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HTTP Alternative Services

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7838.
Authors Mark Nottingham , Patrick McManus , Julian Reschke
Last updated 2014-04-01
Replaces draft-nottingham-httpbis-alt-svc
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd Mark Nottingham
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HTTPbis Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Standards Track                              P. McManus
Expires: October 3, 2014                                         Mozilla
                                                              J. Reschke
                                                           April 1, 2014

                       HTTP Alternative Services


   This document specifies "alternative services" for HTTP, which allow
   an origin's resources to be authoritatively available at a separate
   network location, possibly accessed with a different protocol

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (, which is archived at

   Working Group information can be found at
   <>; that specific to HTTP/2 are at

   The changes in this draft are summarized in Appendix A.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 3, 2014.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
   2.  Alternative Services Concepts  . . . . . . . . . . . . . . . .  4
     2.1.  Host Authentication  . . . . . . . . . . . . . . . . . . .  5
     2.2.  Alternative Service Caching  . . . . . . . . . . . . . . .  6
     2.3.  Requiring Server Name Indication . . . . . . . . . . . . .  6
     2.4.  Using Alternative Services . . . . . . . . . . . . . . . .  6
   3.  The Alt-Svc HTTP Header Field  . . . . . . . . . . . . . . . .  7
     3.1.  Caching Alt-Svc Header Field Values  . . . . . . . . . . .  8
   4.  The Service HTTP Header Field  . . . . . . . . . . . . . . . .  9
   5.  The 421 Not Authoritative HTTP Status Code . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  The Alt-Svc Message Header Field . . . . . . . . . . . . . 10
     6.2.  The Service Message Header Field . . . . . . . . . . . . . 10
     6.3.  The 421 Not Authoritative HTTP Status Code . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     7.1.  Changing Ports . . . . . . . . . . . . . . . . . . . . . . 11
     7.2.  Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 12
     7.3.  Changing Protocols . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 14
     A.1.  Since draft-nottingham-httpbis-alt-svc-05  . . . . . . . . 14
     A.2.  Since draft-ietf-httpbis-alt-svc-00  . . . . . . . . . . . 14

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1.  Introduction

   HTTP [HTTP-p1] conflates the identification of resources with their
   location.  In other words, "http://" (and "https://") URLs are used
   to both name and find things to interact with.

   In some cases, it is desirable to separate these aspects; to be able
   to keep the same identifier for a resource, but interact with it
   using a different location on the network.

   For example:

   o  An origin server might wish to redirect a client to an alternative
      when it needs to go down for maintenance, or it has found an
      alternative in a location that is more local to the client.

   o  An origin server might wish to offer access to its resources using
      a new protocol (such as HTTP/2, see [HTTP2]) or one using improved
      security (such as Transport Layer Security (TLS), see [RFC5246]).

   o  An origin server might wish to segment its clients into groups of
      capabilities, such as those supporting Server Name Indication
      (SNI, see Section 3 of [RFC6066]) and those not supporting it, for
      operational purposes.

   This specification defines a new concept in HTTP, "Alternative
   Services", that allows a resource to nominate additional means of
   interacting with it on the network.  It defines a general framework
   for this in Section 2, along with a specific mechanism for
   discovering them using HTTP header fields in Section 3.

   It also introduces a new status code in Section 5, so that origin
   servers (or their nominated alternatives) can 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",
   document are to be interpreted as described in [RFC2119].

   This document uses the Augmented BNF defined in [RFC5234] along with
   the "OWS", "delta-seconds", "parameter", "port", "token", and "uri-
   host" rules from [HTTP-p1], and uses the "#rule" extension defined in
   Section 7 of that document.

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2.  Alternative Services Concepts

   This specification defines a new concept in HTTP, the "alternative
   service".  When an origin (see [RFC6454]) has resources that are
   accessible through a different protocol / host / port combination, it
   is said to have an alternative service.

   An alternative service can be used to interact with the resources on
   an origin server at a separate location on the network, possibly
   using a different protocol configuration.  Alternative services are
   considered authoritative for an origin's resources, in the sense of
   [HTTP-p1], Section 9.1.

