3rd Edition: Chapter 2 - Concordia University

3rd Edition: Chapter 2 - Concordia University

Application Layer* Jim Kurose and Keith Ross Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition., AddisonWesley, July 2004. * Application layer Principles of network applications Web and HTTP Electronic Mail SMTP, POP3, IMAP Socket programming with TCP Socket programming with UDP Application Layer Our 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 programming network applications socket API Some network apps E-mail

Web Internet telephone Real-time video Instant messaging Remote login conference Massive parallel computing P2P file sharing Multi-user network games Streaming stored video clips Creating a network app Write programs that run on different end systems and communicate over a network.

e.g., Web: Web server software communicates with browser software No software written for devices in network core Network core devices do not function at app layer This design allows for rapid app development applicatio n transport network data link physical applicatio n transport network data link physical applicatio n

transport network data link physical Application layer Principles of network applications Web and HTTP Electronic Mail SMTP, POP3, IMAP Socket programming with TCP Socket programming with UDP Application architectures Application architecture is different from Network Architecture Network architecture (5-layer archit.) provides a specific set of services to the application layer

Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P Client-server architecture A web application: web servers service requests from browsers (clients) server: always-on host permanent IP address server farms (cluster of servers) for scaling clients: communicate with server may have dynamic IP addresses do not communicate directly with each other

(2 browsers dont communicate) Pure P2P architecture no always on server arbitrary end systems directly communicate (hence the name P2P) peers are occasionally connected and change IP addresses example: Gnutella (file sharing application) Highly scalable: millions of users may participate in file sharing Each user increase demand and serving capacity But difficult to manage (because highly distributed) Hybrid of client-server and P2P Napster File transfer P2P

File search centralized: Peers register content at central server Peers query same central server to locate content Instant messaging Chatting between two users is P2P Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies 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 Sockets process sends/receives messages to the network through its socket socket analogous to door host or server sending process sends message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process process

host or server controlled by app developer process socket socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS A socket is the interface between application and transport at a host; also known as Application Programming Interface (API) API: (1) choice of transport protocol; (2) ability to fix a few Addressing processes For a process to receive

messages, it must have an identifier A host has a unique32bit IP address Q: does the IP address of the host on which the process runs suffice for identifying the process? Answer: No, many processes can be running on same host Identifier includes both the IP address and port numbers associated with the process on the host. Example port numbers: HTTP server: 80 Mail server: 25 More on this later App-layer protocol! Defines: Types of messages

exchanged, eg, request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability (developers have to follow the rules in RFCs) eg, HTTP, SMTP Proprietary protocols: eg, KaZaA What transport services 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 Bandwidth some apps (e.g., multimedia) require minimum amount of bandwidth to be effective other apps (elastic apps) make use of whatever bandwidth they get (e.g., e-mail) Transport service requirements of common apps Data loss Bandwidth Time Sensitive file transfer e-mail

Web documents real-time audio/video no loss no loss no loss loss-tolerant no no no yes, 100s msec stored audio/video interactive games instant messaging loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Application

yes, few secs yes, 100s msec yes and no Internet transport protocols services The internet makes two transport protocols available for applications: TCP service: UDP service: connection-oriented: setup connection-less unreliable data transfer required between client and server processes reliable transport between sending and receiving process flow control: sender wont overwhelm receiver congestion control: throttle sender when network

overloaded does not provide: timing, minimum bandwidth guarantees between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? Internet apps: application, transport protocols Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol Underlying transport protocol

SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad) TCP TCP TCP TCP TCP or UDP typically UDP UDP is used for applications that tolerate some data loss but requires a minimum bandwidth guarantee Application layer Principles of network applications app architectures app requirements Web and HTTP Electronic Mail

SMTP, POP3, IMAP Socket programming with TCP Socket programming with UDP Web and HTTP 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 HTTP overview HTTP: hypertext transfer protocol Webs application layer protocol client/server model

implementation: client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HT TP req ue PC running HTT st Pr Explorer esp ons e t es u re q se Server P n T o

running sp HT e r Apache Web TP HT server Mac running Navigator Client and server programs communicate by exchanging HTTP messages HTTP defines the structure of these messages HTTP overview (continued) Uses TCP: HTTP is stateless client initiates TCP server maintains no connection (creates socket) to server, port 80 server accepts TCP connection from client

Server receives messages through its socket HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed information about past client requests aside 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 HTTP connections Nonpersistent HTTP At most one object is

sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode Nonpersistent HTTP (contains text, Suppose user enters URL www.someSchool.edu/someDepartment/home.index references to 10 jpeg images) 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

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

send 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 initiate TCP connection RTT request file RTT file received time time to transmit file time Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object

