1. 6. Naming (name services)
P183
6. Naming (name services)
General issues
Naming in distributed systems
Locating mobile entities

2. DNS is a distributed naming system.
P648
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.

3. 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.
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

4. 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)

5. 2. Name spaces 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="

6. For human use For implementation
1 “usr”
2 “dev”
Purpose?
usr li
liu
dev
nic f1 f2 7 8 2 9 11 12
usr
dev 1 li
liu f1 f2
nic 13
For human use For implementation

7. Example of Unix file system1 n
blocks
… …
inode
inode
inode
… …
data
data
data
data
The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.

8. return
P187
A general naming graph with a single root node.

9. 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

10. Name space distribution
root cn jp
edu
com
MIT cm
ustc
pku
nju
…...
...
….. us
com, edu, gov, mil, net, org, arpa, int, country

11. P196
An example partitioning of the DNS name space, including Internet-accessible files, into three layers.

12. 3. Name resolution A path name ---> information stored in the node.
find
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>

13. For example, n0:<home, steen, mbox>
n5:”keys”
/home/steen/mbox

14. 4. Name service in large distributed system
P194
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

15. The principle of iterative name resolution.

16. The principle of recursive name resolution. ?

17. 6.2 Directory service 1. Directory service
P206
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:

18. Predefined entities and their attributes DIB Entry Attributes Entity
user
name:xxx; tel.no.:xxxxx; addr.:…
name:yyy; tel.no.:yyyyy; addr.:… ……
host
name:xxx; type:www server; OS:Unix; …
name:yyy; type:FTP server; OS:Linux; …
printer …
......
Entity
Attributes
User
Name: …
Tel.no.: …
address:
Dept: ……
Sex:
Host
Name:
Type:
OS:
IP address: ……
Printer …

19. 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

20. 2. X.500 general model for global
P206
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:

21. P207 --
WWW_Server
FTP_Server
, ,
Mail_Servers
Main server CN
CommonName
Math. & Comp. Sc. OU
OrganizationalUnit
Vrije Universiteit O
Organization
Amsterdam L
Locality NL C
Country
Value
Abbr.
Attribute
A simple example of a X.500 directory entry using X.500 naming convention.

22. P207
Naming convention applies to the first five attributes: C, L, O, OU, CN
CommonName is used as a name to identify an entry within a limited part of a directory
The collection of all directory entries in X.500 directory service is called DIB (Directory Information Base)

23. Each naming attribute is called RDN
P207
Each entry in DIB is uniquely named by listing naming attributes in sequence
Each naming attribute is called RDN
C : nl
L : Amsterdam
O : Vrije Universiteit
OU : Math. & Comp. Sc.
CN : Main server
Mail_server : …
FTP_server : …
WWW_server: …

24. /C=NL/O=Vrije Univ/OU=Math.& Sc DIT (Directory Information Tree)
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

25. 1) X.500 name space
DIT
P208
directory entry | node

26. Two directory entries having Host_Name as RDN
P208
Attribute
Value
Country NL
Locality
Amsterdam
Organization
Vrije Universiteit
OrganizationalUnit
Math. & Comp. Sc.
CommonName
Main server
Host_Name
star
zephyr
Host_Address
Two directory entries having Host_Name as RDN
(Relative Distinguished Name).

27. 2) Look up operations read path 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

28. 3) Implementation
P209
DIT is partitioned and distributed across many Directory Service Agent (DSA)
DSA
DSA
DSA
DUA
DSA
DSA
DSA
DUA
Client (DUA—Directory User Agent) interacts with a DSA,
which accesses other DSAs as necessary

29. 2. LDAP 1) LDAP is Internet-based directory services.
Lightweight Directory Access Protocol
1) LDAP is Internet-based directory services.
ASN.1 encoding string
2) Active directory services P539
Microsoft’s LDAP implementation
X.500 protocol
LDAP
ISO
protocol
stack
TCP IP

30. 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 which has a DNS name (ldap.cs.nju.edu.cn)
nju
directory servers … …
soft CS
phy
Domain tree

31. Domain: Users: teacher, student, … Resources: host, printer, DIB Entry
Attributes
user1
user2
resource1
resource2

32. Entity Attributes Teacher Name: … Tel.no.: … Email address: …… Student
Status:
Host
Name:
Type:
OS:
IP address:
Printer

33. Implementation with DNS
DNS Directory service
edu
ustc
pku
nju
...
Domain tree
name server
directory server
nju … …
soft CS
phy
domain has DNS name:
ldap.cs.nju.edu.cn

34. P202 DNS database DIB for domain cs.nju.edu.cn Name Value Entry
Type
Value A
cs.nju.edu.cn
ldap.cs.nju.edu.cn
SRV
TXT
“LDAP server”
DIB for domain
cs.nju.edu.cn
Entry
Attributes
user1
user2
resource1
resource2

35. Use directory service DNS server query(LDAP server) request address
LDAP query
directory
server
client
LDAP reply

36. 6.3 Locating mobile entities
P210
1. Feature of mobile entities
A mobile entity has multiple addresses when mobile host moves.
2. Locating mobile entities
use forwarding pointer
use location-independent identifier to locate mobile entities

37. 1) Forwarding pointer location A location B location C entity entity
When an entity move from A to B, it leaves behind a reference to its new location at B
entity
entity
entity

38. 2) Home-based approaches
Chapter 6.2 Mobile IP

39. 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 smaller subdomains
leaf domain is lowest-level one

40. Top domain D1 D2 D3
D21
D24
D11
D12
D13
D231
D232

41. How organize location service?
Top domain D D1 D2 D3
D21
D24
D11
D12
D13
E31
E22 E1
E12
E32
D231
D232
E33
E13
E23, E232
How organize location service?

42. 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

43. Top domain D D2 D1 D3 D11 D21 D12 D24 D13 E31 E22 E13’ E1 E12 E32 D231

44. dir(D)
dir(D1)
E <E1>
E12 ->dir(D12)
E13 ->dir(D13)
E >dir(D1)
E12 ->dir(D1)
E13 ->dir(D1) ->dir(D2)
E22 ->dir(D2)
E >dir(D2)
E232 ->dir(D2)
E31 ->dir(D3)
E32 ->dir(D3)
E33 ->dir(D3)
dir(D2)
E22 ->dir(D22)
E23 ->dir(D23)
E232 ->dir(D23)
E13 ->dir(D21)
dir(D22)
E <E22>
dir(D23)
E <E23> E232 ->dir(D232)
dir(D232)
E <E232>

45. P218
Hierarchical organization of a location service into domains, each having an associated directory node.

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

47. 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

48. P219
Looking up a location in a hierarchically organized location service.

49. 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.

50. P220

51. 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)

52. 3. Locating mobile entities
use location-independent identifier to locate mobile entities.
ways
home-based approaches
forwarding pointer object(proxy, skeleton)
hierarchical approaches