UNIT-1

Overwiew

• Peer-to-Peer Networks• Server based Networks• Broadcast Networks• Point-to-Point Networks• Circuit Switching.• Packet Switching.• Message Switching.• Networking Devices

What is Peer-to-Peer?

• A model of communication where every node in the network acts alike.

• As opposed to the Client-Server model, where one node provides services and other nodes use the services.

Advantages of P2P Network

• No central point of failure– E.g., the Internet and the Web do not have a central

point of failure.– Most internet and web services use the client-server

model (e.g. HTTP), so a specific service does have a central point of failure.

• Scalability– Since every peer is alike, it is possible to add more

peers to the system and scale to larger networks.

Disadvantages of P2P Computing

• Decentralized coordination– How to keep global state consistent?– Need for distributed coherency protocols.

• All nodes are not created equal.– Computing power, bandwidth have an impact

on overall performance.• Programmability

– As a corollary of decentralized coordination.

P2P Computing Applications

• File sharing

• Process sharing

• Collaborative environments

P2P File Sharing Applications

• Improves data availability• Replication to compensate for failures.• E.g., Napster, Gnutella, Freenet, KaZaA

(FastTrack).

P2P Process Sharing Applications

• For large-scale computations• Data analysis, data mining, scientific

computing• E.g., distributed.net, World-Wide Computer

P2P Collaborative Applications

• For remote real-time human collaboration.• Instant messaging, virtual meetings, shared

whiteboards, teleconferencing, tele-presence.

• E.g., talk, AOL Messenger, Yahoo! Messenger, Jabber, MS Netmeeting, NCSA Habanero, Games

P2P Technical Challenges

• Peer identification • Routing protocols• Network topologies• Peer discovery• Communication/coordination protocols• Quality of service• Security• Fine-grained resource management

P2P Topologies

• Centralized

• Ring

• Hierarchical

• Decentralized• Hybrid

Centralized

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

System is all in one place All information is in one placeX No one can add on to systemX Single point of failure Simply secure one hostX Easy to shut down? One machine. But in practice?

Ring

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

Simple rules for relationships Easy logic for stateX Only ring owner can add Fail-over to next host As long as ring has one ownerX Shut down owner Just add more hosts

Hierarchical

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

½Chain of authority½Cache consistency½Add more leaves, rebalance½Root is vulnerableX Too easy to spoof linksX Just shut down the root Hugely scalable – DNS

Decentralized

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

X Very difficult, many ownersX Difficult, unreliable peers Anyone can join in! RedundancyX Difficult, open research No one to sue? Theory – yes : Practice – no

Centralized + Ring

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

Just manage the ring As coherent as ringX No more than ring Ring is a huge win As secure as ringX Still single place to shut down Ring is a huge win

Common architecture for web applications

Centralized + Decentralized

ManageableCoherent

ExtensibleFault Tolerant

SecureLawsuit-proof

Scalable

X Same as decentralized½Better than decentralized Anyone can still join! Plenty of redundancyX Same as decentralized Still no one to sue? Looking very hopeful

Best architecture for P2P networks?

Napster

• The P2P revolution is started.• Central indexing and searching service• File downloading in a peer-to-peer point-to-

point manner.

Gnutella

• Peer-to-peer indexing and searching service.

• Peer-to-peer point-to-point file downloading using HTTP.

• A gnutella node needs a server (or a set of servers) to “start-up”… gnutellahosts.com provides a service with reliable initial connection points

But introduces a new single point of failure!But introduces a new single point of failure!

The Gnutella protocol (v0.4)

• PING – Notify a peer of your existence• PONG – Reply to a PING request • QUERY – Find a file in the network• RESPONSE – Give the location of a file• PUSHREQUEST – Request a server behind

a firewall to push a file out to a client.

Freenet

• Peer-to-peer indexing and searching service.

• Peer-to-peer file downloading.• Files served use the same route as searches

(not point-to-point)– Provides for anonymity.

KaZaA/Morpheus

• Hybrid indexing/searching model – Not centralized like Napster, not decentralized like

Gnutella.• Peer-to-peer file downloading using HTTP.

– “SmartStream” for incomplete file downloads.– “FastStream” for partial file downloads.

• “SuperNodes” elected dynamically if sufficient bandwidth and processing power – hybrid topology model.

• A central server keeps user registrations, logs usage, and helps bootstrapping peer discovery.

Clienthost

Serverhost

Clienthost

Server Based Networks

Server Based Networks

• Specialization:– Clients specialize in user interface– Servers specialize in managing data and

application logic

• Sharing:– Many clients can be supported by few servers– Often data and logic are shared among

applications and users

Client/server

Peer-to-peerServer

“I want to access some information”

“I want to collaborate

with my colleague”

Client

Distinctions

• Client-server– Asymmetric relationship– Client predominately makes requests, server

makes replies

• Peer-to-peer– Symmetric relationship

ClientServer

Client

Email client sends message to server

Message is stored on POP server

Later, recipient’s email client retrieves message from server

Email application

Client

Server

Client

Chat clients send user’s typing to server

Chat server aggregates typing from all users and sends to all clients

Other user’s clients display aggregated typing from chat server

Chat application

Presentation

Applicationlogic

Shareddata

Local-area network

Note: many clients perapplication server, severalapplication servers per data server

Three-tier client/server

Client

Webbrowser

Webserver

Application logic

Databases and DBMS

Commongatewayinterchange

Host architecture

Application partition

amazon.com

• Broadcast Network: Shared Communication Medium

– Shared Medium can be a set of wires• Inside a computer, this is called a bus• All devices simultaneously connected to devices

– Originally, Ethernet was a broadcast network• All computers on local subnet connected to one another

– More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO

Broadcast Networks

MemoryProcessorI/O

DeviceI/O

DeviceI/O

Device

Broadcast Networks Details

• Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!)– Put header on front of packet: [ Destination | Packet ]– Everyone gets packet, discards if not the target– In Ethernet, this check is done in hardware

• No OS interrupt if not for particular destination– This is layering: we’re going to build complex network protocols by layering

on top of the packet

Header(Dest:2)

Body(Data)Message

ID:1(ignore)

ID:2(receive)

ID:4(ignore)

ID:3(sender)

Broadcast Network Arbitration• Arbitration: Act of negotiating use of shared medium

– What if two senders try to broadcast at same time?– Concurrent activity but can’t use shared memory to coordinate!

