Transcript Chapter 1: Foundation

Computer Networks: A Systems Approach, 5e
Larry L. Peterson and Bruce S. Davie
Chapter 1
Foundation
Copyright © 2010, Elsevier Inc. All rights Reserved
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Chapter 1
Problems
What is a computer network?
How to build a scalable network that will support
different applications?
How is a computer network different from other
types of networks?
What is a computer network architecture?
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Chapter 1
Chapter Outline
Applications
Requirements
Network Architecture
Implementing Network Software
Performance
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Chapter 1
Chapter Goal
Exploring the requirements that different
applications and different communities place on
the computer network
Introducing the idea of network architecture
Introducing some key elements in implementing
Network Software
Define key metrics that will be used to evaluate
the performance of computer network
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Chapter 1
Applications
Most people know about the Internet (a
computer network) through applications
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World Wide Web
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Just a single application of the Internet
Email
Online Social Network
Streaming Audio Video
File Sharing
Instant Messaging
…
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URL
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Uniform resource locater
http://www.just.edu.jo/~misaleh/Teaching/cs342spring2011/main
page.html
www.just.edu.jo is the name of the machine that serves the page
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Chapter 1
Application Protocol (WWW)
IP: 87.236.232.169
nslookup
whois
/~misaleh/Teaching/cs342/mainpage.html uniquely identifies the
required page
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HTTP
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Hyper Text Transfer Protocol
TCP
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Chapter 1
Application Protocol (Cont.)
Transmission Control Protocol
17 messages for one URL request
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6 to find the IP (Internet Protocol) address
3 for connection establishment of TCP
4 for HTTP (GET) request and acknowledgement (Response)
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Request: I got your request and I will send the data
Reply: Here is the data you requested; I got the data
4 messages for tearing down TCP connection
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Chapter 1
Example of an application
A multimedia application including video-conferencing
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HTML requests:
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A couple of seconds delay is acceptable
Pages are complete
Images can be delivered out of order
Etc…
Emails:
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Chapter 1
Expectations!!
No missing parts (i.e., the email is identical to what you sent)
A minute or two delay is acceptable
Etc…
Video conferencing:
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Video and sound MUST be continuous and on a timely matter
Some missing parts are acceptable
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Application Programmer
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List the services that his application needs: delay
bounded delivery of data
Network Designer
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Chapter 1
Requirements
Design a cost-effective network with sharable
resources
Network Provider (usually called Service
Provider, for the Inernet, it ISP)
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List the characteristics of a system that is easy to
administer and manage.
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Add new clients and remove/isolate faulty nodes
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Chapter 1
Requirements
Connectivity
Cost-Effective Resource Sharing
Support for Common Services
Reliability
Manageability
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Chapter 1
Connectivity
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Need to understand the following terminologies
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Scale: A system that is designed to support growth to an
arbitrarily large size
Link: Physical medium that connects nodes
Node: a device (could be a computer, switch, etc) on the network
Point-to-point: direct link between two nodes
Multiple access: multiple nodes share the same link
(a)
(b)
Point-to-point
Multiple access
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Switched Network: a network that uses switches for
forwarding
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Circuit Switched
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Telephone companies
Dedicated/reserved when connection is established
Less utilization of the resources
Packet Switched
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Chapter 1
Connectivity
Overwhelming majority of computer networks
Messages are divided into pieces (discrete blocks) called packets
No dedication is required
More sharing, thus more utilization
Store-and-forward: a switch stores the incoming traffic
(packets) in its own buffers then forwards them.
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Chapter 1
Connectivity
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Cloud: used to represent any kind of network technology
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Point-to-point, multiple-access, or switched network
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Hosts: nodes outside the cloud (usually computer or end
users devices and use the network)
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Switches: nodes inside the cloud and implement the
network (store and forward packets)
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Internetwork (or internet with small “i”): a set of
independent interconnected networks (clouds)
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Internet (with big “I”): is the globally known network
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Router/gateway: connects two or more networks (plays
much the same role as a switch—stores and forwards)
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Host-to-host connectivity: hosts can talk to hosts
