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Understanding Common Application Protocols in Networking

This chapter explores the functionality and design alternatives for common application protocols such as web (HTTP), email (SMTP), FTP, DNS, P2P, and IM. It also discusses the basics of application design and various distributed application designs.

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Understanding Common Application Protocols in Networking

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  1. ELEG 651 and CPEG 419 The Application Layer Lecture 3 – Chapter 2

  2. Goals of this Chapter • To understand common application protocols work • Web (http) • Email (smtp) • FTP • DNS • P2P • IM • To understand how the design alternatives for application design • A network application runs on many hosts, it is a distributed application • This chapter discusses several designs of distributed applications

  3. Road Map • Application basics • Web • Email • FTP • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  4. Road Map • Application basics • Web • Email • FTP • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  5. Creating a network app write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software No need to write software for network-core devices Network-core devices do not run user applications 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

  6. An App-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 allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., Skype

  7. Ports An application is identified by the hosts IP address, transport protocols, and port E.g., A web server has a particular IP address, listens with TCP on port 80. A web browser on a host will connect a request a file from the web server. The browser is identified by the host’s IP address and a TCP port. host (web server) host UDP TCP UDP TCP 0 0 0 0 4567 80 4568 216-1 216-1 216-1 216-1

  8. What transport service does an app need? Data reliability some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Throughput • some apps (e.g., multimedia) require minimum amount of throughput to be “useful” (i.e., in order for the user to gain utility) • other apps (“elastic apps”) make use of whatever throughput they get Security • Encryption, data integrity, …

  9. Transport service requirements of common apps Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games instant messaging Time Sensitive no no not really yes, 100’s msec yes, few secs yes, 100’s msec yes and no Throughput elastic elastic some what elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss

  10. 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: reliability, flow control, congestion control, timing, throughput guarantee, or security Does not require connection set-up Packets can be sent at any rate desired (but this might be cause considerable congestion)

  11. Internet apps: application, transport protocols 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 Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony

  12. Road Map • Application basics • Web • Email • FTP • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  13. Web and HTTP www.someschool.edu/someDept/pic.gif path name host name • Web page consists of objects • Object can be HTML file, JPEG image, Java applet, audio file,… • Web page consists of base HTML-file which includes several referenced objects • The browser first requests the base file • The base file species text and URLs of objects • The browser requests these objects, where ever they are (not always on the same server) • HTTP is used to request the base file and all the other files • Note, that HTTP can be used for other applications besides web • Each object is addressable by a URL • Example URL:

  14. 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 request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator

  15. 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

  16. 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 server.

  17. Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 (contains text, references to 10 jpeg images) 1b. HTTP server at host www.someSchool.edu 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 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 4. HTTP server closes TCP connection. time 6.Steps 1-5 repeated for each of 10 jpeg objects

  18. Non-Persistent HTTP: Response time time to transmit file Definition of RTT: time for a small packet to travel from client to server 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

  19. 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 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects

  20. HTTP request message • two types of HTTP messages: request, response • HTTP request message: • ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu 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

  21. HTTP request message: general format

  22. HTTP response message status line (protocol status code status phrase) HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested HTML file

  23. HTTP response status 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 server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported In first line in server->client response message. A few sample codes:

  24. Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: 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 telnet cis.poly.edu 80 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET /~ross/ HTTP/1.1 Host: cis.poly.edu 3. Look at response message sent by HTTP server!

  25. Wireshark (ethereal) • Wireshark captures all packets that pass through the hosts interface • To run Wireshark , libpcap (linux) or winpcap (windows) must be installed. It comes with wireshark package • Then, run wireshark • Select Capture • Find the active interface • E.g., mot generic dialup, nor vnp, nor packet scheduler, but wireless …. With IP address • Then select prepare • Let’s watch TCP packets on port 80 • Next to capture filter, enter TCP port 80 • Select update in realtime and autoscroll • Might need to enable or disable “capture in promiscuous mode” • Press start • Press close • Load www.eecis.udel.edu page in browser • Press stop in Wireshark • Find http request to 128.4.40.10. • Right click and select follow TCP stream

  26. Web caches (proxy 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 HTTP request HTTP request HTTP response HTTP response HTTP request HTTP response Goal: reduce network utilization by satisfying client request without involving origin server origin server Proxy server client client origin server

  27. More about 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 (but so does P2P file sharing)

  28. Caching example Assumptions average object size = 100,000 bits avg. request rate from institution’s browsers to origin servers = 15/sec delay from institutional router to any origin server 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 + minutes + milliseconds origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

  29. Caching example (cont) possible solution increase bandwidth of access link to, say, 10 Mbps consequence utilization on LAN = 15% utilization on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade origin servers public Internet 10 Mbps access link institutional network 10 Mbps LAN institutional cache

  30. Caching example (cont) possible solution: install cache suppose hit rate is 0.4 consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

  31. Conditional GET Goal: don’t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP response HTTP/1.0 304 Not Modified server cache HTTP request msg If-modified-since: <date> object not modified HTTP request msg If-modified-since: <date> object modified HTTP response HTTP/1.0 200 OK <data>

  32. Road Map • Application basics • Web • FTP • Email • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  33. FTP: the file transfer protocol transfer file to/from remote host client/server model client: side that initiates transfer (either to/from remote) server: remote host ftp: RFC 959 ftp server: listens on port 21 FTP user interface FTP client FTP server file transfer user at host remote file system local file system

