Chapter 5 Naming M Libsie Introduction names play

Chapter 5 - Naming M. Libsie

Introduction § names play an important role to: § share resources § uniquely identify entities § refer to locations § etc. § an important issue is that a name can be resolved to the entity it refers to § to resolve names, it is necessary to implement a naming system § in a distributed system, the implementation of a naming system is itself often distributed, unlike in nondistributed systems § efficiency and scalability of the naming system are the main issues 2

Objectives of the Chapter § we discuss how § human friendly names are organized and implemented; e. g. , those for file systems and the WWW § classes on naming systems § flat naming § structured naming, and § attribute-based naming 3

5. 1 Names, Identifiers, and Addresses § a name in a distributed system is a string of bits or characters that is used to refer to an entity § an entity is anything; e. g. , resources such as hosts, printers, disks, files, objects, processes, users, Web pages, . . . § entities can be operated on; e. g. , a resource such as a printer offers an interface containing operations for printing a document, requesting the status of a job, . . . § to operate on an entity, it is necessary to access it through its access point, itself an entity (special) 4

§ access point § the name of an access point is called an address (such as IP address and port number as used by the transport layer) § the address of the access point of an entity is also referred to as the address of the entity § an entity can have more than one access point (similar to accessing an individual through different telephone numbers) § an entity may change its access point in the course of time (e. g. , a mobile computer getting a new IP address as it moves) 5

§ an address is a special kind of name § it refers to at most one entity § each entity is referred by at most one address; even when replicated such as in Web pages § an entity may change an access point, or an access point may be reassigned to a different entity (like telephone numbers in offices) § separating the name of an entity and its address makes it easier and more flexible; such a name is called location independent § there also other types of names that uniquely identify an entity; in any case an identifier is a name with the following properties § it refers to at most one entity § each entity is referred by at most one identifier § it always refers to the same entity (never reused) § identifiers allow us to unambiguously refer to an entity 6

§ examples § name of an FTP server (entity) § URL of the FTP server § address of the FTP server § IP number: port number § the address of the FTP server may change § there are three classes on naming systems: flat naming, structured naming, and attribute-based naming 7

§ § § 5. 2 Flat Naming a name is a sequence of characters without structure; like human names? may be if it is not Ethiopian name! difficult to be used in a large system since it must be centrally controlled to avoid duplication how are flat names resolved name resolution: mapping a name to an address or an address to a name is called name-address resolution possible solutions: simple, home-based approaches, and hierarchical approaches 8

1. Simple Solutions n two solutions for LANs: Broadcasting and Multicasting, and Forwarding Pointers a. Broadcasting and Multicasting § a computer that wants to access another computer for which it knows its IP address broadcasts this address § the owner responds by sending its Ethernet address § used by ARP (Address Resolution Protocol) in the Internet to find the data link address (MAC address) of a machine § broadcasting is inefficient when the network grows (wastage of bandwidth and too much interruption to other machines) § multicasting is better when the network grows - send only to a restricted group of hosts § multicasting can also be used to locate the nearest replica - choose the one whose reply comes in first 9

b. Forwarding Pointers § how to look mobile entities § when an entity moves from A to B, it leaves behind a reference to its new location § advantage § simple: as soon as the first name is located using traditional naming service, the chain of forwarding pointers can be used to find the current address § drawbacks § the chain can be too long - locating becomes expensive § all the intermediary locations in a chain have to maintain their pointers § vulnerability if links are broken § hence, making sure that chains are short and that forwarding pointers are robust is an important issue 10

2. Home-Based Approaches § broadcasting and multicasting have scalability problems; performance problems and broken links are problems in forwarding pointers § a home location keeps track of the current location of an entity; often it is the place where an entity was created § it is a two-tiered approach § an example where it is used in Mobile IP § each mobile host uses a fixed IP address § all communication to that IP address is initially directly sent to the host’s home agent located on the LAN corresponding to the network address contained in the mobile host’s IP address § whenever the mobile host moves to another network, it requests a temporary address in the new network (called care-of-address) and informs the new address to the home agent 11

§ when the home agent receives a message for the mobile host it forwards it to its new address and also informs the sender the host’s current location for sending other packets home-based approach: the principle of Mobile IP 12