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Address: the way to find nodes. It is a byte string that
identifies a node.
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Routing: the process of determining systematically how
to forward messages toward the destination node based
on its address
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Unicast: send to single destination
Broadcast: send to all nodes on the network
Multicast: send to subset of nodes
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Chapter 1
Connectivity
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Chapter 1
Connectivity
(a)
(a)
(b)
(b)
A switched network
Interconnection of networks
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Resource: links and
nodes
How to share a link?
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Multiplexing
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Multiplexing multiple logical flows
over a single physical link
Chapter 1
Cost-Effective Resource Sharing
Analogy to a timesharing
computer system
De-multiplexing
Synchronous Time-division
Multiplexing (STDM)
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Equal-sized quanta
Round-robin fashion
Time slots/data
transmitted in
predetermined slots
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FDM: Frequency Division Multiplexing
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Diff. TV stations with diff. frequencies.
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Both STDM and FDM waste resources and hard to
accommodate changes (fixed time slots and frequencies)
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Statistical Multiplexing
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Chapter 1
Cost-Effective Resource Sharing
Like STDM: sharing over time but data is
transmitted based on demand rather than
during a predetermined time slot
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Chapter 1
Statistical Multiplexing
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Packets vs. Messages
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Fair decision: FIFO,
Round-Robin, Priorities
(Quality-of-Service (QoS)
allocate bandwidth for a
particular flows)
A switch might run out of
buffers because it receives
data much faster than it can
send on the shared link
 Congestion
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A switch multiplexing packets
from multiple sources onto
one shared link
Start to drop packets
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Chapter 1
LAN, MAN, WAN, and SAN
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Characterize networks according to their
size (they usually use diff. technologies):
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LAN: Local Area Network, typically less than
1km
WAN: Wide Area Network, worldwide
MAN: Metropolitan Area Network, tens of
kilometers
SAN: Systems/Storage Area Network, single
room that has high-performance components
(like leading-edge storage devices) connected
together
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Chapter 1
Support for Common Services
Overly simplistic to think about network as
simple delivering packets among a collection of
computers
But rather a means for application processes to
communicate
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Chapter 1
Support for Common Services
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One option would be for the app designers
to build all that complicated functionality
into each application.
However, since many applications need
common services, it is much more logical
to build those common services once and
let the app designers to use the services.
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Chapter 1
Support for Common Services
Process communicating over an
abstract/logical channel
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The challenge then is recognize what
functionality the channel should provide to
application programs:
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Chapter 1
Support for Common Services
Reliable delivery?!
Same order?!
Eavesdropping parties?!
In general, a network provides a variety of
different types of channels
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An applications can choose the best that
meets its needs
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Chapter 1
Common Communication Patterns
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Designing abstract channels involve
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Understanding representative collection of apps
Extract the common needs
Client/Server
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FTP (file transfer protocol), NFS (network file system),
digital libraries like ACM
Video conferencing
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Two-way traffic (or one-way with video on demand)
No need to guarantee the delivery of ALL messages.
Same order is required
Might need multicast
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Two types of common communication channels
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Request/Reply Channels
Message Stream Channels
It is dangerous to have too much abstractions
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Chapter 1
Common Communication Patterns
Every application has to choose between the provided channels
If you have a hammer then you might see everything looks like a
nail, 
Independent of exactly what functionality a given
channel should provide is where this functionality should
be implemented.
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On switches => dump hosts, like the telephone handsets
On hosts => keep switches as simple as possible
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Chapter 1
Reliability
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Network should hide the errors
Bits are lost
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Bit errors (1 to a 0, and vice versa)
Burst errors – several consecutive errors
Outside forces like: lightning strikes, microwave
ovens, etc.
Packets are lost (Congestion)
Links and Node failures (crashes and misconfig.)
Messages are delayed
Messages are delivered out-of-order
Third parties eavesdrop
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Chapter 1
Manageability
