EECS 489: Computer Networks

Lecture

•Time: MW 12:00-1:30

•Room:1690 CSE

Discussion session:

•Time: Wed 1:30-2:30 (no discussion today)

•Room: 1690 CSE

Instructor: Sugih Jamin

Office: 4737 CSE

Office hours: Mon 1:30-2:30, Tue 2:30-3:30, and by appt.! (but never right before lecture)

Tel: +1 734 763 1583

GSI: Ajay Gopalakrishnan (ajgopala)

Office: 4817 CSE

Office hours: Thu 4-5, Fri 10-11, and by appt.

Required Textbooks

Web Site and Forum

Course Web site: http://www.eecs.umich.edu/~sugih/courses/eecs489/

• Course grade composition

• Policy on collaboration and cheating

• Regrade and late days

Lecture slides will be posted on web site after lecture

Web site and forum are “required readings”

Prerequisites:

• EECS 482

• C

Grading Policy

• 1 Final Exam: 15%

• 1 Midterm Exam: 15%

• 4 Homeworks: 12%

• 4 Programming Assignments: 56%

• Class Participation: 2%

Collaboration:

• All work must be done individually

• Cheating and plagiarizing are not tolerated

Regrade:

• Within 5 working days (except Final Exam and PA4, same day)

• Written request

• Whole work regraded

Late days:

• 4 free late days for the whole term

• 4% per 24 hours or fraction thereof thereafter

What this Course is NOT about

We do NOT cover:

•Homepage design, CSS, PGP, MySQL

•Photoshop, Flash, Silverlight, whatever

•Web site administration

•Web hosting and data center setup and maintenance

•DSL or cable modem setup

•LAN setup and administration

•How to connect to the Internet

•How to become an ISP

•How to run an ISP

What this Course Covers

Network Protocols and Architectures:

• Internet: a network of networks

• Application layer, naming

• Network layer: addressing and routing

• Link layer and wireless

• Transport layer and queue management

• Multimedia networking

• Security in networks

• Focus on the fundamental concepts, not merely the technology

• There will always be new protocols, but how do people design them?

• What are architectural principles?

• Provide high-level overview of computer networks

• Wired, wireless, cellular

• How they are interconnected

What is a Communication Networks

Communication network offers one basic service: move information• bird, fire, messenger, truck, telegraph, telephone, Internet …

Another example, transportation network: move objects• horse, train, truck, airplane ...

• a car carrying a trunk load of tapes can also move information . . . .

What distinguish different types of networks?• the services they provide

What distinguish the services?• Latency

• Bandwidth

• a car carrying a trunk load of tapes has high bandwidth but low latency

• Loss rate

• Number of end systems supported

• Service interface (how to invoke the service?)

• Others

• Reliability, unicast vs. multicast, real-time...

Communication Networks Taxonomy

Communication

networks

Circuit-switched

networks

Packet-switched

networks

POTS Network:• parses number dialed

• sets up a circuit between caller and callee

• sends signal to ring callee’s phone

! a circuit is set up between the two ends

Strengths:

•no end-point intelligence

•excellent voice performance

Weaknesses:

•difficult to add new services

• achieves performance and reliability by over-allocating resources (expensive)

The Internet:• data parceled into packets

• each packet carries a destination address

• each packet is routed independently

• packets can arrive out of order

• packets may not arrive at all

Strengths:

• intelligence at end points

• decentralized control

• operates over heterogeneous access techs

Weaknesses:

• variable performance, no quality of service

• no trusted infrastructure

What’s the Internet

regional ISP

router workstation

server mobile

company

network

local ISP

Internet: “network of networks”

• loosely hierarchical

• public Internet versus private intranet

Network components:

• hosts – communication endpoints: workstations, PDAs, cell phones, toasters, millions of them, running network applications

• Applications: Web browser, Email clients, Chat clients, etc....

