Protocol layers, example: Organization of air travel. ticket (complain) ticket (purchase) baggage (claim) baggage (check) gates (unload) gates (load)

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1 Protocol layers, example: Organization of air travel Layering of airline functionality Föreläsning 2: Applikationslagret ticket (purchase) ticket (complain) baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing a series of steps TDTS04 Fö. : Introduktion -2 ticket (purchase) baggage (check) gates (load) runway (takeoff) departure airport intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) arrival airport Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below Why layering? TDTS04 Fö. : Introduktion - ticket baggage gate takeoff/landing Layering Internet functionality application:supporting network applications FTP, STP, HTTP : process-process data transfer TCP, UDP network:routing of datagrams from source to destination IP, routing protocols link:data transfer between neighbouring network elements PPP, Ethernet physical: bits on the wire Application Transport Network Link Physical message segment H t datagram H n H t frame H l H n H t H t H t H n H t H n H l destination application network link physical source application network link physical H t H n H t H n H l Encapsulation link physical network link physical H t H n switch router Networked software: Architectural Styles () Layered architectures Object-based architectures Data-centered architectures Event-based architectures TDTS04 Fö. : Introduktion TDTS04 Fö. : Introduktion -5

2 Architectural Styles (2) Architectural Styles () Architectural styles (4) Figure 2-. The (a) layered architectural style and Figure 2-. (b) The object-based architectural style. data-centric: e.g. web-based distributed systems shared distributed file system communication through files Architectural Styles (5) Figure 2-2. (a) The event-based architectural style and Architectural Styles (6) Figure 2-2. (b) The shared data-space architectural style. Centralized Architectures Figure 2-. General interaction between a client and a. Event-based and data-centric! 2

3 Application Layering The -interface level The processing level The data level Processing Level -28 The general organization of an Internet search engine into three different layers ultitiered Architectures () The simplest organization is to have only two types of machines: A client machine containing only the programs implementing (part of) the -interface level A machine containing the rest, the programs implementing the processing and data level ultitiered Architectures () ultitiered Architectures (2) odern Architectures Alternative client- organizations (a) (e). An example of a acting as a client. An example of horizontal distribution of a Web service.

4 odern Architectures Vertical distribution Horizontal distribution Peer-to-peer distribution Client example: Networked UI, the X Window System Figure -9. The basic organization of thexwindow System. Client-Side Software for Distribution Transparency Example, Figure -0. Transparent replication of a using a client-side solution. Server design issues Server organization Iterative Concurrent ultithreaded Fork(unix) Stateless or stateful Client contact: End point(port) Well-known Dynamic: daemon; super (unix) End Point, General Design Issues () Figure -. (a) Client-to- binding using a daemon. End Point, General Design Issues (2) Figure -. (b) Client-to- binding using a super. 4

5 Server Clusters () Server Clusters (2) Application layer: Transport service requirements of common apps Application Data loss Bandwidth Time Sensitive Figure -2. The general organization of a three-tiered cluster. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved Figure -. The principle of TCP handoff. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic no elastic no elastic no audio: 5kbps-bps yes, 00s msec video:0kbps-5bps same as above few kbps up elastic yes, few secs yes, 00s msec yes and no What services can the Internet provide these applications? 0 Internet protocols services Internet apps: application vs protocols HTTP overview TCP service: connection-oriented:setup required between client and processes reliable between sending and receiving process flow control:sender won t overwhelm receiver congestion control:throttle sender when network overloaded does not provide:timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Which one to choose if - Data loss/no loss? - Bandwidth elastic/few kbps/bps? - Time sensitive no/yes? Application remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol STP [RFC 282] Telnet [RFC 854] HTTP [RFC 266] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Vonage, Dialpad) Underlying protocol TCP TCP TCP TCP TCP or UDP typically UDP HTTP: hypertext transfer protocol the web s application layer protocol client- model client:browser that requests, receives, displays Web objects :web sends objects in response to requests uses TCP is stateless PC running Explorer ac running Navigator Server running Apache Web 2 5