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Last requirement that is usually neglected
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Making changes as the network grows to
carry more traffic or reach more users
Troubleshooting when something goes wrong
or performance is not as desired
Related to scalability
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Configuring new devices/routers is always
problematic
Needs more automation
Great if it becomes plug-and-play (like home
routers)
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Chapter 1
Requirements Summary
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A computer network must provide general,
cost-effective, fair and robust connectivity
among large number of computers.
The network must be manageable by
number of varying levels of skills
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Chapter 1
Network Architecture
Example of a layered network system
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Chapter 1
Network Architecture
Layered system with alternative abstractions available at a given layer
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Abstraction
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Capture important aspects of the system
Abstractions naturally leads to layering
The general idea
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The hiding of details behind a well-defined interface
Define a model
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Chapter 1
Abstraction and Layering
Start from services offered by the underlying
hardware
Add a sequence of layers, each providing a
higher (i.e., more abstract) level of service.
Manageability and Mudularity
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Chapter 1
Protocoals
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The abstract objects the make up the layers of a
network system are called protocols
Building blocks of a network architecture
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Each protocol object has two different interfaces
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service interface: operations on this protocol
peer-to-peer interface: messages exchanged with
peer (indirect communication, except for the
hardware)
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Chapter 1
Interfaces
Service and Peer Interfaces
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Protocol Specification:
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Chapter 1
Protocols
Written description (prose)
pseudo-code
state transition diagram
Packet format
RFCs: Request For Comments
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IETF: Internet Engineering Task Force
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Standardization body
Ex. RFC 2616 for HTTP protocol
Interoperable: when two or more protocols that
implement the specification accurately
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Chapter 1
Protocol Graph
Example of a protocol graph
nodes are the protocols and links the “depends-on” relation
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Chapter 1
Encapsulation
High-level messages are encapsulated inside of low-level messages
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Header
Payload
Chapter 1
Packets General Format
Trailer
Peers talk to each other
through Headers and
Trailers
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Chapter 1
OSI Architecture
The OSI 7-layer Model
OSI – Open Systems Interconnection
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Physical Layer
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Handles the transmission of raw bits over a communication link
Data Link Layer
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Chapter 1
Description of Layers
Collects a stream of bits into a larger aggregate called a frame
Network adaptor along with device driver in OS implement the
protocol in this layer
Frames are actually delivered to hosts
Network Layer
Handles routing among nodes within a packet-switched network
 Provides Host-to-Host connectivity
 Unit of data exchanged between nodes in this layer is called a
packet
The lower three layers are implemented on all network nodes
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Chapter 1
Description of Layers
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Transport Layer
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Implements a process-to-process channel
Unit of data exchanges in this layer is called a
message
There is a disagreement on the top three
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Session, Presentation, and Application
Mainly because the are not always present
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Chapter 1
Description of Layers
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Session Layer
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Presentation Layer
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Provides a name space that is used to tie together the potentially
different transport streams that are part of a single application
Ex., tie the audio and video together in videoconference
Concerned about the format of data exchanged between peers
Ex., integer formats, audio/video format, most/least significant
Application Layer
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Include things like the Hypertext Transfer Protocol
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Basis the world wide web
Used by web browsers
The transport layer and the higher layers typically run only on endhosts and not on the intermediate switches and routers
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Chapter 1
Internet Architecture
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Sometimes called TCP/IP
Evolved from an earlier packet-switched
network called ARPANET
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Internet and ARPANET were funded by ARPA
(Advanced Research Projects Agency)
Both existed before the OSI architecture
Both affected the OSI model
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Internet Protocol Graph
Chapter 1
Internet Architecture
Alternative view of the
Internet architecture. The
“Network” layer shown here
is sometimes referred to as
the “sub-network” or “link”
layer.
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Chapter 1
Internet Architecture
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Defined by IETF
