Chapter 2 Application Layer Instructor Mohsen Afsharchi Office

Chapter 2 Application Layer Instructor: Mohsen Afsharchi Office: E-122 Email: afsharchim@znu. ac. ir Class Location: Lectures: S, Tu 10: 00 – 11: 30 Notes derived from “Computer Networking: A Top Down Approach Featuring the Internet”, 2005, 3 rd edition, Jim Kurose, Keith Ross, Addison-Wesley. Slides are adapted from the companion web site of the book. 2: Application Layer 1

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P Applications r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 2

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

Some network apps r e-mail r voice over IP r web r real-time video r remote login conferencing r grid computing r P 2 P file sharing r r multi-user network r r instant messaging games r streaming stored video clips r 2: Application Layer 4

Creating a network app write programs that v v v 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 v v network core devices do not run user applications on end systems allows for rapid app development, propagation application transport network data link physical 2: Application Layer 5

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 6

Application architectures r Client-server r Peer-to-peer (P 2 P) r Hybrid of client-server and P 2 P 2: Application Layer 7

Client-server architecture server: v always-on host v permanent IP address v server farms for scaling clients: client/server v v communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 2: Application Layer 8

Pure P 2 P architecture r no always-on server r arbitrary end systems directly communicate peer-peer r peers are intermittently connected and change IP addresses r example: Gnutella Highly scalable but difficult to manage 2: Application Layer 9

Hybrid of client-server and P 2 P Skype v voice-over-IP P 2 P application v centralized server: finding address of remote party: v client-client connection: direct (not through server) Instant messaging v chatting between two users is P 2 P v 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 2: Application Layer 10

Processes communicating Process: program running within a host. r within same host, two processes communicate using inter-process communication (defined 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 P 2 P architectures have client processes & server processes 2: Application Layer 11

Sockets r process sends/receives messages to/from its socket r socket analogous to door v v 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 host or server process controlled by app developer process socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS r API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) 2: Application Layer 12

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 for identifying the process? 2: Application Layer 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 for identifying the process? v A: No, many processes can be running on same host r identifier includes both IP address and port numbers associated with process on host. r Example port numbers: v v HTTP server: 80 Mail server: 25 r to send HTTP message to gaia. cs. umass. edu web server: v v IP address: 128. 119. 245. 12 Port number: 80 r more shortly… 2: Application Layer 14

App-layer protocol defines r Types of messages exchanged, v e. g. , request, response r Message syntax: v what fields in messages & how fields are delineated r Message semantics v meaning of information in fields Public-domain protocols: r defined in RFCs r allows for interoperability r e. g. , HTTP, SMTP Proprietary protocols: r e. g. , Skype r Rules for when and how processes send & respond to messages 2: Application Layer 15

What transport service does an app need? Data loss 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 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 2: Application Layer 16

Transport service requirements of common apps Data loss Bandwidth Time Sensitive file transfer e-mail Web documents real-time audio/video no loss-tolerant no no no yes, 100’s msec stored audio/video interactive games instant messaging loss-tolerant no loss elastic audio: 5 kbps-1 Mbps video: 10 kbps-5 Mbps same as above few kbps up elastic Application yes, few secs yes, 100’s msec yes and no 2: Application Layer 17

Internet transport protocols services TCP service: r connection-oriented: setup r r 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 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? 2: Application Layer 18

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. Real. Networks) proprietary (e. g. , Vonage, Dialpad) TCP TCP TCP or UDP typically UDP 2: Application Layer 19

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 20

Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API r introduced in BSD 4. 1 UNIX, 1981 r explicitly created, used, released by apps r client/server paradigm r two types of transport service via socket API: v unreliable datagram v 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 2: Application Layer 21

Socket-programming using TCP Socket: a door between application process and endend-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 socket TCP with buffers, variables host or server internet socket TCP with buffers, variables controlled by application developer controlled by operating system host or server 2: Application Layer 22

Socket programming with TCP Client must contact server r server process must first be running r server must have created socket (door) that welcomes client’s contact Client contacts server by: r creating client-local TCP socket r specifying IP address, port number of server process r When client creates socket: client TCP establishes connection to server TCP r When contacted by client, server TCP creates new socket for server process to communicate with client v allows server to talk with multiple clients v source port numbers used to distinguish clients (more in Chap 3) application viewpoint TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server 2: Application Layer 23

Client/server socket interaction: TCP Server Client (running on hostid) create socket, port=x, for incoming request: welcome. Socket = Server. Socket() TCP wait for incoming connection request connection. Socket = welcome. Socket. accept() read request from connection. Socket write reply to connection. Socket close connection. Socket setup create socket, connect to hostid, port=x client. Socket = Socket() send request using client. Socket read reply from client. Socket close client. Socket 2: Application Layer 24

