RFC2616学习笔记--第一章（Introduction简介）
Tue Feb 18 2014 18:10:00 GMT+0800 (中国标准时间)
HTTP协议（超文本传输协议）是一个应用层面的协议。
HTTP/0.9: 一个简单的协议,用于在网络上传输原始数据。
HTTP/1.0: 它加强了以下功能：
支持MIME格式的消息
支持包含元数据的支持，这些元数据信息包含数据的传输和修改。
但是HTTP/1.0并没有充分考虑多层代理、缓存、持久化连接的需求、以及虚拟主机等的应用。
HTTP/1.1:它在1.0基础上对进行了严格的需求分析，从而提供了更可靠的功能实现。它参考了 URI、URL、URN来决定使用什么方法来定位资源。
术语：
连接（connection）：传输层虚拟电路。
A transport layer virtual circuit established between two programs for the purpose of communication.
消息（message）：HTTP传输的基本单元。
The basic unit of HTTP communication
请求（request）：一个HTTP请求消息。
An HTTP request message
响应（response):一个HTTP响应消息。
An HTTP response message
资源（resource）：一个能由URI定位的网络数据或服务。
实体（entity）：一个可以被转换为请求/响应的载体的信息。由包含元信息的实体头和包含内容的实体主体组成。
表现（representation）：一个包含在响应中的实体，内容协商机制中使用到。一个特定的响应状态可能关联多个表现。
内容协商（content negotiation）：用于选择合适的表现的机制。
The mechanism for selecting the appropriate representation when servicing a request。 The representation of entities in any response can be negotiated (including error responses).
variant
A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `varriant'. Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.
client
A program that establishes connections for the purpose of sending requests.
user agent
The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools.
server
An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.
origin server
The server on which a given resource resides or is to be created.
proxy
An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies.
gateway
A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.
tunnel
An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed.
cache
A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
cacheable
A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cacheability of HTTP responses are defined in section 13. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular request.
first-hand
A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
explicit expiration time
The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration time is available.
age
The age of a response is the time since it was sent by, or successfully validated with, the origin server.
freshness lifetime
The length of time between the generation of a response and its expiration time.
fresh
A response is fresh if its age has not yet exceeded its freshness lifetime.
stale
A response is stale if its age has passed its freshness lifetime.
semantically transparent
A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.
validator
A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent copy of an entity.
upstream/downstream
Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.
inbound/outbound
Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server", and "outbound" means "traveling toward the user agent"