Three main features
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Does not imply strict layering. The application is free to bypass
the defined transport layers and to directly use IP or other
underlying networks
An hour-glass shape – wide at the top, narrow in the middle and
wide at the bottom. IP serves as the focal point for the
architecture (host-to-host connectivity is separate from all
channel types)
In order for a new protocol to be officially included in the
architecture, there needs to be both a protocol specification and
at least one (and preferably two) representative implementations
of the specification
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Chapter 1
Internet Architecture
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NET1, NET2, …
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Could be Ethernet, Wireless, etc.
Encapsulate both hardware and data link layers from OSI model
IP (Internet Protocol)
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Supports the interconnection of multiple networking technologies
into a single logical internetwork
Analogy to the network layer in the OSI
The routing protocol
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Chapter 1
Internet Architecture
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TCP provides reliable, byte-stream channel (connection
oriented protocol)
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UDP provides unreliable, datagram (message) delivery
channel.
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TCP and UDP are called (besides Transport) end-to-end
protocols
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Chapter 1
Internet Architecture
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Application Protocols
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HTTP, FTP, Telnet (remote login), Simple Mail Transfer Protocol
(SMTP), and much more
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Enables the interoperation of popular applications
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Many different web browsers interoperate with web servers because they all
conform/use the HTTP protocol
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1.8 billion Internet users
Much of its functionality is provided by
software running in general-purpose
computers
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Chapter 1
The Success of the Internet
Small matter of programming
Massive increase in computing power
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Chapter 1
Application Programming Interface
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Interface exported by the network
Since most network protocols are implemented (those in
the high protocol stack) in software and nearly all
computer systems implement their network protocols as
part of the operating system, when we refer to the
interface “exported by the network”, we are generally
referring to the interface that the OS provides to its
networking subsystem
The interface is called the network Application
Programming Interface (API)
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Socket Interface was originally provided by the
Berkeley distribution of Unix
- Now supported in virtually all operating systems
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Each protocol provides a certain set of services,
and the API provides a syntax by which those
services can be invoked in this particular OS
Chapter 1
Application Programming Interface (Sockets)
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What is a socket?
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Chapter 1
Socket
The point where a local application process attaches
to the network
An interface between an application and the network
An application creates the socket
The interface defines operations for
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Creating a socket
Attaching a socket to the network
Sending and receiving messages through the socket
Closing the socket
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Chapter 1
Socket
int sockfd = socket(protocol_family, type, protocol);
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Protocol Family
 PF_INET denotes the Internet family
 PF_PACKET denotes direct access to the network
interface (i.e., it bypasses the TCP/IP protocol stack)
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The socket number returned is the socket descriptor for
the newly created socket
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Chapter 1
Creating a Socket
int sockfd = socket(protocol_family, type, protocol);
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Socket Type
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SOCK_STREAM is used to denote a byte stream (TCP)
SOCK_DGRAM is an alternative that denotes a message
oriented service, such as that provided by UDP
int sockfd = socket (PF_INET, SOCK_STREAM, 0);
int sockfd = socket (PF_INET, SOCK_DGRAM, 0);
The combination of PF_INET and SOCK_STREAM implies TCP
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Chapter 1
Client-Serve Model with TCP
Server
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Passive open
Prepares to accept connection, does not actually establish a
connection
Server invokes
int bind (int socket, struct sockaddr *address,
int addr_len)
int listen (int socket, int backlog)
int accept (int socket, struct sockaddr *address,
int *addr_len)
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Chapter 1
Client-Serve Model with TCP
Bind
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Binds the newly created socket to the specified address i.e. the
network address of the local participant (the server)
Address is a data structure which combines IP and port
Listen
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Defines how many connections can be pending on the specified
socket
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Chapter 1
Client-Serve Model with TCP
Accept
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Carries out the passive open
Blocking operation
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Does not return until a remote participant has established a
connection
When it does, it returns a new socket that corresponds to the
new established connection and the address argument
contains the remote participant’s address
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Chapter 1