Stream jargon r A stream is a sequence of characters that flow into or out of a process. r An input stream is attached to some input source for the process, e. g. , keyboard or socket. r An output stream is attached to an output source, e. g. , monitor or socket. Client process client TCP socket 2: Application Layer 25

Socket programming with TCP Example client-server app: 1) client reads line from standard input (in. From. User stream) , sends to server via socket (out. To. Server 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 (in. From. Server stream) 2: Application Layer 26

Example: Java client (TCP) import java. io. *; import java. net. *; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modified. Sentence; Create input stream Create client socket, connect to server Create output stream attached to socket Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Socket client. Socket = new Socket("hostname", 6789); Data. Output. Stream out. To. Server = new Data. Output. Stream(client. Socket. get. Output. Stream()); 2: Application Layer 27

Example: Java client (TCP), cont. Create input stream attached to socket Buffered. Reader in. From. Server = new Buffered. Reader(new Input. Stream. Reader(client. Socket. get. Input. Stream())); sentence = in. From. User. read. Line(); Send line to server out. To. Server. write. Bytes(sentence + 'n'); Read line from server modified. Sentence = in. From. Server. read. Line(); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 2: Application Layer 28

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 client. Sentence; String capitalized. Sentence; Server. Socket welcome. Socket = new Server. Socket(6789); while(true) { Socket connection. Socket = welcome. Socket. accept(); Buffered. Reader in. From. Client = new Buffered. Reader(new Input. Stream. Reader(connection. Socket. get. Input. Stream())); 2: Application Layer 29

Example: Java server (TCP), cont Create output stream, attached to socket Data. Output. Stream out. To. Client = new Data. Output. Stream(connection. Socket. get. Output. Stream()); Read in line from socket client. Sentence = in. From. Client. read. Line(); capitalized. Sentence = client. Sentence. to. Upper. Case() + 'n'; Write out line to socket out. To. Client. write. Bytes(capitalized. Sentence); } } } End of while loop, loop back and wait for another client connection 2: Application Layer 30

Example: C echo client (TCP) Helpful includes Define socket address structure #include #include <sys/types. h> /* basic system data types */ <sys/socket. h> /* basic socket definitions */ <netinet/in. h> <stdio. h> <unistd. h> int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc != 2) { printf("usage: tcpcli <IPaddress>n"); exit(-1); } Create TCP socket sockfd = socket(AF_INET, SOCK_STREAM, 0); 2: Application Layer 31

Example: C echo client (TCP), cont. Fill in socket structure with server information bzero(&servaddr, sizeof(servaddr)); servaddr. sin_family = AF_INET; servaddr. sin_port = htons(6789); servaddr. sin_addr. s_addr = inet_addr(argv[1]); Establish connection with server connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); Work with the established socket str_cli(stdin, sockfd); /* do it all */ exit(0); } 2: Application Layer 32

Example: C echo client (TCP), cont. Get line of text from stdin void str_cli(FILE *fp, int sockfd) { char sendline[MAXLINE], recvline[MAXLINE]; while (fgets(sendline, MAXLINE, fp) != NULL) { Send line to server write(sockfd, sendline, strlen(sendline)); if (readline(sockfd, recvline, MAXLINE) == 0) { printf("str_cli: server terminated prematurelyn"); exit(-1); } Get line from server Write the line to stdout fputs(recvline, stdout); } } Readline is also user defined. 2: Application Layer 33

Example: C echo server (TCP) Helpful includes #include #include <sys/types. h> /* basic system data types */ <sys/socket. h> /* basic socket definitions */ <netinet/in. h> <stdio. h> <unistd. h> #define MAXLINE 1024 #define LISTENQ 16 /* max size of queue */ Define socket address structure Create TCP socket int main(int argc, char **argv) { int listenfd, connfd; pid_t childpid; int clilen; struct sockaddr_in cliaddr, servaddr; listenfd = socket(AF_INET, SOCK_STREAM, 0); bzero(&servaddr, sizeof(servaddr)); 2: Application Layer 34

Example: C echo server (TCP), cont. Fill in structure to accept conns from any local interface Assign structure to the socket Convert socket to a listening socket Wait until new conn. is established Work with the established conn. servaddr. sin_family = AF_INET; servaddr. sin_addr. s_addr = htonl(INADDR_ANY); servaddr. sin_port = htons(6789); bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); listen(listenfd, LISTENQ); for ( ; ; ) { clilen = sizeof(cliaddr); connfd = accept(listenfd, (struct sockaddr *) &cliaddr, &clilen); str_echo(connfd); /* process the request */ close(connfd); /* close connected socket */ } } /* end main */ 2: Application Layer 35

Example: C echo server (TCP), cont. void str_echo(int sockfd) { ssize_t n; char line[MAXLINE]; Get line from client Write line back to client for ( ; ; ) { if ( (n = readline(sockfd, line, MAXLINE)) == 0) return; /* connection closed by other end */ write(sockfd, line, n); } } /* end str_echo */ Readline is also user defined. 2: Application Layer 36

