Computer Networking: A Top-Down Approach - Chapter 2 Application Layer PDF

Chapter 2 Application Layer A note on the use of these PowerPoint slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your n...

Chapter 2 Application Layer A note on the use of these PowerPoint slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: ▪ If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) ▪ If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Computer Networking: A For a revision history, see the slide note for this page. Top-Down Approach Thanks and enjoy! JFK/KWR 8th edition n All material copyright 1996-2020 Jim Kurose, Keith Ross J.F Kurose and K.W. Ross, All Rights Reserved Pearson, 2020 Application Layer: 2-1 Application layer: overview ▪ Principles of network applications ▪ Web and HTTP ▪ E-mail, SMTP, IMAP ▪ The Domain Name System DNS Application Layer: 2-2 Application layer: overview Our goals: ▪ Learn about protocols by examining popular ▪ Conceptual and application-layer protocols implementation aspects of and infrastructure application-layer protocols HTTP Transport-layer service SMTP, IMAP models DNS Client-server paradigm video streaming systems, CDNs Peer-to-peer paradigm ▪ Programing network applications socket API Application Layer: 2-3 Some network apps ▪ social networking ▪ voice over IP (e.g., Skype) ▪ Web ▪ real-time video conferencing ▪ text messaging (e.g., Zoom) ▪ e-mail ▪ Internet search ▪ multi-user network games ▪… ▪ streaming stored video (YouTube) ▪ P2P file sharing Q: your favorites? Application Layer: 2-4 Creating a network app application transport write programs that: mobile network network data link physical ▪ run on (different) end-systems national or global ISP ▪ communicate over network ▪ e.g., web server software communicates with browser software local or no need to write software for regional ISP network-core devices home network content application ▪ network-core devices do not run user transport network provider network datacenter application applications data link physical transport network network ▪ applications on end-systems allow for data link physical rapid app development and enterprise propagation network Application Layer: 2-5 Client-server paradigm server: mobile network ▪ “always-on” host national or global ISP ▪ permanent IP address ▪ often in data centers clients: local or regional ISP ▪ contact, communicate with server ▪ may be intermittently connected home network content provider ▪ may have dynamic IP addresses network datacenter network ▪ do not communicate directly with each other enterprise ▪ examples: HTTP, IMAP, FTP network Application Layer: 2-6 Data centers ▪ A popular social-networking site can quickly become overwhelmed if it has only one server to handle all requests ▪ For this reason, a data center, housing a large number of hosts, is often used to create a powerful virtual server ▪ The most popular Internet services—such as search engines (e.g., Google, Bing, Baidu), Internet commerce (e.g., Amazon, eBay, Alibaba), Web-based e-mail (e.g., Gmail and Yahoo Mail), social networking (e.g., Facebook, Instagram, Twitter, and WeChat)—employ one or more data centers ▪ Google has 30 to 50 data centers distributed around the world, which collectively handle search, YouTube, Gmail, and other services. A data center can have hundreds of thousands of servers, which must be powered and maintained Application Layer: 2-7 Peer-to-Peer (P2P) architecture ▪ no “always-on” server mobile network ▪ arbitrary end-systems directly national or global ISP communicate ▪ peers request service from other peers, provide service in return to other peers local or regional ISP self scalability – new peers bring new service capacity, as well as new service home network content demands provider network datacenter ▪ peers are intermittently connected network and change IP addresses complex management enterprise ▪ example: P2P file sharing network Application Layer: 2-8 Hybrid architecture ▪ Some applications have hybrid architectures, combining both client- server and P2P elements ▪ For example, for many instant messaging applications, servers are used to track the IP addresses of users, but user-to-user messages are sent directly between user hosts (without passing through intermediate servers) Application Layer: 2-9 Processes communicating In the jargon of operating systems, it clients, servers is actually not programs but client process: process that processes that communicate initiates communication process: program running within a server process: process host that waits to be contacted ▪ processes in different hosts communicate by exchanging ▪ note: applications with messages P2P architectures have client processes & server processes Application Layer: 2-10  Sockets ▪ process sends/receives messages to/from its socket ▪ socket analogous to door sending process