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Application architectures

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Presentation on theme: "Application architectures"— Presentation transcript:

1 Application architectures
Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P 2: Application Layer

2 Client-server architecture
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 2: Application Layer

3 Pure P2P architecture no always-on server
arbitrary end systems directly communicate peers are intermittently connected and change IP addresses example: Gnutella Highly scalable But difficult to manage 2: Application Layer

4 Hybrid of client-server and P2P
Napster File transfer P2P File search centralized: Peers register content at central server Peers query same central server to locate content Instant messaging Chatting between two users is P2P Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies 2: Application Layer

5 HTTP overview HTTP: hypertext transfer protocol
Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web 68% 2005 HTTP response Mac running Navigator 2: Application Layer

6 HTTP overview (continued)
Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer 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 2: Application Layer

7 HTTP connections Nonpersistent HTTP
At most one object is sent over one TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode 2: Application Layer

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

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

10 Persistent HTTP Nonpersistent HTTP issues:
requires 2 RTTs per object OS overhead for each TCP connection browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects 2: Application Layer

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

12 HTTP request message: general format
2: Application Layer

13 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: 2: Application Layer

asks server to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field 2: Application Layer

15 http session captured by ethereal
2: Application Layer

16 User-server state: cookies
Many major Web sites use 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 access Internet always from same PC She visits a specific e-commerce site for first time When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID 2: Application Layer

17 Cookies: keeping “state” (cont.)
client server usual http request msg usual http response + Set-cookie: 1678 cookie: 1678 usual http response msg cookie- specific action spectific Cookie file ebay: 8734 server creates ID 1678 for user entry in backend database Cookie file amazon: 1678 ebay: 8734 access one week later: access Cookie file amazon: 1678 ebay: 8734 2: Application Layer

18 Cookies (continued) Cookies and privacy: What cookies can bring:
aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and to sites search engines use redirection & cookies to learn yet more advertising companies obtain info across sites What cookies can bring: authorization shopping carts recommendations user session state (Web ) 2: Application Layer

19 Web caches (proxy server)
Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client origin server Proxy server HTTP request HTTP request client HTTP response HTTP response HTTP request HTTP response client origin server 2: Application Layer

20 More about Web caching Why Web caching?
Cache acts as both client and server Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? Reduce response time for client request. Reduce traffic on an institution’s access link. Internet dense with caches enables “poor” content providers to effectively deliver content 2: Application Layer

21 Electronic Mail Three major components: SMTP SMTP SMTP user agents
user mailbox outgoing message queue user agent Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent user agent user agent 2: Application Layer

22 Electronic Mail: mail servers
user agent 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 client: sending mail server “server”: receiving mail server mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent user agent user agent 2: Application Layer

23 Electronic Mail: 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 2: Application Layer

24 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 mail server mail server 1 user agent user agent 2 3 6 4 5 2: Application Layer

25 Mail message format header body
SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from SMTP commands! body the “message”, ASCII characters only header blank line body 2: Application Layer

26 Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......base64 encoded data MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data 2: Application Layer

27 Mail access protocols SMTP SMTP access protocol
user agent 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] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. 2: Application Layer

28 POP3 protocol authorization phase C: list transaction phase, client:
S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> C: dele 1 C: retr 2 C: dele 2 C: quit S: +OK POP3 server signing off 2: Application Layer

29 POP3 (more) and IMAP More about POP3 IMAP
Previous example uses “download and delete” mode. Bob cannot re-read 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: names of folders and mappings between message IDs and folder name 2: Application Layer

30 P2P file sharing Alice chooses one of the peers, Bob.
File is copied from Bob’s PC to Alice’s notebook: HTTP While Alice downloads, other users uploading from Alice. Alice’s peer is both a Web client and a transient Web server. All peers are servers = highly scalable! Example Alice runs P2P client application on her notebook computer Intermittently connects to Internet; gets new IP address for each connection Asks for “Hey Jude” Application displays other peers that have copy of Hey Jude. 2: Application Layer

31 P2P: centralized directory
directory server peers Alice Bob 1 2 3 original “Napster” design 1) when peer connects, it informs central server: IP address content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob 2: Application Layer

32 P2P: problems with centralized directory
Single point of failure Performance bottleneck Copyright infringement file transfer is decentralized, but locating content is highly centralized 2: Application Layer

33 Query flooding: Gnutella
fully distributed no central server public domain protocol many Gnutella clients implementing protocol overlay network: graph edge between peer X and Y if there’s a TCP connection all active peers and edges is overlay net Edge is not a physical link Given peer will typically be connected with < 10 overlay neighbors 2: Application Layer

34 Gnutella: protocol Query message sent over existing TCP connections
File transfer: HTTP Query message sent over existing TCP connections peers forward Query message QueryHit sent over reverse path Query QueryHit Scalability: limited scope flooding 2: Application Layer

35 Gnutella: Peer joining
Joining peer X must find some other peer in Gnutella network: use list of candidate peers X sequentially attempts to make TCP with peers on list until connection setup with Y X sends Ping message to Y; Y forwards Ping message. All peers receiving Ping message respond with Pong message X receives many Pong messages. It can then setup additional TCP connections 2: Application Layer

36 Exploiting heterogeneity: KaZaA
Each peer is either a group leader or assigned to a group leader. TCP connection between peer and its group leader. TCP connections between some pairs of group leaders. Group leader tracks the content in all its children. 2: Application Layer

37 KaZaA tricks Limitations on simultaneous uploads Request queuing
Incentive priorities Parallel downloading 2: Application Layer

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