Network Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Standards Track                              P. McManus
Expires: August 15, 2014                                         Mozilla
                                                       February 11, 2014

                        HTTP Alternate Services


   This document proposes a number of changes to HTTP, centered around
   "alternate services", which allow an origin's resources to be
   available at a separate network location, possibly accessed with a
   different protocol configuration.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 15, 2014.

Copyright Notice

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

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
   2.  Use Cases for Alternate Services . . . . . . . . . . . . . . .  3
     2.1.  Upgrading  HTTP/1  . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Using TLS with http:// URIs  . . . . . . . . . . . . . . .  4
     2.3.  Mitigating Load Asymmetry  . . . . . . . . . . . . . . . .  4
     2.4.  Segmenting Clients that Support TLS SNI  . . . . . . . . .  5
   3.  Proposals  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Proposal: Alternate Services . . . . . . . . . . . . . . .  5
       3.1.1.  Host Authentication  . . . . . . . . . . . . . . . . .  7
       3.1.2.  Alternate Service Caching  . . . . . . . . . . . . . .  7
       3.1.3.  Requiring Server Name Indication . . . . . . . . . . .  8
       3.1.4.  Using Alternate Services . . . . . . . . . . . . . . .  8
     3.2.  Proposal: The Alt-Svc HTTP Header Field  . . . . . . . . .  8
       3.2.1.  Caching Alt-Svc Header Field Values  . . . . . . . . .  9
     3.3.  Proposal: ALTSVC Frame . . . . . . . . . . . . . . . . . . 10
     3.4.  Proposal: SETTINGS_UNIVERSAL_SCHEMES (4) . . . . . . . . . 11
     3.5.  Proposal: NOT_AUTHORITATIVE (13) . . . . . . . . . . . . . 11
     3.6.  Proposal: Discovery of TLS Support for http:// URIs  . . . 12
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     4.1.  Changing Ports . . . . . . . . . . . . . . . . . . . . . . 13
     4.2.  Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 13
     4.3.  Changing Protocols . . . . . . . . . . . . . . . . . . . . 14
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Appendix B.  TODO  . . . . . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   [I-D.ietf-httpbis-http2] specifies a few ways to negotiate the use of
   HTTP/2 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 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 explores these use cases in Section 2, and makes
   proposals to address them in Section 3.

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.  Use Cases for Alternate Services

   This section details the use cases for Alternate Services,
   identifying the proposals that would need to be adopted to meet each

2.1.  Upgrading  HTTP/1

   The first use case for Alternate Services is upgrading a client from
   HTTP/1 to HTTP/2 for http:// URIs (i.e., without the use of SSL or

   While HTTP/2 defines how to use the Upgrade header "dance" to do

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   this, it might not always be suitable, since it involves blocking the
   connection until the upgrade succeeds.  Since HTTP/2 is focused on
   improving performance, this is not desirable.

   Furthermore, using Upgrade requires the server that supports HTTP/2
   to be on the same ip:port tuple as the server supporting HTTP/1; this
   can cause deployment issues, as well as operational issues with
   devices that assume that all traffic on port 80 will be HTTP/1.

   Therefore, a means of indicating that a different, new connection can
   be used for HTTP/2 is desirable; this would allow the client to
   continue using the HTTP/1 connection until the new connection is
   established.  It also simplifies deployment considerations by not
   requiring HTTP/1 and HTTP/2 to be "spoken" on the same port, and
   allows issues on port 80 to be avoided.

   This use case can be met if Section 3.1 and Section 3.2 are accepted.
   It can also be met if Section 3.1 is used with a different discovery
   mechanism (e.g., DNS-based).

2.2.  Using TLS with http:// URIs

   As discussed in [I-D.nottingham-http2-encryption], it might be
   desirable to "opportunistically" use TLS when accessing a HTTP URI.

   This case can also be met by Section 3.1 and Section 3.6 with
   Section 3.2 for HTTP/1, and Section 3.1 and Section 3.6 with
   Section 3.2 or Section 3.3 for HTTP/2, with the possible addition of
   Section 3.4.  Section 3.5 might also be useful for handling errors in
   this use case.

2.3.  Mitigating Load Asymmetry

   HTTP/2 fundamentally changes how HTTP uses TCP.  While this has many
   benefits, it also disrupts many server-side practices that take
   advantage of HTTP/1's shorter flows.

   In particular, load balancing among a pool of servers is often used,
   either with DNS ("global" load balancing), or a device (hardware or
   software) that dispatches requests locally.

   HTTP/1's short flows aid in this, because it is easy to shift traffic
   among servers when one becomes overloaded or unavailable.  However,
   in HTTP/2, this is more difficult, due to the protocol's longer

   As a result, a mechanism for re-directing requests for an origin or
   set of origins without making this apparent to the application is

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   This use case can be met if Section 3.1 and Section 3.3 are accepted;
   servers can redirect clients to alternate services as appropriate.
   Section 3.5 might also be useful for handling errors in this use

2.4.  Segmenting Clients that Support TLS SNI

   TLS Server Name Indication (SNI) [RFC6066] was introduced to avoid a
   requirement for a 1:1 mapping between origin hostnames and IP
   addresses (in light of IPv4 address exhaustion), in a manner similar
   to the Host header in HTTP/1.