§ problems: § creates communication latency § the host is unreachable if the home does no more exist (permanently changed); the solution is to register the home at a traditional name service 13

3. Hierarchical Approaches § a generalization of the two-tiered approach into multiple layers § a network is divided into a collection of domains, similar to DNS § a single top-level domain spans the entire network § each domain can be subdivided into multiple, smaller domains § the lowest-level domain is called a leaf domain; typically a LAN § each domain D has an associated directory node dir(D) that keeps track of the entities in that domain leading to a tree of directory nodes § the root (directory) node knows about all entities 14

hierarchical organization of a location service into domains, each having an associated directory node 15

§ each entity is represented by a location record in the directory node dir(D) to keep track of its whereabouts § a location record for an entity in a leaf domain contains the entity’s current address; all other high-level domains will have only pointers to this address; this means the root node will store only pointers to all entities § an entity may have multiple addresses, for instance, if it is replicated; a higher level domain containing the two subdomains where the entity has addresses will have two pointers 16

an example of storing information of an entity having two addresses in different leaf domains 17

§ example of a look up operation § a client (in Domain D) would like to locate an entity E looking up a location in a hierarchically organized location service 18

§ update operations (i. e. , inserting and deleting addresses) § read pages 194 - 195) § another solution is Distributed Hash Tables (DHT) § read pages 188 - 191 19

5. 3 Structured Naming § flat names are not convenient for humans § Name Spaces § names are organized into a name space § each name is made of several parts; the first may define the nature of the organization, the second the name, the third departments, . . . § the authority to assign and control the name spaces can be decentralized where a central authority assigns only the first two parts § a name space is generally organized as a labeled, directed graph with two types of nodes § leaf node: represents the named entity and stores information such as its address or the state of that entity § directory node: a special entity that has a number of outgoing edges, each labeled with a name § each node in a naming graph is considered as another entity with an identifier 20

a general naming graph with a single root node, no § a directory node stores a table in which an outgoing edge is represented as a pair (edge label, node identifier), called a directory table § each path in a naming graph can be referred to by the sequence of labels corresponding to the edges of the path and the first node in the path, such as N: <label-1, label-2, . . . , label-n>, where N refers to the first node in the path 21

§ such a sequence is called a path name § if the first node is the root of the naming graph, it is called an absolute path name; otherwise it is a relative path name § instead of the path name n 0: <home, steen, mbox>, we often use its string representation /home/steen/mbox § there may also be several paths leading to the same node, e. g. , node n 5 can be represented as /keys or /home/steen/keys § although the above naming graph is directed acyclic graph (a node can have more than one incoming edge but is not permitted to have a cycle), the common way is to use a tree (hierarchical) with a single root (as is used in file systems) § in a tree structure, each node except the root has exactly one incoming edge; the root has no incoming edges Ü each node also has exactly one associated (absolute) path name 22

§ Name Resolution § given a path name, the process of looking up a name stored in the node is referred to as name resolution; it consists of finding the address when the name is given (by following the path) § Linking and Mounting § Linking: giving another name for the same entity (an alias) e. g. , environment variables in UNIX such as HOME that refer to the home directory of a user § two types of links (or two ways to implement an alias): § hard link: to allow multiple absolute path names to refer to the same node in a naming graph e. g. , in the previous graph, there are two different path names for node n 5: /keys and /home/steen/keys 23

§ symbolic link: representing an entity by a leaf node and instead of storing the address or state of the entity, the node stores an absolute path name the concept of a symbolic link explained in a naming graph § when first resolving an absolute path name stored in a node (e. g. , /home/steen/keys in node n 6), name resolution will return the path name stored in the node (/keys), at which point it can continue with resolving that new path name 24

§ so far name resolution was discussed as taking place within a single name space § name resolution can also be used to merge different name spaces in a transparent way § the solution is to use mounting Mounting § as an example, consider a mounted file system, which can be generalized to other name spaces as well § let a directory node store the directory node from a different (foreign) name space § the directory node storing the node identifier is called a mount point § the directory node in the foreign name space is called a mounting point, normally the root of a name space § during name resolution, the mounting point is looked up and resolution proceeds by accessing its directory table 25