OS must work and allocate host resources for each TCP connection (i.e., each object) but 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 are sent over connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects HTTP request message two types of HTTP messages: request,

response HTTP request message: ASCII (human-readable format) request line (GET, POST, GET /somedir/page.html HTTP/1.1 HEAD commands) Host: www.someschool.edu User-agent: Mozilla/4.0 header Connection: close lines Accept-language:fr Carriage return, line feed indicates end of message (extra carriage return, line feed) HTTP request message: general format Uploading form input Post method: Web page often includes form input Input is uploaded to server in entity body

e.g., search words for a search engine Method types HTTP/1.0 GET POST HEAD asks server to leave requested object out of response (used for debugging) HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field HTTP response message status line (protocol

status code status phrase) header lines data, e.g., requested HTML file HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 ... Content-Length: 6821 Content-Type: text/html data data data data data ... HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently

requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported Web caches (proxy server) Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns

object to client origin server Proxy HT st TP req server reque H ue se TP client TTP st n T o H res esp r pon se TTP H st e u eq

r se n P o T sp HT e Pr T HT client origin server More about Web caching Cache acts as both client and server Server for browsers Client for original server Typically cache is installed by ISP

(university, company, residential ISP) Why Web caching? Reduce response time for client request. Reduce (substantially) traffic on an institutions access link (and also the Internet) Internet dense with caches enables content providers to effectively deliver content Caching example Assumptions average object size = 100,000 bits avg. request rate from institutions browsers to origin servers = 15/sec delay from institutional router to any origin server and back to router = 2 sec Consequences utilization on LAN = 15% utilization on access link = 100% total delay = Internet delay +

access delay + LAN delay = 2 sec + minutes + milliseconds origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps (15 requests/sec) * LAN (100Kbits/request)/ 100Mbps = 0.15 institutional when traffic intensity cache approaches 1, link gets congested unbounded delays Caching example (cont) Possible solution increase bandwidth of access link to, say, 10 Mbps

origin servers public Internet Consequences utilization on LAN = 15% utilization on access link = 15% institutional Total delay = Internet delay + network access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade! 10 Mbps access link 10 Mbps LAN institutional cache Caching example (cont) origin servers Install cache suppose hit rate is .4 (fraction

of requests satisfied by the cache) public Internet Consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + milliseconds < 1.4 secs 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

Conditional GET Goal: dont send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified server cache HTTP request msg If-modified-since: HTTP response object not modified

HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 200 OK object modified Application layer Principles of network applications Web and HTTP Electronic Mail SMTP, POP3, IMAP Socket programming with TCP Socket programming with UDP

Electronic Mail outgoing message queue user mailbox user agent Three major components: user agents mail servers mail server simple mail transfer SMTP protocol: SMTP User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server user agent

SMTP SMTP mail server user agent user agent mail server user agent user agent Electronic Mail: mail servers user agent Mail Servers mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email

messages client: sending mail server server: receiving mail server mail server user agent SMTP SMTP SMTP mail server user agent user agent mail server user agent user agent

Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server Does not send message to intermediate mail-server three phases of transfer handshaking (greeting) between servers transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII (i.e., limited to text) Scenario: Alice sends message to Bob 4) SMTP client sends Alices message over the TCP connection 5) Bobs mail server places the message in Bobs mailbox 6) Bob (at his convenience)

invokes his user agent to read message 1) Alice uses UA to compose message and to [email protected] 2) Alices UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bobs mail server 1 user agent 2 mail server 3 mail server 4 5 6

user agent Sample SMTP interaction S: C: S: C: S: C: S: C: S: C: C: C: S: C: S: Servers greetings 220 hamburger.edu HELO crepes.fr 250 Hello crepes.fr, pleased to meet you Client initiates this MAIL FROM: 250 [email protected] Senderfor okevery new message RCPT TO: 250 [email protected] ... Recipient ok

DATA 354 Enter mail, end with "." on a line by itself e-mail Do youBody likeof ketchup? How about pickles? Indicates to the server . end of the message 250the Message accepted Closing connection for delivery QUIT 221 hamburger.edu closing connection SMTP: final words SMTP uses persistent connections SMTP requires message (header & body) to be in 7-bit ASCII Server issues replies to each client command Each reply has a code (e.g., 220, etc..) Refer to RFC for

details Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/ response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg (over the same TCP connection) Mail access protocols SMTP SMTP user agent senders mail server access protocol

receivers mail server SMTP: delivery/storage to receivers server Mail access protocol: retrieval from server POP3: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. (web based) user agent POP3 protocol authorization phase client commands: user: declare username pass: password server responses +OK -ERR

transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit User configures POP3: download and delete mode. Bob cannot re-read email if he changes client Download-and-keep: copies of messages on different clients POP3 is stateless across sessions Does not maintain any state information simple implementation IMAP, Web-based e-mail IMAP (another mail access protocol)

