CPS 356: Introduction to Computer Networks Lecture 2: Network Architectures

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CPS 356 Introduction to Computer Networks
Lecture 2 Network Architectures
Reference Chapter 1 of PD

• Xiaowei Yang
• xwy_at_cs.duke.edu

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Overview

• Updated course administrative stuff
• Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works

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Updated Grading Policy

• Old
• Class participation and pop quizzes 20
• Assignments 50
• In a group assignment, both students get the same
  grade for the assignment
• Exams 30
• New
• Class participation and pop quizzes 10
• Assignments 60
• In a group assignment, both students get the same
  grade for the assignment
• Exams 30

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Office hours

• Instructor
• Fridays 3-5pm
• TA
• Tuesdays 7-9pm

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Discussion Forum

• Piazza sign up link
• piazza.com/duke/spring2014/compsci356

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Overview

• Updated course administrative stuff
• Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works

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1st Mission of this course

• Understand the concepts and design principles
  that make the Internet work
• Design paradigm
• Identify requirements, brainstorm design
  choices/mechanisms, make design decisions
• What requirements make sense to you?
• Scalable connectivity
• Cost-effective resource sharing
• Support for different types of services
• Manageability
• It remains an open challenge how to incorporate
  other requirements such as security into the
  Internet design

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Features of computer networks

• Generality
• Carry many different types of data
• Support an unlimited range of applications

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Whats the Internet?

• The Internet is a large-scale general-purpose
  computer network.
• Run more than one applications
• The Internet transfers data between computers.
• The Internet is a network of networks.

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Design requirements and techniques to meet them

1. Scalable connectivity
2. Cost-effective resource sharing
3. Support for common services
4. Manageability

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1. Scalable Connectivity

• A network must provide connectivity among a set
  of computers
• Open vs close to connect all computers or a
  subset of them?
• Internet is an open network
• Scalability A system is designed to grow to an
  arbitrary large size is said to scale
• How to connect an arbitrary large number of
  computers on a network?

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Connectivity recursively occurs at different
levels
Point-to-Point
Multiple-Access

• Link-level connect two or more computers via a
  physical medium
• Computers are referred to as nodes
• The physical medium is referred to as a link

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Switching

• Switching is a mechanism to achieve connectivity
• Nodes that are attached to at least two links
  forward data from one link to another link
• They are called switches
• Computers outside the cloud are called hosts
• A question switch vs router, what can become a
  switch?

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• Circuit switching
• Sets up a circuit before nodes can communicate
• Switches connect circuits on different links
• Packet switching
• Data are split into blocks of data called packets
• Store and forward
• Nodes send packets and switches forward them

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Internetworking Another way to
achieve connectivity

• An internetwork of networks
• Each cloud is a network/a multiple-access link
• A node that is connected to two or more networks
  is commonly called a router
• Speaks different protocols than switches
• An internet can be viewed as a cloud. We can
  recursively build larger clouds by connecting
  smaller ones
• Autonomous system (AS)

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Addressing and routing

• Physical connectivity ! connectivity
• Addressing and routing are mechanisms to achieve
  connectivity
• Nodes are assigned addresses
• Routers compute how to reach them by running
  routing protocols
• intra-AS OSPF, RIP, IS-IS
• Inter-AS BGP

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2. Cost-effective resource sharing

• Question how do all the hosts share the network
  when they want to communicate with each other?
• Use at the same time
• Fair
• Multiplexing a system resource is shared among
  multiple users
• Analogy CPU sharing
• Mechanisms to multiplexing
• Time-division multiplexing (TDM)
• Frequency-division multiplexing (FDM)
• Statistical multiplexing

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Multiplex
Demultiplex
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TDM and FDM
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Problems with FDM and TDM

• What if a user does not have data to send all the
  time?
• Consider web browsing
• ? Inefficient use of resources
• Max of flows is fixed and known ahead of time
• Not practical to change the size of quantum or
  add additional quanta for TDM
• Nor add more frequencies in FDM

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Statistical Multiplexing
10 Mb/s Ethernet
C
A
statistical multiplexing
1.5 Mb/s
B
queue of packets waiting for output link
D
E

• The physical link is shared over time (like TDM)
• But does not have fixed pattern ? statistical
  multiplexing
• Sequence of A B packets are sent on demand, not
  predetermined slots

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Pros and Cons

• Assumption traffic is largely bursty
• Pros Resources are not wasted when hosts are
  idle
• Cons No guarantee flows would have their turns
  to transmit
• Some possible fixes
• Limit maximum packet size
• Scheduling which packets got transmitted, e.g.,
  fair queuing

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Maximum Packet Size

• Divide an application message into blocks of data
  ? packets
• Segments, frames
• Maximum packet size limit
• Flows send on demand
• Must give each flow its turn to send
• Solution defines an upper bound on the size of
  the block of data

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Packet scheduling

• Scheduling which packet to send
• First come first serve (FIFQ)
• Weighted fair queuing

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Switching vs multiplexing

• TDM and FDM are used in circuit switching
• Require a setup as max of flows is fixed
• SM is used in packet switching

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Congestion

• Aggregate incoming rate gt outgoing rate
• An open question
• A large buffer can help temporary congestion

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Packet switching versus circuit switching

• Packet switching allows more users to use network!

