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1. Introduction

HTTP [RFC7230] conflates the identification of resources with their location. In other words, "http://" and "https://" URIs are used to both name and find things to interact with.

In some cases, it is desirable to separate identification and location in HTTP; keeping the same identifier for a resource, but interacting with it at a different location on the network.

For example:

  • An origin server might wish to redirect a client to a different server when it is under load, or it has found a server in a location that is more local to the client.
  • An origin server might wish to offer access to its resources using a new protocol, such as HTTP/2 [RFC7540], or one using improved security, such as Transport Layer Security (TLS) [RFC5246].
  • An origin server might wish to segment its clients into groups of capabilities, such as those supporting Server Name Indication (SNI) (Section 3 of [RFC6066]), for operational purposes.

This specification defines a new concept in HTTP, "Alternative Services", that allows an origin server to nominate additional means of interacting with it on the network. It defines a general framework for this in Section 2, along with specific mechanisms for advertising their existence using HTTP header fields (Section 3) or HTTP/2 frames (Section 4), plus a way to indicate that an alternative service was used (Section 5).

It also endorses the status code 421 (Misdirected Request) (Section 6) that origin servers or their nominated alternatives can use to indicate that they are not authoritative for a given origin, in cases where the wrong location is used.

1.1. Notational Conventions​

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document uses the Augmented BNF defined in [RFC5234] and updated by [RFC7405] along with the "#rule" extension defined in Section 7 of [RFC7230]. The rules below are defined in [RFC5234], [RFC7230], and [RFC7234]:

OWS           = <OWS, see [RFC7230], Section 3.2.3>
delta-seconds = <delta-seconds; see [RFC7234], Section 1.2.1>
port          = <port, see [RFC7230], Section 2.7>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token         = <token, see [RFC7230], Section 3.2.6>
uri-host      = <uri-host, see [RFC7230], Section 2.7>
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