Chapter 2: Application layer r 2. 1 Principles of network applications v v app architectures app requirements r 2. 2 Web and HTTP r 2. 3 FTP r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 37

Web and HTTP First some jargon r Web page consists of objects r Object can be HTML file, JPEG image, Java applet, audio file, … r Web page consists of base HTML-file which includes several referenced objects r Each object is addressable by a URL r Example URL: http: //www. someschool. edu/some. Dept/pic. gif method host name path name 2: Application Layer 38

HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model v client: browser that requests, receives, “displays” Web objects v server: Web server sends objects in response to requests r HTTP 1. 0: RFC 1945 r HTTP 1. 1: RFC 2616 HT TP req ues PC running HT t TP res Explorer pon se st ue q e r P nse Server T o p running HT es r P T Apache Web HT server Mac running Navigator 2: Application Layer 39

HTTP overview (continued) Uses TCP: r client initiates TCP connection (creates socket) to server, port 80 r server accepts TCP connection from client r HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) r TCP connection closed HTTP is “stateless” r server maintains no information about past client requests aside Protocols that maintain “state” are complex! r past history (state) must be maintained r if server/client crashes, their views of “state” may be inconsistent, must be reconciled 2: Application Layer 40

HTTP connections Nonpersistent HTTP r At most one object is sent over a TCP connection. r HTTP/1. 0 uses nonpersistent HTTP Persistent HTTP r Multiple objects can be sent over single TCP connection between client and server. r HTTP/1. 1 uses persistent connections in default mode 2: Application Layer 41

Nonpersistent HTTP (contains text, Suppose user enters URL references to 10 www. some. School. edu/some. Department/home. index jpeg images) 1 a. HTTP client initiates TCP connection to HTTP server (process) at www. some. School. edu on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object some. Department/home. index 1 b. HTTP server at host www. some. School. 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 2: Application Layer 42

Nonpersistent HTTP (cont. ) 4. HTTP server closes TCP 5. HTTP client receives response connection. 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 2: Application Layer 43

Non-Persistent HTTP: Response time Definition of RTT: time to send a small packet to travel from client to server and back. Response time: r one RTT to initiate TCP connection r one RTT for HTTP request and first few bytes of HTTP response to return r file transmission time total = 2 RTT+transmit time initiate TCP connection RTT request file RTT file received time to transmit file time 2: Application Layer 44

Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS overhead for each TCP connection r browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP r server leaves connection open after sending response r subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: r client issues new request only when previous response has been received r one RTT for each referenced object Persistent with pipelining: r default in HTTP/1. 1 r client sends requests as soon as it encounters a referenced object r as little as one RTT for all the referenced objects 2: Application Layer 45

HTTP request message r two types of HTTP messages: request, response r HTTP request message: v ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page. html HTTP/1. 1 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) 2: Application Layer 46

HTTP request message: general format 2: Application Layer 47

Uploading form input Post method: r Web page often includes form input r Input is uploaded to server in entity body URL method: r Uses GET method r Input is uploaded in URL field of request line: www. somesite. com/animalsearch? monkeys&banana 2: Application Layer 48

Method types HTTP/1. 0 r GET r POST r HEAD v asks server to leave requested object out of response HTTP/1. 1 r GET, POST, HEAD r PUT v uploads file in entity body to path specified in URL field r DELETE v deletes file specified in the URL field 2: Application Layer 49

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. . . 2: Application Layer 50

HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK v request succeeded, requested object later in this message 301 Moved Permanently v requested object moved, new location specified later in this message (Location: ) 400 Bad Request v request message not understood by server 404 Not Found v requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 51

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cis. poly. edu 80 Opens TCP connection to port 80 (default HTTP server port) at cis. poly. edu. Anything typed in sent to port 80 at cis. poly. edu 2. Type in a GET HTTP request: GET /~ross/ HTTP/1. 1 Host: cis. poly. edu By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server! 2: Application Layer 52

Let’s look at HTTP in action r telnet example r Ethereal/Wireshark example (in lab) 2: Application Layer 53

User-server state: cookies Example: r Susan always access Internet always from PC r visits specific e 1) cookie header line of HTTP response message commerce site for first 2) cookie header line in time HTTP request message r when initial HTTP 3) cookie file kept on user’s host, managed by requests arrives at site, user’s browser site creates: 4) back-end database at v unique ID Web site v entry in backend database for ID Many major Web sites use cookies Four components: 2: Application Layer 54

Cookies: keeping “state” (cont. ) client ebay 8734 cookie file ebay 8734 amazon 1678 server usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 one week later: usual http response msg Amazon server creates ID 1678 for user create entry cookiespecific action access ebay 8734 amazon 1678 usual http request msg cookie: 1678 usual http response msg backend database cookiespectific action 2: Application Layer 55

