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Naming

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Naming

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  1. Naming Chapter 4

  2. Naming • Names are used to share resources, to uniquely identify entities, or to refer to locations. • Name resolution is used for a process to access the named entity. • Three issues are covered in this chapter: • Naming entities: the organization and implementation of human-friendly names • Locating mobile entities • Removing unreferenced entities

  3. Naming • A name in a distributed system is a string of bits or characters that is used to refer to an entity. • An entity in a distributed system can be practically anything including resources, processes, users, mailboxes, newsgroups, Web pages, and so on. • To operate on an entity, we need to access it at an access point. The name of an access point is called an address. The address of an access point of an entity is the address of that entity. • A location­independent name for an entity E, is independent from the addresses of the access points offered by E. For example, www.yahoo.com has service across several machines.

  4. Naming • A true identifier is a name having the following properties: • Each identifier refers to at most one entity • Each entity is referred to by at most one identifier • An identifier always refers to the same entity (prohibits reusing an identifier) • In many computer systems, addresses and identifiers are represented in machine-readable form only. • Human-friendly names are tailored to be used by humans and generally represented as a character string.

  5. Name Space • Names in a distributed system are organized into a name space. • A name space can be represented as a labeled, directed graph with two types of nodes: • a leaf node represents a (named) entity • a directory node is an entity that refers to other nodes. • A directory node contains a directory table of (edge label, node identifier) pairs. Example: (‘home’, n1), (‘keys’, n5)

  6. Name Space • A node which has only outgoing and no incoming edges is called the root (node). • Each path in a naming graph can be referred to by the sequence of labels corresponding to the edges in the path, such as • N:<label-1, label-2, …, label-n> where N refers to the first node in the path. • If the first node in a path name is the root of the naming graph, it is called an absolute pathname. Otherwise, it is called a relativepath name.

  7. Name Space • A global name is a name that denotes the same entity no matter where that name is used in a system. (e.g. absolute path) • A local name is a name whose interpretation depends on where that name is being used. (e.g. relative path) • Example of path names in file systems (Refer to Figure 4-1) • N0:<home, steen, mbox>  /home/steen/mbox • /home/steen/keys = /keys (The same node represented by different path names.) • Name space can be organized in various ways: a tree or a acyclic graph.

  8. Name Spaces • A general naming graph with a single root node.

  9. Name Space • We can easily store all kinds of attributes in a node, describing aspects of the entity the node represents: • Type of the entity • An identifier for that entity • Address of the entity's location • Nicknames • Directory nodes can also have attributes, besides just storing a directory table with (edge label, node identifier) pairs.

  10. Name Spaces • The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks. • Boot block: loaded when system is booted. • Super block: information on the entire file system • Inode: information on the location of the data of its associated file • Disk block: file data blocks

  11. Name Resolution • The process of looking up a name is called name resolution. • Problem: To resolve a name we need a directory node. How do we actually find that (initial) node? • Knowing how and where to start name resolution is called closure mechanism. The mechanism • selects the implicit context from which to start name resolution. • determine how name resolution should proceed. • Examples: • www.cs.twsu.edu: start at a DNS name server • /home/john/box: start at the local NFS file server (possible recursive search) • 316-978-3156: dial a phone number • 156.26.1.30: route to the WSU Web server

  12. Naming Linking and Mounting • An alias is another name for the same entity. • The hard link approach is to allow multiple absolute paths names to refer to the same node in a naming graph. • The symbolic link approach uses the leaf node to store an entity path. • Name resolution can also be used to merge different name spaces. Mounting is an example (one way) of merging different name spaces.

  13. Linking and Mounting • The concept of a symbolic link explained in a naming graph.

  14. Merging Name Spaces • Mount point is a (Directory) node storing the node identifier. Mounting point is a (Directory) node in the foreign name space. For example, • To mount a foreign name space in a distributed system requires the following information: • The name of an access protocol • The name of the server • The name of the mounting point in the foreign name space • Example: mount nfs://flits.cs.vu.nl//home/steen /remote/vu • nfs://flits.cs.vu.nl//home/steen is the mounting point. • /remote/vu is the mout point.