• Aloha network (70’s): • Blind broadcast, with checksum at end of packet. If received

correctly (not garbled), send back an acknowledgement. If not received correctly, discard.– Sender waits for a while, and if doesn’t get an acknowledgement,

re-transmits.– If two senders try to send at same time, both get garbled, both

simply re-send later.– Problem: Stability: what if load increases?

• More collisions less gets through more resent more load… More collisions…

• Unfortunately: some sender may have started in clear, get scrambled without finishing

Carrier Sense, Multiple Access/Collision Detection• Ethernet (early 80’s): first practical local area network

– It is the most common LAN for UNIX, PC, and Mac – Use wire instead of radio, but still broadcast medium

• Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection– Carrier Sense: don’t send unless idle

• Don’t mess up communications already in process– Collision Detect: sender checks if packet trampled.

• If so, abort, wait, and retry.– Backoff Scheme: Choose wait time before trying again

• How long to wait after trying to send and failing?– What if everyone waits the same length of time? Then, they all

collide again at some time!– Must find way to break up shared behavior with nothing more

than shared communication channel• Adaptive randomized waiting strategy:

– Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc.

– Randomness is important to decouple colliding senders

Point-to-point networks

• Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches?– Didn’t used to be cost-effective– Now, easy to make high-speed switches and routers that can forward packets

from a sender to a receiver.• Point-to-point network: a network in which every physical wire is connected to

only two computers• Switch: a bridge that transforms a shared-bus (broadcast) configuration into a

point-to-point network.• Router: a device that acts as a junction between two networks to transfer data

packets among them.

Router

Internet

Switch

Point-to-Point Networks• Advantages:

– Higher link performance• Can drive point-to-point link faster than broadcast link since less

capacitance/less echoes (from impedance mismatches)– Greater aggregate bandwidth than broadcast link

• Can have multiple senders at once– Can add capacity incrementally

• Add more links/switches to get more capacity– Better fault tolerance

• Disadvantages:– More expensive than having everyone share broadcast link

• Examples– ATM (asynchronous transfer mode)

• The first commercial point-to-point LAN• Inspiration taken from telephone network

– Switched Ethernet• Same packet format and signaling as broadcast Ethernet, but only two

machines on each ethernet.

Switching

Switched network

Taxonomy of switched networks

CIRCUIT-SWITCHED NETWORKS

A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

A trivial circuit-switched network

In circuit switching, the resources need to be reserved during the setup phase;

the resources remain dedicated for the entire duration of data transfer until the teardown phase.

• Circuit switching takes place in Physical layer.• Data transfer between the two nodes is a continuous flow.• No addressing is involved during data transfer.

As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4-kHz voice channels. We assume that each link uses FDM to connect a maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure 8.4 shows the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course the situation may change when new connections are made. The switch controls the connections.

Example

Circuit-switched network used in Example

As another example, consider a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider. There are two 4 × 8 (4 inputs and 8 outputs) switches in this network. For each switch, four output ports are folded into the input ports to allow communication between computers in the same office. Four other output ports allow communication between the two offices.

Example

Circuit-switched network used in Example 2

DATAGRAM NETWORKS

In data communications, we need to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol.

In a packet-switched network, there is no resource reservation;resources are allocated on demand.

A datagram network with four switches (routers)

Routing table in a datagram network

A switch in a datagram network uses a routing table that is based on the destination address.

The destination address in the header of a packet in a datagram network

remains the same during the entire journey of the packet.

Switching in the Internet is done by using the datagram approach to packet

switching at the network layer.

VIRTUAL-CIRCUIT NETWORKS

A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both.

Virtual-circuit identifier

Switch and tables in a virtual-circuit network

Source-to-destination data transfer in a virtual-circuit network

Setup request in a virtual-circuit network

Setup acknowledgment in a virtual-circuit network

In virtual-circuit switching, all packets belonging to the same source and destination travel the same path;

but the packets may arrive at the destination with different delays if resource allocation is on demand.

Switching at the data link layer in a switched WAN is normally implemented by using virtual-circuit techniques.

Message Switching

• A store-and-forward network where the block of transfer is a complete message.

• Since messages can be quite large, this can cause:– buffering problems– high mean delay times

– Repeater– Hub– Bridge– Router– Gateway

Network Devices

Connecting Devices

Networking Devices

Repeaters Bridges

InternetworkingDevices

Routers Gateways

Five categories of connecting devices

A repeater connecting two segments of a LAN

A repeater connects segments of a LAN.

A repeater forwards every frame; it has no filtering capability.

A repeater is a regenerator, not an amplifier.

Function of a repeater

ACTIVE HUB

A bridge connecting two LANs

A bridge has a table used in filtering decisions.

A bridge does not change the physical (MAC) addresses in a frame.

A learning bridge and the process of learning

Loop problem in a learning bridge

A system of connected LANs and its graph representation

Spanning tree in a system of bridges

Forwarding and blocking ports after using spanning tree algorithm

Routers connecting independent LANs and WANs