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e-mail) aside Cookies and privacy: r cookies permit sites to learn a lot about you r you may supply name and e-mail to sites How to keep “state”: r protocol endpoints: maintain state at sender/receiver over multiple transactions r cookies: http messages carry state 2: Application Layer 56

Web caches (proxy server) Goal: satisfy client request without involving origin server r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache v v object in cache: cache returns object else cache requests object from origin server, then returns object to client origin server HT client. HTTP TP req ues Proxy server t res pon se t s ue q re P nse o T p HT es r TP T H client est u q e Pr T nse o p HT res P T HT origin server 2: Application Layer 57

More about Web caching r cache acts as both client and server r typically cache is installed by ISP (university, company, residential ISP) Why Web caching? r reduce response time for client request r reduce traffic on an institution’s access link. r Internet dense with caches: enables “poor” content providers to effectively deliver content (but so does P 2 P file sharing) 2: Application Layer 58

Where is Caching Performed? Assume: there is at least one cache between user and content on server. v v Cache in client browser Workgroup or institutional proxy cache Local ISP proxy cache Remote ISP/CDN proxy cache (for server) *Read A Web Caching Primer for more details. 2: Application Layer 59

Caching example origin servers Assumptions r average object size = 100, 000 bits r avg. request rate from institution’s browsers to origin servers = 15/sec r delay from institutional router to any origin server and back to router = 2 sec Consequences public Internet 1. 5 Mbps access link institutional network 10 Mbps LAN r utilization on LAN = 15% r utilization on access link = 100% r total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds institutional cache 2: Application Layer 60

Caching example (cont) origin servers possible solution r increase bandwidth of access link to, say, 10 Mbps consequence public Internet r utilization on LAN = 15% r utilization on access link = 15% = Internet delay + access delay + LAN delay = 2 sec + msecs r often a costly upgrade 10 Mbps access link r Total delay institutional network 10 Mbps LAN institutional cache 2: Application Layer 61

Caching example (cont) possible solution: install cache r suppose hit rate is 0. 4 consequence origin servers public Internet r 40% requests will be satisfied almost immediately r 60% requests satisfied by origin server r utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) r total avg delay = Internet delay + access delay + LAN delay =. 6*(2. 01) secs +. 4*milliseconds < 1. 4 secs 1. 5 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer 62

Conditional GET r Goal: don’t send object if cache has up-to-date cached version r cache: specify date of cached copy in HTTP request If-modified-since: <date> r 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: <date> HTTP response object not modified HTTP/1. 0 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response object modified HTTP/1. 0 200 OK <data> 2: Application Layer 63

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 64

FTP: the file transfer protocol user at host FTP user client interface file transfer local file system FTP server remote file system r transfer file to/from remote host r client/server model client: side that initiates transfer (either to/from remote) v server: remote host r ftp: RFC 959 r ftp server: port 21 v 2: Application Layer 65

FTP: separate control, data connections r FTP client contacts FTP server r r TCP control connection port 21 at port 21, TCP is transport protocol TCP data connection FTP port 20 client authorized over control client server connection client browses remote r server opens another TCP directory by sending commands data connection to transfer over control connection. another file. when server receives file r control connection: “out of transfer command, server band” opens 2 nd TCP connection (for r FTP server maintains “state”: file) to client current directory, earlier after transferring one file, authentication server closes data connection. 2: Application Layer 66

FTP commands, responses Sample commands: Sample return codes r sent as ASCII text over r status code and phrase (as control channel r USER username r PASS password r LIST return list of file in r r current directory r RETR filename retrieves r r STOR filename stores r (gets) file (puts) file onto remote host in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file 2: Application Layer 67

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 68

Electronic Mail outgoing message queue user mailbox user agent Three major components: r user agents r mail servers mail server SMTP r simple mail transfer protocol: SMTP User Agent r a. k. a. “mail reader” r composing, editing, reading mail messages r e. g. , Eudora, Outlook, elm, Mozilla Thunderbird r outgoing, incoming messages stored on server SMTP mail server user agent SMTP user agent mail server user agent 2: Application Layer 69

Electronic Mail: mail servers user agent Mail Servers r mailbox contains incoming messages for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send email messages v client: sending mail server v “server”: receiving mail server SMTP mail server user agent SMTP user agent mail server user agent 2: Application Layer 70

Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer email message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer v handshaking (greeting) v transfer of messages v closure r command/response interaction v commands: ASCII text v response: status code and phrase r messages must be in 7 -bit ASCII 2: Application Layer 71

Scenario: Alice sends message to Bob 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message 1) Alice uses UA to compose message and “to” bob@someschool. edu 2) Alice’s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 1 user agent 2 mail server 3 mail server 4 5 6 user agent 2: Application Layer 72