6 HTTP request message HTTP request message: general format HTTP response 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/. Host: User-: ozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) status line (protocol status code status phrase) data, e.g., requested HTL file header lines HTTP/. 200 OK Connection close Date: Thu, 06 Aug 998 2:00:5 GT Server: Apache/..0 (Unix) Last-odified: on, 22 Jun Content-Length: 682 Content-Type: text/html data data data data data FTP: the file transfer protocol FTP: separate control, data connections FTP commands, responses at host FTP interface FTP client local file system file transfer FTP remote file system transfer file to/from remote host client/ model client: side that initiates transfer (either to/from remote) : remote host ftp: RFC 959 ftp : port 2 FTP client contacts FTP at port 2, TCP is protocol client authorized over control connection client browses remote directory by sending commands over control connection. when receives file transfer command, opens 2 nd TCP connection (for file) to client after transferring one file, closes data connection. FTP client TCP control connection port 2 TCP data connection port 20 FTP opens another TCP data connection to transfer another file. control connection: out of band FTP maintains state : current directory, earlier authentication Sample commands: sent as ASCII text over control channel USER name PASS password LISTreturn list of file in current directory RETR filenameretrieves (gets) file STOR filenamestores (puts) file onto remote host Sample return codes status code and phrase (as in HTTP) Username OK, password required 25 data connection already open; transfer starting 425 Can t open data connection 452 Error writing file TDTS04 Lecture TDTS04 Lecture TDTS04 Lecture 2 9 6

7 Electronic ail outgoing message queue Electronic ail: mail s Electronic ail: STP [RFC 282] Three major components:. s 2. mail s. simple mail transfer protocol: STP. User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, ozilla Thunderbird outgoing, incoming messages stored on mail STP mail STP STP mail mailbox ail Servers mailboxcontains incoming messages for messagequeueof outgoing (to be sent) mail messages. STP protocol between mail s to send messages client: sending mail : receiving mail mail STP mail STP STP mail 4 uses TCP to reliably transfer message from client to, port 25 direct transfer: sending to receiving 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 sample STP interaction 42 STP: final words ail message format essage format: multimedia extensions STP uses persistent connections STP requires message (header & body) to be in 7-bit ASCII STP uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull STP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg STP: multiple objects sent in multipart msg STP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom STP commands! body the message, ASCII characters only header body blank line IE: ultipurpose Internet ail Extensions, RFC 2045, 2056 additional lines in msg header declare IE content type IE version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. IE-Version:.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data

8 ail access protocols STP STP access protocol sender s mail receiver s mail STP: delivery/storage to receiver s ail access protocol: retrieval from POP: Post Office Protocol [RFC 99] authorization ( <-->) and download IAP: Internet ail Access Protocol [RFC 70] more features (more complex) manipulation of stored msgs on HTTP: gmail, Hotmail, Yahoo! ail, etc. POP protocol authorization phase client commands: : declare name pass: password responses +OK -ERR transaction phase, client: list: list message numbers retr:retrieve message by number dele: delete quit S: +OK POP ready C: bob S: +OK C: pass hungry S: +OK successfully logged on C: list S: 498 S: 2 92 S:. C: retr S: <message contents> S:. C: dele C: retr 2 S: <message contents> S:. C: dele 2 C: quit S: +OK POP signing off POP (more) and IAP ore about POP Previous example uses download and delete mode. Bob cannot re-read if he changes client Download-and-keep : copies of messages on different clients POP is stateless across sessions IAP Keep all messages in one place: the Allows to organize messages in folders IAP keeps state across sessions: names of folders and mappings between message IDs and folder name DNS, Domain Name System A Distributed, Hierarchical Database Root DNS Servers com DNS s org DNS s edu DNS s yahoo.com DNS s amazon.com DNS s pbs.org DNS s poly.edu umass.edu DNS sdns s Client wants IP for st approx: client queries a root to find com DNS client queries com DNS to get amazon.com DNS client queries amazon.com DNS to get IP address for DNS: Root name s contacted by local name that can not resolve name root name : contacts authoritative name if name mapping not known gets mapping returns mapping to local name e NASA t View, CA f Internet Software C. Palo Alto, CA (and 6 other locations) b USC-ISI arina del Rey, CA l ICANN Los Angeles, CA a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U aryland College Park, D g US DoD Vienna, VA h ARL Aberdeen, D j Verisign, ( 2 locations) k RIPE London (also 6 other locations) i Autonomica, Stockholm (plus 28 other locations) m WIDE Tokyo (also Seoul, Paris, SF) root name s worldwide Other DNS s Top-level domain (TLD) s responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Network Solutions maintains s for com TLD Educause for edu TLD Authoritative DNS s organization s DNS s, providing authoritative hostname to IP mappings for organization s s (e.g., Web, mail). can be maintained by organization or service provider Local name s does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one. also called default name when host makes DNS query, query is sent to its local DNS acts as proxy, forwards query into hierarchy TDTS04 Lecture TDTS04 Lecture TDTS04 Lecture 2 5 8

