Lecture 5 Internet Core: Protocol layers

Transcription

1 Internet Core: Protocol layers

2 Application Layer We will learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP application transport network data link physical Focus on client- model Web HTML application transport network data link physical application transport network data link physical

3 Web and HTTP First some jargon What is WWW? Web page consists of objects Objects can be HTML file, JPEG image, JavaScript, video file, 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

4 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol PC running Explorer Resources to be accessed by HTTP are identified using Uniform Resource Identifiers (URIs) or, more specifically, Uniform Resource Locators (URLs) Mac running Navigator Server running Apache Web

5 HTTP overview (continued) HTTP requires guarantee: HTTP (app.) runs over TCP (transport) client initiates TCP connection (creates socket) to, port 80 accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web (HTTP ) TCP connection closed

6 HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection. Persistent HTTP Multiple objects can be sent over single TCP connection between client and.

7 Nonpersistent HTTP Suppose enters URL (contains text, references to 10 jpeg images) time 1a. HTTP client initiates TCP connection to HTTP (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 at host waiting for TCP connection at port 80. accepts connection, notifying client 3. HTTP receives request message, forms response message containing requested object, and sends message into its socket

8 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 closes TCP connection.

9 Non-Persistent HTTP: Response time Definition of RTT: time for a small packet to travel from client to and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time initiate TCP connection RTT request file RTT file received time time time to transmit file

10 Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP connection Improved version: browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP leaves connection open after sending response subsequent HTTP messages between same client/ sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects

11 HTTP Message Format 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)

12 HTTP request message: general format

13 Request Method types GET Requests a representation of the specific resource. POST Submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both. PUT Uploads a representation of the specific resource. DELETE Deletes the specific resource. TRACE Echoes back the received request, so that a client can see what intermediate s are adding or changing in the request. 1-13

14 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...

15 HTTP response status codes In first line in ->client response message. A few 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 (Location:) 400 Bad Request request message not understood by 404 Not Found requested document not found on this 505 HTTP Version Not Supported

16 HTTP overview (continued) HTTP is stateless maintains no information about past client requests If HTTP is stateless, how does it remember our session information? HTTP Cookies

17 User- state: cookies Many major Web sites use cookies 1) cookie file kept on s host, managed by s browser 2) back-end database at Web site Example: When initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID Cookies are not cache!

18 Cookies: keeping state (cont.) client cookie file amazon 1678 usual http request msg usual http response Set-cookie: 1678 Amazon creates ID 1678 for create entry one week later: amazon 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg cookiespecific action cookiespectific action access access backend database

19 Cookies (continued) Remember HTTP is stateless What cookies can bring: Authorization Previous session (e.g., shopping carts) aside Cookies and privacy: cookies permit sites to learn a lot about you!!! Security???

20 Category of cookies First party cookies Same domain Third party cookies Different domain than what is being shown Mostly used for tracking s browsing websites (spyware) Cookies are not malwares but they can violate privacy rules

21 How to speed up Internet access? An example: average object size = 100,000 bits origin s avg. request rate from institution s browsers to origin s = 15/sec public Internet Internet delay from institutional router to any origin and back to router = 2 sec Consequences utilization on LAN = 15% utilization on access link = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + seconds + milliseconds Huge delay institutional network 1.5 Mbps access link 10 Mbps LAN

22 How to speed up Internet access? (cont) possible upgrade increase bandwidth of access link to, say, 10 Mbps public Internet origin s consequence utilization on LAN = 15% utilization on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs institutional network 10 Mbps access link 10 Mbps LAN often a costly upgrade

23 A better solution: Web Cache Install web cache suppose hit rate is 0.4 public Internet origin s consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin utilization of access link reduced to 60%, resulting in negligible delays institutional network 1.5 Mbps access link 10 Mbps LAN institutional cache

24 Web caches (proxy ) Goal: satisfy client request without involving origin sets browser: Web accesses via cache browser sends all HTTP requests to cache client Proxy origin object in cache: cache returns object else cache requests object from origin, then returns object to client client origin

25 More about Web caching 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 client client Proxy origin origin

26 Application layer 2.1 Web browsing and HTTP 2.2 File transferring and FTP (File Transfer Protocol) 2.3 and SMTP

27 FTP: the file transfer protocol 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 : port 21

28 FTP: separate control, data connections 1. FTP client contacts FTP at port client authorized over control connection 3. client browses remote directory by sending commands over control connection. 4. when receives file transfer command, opens 2 nd TCP connection (for file) to client 5. after transferring one file, closes data connection. FTP client TCP control connection port 21 TCP data connection port 20 FTP

29 Application layer 2.1 Web browsing and HTTP 2.2 File transferring and FTP 2.3 and SMTP (Simple Mail Transfer Protocol)

30 Electronic Mail Three major components: agents mail s simple mail transfer protocol: SMTP User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Outlook, Mozilla Thunderbird outgoing, incoming messages stored on mail SMTP mail agent agent SMTP SMTP agent mail outgoing message queue mailbox agent agent agent

31 Electronic Mail: mail s Mail Servers mailbox contains incoming messages for message queue of outgoing (to be sent) mail messages SMTP protocol between mail s to send messages client: sending mail mail SMTP mail agent SMTP SMTP mail agent agent agent : receiving mail agent agent

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

33 SMTP: comparison with HTTP Comparison with HTTP: SMTP uses persistent connections HTTP: pull SMTP: push HTTP: port 80 SMTP: port 25 Both require guarantee Both use TCP connection

34 Mail access protocols agent SMTP SMTP access protocol agent sender s mail receiver s mail Post Office Protocol (POP3) Internet Message Access Protocol (IMAP)

35 POP3 and IMAP: Comparison POP3 Uses download and delete mode. Bob cannot re-read e- mail if he changes client POP3 is stateless across sessions IMAP Keep all messages in one place: the Allows to organize messages in folders IMAP keeps state across sessions: names of folders and mappings between message IDs and folder name