Network Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Standards Track                              P. McManus
Expires: June 14, 2014                                           Mozilla
                                                       December 11, 2013

                        HTTP Alternate Services


   This document introduces "alternate services" to allow an HTTP
   origin's resources to be available at a separate network location,
   possibly accessed with a different protocol configuration.

   It also specifies one means of discovering alternate services, the
   "Alt-Svc" header field.

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   This Internet-Draft will expire on June 14, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   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.  Alternate Services . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Client Handling for Alternate Services . . . . . . . . . .  5
       2.1.1.  Host Authentication  . . . . . . . . . . . . . . . . .  5
       2.1.2.  Alternate Service Caching  . . . . . . . . . . . . . .  5
       2.1.3.  Alternate Service Priorities . . . . . . . . . . . . .  6
       2.1.4.  Requiring Server Name Indication . . . . . . . . . . .  6
       2.1.5.  Using Alternate Services . . . . . . . . . . . . . . .  6
   3.  The Alt-Svc HTTP Header Field  . . . . . . . . . . . . . . . .  7
     3.1.  Caching Alt-Svc Header Field Values  . . . . . . . . . . .  7
     3.2.  Indicating Alt-Svc Header Field Priority . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     4.1.  Changing Ports . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Changing Hosts . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Changing Protocols . . . . . . . . . . . . . . . . . . . . 10
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Appendix B.  TODO  . . . . . . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   [I-D.ietf-httpbis-http2] specifies a few ways to negotiate the use of
   HTTP/2.0 without changing existing URIs.  However, several
   deficiencies in using the "upgrade dance" for "http://" URIs have
   become apparent.  While that mechanism is still being investigated,
   some have expressed interest in an alternate approach.

   Furthermore, some implementers have expressed a strong desire utilize
   HTTP/2 only in conjunction with TLS.  Alternate-Services provides a
   potential mechanism for achieving that for "http://" URIs; see
   [I-D.nottingham-http2-encryption] for details.

   Finally, HTTP/2.0 is designed to have longer-lived, fewer and more
   active TCP connections.  While these properties are generally
   "friendlier" for the network, they can cause problems for servers
   that currently exploit the short-lived flow characteristics of
   HTTP/1.x for load balancing, session affinity and maintaining
   locality to the user.

   This document specifies a new concept in HTTP, the "alternate
   service," to address these use cases.  An alternate 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

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", "uri-host", "port" and "delta-
   second" rules from [I-D.ietf-httpbis-p1-messaging], and uses the
   "#rule" extension defined in Section 7 of that document.

2.  Alternate Services

   On the Web, a resource is accessed through a scheme (e.g., "https" or
   "http") on a nominated host / port combination.

   These three pieces of information collectively can be used to
   establish the authority for ownership of the resource (its "origin";
   see [RFC6454]), as well as providing enough information to bootstrap
   access to it.

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   This document introduces the notion of an "Alternate Service"; when
   an origin's resources are accessible through a different protocol /
   host / port combination, it is said to have an alternate service.

   For example, an origin:

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

   might declare that its resources are also accessible at the alternate

   ("http2-tls", "", "443")

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

   Alternate 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 alternate service is essentially alternate
   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.

   Furthermore, it is important to note that the first member of an
   alternate 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 alternate 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

   Importantly, this includes its security context; in particular, when
   TLS [RFC5246] is in use, the alternate server will need to present a
   certificate for the origin's host name, not that of the alternate.
   Likewise, the Host header is still derived from the origin, not the
   alternate 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 alternate service is identified by the combination of:

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   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 alternate service MUST have:

   o  A freshness lifetime, expressed in seconds; see Section 2.1.2
   o  A numeric priority; see Section 2.1.3

   Potentially, there are many ways that a client could discover the
   alternate service(s) associated with an origin; this document
   currently defines one, the Alt-Svc HTTP Header Field (Section 3).

2.1.  Client Handling for Alternate Services

2.1.1.  Host Authentication

   Clients MUST NOT use alternate services with a host other than the
   origin's without strong server authentication; this mitigates the
   attack described in Section 4.2.  One way to achieve this is for the
   alternate to use TLS with a certificate that is valid for that

   For example, if the origin's host is "" and an
   alternate is offered on "" with the "http2-tls"
   protocol, and the certificate offered is valid for "",
   the client can use the alternate.  However, if "" is
   offered with the "http2" protocol, the client cannot use it, because
   there is no mechanism in that protocol to establish strong server

2.1.2.  Alternate Service Caching

   Mechanisms for discovering alternate 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 alternate service instead of the origin
   at any time when it is considered fresh; see Section 2.1.5 for
   specific recommendations.

   To mitigate risks associated with caching compromised values (see
   Section 4.2 for details), user agents SHOULD examine cached alternate
   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.

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2.1.3.  Alternate Service Priorities

   Mechanisms for discovering alternate services can associate a
   priority with them; for example, the Alt-Svc header field uses the
   "pr" parameter.

   Priorities are numeric, with a range of 1-64, and are relative to the
   origin server, which has a static priority of 32.  Higher values are

   Therefore, an alternate with a priority of 48 will be used in
   preference to the origin server, whereas one with a priority of 10
   will be used only when the origin server becomes unavailable.

