Introduction to Computer Networking. Gang Wang Fall 2016
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1 Introduction to Computer Networking Gang Wang Fall 2016
2 Data Networks A set of interconnected nodes exchange information Sharing of the transmission circuits= "switching". Links allow more than one path between every 2 nodes. Network must select an appropriate path for each required connection. 2
3 Internet History : Early packet-switching principles 1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: ARPAnet public demonstration NCP (Network Control Protocol) first host-host protocol First program ARPAnet has 15 nodes 1-3
4 Internet History : Internetworking, new and proprietary nets 1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn - architecture for interconnecting networks 1976: Ethernet at Xerox PARC late70 s: proprietary architectures: DECnet, SNA, XNA late 70 s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf & Kahn s internetworking principles minimalism, autonomy - no internal changes required to interconnect networks best effort service model stateless routers decentralized control define today s Internet architecture 1-4
5 Internet History : new protocols, a proliferation of networks 1983: deployment of TCP/IP 1982: smtp protocol 1983: DNS defined for nameto-ip-address translation 1985: ftp protocol defined 1988: TCP congestion control new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks 1-5
6 Internet History s: commercialization, the Web, new apps Early 1990s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext [Bush 1945, Nelson 1960 s] HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990 s: commercialization of the Web Late 1990s 2000s: More killer apps: instant messaging, P2P file sharing Network security to forefront 50 million host 100 million+ users 1-6
7 Document Object Model (DOM) HTML page is structured data; DOM provides representation of this hierarchy DOM is browser s view of an HTML page as an object hierarchy Tim Berners-Lee (HTML/WWW inventor) US NAS member JavaScript is used to change DOM e.g., change colors on user events Who invented Mosaic (Netscape) browser in 93? Marc Andressen and others (UIUC/NCSA) 7
8 INTERNET PROTOCOL 8
9 Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing datagrams IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits on the wire application transport network link physical 1-9
10 segment datagram frame message H l H n H n H t H t H t M M M M source application transport network link physical Encapsulation link physical switch H l H n H n H t H t H t M M M M destination application transport network link physical H n H t H l H n H t M M network link physical H n H t M router 1-10
11 Application Layer: internet apps web instant messaging remote login P2P file sharing multi-user network games Video streaming voice over IP real-time video grid computing online social networks 11
12 Client-server architecture client/server server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 12
13 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses peer-peer Highly scalable but difficult to manage 13
14 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 which brings message to socket at receiving process host or server process socket TCP with buffers, variables controlled by app developer Internet Controlled by OS host or server process socket TCP with buffers, variables API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) 14
15 Addressing Processes To receive messages, process must have identifier Host device has unique 32-bit IP address Q: does IP address of host suffice for identifying the process? A host have many processes identifier includes both IP address and port numbers Example port numbers: HTTP server: 80 Mail server: 25 To send HTTP message to cs.vt.edu web server: IP address: Port number: 80 15
16 Application-layer protocol Defines Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs e.g., HTTP, SMTP Proprietary protocols: e.g., Skype 16
17 Quick Announcements Programming assignment 1 Grades available this Friday Programming assignment 2 Due Oct 14 (next Friday) 5:00 pm on CANVAS Midterm: October 11 (Tuesday) Close book, close note, 75 minutes No questions about network security You cannot leave the room in the middle of your test Those with conflicted schedules Midterm in my office (CRC, KnowledgeWorks II, room 2223) October 10 (Monday): 10:00 am - 11:15 am 17
18 Internet Transport Protocols Services TCP service: connection-oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum 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? 18
19 HTTP & WEB 19
20 First some jargon Web page consists of objects Web and HTTP Object can be HTML file, JPEG image, Java applet, audio files Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: host name path name 20
21 HTTP overview HTTP: hypertext transfer protocol Application layer protocol client/server model Browser1 client: browser that requests, receives, displays Web objects server: web server sends objects in response to requests Server running Apache Web server Browser2 21
22 HTTP overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server port 80 server accepts TCP connection from client HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is stateless Server maintains no information about past client requests aside Protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent, must be reconciled 22
23 HTTP request message two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) Carriage return, line feed indicates end of message header lines GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) 23
24 Uploading form input Post method: Web page often includes form input Input is uploaded to server in entity body URL method: Uses GET method Input is uploaded in URL field of request line Example of URL method monkeys&banana 24
25 HTTP response message status line (protocol status code status phrase) 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... 25
26 HTTP response status codes In first line in server->client response message. 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported 26
27 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cs.vt.edu 80 Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu 2. Type in a GET HTTP request: GET /about/ HTTP/1.1 Host: cs.vt.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! 27
