Chapter 2 Application Layer

Transcription

1 Chapter 2 Application Layer Slides are adopted from slides by J.F Kurose and K.W. Ross All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012

2 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 Electronic Mail v SMTP, POP3, IMAP 2.4 DNS 2.5 P2P applications 2: Application Layer 2

3 Creating a network app write programs that v run on (different) end systems v communicate over network v e.g., web server software communicates with browser software No need to write software for network-core devices v Network-core devices do not run user applications v applications on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical application transport network data link physical 2: Application Layer 3

4 Application architectures v Including data centers / cloud computing 2: Application Layer 4

5 Client-server architecture server: v v v server farms for scaling clients: v communicate with server v do not communicate directly with each other v v 2: Application Layer 5

6 Google Data Centers Estimated cost of data center: $600M Google spent $2.4B in 2007 on new data centers Each data center uses megawatts of power

7 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP Pros Cons 2: Application Layer 7

8 Hybrid of client-server and P2P Skype v voice-over-ip P2P application v centralized server: v client-client connection: Instant messaging v chatting between two users is P2P v centralized service: client presence detection/ location 2: Application Layer 8

9 What transport service does an app need? Data loss some apps tolerate loss v Ex: other apps require 100% reliable data transfer v Ex: Timing some apps require low delay to be effective v Ex: Throughput some apps require minimum amount of throughput to be effective Ex: other apps use whatever throughput they get Security Encryption, data integrity, 2: Application Layer 9

10 Internet transport protocols services TCP service: : setup required between client and server processes between sending and receiving process : sender won t overwhelm receiver : throttle sender when network overloaded does not provide: timing, minimum throughput guarantees, security UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP? 2: Application Layer 10

11 Internet apps: application, transport protocols Application remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP 2: Application Layer 11

12 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail v SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2: Application Layer 12

13 Web and HTTP First some jargon Web page consists of v HTML file, JPEG image, Java applet, audio file, Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a v Example 2: Application Layer 13

14 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol client/server model v client: browser that requests, receives, displays Web objects v server: Web server sends objects in response to requests PC running Explorer Mac running Navigator Server running Apache Web server 2: Application Layer 14

15 HTTP overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port server accepts TCP connection from client TCP connection closed HTTP is stateless server maintains no information about past client requests Nonpersistent HTTP Persistent HTTP (not covered) 2: Application Layer 15

16 Nonpersistent HTTP Suppose user enters URL somedepartment/home.index (contains text, references to 10 jpeg images) time 1a. HTTP client initiates TCP connection to HTTP server (process) at on port HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object somedepartment/home.index 1b. HTTP server at host waiting for TCP connection at port 80. accepts connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket 2: Application Layer 16

17 Nonpersistent HTTP (cont.) time 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects 4. HTTP server closes TCP connection. 2: Application Layer 17

18 Non-Persistent HTTP: Response time Definition of RTT: time for a small packet to travel from client to server and back. Response time: file transmission time total = 2: Application Layer 18

19 HTTP request message two types of HTTP messages:, HTTP request message: v ASCII (human-readable format) header lines GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr Carriage return, line feed indicates end of message (extra carriage return, line feed) 2: Application Layer 19

20 HTTP response message data, e.g., requested HTML file header lines HTTP/ OK Connection close Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html data data data data data... 2: Application Layer 20

21 HTTP response status codes In first line in server->client response message. A few sample codes: 200 v request succeeded, requested object later in this message 301 v requested object moved, new location specified later in this message (Location:) 400 v request message not understood by server 404 v requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 21

22 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 Electronic Mail v SMTP, POP3, IMAP 2.4 DNS 2.5 P2P applications 2: Application Layer 22

23 Electronic Mail outgoing message queue user mailbox Three major components: mail server user agent user agent User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Mozilla Thunderbird outgoing, incoming messages stored on server SMTP mail server user agent SMTP SMTP user agent mail server user agent user agent 2: Application Layer 23

24 Electronic Mail: mail servers Mail Servers mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send messages v client: sending mail server v server : receiving mail server mail server SMTP mail server user agent user agent SMTP SMTP user agent mail server user agent user agent user agent 2: Application Layer 24

25 Electronic Mail: SMTP [RFC 2821] uses to reliably transfer message from client to server, port direct transfer: sending server to receiving server three phases of transfer v handshaking (greeting) v transfer of messages v closure 2: Application Layer 25