Sample SMTP interaction S: C: S: C: C: C: S: 220 hamburger. edu HELO crepes. fr 250 Hello crepes. fr, pleased to meet you MAIL FROM: <alice@crepes. fr> 250 alice@crepes. fr. . . Sender ok RCPT TO: <bob@hamburger. edu> 250 bob@hamburger. edu. . . Recipient ok DATA 354 Enter mail, end with ". " on a line by itself Do you like ketchup? How about pickles? . 250 Message accepted for delivery QUIT 221 hamburger. edu closing connection 2: Application Layer 73

Try SMTP interaction for yourself: r telnet servername 25 r see 220 reply from server r enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader) 2: Application Layer 74

SMTP: final words r SMTP uses persistent connections r SMTP requires message (header & body) to be in 7 bit ASCII r SMTP server uses CRLF to determine end of message Comparison with HTTP: r HTTP: pull r SMTP: push r both have ASCII command/response interaction, status codes r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in multipart msg 2: Application Layer 75

Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: r header lines, e. g. , To: v From: v Subject: different from SMTP commands! v header blank line body r body v the “message”, ASCII characters only 2: Application Layer 76

Message format: multimedia extensions r MIME: multimedia mail extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: alice@crepes. fr To: bob@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data 2: Application Layer 77

MIME types Content-Type: type/subtype; parameters Text r example subtypes: plain, html Image r example subtypes: jpeg, gif Audio r example subtypes: basic (8 -bit mu-law encoded), 32 kadpcm (32 kbps coding) Video r example subtypes: mpeg, quicktime Application r other data that must be processed by reader app before “viewable” r example subtypes: msword, octet-stream 2: Application Layer 78

Multipart Type From: alice@crepes. fr To: bob@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Type: multipart/mixed; boundary=Start. Of. Next. Part --Start. Of. Next. Part Dear Bob, Please find a picture of a crepe. --Start. Of. Next. Part Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data --Start. Of. Next. Part Do you want the recipe? 2: Application Layer 79

Mail access protocols user agent SMTP sender’s mail server access protocol user agent receiver’s mail server r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server v v v POP: 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: Gmail, Hotmail, Yahoo! Mail, Lehigh’s IMP, etc. 2: Application Layer 80

POP 3 protocol authorization phase r client commands: v v user: declare username pass: password r server responses v v +OK -ERR transaction phase, client: r list: list message numbers r retr: retrieve message by number r dele: delete r quit S: C: S: +OK POP 3 server ready user bob +OK pass hungry +OK user successfully logged C: S: S: S: C: C: S: list 1 498 2 912. retr 1 <message 1 contents>. dele 1 retr 2 <message 1 contents>. dele 2 quit +OK POP 3 server signing off 2: Application Layer 81 on

POP 3 (more) and IMAP More about POP 3 r Previous example uses “download and delete” mode. r Bob cannot re-read email if he changes client r “Download-and-keep”: copies of messages on different clients r POP 3 is stateless across sessions IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders r IMAP keeps user state across sessions: v names of folders and mappings between message IDs and folder name 2: Application Layer 82

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP r 2. 9 Building a Web server 2: Application Layer 83

DNS: Domain Name System People: many identifiers: v SSN, name, passport # Internet hosts, routers: v v IP address (32 bit) used for addressing datagrams “name”, e. g. , ww. yahoo. com - used by humans Q: map between IP addresses and name ? Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) v note: core Internet function, implemented as application-layer protocol v complexity at network’s “edge” 2: Application Layer 84

DNS services r hostname to IP address translation r host aliasing v Canonical, alias names r mail server aliasing r load distribution v replicated Web servers: set of IP addresses for one canonical name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! 2: Application Layer 85

Distributed, Hierarchical Database Root DNS Servers com DNS servers yahoo. com amazon. com DNS servers org DNS servers pbs. org DNS servers edu DNS servers poly. edu umass. edu DNS servers Client wants IP for www. amazon. com; 1 st approx: r client queries a root server to find com DNS server r client queries com DNS server to get amazon. com DNS server r client queries amazon. com DNS server to get IP address for www. amazon. com 2: Application Layer 86

DNS: Root name servers r contacted by local name server that can not resolve name r root name server: v v v contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US Do. D Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) e NASA Mt View, CA f Internet Software C. Palo Alto, k RIPE London (also 16 other locations) i Autonomica, Stockholm (plus 28 other locations) m WIDE Tokyo (also Seoul, Paris, SF) CA (and 36 other locations) 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 2: Application Layer 87

TLD and Authoritative Servers r Top-level domain (TLD) servers: v responsible for com, org, net, edu, etc, and all top -level country domains uk, fr, ca, jp. v Network Solutions maintains servers for com TLD v Educause for edu TLD r Authoritative DNS servers: v organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e. g. , Web, mail). v can be maintained by organization or service provider 2: Application Layer 88

Local Name Server r does not strictly belong to hierarchy r each ISP (residential ISP, company, university) has one. v also called “default name server” r when host makes DNS query, query is sent to its local DNS server v acts as proxy, forwards query into hierarchy 2: Application Layer 89

