Naming

Chapter 5

Most of the lecture notes are based on slides by Prof. Jalal Y. Kawash at Univ. of Calgary

Some slides are from Brennen Reynolds at University of California, Davis

I have modified them and added new slides

Giving credit where credit is due:

CSCE455/855 Distributed Operating Systems

Names (1)

Name: a string of bits or characters Why we need names in a computer system?

To refer to and operate on entities such as hosts, printers, disks, and files etc.

To operate on an entity, we need to access it i.e., to obtain an entity’s access points (addresses)

for example, the following are my access points: ylu@cse.unl.edu, (402)472-5793

for a running server process, its transport-level address (IP address, port number) serves as its access point

Names (2)

How about use an address as the name to refer to an entity? Use (129.93.165.4, TCP port 21) to refer to our dept.

ftp service? Use (402) 472 – 5793 to refer to me?

Names (3)

Using address to refer to an entity is Inflexible, human unfriendly, subject to changes

an entity changes address, address reassigned to another entity

An entity has more than one address (access point), which one to use as a reference?

Easier and more flexible to use location independent names

A Special Type of Name: Identifier

An 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)

What could serve as a person’s identifier?

Name Types

• Identifier– unambiguously refers to an entity

• Address– access point

• Human Friendly Name– a name tailored to be used by humans, often

a character string

What is a Naming System

• Name system maintains a name-to-address binding to resolve name (or identifier) to address

• The implementation of naming systems in distributed and non-distributed systems are quite different– Say, how to locate a process in a uniprocessor

system?

Naming Systems

• Part I: Flat Naming– Resolves identifiers to addresses

• Part II: Structured Naming– Resolves structured human-friendly names

to addresses

• Part III: Attributed-based Naming– Resolves descriptive names to addresses

Flat Naming

• Simple Solutions– Broadcasting– Forwarding pointers

• Home-based Solutions

• Hierarchical Solutions

• Distributed Hash Tables

Broadcasting (I)

Entity(A)

Need Address for Entity(A)

Reply toRequest

Address of Entity(A)

IGNORERequest

Internet ARP: Network IP addresses data-link MAC addresses

Broadcasting (II)

• Broadcasting is not suitable for larger networks

– Bandwidth is wasted– Hosts are interrupted for no reason

Forwarding Pointers

• A popular method to locate mobile entities

• When an entity moves from A to B, it leaves a pointer to B at A

• Simple: use traditional naming service to locate A, then follow the pointer chain

• Example: postal service– One person moves from Florida Virginia

Forwarding Pointers

• Figure 5-1. The principle of forwarding pointers using (client stub, server stub) pairs.

Forwarding Pointers

• Expensive if chain gets too large

• Lost pointer?

Forwarding Pointers

• Figure 5-2. Redirecting a forwarding pointer by storing a shortcut in a client stub.

Forwarding Pointers

• Figure 5-2. Redirecting a forwarding pointer by storing a shortcut in a client stub.

Comparison

• Broadcasting:– Scalability problems– Efficiency problems in large scale systems

• Forwarding Pointers:– Geographical scalability problems

• Long chains: performance problem

• Prone to failure

Home-Based Approaches

• Home-location: popular for supporting mobile entities in large-scale networks

• Keeps track of the current location

Mobile IP

• Assign a fixed IP (home location) to a mobile host• Contact host through home location

Flat Naming

• Simple Solutions– Broadcasting– Forwarding pointers

• Home-based Solutions

• Hierarchical Solutions

• Distributed Hash Tables

Hierarchical Approaches

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

Hierarchical Approaches – Lookup Operation

Looking up a location in a hierarchically organized location service.

Hierarchical Approaches – Update Operation

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

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

Hierarchical Approaches – Replicating Entities

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

Hierarchical Approaches – Delete Operation

•Delete a replica of entry E?

Hierarchical Approaches – Delete Operation

• Delete Replica R of Entity E from domain D• Delete pointer from dir(D) to R• If location record of E at dir(D) is empty,

delete record• Apply recursively, going up the tree