http2

http2:链接地址：http://http2.github.io/http2-spec/index.htmlhttp1.1:采用文本协议,虽然支持连接复用，但是在返回的时候仍然是顺序串行话的，导致很多网站采用多个域名同时建立多个链接来提升性能，造成冗长和重复，导致不必要的网络io及tcp消耗http2:由于上一个版本存在的问题，采用了spdy的思想，使用binary 协议，加上frame、stream的概念1、header压缩2、connection共用3、使用二进制frame作为单元传输4、push promise 服务器端的预推送5、增加优先级概念及flow controlhttp2有两个版本通过h2、h2c来标识，h2标识通过TLS传输、h2c标识cleartext明文传输在查询server是否使用http2时可以使用如下头

GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

A server 如果不支持那么upgrade头不会出现如下：

HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html

支持的话如下:

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c

[ HTTP/2 connection ...

如果一个请求升级到http2的头必须包含一个HTTP2-Settings header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

HTTP2-Settings    = token68

Frame的格式

All frames begin with a fixed 9-octet header followed by a variable-length payload.

+-----------------------------------------------+
|                 Length (24)                   |
+---------------+---------------+---------------+
|   Type (8)    |   Flags (8)   |
+-+-------------+---------------+-------------------------------+
|R|                 Stream Identifier (31)                      |
+=+=============================================================+
|                   Frame Payload (0...)                      ...
+---------------------------------------------------------------+

Figure 1: Frame LayoutThe fields of the frame header are defined as:Length:
The length of the frame payload expressed as an unsigned 24-bit integer. Values greater than 214(16,384) MUST NOT be sent unless the receiver has set a larger value forSETTINGS_MAX_FRAME_SIZE.The 9 octets of the frame header are not included in this value.Type:
The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.Flags:
An 8-bit field reserved for boolean flags specific to the frame type.Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored and MUST be left unset (0x0) when sending.R:
A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving.Stream Identifier:
A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.The structure and content of the frame payload is dependent entirely on the frame type.frame的大小由SETTINGS_MAX_FRAME_SIZE settings 决定HEADER传输采用由一系列不中断的header blocks，最后一个headers或者CONTINUATION包含end_headers 标识，如下组成

+---------------+
|Pad Length? (8)|
+-+-------------+-----------------------------------------------+
|E|                 Stream Dependency? (31)                     |
+-+-------------+-----------------------------------------------+
|  Weight? (8)  |
+-+-------------+-----------------------------------------------+
|                   Header Block Fragment (*)                 ...
+---------------------------------------------------------------+
|                           Padding (*)                       ...
+---------------------------------------------------------------+

DATA:如下组成

+---------------+
|Pad Length? (8)|
+---------------+-----------------------------------------------+
|                            Data (*)                         ...
+---------------------------------------------------------------+
|                           Padding (*)                       ...
+---------------------------------------------------------------+

http2还包含各种frame如SETTINGS(包含全局设置如flow control大小，也包含stream设置)、RST_STREAM(取消或重置当前的stream)、PUSH_PROMISE(预推送)、PING(侦测链接是否可用)、GOAWAY(关闭链接一般包含一个比当前链接的stream都要大的id，可以优雅的结束当前的小于id的stream)、WINDOW_UPDATE(窗口大小用于flow control)http1.1与http2例子对比：

request:
GET /resource HTTP/1.1           HEADERS
Host: example.org          ==>     + END_STREAM
Accept: image/jpeg                 + END_HEADERS
:method = GET
:scheme = https
:path = /resource
host = example.org
accept = image/jpeg
reponse:

HTTP/1.1 304 Not Modified        HEADERS
ETag: "xyzzy"              ==>     + END_STREAM
Expires: Thu, 23 Jan ...           + END_HEADERS
:status = 304
etag = "xyzzy"
expires = Thu, 23 Jan ...

request:
POST /resource HTTP/1.1          HEADERS
Host: example.org          ==>     - END_STREAM
Content-Type: image/jpeg           - END_HEADERS
Content-Length: 123                  :method = POST
:path = /resource
{binary data}                        :scheme = https

CONTINUATION
+ END_HEADERS
content-type = image/jpeg
host = example.org
content-length = 123

DATA
+ END_STREAM
{binary data}
reponse:

HTTP/1.1 200 OK                  HEADERS
Content-Type: image/jpeg   ==>     - END_STREAM
Content-Length: 123                + END_HEADERS
:status = 200
{binary data}                        content-type = image/jpeg
content-length = 123

DATA
+ END_STREAM
{binary data}

HTTP/1.1 100 Continue            HEADERS
Extension-Field: bar       ==>     - END_STREAM
+ END_HEADERS
:status = 100
extension-field = bar

HTTP/1.1 200 OK                  HEADERS
Content-Type: image/jpeg   ==>     - END_STREAM
Transfer-Encoding: chunked         + END_HEADERS
Trailer: Foo                         :status = 200
content-length = 123
123                                  content-type = image/jpeg
{binary data}                        trailer = Foo
0
Foo: bar                         DATA
- END_STREAM
{binary data}

HEADERS
+ END_STREAM
+ END_HEADERS
foo = bar