6. Naming (name services)

• General issues

• Naming in distributed systems

• Locating mobile entities

P183

• WWW can be viewed as a huge distributed system consisting of millions of clients and servers for accessing linked documents.

• DNS is a distributed naming system.

P648

6.1 Naming entities

1. Three kinds of names• human-friendly name: f1, cs.nju.edu.cn• address: a special kind of name – name of an access

point. 202.119.32.6

• identifier: a name has properties:– an identifier refers to at most one entity– each entity is referred to by at most one identifier

RFC:2648

Names are used to identify and refer to entities.

• Address is not well suited for referring an entity– an entity may be moved changes address– an entity can offer more than one access points.

• Identifier is location independent (PID)

Name space is a collection of all valid names which

can be represented as a labeled, directed graph.(recognized by a particular service)

C++: namespace A { … }XML:

<student xmlns = “student.dtd” xmlns:teacher =“teacher.dtd”><xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

2. Name spaces

For human use For implementation

1 “usr”2 “dev”

0

7 8

2

9

11 12

usr dev

1

li liu

f1 f2

nic

0

13

f1

usr

li liu

dev

nic

f1 f2 f1

Purpose?

Example of Unix file system

The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.

inode inode inode0 1 n

data data data data

… …

… …

blocks

P188

A general naming graph with a single root node.

return P187

• Leaf node

represent named entity

store information (address or state) on the entity• directory node

represent a collection of entities

store a directory table of (edge label, node identifier).

• path name

N:<label-1, label-2, …, label-n> ..

no:<home,steen,mbox> /home/steen/mbox

com, edu, gov, mil, net, org, arpa, int, country

root

cn jp edu com

edu MIT cmcom

ustcpku nju

…...

...

….. ...

…...

…...

…...

...

us ...

edu com

Name space distribution

An example partitioning of the DNS name space, including Internet-accessible files, into three layers.

P196

3. Name resolution

A path name ---> information stored in the node.

or entity

• Name resolution:

the process of traversing the naming graph by looking up the components of a path name, one at a time.

N:<label-1, label-2, …, label-n>

P198

find

For example, n0:<home, steen, mbox>n1:”home”n5:”keys”

/home/steen/mbox

4. Name service in large distributed system

• Name service Chapter 6.1 DNS

name entity (or address)• Design issue: structure of name space

A large-scale name space (naming graph) is implemented

by distributing its nodes across multiple name servers and organized hierarchically.

• Distribution of a name space across multiple name servers affects the implementation of name resolution

P194

The principle of iterative name resolution. P198

The principle of recursive name resolution. ?

6.2 Directory service

1. Directory service

If we wish to find a particular person or resource, but we don’t know its name, only some of its other attributes.

For example,– Query the name of each Web server running Unix– the person with CS department whose family name is

Li, man

If we have directory service with information:

P206

Entity Attributes User Name: …

Tel.no.: …

Email address:

Dept: ……

Sex:

Host Name:

Type:

OS:

IP address:

……

Printer Name:

Type:

……

… ……

Predefined entities and their attributes

Entry Attributes user name:xxx; tel.no.:xxxxx; email addr.:…

name:yyy; tel.no.:yyyyy; email addr.:…

……

host name:xxx; type:www server; OS:Unix; …

name:yyy; type:FTP server; OS:Linux; …

……

printer ……

… ......

DIB

Directory service

• store collections of binding between names and attributes

• look for an entity based on a description of attributes

some attributes ----> information of entities type=Web server&OS=Unix ----> entities (or names)

Examples:

X.500, LDAP(P209), Active directory services

• an X.500 directory service consists of a number of directory entries

• a directory entry is made up of a collection of (attribute, value) pairs

……

Name: cs.nju.edu.cn

Type: www server

OS: Linux

IP address: 202.119.36.1

……

P2062. X.500 general model for global

130.37.21.11--WWW_Server

130.37.21.11--FTP_Server

130.37.24.6, 192.31.231.42,192.31.231.66--Mail_Servers

Main serverCNCommonName

Math. & Comp. Sc.OUOrganizationalUnit

Vrije UniversiteitOOrganization

AmsterdamLLocality

NLCCountry

ValueAbbr.Attribute

A simple example of a X.500 directory entry using X.500 naming convention.

P207

• Naming convention applies to the first five attribut

es: C, L, O, OU, CN • CommonName is used as a name to identify an ent

ry within a limited part of a directory

• The collection of all directory entries in X.500 directory service is called DIB (Directory Information Base)

P207

• Each entry in DIB is uniquely named by listing

naming attributes in sequence• Each naming attribute is called RDN

C : nlL : AmsterdamO : Vrije UniversiteitOU : Math. & Comp. Sc.CN : Main serverMail_server : …FTP_server : …WWW_server: …

P207

• C, O, OU could be used to form the global unique name, analog to DNS name

/C=NL/O=Vrije Univ/OU=Math.& Sc

~ nl.vu.cs

• DIT (Directory Information Tree)

use of globally unique names leads to a hierarchy of the collection of directory entries

1) X.500 name space

directory entry | node

DIT P208

Attribute Value Attribute Value

Country NL Country NL

Locality Amsterdam Locality Amsterdam

Organization Vrije Universiteit Organization Vrije Universiteit

OrganizationalUnit Math. & Comp. Sc. OrganizationalUnitMath. & Comp. Sc.