   For example, an origin:

   ("http", "", "80")

   might declare that its resources are also accessible at the
   alternative service:

   ("h2", "", "81")

   By their nature, alternative services are explicitly at the
   granularity of an origin; i.e., they cannot be selectively applied to
   resources within an origin.

   Alternative services do not replace or change the origin for any
   given resource; in general, they are not visible to the software
   "above" the access mechanism.  The alternative service is essentially
   alternative routing information that can also be used to reach the
   origin in the same way that DNS CNAME or SRV records define routing
   information at the name resolution level.  Each origin maps to a set
   of these routes -- the default route is derived from origin itself
   and the other routes are introduced based on alternative-protocol

   Furthermore, it is important to note that the first member of an
   alternative service tuple is different from the "scheme" component of
   an origin; it is more specific, identifying not only the major
   version of the protocol being used, but potentially communication
   options for that protocol.

   This means that clients using an alternative service will change the
   host, port and protocol that they are using to fetch resources, but
   these changes MUST NOT be propagated to the application that is using
   HTTP; from that standpoint, the URI being accessed and all
   information derived from it (scheme, host, port) are the same as

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   Importantly, this includes its security context; in particular, when
   TLS [RFC5246] is in use, the alternative server will need to present
   a certificate for the origin's host name, not that of the
   alternative.  Likewise, the Host header field is still derived from
   the origin, not the alternative service (just as it would if a CNAME
   were being used).

   The changes MAY, however, be made visible in debugging tools,
   consoles, etc.

   Formally, an alternative service is identified by the combination of:

   o  An Application Layer Protocol Negotiation (ALPN) protocol, as per

   o  A host, as per [RFC3986], Section 3.2.2

   o  A port, as per [RFC3986], Section 3.2.3

   Additionally, each alternative service MUST have:

   o  A freshness lifetime, expressed in seconds; see Section 2.2

   There are many ways that a client could discover the alternative
   service(s) associated with an origin.

2.1.  Host Authentication

   Clients MUST NOT use alternative services with a host other than the
   origin's without strong server authentication; this mitigates the
   attack described in Section 7.2.  One way to achieve this is for the
   alternative to use TLS with a certificate that is valid for that

   For example, if the origin's host is "" and an
   alternative is offered on "" with the "h2" protocol,
   and the certificate offered is valid for "", the
   client can use the alternative.  However, if "" is
   offered with the "h2c" protocol, the client cannot use it, because
   there is no mechanism in that protocol to establish strong server

   Furthermore, this means that the HTTP Host header field and the SNI
   information provided in TLS by the client will be that of the origin,
   not the alternative.

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2.2.  Alternative Service Caching

   Mechanisms for discovering alternative services can associate a
   freshness lifetime with them; for example, the Alt-Svc header field
   uses the "ma" parameter.

   Clients MAY choose to use an alternative service instead of the
   origin at any time when it is considered fresh; see Section 2.4 for
   specific recommendations.

   Clients with existing connections to alternative services are not
   required to fall back to the origin when its freshness lifetime ends;
   i.e., the caching mechanism is intended for limiting how long an
   alternative service can be used for establishing new requests, not
   limiting the use of existing ones.

   To mitigate risks associated with caching compromised values (see
   Section 7.2 for details), user agents SHOULD examine cached
   alternative services when they detect a change in network
   configuration, and remove any that could be compromised (for example,
   those whose association with the trust root is questionable).  UAs
   that do not have a means of detecting network changes SHOULD place an
   upper bound on their lifetime.

2.3.  Requiring Server Name Indication

   A client MUST only use a TLS-based alternative service if the client
   also supports TLS Server Name Indication (SNI) ([RFC6066], Section
   3).  This supports the conservation of IP addresses on the
   alternative service host.

2.4.  Using Alternative Services

   By their nature, alternative services are optional; clients are not
   required to use them.  However, it is advantageous for clients to
   behave in a predictable way when they are used by servers (e.g., for
   load balancing).

   Therefore, if a client becomes aware of an alternative service, the
   client SHOULD use that alternative service for all requests to the
   associated origin as soon as it is available, provided that the
   security properties of the alternative service protocol are
   desirable, as compared to the existing connection.