• 1 Mb/s link
• each user
• 100 kb/s when active
• active 10 of time
• circuit-switching fixed capacity
• 10 users
• packet switching
• with 35 users, probability gt 10 active less than
  .0004

N users
1 Mbps link
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3. Support for common services

• Application developers want a network to provide
  services that make application programs
  communicate with each other, not just sending
  packets
• E.g. reliably delivering an email message from a
  sender to a receiver
• Many complicated things need to happen
• Can you name a few?
• Design choices
• Application developers build all functions they
  need
• Network provides common services ? a layered
  network architecture
• Build it once, and shared many times

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• Interactive request/reply
• Streaming of data
• Bulk data transfer

• Key challenges what services/channels to provide
  that can satisfy most applications at lowest
  costs?
• Approach identify common patterns, then decide
• What functions to implement
• Where to implement those functions
• We will discuss end-to-end arguments in future
  class

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Ex how to provide reliability as a common service

• Failures may occur at different scopes
• Bit transmission errors
• Packet loss
• Component failures link, node
• Design choices
• Link layer
• Every hop in the router
• End systems
• In future classes, we will discuss how to cope
  with these failures

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4. Manageability

• Manage the network as it grows and when things go
  wrong
• An open research challenge
• Datacenter networks
• Backbones
• Home networks
• IP cameras, printers, network attached storage

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Overview

• Updated course administrative stuff
• Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of how the Internet works

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Network Architectures

• Many ways to build a network
• Use network architectures to characterize
  different ways of building a network
• The general blueprints that guide the design and
  implementation of networks are referred to as
  network architectures

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Central concepts

• Layering
• Protocols

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Layering
Not so strict

• An abstraction to handle complexity
• A unifying model that capture important aspect of
  a system
• Encapsulate the model in an object that has an
  interface for others to interact with
• Hide the details from the users of the object

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Advantages of layering

• Simplify the design tasks
• Each layer implements simpler functions
• Modular design
• Can provide new services by modifying one layer

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Protocols

• The abstract objects that make up the layers of a
  network system are called protocols
• Each protocol defines two different interfaces
• Service interface
• Peer interface

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A protocol graph

• Peer-to-peer communication is indirect
• Except at the hardware level
• Potentially multiple protocols at each level
• Show the suite of protocols that make up a
  network system with a protocol graph

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A sample protocol graph
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Protocol standardization

• Standard bodies such as IETF govern procedures
  for introducing, validating, and approving
  protocols
• The Internet protocol suite uses open standard
• Set of rules governing the form and content of a
  protocol graph are called a network architecture

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We reject kings, presidents, and voting. We
believe in rough consensus and running code-
David Clark
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Encapsulation

• Upper layer sends a message using the service
  interface
• A header, a small data structure, to add
  information for peer-to-peer communication, is
  attached to the front message
• Sometimes a trailer is added to the end
• Message is called payload or data
• This process is called encapsulation

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(No Transcript)
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Multiplexing Demultiplexing

• Same ideas apply up and down the protocol graph

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Examples of Network Architectures
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The protocol graph of Internet
Applicatoin layer
Transport layer
Network layer
Link layer

• No strict layering. One can do cross-layer design
• Hourglass shaped IP defines a common method for
  exchanging packets among different networks
• To propose a new protocol, one must produce both
  a spec and one/two implementations

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Functions of the Layers

• Link Layer
• Service Reliable transfer of frames over a
  link Media Access Control on a LAN
• Functions Framing, media access control, error
  checking
• Network Layer
• Service Move packets from source host to
  destination host
• Functions Routing, addressing
• Transport Layer
• Service Delivery of data between hosts
• Functions Connection establishment/termination,
  error control, flow control
• Application Layer
• Service Application specific (delivery of
  email, retrieval of HTML documents, reliable
  transfer of file)
• Functions Application specific

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The Open Systems Interconnection (OSI)
architecture
Seven-layer
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• International Telecommunications Union (ITU)
  publishes protocol specs based on the OSI
  reference model
• X dot series
• Physical layer handles raw bits
• Data link layer aggregate bits to frames.
  Network adaptors implement it
• Network layer handles host-to-host packet
  delivery. Data units are called packets
• Transport implements process channel. Data units
  are called messages
• Session layer handles multiple transport streams
  belong to the same applications
• Presentation layer data format, e.g., integer
  format, ASCII string or not
• Application layer application specific protocols

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Summary of New Terms

• Layering is an abstraction that captures
  important aspects of the system, provides service
  interfaces, and hides implementation details
• Protocols are abstract objects that make up the
  layers of a network system are
• A protocol graph represents protocols that make
  up a system
• Nodes are protocols
• Links are depend-on relations
• Set of rules governing the form and content of a
  protocol graph are called a network architecture
• Attaching a header/trailer to an upper layer data
  unit is referred to as encapsulation

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An Example
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A simple TCP/IP Example

• A user on host argon.tcpip-lab.edu (Argon)
  makes web access to URL
• http//neon. tcpip-lab.edu/index.html.
  