Keep all messages in one place: the server Allows user to organize messages in folders IMAP keeps user state across sessions: names of folders and mappings between message IDs and folder name Web-based e-mail User agent is ordinary web browser User communicate with its mail box through HTTP Bobs mail server sends a message to Bobs client through HTTP (not POP3 or IMAP) Alice sends message to her mail server through HTTP SMTP is used between mail servers. Application layer Principles of network

applications Web and HTTP FTP Electronic Mail SMTP, POP3, IMAP Socket programming with TCP Socket programming with UDP Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API introduced in BSD4.1 UNIX, 1981 explicitly created, used, released by apps (standard vs. proprietary) client/server paradigm two types of transport service via socket API: unreliable datagram reliable, byte streamoriented socket

a host-local, application-created, OS-controlled interface (a door) into which application process can both send and receive messages to/from another application process Socket-programming using TCP Processes on different machines communicate by sending messages into sockets Socket: a door between application process and end-end-transport protocol (UCP or TCP) TCP service: reliable transfer of bytes from one process to another controlled by application developer controlled by operating system process process socket TCP with buffers,

variables socket TCP with buffers, variables host or server internet host or server controlled by application developer controlled by operating system Socket programming with TCP Client must contact server server process must first be running server must have created socket (door) that welcomes clients contact Client contacts server by: creating client-local TCP

socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket dedicated for server process to communicate with client allows server to talk with multiple clients source port numbers used to distinguish clients (more later) New socket is called connection socket Socket programming with TCP From application perspective, the TCP connection is a virtual pipe between the client socket and the server connection socket

application viewpoint TCP provides reliable, in-order transfer of bytes (pipe) between client and server Stream notion A stream is a sequence of characters that flow into or out of a process. An input stream is attached to some input source for the process, eg, keyboard or socket. An output stream is attached to an output source, eg, monitor or socket. Socket programming with TCP input stream Client Process process output stream

inFromServer 1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream) 2) server reads line from socket 3) server converts line to uppercase, sends back to client 4) client reads, prints modified line from socket (inFromServer stream) outToServer Example client-server app: monitor inFromUser keyboard input stream client TCP clientSocket

socket to network from network TCP socket Socket programming with TCP This process contacts the server and establish a TCP connection with it. input stream Client Process process output stream inFromServer 1) A server program is first executed (to create the server process) at the

server host 2) server process waits to be contacted by client process 3) When the client program is executed, a process is created at the client outToServer NOTE monitor inFromUser keyboard input stream client TCP clientSocket socket to network from network TCP socket

Client/server socket interaction: TCP Server Client (running on hostid) create socket, port=x, for incoming request: welcomeSocket = ServerSocket() TCP wait for incoming connection request connection connectionSocket = welcomeSocket.accept() read request from connectionSocket write reply to connectionSocket close connectionSocket setup create socket, connect to hostid, port=x clientSocket =

Socket() send request using clientSocket read reply from clientSocket close clientSocket Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; Create input stream Create client socket, connect to server Create output stream attached to socket BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Example: Java client (TCP), cont. Create input stream attached to socket BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); Send line to server outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); Read line from server System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } } Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer {

Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Example: Java server (TCP), cont Create output stream, attached to socket Read in line from socket DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; Write out line to socket outToClient.writeBytes(capitalizedSentence); } } } End of while loop, loop back and wait for another client connection Application layer Principles of network applications Web and HTTP FTP Electronic Mail SMTP, POP3, IMAP Socket programming with TCP Socket programming

with UDP Socket programming with UDP UDP: no connection between client and server no handshaking sender explicitly attaches IP address and port of destination to each packet server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost application viewpoint UDP provides unreliable transfer of groups of bytes (datagrams) between client and server Client/server socket interaction: UDP Server (running on hostid) create socket, port=x, for incoming request: serverSocket =

DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port number Client create socket, clientSocket = DatagramSocket() Create, address (hostid, port=x) send datagram request using clientSocket read reply from clientSocket close clientSocket Example: Java client (UDP) input stream Client Process monitor

inFromUser keyboard Input: receives process packet (TCP received byte stream) UDP packet receivePacket packet (TCP sent byte stream) sendPacket Output: sends UDP packet client UDP clientSocket socket

to network from network UDP socket Example: Java client (UDP) import java.io.*; import java.net.*; Create input stream Create client socket Translate hostname to IP address using DNS class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes();

Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } Example: Java server (UDP) import java.io.*; import java.net.*; Create datagram socket at port 9876

class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { Create space for received datagram Receive datagra m DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Example: Java server (UDP), cont String sentence = new String(receivePacket.getData()); Get IP addr port #, of sender InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); Create datagram to send to client Write out datagram to socket } DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } } End of while loop, loop back and wait for another datagram Summary Our study of network apps now complete! Application architectures client-server P2P hybrid application service requirements:

reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP specific protocols: HTTP FTP SMTP, POP, IMAP DNS socket programming Summary Most importantly: learned about protocols typical request/reply message exchange: client requests info or service

server responds with data, status code message formats: headers: fields giving info about data data: info being communicated control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer complexity at network edge

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