DNS name resolution example root DNS server 2 r Host at cis. poly. edu wants IP address for gaia. cs. umass. edu iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” 3 4 TLD DNS server 5 local DNS server dns. poly. edu 1 8 requesting host 7 6 authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 2: Application Layer 90

DNS name resolution example recursive query: root DNS server 2 r puts burden of name resolution on contacted name server r heavy load? 3 7 local DNS server dns. poly. edu 1 6 TLD DNS server 5 4 8 requesting host authoritative DNS server dns. cs. umass. edu cis. poly. edu gaia. cs. umass. edu 2: Application Layer 91

DNS: caching and updating records r once (any) name server learns mapping, it caches mapping v cache entries timeout (disappear) after some time v TLD servers typically cached in local name servers • Thus root name servers not often visited r update/notify mechanisms under design by IETF v RFC 2136 v http: //www. ietf. org/html. charters/dnsind-charter. html 2: Application Layer 92

DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) r Type=A v name is hostname v value is IP address r Type=CNAME v name is alias name for some “canonical” (the real) name www. ibm. com is really r Type=NS servereast. backup 2. ibm. com v name is domain (e. g. v value is canonical name foo. com) v value is hostname of r Type=MX authoritative name server v value is name of mailserver for this domain associated with name 2: Application Layer 93

DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: v query or reply v recursion desired v recursion available v reply is authoritative 2: Application Layer 94

DNS protocol, messages Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer 95

Inserting records into DNS r example: new startup “Network Utopia” r register name networkuptopia. com at DNS registrar (e. g. , Network Solutions) v v provide names, IP addresses of authoritative name server (primary and secondary) registrar inserts two RRs into com TLD server: (networkutopia. com, dns 1. networkutopia. com, NS) (dns 1. networkutopia. com, 212. 1, A) r create authoritative server Type A record for www. networkuptopia. com; Type MX record for networkutopia. com r How do people get IP address of your Web site? 2: Application Layer 96

DNS Tools nslookup: r can find IP given name r can find name given IP r can show other RR dig and host: Show more detail Often available on Suns, Linux q Online: q q http: //www. ip-plus. net/tools/ dns_config. en. html q whois: r can show information about domain and owner r can show information about “owner” of an IP address or network r online whois servers v v http: //www. arin. net/whois/index. html And others… 2: Application Layer 97

Content distribution networks (CDNs) Content replication origin server in North America r challenging to stream large files (e. g. , video) from single origin server in real time r solution: replicate content at hundreds of servers throughout Internet v content downloaded to CDN servers ahead of time v placing content “close” to user avoids impairments (loss, delay) of sending content over long paths v CDN server typically in edge/access network CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia 2: Application Layer 98

Content distribution networks (CDNs) Content replication r CDN (e. g. , Akamai) customer is the content provider (e. g. , CNN) r CDN replicates customers’ content in CDN servers. r when provider updates content, CDN updates servers origin server in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia 2: Application Layer 99

CDN example 1 HTTP request for www. foo. com/sports. html origin server 2 client 3 DNS query for www. cdn. com CDN’s authoritative DNS server HTTP request for www. cdn. com/www. foo. com/sports/ruth. gif CDN server near client origin server (www. foo. com) r distributes HTML r replaces: CDN company (cdn. com) r distributes gif files r uses its authoritative DNS http: //www. foo. com/sports. ruth. gif with server to route redirect requests http: //www. cdn. com/www. foo. com/sports/ruth. gif 2: Application Layer 100

More about CDNs routing requests r CDN creates a “map”, indicating distances from leaf ISPs and CDN nodes r when query arrives at authoritative DNS server: server determines ISP from which query originates v uses “map” to determine best CDN server v r CDN nodes create application-layer overlay network 2: Application Layer 101

Chapter 2: Application layer r 2. 1 Principles of network applications v v app architectures app requirements r 2. 2 Web and HTTP r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP r 2. 5 DNS 2: Application Layer 102

P 2 P file sharing Example r Alice runs P 2 P client application on her notebook computer r intermittently connects to Internet; gets new IP address for each connection r asks for “Hey Jude” r application displays other peers that have copy of Hey Jude. r Alice chooses one of the peers, Bob. r file is copied from Bob’s PC to Alice’s notebook: HTTP r while Alice downloads, other users uploading from Alice. r Alice’s peer is both a Web client and a transient Web server. All peers are servers = highly scalable! 2: Application Layer 103

P 2 P: centralized directory original “Napster” design 1) when peer connects, it informs central server: v v Bob centralized directory server 1 peers IP address content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob 1 3 1 2 1 Alice 2: Application Layer 104

P 2 P: problems with centralized directory r single point of failure r performance bottleneck r copyright infringement: “target” of lawsuit is obvious file transfer is decentralized, but locating content is highly centralized 2: Application Layer 105

