Reliable Distributed Systems

1
Reliable Distributed Systems

• Naming(Communication Basics Part II)
• Slide set based on one by Prof. Paul Francis,
  Cornell University.
• Updated by Bina Ramamurthy

2

• Any problem in computer science can be solved
  with another layer of indirection
• David Wheeler

3
Naming is a layer of indirection

• What problems does it solve?
• Makes objects human readable
• Hides complexity and dynamics
• Multiple lower-layer objects can have one name
• Changes in lower-layer objects hidden
• Allows an object to be found in different ways
• One object can have multiple names

4
Names map to objects through a resolution service
Name
Distributed Name Resolution Service
Object
5
Identifiers and Locators

• A name is always an identifier to a greater or
  lesser extent
• Can be persistent or non-persistent
• Can be globally unique, locally unique, or even
  non-unique
• If a name has structure that helps the resolution
  service, then the name is also a locator

6
Naming in networks
Address
Name
Route
7
DNS names map into addresses
Domain Name System (DNS)
Address
Many-to-many
Name
Route

• Hierarchical
• User-friendly
• Location independent
• But not org independent

Domain Name (www.cnn.com)
8
Addresses map into routes
IP address (128.94.2.17)
Routing algorithm (BGP, OSPF, RIP)
Address
One-to-many
Name

• Hierarchical
• Location Dependent
• Non-unique
• Can change often
• Refers to an interface, not a host

Route
9
Routes get packets to interfaces
Address
Name
Route

• A path
• Source dependent
• Can change often

10
DNS names and IP addresses are identifiers and
locators

• Both are typically non-persistent
• Private IP addresses identify only in the context
  of an IP realm
• Domain names are good identifiers
• woodstock.cs.cornell.edu identifies a host
• www.cnn.com identifies a service
• URLs are good identifiers

11
Domain Name System (DNS)

• Distributed directory service
• Hierarchical name space
• Each level separated by .
• Analogous to / separator in file systems
• One global root
• Replicated across
• There have been Denial of Service (DoS) attacks
  on these root servers, none real successful
• Because of caching, queries to root servers
  relatively rare
• DNS maybe only global directory service???

12
DNS is the core of the Internet

• Global name space
• Can be the core of a naming or identifying scheme
• Global directory service
• Can resolve a name to nearly every computer on
  the planet

13
DNS tree structure
NS RR pointers
.
edu.
com.
jp.
us.
cornell.edu.
cmu.edu.
mit.edu.
cs.cornell.edu.
eng.cornell.edu.
foo.cs.cornell.edu A 10.1.1.1 bar.cs.cornell.edu
A 10.1.1.1
14
LDAP is another popular distributed directory
service

• Richer and more general than DNS
• Has generalized attribute/value scheme
• Can search on attribute, not just name
• Simpler and more efficient than a full relational
  database
• Not a global directory service, though namespace
  is global
• Its predecessor, X.500, was meant to be
• But local LDAP services can point to each other
• Commonly used for personnel RR databases,
  subscriber databases

15
URL

• Consists of

16
URL

• Consists of

A protocol
Information the protocol needs
17
URL examples

• HTTP (web)
• http//www.cnn.com/news/story.html
• Email
• mailto//francis_at_cs.cornell.edu
• Newsgroups
• newscornell/class/cs514
• SIP (Session Initiation Protocol)
• sip//service_at_phone.verizon.com

18
Note the central role of DNS

• HTTP (web)
• http//www.cnn.com/news/story.html
• Email
• mailto//francis_at_cs.cornell.edu
• Newsgroups
• newscornell/class/cs514
• SIP (Session Initiation Protocol)
• sip//service_at_phone.verizon.com

19
Locating mobile entities (section 4.2, AT/MvS)

• What is a mobile entity?
• From naming perspective, it is an entity whose
  address changes often
• This doesnt require physical mobility!
• Every time you dial up, you may get a new address
• So, mobility existed well before laptops became
  common
• Though laptops create more mobility

20
Is mobility a problem for DNS?

• Not really
• Even though DNS was designed with relatively
  stable IP addresses in mind
• Because mobility only effects leaf DNS servers
• Recall A RR TTL is short, but NS RR TTL is long
• Note non-mobile web servers A RRs often have
  very short TTLs
• To allow quick failover to another web server

21
Is mobility a problem at all?

• Less than youd think
• Most mobile systems are clients servers are
  rarely mobile
• Clients are initiators of connections, not
  recipients
• Therefore, there is not a client locating problem
• What about email, instant messaging, and VoIP
  (Voice over IP)?
• Clients receive emails, instant messages, and
  phone calls

22
Identifiers must be made hard to spoof

• That is why drivers licenses have pictures and
  credit cards have signatures
• In networking, two ways
• Identifier is also a locator
• Reverse routability
• Some kind of secret-protected signature

23
Summary of Lecture

• Introduction to Naming
• Naming basics
• Names, Addresses, Routes
• Identifiers and Locators
• DNS is the global directory service
• LDAP is a popular local directory service
• URLs build on DNS
• Identifiers must be hard to spoof
• Reverse routability, cryptographic signatures