§ consider a collection of name spaces distributed across different machines (each name space implemented by a different server) § to mount a foreign name space in a DS, the following are at least required § the name of an access protocol (for communication) § the name of the server § the name of the mounting point in the foreign name space § each of these names needs to be resolved § to the implementation of the protocol § to an address where the server can be reached § to a node identifier in the foreign name space § the three names can be listed as a URL 26

§ example: Sun’s Network File System (NFS) is a distributed file system with a protocol that describes how a client can access a file stored on a (remote) NFS file server § an NFS URL may look like nfs: //flits. cs. vu. nl/home/steen - nfs is an implementation of a protocol - flits. cs. vu. nl is a server name to be resolved using DNS - /home/steen is resolved by the server § e. g. , the subdirectory /remote includes mount points foreign name spaces on the client machine § a directory node named /remote/vu is used to store nfs: //flits. cs. vu. nl/home/steen § consider /remote/vu/mbox § this name is resolved by starting at the root directory on the client’s machine until node /remote/vu, which returns the URL nfs: //flits. cs. vu. nl/home/steen § this leads the client machine to contact flits. cs. vu. nl using the NFS protocol § then the file mbox is read in the directory /home/steen 27

mounting remote name spaces through a specific process protocol 28

§ distributed systems that allow mounting a remote file system also allow to execute some commands § example commands to access the file system cd /remote/vu ls -l § by doing so the user is not supposed to worry about the details of the actual access; the name space on the local machine and that on the remote machine look to form a single name space 29

§ The Implementation of a Name Space § a name space forms the heart of a naming service § a naming service allows users and processes to add, remove, and lookup names § a naming service is implemented by name servers § for a distributed system on a single LAN, a single server might suffice; for a large-scale distributed system the implementation of a name space is distributed over multiple name servers § Name Space Distribution § in large scale distributed systems, it is necessary to distribute the name service over multiple name servers, usually organized hierarchically § a name service can be partitioned into logical layers § the following three layers can be distinguished (according to Cheriton and Mann) 30

§ global layer § formed by highest level nodes (root node and nodes close to it or its children) § nodes on this layer are characterized by their stability, i. e. , directory tables are rarely changed § they may represent organizations, groups of organizations, . . . , where names are stored in the name space § administrational layer § groups of entities that belong to the same organization or administrational unit, e. g. , departments § relatively stable § managerial layer § nodes that may change regularly, e. g. , nodes representing hosts of a LAN, shared files such as libraries or binaries, … § nodes are managed not only by system administrators, but also by end users 31

an example partitioning of the DNS name space, including Internetaccessible files, into three layers 32

§ the name space is divided into nonoverlapping parts, called zones in DNS § a zone is a part of the name space that is implemented by a separate name server § some requirements of servers at different layers § performance (responsiveness to lookups), availability (failure rate), etc. § high availability is critical for the global layer, since name resolution cannot proceed beyond the failing server; it is also important at the administrational layer for clients in the same organization § performance is very important in the lowest layer, since results of lookups can be cached and used due to the relative stability of the higher layers § they may be enhanced by client side caching (global and administrational layers since names do not change often) and replication; they create implementation problems since they may introduce inconsistency problems (see Chapter 7) 33

Item Global Administrational Managerial Geographical scale of network Worldwide Organization Department Total number of nodes Few Many Vast numbers Responsiveness to lookups Seconds Milliseconds Immediate Update propagation Lazy Immediate Availability requirement Very High low Number of replicas Many None or few None Is client-side caching applied? Yes Sometimes 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 34

§ Implementation of Name Resolution § recall that name resolution consists of finding the address when the name is given § assume that name servers are not replicated and that no client-side caches are allowed § each client has access to a local name resolver, responsible for ensuring that the name resolution process is carried out § e. g. , assume the path name root: <nl, vu, cs, ftp, pub, globe, index. txt> is to be resolved or using a URL notation, this path name would correspond to ftp: //ftp. cs. vu. nl/pub/globe/index. txt 35

§ Resolution § mapping a name to an address or an address to a name is § called name-address resolution Resolver § a host that needs to map an address to a name or a name to an address calls a DNS client named a resolver § the resolver accesses the closest DNS server with a mapping request § if the server has the information it satisfies the resolver; otherwise, it either refers the resolver to other servers (called Iterative Resolution) or asks other servers to provide the information (called Recursive Resolution) 36