26 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and to 2) Alice s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob s mail server 4) SMTP client sends Alice s message over the TCP connection 5) Bob s mail server places the message in Bob s mailbox 6) Bob invokes his user agent to read message user agent mail server mail server user agent 2: Application Layer 26

27 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 Sender ok C: RCPT TO: S: 250 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 2: Application Layer 27

28 Mail access protocols user agent SMTP SMTP access protocol The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. user agent sender s mail server receiver s mail server SMTP: delivery/storage to receiver s server Mail access protocol: retrieval from server v POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download v IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server v HTTP: gmail, Hotmail, Yahoo! Mail, etc. 2: Application Layer 28

29 POP3 protocol authorization phase client commands: v user: declare username v pass: password server responses v +OK v -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: S: S:. C: retr 1 S: <message 1 contents> S:. C: dele 1 C: retr 2 S: <message 1 contents> S:. C: dele 2 C: quit S: +OK POP3 server signing off 2: Application Layer 29

30 POP3 (more) and IMAP More about POP3 Previous example uses download and delete mode. Bob cannot re-read e- mail if he changes client Download-and-keep : copies of messages on different clients POP3 is stateless across sessions IMAP Keep all messages in one place: the server Allows user to organize messages in folders IMAP keeps user state across sessions: v names of folders and mappings between message IDs and folder name 2: Application Layer 30

31 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 Electronic Mail v SMTP, POP3, IMAP 2.4 DNS 2.5 P2P applications 2: Application Layer 31

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

33 Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers msu.edu umass.edu DNS servers DNS servers Client wants IP for 1 st approx: client queries a 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 2: Application Layer 33

34 DNS: Root name servers contacted by local name server that can not resolve name root name server: v contacts authoritative name server if name mapping not known v gets mapping v returns mapping to local name server e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) k RIPE London (also 16 other locations) i Autonomica, Stockholm (plus 28 other locations) m WIDE Tokyo (also Seoul, Paris, SF) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 13 root name servers worldwide 2: Application Layer 34

35 TLD and Authoritative Servers Top-level domain (TLD) servers: v responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. v Network Solutions maintains servers for com TLD v Educause for edu TLD Authoritative DNS servers: v organization s DNS servers, providing authoritative hostname to IP mappings for organization s servers (e.g., Web, mail). v can be maintained by organization or service provider 2: Application Layer 35

36 Local Name Server does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one. v also called default name server when host makes DNS query, query is sent to its local DNS server v acts as proxy, forwards query into hierarchy 2: Application Layer 36

37 DNS name resolution example root DNS server Host at cis.poly.edu wants IP address for gaia.cs.umass.edu TLD DNS server iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server local DNS server dns.poly.edu 1 8 requesting host cis.poly.edu 7 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu 2: Application Layer 37

38 DNS: caching and updating records once (any) name server learns mapping, it caches mapping v cache entries timeout (disappear) after some time v TLD servers typically cached in local name servers Thus root name servers not often visited update/notify mechanisms under design by IETF v RFC 2136 v 2: Application Layer 38

39 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 Electronic Mail v SMTP, POP3, IMAP 2.4 DNS 2.5 P2P applications 2: Application Layer 39

40 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses peer-peer Three topics: v File distribution v Searching for information v Case Study: Skype 2: Application Layer 40

41 File Distribution: Server-Client vs P2P Question : How much time to distribute file from one server to N peers? File, size F Server u 1 d 1 u 2 u d 2 s u s : server upload bandwidth u i : peer i upload bandwidth d i : peer i download bandwidth d N u N Network (with abundant bandwidth) 2: Application Layer 41

42 File distribution time: server-client server sequentially sends N copies: v time client i takes F/d i time to download F Server d N u N u u 2 1 d 1 u d s 2 Network (with abundant bandwidth) Time to distribute F to N clients using client/server approach 2: Application Layer 42

43 File distribution time: P2P server must send one copy: time client i takes time to download NF bits must be downloaded (aggregate) fastest possible upload rate: F Server d N u N u u 2 1 d 1 u d s 2 Network (with abundant bandwidth) 2: Application Layer 43

44 Server-client vs. P2P: example Client upload rate = u, F/u = 1 hour, u s = 10u, d min u s Minimum Distribution Time P2P Client-Server N 2: Application Layer 44

45 P2P Case study: Skype inherently P2P: pairs of users communicate. proprietary application-layer protocol (inferred via reverse engineering) hierarchical overlay with SNs Index maps usernames to IP addresses; distributed over SNs Skype login server 2: Application Layer 45