CPS 356: Introduction to Computer Networks

Lecture 2: Network Architectures

Xiaowei Yang

xwy@cs.duke.edu

Reference: Chapter 1 of [PD]

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

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%

Office hours

• Instructor– Fridays: 3-5pm

• TA– Tuesdays 7-9pm

Discussion Forum

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

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

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

Features of computer networks

• Generality

• Carry many different types of data

• Support an unlimited range of applications

What’s 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.

Design requirements and techniques to meet them

1. Scalable connectivity

2. Cost-effective resource sharing

3. Support for common services

4. Manageability

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?

Connectivity recursively occurs at different levels

• 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

Point-to-Point

Multiple-Access

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?

• 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

• 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)

Internetworking:Another way to achieveconnectivity

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

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

Multiplex Demultiplex

TDM and FDM

TDM

frequency

time

4 users

Example:

FDM

frequency

time

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

Statistical Multiplexing

• 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

A

B

C10 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

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

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

Packet scheduling

• Scheduling: which packet to send

• First come first serve (FIFQ)

• Weighted fair queuing

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

Congestion

• Aggregate incoming rate > outgoing rate

• An open question

• A large buffer can help temporary congestion

Packet switching versus circuit switching

• 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 > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

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

• 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

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

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

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

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

Central concepts

• Layering

• Protocols

Layering

• 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

Not so strict

Advantages of layering

• Simplify the design tasks– Each layer implements simpler functions

• Modular design– Can provide new services by modifying one layer

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

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

A sample protocol graph

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

We reject kings, presidents, and voting. We believe in rough consensus and running code

- David Clark

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

Multiplexing & Demultiplexing

• Same ideas apply up and down the protocol graph

Examples of Network Architectures

The protocol graph of Internet

• 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

Link layer

Network layer

Transport layer

Applicatoin layer

Functions of the Layers• Link Layer:

– Service: Reliable transfer of frames over a linkMedia 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

The Open Systems Interconnection (OSI) architecture

Seven-layer

• 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

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

An 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?

A simple TCP/IP Example

argon.tcpip-lab.edu("Argon")

neon.tcpip-lab.edu("Neon")

Web request

Web page

Web client Web server

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

HTTP client

Argon

HTTP server

Neon

HTTP request

HTTP response

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

HTTP Response

HTTP/1.1 200 OK

Date: Sat, 25 May 2002 21:10:32 GMT

Server: Apache/1.3.19 (Unix)

Last-Modified: Sat, 25 May 2002 20:51:33 GMT

ETag: "56497-51-3ceff955"

Accept-Ranges: bytes

Content-Length: 81

Keep-Alive: timeout=15, max=100

Connection: Keep-Alive

Content-Type: text/html

<HTML>

<BODY>

<H1>Internet Lab</H1>

Click <a href="http://www.tcpip-lab.net/index.html">here</a> for the Internet Lab webpage.

</BODY>

</HTML>

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

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

HTTP client

TCP client

Argon

HTTP server

TCP server

Neon

HTTP request / HTTP response

TCP connection

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.

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 128.143.137.144neon.tcpip-lab.edu 128.143.71.21

HTTP client DNS Server

argon.tcpip-lab.edu 128.143.136.15

neon.tcpip-lab.edu

128.143.71.21

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 79telnet 23 http 80smtp 25 nntp 119

Requesting a TCP Connection

• 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

HTTP client

TCP client

argon.tcpip-lab.edu

Establish a TCP connectionto port 80 of 128.143.71.21

connect(s, (struct sockaddr*)&sin, sizeof(sin))

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)

TCP client

argon.tcpip-lab.edu

IP

Send an IP datagram to128.143.71.21

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).

Invoking the device driver

• The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20

• Ethernet address of the default router is found out via ARP

argon.tcpip-lab.edu

IP module

Ethernet

Send an Ethernet frameto 00:e0:f9:23:a8:20

The route from Argon to Neon

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

neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

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

argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44

IP Datagram for Neon

router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20

Forwarding the IP datagram

• The IP router receives the Ethernet frame at interface 128.143.137.11. recovers the IP datagram2. 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

neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

• The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28

router71.tcpip-lab.edu

IP module

Ethernet

Send a frame to00:20:af:03:98:28

Invoking the Device Driver at the Router

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.

IP Datagram for Neon

neon.tcpip-lab.edu128.143.71.21

00:20:af:03:98:28

router71.tcpip-lab.edu128.143.71.1

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

HTTP server

neon.tcpip-lab.edu

TCP server

IP module

Ethernet

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