  15. Linking and Mounting • Mounting remote name spaces through a specific process protocol.

  16. Merging Name Spaces • We have different name spaces that we wish to access from any given name space. • Solution 1: Introduce a naming scheme by which pathnames of different name spaces are simply concatenated (URLs). • Solution 2: Introduce (specific) nodes that contain the name of a node in a foreign name space, along with the information how to select the initial context in that foreign name space (Jade). • Solution 3: Use only full pathnames, in which the starting context is explicitly identified, and merge by adding a new root node (DEC's Global Name Space). • n0:/home/stten/keys, m0:/home/steen

  17. Linking and Mounting • Organization of the DEC Global Name Service

  18. Name Space Implementation • A name space forms the heart of a naming service. A naming service is implemented by name server. In distributed systems, it is necessary to distribute the name resolution process as well as name space management across multiple machines, by distributing nodes of the naming graph. • Consider a hierarchical naming graph and distinguish three levels: • Global level: Consists of the high­level directory nodes. Main aspect is that these directory nodes have to be jointly managed by different administrations • Administrational level: Contains mid­level directory nodes that can be grouped in such a way that each group can be assigned to a separate administration. • Managerial level: Consists of low­level directory nodes within a single administration. Main issue is effectively mapping directory nodes to local name servers.

  19. Name Space Distribution • An example partitioning of the DNS name space, including Internet-accessible files, into three layers.

  20. Name Space Distribution • A comparison between name servers for implementing nodes from a large-scale name space partitioned into a global layer, an administrational layer, and a managerial layer. • Name servers in the global and administrational layer are the most difficult to implement because of replication and caching.

  21. Implementation of Name Resolution • Each client has access to a local name resolver, which is responsible for ensuring that the name resolution process is carried out. • There are two ways to implement name resolution: • In iterative name resolution, a name resolver hands over the complete name to the root name server, returning each intermediate result back to the client’s name resolver. • In recursive name resolution, a name server passes the result to the next name server it finds.

  22. Implementation of Name Resolution • The principle of iterative name resolution.

  23. Implementation of Name Resolution • The principle of recursive name resolution.

  24. Implementation of Name Resolution • Recursive name resolution • Drawback: it demands high performance on each name server. • Advantages: • Caching results is more effective. • Communication cost is reduced. • One of the largest distributed naming services is the Internet Domain Name System (DNS).

  25. Implementation of Name Resolution • Recursive name resolution of <nl, vu, cs, ftp>. Name servers cache intermediate results for subsequent lookups.

  26. Implementation of Name Resolution • The comparison between recursive and iterative name resolution with respect to communication costs.

  27. The DNS Name Space • The DNS name space is hierarchically organized as a rooted tree. • The string representation of a path name consists of listing its labels, starting with the rightmost one, and separating the labels by a dot. • For example, www.cs.twsu.edu., which includes the rightmost dot to indicate the root node. • The label attached to a node’s incoming edge is used as the name for that node. • A subtree is a called a domain; a path name to its root node is called a domain name. • The contents of a node is formed by a collection of resource records. Different types of resource records are shown in Figure 4-12. • DNS distinguishes aliases from what are called canonical names or primary name

  28. The DNS Name Space • The most important types of resource records forming the contents of nodes in the DNS name space.

  29. DNS Implementation • The DNS name space can be divided into a global layer and administrational layer. • Secondary name servers do not access the database directly but request the primary server to transfer its content. The latter is called a zone transfer in DNS terminology. • A DNS database is implemented as a small collection of files. • DNS is comparable to a telephone book for looking up phone numbers.

  30. DNS Implementation • An excerpt from the DNS database for the zone cs.vu.nl.

  31. DNS Implementation • Part of the description for the vu.nl domain which contains the cs.vu.nl domain.

  32. X.500 • X.500 Directory Service is a standard way to develop an electronic directory so that it can be part of a global directory available with Internet access. • The idea is to be able to look up people in a user-friendly way by name, department, or organization. It is similar to use the yellow pages. • Because these directories are organized as part of a single global directory, you can search for hundreds of thousands of entries from a single place on the World Wide Web.

  33. X.500 • An X.500 directory service consists of a number of records, usually referred to as directory entries. • The collection of all directory entries in an X.500 directory service is called a Directory Information Base (DIB). • Each record is uniquely named. Each naming attribute is called a Relative Distinguished Name (RDN). • Calling read will return a directory entry. Calling list will return a list of entries.

  34. The X.500 Name Space • A simple example of a X.500 directory entry using X.500 naming conventions.

  35. The X.500 Name Space • Part of the directory information tree.

  36. The X.500 Name Space • Two directory entries having Host_Name as RDN.

  37. X.500 Implementation • In X.500, each local directory is called a Directory System Agent (DSA). A DSA can represent one organization or a group of organizations. • The DSAs are interconnected from the Directory Information Tree (DIT). The user interface program for access to one or more DSAs is a Directory User Agent (DUA). • DUAs include whois, finger, and programs that offer a graphical user interface. X.500 is implemented as part of the Distributed Computing Environment (DCE) in its Global Directory Service (GDS). • The Lightweight Directory Access Protocol (LDAP) is a simplified protocol to accommodate X.500 directory services in the Internet.