9 Host at cis.poly.edu wants IP address for gaia.cs.umass.edu iterated query: contacted replies with name of to contact I don t know this name, but ask this DNS name resolution example local DNS dns.poly.edu requesting host cis.poly.edu root DNS gaia.cs.umass.edu TDTS04 Lecture TLD DNS 6 authoritative DNS dns.cs.umass.edu DNS name resolution example recursive query: puts burden of name resolution on contacted name heavy load? local DNS dns.poly.edu requesting host cis.poly.edu root DNS gaia.cs.umass.edu TDTS04 Lecture authoritative DNS dns.cs.umass.edu TLD DNS DNS records DNS: distributed database storing Resource Records(RR) Type=A nameis hostname valueis IP address RR format: (name, value, type, ttl) Type=NS name is domain (e.g. foo.com) valueis hostname of authoritative name for this domain Type=CNAE nameis alias name for some canonical (the real) name is really east.backup2.ibm.com valueis canonical name Type=X valueis name of mail associated with name TDTS04 Lecture 2 54 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header identification:6 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative Name, type fields for a query DNS protocol, messages RRs in response to query records for authoritative s additional helpful info that may be used Inserting records into DNS example: new startup Network Utopia register name networkutopia.com at DNS registrar(e.g., Network Solutions) provide names, IP addresses of authoritative name (primary and secondary) registrar inserts two RRs into com TLD : (networkutopia.com, dns.networkutopia.com, NS) (dns.networkutopia.com, , A) create authoritative Type A record for Type X record for networkutopia.com How do people get IP address of your Web site? see previous name resolution example TDTS04 Lecture TDTS04 Lecture TDTS04 Lecture

10 peer-topeer Application architectures client- Also: Hybrid of client and P2P Hybrid of client- and P2P Skype voice-over-ip P2P application centralized : finding address of remote party: client-client connection: direct (not through ) Instant messaging chatting between two s is P2P centralized service: client presence detection/location registers its IP address with central when it comes online contacts central to find IP addresses of buddies Napster s P2P: centralized directory original Napster design centralized directory ) when peer connects, it informs central : IP address content 2 2) Alice queries for Hey Jude ) Alice requests file from Bob file transfer is decentralized, but locating content is highly centralized Alice Bob peers File distribution: BitTorrent (hybrid p2p) tracker:tracks peers participating in torrent obtain list of peers peer trading chunks torrent:group of peers exchanging chunks of a file BitTorrent () file divided into 256KB chunks. peer joining torrent: has no chunks, but will accumulate them over time registers with tracker to get list of peers, connects to subset of peers ( neighbors ) while downloading, peer uploads chunks to other peers. peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain BitTorrent (2) Pulling Chunks Sending Chunks: tit-for-tat at any given time, different peers Alice sends chunks to four have different subsets of file neighbors currently sending chunks her chunks at the highest rate periodically, a peer (Alice) asks each neighbor for list of chunks re-evaluate top 4 every 0 that they have. secs Alice sends requests for her every 0 secs: randomly select missing chunks another peer, starts sending rarest first chunks newly chosen peer may join top 4 optimistically unchoke 7-6 2: Application Layer : Application Layer : Application Layer

11 BitTorrent: Tit-for-tat () Alice optimistically unchokes Bob (2) Alice becomes one of Bob s top-four providers; Bob reciprocates () Bob becomes one of Alice s top-four providers With higher upload rate, can find better trading partners & get file faster! : Application Layer Distributed Hash Table (DHT) DHT = distributed P2P database Database has (key, value) pairs; key: ssnumber; value: human name key: content type; value: IP address Peers querydb with key DB returns values that match the key Peers can also insert(key, value) pairs DHT Identifiers Assign integer identifier to each peer in range [0,2 n -]. Each identifier can be represented by n bits. Require each key to be an integer in same range. To get integer keys, hash original key. eg, key = h( Led Zeppelin IV ) This is why they call it a distributed hash table How to assign keys to peers? Central issue: Assigning (key, value) pairs to peers. Rule: assign key to the peer that has the closest ID. Convention in lecture: closest is the immediate successor of the key. Ex: n=4; peers:,,4,5,8,0,2,4; key =, then successor peer = 4 key = 5, then successor peer = Circular DHT () 0 8 Each peer onlyaware of immediate successor and predecessor. Overlay network O(N) messages on avg to resolve query, when there are N peers Define closest as closest successor 00 I am 0 00 Circle DHT (2) Who s resp for key 0?

12 Circular DHT with Shortcuts Each peer keeps track of IP addresses of predecessor, successor, short cuts. Reduced from 6 to 2 messages. Possible to design shortcuts so O(log N) neighbors, O(log N) messages in query 5 Who s resp for key 0? Peer Churn 5 4 To handle peer churn, require each peer to know the IP address of its two successors. Each peer periodically pings its two successors to see if they are still alive. 0 8 Peer 5 abruptly leaves Peer 4 detects; makes 8 its immediate successor; asks 8 who its immediate successor is; makes 8 s immediate successor its second successor. What if peer wants to join?