Chapter 2, slide: 1 CS 372 – introduction to computer networks* Monday June 28 Announcements: r Lab 1 is due today r Lab 2 is posted today and is due next Tuesday r Assignment 1 is due tomorrow r Quiz 1 is tomorrow and it covers chapter 1 Acknowledgement: slides drawn heavily from Kurose & Ross * Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.

Chapter 2, slide: 2 Chapter 1: recap By now, you should know: r the Internet and its components r circuit-switching networks vs. packet-switching networks r different network access technologies r the three Tiers 1, 2, and 3 r layered architecture of networks r types of delays

Chapter 2, slide: 3 Chapter 2: Application Layer Our goals: r conceptual and implementation aspects of network application protocols  transport-layer service models  client-server paradigm  peer-to-peer paradigm r learn about protocols by examining popular application-level protocols  HTTP, FTP, P2P r programming network applications  socket API

Chapter 2, slide: 4 Some network apps r r web r instant messaging r remote login r P2P file sharing r multi-user network games r streaming stored video clips r voice over IP r real-time video conferencing r grid computing

Chapter 2, slide: 5 Creating a network app write programs that  run on (different) end systems  communicate over network  e.g., web server software communicates with browser software little software written for devices in network core  network core devices do not run user applications

Chapter 2, slide: 6 Chapter 2: Application layer r Principles of network applications r Web and HTTP r File transfer: FTP r P2P file sharing r Socket programming with TCP

Chapter 2, slide: 7 Application architectures There are 3 types of architectures: r Client-server r Peer-to-peer (P2P) r Hybrid of client-server and P2P

Chapter 2, slide: 8 Client-server architecture server:  always-on  fixed/known IP address  serves many clients at same time clients:  communicate with server only  may be intermittently connected  may have dynamic IP addresses  do not communicate directly with each other E.g., of client-server archit.:  Google, Amazon, MySpace, YouTube,

Chapter 2, slide: 9 Pure P2P architecture r no always-on server r arbitrary end systems directly communicate r peers are intermittently connected and change IP addresses r example: BitTorrent Pros and cons: r scalable and distributive r difficult to manage r not secure peer-peer

Chapter 2, slide: 10 Hybrid of client-server and P2P Skype  voice-over-IP P2P application  centralized server: finding address of remote party  client-client connection: direct (not through server) Instant messaging  chatting between two users is P2P  centralized service: client presence location user registers its IP address with central server when it comes online user contacts central server to find IP addresses of buddies

Chapter 2, slide: 11 Processes communicating Process: is program running within a host. r processes in same host communicate using inter-process communication (managed by OS). r processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted r Note: applications with P2P architectures have client processes & server processes

Chapter 2, slide: 12 Sockets r process sends/receives messages to/from its socket r socket analogous to door  sending process shoves message out door  sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process controlled by OS process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server Internet controlled by app developer r App.Prog.Interf (API): (1) choice of transport protocol; (2) ability to fix a few parameters

Chapter 2, slide: 13 Addressing processes r to receive messages, process must have identifier r host device has unique 32-bit IP address r Q: does IP address of host on which process runs suffice to identify the process? r identifier consists of:  IP address (host)  port numbers (process) r Example port numbers:  HTTP server: 80  Mail server: 25 r to send HTTP message to gaia.cs.umass.edu web server:  IP address:  Port number: 80 r more shortly… r A: No, many processes can be running on same host

Chapter 2, slide: 14 App-layer protocol defines r Types of messages exchanged,  e.g., request, response r Message syntax:  what fields in messages & how fields are delineated r Message semantics  meaning of information in fields r Rules for when and how processes send & respond to messages Public-domain protocols: r defined in RFCs r allows for interoperability r e.g., HTTP, SMTP Proprietary protocols: r e.g., Skype Question: why do we need an “App-layer protocol” ?

Chapter 2, slide: 15 What transport service does an app need? Data loss/reliability r some apps (e.g., audio) can tolerate some loss r other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing r some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Bandwidth r Bandwidth sensitive apps: some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” r other apps (“elastic apps”) make use of whatever bandwidth they get Security  Data encryption/ Decryption  Data Integrity  End-point authentication

Chapter 2, slide: 16 Transport service requirements of common apps Application file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss loss-tolerant no loss Bandwidth elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Time Sensitive no yes, 100’s msec yes, few secs yes, 100’s msec yes and no

Chapter 2, slide: 17 What services do Internet transport protocols provide? TCP service: r connection-oriented: setup required between client and server processes r reliable transport between sending and receiving process r flow control: sender won’t overwhelm receiver r congestion control: throttle sender when network overloaded r does not provide: timing, minimum bandwidth guarantees UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP?

Chapter 2, slide: 18 Internet apps: application, transport protocols Application remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. Youtube) proprietary (e.g., Skype) Underlying transport protocol TCP TCP or UDP typically UDP

Chapter 2, slide: 19 Principles of network applications: Review Questions Question 1: In P2P architecture, which process is the client and which one is the server? Question 2: What information is used by a process to identify another process to communicate with? Question 3: List 2 types of services an application may need from a transport protocol. Indicate whether TCP, UDP, neither, or both provide such a service Answer 1: Client: who requests Server: who responds Answer 2:  IP address of host  Port number of process Answer 3:  Reliability: TCP  Timing: none  Bandwidth: none