CommonName Main server CommonName Main server

Host_Name star Host_Name zephyr

Host_Address 192.31.231.42 Host_Address 192.31.231.66

Two directory entries having Host_Name as RDN

(Relative Distinguished Name).

P208

2) Look up operations

• readpath name an entity

• search base name list of names

record=read(“&(C=NL)(O=Vrije Univ.)(OU=Math. & Comp. Sc.)(CN=Main server)”) P207

list=search(“&(C=NL)(O=Vrije Univ.)(OU= Math. & Comp. Sc.) (CN=Main server)”) star, zephyr

filter expression for all of entries below the base node

list

3) Implementation

DIT is partitioned and distributed across many Directory Service Agent (DSA)

P209

DSA

DSA DSA

DSADSA

DSADUA

DUA

Client (DUA—Directory User Agent) interacts with a DSA, which accesses other DSAs as necessary

2. LDAP

1) LDAP is Internet-based directory services.

ASN.1 encoding string

2) Active directory services P539

Microsoft’s LDAP implementation

P209

X.500 protocol

ISOprotocol stack

LDAP

TCP IP

Lightweight Directory Access Protocol

Active directory services

• a distributed system is partitioned into domains

• each domain consists of a number of users and resources• each domain has one or more directory servers, each of wh

ich has a DNS name (ldap.cs.nju.edu.cn)

nju

soft CS phy… …

Domain tree

directory servers

Domain:

• Users: teacher, student, …

• Resources: host, printer,

Entry Attributes user1

user2

resource1

resource2

DIB

Entity Attributes Teacher Name: …

Tel.no.: …

Email address:

……

Student Name: …

Email address:

Status:

……

Host Name:

Type:

OS:

IP address:

……

Printer Name:

Type:

……

Implementation with DNS

DNS Directory service

edu

ustcpku nju

...

nju

soft CS phy… …

directory server

Domain tree

domain has DNS name: ldap.cs.nju.edu.cn

name server

Name Type Value

www.nju.edu.cn A 202.119.32.6

cs.nju.edu.cn A 202.119.36.1

ldap.cs.nju.edu.cn SRV 202.119.36.2

ldap.cs.nju.edu.cn TXT “LDAP server”

DNS database

Entry Attributes user1

user2

resource1

resource2

DIB for domain cs.nju.edu.cn

P202

Use directory service

client

DNSserver

query(LDAP server) request address

directoryserver

LDAP query

LDAP reply

6.3 Locating mobile entities

1. Feature of mobile entitiesA mobile entity has multiple addresses when mobile host moves.

2. Locating mobile entities• use forwarding pointer• use location-independent identifier to locate mobile

entities

P210

1) Forwarding pointer

location A location B location C

entity

When an entity move from A to B, it leaves behind a reference to its new location at B

entity entityentity

P214

2) Home-based approaches

Chapter 6.2 Mobile IP

P216

3) Hierarchical approaches

It is used in Globe location service.• A network is divided into a collection of domains.• There is a single top-domain that spans the entire

network• each domain can be subdivided into multiple small

er subdomains• leaf domain is lowest-level one

P217

D1D2

D3

D21D24

D11

D12

D13

Top domain

D231D232

D1D2

D3

D21D24

D11

D12

D13

Top domain D

D231D232

E1

E12

E13

E31

E32

E33

E23, E232

E22

How organize location service?

• Directory node:

each domain D has a directory node dir(D) that stores location records for all entities in D

• Location record:

For leaf domain, the location record for entity E contains E’s current address in the domain;

For higher-level domain D’, the location record for entity E contains a pointer to directory node of next lower-level domain that has E

• root directory node knows about all entities

D1D2

D3

D21D24

D11

D12

D13

Top domain D

D231D232

E1

E12

E13

E31

E32

E33

E23, E232

E22E13’

dir(D)

E1 ->dir(D1)E12 ->dir(D1)E13 ->dir(D1) ->dir(D2)E22 ->dir(D2)E23 ->dir(D2)E232 ->dir(D2)E31 ->dir(D3)E32 ->dir(D3)E33 ->dir(D3)

dir(D1)

E1 <E1>E12 ->dir(D12)E13 ->dir(D13)

dir(D2)

E22 ->dir(D22) E23 ->dir(D23)E232 ->dir(D23)E13 ->dir(D21)

dir(D22)

E22 <E22>

dir(D23)

E23 <E23>

E232 ->dir(D232)

dir(D232)

E232 <E232>

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

P218

• An example of storing information of an entity having two addresses in different leaf domains.

Look up operation

• Client issues the request for E to the directory node of the leaf domain D in which the client resides

• search the tree of directory nodes• E’ address is returned to the client

method 1: from the root if E is not in the leaf domain method 2: from its parents if … and there are copies

• Looking up a location in a hierarchically organized location service.

P219

Insert operation

• An insert request is forwarded to the first node that knows about entity E.

• A chain of forwarding pointers to the leaf node is created.

For example, An entity E has created a replica in the leaf domain

D.

P220

Summary

1. Naming entity and name service• three kinds of name• name space• name resolution • name service in large distributed system(DNS)2. Directory service• difference between directory service and name

service• X.500 and LDAP (Active directory services)

3. Locating mobile entities• use location-independent identifier to locate

mobile entities.• ways

– home-based approaches– forwarding pointer object(proxy,

skeleton)– hierarchical approaches