shoves the message outdoor sending process relies on the transport infrastructure on the other side of the door to deliver the message to the socket at the receiving process two sockets involved: one on each side application application socket controlled by process process app developer transport transport network network controlled link by OS link Internet physical physical Application Layer: 2-11 Addressing processes ▪ to receive messages, process ▪ identifier includes both IP address must have an identifier and port numbers associated with ▪ host device has unique 32-bit process on host IP address ▪ example port numbers: ▪ Q: does IP address of host on HTTP server: 80 which process runs suffice for mail server: 25 identifying the process? ▪ to send HTTP message to ▪ A: no, many processes gaia.cs.umass.edu web server: can be running on IP address: 128.119.245.12 same host port number: 80 Application Layer: 2-12 Processes ▪ The application developer (programmer) has control of everything on the application layer side of the socket but has little control on the transport-layer side of the socket ▪ The only control that the application developer has on the transport- layer side is (1) the choice of transport protocol and (2) perhaps the ability to fix a few transport-layer parameters such as maximum buffer and maximum segment sizes Application Layer: 2-13 An application-layer protocol defines: ▪ types of messages exchanged, open protocols: e.g., request, response ▪ defined in RFCs, everyone ▪ message syntax: has access to protocol what fields in messages & how definition fields are delineated ▪ allows for interoperability ▪ message semantics ▪ e.g., HTTP, SMTP meaning of information in fields proprietary protocols: ▪ rules for when and how processes ▪ e.g., Skype, Zoom send & respond to messages Application Layer: 2-14 What transport service does an app need? data integrity throughput ▪ some apps (e.g., file transfer, ▪ some apps (e.g., multimedia) web transactions) require require minimum amount of 100% reliable data transfer throughput to be “effective” ▪ other apps (e.g., audio) can ▪ other apps (“elastic apps”) tolerate some loss make use of whatever throughput they get timing ▪ some apps (e.g., Internet security telephony, interactive games) ▪ encryption, data integrity, etc. require low delay to be “effective” Application Layer: 2-15  Transport service requirements: common apps application data loss throughput time sensitive? file transfer/download no loss elastic no e-mail no loss elastic no Web documents no loss elastic no real-time audio/video loss-tolerant audio: 5Kbps-1Mbps yes, 10’s msec video:10Kbps-5Mbps streaming audio/video loss-tolerant same as above yes, few secs interactive games loss-tolerant Kbps+ yes, 10’s msec text messaging no loss elastic yes and no Application Layer: 2-16 TCP and UDP ▪ Internet makes two distinct transport-layer protocols available to the application layer ▪ One of these protocols is UDP (User Datagram Protocol), which provides an unreliable, connectionless service to the invoking application. ▪ The second of these protocols is TCP (Transmission Control Protocol), which provides a reliable, connection-oriented service to the invoking application Application Layer: 2-17 Internet transport protocols services TCP service: UDP service: ▪ reliable transport between sending ▪ unreliable data transfer and receiving processes between sending and receiving ▪ flow control: sender won’t processes overwhelm receiver ▪ does not provide: reliability, ▪ congestion control: throttle sender flow control, congestion when network is overloaded control, timing, throughput guarantee, security, or ▪ connection-oriented: setup required connection setup. between client and server processes ▪ does not provide: timing, minimum Q: why bother? Why throughput guarantee, security is there a UDP? Application Layer: 2-18  Internet applications, and transport protocols application application layer protocol transport protocol file transfer/download FTP [RFC 959] TCP e-mail SMTP [RFC 5321] TCP Web documents HTTP 1.1 [RFC 7320] TCP Internet telephony SIP [RFC 3261], RTP [RFC TCP or UDP 3550], or proprietary streaming audio/video HTTP [RFC 7320], DASH TCP interactive games WOW, FPS (proprietary) UDP or TCP Application Layer: 2-19 Securing TCP TCP & UDP sockets: TLS implemented in ▪ no encryption application layer ▪ cleartext passwords sent into socket ▪ apps use TSL libraries, that traverse Internet in cleartext (!) use TCP in turn Transport Layer Security (TLS) ▪ cleartext sent into “socket” ▪ provides encrypted TCP connections traverse Internet encrypted ▪ data integrity ▪ end-point authentication Application Layer: 2-20  21 Socket Programming using Python: Example App (For information only) Python UDPServer “UDPServer.py” from socket import * serverPort = 12000 create UDP socket serverSocket = socket(AF_INET, SOCK_DGRAM) bind socket to local port serverSocket.bind((‘ ', serverPort)) number 12000 The method recvfrom takes print (‘The server is ready to receive’) the buffer size 2048 as input loop forever while 1: Read from UDP socket into message, clientAddress = serverSocket.recvfrom(2048) message, getting client’s address (client IP and port) modifiedMessage = message.upper() send upper case string serverSocket.sendto(modifiedMessage, clientAddress) back to this client To just show the IP address and print(clientAddress) port number of client (optional) Socket Programming using Python: Example App 22 (For information only) Python UDPClient “UDPClient.py” include Python’s socket library from socket import * serverName = ‘hostname’ serverPort = 12000 create UDP socket for clientSocket = socket(AF_INET,SOCK_DGRAM) client get user keyboard message = input(’Input lowercase sentence:’) input clientSocket.sendto(message.encode('ascii'),(serverName, serverPort)) Attach server name, port to message; send into socket modifiedMessage, serverAddress = read reply characters from clientSocket.recvfrom(2048) socket into string print (modifiedMessage.decode(‘ascii’)) print out received string clientSocket.close() and close socket Application layer: overview ▪ Principles of network applications ▪ Web and HTTP ▪ E-mail, SMTP, IMAP ▪ The Domain Name System DNS Application Layer: 2-23 Web and HTTP First, a quick review… ▪ web page consists of objects, each of which can be stored on different Web servers ▪ object can be HTML file, JPEG image, Java applet, audio file,… ▪ web page consists of base HTML-file which includes several referenced objects, each addressable by a URL, e.g., www.someschool.edu/someDept/pic.gif host name path name Application Layer: 2-24 HTTP overview HTTP: hypertext transfer protocol ▪ Web’s application-layer protocol ▪ client/server model: PC running client: browser that requests, Firefox browser receives, (using the HTTP protocol) and “displays” Web objects server running server: Web server sends (using Apache Web the HTTP protocol) objects in server response to requests iPhone running Safari browser Application Layer: 2-25 HTTP overview (continued) HTTP uses TCP: HTTP is “stateless” ▪ client initiates TCP connection ▪ server maintains no (creates socket) to server, port 80 information about past client ▪ server accepts TCP connection requests from client aside protocols that maintain “state” ▪ HTTP messages (application-layer are complex! protocol messages) exchanged ▪ past history (state) must be between browser (HTTP client) and maintained Web server (HTTP server) ▪ if server/client crashes, their views ▪ TCP connection closed of “state” may be inconsistent, must be reconciled Application Layer: 2-26 HTTP connections: two types Non-persistent HTTP Persistent HTTP 1. TCP connection opened ▪TCP connection opened to 2. at most one object sent a server over TCP connection ▪multiple objects can be 3. TCP connection closed sent over single TCP connection between client, downloading multiple and that server objects required multiple ▪TCP connection closed connections Application Layer: 2-27  Non-persistent HTTP: example User enters URL: www.someSchool.edu/someDepartment/home.index (containing text, references to 10 jpeg images) 1a. HTTP client initiates TCP connection to HTTP server 1b. HTTP server at host (process) at www.someSchool.edu on www.someSchool.edu waiting for TCP port 80 connection at port 80 “accepts” connection, notifying client 2. HTTP client sends HTTP request message (containing URL) into TCP connection 3. HTTP server receives request message, socket. Message indicates forms response message containing time that client wants object requested object, and sends message someDepartment/home.index into its socket Application Layer: 2-28  Non-persistent HTTP: example (cont.) User enters URL: www.someSchool.edu/someDepartment/home.index (containing text, references to 10 jpeg images) 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 6. Steps 1-5 repeated for each of 10 jpeg objects time Application Layer: 2-29 Non-persistent HTTP: response time RTT (definition): time for a small packet to travel from client to initiate TCP server and back connection RTT HTTP response time (per object): ▪ one RTT to initiate TCP connection request file ▪ one RTT for HTTP request and first few RTT time to transmit bytes of HTTP response to return file file received ▪ object/file transmission time time time Non-persistent HTTP response time = 2RTT+ file transmission time Application Layer: 2-30 Persistent HTTP (HTTP 1.1) Non-persistent HTTP issues: Persistent HTTP (HTTP1.1): ▪ requires 2 RTTs per object ▪ server leaves connection open after ▪ OS overhead for each TCP sending response connection ▪ subsequent HTTP messages ▪ browsers often open multiple between the same client and server parallel