28 User-server state: cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host, managed by user s browser 4) back-end database at Web site Example: Susan always access Internet always from PC visits specific e-commerce site for first time when initial HTTP requests arrives at site, site creates unique ID entry in backend database for ID 28
29 Cookies: keeping state (cont.) client server ebay 8734 cookie file ebay 8734 amazon 1678 one week later: ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg Amazon server creates ID 1678 for user create entry cookiespecific action cookiespectific action access access backend database 29
30 Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state aside Cookies and privacy: cookies permit sites to learn about you you may supply name and How to keep state : Maintain state at sender/receiver over multiple transactions Cookies: http messages carry state 30
31 31
32 outgoing message queue user mailbox Three major components: user agents mail servers mail server user agent user agent mail transfer protocol: SMTP User Agent a.k.a. mail reader composing, editing, reading mail messages e.g. Outlook, SMTP mail server SMTP SMTP mail server user agent user agent outgoing, incoming messages stored on server user agent user agent 32
33 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 mail server SMTP user agent SMTP mail server user agent user agent client: sending mail server server: receiving mail server mail server SMTP user agent user agent user agent 33
34 SMTP [RFC 2821] Uses TCP to reliably transfer message from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII 34
35 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 (e.g. crepes.fr); msg 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 1 user agent mail server mail server 5 6 user agent 35
36 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 36
37 Try SMTP interaction for yourself telnet smtp.gmail.com 587 Why not port 25? Helo google see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands Model SMTP server usually require TLS to encrypt traffic 37
38 Mail access protocols user agent SMTP SMTP access protocol user agent Sender s mail server Receiver s mail server SMTP: delivery/storage to receiver s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] o authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] o more features (more complex) o manipulation of stored msgs on server HTTP(s): gmail, Hotmail, Yahoo! Mail, etc. 38
39 Slides credit to J.F Kurose and K.W. Ross NDS 39
40 DNS: Domain Name System Internet hosts, routers: IP address (32 bit) - used for addressing datagrams name, e.g., ww.yahoo.com - used by humans Q: map between IP addresses and name? Domain Name System Distributed database implemented in hierarchy of many name servers Application-layer protocol host, routers, name servers to communicate to resolve names Core Internet function, implemented as application-layer protocol 40
41 DNS DNS services Hostname -> IP translation Host aliasing Mail server aliasing Load distribution replicated Web servers: set of IP addresses for one canonical name E.g., amazon.com Why not centralize DNS? single point of failure traffic volume distant centralized database Doesn t scale! 41
42 Domain Name Hierarchy com edu net gov int mil org ae... us... za yahoo cnn vt yale Country Domains cs www Generic Domains 42
43 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 poly.edu DNS servers umass.edu 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 client queries amazon.com DNS server to get IP address for 43
44 DNS: Root name servers Contacted by local name server that can not resolve name Root name server: contacts authoritative name server if name mapping not known gets mapping 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 (much more than 13 physical servers for reliability and redundancy) 44
45 TLD and Authoritative Servers Top-level domain (TLD) servers: Responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Network Solutions maintains servers for com TLD Education for edu TLD Authoritative DNS servers: Organization s DNS servers, providing authoritative hostname to IP mappings for organization s servers (e.g., Web, mail). Can be maintained by organization or service provider 45
46 Local Name Server 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 local DNS server acts as proxy, forwards query into hierarchy If a host name cannot be resolved by local name server, then forward the query to upper-level name server 46
47 DNS name resolution example root DNS server Host at me.cs.vt.edu wants IP address for gaia.cs.umass.edu TLD DNS server iterated query: contacted server replies with name of server to contact local DNS server dns.vt.edu I don t know this name, but ask this server requesting host me.cs.vt.edu authoritative DNS server dns.umass.edu gaia.cs.umass.edu 47
48 DNS name resolution example root DNS server recursive query: puts burden of name resolution on contacted name server heavy load? TLD DNS server local DNS server dns.vt.edu requesting host me.cs.vt.edu authoritative DNS server dns.umass.edu gaia.cs.umass.edu 48
49 DNS: caching and updating records once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time TLD servers typically cached in local name servers o Thus root name servers not often visited update/notify mechanisms under design by IETF RFC
50 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 two RRs into com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, , A) How do people get IP address of your Web site? 50
51 Bootstrapping DNS How does a host contact the name server if all it has is the name and no IP address? IP address of at least 1 nameserver must be given a priori o or with another protocol (DHCP, bootp) File /etc/resolv.conf in unix Start -> settings-> control panel-> network ->TCP/IP -> properties in windows 51
52 Interface to DNS The dig and nslookup programs provide an interface to DNS nslookup cs.vt.edu Server: Address: #53 Non-authoritative answer: Name: cs.vt.edu Address:
53 Reverse DNS We have the IP address, but want the name Use DNS to perform the lookup function Special domain, in-addr.arpa domain for reverse lookups Internet address is reversed in the lookup nslookup in-addr.arpa name = cs.vt.edu 53
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