Chapter2-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 ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-2 Application layer: overview Our goals: ▪ learn about protocols by ▪ conceptual and examining popular implementation aspects of application-layer protocols application-layer protocols HTTP transport-layer service SMTP, IMAP models DNS client-server paradigm ▪ programming network peer-to-peer paradigm 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 ▪ Internet search ▪ e-mail ▪ remote login ▪ multi-user network games ▪ … ▪ streaming stored video (YouTube, Hulu, Netflix) ▪ 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 allows data link physical for rapid app development, 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, for scaling 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 Peer-peer 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: 2-7 Processes communicating process: program running clients, servers within a host client process: process that initiates communication ▪within same host, two server process: process processes communicate that waits to be contacted using inter-process communication (defined by OS) ▪ note: applications with P2P architectures have ▪processes in different hosts client processes & communicate by exchanging server processes messages Application Layer: 2-8 Sockets ▪ process sends/receives messages to/from its socket ▪ socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door to deliver message to socket at 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: 2-9 Addressing processes ▪ to receive messages, process ▪ identifier includes both IP address must have 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 ▪ more shortly… Application Layer: 2-10 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 & definition how fields are delineated ▪ allows for interoperability ▪ message semantics ▪ e.g., HTTP, SMTP meaning of information in proprietary protocols: fields ▪ e.g., Skype ▪ rules for when and how processes send & respond to messages Application Layer: 2-11 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, require low delay to be “effective” … Application Layer: 2-12  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: 2-13 Internet transport protocols services TCP service: UDP service: ▪ reliable transport between sending ▪ unreliable data transfer and receiving process between sending and receiving ▪ flow control: sender won’t process overwhelm receiver ▪ does not provide: reliability, ▪ congestion control: throttle sender flow control, congestion when network overloaded control, timing, throughput guarantee, security, or ▪ does not provide: timing, minimum connection setup. throughput guarantee, security ▪ connection-oriented: setup required Q: why bother? Why between client and server processes is there a UDP? Application Layer: 2-14  Internet transport protocols services 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: 2-15 Securing TCP Vanilla TCP & UDP sockets: TSL 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) TLS socket API ▪ provides encrypted TCP connections ▪ cleartext sent into socket ▪ data integrity traverse Internet encrypted ▪ end-point authentication ▪ see Chapter 8 Application Layer: 2-16 Application layer: overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-17 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: 2-18 HTTP overview HTTP: hypertext transfer protocol ▪ Web’s application layer protocol PC running ▪ client/server model: Firefox browser client: browser that requests, receives, (using HTTP protocol) and “displays” Web objects server running Apache Web server: Web server sends (using server HTTP protocol) objects in response to requests iPhone running Safari browser Application Layer: 2-19 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: 2-20 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: 2-21  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: 2-22  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: 2-23 Non-persistent HTTP: response time RTT (definition): time for a small packet to travel from client to server and back initiate TCP connection HTTP response time (per object): RTT ▪ one RTT to initiate TCP connection request file ▪ one RTT for HTTP request and first few RTT time to bytes of HTTP response to return transmit ▪ obect/file transmission time file received file time time Non-persistent HTTP response time = 2RTT+ file transmission time Application Layer: 2-24 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 same client/server sent parallel TCP connections to over open connection fetch referenced objects in ▪ client sends requests as soon as it parallel encounters a referenced object ▪ as little as one RTT for all the referenced objects (cutting response time in half) Application Layer: 2-25  