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COMP 150-IDS: Internet Scale Distributed Systems (Spring 2016)

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Presentation on theme: "COMP 150-IDS: Internet Scale Distributed Systems (Spring 2016)"— Presentation transcript:

1 COMP 150-IDS: Internet Scale Distributed Systems (Spring 2016)
URIs and RFC 3986 Noah Mendelsohn Tufts University Web:

2 Goals What is named when we use the Web
Learn the detailed design of URIs See how the naming principles we’ve explored are reflected in Web architecture and URIs Learn to read RFCs and to study the art of writing specifications Understand why grammars are important

3 Review: Naming Questions
3 3

4 Some characteristics of names
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?

5 Review Web Architecture Basics

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

7 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?

8 The user clicks on a link
URI is URI is

9 The http “scheme” tells client to send HTTP GET msg
URI is URI is HTTP GET

10 The client sends an HTTP GET
URI is HTTP GET 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 Host: webarch.noahdemo.com

11 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :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 Code 200 Means Success! demo1/test.html Host: webarch.noahdemo.com HTTP RESPONSE

12 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :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 The “representation” returned is an HTML document Host: webarch.noahdemo.com HTTP RESPONSE

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

14 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.

15 The Structure of URIs 16 16

16 A simple URI

17 A simple URI

18 A simple URI Scheme

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

20 Authority: who controls allocation of this name?
A simple URI Authority Authority: who controls allocation of this name?

21 // Fixed in grammar to indicate authority follows
A simple URI // Fixed in grammar to indicate authority follows

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

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

24 A more complex URI

25 A more complex URI http://www.tufts.edu?student=smith Query component
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
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

28 Some characteristics of URIs
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?

29 Some characteristics of URIs
Both supported Some characteristics of URIs 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?

30 Some characteristics of URIs
Depends on scheme Some characteristics of URIs 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?

31 Some characteristics of URIs
Yes… URIs “on the side of a bus” is an important goal… but some URIs are complex Some characteristics of URIs 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?

32 Some characteristics of URIs
Allowed but not required 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?

33 Some characteristics of URIs
With most schemes, absolute URIs are global Some characteristics of URIs 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?

34 Some characteristics of URIs
FILE: scheme is not global! Some characteristics of URIs 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?

35 Some characteristics of URIs
NO!! Status code 404 is key to Web scalability Some characteristics of URIs 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?

36 Some characteristics of URIs
Some aliases required e.g.: http vs. HTTP… worst cases depend on users Some characteristics of URIs 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?

37 Some characteristics of URIs
Depends on scheme and user… see Metadata in URI finding Some characteristics of URIs 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?

38 Some characteristics of URIs
URIs are the structuring mechanism for the Web as a whole Some characteristics of URIs 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?

39 Some characteristics of URIs
Designed to allow mappings to hierarchical systems Some characteristics of URIs 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?

40 Some characteristics of URIs
Decentralized allocation except: 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?

41 Some characteristics of URIs
scheme names centrally registered with IANA 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?

42 Some characteristics of URIs
For http and mailto schemes: central Domain Name (DNS) registration required for authority 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?

43 Some characteristics of URIs
Yes. E.g.: ASCII-only, spaces and some punctuation must be %encoded 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?

44 Some characteristics of URIs
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 silent on this…but HTTP redirection provides for indirect identification

45 Grammars 46 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:

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

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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