Client-Serve Model with TCP
Client
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Application performs active open
It says who it wants to communicate with
Client invokes
int connect (int socket, struct sockaddr *address,
int addr_len)
Connect
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Does not return until TCP has successfully established a
connection at which application is free to begin sending data
Address contains remote machine’s address
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Chapter 1
Client-Serve Model with TCP
In practice
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The client usually specifies only remote participant’s
address and let’s the system fill in the local
information
Whereas a server usually listens for messages on a
well-known port
A client does not care which port it uses for itself, the
OS simply selects an unused one
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Chapter 1
Client-Serve Model with TCP
Once a connection is established, the application
process invokes two operation
int send (int socket, char *msg, int msg_len,
int flags)
int recv (int socket, char *buff, int buff_len,
int flags)
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#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
Chapter 1
Example Application: Client
#define SERVER_PORT 5432
#define MAX_LINE 256
int main(int argc, char * argv[])
{
FILE *fp;
struct hostent *hp;
struct sockaddr_in sin;
char *host;
char buf[MAX_LINE];
int s;
int len;
if (argc==2) {
host = argv[1];
}
else {
fprintf(stderr, "usage: simplex-talk host\n");
exit(1);
}
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/* translate host name into peer’s IP address */
hp = gethostbyname(host);
if (!hp) {
fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
exit(1);
}
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
sin.sin_port = htons(SERVER_PORT);
/* active open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
exit(1);
}
if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
perror("simplex-talk: connect");
close(s);
exit(1);
}
/* main loop: get and send lines of text */
while (fgets(buf, sizeof(buf), stdin)) {
buf[MAX_LINE-1] = ’\0’;
len = strlen(buf) + 1;
send(s, buf, len, 0);
}
Chapter 1
Example Application: Client
}
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#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define SERVER_PORT 5432
#define MAX_PENDING 5
#define MAX_LINE 256
Chapter 1
Example Application: Server
int main()
{
struct sockaddr_in sin;
char buf[MAX_LINE];
int len;
int s, new_s;
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(SERVER_PORT);
/* setup passive open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
exit(1);
}
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if ((bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0) {
perror("simplex-talk: bind");
exit(1);
}
listen(s, MAX_PENDING);
/* wait for connection, then receive and print text */
while(1) {
if ((new_s = accept(s, (struct sockaddr *)&sin, &len)) < 0) {
perror("simplex-talk: accept");
exit(1);
}
while (len = recv(new_s, buf, sizeof(buf), 0))
fputs(buf, stdout);
close(new_s);
}
Chapter 1
Example Application: Server
}
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Old programming adage: “first get it right and then make
it fast”.
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Chapter 1
Performance
In networking: necessary to “design for performance”
Two fundamental ways to measure the performance
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Bandwidth
Latency
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Chapter 1
Bandwidth
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Frequency band (measured in Hertz): we don’t mean that
Number of bits per second that can be transmitted over a
communication link
Throughput vs. bandwidth (from the most confusing
terms in computer networks.
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Bandwidth: the maximum data rate (bits per second)
Throughput: number of bits per second that we actually transmit
over the link in practice
1 Mbps: 1 x 106 bits/second = 1x220 bits/sec
1 x 10-6 seconds to transmit each bit or imagine that a
timeline, now each bit occupies 1 micro second space.
On a 2 Mbps link the width is 0.5 micro second.
Smaller the width more will be transmission per unit time.
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Chapter 1
Bandwidth
Bits transmitted at a particular bandwidth can be regarded as
having some width:
(a) bits transmitted at 1Mbps (each bit 1 μs wide);
(b) bits transmitted at 2Mbps (each bit 0.5 μs wide).
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MB, Mbps, KB, Kbps
Mbps is always 106. Kbps is always 103
MB you can use 220 or 106
KB you can use 210 or 103
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Chapter 1
Units Confusion
5% error
Bits and bytes
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b (small letter) for bits
B (capital letter) for bytes (each byte is 8 bits,
you can use 10 sometimes, but with 20%
errors)
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Chapter 1
Latency
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How long it takes a message to travel from
one end of a network to the other.
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Measured in time
69
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Chapter 1
Latency
Three components
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Speed-of-light propagation delay
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Different media at different speeds
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3.0 × 108 m/s in a vacuum
2.3 × 108 m/s in copper cable
2.0 × 108 m/s in optical fiber
Amount of time to transmit a unit of data
Queuing delays (switches store packets)
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Latency = Propagation + transmit + queue
Propagation = distance/speed of light
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Distance: the length of the wire
Transmit = size/bandwidth
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Chapter 1
Performance
Size: the size of the packet
One bit transmission => propagation is important
Large bytes transmission => bandwidth is important
71
Chapter 1
Performance (ex1)
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A client sends1-byte message to a server
and receives a 1-byte message