§ Iterative § a name resolver hands over the complete name to the root name server § the root name server will resolve the name as far as it can and return the result to the client; at the minimum it can resolve the first level and sends the name of the first level name server to the client § the client calls the first level name server, then the second, . . . , until it finds the address of the entity the principle of iterative name resolution 37

§ Recursive § a name resolver hands over the whole name to the root name server § the root name server will try to resolve the name and if it can’t, it requests the first level name server to resolve it and to return the address § the first level will do the same thing recursively the principle of recursive name resolution 38

§ Advantages and drawbacks § recursive name resolution puts a higher performance demand on each name server; hence name servers in the global layer support only iterative name resolution § caching is more effective with recursive name resolution; each name server gradually learns the address of each name server responsible for implementing lower-level nodes; eventually lookup operations can be handled efficiently Server for node Should resolve Looks up Passes to child Receives and caches Returns to requester cs vu <ftp> <cs, ftp> #<cs> -<ftp> -#<ftp> nl <vu, cs, ftp> #<vu> <cs, ftp> #<cs, ftp> root <nl, vu, cs, ftp> #<nl> <vu, cs, ftp> #<vu, cs> #<vu, cs, ftp> #<cs> #<cs, ftp> #<vu, cs> #<vu, cs, ftp> #<nl, vu> #<nl, vu, cs, ftp> recursive name resolution of <nl, vu, cs, ftp>; name servers cache intermediate results for subsequent lookups 39

§ communication costs may be reduced in recursive name resolution the comparison between recursive and iterative name resolution with respect to communication costs; assume the client is in Ethiopia and the name servers in the Netherlands § Summary Method Advantage(s) Recursive Less Communication cost; Caching is more effective Iterative Less performance demand on name servers 40

§ Example - The Domain Name System (DNS) § one of the largest distributed naming services is the Internet DNS § it is used for looking up host addresses and mail servers § hierarchical, defined in an inverted tree structure with the root at the top § the tree can have only 128 levels 41

§ Label § each node has a label, a string with a maximum of 63 characters (case insensitive) § the root label is null § children of a node must have different names (to guarantee uniqueness) § Domain Name § each node has a domain name § a full domain name is a sequence of labels separated by dots (the last character is a dot; null string is nothing) § domain names are read from the node up to the root § full path names must not exceed 255 characters 42

§ Fully Qualified Domain Name (FQDN) or Absolute § terminated by a null string § contains the full name of a host, e. g. , cs. aau. edu. et. § usually the last dot is omitted for readability § Partially Qualified Domain Name (PQDN) or Relative § not terminated with a null string § it starts from a node but does not reach the root § used when the name to be resolved belongs to the same site as the client (the resolver supplies the missing part, called the suffix to create an FQDN) 43

§ Domain § a domain is a subtree of the domain name space § the name of the domain is the domain name of the node at the top of the subtree § the Internet is divided into over 200 top-level domains; each partitioned into subdomains, . . . ; the leaves represent domains that have no subdomains; a leaf domain may contain a single host or represent a company and contain thousands of hosts 44

§ Hierarchy of Name Servers § storing the information contained in the domain name space § § in a single computer is inefficient and unreliable distribute the information among many computers called DNS servers there is a hierarchy of name servers as we have a hierarchy of names 45

§ Zone § what a server is responsible for, or has authority over, is called a zone; zones are nonoverlapping § the server makes a database called a zone file and keeps all the information for every node under that domain § it can divide its domain into subdomains and delegate part of its authority to other servers 46

§ Root Server § a server whose zone consists of the whole tree § it usually does not store the whole information about domains but delegates its authority to other servers and keeps references to those servers § there are currently more than 13 root servers, each covering the whole domain name space and distributed all around the world § Primary and Secondary Servers § a primary server is one that stores a file about the zone for which it is an authority; it is responsible for creating, maintaining, and updating the zone file § a secondary server is one that transfers the complete information about a zone from another server (primary or secondary); it does not create or update the file § such arrangement is to create redundancy so that if one server fails, the other can still serve clients 47

