Application Layer

1 .Application Layer CS 3516 – Computer Networks

2 .2: Application Layer 2

3 .3 Chapter 2: Application Layer Goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS programming network applications socket API

4 .4 Some network apps e-mail web instant messaging remote login P2P file sharing multi-user network games streaming stored video clips social networks voice over IP real-time video conferencing grid computing

5 .5 Creating a Network A pp Write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software No need to write software for network-core devices Network-core devices do not run user applications applications on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical application transport network data link physical

6 .6 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 Socket programming with UDP 2.8 Socket programming with TCP

7 .Application architectures Client-server ( CS ) Including data centers / cloud computing Peer-to-peer ( P2P ) Hybrid of client-server and P2P

8 .8 Client-server Architecture server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other client/server

9 .Server Example - Google Data Centers Estimated cost of data center: $600M Google spent $2.4B in 2007 on new data centers Each data center uses 50-100 megawatts of power

10 .Pure P2P Architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses Highly scalable but difficult to manage peer-peer

11 .Hybrid of Client-server and P2P E.g. Skype voice-over-IP P2P application centralized server: finding address of remote party client-client connection: often direct (not through server) E.g. Instant messaging chatting between two users is P2P centralized service: client presence detection/location user registers its IP address with central server when it comes online user contacts central server to find IP addresses of buddies

12 .Processes Communicating Process: program running within a host. Within same host, two processes communicate using inter-process communication (defined by OS). Processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted Note: applications with P2P architectures have client processes & server processes

13 .Sockets Process sends/receives messages to/from its socket 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 process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server Internet controlled by OS controlled by app developer API: (1) choice of transport protocol; (2) ability to fix a few parameters (see Sockets slide deck)

14 .Addressing Processes To receive messages, process must have identifier Host device has unique 32-bit IP address Exercise: use ipconfig from command prompt to get your IP address (Windows) Q: does IP address of host on which process runs suffice for identifying the process? A: No, many processes can be running on same Identifier includes both IP address and port numbers associated with process on host. Example port numbers: HTTP server: 80 Mail server: 25

15 .App-layer Protocol Defines Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: Defined in RFCs allows for interoperability e.g., HTTP, SMTP, BitTorrent Proprietary protocols: e.g., Skype, ppstream

16 .What Transport Service Does an App Need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Throughput some apps (e.g., multimedia) require minimum amount of throughput to be “effective” other apps (“elastic apps”) make use of whatever throughput they get Security e ncryption , data integrity, …

17 .Transport Service Requirements of Common Apps Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss Throughput elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no

18 .Internet Transport Protocols Services TCP service: connection-oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantees, security UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP?

19 .Internet Apps: Application, Transport Protocols Application e-mail 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] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP

20 .Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 Socket programming with UDP 2.8 Socket programming with TCP

21 .Web and HTTP First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file,… Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: www.someschool.edu/someDept/pic.gif host name path name

22 .HTTP Overview HTTP: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests PC running Explorer Server running Apache Web server Mac running Navigator HTTP request HTTP request HTTP response HTTP response

23 .HTTP Overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is “stateless” server maintains no information about past client requests Protocols that maintain “state” are complex! past history (state) must be maintained if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside

24 .HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection. Persistent HTTP Multiple objects can be sent over single TCP connection between client and server.

25 .Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a . HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment / home.index 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time (contains text, references to 10 jpeg images)

26 .Nonpersistent HTTP (cont.) 5 . HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects 4. HTTP server closes TCP connection. time

27 .Nonpersistent HTTP: Response time Definition of RTT: time for a small packet to travel from client to server and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time time to transmit file initiate TCP connection RTT request file RTT file received time time

28 .Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP connection browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects

29 .HTTP request message two types of HTTP messages: request , response HTTP request message: ASCII (human-readable format) GET / somedir /page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept- language:fr (extra carriage return, line feed) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message