Suppose RTT = 100ms


Case 1: bandwidth 1 Mbps


=> Transmit = 8 bits / 1Mbps = 8 micro seconds
Case 2: bandwidth 100 Mbps


RTT = propagation delay X 2
=> Transmit = 8 bits / 100Mbps = 0.08 micro
seconds
Insignificant difference between Case 1 and 2
72

Chapter 1
Performance (ex2)
A client wants to download a 25MB image



Bandwidth 10Mbps
=>Transmit = 25 × 106 × 8 bits / 10 × 106
bits/sec = 20 seconds
Case 1: RTT = 100ms


Case 2: RTT = 1ms


Latency = 20.1 sec
Latency = 20.001 sec
Propagation delay is insignificant in this case
73
Copyright © 2012, Elsevier Inc. All rights Reserved
Chapter 1
FIGURE 1.17 Perceived latency (response time) versus round-trip time for
various object sizes and link speeds.
74





Chapter 1
Delay X Bandwidth
We think the channel between a pair of processes as a
hollow pipe
Latency is the length of the pipe and bandwidth is the
width of the pipe
Delay of 50 ms and bandwidth of 45 Mbps
50 x 10-3 seconds x 45 x 106 bits/second
2.25 x 106 bits ≈ 280 KB data.
Network as a pipe
75

Chapter 1
Delay X Bandwidth
Relative importance of bandwidth and latency
depends on application


For large file transfer, bandwidth is critical
For small messages (HTTP, NFS, etc.), latency is
critical
76




Chapter 1
Delay X Bandwidth
The max number of bits that could be in transit
through the pipe at any given instant
How many bits the sender must transmit
before the first bit arrives at the receiver if the
sender keeps the pipe full
Takes another one-way latency to receive a
response from the receiver
If the sender does not fill the pipe (i.e., send a
whole delay × bandwidth product’s worth of
data before it stops to wait for a signal), then
the sender will not fully utilize the network
77
Chapter 1
Delay X Bandwidth

One-way latency of 50ms and a bandwidth
of 45 Mbps is able to hold


This number is doubled if we consider the
RTT time instead of the one-way


50 × 10-3s × 45 × 106 bits/s = 2.25 × 106 bits
Until the send hears the first bit from the
receiver to stop sending or something
Most of the time, we will be interested in
the RTT × Bandwidth
78


Bandwidths are increasing dramatically
What is the impact network design of having infinite
bandwidth available??!!


Chapter 1
High-Speed Networks
RTT dominates
What does not change as bandwidth increases: the
speed of light

You can not change the laws of physics
79
Chapter 1
Relationship between bandwidth and latency
A 1-MB file would fill the 1-Mbps link 80 times,
but only fill the 1-Gbps link 1/12 of one time
80
Chapter 1
High-Speed Networks





Delay is 100 x 10-3 sec
File is 1MB
Bandwidth1 = 1Mbps
Bandwidth2 = 1Gbps
Delay x Bandwidth1 = 100 x 10-3 sec x 106 bits/sec = 105
bits



1MB / 105bits = 8 x 106 / 105 = 80
=> the file is 80 times larger than the channel
Delay x Bandwidth2 = 100 x 10-3 sec x 109 bits/sec = 108
bits


1MB / 108 bits = 8 x 106 / 108 = 1 / 12.5
=> the channel is 12.5 times larger than the file
81

The effective end-to-end throughput


Throughput = TransferSize / Latency
Latency= RTT + Size / Bandwidth


Chapter 1
Throughput and TransferTime
Insert the two-way, RTT to the formula instead of Prop. Delay
Ex., a user wants to fetch 1MB file across 1Gbps network
with a round-trip time of 100ms.



Transmit time = 1MB / 1Gbps = 8ms
Latency = RTT + transmit = 108ms
Effective throughput = 1MB/108ms = 74.1 Mbps



Much less than the available bandwidth
Transferring a larger amount of data will help improve the effective
throughput
large transfer size => throughput approaches the bandwidth
82




Chapter 1
Summary
We have identified what we expect from a computer
network
We have defined a layered architecture for computer
network that will serve as a blueprint for our design
We have discussed the socket interface which will be
used by applications for invoking the services of the
network subsystem
We have discussed two performance metrics using
which we can analyze the performance of computer
networks
83