HTTP斷點續傳的基本原理

 HTTP斷點續傳的基本原理

斷點續傳是咱們如今常常接觸的概念，那麼HTTP協議是如何支持斷點續傳的呢。咱們先從一個例子來看看。

下面是一個斷點續傳的例子：（使用Net Vampire獲得）

I 01-7-12 19:19:23 ------------------------- Attempt 1 -------------------------
P 01-7-12 19:19:24 Connecting to 127.0.0.3 ...
P 01-7-12 19:19:24 Connected to 127.0.0.3 [127.0.0.3]
S 01-7-12 19:19:24 GET /VS0515AI.EXE HTTP/1.1
S 01-7-12 19:19:24 Connection: close
S 01-7-12 19:19:24 Host: 127.0.0.3
S 01-7-12 19:19:24 Accept: */*
S 01-7-12 19:19:24 Pragma: no-cache
S 01-7-12 19:19:24 Cache-Control: no-cache
S 01-7-12 19:19:24 Referer: http://127.0.0.3/
S 01-7-12 19:19:24 User-Agent: Mozilla/4.04 [en] (Win95; I ;Nav)
S 01-7-12 19:19:24
R 01-7-12 19:19:24 HTTP/1.1 200 OK
R 01-7-12 19:19:24 Server: Zero Http Server/1.0
R 01-7-12 19:19:24 Date: Thu, 12 Jul 2001 11:19:24 GMT
R 01-7-12 19:19:24 Cache-Control: no-cache
R 01-7-12 19:19:24 Last-Modified: Tue, 30 Jan 2001 13:11:30 GMT
R 01-7-12 19:19:24 Content-Type: application/octet-stream
R 01-7-12 19:19:24 Content-Length: 15143086
R 01-7-12 19:19:24 Connection: close
R 01-7-12 19:19:24
P 01-7-12 19:19:25 Data transfer started
I 01-7-12 19:19:32 Job Stopped by user
I 01-7-12 19:19:33 Received 5 275 648 bytes in 0:00:07 (691 435 bytes/s)

 

I 01-7-12 19:19:40 ------------------------- Attempt 2 -------------------------
P 01-7-12 19:19:40 Connecting to 127.0.0.3 ...
P 01-7-12 19:19:40 Connected to 127.0.0.3 [127.0.0.3]
S 01-7-12 19:19:40 GET /VS0515AI.EXE HTTP/1.1
S 01-7-12 19:19:40 Connection: close
S 01-7-12 19:19:40 Host: 127.0.0.3
S 01-7-12 19:19:40 Accept: */*
S 01-7-12 19:19:40 Pragma: no-cache
S 01-7-12 19:19:40 Cache-Control: no-cache
S 01-7-12 19:19:40 Referer: http://127.0.0.3/
S 01-7-12 19:19:40 User-Agent: Mozilla/4.04 [en] (Win95; I ;Nav)
S 01-7-12 19:19:40 Range: bytes=5275648-
S 01-7-12 19:19:40
R 01-7-12 19:19:40 HTTP/1.1 206 Partial Content
R 01-7-12 19:19:40 Server: Zero Http Server/1.0
R 01-7-12 19:19:40 Date: Thu, 12 Jul 2001 11:19:40 GMT
R 01-7-12 19:19:40 Cache-Control: no-cache
R 01-7-12 19:19:40 Last-Modified: Tue, 30 Jan 2001 13:11:30 GMT
R 01-7-12 19:19:40 Content-Type: application/octet-stream
R 01-7-12 19:19:40 Content-Range: bytes 5275648-15143085/15143086
R 01-7-12 19:19:40 Content-Length: 9867438
R 01-7-12 19:19:40 Connection: close
R 01-7-12 19:19:40
P 01-7-12 19:19:40 Data transfer started
I 01-7-12 19:19:41 Job Stopped by user
I 01-7-12 19:19:41 Received 1 124 756 bytes in 0:00:01 (969 617 bytes/s)

 

第一次是普通的傳輸；第二次因爲沒有傳徹底，就發出了Range這個頭部，從5275648字節開始傳輸（默認是按字節算），迴應使用206狀態值，表示如今開始部分傳輸，回覆Content-Length頭部，表示傳輸的部分，用字節記，而後就與普通傳輸沒有區別了。