Query flooding: Gnutella r fully distributed v no central server r public domain protocol r many Gnutella clients implementing protocol overlay network: graph r edge between peer X and Y if there’s a TCP connection r all active peers and edges form overlay net r edge: virtual (not physical) link r given peer typically connected with < 10 overlay neighbors 2: Application Layer 106

Gnutella: protocol r Query message sent over existing TCP connections r peers forward Query message ry e r Query. Hit it Qu H ry sent over e Qu reverse Query path File transfer: HTTP Query. Hit Qu ery Query. Hit Scalability: limited scope flooding Qu er y 2: Application Layer 107

Gnutella: Peer joining peer Alice must find another peer in Gnutella network: use list of candidate peers 2. Alice sequentially attempts TCP connections with candidate peers until connection setup with Bob 3. Flooding: Alice sends Ping message to Bob; Bob forwards Ping message to his overlay neighbors (who then forward to their neighbors…. ) r peers receiving Ping message respond to Alice with Pong message 4. Alice receives many Pong messages, and can then setup additional TCP connections Peer leaving: see homework problem! 1. 2: Application Layer 108

Comparing Client-server, P 2 P architectures Question : How much time to distribute file initially at one server to N other computers? us: server upload bandwidth Server us File, size F d. N u 1 d 1 u 2 ui: client/peer i upload bandwidth d 2 di: client/peer i download bandwidth Network (with abundant bandwidth) 2: Application Layer 109

Client-server: file distribution time r server sequentially sends N copies: v NF/us time r client i takes F/di time to download Server F us d. N u 1 d 1 u 2 d 2 Network (with abundant bandwidth) Time to distribute F to N clients using = dcs = max { NF/us, F/min(di) } i client/server approach increases linearly in N (for large N) 2: Application Layer 110

P 2 P: file distribution time r server must send one Server F u 1 d 1 u 2 d 2 copy: F/us time us r client i takes F/di time Network (with d. N to download abundant bandwidth) u. N r NF bits must be downloaded (aggregate) r fastest possible upload rate (assuming all nodes sending file chunks to same peer): us + Sui i=1, N S ui) } i=1, N d. P 2 P = max { F/us, F/min(di) , NF/(us + i 2: Application Layer 111

Comparing Client-server, P 2 P architectures 2: Application Layer 112

P 2 P Case Study: Bit. Torrent r P 2 P file distribution tracker: tracks peers participating in torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer 2: Application Layer 113

Bit. Torrent (1) r file divided into 256 KB chunks. r peer joining torrent: has no chunks, but will accumulate them over time v registers with tracker to get list of peers, connects to subset of peers (“neighbors”) r while downloading, peer uploads chunks to other peers may come and go r once peer has entire file, it may (selfishly) leave or (altruistically) remain v 2: Application Layer 114

Bit. Torrent (2) Pulling Chunks r at any given time, different peers have different subsets of file chunks r periodically, a peer (Alice) asks each neighbor for list of chunks that they have. r Alice issues requests for her missing chunks v rarest first Sending Chunks: tit-for-tat r Alice sends chunks to four neighbors currently sending her chunks at the highest rate v re-evaluate top 4 every 10 secs r every 30 secs: randomly select another peer, starts sending chunks v newly chosen peer may join top 4 2: Application Layer 115

P 2 P Case study: Skype clients (SC) r P 2 P (pc-to-pc, pc-to- phone, phone-to-pc) Voice-Over-IP (Vo. IP) Skype application login server v also IM r proprietary application -layer protocol (inferred via reverse engineering) r hierarchical overlay Supernode (SN) 2: Application Layer 116

Skype: making a call r User starts Skype r SC registers with SN v list of bootstrap SNs r SC logs in Skype login server (authenticate) r Call: SC contacts SN with callee ID v SN contacts other SNs (unknown protocol, maybe flooding) to find addr of callee; returns addr to SC r SC directly contacts callee, over TCP 2: Application Layer 117

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP r 2. 3 FTP r 2. 4 Electronic Mail v r 2. 6 P 2 P file sharing r 2. 7 Socket programming with TCP r 2. 8 Socket programming with UDP SMTP, POP 3, IMAP r 2. 5 DNS 2: Application Layer 118

Socket programming with UDP: no “connection” between client and server r no handshaking r sender explicitly attaches IP address and port of destination to each packet r server must extract IP address, port of sender from received packet application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server UDP: transmitted data may be received out of order, or lost 2: Application Layer 119

Client/server socket interaction: UDP Server (running on hostid) create socket, port=x, for incoming request: server. Socket = Datagram. Socket() read request from server. Socket write reply to server. Socket specifying client host address, port number Client create socket, client. Socket = Datagram. Socket() Create, address (hostid, port=x, send datagram request using client. Socket read reply from client. Socket close client. Socket 2: Application Layer 120