• links – carry bits from one place to another: fiber, copper, satellite, …

• routers/switches – interconnect links: store and forward packets

History of the Internet

! 68-70’s: started as a research project, 56 kbps, initially 4 nodes (UCLA, UCSB, SRI, Utah) then < 100 computers

! 80-83: TCP/IP, DNS; ARPANET and MILNET split

! 85-86: NSF builds NSFNET as backbone, links 6 Supercomputer centers, 1.5 Mbps, 10,000 computers

! 87-90: link regional networks, NSI (NASA), ESNet (DOE), DARTnet, TWBNet (DARPA), 100,000 computers

! 90-92: NSFNET moves to 45 Mbps, 16 mid-level networks

! 94: NSF backbone dismantled, multiple private backbones; Introduction of Commercial Internet

! Today: backbones run at 10 Gbps, close to 200 millions computers in 150 countries

The ARPANet

Paul Baran

• RAND Corp, early 1960s

• Communications networks that would survive a major enemy attack

ARPANet: Research vehicle for “Resource Sharing Computer Networks”

• 2 September 1969: UCLA first node on the ARPANet

• December 1969: 4 nodes connected by phone lines

SRI940

UCLASigma 7

UCSBIBM 360

UtahPDP 10

IMPs

BBN team that implementedthe interface message processor

ARPANet Evolves into Internet

Web HostingMultiple ISPs

Internet2 BackboneInternet Exchanges

Application Hosting, Data Center

ARPANetSATNetPRNet

TCP/IP NSFNet Deregulation &Commercialization

1965 1975 1985 1995 2005

WWW

ISPData Center

15 16

Parallel Backbones

Qwest IP Backbone (Late 1999) Digex Backbone

GTE Internetworking Backbone

28

Parallel Backbones

30

What’s the Internet

Earlier we said . . . .

• data parceled into packets

• each packet carries a destination address

• each packet is routed independently

• packets can arrive out of order

• packets may not arrive at all

However, the Internet provides two types of delivery services:

1. connectionless (datagram, UDP, e.g., streaming media app, games)

2. connection oriented (byte stream, TCP, e.g., web, email)

Connection oriented service provides:

1. end-to-end reliability (sender retransmit lost packets)

2. in-sequence delivery (receiver buffers incoming packets until it can deliver them in order)

How does a router know which router to forward a packet to?

How does a receiver know the correct ordering of packets?

How does a sender know which packet is lost and must be retransmitted?

Protocols – rules governing communication between nodes, e.g., TCP/IP

• a “node” is one of sender, router, or receiver

protocols define the format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

Internet Protocol Stack

application: supporting network applications

•HTTP, SMTP, FTP, etc.

transport: endhost-endhost data transfer

•TCP, UDP

network: routing of datagrams from source to destination

•IP, routing protocols

link: data transfer between neighboring network elements

•Ethernet, WiFi

physical: bits “on the wire”

application

transport

network

link

physical

Layering in the IP Protocols

Internet Protocol

Transmission ControlProtocol (TCP)

User Datagram Protocol (UDP)

TelnetHTTP

SONET ATMEthernet

RTPDNSFTP

Application-Layer Protocols

• Messages exchanged between applications

• Syntax and semantics of the messages between end-hosts

• Tailored to the specific application (e.g., Web, e-mail)

• Messages transferred over transport connection (e.g., TCP)

• Popular application-layer protocols

• HTTP, SMTP, FTP, SSH, …

Client Server

GET /index.html HTTP/1.1

HTTP/1.1 200 OK

sourceapplication

transport

network

link

physical

destination

application

transport

network

link

physical

network

link

physical

link

physical

router

switch

Not all Network Elements “Speak” All Layers

Why Layering?

Networks are complex! Many “pieces”:

• applications

• hosts

• routers

• links of various media

Dealing with complex systems:

• explicit structure allows identification of well-defined, specific parts of a large and complex system

• modularization eases maintenance, updating of system

• change of implementation of layer’s service transparent to rest of system

• note that change of implementation is different from change of service definition!

How Do You “Use” the Internet?

You want to send email to friend@cs.usc.edu

At your end, your mailer

• translates cs.usc.edu to its IP address (128.125.1.45)

• decides to use TCP as the transport protocol (Why?)