  38. X.500 Implementation • Providing an X.500 directory allows an organization to make itself and selected members known on the Internet. • Two of the largest directory service providers are InterNIC, the organization that supervises domain name registration in the U.S., and ESnet, which maintains X.500 data for all the U.S. national laboratories. • ESNet and similar providers also provide access to looking up names in the global directory, using a number of different user interfaces including designated Web sites, whois, and finger. • These organizations also provide assistance to organizations that are creating their own Directory Information Tree (DIT).

  39. Naming & Locating Objects • Observation: If a traditional naming service is used to locate entities, we also have to assume that node contents at the managerial level is stable, as we can use only names as identifiers (think of Web pages). • Problem: It is not realistic to assume stable node contents down to the local naming level • Solution: Decouple naming from locating entities • Location service: • Solely aimed at providing the addresses of the current locations of entities. • Assumption: Entities are mobile, so that their current address may change frequently.

  40. Naming & Locating Objects • Naming service: • Aimed at providing the content of nodes in a name space, given a (compound) name. Content consists of different (attribute,value) pairs. • Assumption: Node contents at global and administrational level is relatively stable for scalability reasons. • Two-level mapping is used to local mobile objects: • Name: Any name in a traditional naming space • Entity ID: A true identifier • Address: Provides all information necessary to contact an entity • Observation: An entity's name is now completely independent from its location.

  41. Naming & Locating Objects • Observation: If a traditional naming service is used to locate entities, we also have to assume that node contents at the managerial level is stable, as we can use only names as identifiers (think of Web pages). • Problem: It is not realistic to assume stable node contents down to the local naming level • Solution: Decouple naming from locating entities

  42. Naming versus Locating Entities • Direct, single level mapping between names and addresses. • T-level mapping using identities.

  43. Simple Solutions for Locating Entities • Broadcasting: Simply broadcast the ID, requesting the entity to return its current address. • Can never scale beyond local­area networks (think of ARP/RARP) • Requires all processes to listen to incoming location requests • One possible solution is to switch multicasting. • Forwarding pointers: Each time an entity moves, it leaves behind a pointer telling where it has gone to. • Referencing can be made entirely transparent to clients by simply following the chain of pointers • Update a client's reference as soon as present location has been found • Geographical scalability problems: • Long chains are not fault tolerant • Increased network latency

  44. Forwarding Pointers • The principle of forwarding pointers using (proxy, skeleton) pairs.

  45. Forwarding Pointers • Redirecting a forwarding pointer, by storing a shortcut in a proxy.

  46. Home­Based Approaches • The use of broadcasting or multicasting is difficult to implement. Long chains of forwarding pointers introduce performance problems and are susceptible to broken link. • A popular approach to supporting mobile entities in large-scale networks is to introduce a home location, which keeps track of the current location of an entity. • Single­tiered scheme: Let a home keep track of where the entity is: • An entity's home address is registered at a naming service • The home registers the foreign address (care-of-address) of the entity • Clients always contact the home first, and then continues with the foreign location

  47. Home­Based Approaches • Problems with home­based approaches: • The home address has to be supported as long as the entity lives. • The home address is fixed, which means an unnecessary burden when the entity permanently moves to another location • Poor geographical scalability (the entity may be next to the client) • Possible solution to home-based approaches: Two­tiered scheme (similar to mobile telephony): • Check local visitor registry first • Fall back to home location if local lookup fails

  48. Home-Based Approaches • The principle of Mobile IP.

  49. Hierarchical Location Services • The two-tiered home-based approach can be generalized to multiple layers. A network is divided into a collection of domains. Each domain can be subdivided into multiple subdomain. A lowest-level domain, called a leaf domain. • Each domain has an associated directory node that keeps track of the entities in that domain. • HLS (Hierarchical Location Services): • Each entity in a domain is represented by a location record in that directory node. • The address of an entity is stored in a leaf node. • Intermediate nodes contain a pointer to a child if and only if the subtree rooted at the child stores an address of the entity • The root knows about all entities

  50. Hierarchical Approaches • Hierarchical organization of a location service into domains, each having an associated directory node.