一、HEAD HPACK壓縮協議格式說明,HPACK壓縮的二進制格式採用以下2種格式：
1、用索引標識頭域，如下所示，0位固定爲1，其它7bit標識索引值，索引參考下面的靜態表格，如82代表 -> :method: GET。
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Index (7+) |
+---+---------------------------+
Figure 5: Indexed Header Field
+-------+-----------------------------+---------------+
| Index | Header Name | Header Value |
+-------+-----------------------------+---------------+
| 1 | :authority | |
| 2 | :method | GET |
| 3 | :method | POST |
| 4 | :path | / |
| 5 | :path | /index.html |
| 6 | :scheme | http |
| 7 | :scheme | https |
| 8 | :status | 200 |
| 9 | :status | 204 |
| 10 | :status | 206 |
| 11 | :status | 304 |
| 12 | :status | 400 |
| 13 | :status | 404 |
| 14 | :status | 500 |
| 15 | accept-charset | |
| 16 | accept-encoding | gzip, deflate |
| 17 | accept-language | |
| 18 | accept-ranges | |
| 19 | accept | |
| 20 | access-control-allow-origin | |
| 21 | age | |
| 22 | allow | |
| 23 | authorization | |
| 24 | cache-control | |
| 25 | content-disposition | |
| 26 | content-encoding | |
| 27 | content-language | |
| 28 | content-length | |
| 29 | content-location | |
| 30 | content-range | |
| 31 | content-type | |
| 32 | cookie | |
| 33 | date | |
| 34 | etag | |
| 35 | expect | |
| 36 | expires | |
| 37 | from | |
| 38 | host | |
| 39 | if-match | |
| 40 | if-modified-since | |
| 41 | if-none-match | |
| 42 | if-range | |
| 43 | if-unmodified-since | |
| 44 | last-modified | |
| 45 | link | |
| 46 | location | |
| 47 | max-forwards | |
| 48 | proxy-authenticate | |
| 49 | proxy-authorization | |
| 50 | range | |
| 51 | referer | |
| 52 | refresh | |
| 53 | retry-after | |
| 54 | server | |
| 55 | set-cookie | |
| 56 | strict-transport-security | |
| 57 | transfer-encoding | |
| 58 | user-agent | |
| 59 | vary | |
| 60 | via | |
| 61 | www-authenticate | |
+-------+-----------------------------+---------------+
2、字符頭域表示法
2.1字符頭域索引法，包括插入索引法和新命名法
直接插入索引法：
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
新命名法：
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
40 | == Literal indexed ==
0a | Literal name (len = 10)
6375 7374 6f6d 2d6b 6579 | custom-key
0d | Literal value (len = 13)
6375 7374 6f6d 2d68 6561 6465 72 | custom-header
| -> custom-key: custom-head\
| er
2.1字符頭域無索引法，不插入新索引和新命名法
不插入新索引：
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
04 | == Literal not indexed ==
| Indexed name (idx = 4)
| :path
0c | Literal value (len = 12)
2f73 616d 706c 652f 7061 7468 | /sample/path
| -> :path: /sample/path
新命名法：
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
2.2從不索引，不插入新索引和新命名法
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | 0 |
+---+---+-----------------------+
| H | Name Length (7+) |
+---+---------------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
10 | == Literal never indexed ==
08 | Literal name (len = 8)
7061 7373 776f 7264 | password
06 | Literal value (len = 6)
7365 6372 6574 | secret
| -> password: secret
二、實例分析：HTTP2數據帶Haffman Code
00000000 50 52 49 20 2a 20 48 54 54 50 2f 32 2e 30 0d 0a PRI * HT TP/2.0..
00000010 0d 0a 53 4d 0d 0a 0d 0a 00 0c 04 00 00 00 00 00 ..SM.... ........
00000020 00 03 00 00 00 64 00 04 00 00 ff ff 00 39 01 05 .....d.. .....9..
00000030 00 00 00 01 41 8a a0 e4 1d 13 9d 09 b8 c8 00 0f ....A... ........
00000040 82 04 94 62 43 91 8a 47 55 a3 a1 89 d3 4d 0c 1a ...bC..G U....M..
00000050 a9 0b e5 79 d3 4d 1f 86 53 03 2a 2f 2a 90 7a 8d ...y.M.. S.*/*.z.
00000060 aa 69 d2 9a c4 c0 17 6d 71 2d 7f 07 1f .i.....m q-...
1、頭部：
00000000 50 52 49 20 2a 20 48 54 54 50 2f 32 2e 30 0d 0a PRI * HT TP/2.0..
00000010 0d 0a 53 4d 0d 0a 0d 0a ..SM....
2、設置幀：
長度：00 0c
TYPE: 04
FLAG: 00
FlowID: 00 00 00 00
MAXFLOWNUM FLAG:00 03
MAXFLOWNUM : 00 00 00 64
MAX Win FLAG: 00 04
MAX Win Size: 00 00 ff ff
3、報頭幀
長度：00 39
TYPE: 01
FLAG: 05
FlowID: 00 00 00 01
報頭數據,值部分數據採用了Haffman編碼：
41 8a a0 e4 1d 13 9d 09 b8 c8 00 0f ....A... ........
00000040 82 04 94 62 43 91 8a 47 55 a3 a1 89 d3 4d 0c 1a ...bC..G U....M..
00000050 a9 0b e5 79 d3 4d 1f 86 53 03 2a 2f 2a 90 7a 8d ...y.M.. S.*/*.z.
00000060 aa 69 d2 9a c4 c0 17 6d 71 2d 7f 07 1f .i.....m q-...
41：:authority
8a: len = 10 data = a0 e4 1d 13 9d 09 b8 c8 00 0f data dec = localhost:3000
82: :method = Get
04: :path
94: len = 20 data 62 43 91 8a 47 55 a3 a1 89 d3 4d 0c 1a a9 0b e5 79 d3 4d 1f data dec /doc/manual/html/index.html
86: :scheme = http
53: :accept
03 len = 3 data = 2a 2f 2a */*
90: :accept-encoding = gzip,deflate
7a: :user-agent
8d: len = 13 data = aa 69 d2 9a c4 c0 17 6d 71 2d 7f 07 1f data dec = nghttp
/*
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 1 1 1 1 1 |
+---+---+---+-------------------+
| 1 | Value-(2^N-1) LSB |
+---+---------------------------+
...
+---+---------------------------+
| 0 | Value-(2^N-1) MSB |
+---+---------------------------+
*/
