, Paul Krzyzanowski 42

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