HTTP - timeline

1. HTTP - timeline • Mar 1990 CERN labs document proposing Web • Jan 1992 HTTP/0.9 specification • Dec 1992 Proposal to add MIME to HTTP • Feb 1993 UDI (Universal Document Identifier) Network • Mar 1993 HTTP/1.0 first draft • Jun 1993 HTML (1.0 Specification) • Oct 1993 URL specification • Nov 1993 HTTP/1.0 second draft • Mar 1994 URI in WWW • May 1996 HTTP/1.0 Informational, RFC 1945 • Jan 1997 HTTP/1.1 Proposed Standard, RFC 2068 • Jun 1999 HTTP/1.1 Draft Standard, RFC 2616 • 2001 HTTP/1.1 Formal Standard HTTP Protocol Design

2. Uniform Resource Identifier (URI) • Resource independent of its current location or name by which it is known • URI combination of : • Uniform Resource Locator (URL) - Several alternatives (e.g., http://, ftp://) - Most popular • Uniform Resource Name (URN) - Globally unique - Like ISBN for a book • URI characteristics • Absolute: if scheme:string (scheme: file news, http, telnet,…) • Relative: if no scheme HTTP Protocol Design

3. MIME and HTTP • Original proposal • All resources MIME encapsulated • Protocols such as Web should only handle MIME-compliant data • Adopted • Classification of data formats (MIME types) • Formats for multipart messages • Not adopted • Rich text markup mechanism (rather used HTML) • Addressing external documents (rather used URLs) HTTP Protocol Design

4. MIME and HTTP differences • MIME defined for e-mail • HTTP high performance • Interpretation of header fields (content- length) • Limitation on line length • HTTP is not MIME-compliant (content- encoding) • Different kinds of entities HTTP Protocol Design

5. HTTP terms • Message • Sequence of octets • Syntax: Request • Request-Line • General/Request/Entity Header(s) • CRLF • Optional Message Body • Syntax: Response • Status-Line • General/Response/Entity Header(s) • CRLF • Optional Message Body HTTP Protocol Design

6. HTTP terms (cont.) • Entity • Representation of a resource from request or response message • Includes entity headers and an optional entity body • Resource • “Network data object or service that can be identified by a URI” • User agent HTTP Protocol Design

7. HTTP/1.0 request methods • Safety: examines the state of a resource • Idempotent: side effects of one request == those of multiple requests • GET (safe, idempotent) • HEAD • POST (not safe, not idempotent) • PUT (not safe, idempotent) • Delete • LINK/UNLINK HTTP Protocol Design

8. HTTP/1.0 headers • General • Date • Pragma (no-cache) • Request • Authorization • From • If-Modified-Since • Referer • User-Agent • Response • Location (redirects) • Server • WWW-Authenticate (issues challenge) HTTP Protocol Design

9. HTTP/1.0 headers (cont.) • Entity • Allow (valid methods) • Content-Type • Content-Encoding • Content-Length • Expires • Last-Modified HTTP Protocol Design

10. HTTP/1.0 response classes? • From SMTP reply codes (yet no specific meaning) • X00: default response • 1XX: Informational • 2XX: Success 200 OK, 201 Created, 202 Accepted, 204 No Content • 3XX: Redirection 300 Multiple Choices, 301 Moved Permanently, 302 Moved Temporarily, 304 Not Modified • 4XX: Client error 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found • 5XX: Server error 500 Internal Server Error 502 Not Implemented HTTP Protocol Design

11. Problems with HTTP/1.0 • Lack of control: cache duration, cache location, selection among cached variants, … • Ambiguity of rules for proxies and caches • Download of full resource instead of needed part • Poor use of TCP: short Web responses • No guarantee for full receipt for dynamically generated responses • Depletion of IP addresses • Inability to tailor request, responses according to client, server preference • Poor level of security • Miscellaneous HTTP Protocol Design

12. New concepts • Hop-by-hop mechanism • Headers valid only for a single transport-level connection: Transfer-Encoding, Connection • Cannot be stored by caches or forwarded by proxies • Transfer coding • Split: message vs. entity (including headers) • Content coding is applied to whole entity • Transfer coding applies to entity-body • Property of message not original entity • TE, Transfer-Encoding • Virtual hosting • Semantic transparency for caching • Support for variants of a resource HTTP Protocol Design

13. HTTP/1.1 methods • GET, HEAD, POST • PUT, DELETE: formalized • OPTIONS: purpose extensibility • Learn about a server’s capability • Learn about intermediate servers in the path (Max-Forwards header == TTL) • TRACE: purpose extensibility • Returns the content of the message from the receiver (Via header == records intermediaries) • CONNECT: future use (extensibility: Upgrade header allows switch to other protocols) HTTP Protocol Design

14. New headers: General • Old: • Date, Pragma • New: • Cache-Control Caching • Connection Hop-by-hop • Trailer List of headers at end • Transfer-Encoding Transformation to message body • Upgrade Upgrade to other protocols • Via Intermediate servers • Warning Error-notification HTTP Protocol Design