• creates a socket

• connects to 128.125.1.45 at the well-known SMTP

port # (25)

• parcels out your email into packets

• sends the packets out

On the Internet, your packets got:

• transmitted

• routed

• buffered

• forwarded, or

• dropped

At the receiver, smtpd

• must make a “receiver” ahead of time:

• creates a socket

• decides on TCP

• binds the socket to smtp’s well-

known port #

• listens on the socket

• accepts your smtp connection requests

• recves your email packets

Client-Server Computing

Email (SMTP) uses the client-server paradigm

What is a client?

What is a server?

Process vs. machine

Alternative(s) to client-server?

Sockets API Programming

What is a socket?

Review of sockets (assume familiarity from EECS 482)

(learn how to read man pages!)

Applications need Application Programming Interface (API) to use the network

API: set of function types, data structures, and constants

• allows programmer to learn once, write anywhere

• greatly simplifies job of application programmer

physical

data-link

network

transport

application

socket API

Socket-programming using TCP

Socket: a “door” between application process and end-end transport protocol (UDP or TCP)

process

TCP with

buffers,

variables

socket

controlled by

application

developer

controlled by

operating

system

host or

server

process

TCP with

buffers,

variables

socket

controlled by

application

developer

controlled by

operating

system

host or

server

internet

Sockets

What exactly are sockets?

• an endpoint of a connection

• similar to UNIX file I/O API (provides a file descriptor)

• associated with each end-point (end-host) of a connection

• identified by the IP address and port number of both the sender and receiver

Berkeley sockets is the most popular network API

• runs on Linux, FreeBSD, OS X, Windows

• fed/fed off the popularity of TCP/IP

• can build higher-level interfaces on top of sockets

• e.g., Remote Procedure Call (RPC)

Based on C, single threaded model

• does not require multiple threads

Useful sample code available at

• http://www.kohala.com/start/unpv12e.html

Types of Sockets

Different types of sockets implement different service models

• Stream v.s. datagram

Stream socket (aka TCP)

• connection-oriented

• reliable, in order delivery

• at-most-once delivery, no duplicates

• used by e.g., ssh, http

Datagram socket (aka UDP)

• connectionless (just data-transfer)

• “best-effort” delivery, possibly lower variance in delay

• used by e.g., IP telephony, streaming audio, streaming video, Internet gaming, etc.

Stream/TCP Sockets

socket ()

bind ()

listen ()

accept ()

recv ()

close ()

socket ()

connect ()

send ()

send ()recv ()

close ()time

initialize

establish

dataxfer

terminate

Client Server

Stream/TCP Socket

Server:

! server process must first be running

! server must have created socket (door) that welcomes client’s contact

Client:

! creates client-local TCP socket

! specifies IP address, port number of server process

! When client contacts server: client TCP establishes connection to server TCP

! When contacted by client, server TCP creates new socket for server process to communicate with client

- allows server to talk with multiple clients

- source port numbers used to distinguish clients

Initialize (Client)

int sd;

if ((sd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {

perror("socket");

printf("Failed to create socket\n");

abort();

}

socket creates a socket data structure and attaches it to the process’s file descriptor table

Handling errors that occur rarely usually consumes most of systems code

Establish (Client)

struct sockaddr_in sin;

struct hostent *host = gethostbyname(argv[1]);unsigned int server_addr = *(unsigned long *) host->h_addr_list[0];unsigned short server_port = atoi(argv[2]);

memset(&sin, 0, sizeof(sin));

sin.sin_family = AF_INET;sin.sin_addr.s_addr = server_addr;sin.sin_port = htons(server_port);

if (connect(sd, (struct sockaddr *) &sin, sizeof (sin)) < 0) { perror("connect"); printf("Cannot connect to server\n"); abort();}

connect initiates connection (for TCP)

Sending Data Stream (Client)

int send_packets(char *buffer, int buffer_len) { sent_bytes = send(sd, buffer, buffer_len, 0);

if (send_bytes < 0) perror(“send”);

return 0;}

send

• returns how many bytes are actually sent

• must loop to make sure that all is sent(except for blocking I/O, see UNP Section 6.2)

What is blocking and non-blocking I/O?

Why do you want to use non-blocking I/O?