TCP connections to are sent over that open connection fetch referenced objects in ▪ client sends requests as soon as it parallel encounters a referenced object ▪ Reduces the response time remarkably Application Layer: 2-31  HTTP request message ▪ two types of HTTP messages: request, response ▪ HTTP request message: ASCII (human-readable format) carriage return character line-feed character request line (GET, POST, GET /index.html HTTP/1.1\r\n HEAD commands) Host: www-net.cs.umass.edu\r\n User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:80.0) Gecko/20100101 Firefox/80.0 \r\n header Accept: text/html,application/xhtml+xml\r\n lines Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Connection: keep-alive\r\n \r\n carriage return, line feed at start of line indicates end of header lines * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Application Layer: 2-32 HTTP request message ▪ A request message can have many lines or just one line ▪ The method field can take on several different values, including GET, POST, HEAD, PUT, and DELETE ▪ The header line Host: www.someschool.edu specifies the host on which the object resides ▪ The majority of HTTP request messages use the GET method indicating that the browser requesting an object ▪ In this example, the browser is requesting the object /somedir/page.html and implements version HTTP/1.1 ▪ By including the Connection: close header line, the browser wants the server to close the connection after sending the requested object (non- persistent) Transport Layer: 3-33 HTTP request message ▪ The User-agent: header line specifies the user agent, that is, the browser type that is making the request to the server ▪ Here the user agent is Mozilla/5.0, a Firefox browser ▪ Finally, the Accept-language: header indicates that the user prefers to receive an English version of the object, if such an object exists on the server ▪ The Accept-language: header is just one of many content negotiation headers available in HTTP Transport Layer: 3-34 HTTP request message: general format method sp URL sp version cr lf request line header field name value cr lf header ~ ~ ~ ~ lines header field name value cr lf cr lf ~ ~ entity body ~ ~ body Application Layer: 2-35 Other HTTP request messages POST method: HEAD method: ▪ web page often allows to input a ▪ requests headers (only) that message/content would be returned if specified ▪ user input sent from client to URL were requested with an server in entity body of HTTP HTTP GET method. POST request message PUT method: ▪ uploads new file (object) to server GET method (for sending data to server): ▪ completely replaces file that exists ▪ include user data in URL field of HTTP at specified URL with content in GET request message (following a ‘?’): entity body of POST HTTP request www.somesite.com/animalsearch?monkeys&banana message Application Layer: 2-36 HTTP response message status line (protocol HTTP/1.1 200 OK status code status phrase) Date: Tue, 08 Sep 2020 00:53:20 GMT Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips PHP/7.4.9 mod_perl/2.0.11 Perl/v5.16.3 header Last-Modified: Tue, 01 Mar 2016 18:57:50 GMT lines ETag: "a5b-52d015789ee9e" Accept-Ranges: bytes Content-Length: 2651 Content-Type: text/html; charset=UTF-8 \r\n data, e.g., requested data data data data data... HTML file Application Layer: 2-37 HTTP response status codes ▪ status code appears in 1st line in server-to-client response message. ▪ some 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 (in Location: field) 400 Bad Request request msg not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported Application Layer: 2-38 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: (Telnet is s network protocol that uses command-line interface) telnet cis.poly.edu 80 opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. anything typed in will be sent to port 80 at cis.poly.edu 2. type in a GET HTTP request: Note: online API GET /~ross/ HTTP/1.1 by typing this in (hit carriage tester tools can also Host: cis.poly.edu return twice), you send help in observing this minimal (but complete) HTTP headers GET request to HTTP server 3. look at response message sent by HTTP server! (or use Wireshark to look at captured HTTP request/response) Transport Layer: 3-39 Maintaining user/server state: cookies Websites and client browser use Example: cookies to maintain some state ▪ Susan uses browser on laptop, visits specific e-commerce site between transactions for first time four components: ▪ when initial HTTP requests 1) cookie header line of HTTP response arrives at site, site creates: message unique ID (aka “cookie”) entry in backend database 2) cookie header line in next HTTP for ID request message subsequent HTTP requests 3) cookie file kept on user’s host, from Susan to this site will managed by user’s browser contain a cookie ID value, 4) back-end