15. New headers: Request • Response preference • New: Accept (charset, encoding, language), TE • Information • Old: Authorization, From, Referer, User-Agent • New: Proxy-Authorization • Conditional request • Old: If-Modified-Since • New: If-Match, If-None-Match, If-Unmodified- Since, If-Range • Constraint on server • New: Expect, Host, Max-Forwards, Range HTTP Protocol Design

16. New headers: Response • Redirection: • Old: Location • Information • Old: Server • New: Retry-After, Accept-Ranges • Security related • Old: WWW-Authenticate • New: Proxy-Authenticate • Caching related • New: Etag, Age, Vary HTTP Protocol Design

17. New headers: Entity • Old: • Allow • Content-Encoding, -Length, -Type • Expires • Last-Modified • New: • Content-Language, -Location, -MD5, -Range HTTP Protocol Design

18. Response codes: Examples • Informational • 100 Continue, 101 Switching Protocols • Success: 206 Partial Content, … • Redirection: 305 Use Proxy, … • Client errors • 14 new ones • Error codes: 400 bad request, 404 not found • Clarification status codes: 405 method not allowed, 410 gone • Using negotiation: 406 not acceptable, 412 unsupported media type • Length related: 411 length required • Other features: 402 Payment Required, 417 expectation failed • Server errors: 504 gateway Timeout, … HTTP Protocol Design

19. Caching HTTP/1.0 • Control options • Request directive (Pragma: no-cache) • Modifier to GET (If-Modified-Since) • Response header (Expires) • Cache busting • Expire header forced immediate expiry of resource • Last-Modified typically means not dynamically generated • Absolute clock values HTTP Protocol Design

20. Caching HTTP/1.1 • Issues: • Separation of cacheable and save use of copy • Ensure correctness (no cache should unknowingly return a stale value) • More control by server over cacheability • No absolute timestamps (no synch) • Caching of negotiated responses • Headers: • Age, Cache-control, Etag, Vary HTTP Protocol Design

21. Cache-control HTTP/1.1 request • No-cache forcible revalidation • Only-if-cached resource only from cache • No-store cache cannot store • Max-age age <= value • Max-stale expired OK but <= value • Min-fresh remain fresh for value • No-transform no change of media type • Extension new tokens HTTP Protocol Design

22. Cache-control HTTP/1.1 response • Public OK to cache • Private Response for specific user only • No-store not permitted to store • No-cache do not serve from cache without • revalidation • No-transform proxy cannot change media type, etc • Must-revalidate cached but revalidate if stale • Proxy-revalidate shared caches need revalidation • Max-age response age should be <= age • S-max age shared caches use value as max-age HTTP Protocol Design

23. Etag • Opaque value • Different versions of resources => different etag values • Decoupled from cache validation • If-Match, If-None-Match • E.g. If-None-Match in PUT to avoid overwriting HTTP Protocol Design

24. Vary • E.g., Accept-Language HTTP Protocol Design

25. Bandwidth Optimization :Factors • Resource sizes are growing • Embedded Images • More users, better connected • Multiple parallel connections HTTP Protocol Design

26. Bandwidth Optimization: Solutions • Only transfer necessary pieces of resource • Range request • Only transfer if receiver can handle response • Expect/continue • Transform resource before sending • Compression HTTP Protocol Design

27. Connection management • Problem: • TCP not optimized for typical short-lived HTTP • message exchange • Use of parallel connections • Solution: • Persistent connections - Keep-Alive • Pipelined connections - Connection header • Problems: - Head of line blocking - Unexpected close (aborts) HTTP Protocol Design

28. Message transmission • HTTP/1.0 • Content-length field • HTTP/1.1 • Chunked encoding (ends with zero-length chunk) • Response: Transfer-Encoding: chunked • Request: TE: trailers HTTP Protocol Design

29. Internet address conservation • Many Web server on a single host • HTTP/1.0 one IP address per Web server • HTTP/1.1 Host header • Host: www.foo.com HTTP Protocol Design

30. Content negotiation • Different formats for each resource • Client and server negotiate about preferred representation • Agent-driven • Server-driven HTTP Protocol Design

31. Proxies in HTTP/1.1: syntax • Requirement dealing with forwarding messages • HTTP/1.1 vs. HTTP/1.0 • Forward non understood headers • Treat hop-by-hop headers • Remove Connection header • Requirement dealing with modifying existing headers, adding new ones • Add Via header • Do not alter the order of field values • Adhere to cache control directive • Do not modify From and Server • Do not alter fully qualified domain names • Do not generate certain headers: Content-MD5 HTTP Protocol Design

32. Proxies in HTTP/1.1: semantic • Caching requirements • See cache control • Obligated to send Age header • Connection management requirements • HTTP/1.1 proxies may not establish persistent connections with HTTP/1.0 clients • Different guidelines regarding persistent connections (2*simultaneously active users) • Bandwidth management requirements • Range requests • Forward expect header/417 expectation failed response HTTP Protocol Design