   When a client uses an alternate service, it MUST emit the Service
   header field (Section 4) on every request using that alternate

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   The client is not required to block requests; the origin's connection
   can be used until the alternative connection is established.
   However, if the security properties of the existing connection are
   weak (e.g. cleartext HTTP/1.1) then it might make sense to block
   until the new connection is fully available in order to avoid
   information leakage.

   Furthermore, if the connection to the alternative service fails or is
   unresponsive, the client MAY fall back to using the origin.  Note,
   however, that this could be the basis of a downgrade attack, thus
   losing any enhanced security properties of the alternative service.

3.  The Alt-Svc HTTP Header Field

   An HTTP(S) origin server can advertise the availability of
   alternative services to clients by adding an Alt-Svc header field to

   Alt-Svc     = 1#( alternative *( OWS ";" OWS parameter ) )
   alternative = protocol-id "=" port
   protocol-id = token ; percent-encoded ALPN protocol identifier

   ALPN protocol names are octet sequences with no additional
   constraints on format.  Octets not allowed in tokens ([HTTP-p1],
   Section 3.2.6) MUST be percent-encoded as per Section 2.1 of
   [RFC3986].  Consequently, the octet representing the percent
   character "%" (hex 25) MUST be percent-encoded as well.

   In order to have precisely one way to represent any ALPN protocol
   name, the following additional constraints apply:

   1.  Octets in the ALPN protocol MUST NOT be percent-encoded if they
       are valid token characters except "%", and

   2.  When using percent-encoding, uppercase hex digits MUST be used.

   With these constraints, recipients can apply simple string comparison
   to match protocol identifiers.

   For example:

   Alt-Svc: http2=8000

   This indicates that the "http2" protocol on the same host using the
   indicated port (in this case, 8000).

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   Examples for protocol name escaping:

   | ALPN protocol name | protocol-id | Note                |
   | http2              | http2       | No escaping needed  |
   | w=x:y#z            | w%3Dx%3Ay#z | "=" and ":" escaped |
   | x%y                | x%25y       | "%" needs escaping  |

   Alt-Svc MAY occur in any HTTP response message, regardless of the
   status code.

   Alt-Svc does not allow advertisement of alternative services on other
   hosts, to protect against various header-based attacks.

   It can, however, have multiple values:

   Alt-Svc: h2c=8000, h2=443

   The value(s) advertised by Alt-Svc can be used by clients to open a
   new connection to one or more alternative services immediately, or
   simultaneously with subsequent requests on the same connection.

   Intermediaries MUST NOT change or append Alt-Svc field values.

3.1.  Caching Alt-Svc Header Field Values

   When an alternative service is advertised using Alt-Svc, it is
   considered fresh for 24 hours from generation of the message.  This
   can be modified with the 'ma' (max-age) parameter;

   Alt-Svc: h2=443;ma=3600

   which indicates the number of seconds since the response was
   generated the alternative service is considered fresh for.

   ma = delta-seconds

   See Section 4.2.3 of [HTTP-p6] for details of determining response

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   For example, a response:

     HTTP/1.1 200 OK
     Content-Type: text/html
     Cache-Control: 600
     Age: 30
     Alt-Svc: h2c=8000; ma=60

   indicates that an alternative service is available and usable for the
   next 60 seconds.  However, the response has already been cached for
   30 seconds (as per the Age header field value), so therefore the
   alternative service is only fresh for the 30 seconds from when this
   response was received, minus estimated transit time.

   When an Alt-Svc response header field is received from an origin, its
   value invalidates and replaces all cached alternative services for
   that origin.

   See Section 2.2 for general requirements on caching alternative

   Note that the freshness lifetime for HTTP caching (here, 600 seconds)
   does not affect caching of Alt-Svc values.

4.  The Service HTTP Header Field

   The Service HTTP header field is used in requests to indicate the
   identity of the alternate service in use, just as the Host header
   field identifies the host and port of the origin.