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: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n header Accept-Language: en-us,en;q=0.5\r\n lines Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n Connection: keep-alive\r\n carriage return, line feed \r\n 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: 2-26 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: 2-27 Other HTTP request messages POST method: HEAD method: ▪ web page often includes form ▪ requests headers (only) that input 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: 2-28 HTTP response message status line (protocol HTTP/1.1 200 OK\r\n status code status phrase) Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n Server: Apache/2.0.52 (CentOS)\r\n Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n header ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n lines Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=ISO-8859- 1\r\n \r\n data, e.g., requested data data data data data... HTML file * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ Application Layer: 2-29 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: 2-30 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet gaia.cs.umass.edu 80 ▪ opens TCP connection to port 80 (default HTTP server port) at gaia.cs.umass. edu. ▪ anything typed in will be sent to port 80 at gaia.cs.umass.edu 2. type in a GET HTTP request: GET /kurose_ross/interactive/index.php HTTP/1.1 Host: gaia.cs.umass.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! (or use Wireshark to look at captured HTTP request/response) Application Layer: 2-31 Maintaining user/server state: cookies a stateful protocol: client makes Recall: HTTP GET/response two changes to X, or none at all interaction is stateless X ▪ no notion of multi-step exchanges of HTTP messages to complete a Web X “transaction” no need for client/server to track X’ “state” of multi-step exchange t’ all HTTP requests are independent of X’’ each other no need for client/server to “recover” X’’ from a partially-completed-but-never- time time completely-completed transaction Q: what happens if network connection or client crashes at t’ ? Application Layer: 2-32 Maintaining user/server state: cookies Web sites 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 cookie ID value, 4) back-end database at Web site allowing site to “identify” Susan Application Layer: 2-33 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: 2-34 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 Challenge: How to keep state: tracked across multiple ▪ protocol endpoints: maintain state at web sites sender/receiver over multiple transactions ▪ cookies: HTTP messages carry state Application Layer: 2-35 Web caches (proxy servers) Goal: satisfy client request without involving origin server ▪ user configures browser to point to a Web cache proxy ▪ browser sends all HTTP server requests to cache client origin if object in cache: cache server returns object to client else cache requests object from origin server, caches received object, then client returns object to client origin server Application Layer: 2-36 Web caches (proxy servers) ▪ Web cache acts as both Why Web caching? client and server ▪ reduce response time for client server for original request requesting client cache is closer to client client to origin server ▪ reduce traffic on an institution’s ▪ typically cache is access link installed by ISP (university, company, ▪ Internet is dense with caches residential ISP) enables “poor” content providers to more effectively deliver content Application Layer: 2-37 Caching example 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 ▪ average data rate to browsers: 1.50 Mbps 1.54 Mbps Performance: access link problem: large ▪ LAN utilization:.0015 delays at high institutional network ▪ access link utilization =.97 utilization! 1 Gbps LAN ▪ end-end delay = Internet delay + access link delay + LAN delay = 2 sec + minutes + usecs Application Layer: 2-38 Caching example: 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 ▪ Avg request rate from browsers to origin servers: 15/sec ▪ avg data rate to browsers: 1.50 Mbps 154 Mbps 1.54 Mbps Performance: access link ▪ LAN utilization:.0015 institutional network ▪ access link utilization =.97.0097 1 Gbps LAN ▪ end-end delay = Internet delay + access link delay + LAN delay = 2 sec + minutes + usecs Cost: faster access link (expensive!) msecs Application Layer: 2-39 Caching example: 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 ▪ Avg request rate from browsers to origin servers: 15/sec ▪ avg data rate to browsers: 1.50 Mbps 1.54 Mbps Performance: access link ▪ LAN utilization:.? How to compute link institutional network ▪ access link utilization = ? utilization, delay? 