database at Web site allowing site to “identify” Susan Application Layer: 2-40 Maintaining user/server state: cookies client server ebay 8734 usual HTTP request msg Amazon server cookie file creates ID usual HTTP response 1678 for user backend create ebay 8734 set-cookie: 1678 entry database amazon 1678 usual HTTP request msg cookie: 1678 cookie- access specific usual HTTP response msg action one week later: access ebay 8734 usual HTTP request msg amazon 1678 cookie: 1678 cookie- specific usual HTTP response msg action time time Application Layer: 2-41 HTTP cookies: comments aside What cookies can be used for: cookies and privacy: ▪ authorization ▪ cookies permit sites to ▪ shopping carts learn a lot about you on their site ▪ recommendations ▪ third party persistent ▪ user session state (Web e-mail) cookies (tracking cookies) allow common identity (cookie value) to be tracked across multiple web sites Application Layer: 2-42 Web caches Goal: satisfy client requests without involving origin server ▪ user configures browser to point to a (local) Web cache Web cache ▪ browser sends all HTTP client origin server requests to cache if object in cache: cache returns object to client else cache requests object client from the origin server, caches received object, then returns object to client Application Layer: 2-43 Web caches (aka proxy servers) ▪ Web cache acts as both Why Web caching? client and server ▪ reduce response time for client server for original requesting client request client to origin server cache is closer to the client ▪ reduce traffic on an institution’s ▪ server tells cache about object’s allowable caching in access link response header: ▪ Internet is dense with caches enables “poor” content providers to more effectively deliver content Application Layer: 2-44 Caching example origin servers Scenario: public ▪ access link rate: 1.54 Mbps Internet ▪ RTT from institutional router to server: 2 sec ▪ web object size: 100K bits ▪ average request rate from browsers to origin 1.54 Mbps access link servers: 15 request/sec institutional ▪ avg data rate to browsers: 1.50 Mbps network 1 Gbps LAN Performance: ▪ access link utilization =(100Kb x 15) /1.54Mbps ▪ access link utilization =.97 problem: large queueing ▪ LAN utilization:.0015 delays at high utilization! ▪ end-end delay = Internet delay + access link delay + LAN delay = 2 sec + minutes + micro secs Application Layer: 2-45 Option 1: buy a faster access link Scenario: 154 Mbps ▪ access link rate: 1.54 Mbps origin ▪ RTT from institutional router to server: 2 sec servers ▪ web object size: 100K bits public Internet ▪ average request rate from browsers to origin servers: 15/sec ▪ avg data rate to browsers: 1.50 Mbps 154 Mbps 1.54 Mbps access link Performance: ▪ access link utilization =.97.0097 institutional network 1 Gbps LAN ▪ LAN utilization:.0015 ▪ end-end delay = Internet delay + access link delay + LAN delay = 2 sec + minutes + micro secs Cost: faster access link (expensive!) msecs Application Layer: 2-46 Option 2: install a web cache Scenario: ▪ access link rate: 1.54 Mbps origin ▪ RTT from institutional router to server: 2 sec servers ▪ web object size: 100K bits public Internet ▪ average request rate from browsers to origin servers: 15/sec ▪ avg data rate to browsers: 1.50 Mbps 1.54 Mbps access link Cost: web cache (cheap!) institutional network Performance: 1 Gbps LAN ▪ LAN utilization:.? How to compute link ▪ access link utilization = ? utilization, delay? ▪ average end-end delay = ? local web cache Application Layer: 2-47 Calculating access link utilization, end-end delay with cache: suppose cache hit rate is 0.4: ▪ 40% requests served by cache, with low origin servers (msec) delay public ▪ 60% requests satisfied at origin Internet rate to browsers over access link = 0.6 x 1.50 Mbps = 0.9 Mbps 1.54 Mbps access link utilization = 0.9/1.54 =.58 means access link low (msec) queueing delay at access link institutional ▪ average end-to-end delay: network 1 Gbps LAN = 0.6 (delay from origin servers) + 0.4 (delay when satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs local web cache lower average end-end delay than with 154 Mbps link (and cheaper too!) Application Layer: 2-48 Conditional GET client server Goal: don’t send object if cache has HTTP request msg up-to-date cached version If-modified-since: object not no object transmission delay (or use modified of network resources) HTTP response before HTTP/1.0 ▪ client: specify date of cached copy 304 Not Modified in HTTP request If-modified-since: ▪ server: response contains no HTTP request msg If-modified-since: object object if cached copy is up-to-date: modified HTTP/1.0 304 Not Modified HTTP response after HTTP/1.0 200 OK Application Layer: 2-49 HTTP Types HTTP/1.0: HTTP/1.1: ▪ GET ▪ GET, POST, HEAD ▪ POST ▪ PUT ▪ HEAD uploads file in entity body Similar to GET but asks the to path