   Service = uri-host [ ":" port ]

   Service is intended to allow alternate services to detect loops,
   differentiate traffic for purposes of load balancing, and generally
   to ensure that it is possible to identify the intended destination of
   traffic, since introducing this information after a protocol is in
   use has proven to be problematic.

   When using an Alternate Service, clients MUST include a Service
   header in all requests.

   For example:

     GET /thing
     User-Agent: Example/1.0

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5.  The 421 Not Authoritative HTTP Status Code

   The 421 (Not Authoritative) status code indicates that the current
   origin server (usually, but not always an alternative service; see
   Section 2) is not authoritative for the requested resource, in the
   sense of [HTTP-p1], Section 9.1.

   Clients receiving 421 (Not Authoritative) from an alternative service
   MUST remove the corresponding entry from its alternative service
   cache (see Section 2.2) for that origin.  Regardless of the
   idempotency of the request method, they MAY retry the request, either
   at another alternative server, or at the origin.

   421 (Not Authoritative) MAY carry an Alt-Svc header field.

   This status code MUST NOT be generated by proxies.

   A 421 response is cacheable by default; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [HTTP-p6]).

   [[apr: This really ought to be 420.]]

6.  IANA Considerations

6.1.  The Alt-Svc Message Header Field

   This document registers Alt-Svc in the Permanent Message Header
   Registry [RFC3864].

   o  Header Field Name: Alt-Svc

   o  Application Protocol: http

   o  Status: standard

   o  Author/Change Controller: IETF

   o  Specification Document: [this document]

   o  Related Information:

6.2.  The Service Message Header Field

   This document registers Alt-Svc in the Permanent Message Header
   Registry [RFC3864].

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   o  Header Field Name: Service

   o  Application Protocol: http

   o  Status: standard

   o  Author/Change Controller: IETF

   o  Specification Document: [this document]

   o  Related Information:

6.3.  The 421 Not Authoritative HTTP Status Code

   This document registers the 421 (Not Authoritative) HTTP Status code
   in the Hypertext Transfer Protocol (HTTP) Status Code Registry
   ([HTTP-p2], Section 8.2).

      Status Code: 421

      Short Description: Not Authoritative

      Specification: Section 5 of this document

7.  Security Considerations

   [[anchor1: Identified security considerations should be enumerated in
   the appropriate documents depending on which proposals are accepted.
   Those listed below are generic to all uses of alternative services;
   more specific ones might be necessary.]]

7.1.  Changing Ports

   Using an alternative service implies accessing an origin's resources
   on an alternative port, at a minimum.  An attacker that can inject
   alternative services and listen at the advertised port is therefore
   able to hijack an origin.

   For example, an attacker that can add HTTP response header fields can
   redirect traffic to a different port on the same host using the Alt-
   Svc header field; if that port is under the attacker's control, they
   can thus masquerade as the HTTP server.

   This risk can be mitigated by restricting the ability to advertise
   alternative services, and restricting who can open a port for
   listening on that host.

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7.2.  Changing Hosts

   When the host is changed due to the use of an alternative service, it
   presents an opportunity for attackers to hijack communication to an

   For example, if an attacker can convince a user agent to send all
   traffic for "" to "" by
   successfully associating it as an alternative service, they can
   masquerade as that origin.  This can be done locally (see mitigations
   above) or remotely (e.g., by an intermediary as a man-in-the-middle

   This is the reason for the requirement in Section 2.1 that any
   alternative service with a host different to the origin's be strongly
   authenticated with the origin's identity; i.e., presenting a
   certificate for the origin proves that the alternative service is
   authorized to serve traffic for the origin.

   However, this authorization is only as strong as the method used to
   authenticate the alternative service.  In particular, there are well-
   known exploits to make an attacker's certificate appear as

   Alternative services could be used to persist such an attack; for
   example, an intermediary could man-in-the-middle TLS-protected
   communication to a target, and then direct all traffic to an
   alternative service with a large freshness lifetime, so that the user
   agent still directs traffic to the attacker even when not using the

   As a result, there is a requirement in Section 2.2 to examine cached
   alternative services when a network change is detected.

