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URIs and RFC 3986

COMP 150-IDS: Internet Scale Distributed Systems (Fall 2012). URIs and RFC 3986. Noah Mendelsohn Tufts University Email: noah@cs.tufts.edu Web: http://www.cs.tufts.edu/~noah. Goals. Learn the detailed design of URIs See how the naming principles we’ve explored are reflected in URIs

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URIs and RFC 3986

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  1. COMP 150-IDS: Internet Scale Distributed Systems (Fall 2012) URIsandRFC 3986 Noah Mendelsohn Tufts UniversityEmail: noah@cs.tufts.edu Web: http://www.cs.tufts.edu/~noah

  2. Goals • Learn the detailed design of URIs • See how the naming principles we’ve explored are reflected in URIs • Learn to read RFCs and to study the art of writing specifications • Understand why grammars are important

  3. ReviewWeb Architecture Basics

  4. Architecting a universal Web • Identification: URIs • Interaction: HTTP • Data formats: HTML, JPEG, etc.

  5. What Happens When We Browse a Web Page? Consider:What are all the things that are “named”in the interaction between browser and Web Server?

  6. The user clicks on a link URI is http://webarch.noahdemo.com/demo1/test.html URI is http://webarch.noahdemo.com/demo1/test.html

  7. The http “scheme” tells client to send HTTP GET msg URI is http://webarch.noahdemo.com/demo1/test.html URI is http://webarch.noahdemo.com/demo1/test.html HTTP GET

  8. demo1/test.html GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us The client sends an HTTP GET URI is http://webarch.noahdemo.com/demo1/test.html HTTP GET Host: webarch.noahdemo.com

  9. HTTP/1.1 200 OK Date: Tue, 28 Aug 2007 01:49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET HTTP Status Code200 Means Success! demo1/test.html Host: webarch.noahdemo.com HTTP RESPONSE

  10. HTTP/1.1 200 OK Date: Tue, 28 Aug 2007 01:49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET demo1/test.html Host: webarch.noahdemo.com The “representation” returned is an HTML document HTTP RESPONSE

  11. Architecting a universal Web • Identification: URIs • Interaction: HTTP • Data formats: HTML, JPEG, etc.

  12. Assign URIs for all Resources • A resource is something that has information (e.g. a Web page) • If a resource doesn’t have a URI, you can’t link to it…it’s not part of the Web.

  13. Review:Naming Questions 14

  14. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too few names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of names

  15. The Structure of URIs 16

  16. A simple URI http://uss.tufts.edu/stuserv/acadcal/

  17. A simple URI http://uss.tufts.edu/stuserv/acadcal/

  18. A simple URI Scheme http://uss.tufts.edu/stuserv/acadcal/

  19. Schemes Scheme http://uss.tufts.edu/stuserv/acadcal/ mailto:noah@cs.tufts.edu Schemes let us name different kinds of things, accessed in different ways.

  20. A simple URI Authority http://uss.tufts.edu/stuserv/acadcal/ Authority: who controls allocation of this name?

  21. A simple URI // Fixed in grammar to indicate authority follows http://uss.tufts.edu/stuserv/acadcal/

  22. A simple URI Path http://uss.tufts.edu/stuserv/acadcal/ Path: provides for hierarchical naming…… also supports “../xxx” relative syntax?

  23. A simple URI Path http://uss.tufts.edu/stuserv/acadcal/ Path: provides for hierarchical naming…… maps well to heirarchicalinformation systems

  24. A more complex URI http://www.tufts.edu?student=smith

  25. Query component A more complex URI http://www.tufts.edu?student=smith The query is part of the URI... However, in many cases, all URIs with a common path are processed by the same server-side code Also…HTML forms are useful for filling in the query components

  26. Fragments identify parts of documents Fragments http://tools.ietf.org/html/rfc3986#section-3.5 Fragment interpretation depends on the media type of the returned representation (text/html)…this is useful but tricky and causes a variety of problems.

  27. Characteristics of URIs 28

  28. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs

  29. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Both supported Some characteristics of URIs

  30. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Depends on scheme Some characteristics of URIs

  31. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Yes…URIs “on the side of a bus”is an important goal…but some URIs are complex Some characteristics of URIs

  32. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs Allowed but not required

  33. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? With most schemes, absolute URIs are global Some characteristics of URIs

  34. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? FILE: scheme is not global! Some characteristics of URIs

  35. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? NO!!Status code 404 is key to Web scalability Some characteristics of URIs

  36. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some aliases requirede.g.: http vs. HTTP…worst cases depend on users Some characteristics of URIs

  37. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Depends on scheme and user…see Metadata in URI finding Some characteristics of URIs

  38. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? URIs are the structuringmechanism for the Web as a whole Some characteristics of URIs

  39. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Designed to allow mappings to hierarchical systems Some characteristics of URIs

  40. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs Decentralized allocation except:

  41. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs scheme names centrally registered with IANA

  42. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs For http and mailto schemes:central Domain Name (DNS) registration required for authority

  43. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs Yes. E.g.: ASCII-only, spaces and some punctuation must be %encoded

  44. Absolute vs. relative Address (locator)? Human readable? Short/convenient? Global (context independent)? Ensures referent exists? Aliases? (too many names) Opaque vs. data-carrying? Reflect structure of system? Supports navigation: e.g. “..”? Who can generate them? Constraints from environment E.g. no “-” in C/C++ variable names Indirect identification allowed? Some characteristics of URIs URIs are silent on this…but HTTP redirection provides for indirect identification

  45. Grammars 46

  46. What are formal grammars? • Grammars are formal languages for specifying other languages • A grammar allows you to: • Always: determine whether a given string is “in” the specified language • Often: associate structures in the grammar with parts of the string • The Chomsky hierarchy: • Different grammars have different expressive power • Regular expressions recognize “regular languages” (ab*)  a, ab, abb, abbb • Context-free grammars are more powerful: typically used for programming languages • The ABNF used in RFC’s is a context-free grammar • Context-free grammars can be recognized (parsed) by a finite-state pushdown automaton Tutorial at: http://en.wikipedia.org/wiki/Chomsky_hierarchy

  47. Why use formal grammars for specifying languages? • Precise and rigorous • Less ambiguous than an explanation in English • Membership of a string in a language can be checked automatically • Tools to process the language can often be constructed automatically from the grammar

  48. ABNF: the grammar for IETF RFCs • ABNF example from RFC 3986:URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] • ABNF is itself specified in RFC 2234 • ABNF is convenient to use in fixed-font specification documents like RFCs

  49. Summary 50

  50. Summary • The structure and interpretation of URIs is set out in RFC 3986 • URIs embody many of the principles we have studied • Formal grammars are powerful tools for specifying names • The design decisions embodied in URIs are keys to the success of the Web!!

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