1 Gbps LAN ▪ average end-end delay = ? Cost: web cache (cheap!) local web cache Application Layer: 2-40 Caching example: install a web cache Calculating access link utilization, end- end delay with cache: origin ▪ suppose cache hit rate is 0.4: 40% requests servers satisfied at cache, 60% requests satisfied at public Internet origin ▪ access link: 60% of requests use access link ▪ data rate to browsers over access link 1.54 Mbps = 0.6 * 1.50 Mbps =.9 Mbps access link ▪ utilization = 0.9/1.54 =.58 institutional network ▪ average end-end delay 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: 2-41 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 modified lower link utilization HTTP response before HTTP/1.0 ▪ cache: specify date of cached copy 304 Not Modified in HTTP request If-modified-since: HTTP request msg ▪ server: response contains no If-modified-since: object object if cached copy is up-to-date: modified HTTP response after HTTP/1.0 304 Not Modified HTTP/1.0 200 OK Application Layer: 2-42 HTTP/2 Key goal: decreased delay in multi-object HTTP requests HTTP1.1: introduced multiple, pipelined GETs over single TCP connection ▪ server responds in-order (FCFS: first-come-first-served scheduling) to GET requests ▪ with FCFS, small object may have to wait for transmission (head-of- line (HOL) blocking) behind large object(s) ▪ loss recovery (retransmitting lost TCP segments) stalls object transmission Application Layer: 2-43 HTTP/2 Key goal: decreased delay in multi-object HTTP requests HTTP/2: [RFC 7540, 2015] increased flexibility at 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) ▪ push unrequested objects to client ▪ divide objects into frames, schedule frames to mitigate HOL blocking Application Layer: 2-44 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: 2-45 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: 2-46 HTTP/2 to HTTP/3 Key goal: decreased delay in multi-object HTTP requests HTTP/2 over single TCP connection means: ▪ recovery from packet loss still stalls all object transmissions as in HTTP 1.1, browsers have incentive to open multiple parallel TCP connections to reduce stalling, increase overall throughput ▪ no security over vanilla TCP connection ▪ HTTP/3: adds security , per object error- and congestion- control (more pipelining) over UDP more on HTTP/3 in transport layer Application Layer: 2-47 Application layer: overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-48  outgoing E-mail message queue user mailbox user Three major components: agent ▪ user agents mail user server ▪ mail servers agent ▪ simple mail transfer protocol: SMTP SMTP mail user server agent SMTP User Agent SMTP user ▪ a.k.a. “mail reader” mail agent server ▪ composing, editing, reading mail messages user ▪ e.g., Outlook, iPhone mail client agent user ▪ outgoing, incoming messages stored on agent server Application Layer: 2-49  outgoing E-mail: mail servers message queue user mailbox user mail servers: agent ▪ mailbox contains incoming mail server user agent messages for user SMTP mail user ▪ message queue of outgoing (to server agent be sent) mail messages SMTP ▪ SMTP protocol between mail SMTP user agent servers to send email messages mail server client: sending mail server user agent “server”: receiving mail server user agent Application Layer: 2-50 E-mail: the RFC (5321) ▪ uses TCP to reliably transfer email message from client (mail server initiating connection) to server, port 25 ▪ direct transfer: sending server (acting like client) to receiving server ▪ three phases of transfer handshaking (greeting) transfer of messages closure ▪ command/response interaction (like HTTP) commands: ASCII text response: status code and phrase ▪ messages must be in 7-bit ASCI Application Layer: 2-51 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; message placed in the message in Bob’s message queue mailbox 3) client side of SMTP opens TCP 6) Bob invokes his user connection with Bob’s mail server agent to read message 1 user mail user mail agent agent server server 2 3 6 4 5 Alice’s mail server Bob’s mail server Application Layer: 2-52 Sample SMTP interaction 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 Application Layer: 2-53 Try SMTP interaction for yourself: telnet 25 ▪ see 220 reply from server ▪ enter HELO, MAIL FROM:, RCPT TO:, DATA, QUIT commands above lets you send email without using e-mail client (reader) Note: this will only work if allows telnet connections to port 25 (this is becoming increasingly rare because of security concerns) Application Layer: 2-54 SMTP: closing observations comparison with HTTP: ▪ SMTP uses persistent connections ▪ HTTP: pull ▪ SMTP requires message ▪ SMTP: push (header & body) to be in ▪ both have ASCII command/response 7-bit ASCII interaction, status codes ▪ SMTP server uses CRLF.CRLF to determine ▪ HTTP: each object encapsulated in its end of message own response message ▪ SMTP: multiple objects sent in multipart message Application Layer: 2-55 Mail message format SMTP: protocol for exchanging e-mail messages, defined in RFC 531 (like HTTP) RFC 822 defines syntax for e-mail message itself (like HTML) ▪ header lines, e.g., header To: blank 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: 2-56  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 STMP (to send), IMAP (or POP) to retrieve e-mail messages Application Layer: 2-57 Application Layer: Overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-58 DNS: Domain Name System people: many identifiers: Domain Name System: 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, addressing datagrams name 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 name, and vice protocol versa ? complexity at network’s “edge” Application Layer: 2-59 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 addresses correspond to one alone: 600B DNS queries name per day Application Layer: 2-60  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: 2-61 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 and message integrity) ▪ ICANN (Internet Corporation for Assigned Names and Numbers) manages root DNS domain Application Layer: 2-62 TLD: 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: 2-63 Local DNS name servers ▪ does not strictly belong to hierarchy ▪ each ISP (residential ISP, company, university) has one also called “default name server” ▪ when host makes DNS query, query is sent to its local DNS server has local cache of recent name-to-address translation pairs (but may be out of date!) acts as proxy, forwards query into hierarchy Application Layer: 2-64 DNS name resolution: iterated query root DNS server Example: host at engineering.nyu.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.nyu.edu dns.nyu.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: 2-65 DNS name resolution: recursive query root DNS server Example: host at engineering.nyu.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.nyu.edu dns.nyu.edu contacted name gaia.cs.umass.edu server ▪ heavy load at upper authoritative DNS server levels of hierarchy? dns.cs.umass.edu Application Layer: 2-66 Caching, Updating DNS Records ▪ once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers thus root name servers not often visited ▪ cached entries may be out-of-date (best-effort name-to- address translation!) if name host changes IP address, may not be known Internet-wide until all TTLs expire! ▪ update/notify mechanisms proposed IETF standard RFC 2136 Application Layer: 2-67 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 mailserver this domain associated with name Application Layer: 2-68 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 uses 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: 2-69  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: 2-70 Inserting records into 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, 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: 2-71 DNS security DDoS attacks Redirect attacks ▪ bombard root servers with ▪ man-in-middle traffic intercept DNS queries not successful to date ▪ DNS poisoning traffic filtering send bogus relies to DNS DNSSEC local DNS servers cache IPs of TLD server, which caches [RFC 4033] servers, allowing root server Exploit DNS for DDoS bypass ▪ send queries with spoofed ▪ bombard TLD servers source address: target IP potentially more dangerous ▪ requires amplification Application Layer: 2-72 Application Layer: Overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-73 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, and new service demands home network content provider ▪ peers are intermittently connected network datacenter network and change IP addresses complex management ▪ examples: P2P file sharing (BitTorrent), enterprise network streaming (KanKan), VoIP (Skype) Application Layer: 2-74 File distribution: client-server vs P2P Q: how much time to distribute file (size F) from one server to N peers? peer upload/download capacity is limited resource us: server upload capacity di: peer i download file, size F u1 d1 u2 capacity us d2 server di uN network (with abundant bandwidth) ui dN ui: peer i upload capacity Introduction: 1-75  File distribution time: client-server ▪ server transmission: must sequentially send (upload) N file copies: F time to send one copy: F/us us time to send N copies: NF/us di network ui ▪ client: each client must download file copy dmin = min client download rate min client download time: F/dmin time to distribute F to N clients using Dc-s > max{NF/us,,F/dmin} client-server approach increases linearly in N Introduction: 1-76 File distribution time: P2P ▪ server transmission: must upload at least one copy: F time to send one copy: F/us us ▪ client: each client must download di network file