17. 내용 협상과 트랜스코딩

HTTP provides content-negotiation methods that allow clients and servers to make determination about which version of the document to send by the server, English or French.
A single URL can correspond to different resources.
These different versions are called variants.
Servers can automatically generate customized pages.
- WML
- transcodings.

Content-Negotiation techniques

three distinct methods for deciding which page at a server is the right one for a client.
present the choice to the client - client-driven negotiation
- client makes a request, server sends list of choices to client, client chooses.
- easiest to implement.
- adds latency.
decide automatically at the server - server-driven negotiation
- server examines request headers and decide what version to serve.
- q-value mechanism, Vary header
- server might have to guess.
ask an intermediary to select - transparent negotiation
- a proxy cache does the request negotiation.
- offloads the negotiation from the server.
- no formal specifications.

server sends back a response listing the available pages and let the client decide.
easiest to implement at the server.
two requests are needed.
two ways for server to present the choices
- sending back HTML document with links to the different versions of the page.
- 300 Multiple Choices
Client may popup a dialog window for a user to choose.
This method requires multiple URLs.
- www.joes-hardware.com/english, www.joes-hardware.com/french

let the server decide which page to send back.
two mechanisms that HTTP servers use to evaluate the proper response
- examine content-negotiation headers
  - Accept headers.
- Varying on other headers.
  - User-Agent header

Accept - media types
Accept-Language - languages
Accept-Charset - charsets
Accept-Encoding - encodings
similar to entity headers.
Content-negotiation headers are used by clients and servers
- to exchange preference information
- to help choose between different versions of a document.
clients must send their preference information with every request.
reduces latency with the back-and-forth communication in client-driven model.
Spaniard requests, then English or French?
- guess or fall back to client-driven.
- you can use quality values(q values)

Accept-Language: en;q=0.5, fr;q=0.0, nl;q=1.0, tr;q=0.0
q values can range from 0.0 to 1.0
the order are not important, only q values are.
server may change or transcode the document to match the client's preference.
- Q. google translate

Server can attempt to match up responses with other client request such as User-Agent
- remove JavaScript from the page if not supports
Caches must attempt to serve correct "best" versions of cached documents
- Vary hjeader tells caches which headers the server is using to determine the best version.

intermediary proxy negotiate on behalf of client.
- move the load of server-driven negotiation away from the server.
- minimize message exchanges with the client.
server must be able to tell proxies what request headers the server examines - Vary header.
Caching proxies can store different copies of documents accessed via a single URL.
- the caches can negotiate with clients on behalf of the servers.
- also great place to transcode content - can trascode content from any server.
  - Q - modern web examples?

caches must employ much of the decision-making logic that servers do.
- choose correct document to serve.
Accept headers to decide which cached response to send back.

Figure 17-1

Cache should check Accept-Language header to send back correct response.
if cache doesn't have spanish version, should forward the second request to server, and store both the response and an "alternate" response for that URL.
two different documents for the same URL - variants or alternates

Q - Accept 헤더의 앞부분에 있을 수록 우선순위가 높은 것일까?
if servers are using other headers to make their decisions, caches must know.
the Vary header lists all of the client request headers that server considers.
- if served document depends on User-Agent, Vary must include it.
before cache can serve document to the client, it must see whether the server sent a Vary header in the cached response.
- if present, header values in the new request must match the header values in the old, cached request.
caches must store both the client request headers and the corresponding server response headers with each cached variant.
Vary: User-Agent, Cookie
- the number of User-Agent and Cookie values could generate many variants.
- cache would have to store each document version corresponding to each variant.

when a server does not have a document that matches the client's needs at all?
may have to respond with an error, but may be able to transform one of its existing documents into something that client can use. transcoding

transformation of data from one format to another to make it viewable by client.
HTML-WML
different from content encoding or transfer encoding.
- latter two are used for more efficient or safe transport of content.
- the former is used to make content viewable on the access device.

the extraction of key pieces of information from a document.
generation of an outline of a document based on section headings, removal of advertisements, and logos from a page.
categorizing pages based on keywords in content
Q. vs og-tags?
Q. modern web technologies to handle these?

may increase the amount of content.
automatic ad generators, user-tracking systems.
- has to be dynamic - be relevant, target for a particular user.

performance limits
- searching through many variants for appropriate content.
- trying to guess the best match.
straming media and fax
- client and server need to discuss the best answer to the client's request. - not HTTP.
- can a general content-negotiation protocol be developed on top of TCP/IP application protocols?

Last updated 4 years ago