§ Types of Top-Level Domains § two types: generic domains and country domains; there is a § third one called Inverse Domain (used to map an address to a name; we will not discuss it further) Generic Domains § define registered hosts according to their generic behaviour Label com Description Commercial organizations edu Educational institutions gov int mil net org Government institutions International organizations Military groups Network support centers Nonprofit organizations 48

§ newly introduced first-level domains Label aero biz coop info museum name pro Description Airlines and aerospace companies Businesses or firms (similar to com) Cooperative business organizations Information service providers Museums and other nonprofit organizations Personal names (individuals) Professional individual organizations § Country Domains § include one entry for every country (as defined by ISO) two character abbreviations 49

§ the contents of a node is formed by a collection of resource records; the important ones are the following Type of record Associated Description entity SOA (start of authority) Zone Holds information on the represented zone, such as an e-mail address of the system administrator A (address) Host Contains an IP address of the host this node represents MX (mail exchange) Domain Refers to a mail server to handle mail addressed to this node; it is a symbolic link; e. g. name of a mail server SRV Domain Refers to a server handling a specific service NS (name server) Zone Refers to a name server that implements the represented zone CNAME Node Contains the canonical name of a host PTR (pointer) Host Symbolic link with the primary name of the represented node HINFO (host info) Host Holds information on the host this node represents; such as machine type and OS TXT Any kind Contains any entity-specific information considered useful 50

§ cs. vu. nl represents the domain as well as the zone; it has 3 name servers (star, top, solo) and 3 mail servers § name server for this zone with 2 network addresses § mail server § Web server § FTP server § a single machine implementing Web server and FTP server § laser printer § inverse mapping an excerpt from the DNS database for the zone cs. vu. nl 51

5. 4 Attribute-Based Naming § flat naming: provides a unique and location-independent way of referring entities § structured naming: also provides a unique and locationindependent way of referring entities as well as human-friendly names § but do not allow searching entities by giving a description of an entity § each entity is assumed to have a collection of attributes that say something about the entity § then a user can search an entity by specifying (attribute, value) pairs known attribute-based naming § Directory Services § attribute-based naming systems are also called directory services 52

§ how are resources described? one possibility is to use RDF (Resource Description Framework) that uses triplets consisting of a subject, a predicate, and an object § e. g. , (person, name, Alice) to describe a resource Person whose Name is Alice § Hierarchical Implementations: LDAP § distributed directory services are implemented by combining structured naming with attribute-based naming § e. g. , Microsoft’s Active directory service § such systems rely on the lightweight directory access protocol or LADP which is derived from OSI’s X. 500 directory service § a LADP directory service consists of a number of records called directory entries (attribute, value) pairs, similar to a resource record in DNS; could be single- or multiple-valued (e. g. , Mail_Servers) 53

Attribute Abbr. Value Country C NL Locality L Amsterdam Organization O Vrije Universiteit Organizational. Unit OU Comp. Sc. Common. Name CN Main server Mail_Servers -- 137. 20. 3, 130. 37. 24. 6, 137. 20. 10 FTP_Server -- 130. 37. 20 WWW_Server -- 130. 37. 20 a simple example of an LDAP directory entry using LDAP naming conventions to identify the network addresses of some servers 54

§ the collection of all directory entries is called a Directory Information Base (DIB) § each record is uniquely named so that it can be looked up § each naming attribute is called a Relative Distinguished Name (RDN); the first 5 entries above § a globally unique name is formed using abbreviations of naming attributes, e. g. , /C=NL/O=Vrije Universiteit/OU=Comp. Sc. § this is similar to the DNS name nl. vu. cs § listing RDNs in sequence leads to a hierarchy of the collection of directory entries, called a Directory Information Tree (DIT) § a DIT forms the naming graph of an LDAP directory service where each node represents a directory entry 55

§ node N corresponds to the directory entry shown earlier; it also acts as a parent of other directory entries that have an additional attribute, Host_Name; such entries may be used to represent hosts part of the directory information tree 56

Attribute Value Country NL Locality Amsterdam Organization Vrije Universiteit Organizational. Unit Comp. Sc. Common. Name Main server Host_Name star Host_Name zephyr Host_Address 192. 31. 231. 42 Host_Address 137. 20. 10 two directory entries having Host_Name as RDN § read pages 222 - 226 about Decentralized Implementations 57