copy ui min client download time: F/dmin ▪ clients: as aggregate must download NF bits max upload rate (limiting max download rate) is us + Sui time to distribute F to N clients using DP2P > max{F/us,,F/dmin,,NF/(us + Sui)} P2P approach increases linearly in N … … but so does this, as each peer brings service capacity Application Layer: 2-77 Client-server vs. P2P: example client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us 3.5 P2P Minimum Distribution Time 3 Client-Server 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 N Application Layer: 2-78 P2P file distribution: BitTorrent ▪ file divided into 256Kb chunks ▪ peers in torrent send/receive file chunks tracker: tracks peers torrent: group of peers participating in torrent exchanging chunks of a file Alice arrives … … obtains list of peers from tracker … and begins exchanging file chunks with peers in torrent Application Layer: 2-79 P2P file distribution: BitTorrent ▪ peer joining torrent: has no chunks, but will accumulate them over time from other peers registers with tracker to get list of peers, connects to subset of peers (“neighbors”) ▪ while downloading, peer uploads chunks to other peers ▪ peer may change peers with whom it exchanges chunks ▪ churn: peers may come and go ▪ once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent Application Layer: 2-80 BitTorrent: requesting, sending file chunks Requesting chunks: Sending chunks: tit-for-tat ▪ at any given time, different ▪ Alice sends chunks to those four peers have different peers currently sending her chunks subsets of file chunks at highest rate ▪ periodically, Alice asks other peers are choked by Alice (do each peer for list of chunks not receive chunks from her) that they have re-evaluate top 4 every10 secs ▪ Alice requests missing ▪ every 30 secs: randomly select chunks from peers, rarest another peer, starts sending first chunks “optimistically unchoke” this peer newly chosen peer may join top 4 Application Layer: 2-81 BitTorrent: tit-for-tat (1) Alice “optimistically unchokes” Bob (2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers higher upload rate: find better trading partners, get file faster ! Application Layer: 2-82 Application layer: overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-83 Video Streaming and CDNs: context ▪ stream video traffic: major consumer of Internet bandwidth Netflix, YouTube, Amazon Prime: 80% of residential ISP traffic (2020) ▪ challenge: scale - how to reach ~1B users? single mega-video server won’t work (why?) ▪ challenge: heterogeneity ▪ different users have different capabilities (e.g., wired versus mobile; bandwidth rich versus bandwidth poor) ▪ solution: distributed, application-level infrastructure Application Layer: 2-84 Multimedia: video spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and ▪ video: sequence of images number of repeated values (N) displayed at constant rate …………………….. ……………….……. e.g., 24 images/sec ▪ digital image: array of pixels each pixel represented by bits ▪ coding: use redundancy within and frame i between images to decrease # bits used to encode image spatial (within image) temporal coding example: instead of sending temporal (from one image to complete frame at i+1, send only differences from next) frame i frame i+1 Application Layer: 2-85 Multimedia: video spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and ▪ CBR: (constant bit rate): video number of repeated values (N) encoding rate fixed …………………….. ……………….……. ▪ VBR: (variable bit rate): video encoding rate changes as amount of spatial, temporal coding changes ▪ examples: frame i MPEG 1 (CD-ROM) 1.5 Mbps MPEG2 (DVD) 3-6 Mbps temporal coding example: MPEG4 (often used in instead of sending complete frame at i+1, Internet, 64Kbps – 12 Mbps) send only differences from frame i frame i+1 Application Layer: 2-86 Streaming stored video simple scenario: Internet video server client (stored video) Main challenges: ▪ server-to-client bandwidth will vary over time, with changing network congestion levels (in house, in access network, in network core, at video server) ▪ packet loss and delay due to congestion will delay playout, or result in poor video quality Application Layer: 2-87 Streaming stored video 2. video sent 1. video 3. video received, played out at client recorded (30 frames/sec) (e.g., 30 network delay time frames/sec) (fixed in this example) streaming: at this time, client playing out early part of video, while server still sending later part of video Application Layer: 2-88 Streaming stored video: challenges ▪ continuous playout constraint: once client playout begins, playback must match original timing … but network delays are variable (jitter), so will need client-side buffer to match playout requirements ▪ other challenges: client interactivity: pause, fast-forward, rewind, jump through