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COS 420

COS 420. DAY 26. Agenda. Group Project Discussion Final Paper Due Monday, May 3, 8AM User Manual Protocol Definition Program requirements Technical Specifications Applications and Presentation Due May 5 Noon Final Exam available Thursday April 29 Due Tuesday May 5 noon

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COS 420

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  1. COS 420 DAY 26

  2. Agenda • Group Project Discussion • Final Paper Due Monday, May 3, 8AM • User Manual • Protocol Definition • Program requirements • Technical Specifications • Applications and Presentation Due May 5 Noon • Final Exam available Thursday April 29 • Due Tuesday May 5 noon • Assignment 4 is Due • Today we will discuss HTTP, RTP, SNMP and Protocol Dependencies

  3. PART XXVIII APPLICATIONS: WORLD WIDE WEB (HTTP)

  4. World Wide Web • Distributed hypermedia paradigm • Major service on the Internet • Use surpassed file transfer in 1995

  5. Web Page Identifier • Known as Uniform Resource Locator (URL) • Encodes • Access protocol to use • Domain name of server • Protocol port number (optional) • Path through server’s file system (optional) • Parameters (optional) • Query (optional) • Format • http: // hostname [: port] / path [; parameters] [? query]

  6. Web Standards • Separate standards for • Representation • Transfer

  7. Representation • HyperText Markup Language (HTML) • Document contains text plus embedded links • HTML gives guidelines for display, not details • Consequence: two browsers may choose to display same document differently

  8. Transfer • Used between browser and web server • Protocol is HyperText Transfer Proto (HTTP) • Runs over TCP

  9. HTTP Characteristics • Application level • Request/response paradigm • Stateless • Permits bi-directional transfer • Offers capability negotiation • Support for caching • Support for intermediaries

  10. HTTP Operation • Browser sends requests to which server replies • Typical request: GET used to fetch document • Example GET http://www.cs.purdue.edu/people/comer/ HTTP/1.1 • Relative URL also permitted GET /people/comer/ HTTP/1.1

  11. Error Messages • HTTP includes set of error responses • Server can format error as HTML message for user or use internal form and allow browser to format message

  12. Persistent Connections • HTTP version 1.0 uses one TCP connection per transfer • Browser forms TCP connection to server • Browser sends GET request • Server returns header describing item • Server returns item • Server closes connection • HTTP version 1.1 permits connection to persist across multiple requests

  13. HTTP Headers • HTTP uses MIME-like headers to carry meta information. Both browsers and servers send headers that allow them to negotiate agreement on the document representation and encoding to be used.

  14. Handing Persistence • To allow a TCP connection to persist through multiple requests and responses, HTTP sends a length before each response. If it does not know the length, a server informs the client, sends the response, and then closes the connection.

  15. Headers And Length Encoding • HTTP headers use same syntax as email headers • Lines of text followed by blank line • Lines of text have form keyword:information • For persistent connections header specifies length (in octets) of data item that follows

  16. Items That Can Appear In An HTTP Header

  17. Example Of Header Content-Length: 34 Content-Language: english Content-Encoding: ascii <HTML> A trivial example. </HTML> • Note: if length is not known in advance, server can inform browser that connection will close following transfer • Connection: close

  18. Negotiation • Either server or browser can initiate • Items sent in headers • Can specify representations that are acceptable with preference value assigned to each • Example Accept: text/html, text/plain; q=0.5, text/x-dvi; q=0.8

  19. Items For Negotiation • Accept-Encoding: • Accept-Charset: • Accept-Language:

  20. Conditional Request • Allows browser to check cached copy for freshness • Eliminates useless latency • Sends If-Modified-Since in header of GET request • Example If-Modified-Since: Wed, 31 Dec 2003 05:00:01 GMT

  21. Proxy Servers • Browser can be configured to contact proxy • Permits caching for entire organization • Server can specify maximum number of proxies along path (including none)

  22. Caching Of Web Pages • Caching essential to efficiency • Server specifies • Whether page can be cached • Maximum time page can be kept • Intermediate caches and browser cache web pages • Browser can specify maximum age of page (forces intermediate caches to revalidate)

  23. Summary • Web is major application in the Internet • Standard for representation is HTML • Standard for transfer is HTTP • Request-response protocol • Header precedes item • Version 1.1 permits persistent connections • Server specifies length of time item can be cached • Browser can issue conditional request to validate cached item

