5. The Link Header Field

The Link header field provides a means for serializing links in HTTP headers.

Overview

Semantic Equivalence:

• Equivalent to HTML <LINK> element
• Equivalent to Atom atom:link feed-level element

ABNF Syntax

Link           = "Link" ":" #link-value
link-value     = "`<" URI-Reference ">`" *( ";" link-param )
link-param     = ( ( "rel" "=" relation-types )
               | ( "anchor" "=" <"> URI-Reference <"> )
               | ( "rev" "=" relation-types )
               | ( "hreflang" "=" Language-Tag )
               | ( "media" "=" ( MediaDesc | ( <"> MediaDesc <"> ) ) )
               | ( "title" "=" quoted-string )
               | ( "title*" "=" ext-value )
               | ( "type" "=" ( media-type | quoted-mt ) )
               | ( link-extension ) )
relation-types = relation-type
               | <"> relation-type *( 1*SP relation-type ) <">
relation-type  = reg-rel-type | ext-rel-type
reg-rel-type   = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-rel-type   = URI

Components

5.1. Target IRI

• Conveyed as URI-Reference in angle brackets <>
• Relative URIs resolved per RFC 3986, Section 5
• Base IRI from message content NOT applied

5.2. Context IRI

• Default: IRI of requested resource
• Override: Use anchor parameter
• Can specify fragment or third resource
• Relative URIs resolved per RFC 3986
• Implementations may ignore anchored links

5.3. Relation Type

• Specified in rel parameter
• MUST NOT appear more than once per link-value
• rev parameter: DEPRECATED (reverse direction)
• Extension types: must be absolute URIs, quoted if containing ; or ,

5.4. Target Attributes

Standard Parameters:

Parameter	Description
hreflang	Language hint for target resource
media	Intended media/medium for style info
title	Human-readable label
title*	Encoded label (RFC 5987)
type	Media type hint

Usage Rules:

• hreflang: Can appear multiple times
• media: Must be quoted if contains ; or ,
• title: Must NOT appear more than once
• title*: Preferred over title if both present
• type: Media type hint, doesn't override Content-Type

5.5. Examples

Simple Navigation:

Link: `\`http://example.com/TheBook/chapter2\``; rel="previous";
      title="previous chapter"

Extension Relation:

Link: `</>`; rel="http://example.net/foo"

Multiple Links with Encoding:

Link: `</TheBook/chapter2>`;
      rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
      `</TheBook/chapter4>`;
      rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel

Multiple Relations:

Link: `\`http://example.org/\``;
      rel="start http://example.net/relation/other"

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See Also:

• Section 4: Relation Types
• Section 6: IANA Considerations

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