Network Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Standards Track                              P. McManus
Expires: September 22, 2014                                      Mozilla
                                                          March 21, 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

Status of this Memo

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

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   This Internet-Draft will expire on September 22, 2014.

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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  . . . . . . . . . . . . . . .  5
     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  . . . . . . . . . . .  7
   4.  The 4NN Not Authoritative HTTP Status Code . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  The Alt-Svc Message Header Field . . . . . . . . . . . . .  9
     5.2.  The 4NN Not Authoritative HTTP Status Code . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     6.1.  Changing Ports . . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 10
     6.3.  Changing Protocols . . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   HTTP [I-D.ietf-httpbis-p1-messaging] 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

   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 [I-D.ietf-httpbis-http2]) or one
      using improved security (such as TLS {{RFC5246}).
   o  An origin server might wish to segment its clients into groups of
      capabilities, such as those supporting SNI (see [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 headers in Section 3.

   It also introduces a new status code in Section 4, 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", "DIGIT", "parameter", "port" and "delta-second" rules from
   [I-D.ietf-httpbis-p1-messaging], 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 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
   [I-D.ietf-httpbis-p1-messaging] 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 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 ALPN protocol, as per [I-D.ietf-tls-applayerprotoneg]
   o  A host, as per [RFC3986]
   o  A port, as per [RFC3986]

   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 6.2.  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 and the SNI
   information provided in TLS by the client will be that of the origin,
   not the alternative.

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.

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   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 6.2 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].  This
   supports the conservation of IP addresses on the alternative service

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.

   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

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   losing any enhanced security properties of the alternative service.

3.  The Alt-Svc HTTP Header Field

   A HTTP(S) origin server can advertise the availability of alternative
   services (see Section 2) to clients by adding an Alt-Svc header field
   to responses.

   Alt-Svc     = 1#( alternative *( OWS ";" OWS parameter ) )
   alternative   = <"> protocol-id <"> "=" port
   protocol-id = <ALPN protocol identifier>

   For example:

   Alt-Svc: "http2"=8000

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

   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 values.

   Finally, note that while it may be technically possible to put
   content other than printable ASCII in a HTTP header, some
   implementations only support ASCII (or a superset of it) in header
   field values.  Therefore, this field SHOULD NOT be used to convey
   protocol identifiers that are not printable ASCII, or those that
   contain quote characters.

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;

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   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 [I-D.ietf-httpbis-p6-cache] Section 4.2.3 for details of
   determining response age.  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 is received from an origin, its value
   invalidates and replaces all cached alternative services for that

   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 4NN Not Authoritative HTTP Status Code

   The 4NN (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 [I-D.ietf-httpbis-p1-messaging], Section 9.1.

   Clients receiving 4NN (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.

   4NN (Not Authoritative) MAY carry an Alt-Svc header field.

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   This status code MUST NOT be generated by proxies.

   A 4NN response is cacheable by default; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [I-D.ietf-httpbis-p6-cache]).

5.  IANA Considerations

5.1.  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:

5.2.  The 4NN Not Authoritative HTTP Status Code

   This document registers the 4NN (Not Authoritative) HTTP Status code

   o  Status Code: 4NN
   o  Short Description: Not Authoritative
   o  Specification: [this document], Section 4

6.  Security Considerations

   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.

6.1.  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

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   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.

6.2.  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.

6.3.  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

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   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
   end users) are met.

7.  References

7.1.  Normative References

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-26 (work in progress),
              February 2014.

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

              Friedl, S., Popov, A., Langley, A., and S. Emile,
              "Transport Layer Security (TLS) Application Layer Protocol
              Negotiation Extension", draft-ietf-tls-applayerprotoneg-05
              (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
              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.

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7.2.  Informative References

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

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-26 (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.  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 in SPDY.

Authors' Addresses

   Mark Nottingham


   Patrick McManus


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