  34. FTP is weird: separate control and data connections FTP client contacts FTP server at port 21, TCP is transport protocol client authorized over control connection This is done in “clear text” (i.e., unencrypted) So if some one if sniffing packets, your password might be learned. Sniffing packets is difficult on ethernet, encrypted wifi, and DSL, but is possible on cable modems client browses remote directory by sending commands over control connection. Data is transferred over different connections. Two approaches Active Passive TCP control connection port 21 TCP data connection port 20 FTP client FTP server • Active mode is a problem for firewalls • If my desktop is not a server, if should not receive any requests for connections. • But FTP servers will make such a requests • Active • The client opens a TCP socket with on some port (port number >1024) • The client sends the server the port • The server connects to the client’s port where the servers source port is 20

  35. FTP Passive mode When a file is to be transferred, the server opens a port (number>1024 and not 20) The server sends this port number information over the command connection The client connects to the servers over this port. TCP control connection port 21 TCP data connection high port FTP client FTP server • Drawback of passive • Some enterprises (companies) like to control which applications are used • E.g., web browsing is ok, but skype is not • One way to do this is to block out going connections based on the port. • However, this will cause FTP to fail, unless the device that blocks connections is smart

  36. Road Map • Application basics • Web • FTP • Email • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  37. Email Protocol Design user agent user agent mail server mail server • Basic assumption: weak user agents and strong mail servers • The user wants to send the mail and leave • The user wants to get the mail • The user may come and go whenever (e.g., roaming laptop) • It should be possible to send mail to a user even if neither user is online at the same time. • We conclude that there must be a middle man/mail server. • Servers are not that strong: The protocol must be as robust as possible to servers being offline • No single server – why • Single point of failure • The server would have to be too big (congestion) • We conclude that there should be many mail servers • Two types of hosts • Users • Mail servers • Each user has a mail box in its mail server • Users retrieve mail from their mail server at there convenience • Users give mail to their mail servers to deliver the mail • Mail servers communicate with • The users that have mail boxes in the server • Other mail servers

  38. Email Protocol Design user agent user agent mail server mail server • Two types of hosts • Users • Mail servers • Each user has a mail box in its mail server • Users retrieve mail from their mail server at there convenience • Users give mail to their mail servers to deliver the mail • Mail servers communicate with • The users that have mail boxes in the server • Other mail servers Destination user requests emails from mailbox User composes mail and sends it to its mail server (or a mail server that will send mail for it) Mail server finds the destination mail server and attempts to send the mail Destination server gives mails to user

  39. Email Protocol Design user agent user agent mail server mail server • Two types of hosts • Users • Mail servers • Each user has a mail box in its mail server • Users retrieve mail from their mail server at there convenience • Users give mail to their mail servers to deliver the mail • Mail servers communicate with • The users that have mail boxes in the server • Other mail servers Destination user requests emails from mailbox User composes mail and sends it to its mail server (or a mail server that will send mail for it) Mail server finds the destination mail server and attempts to send the mail Destination server gives mails to user SMTP SMTP POP3 IMAP …

  40. Electronic Mail: Details 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, Mozilla Thunderbird Put outgoing on server (with SMTP) Get incoming messages from server user agent user agent user agent user agent user agent user agent mail server mail server outgoing message queue user mailbox SMTP mail server SMTP SMTP

  41. Electronic Mail: mail servers Mail Servers mailbox contains incoming messages for user messagequeue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages client: sending mail server “server”: receiving mail server Reliable: several attempts and provide notification if delivery fails user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server

  42. Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server Emails are pushed to servers (but users pull messages from servers) 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 Makes it difficult to send attachments

  43. Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” bob@someschool.edu 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 user agent mail server mail server 1 2 6 3 4 5

  44. Sample SMTP interaction Client connects to server S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... 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

  45. Try SMTP interaction for yourself: • telnet mail.eecis.udel.edu 25 • see 220 reply from server • enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader)

  46. SMTP: final words SMTP uses persistent connections SMTP requires message (header & body) to be in 7-bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg

  47. Mail access • POP3 and IMAP are two protocols for access mail on a mail server • Web-based mail works differently, the web mail server and the mail server can be integrated, so that there is no user agent.

  48. Mail access protocols 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: gmail, Hotmail, Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP SMTP access protocol receiver’s mail server

  49. Road Map • Application basics • Web • FTP • Email • DNS • P2P • Graph theory • State diagrams • P2P design • IM

  50. DNS – domain name system • Change names, like www.yahoo.com into IP address. • Services provided by DNS • Name to address translation • Host aliasing • A host relay1.west-coast.yahoo.com could have two aliases, yahoo.com and www.yahoo.com. • In this case, the canonical hostname is relay1.west-coast.yahoo.com. • DNS can provide canonical host names • Mail server aliasing • When a mail server wants to send a mail to Me@udel.edu, it does not send it to www.udel.edu, but to mail.udel.edu. Or maybe udmail.udel.edu. DNS can translate udel.edu to mail.udel.edu • (Cheap) Load distribution • Cnn.com has several servers. • DNS will respond with all address, • but it will reorder the addresses every time. • If the client uses the first address listed, then each client will use different servers. • Content distribution networks (CDN) are better ways of load balancing

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