The Trailer HTTP Header is a response header that indicates the presence of a specified set of header fields in the trailer of a message encoded using chunked transfer-coding. It enables senders to put additional fields at the end of chunked messages to deliver metadata. The chunked transfer encoding is a data transport technique included in HTTP version 1.1. To enable the trailer fields in the Trailer HTTP Header, the TE request header must be set to trailers. The transfer encoding that the user agent is ready to accept is specified in the TE request header. The Trailer HTTP Header has only one value. The Trailer HTTP Header syntax is the header-name, which is a collection of header fields found in the trailer of a message encoded using chunked transfer-coding. The example of the Trailer HTTP Header is shown below.

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked
Trailer: Expires
7\r\n
Mozilla\r\n
9\r\n
Developer\r\n
7\r\n
Network\r\n
0\r\n
Expires: Wed, 21 Oct 2015 07:28:00 GMT\r\n
\r\n

The Trailer HTTP Header Response Header is seen above. In the article, the Trailer HTTP Header Syntax, Directives, and Uses with examples will be processed.

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What is a Trailer HTTP Header?

The Trailer HTTP header is known as the Trailer HTTP Header. The Trailer HTTP Header is a response header that informs the server that a particular set of header fields have been included in the trailer of a message that has been encoded using chunked transfer-coding. It enables senders to include additional fields at the end of chunked messages in order to give metadata to the recipient.

What is the Syntax of the Trailer HTTP Header?

The syntax for the Trailer HTTP Header is the header-name. The syntax for using the Trailer HTTP Header is written below.

Trailer: header-names

What is the Directive of Trailer HTTP Header?

There is only one directive that used in the Trailer HTTP Header. The directives for using the Trailer HTTP Header are the header-names that will be present in the trailer part of the chunked messages. However, the Trailer HTTP Header field is disallowed in message framing headers such as transfer-encoding and content-length, routing headers such as host, request modifiers such as control and conditional, like cache-control, Max-Forwards, or TE, and lastly for authentication headers such as Authorization or Set-Cookie, or Content-Encoding, Content-Type, Content-Range, and the Trailer itself. The directives for using the Trailer HTTP Header are written below.

The request modifier Max-Forwards is being used in this case.

Trailer: Max-Forwards

In its case, the header that is being used is the Host header which is a routing header.

Trailer: Host

A trailer header with a chunked response from a database query is shown here. In its case, the Expires header is used.

HTTP/1.0 500 OK 
Content-Type: text/plain 
Transfer-Encoding: chunked
Trailer: Expires
0\r\n
Mozilla\r\n 
7\r\n
Developer\r\n
9\r\n
Network\r\n
0\r\n 
\r\n

How to use a Trailer HTTP Header?

A message body that is encoded with chunked transfer-coding and the sender wants to send metadata in the form of trailer fields at the end of the message. The sender should add a Trailer HTTP header field before the message body. The Trailer HTTP Header will tell the receiver which fields will be in the trailer. As a result, the recipient prepares for that metadata before it even starts processing the body. It will be useful if the message is being streamed and the recipient wants to make sure the integrity of the message is maintained on the fly.

Examples of Trailer HTTP Header Use

In the example, the Expires header is used at the end of the chunked message and serves as a trailing header. The Expires HTTP header contains the date and time after which the response is considered expired.

HTTP/1.1 200 OK
Content-Type: text/plain
Transfer-Encoding: chunked
Trailer: Expires
7\r\n
Mozilla\r\n
9\r\n
Developer\r\n
7\r\n
Network\r\n
0\r\n
Expires: Wed, 25 Nov 2015 07:16:00 GMT\r\n
\r\n

What is the Specification Documents for Trailer HTTP Header?

There is only one specification document for the Trailer HTTP Header which is the Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing (HTTP/1.1 Message Syntax and Routing). The Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing (HTTP/1.1 Message Syntax and Routing) mention the definition and use of the Trailer HTTP Header. Additionally, the article discusses the Trailer HTTP Header’s examples and values.

What are the types of Trailer HTTP Header?

The Trailer HTTP Header HTTP Type is listed below.

Request Header – The Trailer HTTP Header HTTP header is a Request Header because it provides information about the request context so that the server can tailor the response.
Payload Header- The Trailer HTTP Header HTTP Header is a Payload Header because it describes the payload information related to safe transmission and reconstruction of the original resource representation from one or more messages.

What are the similar HTTP Headers to the Trailer HTTP Header?

The similar HTTP Headers to the Trailer HTTP Header are listed below.

Transfer-Encoding HTTP Header: Similar to the Trailer HTTP Header, the Transfer-Encoding HTTP Header provides the type of encoding required to safely transfer the payload body to the user.
TE HTTP Header: As with the Trailer HTTP Header, the TE HTTP Header specifies the type of encoding required for the payload body to be successfully transmitted to the user.

Which Browsers Support Trailer HTTP Header?

There are multiple browsers that support Trailer HTTP Header. The following browsers are listed below.

Chrome Browser is compatible with the Trailer HTTP Header
Edge Browser is compatible with the Trailer HTTP Header
Firefox Browser is compatible with the Trailer HTTP Header
Internet Explorer Browser is compatible with the Trailer HTTP Header
Opera Browser is compatible with the Trailer HTTP Header
Safari Browser is compatible with the Trailer HTTP Header
WebView Android Browser is compatible with the Trailer HTTP Header
Chrome Android Browser is compatible with the Trailer HTTP Header
Firefox Android Browser is compatible with the Trailer HTTP Header
Opera Android Browser is compatible with the Trailer HTTP Header
Safari IOS Browser is compatible with the Trailer HTTP Header
Samsung Internet Browser is compatible with the Trailer HTTP Header

You can see an image that shows cross-browser compatibility of Trailer HTTP Headers below.

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