video video packets may be lost, retransmitted Application Layer: 2-89 Streaming stored video: playout buffering constant bit rate video client video constant bit transmission reception rate video playout at client variable buffered network video delay client playout time delay ▪client-side buffering and playout delay: compensate for network-added delay, delay jitter Application Layer: 2-90 Streaming multimedia: DASH ▪ DASH: Dynamic, Adaptive Streaming over HTTP ▪ server: divides video file into multiple chunks each chunk stored, encoded at different rates manifest file: provides URLs for different chunks Internet client ▪ client: periodically measures server-to-client bandwidth consulting manifest, requests one chunk at a time chooses maximum coding rate sustainable given current bandwidth can choose different coding rates at different points in time (depending on available bandwidth at time) Application Layer: 2-91 Streaming multimedia: DASH ▪“intelligence” at client: client determines when to request chunk (so that buffer starvation, or overflow does not occur) Internet what encoding rate to request (higher client quality when more bandwidth available) where to request chunk (can request from URL server that is “close” to client or has high available bandwidth) Streaming video = encoding + DASH + playout buffering Application Layer: 2-92 Content distribution networks (CDNs) ▪ challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users? ▪ option 1: single, large “mega-server” single point of failure point of network congestion long path to distant clients multiple copies of video sent over outgoing link ….quite simply: this solution doesn’t scale Application Layer: 2-93 Content distribution networks (CDNs) ▪ challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users? ▪ option 2: store/serve multiple copies of videos at multiple geographically distributed sites (CDN) enter deep: push CDN servers deep into many access networks close to users Akamai: 240,000 servers deployed in more than 120 countries (2015) bring home: smaller number (10’s) of larger clusters in POPs near (but not within) access networks used by Limelight Application Layer: 2-94 Content distribution networks (CDNs) ▪ CDN: stores copies of content at CDN nodes e.g. Netflix stores copies of MadMen ▪ subscriber requests content from CDN directed to nearby copy, retrieves content may choose different copy if network path congested manifest file where’s Madmen? Application Layer: 2-95 Content distribution networks (CDNs) OTT: “over the top” Internet host-host communication as a service OTT challenges: coping with a congested Internet ▪ from which CDN node to retrieve content? ▪ viewer behavior in presence of congestion? ▪ what content to place in which CDN node? Application Layer: 2-96 CDN content access: a closer look Bob (client) requests video http://netcinema.com/6Y7B23V ▪ video stored in CDN at http://KingCDN.com/NetC6y&B23V 1. Bob gets URL for video http://netcinema.com/6Y7B23V from netcinema.com web page 2. resolve http://netcinema.com/6Y7B23V 2 via Bob’s local DNS 1 6. request video from 5 Bob’s KINGCDN server, local DNS streamed via HTTP server 3. netcinema’s DNS returns CNAME for netcinema.com 4 http://KingCDN.com/NetC6y&B23V 3 netcinema’s authoratative DNS KingCDN.com KingCDN authoritative DNS Application Layer: 2-97 Case study: Netflix Amazon cloud upload copies of multiple versions of video to CDN servers CDN server Netflix registration, accounting servers Bob browses Netflix video CDN 2 Manifest file, server requested 1 3 returned for Bob manages specific video Netflix account CDN 4 server DASH server selected, contacted, streaming begins Application Layer: 2-98 Application Layer: Overview ▪ P2P applications ▪ Principles of network ▪ video streaming and content applications distribution networks ▪ Web and HTTP ▪ socket programming with ▪ E-mail, SMTP, IMAP UDP and TCP ▪ The Domain Name System DNS Application Layer: 2-99 Socket programming goal: learn how to build client/server applications that communicate using sockets socket: door between application process and end-end-transport protocol application application socket controlled by process process app developer transport transport network network controlled link by OS link Internet physical physical Application Layer: 2-100 Socket programming Two socket types for two transport services: ▪ UDP: unreliable datagram ▪ TCP: reliable, byte stream-oriented Application Example: 1. client reads a line of characters (data) from its keyboard and sends data to server 2. server receives the data and converts characters to uppercase 3. server sends modified data to client 4. client receives modified data and displays line on its screen Application Layer: 2-101 Socket programming with UDP UDP: no “connection” between client & server ▪ no