7.3.  Changing Protocols

   When the ALPN protocol is changed due to the use of an alternative
   service, the security properties of the new connection to the origin
   can be different from that of the "normal" connection to the origin,
   because the protocol identifier itself implies this.

   For example, if a "https://" URI had a protocol advertised that does
   not use some form of end-to-end encryption (most likely, TLS), it
   violates the expectations for security that the URI scheme implies.

   Therefore, clients cannot blindly use alternative services, but
   instead evaluate the option(s) presented to assure that security
   requirements and expectations (of specifications, implementations and

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   end users) are met.

8.  Acknowledgements

   Thanks to Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin,
   Erik Nygren, Paul Hoffman, Adam Langley, Will Chan and Richard Barnes
   for their feedback and suggestions.

   The Alt-Svc header field was influenced by the design of the
   Alternative-Protocol header field in SPDY.

9.  References

9.1.  Normative References

   [HTTP-p1]                        Fielding, R., Ed. and J. Reschke,
                                    Ed., "Hypertext Transfer Protocol
                                    (HTTP/1.1): Message Syntax and
                                    (work in progress), February 2014.

   [HTTP-p6]                        Fielding, R., Ed., Nottingham, M.,
                                    Ed., and J. Reschke, Ed., "Hypertext
                                    Transfer Protocol (HTTP/1.1):
                                    draft-ietf-httpbis-p6-cache-26 (work
                                    in progress), February 2014.

   [I-D.ietf-tls-applayerprotoneg]  Friedl, S., Popov, A., Langley, A.,
                                    and S. Emile, "Transport Layer
                                    Security (TLS) Application Layer
                                    Protocol Negotiation Extension",
                                    (work in progress), March 2014.

   [RFC2119]                        Bradner, S., "Key words for use in
                                    RFCs to Indicate Requirement
                                    Levels", BCP 14, RFC 2119,
                                    March 1997.

   [RFC3986]                        Berners-Lee, T., Fielding, R., and
                                    L. Masinter, "Uniform Resource
                                    Identifier (URI): Generic Syntax",
                                    STD 66, RFC 3986, January 2005.

   [RFC5234]                        Crocker, D. and P. Overell,
                                    "Augmented BNF for Syntax

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                                    Specifications: ABNF", STD 68,
                                    RFC 5234, January 2008.

   [RFC6066]                        Eastlake, D., "Transport Layer
                                    Security (TLS) Extensions: Extension
                                    Definitions", RFC 6066,
                                    January 2011.

   [RFC6454]                        Barth, A., "The Web Origin Concept",
                                    RFC 6454, December 2011.

9.2.  Informative References

   [HTTP-p2]                        Fielding, R., Ed. and J. Reschke,
                                    Ed., "Hypertext Transfer Protocol
                                    (HTTP/1.1): Semantics and Content",
                                    (work in progress), February 2014.

   [HTTP2]                          Belshe, M., Peon, R., and M.
                                    Thomson, Ed., "Hypertext Transfer
                                    Protocol version 2",
                                    draft-ietf-httpbis-http2-10 (work in
                                    progress), February 2014.

   [RFC3864]                        Klyne, G., Nottingham, M., and J.
                                    Mogul, "Registration Procedures for
                                    Message Header Fields", BCP 90,
                                    RFC 3864, September 2004.

   [RFC5246]                        Dierks, T. and E. Rescorla, "The
                                    Transport Layer Security (TLS)
                                    Protocol Version 1.2", RFC 5246,
                                    August 2008.

Appendix A.  Change Log (to be removed by RFC Editor before publication)

A.1.  Since draft-nottingham-httpbis-alt-svc-05

   This is the first version after adoption of
   draft-nottingham-httpbis-alt-svc-05 as Working Group work item.  It
   only contains editorial changes.

A.2.  Since draft-ietf-httpbis-alt-svc-00

   Selected 421 as proposed status code for "Not Authoritative".

   Changed header field syntax to use percent-encoding of ALPN protocol

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   names (<>).

Authors' Addresses

   Mark Nottingham


   Patrick McManus


   Julian F. Reschke
   greenbytes GmbH


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