   Note that priorities are not specific to the mechanism that an
   alternate was discovered with; i.e., there is only one "pool" of
   priorities for an origin.

2.1.4.  Requiring Server Name Indication

   A client must only use a TLS based alternate service if the client
   also supports TLS Server Name Indication (SNI) [RFC6066].  This
   supports the conservation of IP addresses on the alternate service

2.1.5.  Using Alternate Services

   By their nature, alternate 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 alternate service that has
   a higher priority than a connection currently in use, the client
   SHOULD use that alternate service as soon as it is available,
   provided that the security properties of the alternate service
   protocol are desirable, as compared to the existing connection.

   For example, if an origin advertises a "http2-tls" alternate service
   using an "Alt-Svc" response header field, the client ought to
   immediately establish a connection to the most preferable alternate
   service, and use it in preference to the origin connection once

   The client is not required to block requests; the origin's connection
   can be used until the alternate 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

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   connection is fully available in order to avoid information leakage.

   Furthermore, if the connection to the alternate service fails or is
   unresponsive, the client MAY fall back to using the origin, or a less
   preferable alternate service.

3.  The Alt-Svc HTTP Header Field

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

   Alt-Svc     = 1#( alternate *( OWS ";" OWS parameter ) )
   alternate   = protocol-id "=" [ uri-host ] ":" 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 can also contain a host:


   This indicates that all resources on the origin are available using
   the "http2-tls" profile on port 443.

   It can also have multiple values:

   Alt-Svc: http2=:8000,

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

   Intermediaries MUST NOT change or append Alt-Svc values.

3.1.  Caching Alt-Svc Header Field Values

   When an alternate 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: http2-tls=:443;ma=3600

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   which indicates the number of seconds since the response was
   generated the alternate 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: http2=:8000; ma=60

   indicates that an alternate 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
   alternate 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 alternate services for that
   origin.  This includes the empty Alt-Svc header, which clears all
   cached alternate services for an origin.

   See Section 2.1.2 for general requirements on caching alternate

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

3.2.  Indicating Alt-Svc Header Field Priority

   Finally, an explicit priority can be associated with an Alt-Svc
   header field value by using the "pr" parameter:

   Alt-Svc: http2-tls:8000 ;pr=64

   See Section 2.1.3 for details of the priority mechanism.

   pr = 1*2DIGIT

   If the "pr" parameter is not present or is invalid, the default
   priority for alternate services discovered with the Alt-Svc header
   field is 48.

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4.  Security Considerations

4.1.  Changing Ports

   Using an alternate service implies accessing an origin's resources on
   an alternate port, at a minimum.  An attacker that can inject
   alternate 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 set the Alt-
   Svc response header field on the origin, and restricting who can open
   a port for listening on that host.

4.2.  Changing Hosts

   When the host is changed due to the use of an alternate 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 alternate 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 2.1.1 that any
   alternate 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 alternate service is
   authorized to serve traffic for the origin.

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

   Alternate 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
   alternate service with a large freshness lifetime, so that the user
   agent still directs traffic to the attacker even when not using the

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   As a result, there is a requirement in Section 2.1.2 to examine
   cached alternate services when a network change is detected.

4.3.  Changing Protocols

   When the ALPN protocol is changed due to the use of an alternate
   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 alternate services, but instead
   evaluate the option(s) presented to assure that security requirements
   and expectations (of specifications, implementations and end users)
   are met.

5.  References

5.1.  Normative References

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-25 (work in progress),
              November 2013.

              Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching",
              draft-ietf-httpbis-p6-cache-25 (work in progress),
              November 2013.

              Friedl, S., Popov, A., Langley, A., and S. Emile,
              "Transport Layer Security (TLS) Application Layer Protocol
              Negotiation Extension", draft-ietf-tls-applayerprotoneg-03
              (work in progress), October 2013.

   [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.

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   [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.

5.2.  Informative References

              Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
              "Hypertext Transfer Protocol version 2.0",
              draft-ietf-httpbis-http2-08 (work in progress),
              November 2013.

              Nottingham, M., "Opportunistic Encryption for HTTP URIs",
              draft-nottingham-http2-encryption-01 (work in progress),
              October 2013.

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

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

Appendix A.  Acknowledgements

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

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

Appendix B.  TODO

   o  GOAWAY: A GOAWAY-like frame (or just a GOAWAY modification) that
      allows an alternate service to be switched to might be suggested
      in a future revision.
   o  DNS: Alternate services are also amenable to DNS-based discovery.
      If there is sufficient interest, a future revision may include a
      proposal for that.

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   o  Indicating Chosen Service: It's likely necessary for the server to
      know which protocol the user agent has chosen, and perhaps even
      the hostname (for load balancing).  This could be conveyed as part
      of the "magic", or as a request header.
   o  IPV6: The intersection between Alternate Services and Happy
      Eyeballs [RFC6555] should be investigated.
   o  ALPN strings: all of the ALPN strings in this document are
      fictional; they need to be updated based upon that specification's
      progress (and the registry, eventually).
   o  Advice for setting headers: guidelines for servers that use the
      Alt-Svc header field.

Authors' Addresses

   Mark Nottingham


   Patrick McManus


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