specified in URL server to leave requested field object out of response ▪ DELETE (often used for debugging) deletes file specified in the URL field Transport Layer: 3-50 HTTP/2 Key goal: decreased delay in multi-object HTTP requests HTTP1.1: introduced multiple, pipelined GETs over a single TCP connection ▪ server responds in-order (FCFS: first-come-first-serve scheduling) to GET requests ▪ with FCFS, small object may have to wait for transmission (head-of- line (HOL) blocking) behind large object(s) Application Layer: 2-51 HTTP/2 Key goal: decreased delay in multi-object HTTP requests HTTP/2: [RFC 7540, 2015] increased flexibility at the server in sending objects to client: ▪ methods, status codes, most header fields unchanged from HTTP 1.1 ▪ transmission order of requested objects based on client-specified object priority (not necessarily FCFS) ▪ divide objects into frames, schedule frames to mitigate HOL blocking Application Layer: 2-52 HTTP/2: mitigating HOL blocking HTTP 1.1: client requests 1 large object (e.g., video file) and 3 smaller objects server GET O4 GET O3 GET O 2 GET O1 object data requested client O1 O2 O1 O3 O2 O3 O4 O4 objects delivered in order requested: O2, O3, O4 wait behind O1 Application Layer: 2-53 HTTP/2: mitigating HOL blocking HTTP/2: objects divided into frames, frame transmission interleaved server GET O4 GET O3 GET O 2 GET O1 object data requested client O2 O4 O3 O1 O2 O3 O1 O4 O2, O3, O4 delivered quickly, O1 slightly delayed Application Layer: 2-54 HTTP/2 to HTTP/3 ▪ HTTP/3: adds security, per object error- and congestion- control (more pipelining) over UDP Application Layer: 2-55 Application layer: overview ▪ Principles of network applications ▪ Web and HTTP ▪ E-mail, SMTP, IMAP ▪ The Domain Name System DNS Application Layer: 2-56 E-mail user agent Three major components: mail user ▪ user agents server agent ▪ mail servers SMTP mail user agent ▪ simple mail transfer protocol: SMTP SMTP server SMTP user User Agent mail agent ▪ a.k.a. “mail reader” server user ▪ composing, editing, reading mail messages agent user ▪ e.g., Outlook, iPhone mail client agent outgoing ▪ outgoing, incoming messages stored on message queue server user mailbox Application Layer: 2-57 E-mail: mail servers user agent mail servers: mail user server ▪ mailbox contains incoming agent messages for user SMTP mail user server agent ▪ message queue of outgoing (to be SMTP sent) mail messages user SMTP agent SMTP protocol between mail mail server servers to send email messages user agent ▪ client: sending mail server user ▪ “server”: receiving mail server agent outgoing message queue user mailbox Application Layer: 2-58 SMTP RFC (5321) “client” SMTP server “server” SMTP server ▪ uses TCP to reliably transfer email message initiate TCP from the client (mail server initiating connection connection) to server, port 25 RTT TCP connection ▪ direct transfer: sending server (acting like client) initiated to receiving server ▪ three phases of transfer 220 SMTP handshaking (greeting) SMTP HELO handshaking SMTP transfer of messages 250 Hello SMTP closure ▪ command/response interaction (like HTTP) SMTP commands: ASCII text transfers response: status code and phrase time Application Layer: 2-59 Scenario: Alice sends e-mail to Bob 1) Alice uses UA to compose e-mail 4) SMTP client sends Alice’s message message “to” [email protected] over the TCP connection 2) Alice’s UA sends message to her 5) Bob’s mail server places mail server using SMTP; message the message in Bob’s placed in message queue mailbox 3) client side of SMTP at mail server 6) Bob invokes his user opens TCP connection with Bob’s mail agent to read message server 1 user mail user mail agent agent server server 2 3 6 4 5 Alice’s mail server Bob’s mail server Application Layer: 2-60 Sample SMTP interaction (For information only) S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 [email protected]... Sender ok C: RCPT TO: S: 250 [email protected]... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection s: server c: client Application Layer: 2-61 SMTP: observations comparison with HTTP: ▪ SMTP uses persistent connections ▪ HTTP: client pull ▪ SMTP requires message ▪ SMTP: client push (header & body) to be in ▪ both have ASCII command/response 7-bit ASCII interaction, status codes ▪ SMTP server uses carriage return line feed (CRLF.CRLF) to determine end of message Application Layer: 2-62 Mail message format SMTP: protocol for exchanging e-mail messages, defined in RFC 5321 (like RFC 7231 defines HTTP) RFC 2822 defines syntax for e-mail message itself (like HTML defines syntax for web documents) ▪ header lines, e.g., header blank To: line From: Subject: these lines, within the body of the email body message area different from SMTP MAIL FROM:, RCPT TO: commands! ▪ Body: the “message” , ASCII characters only Application Layer: 2-63  Retrieving email: mail access protocols user e-mail access user SMTP SMTP protocol agent agent (e.g., IMAP, HTTP) sender’s e-mail receiver’s e-mail server server ▪ SMTP: delivery/storage of e-mail messages to receiver’s server ▪ mail access protocol: retrieval from server IMAP: Internet Mail Access Protocol [RFC 3501]: messages stored on server, IMAP provides retrieval, deletion, folders of stored messages on server ▪ HTTP: gmail, Hotmail, Yahoo! Mail, etc. provides web-based interface on top of SMTP (to send), IMAP (or POP) to retrieve e-mail messages Application Layer: 2-64 More on E-mail Protocols ▪ Post Office Protocol (POP3) is an extremely simple mail access protocol. Because the protocol is so simple, its functionality is rather limited. POP3 begins when the user agent (the client) opens a TCP connection to the mail server (the server) on port 110 ▪ With the TCP connection established, POP3 progresses through three phases: authorization (password), transaction (send message), and update (end session) ▪ Internet Mail Access Protocol (IMAP): has advanced features compared to POP3, more complex ▪ Web-based email: user access the mailbox using HTTP, however, messages are exchanged between mail servers using SMTP Application Layer: 2-65 Application Layer: Overview ▪ Principles of network applications ▪ Web and HTTP ▪ E-mail, SMTP, IMAP ▪ The Domain Name System DNS Application Layer: 2-66 DNS: Domain Name System people: many identifiers: Domain Name System (DNS): SSN, name, passport # ▪ distributed database implemented in Internet hosts, routers: hierarchy of many name servers IP address (32 bit) - used for ▪ application-layer protocol: hosts, DNS addressing datagrams servers communicate to resolve “name”, e.g., cs.umass.edu - names (address/name translation) used by humans note: core Internet function, Q: how to map between IP implemented as application-layer address and website name, protocol and vice versa ? complexity at the network’s “edge” (at servers not core network routers) Application Layer: 2-67 DNS: services, structure DNS services: Q: Why not centralize DNS? ▪ hostname-to-IP-address translation ▪ single point of failure ▪ traffic volume ▪ host aliasing ▪ distant centralized database canonical, alias names ▪ maintenance ▪ mail server aliasing ▪ load distribution A: doesn‘t scale! replicated Web servers: many IP ▪ Comcast DNS servers alone: addresses correspond to one 600B DNS queries/day name ▪ Akamai DNS servers alone: 2.2T DNS queries/day Application Layer: 2-68 Thinking about the DNS humongous distributed database: ▪ ~ billion records, each simple handles many trillions of queries/day: ▪ many more reads than writes ▪ performance matters: almost every Internet transaction interacts with DNS - milliseconds count! organizationally, physically decentralized: ▪ millions of different organizations responsible for their records reliability, security Application Layer: 2-69  DNS: a distributed, hierarchical database Root DNS Servers Root … ….com DNS servers.org DNS servers.edu DNS servers Top Level Domain … … … … yahoo.com amazon.com pbs.org nyu.edu umass.edu DNS servers DNS servers DNS servers DNS servers DNS servers Authoritative Client wants IP address for www.amazon.com; 1st approximation: ▪ client queries root server to find.com DNS server ▪ client queries.com DNS server to get amazon.com DNS server ▪ client queries amazon.com DNS server to get IP address for www.amazon.com Application Layer: 2-70 DNS: root name servers ▪ official, contact-of-last-resort by name servers that can not resolve name Application Layer: 2-71 DNS: root name servers ▪ official, contact-of-last-resort by name servers that can not 13 logical root name “servers” worldwide each “server” replicated resolve name many times (~200 servers in US) ▪ incredibly important Internet function Internet couldn’t function without it! DNSSEC – provides security (authentication, message integrity) ▪ ICANN (Internet Corporation for Assigned Names and Numbers) manages root DNS domain https://lookup.icann.org/en https://root-servers.org/ Top-Level Domain, and authoritative servers Top-Level Domain (TLD) servers: ▪ responsible for.com,.org,.net,.edu,.aero,.jobs,.museums, and all top-level country domains, e.g.:.cn,.uk,.fr,.ca,.jp ▪ Network Solutions: authoritative registry for.com,.net TLD ▪ Educause:.edu TLD authoritative DNS servers: ▪ organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts ▪ can be maintained by organization or service provider Application Layer: 2-73 Local DNS name servers ▪ when host makes DNS query, it is sent to its local DNS server Local DNS server returns reply, answering: from its local cache of recent name-to-address translation pairs (possibly out of date!) forwarding request into DNS hierarchy for resolution each ISP has local DNS name server; to find yours: MacOS: % scutil --dns Windows: >ipconfig/all ▪ local DNS