unit-1. overview peer-to-peer networks server based networks broadcast networks point-to-point...
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UNIT-1
Overview• Peer-to-Peer Networks• Server based Networks• Broadcast Networks• Point-to-Point Networks• Circuit Switching.• Packet Switching.• Message Switching.• Datagram & Virtual Circuit Network• 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!
Freenet• Peer-to-peer indexing and searching service.• Peer-to-peer file downloading.• uses a decentralized distributed data store
to keep and deliver information, and has a suite of free software for publishing and communicating on the Web without fear of censorship
• Files served use the same route as searches (not point-to-point)– Provides for anonymity.
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 Networks• 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
MemoryProcessorI/O
DeviceI/O
DeviceI/O
Device
Broadcast Networks Details
Header(Dest:2)
Body(Data)Message
ID:1(ignore)
ID:2(receive)
ID:4(ignore)
ID:3(sender)
Broadcast Network Arbitration• 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
• 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!
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
A switched network consists of a series of interlinked nodes, called switches. Switches are devices capable of creating temporary connectionsbetween two or more devices linked to the switch.For example: computers or telephones
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
Efficiency
• Less Because resources are allocated during the entire duration of the connection.
• In a telephone network, people normally terminate the communication when they have finished their conversation.
• However, in computer networks, a computer can be connected to another computer even if there is no activity for a long time
Delay: Transmission and Propagation Delay
Propagation delay between Host 1 and Switch1
Transmission delay
Host 1 Switch 1
1. Size of transfer2. Bandwidth of link
1. Speed of light2. Physical distance
• Delay Minimal• Because resources are allocated for the duration of the connection• But delay due to transmission and propagations
Circuit-Switched Technology in Telephone Networks
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.
Datagram switching is normally done at the network layer.
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.
Efficiency
• The efficiency of a datagram network is better than that of a circuit-switched network;
• resources are allocated only when there are packets to be transferred..
Delay• There may be greater delay in a datagram network than in a virtual-circuit
network.• The packet travels through two switches. There are three transmission
times (3T),three propagation delays (slopes 3't of the lines), and two waiting times (WI + w2)' Weignore the processing time in each switch.
• Total delay =3T + 3t + WI + W2
Datagram Networks in the Internet
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.
A source or destination can be a computer,packet switch, bridge, or any other device that connects other networks
Virtual-circuit identifierThe identifier that is actually used for data transfer is called the virtual-circuit identifier(VCI)
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.
Efficiency
• 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.
• Ex: Consider a family that wants to dine at a restaurant. Although the restaurant may not accept reservations (allocation of the tables is on demand), the family can call and find out the waiting time. This can save the family time and effort.
Delay• a one-time delay for setup and a one-time delay for teardown.• The packet is traveling through two switches (routers). There are three
transmission times (3T), three propagation times (3't), data transfer depicted by the sloping lines, a setup delay (which includes transmission and propagation in two directions), and a teardown delay (which includes transmission and propagation in one direction).
• Total delay = 3T+ 3't + setup delay + teardown delay
Circuit-Switched Technology in WANs
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
Network Devices
– Repeater– Hub– Bridge– Router– Gateway
Connecting Devices
Networking Devices
Repeaters Bridges
InternetworkingDevices
Routers Gateways
Connecting Devices
Hub
Repeater
• Extend the physical length• No network function has been changed• Location is matter
Function of repeater
Repeater is not same as Amplifier
Hub
• Actually is a multiport repeater• Star / Tree Topology
Bridge
Bridge
• Divide a large network into smaller segment• Isolating and controlling the link problems
(e.g. congestion)• Regenerate signal + Checking Physical Address
and forward only to the specified segment
Function of a bridge
Multiport bridge
74/ 25
Routers
Routers in an internet
76/ 25
Routers
• Act like stations on a network• Multi-home• Definition (Goal)
– “Learning how to get from here to there." – “Process of discovering, selecting, and employing
paths from one place to another (or to many others) in a network”
Routing Principle
• Goal: Arriving at the destination• Considerations:
– Direct route (shortest)– Reliable route– Cheap route– Safe route– Scenic route
Gateways (protocol converter)
A gateway
SNA network (IBM)Netware network (Novell)
Other devices
• Multiprotocol routers• Brouters• Switches• Routing switches
Single VS. Multiprotocol router
Brouter
Backbone Network
84/ 25
Bus Backbone
Star Backbone
Connecting Remote LAN
A point-to-point link acts as a LAN in a remote backbone connected by remote bridges
Virtual LAN (VLAN)
VLANs create broadcast domains
VLAN with backbone switch
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