• What actually happens in the network?

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HTTP Request and HTTP response

• Web server runs an HTTP server program
• HTTP client Web browser runs an HTTP client
  program
• sends an HTTP request to HTTP server
• HTTP server responds with HTTP response

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HTTP Request
GET /example.html HTTP/1.1 Accept image/gif,
/ Accept-Language en-us Accept-Encoding gzip,
deflate User-Agent Mozilla/4.0 Host
192.168.123.144 Connection Keep-Alive
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HTTP Response
HTTP/1.1 200 OK Date Sat, 25 May 2002 211032
GMT Server Apache/1.3.19 (Unix) Last-Modified
Sat, 25 May 2002 205133 GMT ETag
"56497-51-3ceff955" Accept-Ranges
bytes Content-Length 81 Keep-Alive timeout15,
max100 Connection Keep-Alive Content-Type
text/html ltHTMLgt ltBODYgt ltH1gtInternet
Lablt/H1gt Click lta href"http//www.tcpip-lab.net/i
ndex.html"gtherelt/agt for the Internet Lab
webpage. lt/BODYgt lt/HTMLgt

• How does the HTTP request get from Argon to Neon
  ?

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From HTTP to TCP

• To send request, HTTP client program establishes
  an TCP connection to the HTTP server Neon.
• The HTTP server at Neon has a TCP server running

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Resolving hostnames and port numbers

• Since TCP does not work with hostnames and also
  would not know how to find the HTTP server
  program at Neon, two things must happen
• 1. The name neon.tcpip-lab.edu must be
  translated into a 32-bit IP address.
• 2. The HTTP server at Neon must be identified by
  a 16-bit port number.

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Translating a hostname into an IP address

• The translation of the hostname
  neon.tcpip-lab.edu into an IP address is done via
  a database lookup
• gethostbyname(host)
• The distributed database used is called the
  Domain Name System (DNS)
• All machines on the Internet have an IP
  address argon.tcpip-lab.edu 128.143.137.144 ne
  on.tcpip-lab.edu 128.143.71.21

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Finding the port number

• Note Most services on the Internet are reachable
  via well-known ports. E.g. All HTTP servers on
  the Internet can be reached at port number 80.
• So Argon simply knows the port number of the
  HTTP server at a remote machine.
• On most Unix systems, the well-known ports are
  listed in a file with name /etc/services. The
  well-known port numbers of some of the most
  popular services are
• ftp 21 finger 79
• telnet 23 http 80
• smtp 25 nntp 119

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Requesting a TCP Connection
connect(s, (struct sockaddr)sin, sizeof(sin))

• The HTTP client at argon.tcpip-lab.edu requests
  the TCP client to establish a connection to port
  80 of the machine with address 128.141.71.21

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Invoking the IP Protocol

• The TCP client at Argon sends a request to
  establish a connection to port 80 at Neon
• This is done by asking its local IP module to
  send an IP datagram to 128.143.71.21
• (The data portion of the IP datagram contains the
  request to open a connection)

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Sending the IP datagram to the default router

• Argon sends the IP datagram to its default router
• The default gateway is an IP router
• The default gateway for Argon is
  Router137.tcpip-lab.edu (128.143.137.1).

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Invoking the device driver

• The IP module at Argon, tells its Ethernet device
  driver to send an Ethernet frame to address
  00e0f923a820
• Ethernet address of the default router is found
  out via ARP

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The route from Argon to Neon

• Note that the router has a different name for
  each of its interfaces.

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Sending an Ethernet frame

• The Ethernet device driver of Argon sends the
  Ethernet frame to the Ethernet network interface
  card (NIC)
• The NIC sends the frame onto the wire

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Forwarding the IP datagram

• The IP router receives the Ethernet frame at
  interface 128.143.137.1
• recovers the IP datagram
• determines that the IP datagram should be
  forwarded to the interface with name 128.143.71.1
• The IP router determines that it can deliver the
  IP datagram directly

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Invoking the Device Driver at the Router

• The IP protocol at Router71, tells its Ethernet
  device driver to send an Ethernet frame to
  address 0020af039828

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Sending another Ethernet frame

• The Ethernet device driver of Router71 sends the
  Ethernet frame to the Ethernet NIC, which
  transmits the frame onto the wire.

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Data has arrived at Neon

• Neon receives the Ethernet frame
• The payload of the Ethernet frame is an IP
  datagram which is passed to the IP protocol.
• The payload of the IP datagram is a TCP segment,
  which is passed to the TCP server

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Summary

• Updated course administrative stuff
• Grading policy, office hours, piazza
• Design requirements of the original Internet
• Concepts of Network Architectures
• An Example of
• how the Internet works