server doesn’t strictly belong to hierarchy Application Layer: 2-74 DNS name resolution: iterated query root DNS server Example: host at engineering.uni.edu wants IP address for gaia.cs.umass.edu 2 3 TLD DNS server Iterated query: 1 4 ▪ contacted server replies 8 5 with name of server to requesting host at local DNS server contact engineering.uni.edu dns.uni.edu gaia.cs.umass.edu ▪ “I don’t know this name, 7 6 but ask this server” authoritative DNS server dns.cs.umass.edu Application Layer: 2-75 DNS name resolution: recursive query root DNS server Example: host at engineering.uni.edu wants IP address for gaia.cs.umass.edu 2 3 7 6 Recursive query: 1 TLD DNS server ▪ puts burden of name 8 resolution on requesting host at local DNS server 5 4 engineering.uni.edu dns.uni.edu contacted name gaia.cs.umass.edu server ▪ heavy load at upper authoritative DNS server levels of hierarchy? dns.cs.umass.edu Application Layer: 2-76 Caching DNS Information ▪ once (any) name server learns mapping, it caches mapping, and immediately returns a cached mapping in response to a query caching improves response time cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers ▪ cached entries may be out-of-date if named host changes IP address, may not be known Internet- wide until all TTLs expire! best-effort name-to-address translation! Application Layer: 2-77 DNS records DNS: distributed database storing resource records (RR) RR format: (name, value, type, ttl) type=A type=CNAME ▪ name is hostname ▪ name is alias name for some “canonical” ▪ value is IP address (the real) name ▪ www.ibm.com is really servereast.backup2.ibm.com type=NS ▪ value is canonical name ▪ name is domain (e.g., foo.com) ▪ value is hostname of type=MX authoritative name server for ▪ value is name of SMTP mail this domain server associated with name Application Layer: 2-78 DNS: services, structure ▪ Host aliasing: A host with a complicated hostname can have one or more alias names. For example, a hostname such as relay1.west- coast.enterprise.com could have, say, two aliases such as enterprise.com and www.enterprise.com ▪ In this case, the hostname relay1.west-coast.enterprise.com is said to be a canonical hostname ▪ DNS can be invoked by an application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host Application Layer: 2-79 DNS protocol messages DNS query and reply messages, both have same format: 2 bytes 2 bytes message header: identification flags ▪ identification: 16 bit # for query, # questions # answer RRs reply to query using same # # authority RRs # additional RRs ▪ flags: query or reply questions (variable # of questions) recursion desired answers (variable # of RRs) recursion available reply is authoritative authority (variable # of RRs) additional info (variable # of RRs) Application Layer: 2-80  DNS protocol messages DNS query and reply messages, both have same format: 2 bytes 2 bytes identification flags # questions # answer RRs # authority RRs # additional RRs name, type fields for a query questions (variable # of questions) RRs in response to query answers (variable # of RRs) records for authoritative servers authority (variable # of RRs) additional “ helpful” info that may additional info (variable # of RRs) be used Application Layer: 2-81 Getting your info into the DNS example: new startup “Network Utopia” ▪ register name networkuptopia.com at DNS registrar (e.g., Network Solutions) provide names, IP addresses of authoritative name server (primary and secondary) registrar inserts NS and A RRs into.com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, 212.212.212.1, A) ▪ create authoritative server locally with IP address 212.212.212.1 type A record for www.networkuptopia.com type MX record for networkutopia.com Application Layer: 2-82 DNS security DDoS attacks Spoofing attacks ▪ bombard root servers with ▪ intercept DNS queries, traffic returning bogus replies not successful to date ▪ DNS cache poisoning traffic filtering ▪ RFC 4033: DNSSEC authentication services local DNS servers cache IPs of TLD servers, allowing root server bypass ▪ bombard TLD servers potentially more dangerous Application Layer: 2-83 Chapter 2: Summary our study of network application layer is now complete! ▪ application architectures ▪ specific protocols: client-server HTTP P2P SMTP, IMAP DNS ▪ application service requirements: reliability, bandwidth, delay ▪ Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP Application Layer: 2-84 Chapter 2: Summary Most importantly: learned about protocols! ▪ typical request/reply message important themes: exchange: ▪ centralized vs. decentralized client requests info or service ▪ stateless vs. stateful server responds with data, status code ▪ scalability ▪ message formats: ▪ reliable vs. unreliable headers: fields giving info about data message transfer data: info (payload) being communicated Application Layer: 2-85