Description
CS6676: Lecture 2
Peer-to-Peer(P2P)
Peer 2
Peer 1
App
App
Sharable
objects
Peer 3
Peer 4
App
App
P2P: Napster
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Centralized communication to decentralized communication
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3
1, 2
4
Problems?
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1) Peers register content directory to the central server
? IP address, content
2) Peer 1 send query an object
3) Central server let Peer 1 know that Peer 2 has the object
? Let Peer 1 know about Peer 2
4) Peer 1 establish connection to Peer 2 and request
the object
5) Peer 2 replies Peer 1 with the object
Single point of failure
Performance bottleneck
Mixed system
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1, 2, 3 (centralized)
4, 5 (decentralized)
Peer 1
5
1
Peer 2
P2P: Gnutella
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Fully distributed
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No Central server
Query message sent over existing TCP connection
Peers forward Query message
QueryHit sent over reverse path
Flooding cause excessive traffic
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Limits Scope of TimeToLive (TTL)
Forward!
Peer 2
Peer 1
Query
QueryHit
Query
QueryHit
Found!
Peer 3
Multiple Servers
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Increase performance and resilience
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Data replicated in multiple servers distribute the load
Ex. Sun NIS(Network Information System)
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Each NIS server has own replica of password file containing list of users? login
name and encrypted password
Web Proxy Server
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Proxy caching, store, recently used data objects
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Increase availability and performance of the service by reducing the load on
the wide-area network and Web servers
Web Applets
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Client requests and downloads applet code
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Client interact with the applet, locally
Thin clients and computer servers
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Thin client
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A software layer that supports a window-based user interface on a computer
(local), while executing application program on a remote computer
Drawback?
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In case of high interactivities, (CAD, image processing), delay will be increased
Compute server
Network computer or PC
Thin
Client
network
Application
Process
Networking and Internetworking:
Introduction
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Networks used in distributed systems,
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Networking issues for distributed systems,
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Hardware devices (e.g., router, switch, bridges, hub, repeaters, etc)
Software components (e.g., protocol stacks, communication handlers and
devices, etc)
Performance (e.g., latency, data transfer rate, etc.)
Scalability/reliability
Security
Mobility
QoS
Multicasting ? one to many communication
Users? insatiable access requirements,
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Wireless networks, high-performance networks, etc
Types of Network
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Personal Area Networks (PANs)
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Digital devices connected by low-cost, low energy network
Wireless Personal Network is increasingly popular
? Bluetooth
Types of Network
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Local Area Networks
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Computers connected by a single communication medium
(twisted copper wire, coaxial cable, optical fiber)
Segment: section of cable that have many computer connection
Bandwidth shared among computers connected
Larger local networks (such as campus, office building) have multiple segments
interconnected by switches and hubs
Total bandwidth is high, latency is low (May change depending on message traffic)
Wireless Local Area Networks
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Provide connectivity for mobile devices
Types of Network
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Wide Area Networks
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Carry messages at lower speed
Nodes communicating separated by large distances (different cities, countries ?)
Wireless Wide Area Networks
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Most mobile phones connected through wireless network
Types of Network
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Metropolitan Area Networks
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Provide connectivity for cities blocks
Wireless Metropolitan Area Networks
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Provides wireless connectivity in homes, office buildings
Types of Network
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Internetworks
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LAN
Communication subsystem which several networks
linked together
Open system, extensible
WAN
LAN
LAN
Transporting messages through network
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Protocol Layers: Network software is arranged in hierarchy of layers
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Each layer presents an interface to the layers above, extends underlying
communication system
Message received
Message sent
Layer n
Layer 2
Layer 1
Sender
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Protocol
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Communication
medium
Recipient
Set of rules and formats to be used for communications between processes
Transporting messages through network
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Protocol Stack: Complete set of Protocol layers
Layer
Description
Examples
Application
Protocols that are designed to meet the communication requirements of
specific applications, often defining the interface to a service.
HTTP, FTP , SMTP,
CORBA IIOP
Presentation
Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which may
differ. Encryption is also performed in this layer, if required.
Secure Sockets
( SSL),CORBA Data
Rep.
Session
At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
Transport
This is the lowest level at which messages (rather than packets) are handled.
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
TCP,
Network
Transfers data packets between computers in a specific network. In a WAN
or an internetwork this involves the generation of a route passing through
routers. In a single LAN no routing is required.
IP, ATM virtual
circuits
Data link
Responsible for transmission of packets between nodes that are directly
connected by a physical link. In a WAN transmission is between pairs of
routers or between routers and hosts. In a LAN it is between any pair of hosts.
Ethernet MAC,
ATM cell transfer,
PPP
Physical
The circuits and hardware that drive the network. It transmits sequences of
binary data by analogue signalling, using amplitude or frequency modulation
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).
Ethernet base- band
signalling,
ISDN
UDP
Source
Encapsulation
Application
Transport
Network
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Maximum transfer unit
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Destination
Application
Transport
Network
Data Link
Physical
If message exceeds MTU, must be
segmented
Data Link
M message
Segment
?? M
Diagram
?? ?? M
?? ?? ?? M
Frame
Physical
Packet switching vs circuit switching
Data Link
M message
Segment
?? M
Diagram
?? ?? M
Data Link
?? ?? ?? M
Physical
Frame
Network
Physical
Transport Protocol
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Provide logical communication between app processes running on
different hosts
Transport protocols run in end systems
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Sender
? Breaks app messages into segments, passes to network layer
Receiver
`Reassembles segments into messages, passes to app layer
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Reliable, in-order delivery: Transmission Control Protocol(TCP)
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Congestion control
Flow control
Connection setup
Transport Layer vs Network Layer
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Transport layer:
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Network layer:
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logical communication between processes
relies on, enhances, network layer services
logical communication between hosts
For example, household analogy: 12 kids (Ann) sending letters to 12
kids (Bill)
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processes = kids
app messages = letters in envelopes
hosts = houses
transport protocol = Ann and Bill
network-layer protocol = postal service
Network Layer
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Transport segment from sending to receiving host
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on sending side
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encapsulates segments into datagrams
on receiving side,
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delivers segments to transport layer
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network layer protocols in every host, router
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router examines header fields in all IP datagrams passing through it
Network Layer:Two key functions
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Forwarding:
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Routing:
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Move packets from router?s input to appropriate router output
Determine route taken by packets from source to destination
Routing algorithms
Analogy:
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Routing: process of planning trip from source to destination
Forwarding: process of getting through single interchange
Link Layer
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Layer-2 packet is a frame, encapsulates datagram
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datagram transferred by different link protocols over different links:
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Link layer implemented in ?adaptor? (a.k.a., network interface card
NIC)
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e.g. Ethernet on first link, frame relay on intermediate links, 802.11 on last link
Ethernet card, PCMCI card, 802.11 card
Implements link, physical layer
Attaches into host?s system buses
Has responsibility of transferring datagram from one node to adjacent
node over a link
Adaptors Communication
CPU
Memory
physical
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physical
Sending side:
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CPU
Encapsulates datagram in frame
Adds error checking bits, flow control, etc
Receiving side
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Looks for errors, flow control, etc
Extracts datagram, passes to upper layer at receiving side
Memory
TCP Flow Control
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sender won?t overflow receiver?s buffer by transmitting too much, too
fast
Receiver side of the TCP connection has a receive buffer
app process may be slow at reading from buffer
speed-matching service: matching the send rate to the receiving app?s
drain rate
TCP Flow Control
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spare room in buffer
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LastByteRcvd ? LastByteRead
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