  24. PART XXIX APPLICATIONS: VOICE AND VIDEO OVER IP (VOIP, RTP, RSVP)

  25. TCP/IP Protocols • Designed for data • Can also handle voice and video • Upcoming application is Voice Over IP (VOIP)

  26. Representation • Voice and video must be converted between analog and digital forms • Typical device is codec (coder/decoder) • Example encoding used by phone system is Pulse Code Modulation (PCM) • Note: 128 second audio clip encoded in PCM requires one megabyte of memory • Codec for voice, known as vocodec, attempts to recognize speech rather than just waveforms

  27. Playback • Internet introduces burstiness • Jitter buffer used to smooth bursts • Protocol support needed

  28. Requirements For Real-Time • Because an IP Internet is not isochronous, additional protocol support is required when sending digitized real-time data. In addition to basic sequence information that allows detection of duplicate or reordered packets, each packet must carry a separate timestamp that tells the receiver the exact time at which the data in the packet should be played.

  29. Illustration Of Jitter Buffer

  30. Real-Time Transport Protocol (RTP) • Internet standard • Provides playback timestamp along with data • Allows receiver to playback items in sequence

  31. RTP Message Format • Each message begins with same header

  32. Terminology And Layering • Name implies that RTP is a transport-layer protocol • In fact • RTP is an application protocol • RTP runs over UDP

  33. Mixing • RTP can coordinate multiple data streams • Intended for combined audio and video • Up to 15 sources • Header specifies mixing

  34. RTP Control Protocol (RTCP) • Required part of RTP • Allows sender and receiver to exchange information about sessions that are in progress • Separate data stream • Uses protocol port number one greater than port number of data stream

  35. RTCP Message Types

  36. RTCP Interaction • Receivers generate receiver report messages • Inform sender about reception and loss • Senders generate sender report • Provide absolute timestamp and relate real time to relative playback timestamp

  37. VOIP • RTP used for encoding and transfer • Also need signaling protocol for • Dialing • Answering a call • Call forwarding • Gateway used to connect IP telephone network to Public Switched Telephone Network (PSTN) • PSTN uses SS7 for signaling

  38. Standards For IP Telephony • H.323 • SIP

  39. H.323 ITU standard • Set of many protocols • Major protocols specified by H.323 include

  40. How H.323 Protocols Fit Together

  41. Session Initiation Protocol (SIP) • IETF standard • Alternative to H.323 • Less functionality • Much smaller • Permits SIP telephone to make call • Does not require RTP for encoding

  42. Session Description Protocol (SDP) • Companion to SIP • Specified detail such as • Media encoding • Protocol port numbers • Multicast addresses

  43. Quality Of Service (QoS) • Statistical guarantee of performance • Requires changes to underlying Internet infrastructure • Proponents claim it is needed for telephony • Others claim only larger bandwidth will solve the problem

  44. Resource ReSerVation Protocol (RSVP) • IETF response to ATM • End-to-end QoS guarantees • Abstraction is unidirectional flow • Initiated by endpoint

  45. RSVP Requests • An endpoint uses RSVP to request a simplex flow through an IP internet with specified QoS bounds. If routers along the path agree to honor the request, they approve it; otherwise, they deny it. If an application needs QoS in two directions, each endpoint must use RSVP to request a separate flow.

  46. Note About RSVP • RSVP defines • Messages endpoint sends to router to request QoS • Messages routers send to other routers • Replies • RSVP does not specify how enforcement done • Separate protocol needed

  47. Common Open Policy Services (COPS) • Proposed enforcement protocol for RSVP • Known as traffic policing • Uses policy server • Checks data sent on flow to ensure the flow does not exceed pre-established bounds

  48. Summary • Codec translates between analog and digital forms • RTP used to transfer real-time data • RTP adds timestamp that sender uses to determine playback time • RTCP is companion protocol for RTP that senders and receivers use to control and coordinate data transfer • RSVP and COPS provide quality of service guarantees

  49. PART XXX APPLICATIONS: INTERNET MANAGEMENT (SNMP)

  50. Management Protocols • Early network systems used two approaches • Separate, parallel management network • Link-level management commands • TCP/IP pioneered running management protocols at the application layer • Motivation: provide internet-wide capability instead of single network capability

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