Example: Java client (UDP) Client process Input: receives packet (recall that. TCP received “byte stream”) Output: sends packet (recall that TCP sent “byte stream”) client UDP socket 2: Application Layer 121

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 { Buffered. Reader in. From. User = new Buffered. Reader(new Input. Stream. Reader(System. in)); Datagram. Socket client. Socket = new Datagram. Socket(); Inet. Address IPAddress = Inet. Address. get. By. Name("hostname"); byte[] send. Data = new byte[1024]; byte[] receive. Data = new byte[1024]; String sentence = in. From. User. read. Line(); send. Data = sentence. get. Bytes(); 2: Application Layer 122

Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Send datagram to server Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, 9876); client. Socket. send(send. Packet); Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); Read datagram from server client. Socket. receive(receive. Packet); String modified. Sentence = new String(receive. Packet. get. Data()); System. out. println("FROM SERVER: " + modified. Sentence); client. Socket. close(); } } 2: Application Layer 123

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 { Datagram. Socket server. Socket = new Datagram. Socket(9876); byte[] receive. Data = new byte[1024]; byte[] send. Data = new byte[1024]; while(true) { Create space for received datagram Receive datagram Datagram. Packet receive. Packet = new Datagram. Packet(receive. Data, receive. Data. length); server. Socket. receive(receive. Packet); 2: Application Layer 124

Example: Java server (UDP), cont String sentence = new String(receive. Packet. get. Data()); Get IP addr port #, of sender Inet. Address IPAddress = receive. Packet. get. Address(); int port = receive. Packet. get. Port(); String capitalized. Sentence = sentence. to. Upper. Case(); send. Data = capitalized. Sentence. get. Bytes(); Create datagram to send to client Write out datagram to socket } Datagram. Packet send. Packet = new Datagram. Packet(send. Data, send. Data. length, IPAddress, port); server. Socket. send(send. Packet); } } End of while loop, loop back and wait for another datagram 2: Application Layer 125

Example: C echo client (UDP) […headers…] int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc != 2) { printf("usage: udpcli <IPaddress>"); exit(-1); } bzero(&servaddr, sizeof(servaddr)); servaddr. sin_family = AF_INET; servaddr. sin_port = htons(6789); servaddr. sin_addr. s_addr = inet_addr(argv[1]); Set up socket parameters Create UDP socket sockfd = socket(AF_INET, SOCK_DGRAM, 0); Work with the established socket dg_cli(stdin, sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); exit(0); } 2: Application Layer 126

Example: C echo client (UDP) cont. Get line from STDIN void dg_cli(FILE *fp, int sockfd, const struct sockaddr *pservaddr, int servlen) { int n; char sendline[MAXLINE], recvline[MAXLINE + 1]; while (fgets(sendline, MAXLINE, fp) != NULL) { Write out datagram to socket sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen); n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL); Receive datagram From socket Send line to STDOUT recvline[n] = 0; /* null terminate */ fputs(recvline, stdout); } } 2: Application Layer 127

Example: C echo server (UDP) #include <sys/types. h> /* basic system data types */ #include <sys/socket. h> /* basic socket definitions */ #include <netinet/in. h> #include <stdio. h> #include <unistd. h> #define MAXLINE 1024 Create UDP socket Bind socket to specified address Work with the established socket int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr, cliaddr; sockfd = socket(AF_INET, SOCK_DGRAM, 0); bzero(&servaddr, sizeof(servaddr)); servaddr. sin_family = AF_INET; servaddr. sin_addr. s_addr = htonl(INADDR_ANY); servaddr. sin_port = htons(6789); bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); dg_echo(sockfd, (struct sockaddr *) &cliaddr, sizeof(cliaddr)); } 2: Application Layer 128

Example: C echo server (UDP) cont. void dg_echo(int sockfd, struct sockaddr *pcliaddr, int clilen) { int n; int len; char mesg[MAXLINE]; for ( ; ; ) { len = clilen; n = recvfrom(sockfd, mesg, MAXLINE, 0, pcliaddr, &len); Read next datagram Send datagram back to client sendto(sockfd, mesg, n, 0, pcliaddr, len); } } 2: Application Layer 129

Chapter 2: Summary our study of network apps now complete! r application architectures v client-server v P 2 P v hybrid r application service requirements: v reliability, bandwidth, delay r specific protocols: v HTTP v FTP v SMTP, POP, IMAP v DNS v P 2 P: Bit. Torrent, Skype r socket programming r Internet transport service model v v connection-oriented, reliable: TCP unreliable, datagrams: UDP 2: Application Layer 130

Chapter 2: Summary Most importantly: learned about protocols r typical request/reply message exchange: v v client requests info or service server responds with data, status code r message formats: v headers: fields giving info about data v data: info being communicated Important themes: r control vs. data msgs v in-band, out-of-band r centralized vs. decentralized r stateless vs. stateful r reliable vs. unreliable msg transfer r “complexity at network edge” 2: Application Layer 131