handshaking before sending data ▪ sender explicitly attaches IP destination address and port # to each packet ▪ receiver extracts sender IP address and port# from received packet UDP: transmitted data may be lost or received out-of-order Application viewpoint: ▪ UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server Application Layer: 2-102 Client/server socket interaction: UDP server (running on serverIP) client create socket: create socket, port= x: clientSocket = serverSocket = socket(AF_INET,SOCK_DGRAM) socket(AF_INET,SOCK_DGRAM) Create datagram with server IP and port=x; send datagram via read datagram from clientSocket serverSocket write reply to serverSocket read datagram from specifying clientSocket client address, port number close clientSocket Application Layer: 2-103  Example app: UDP client Python UDPClient include Python’s socket library from socket import * serverName = ‘hostname’ serverPort = 12000 create UDP socket for server clientSocket = socket(AF_INET, SOCK_DGRAM) get user keyboard input message = raw_input(’Input lowercase sentence:’) attach server name, port to message; send into socket clientSocket.sendto(message.encode(), (serverName, serverPort)) read reply characters from socket into string modifiedMessage, serverAddress = clientSocket.recvfrom(2048) print out received string and close socket print modifiedMessage.decode() clientSocket.close() Application Layer: 2-104 Example app: UDP server Python UDPServer from socket import * serverPort = 12000 create UDP socket serverSocket = socket(AF_INET, SOCK_DGRAM) bind socket to local port number 12000 serverSocket.bind(('', serverPort)) print (“The server is ready to receive”) loop forever while True: Read from UDP socket into message, getting message, clientAddress = serverSocket.recvfrom(2048) client’s address (client IP and port) modifiedMessage = message.decode().upper() send upper case string back to this client serverSocket.sendto(modifiedMessage.encode(), clientAddress) Application Layer: 2-105 Socket programming with TCP Client must contact server ▪ when contacted by client, server ▪ server process must first be TCP creates new socket for server running process to communicate with that ▪ server must have created socket particular client (door) that welcomes client’s allows server to talk with multiple contact clients Client contacts server by: source port numbers used to distinguish clients (more in Chap 3) ▪ Creating TCP socket, specifying IP address, port number of server process Application viewpoint ▪ when client creates socket: client TCP provides reliable, in-order TCP establishes connection to byte-stream transfer (“pipe”) server TCP between client and server Application Layer: 2-106 Client/server socket interaction: TCP server (running on hostid) client create socket, port=x, for incoming request: serverSocket = socket() wait for incoming create socket, connection request TCP connect to hostid, port=x connectionSocket = connection setup clientSocket = socket() serverSocket.accept() send request using read request from clientSocket connectionSocket write reply to connectionSocket read reply from clientSocket close connectionSocket close clientSocket Application Layer: 2-107 Example app: TCP client Python TCPClient from socket import * serverName = ’servername’ serverPort = 12000 create TCP socket for server, clientSocket = socket(AF_INET, SOCK_STREAM) remote port 12000 clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) clientSocket.send(sentence.encode()) No need to attach server name, port modifiedSentence = clientSocket.recv(1024) print (‘From Server:’, modifiedSentence.decode()) clientSocket.close() Application Layer: 2-108 Example app: TCP server Python TCPServer from socket import * serverPort = 12000 create TCP welcoming socket serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort)) server begins listening for incoming TCP requests serverSocket.listen(1) print ‘The server is ready to receive’ loop forever while True: server waits on accept() for incoming connectionSocket, addr = serverSocket.accept() requests, new socket created on return read bytes from socket (but sentence = connectionSocket.recv(1024).decode() not address as in UDP) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence. encode()) close connection to this client (but not connectionSocket.close() welcoming socket) Application Layer: 2-109 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: P2P: BitTorrent reliability, bandwidth, delay ▪ video streaming, CDNs ▪ Internet transport service model ▪ socket programming: connection-oriented, reliable: TCP TCP, UDP sockets unreliable, datagrams: UDP Application Layer: 2-110 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 ▪ “complexity at network communicated edge” Application Layer: 2-111 Additional Chapter 2 slides Application Layer: 2-112