Trailer
The Trailer header allows a method that the Web server or Web client can use in HTTP messages to include some HTTP headers after the content. Recall that an HTTP message is constructed as follows:
Request line (HTTP requests)/Status line (HTTP responses)
HTTP headers
Content
The use of the Trailer header allows for the following format to be used:
Request line (HTTP requests)/Status line (HTTP responses)
HTTP headers
Content
HTTP headers
This allocation is only allowed when a chunked transfer encoding is used, as is specified with the use of the following general header:
Transfer-Encoding: chunked
The headers that are included after the content must be specified by name in the Trailer header. The following example shows an HTTP response using this method:
HTTP/1.1 200 OK Content-Type: text/html Transfer-Encoding: chunked Trailer: Date 7f <html> <head> <title>Transfer-Encoding Example</title> </head> <body> <p>Please wait while we complete your transaction ...</p> 2c <p>Transaction complete!</p> </body> </html> 0 Date: Tue, 21 May 2002 12:34:56 GMT
This allocation is helpful for information that cannot be accurately determined until the response has been completely generated. For example, the previous response may have a significant wait between the first and second chunks.