   However, there are still clients in common use that do not send SNI.
   As a result, servers have no way to take practical advantage of the
   extension, because there is no way to segment those clients that
   support SNI from those that do not.

   As a result, they need to build their infrastructure as if SNI did
   not exist.

   This use case can be met if Section 3.1 and Section 3.3 are accepted;
   servers can advertise an alternate service and direct clients that
   support SNI and HTTP/2 to the optimal server, while still maintaining
   a smaller set of legacy servers for those clients that do not support
   SNI (since HTTP/2 requires SNI support when TLS is in use).

3.  Proposals

   This section enumerates proposals made to meet the use cases in
   Section 2.  Note that they all need not be accepted together,
   depending on the use cases that are determined as in-scope.

3.1.  Proposal: Alternate Services

   NOTE: This section can be incorporated into HTTP/2 directly, or it
   can be published as a standalone specification.  However, if
   Section 3.3 is accepted, it will need to be included or referenced
   from the spec, since frame type extensibility has been ruled out.

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

   An alternate service can be used to interact with the resources on an

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   origin server at a separate location on the network, possibly using a
   different protocol configuration.

   For example, an origin:

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

   might declare that its resources are also accessible at the alternate

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

   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.  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 alternate-protocol information.

   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 3.1.2

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

3.1.1.  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 "h2t" protocol,
   and the certificate offered is valid for "", the
   client can use the alternate.  However, if "" is
   offered with the "h2" 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 alternate.

3.1.2.  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 3.1.4 for
   specific recommendations.

   Clients with existing connections to alternate 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
   alternate service can be used for establishing new requests, not
   limiting the use of existing ones.

   To mitigate risks associated with caching compromised values (see

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   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.

3.1.3.  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

3.1.4.  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, the
   client SHOULD use that alternate service for all requests to the
   associated origin as soon as it is available, provided that the
   security properties of the alternate 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 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
   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.  Note,
   however, that this could be the basis of a downgrade attack, thus
   losing any enhanced security properties of the alternate service.

3.2.  Proposal: The Alt-Svc HTTP Header Field

   NOTE: Because this header is mostly defined for use in HTTP/1, it is
   most likely most appropriate to put it in a separate specification.
   However, it will need to reference Section 3.1, wherever that is

   A HTTP(S) origin server can advertise the availability of alternate
   services (see Section 3.1) to clients by adding an Alt-Svc header
   field to responses.

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   Alt-Svc     = 1#( alternate *( OWS ";" OWS parameter ) )
   alternate   = <"> 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 does not allow advertisement of alternate services on other
   hosts, to protect against various header-based attacks.

   It can, however, have multiple values:

   Alt-Svc: "h2"=8000, "h2t"=443

   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.

   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.

3.2.1.  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: "h2t"=443;ma=3600

   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:

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   HTTP/1.1 200 OK
   Content-Type: text/html
   Cache-Control: 600
   Age: 30
   Alt-Svc: "h2"=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

   See Section 3.1.2 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.

3.3.  Proposal: ALTSVC Frame

   NOTE: Because of the current approach to HTTP/2 extensibility, this
   section will need to be incorporated to the frame type listing in
   HTTP/2 if accepted.

   The ALTSVC frame (type=0xa) advertises the availability of an
   alternate service to the client.  It can be sent at any time for an
   existing client-initiated stream or stream 0, and is intended to
   allow servers to load balance or otherwise segment traffic; see
   Section 3.1 for details.

   An ALTSVC frame on a client-initiated stream indicates that the
   conveyed alternate service is associated with the origin of that

   An ALTSVC frame on stream 0 indicates that the conveyed alternate
   service is associated with all origins that map to the server's
   address (i.e., host and port).

   The ALTSVC frame is intended for receipt by clients; a server that
   receives an ALTSVC frame MUST treat it as a connection error of type

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  PID_LEN (8)  | Reserved (8)  |            Port (16)          |
   |                          Max-Age (32)                         |
   |                        Protocol-ID (*)                        |
   |                           Host (*)                            |

   The ALTSVC frame contains the following fields:

   o  PID_LEN: An unsigned, 8-bit integer indicating the length, in
      bytes, of the PROTOCOL-ID field.
   o  Reserved: for future use.
   o  Port: An unsigned, 16-bit integer indicating the port that the
      alternate service is available upon.
   o  Max-Age: An unsigned, 32-bit integer indicating the freshness
      lifetime of the alternate service association, as per
      Section 3.1.2.
   o  Protocol-ID: A sequence of bytes (length determined by PID_LEN)
      containing the ALPN protocol identifier of the alternate service.
   o  Host: A sequence of bytes containing an ascii string indicating
      the host that the alternate service is available upon.

   The ALTSVC frame does not define any flags.


   NOTE: This is a proposal for a new SETTINGS value, to be incorporated
   in that list if accepted.

   A non-zero value indicates the sender MAY accept requests with
   schemes that do not match the default port for the server.  A non-
   zero value is a pre-requisite for sending http:// over TLS via the
   previous https connection method described in Section 3.6.

3.5.  Proposal: NOT_AUTHORITATIVE (13)

   NOTE: This is a proposal for a new error code, which should be
   incorporated into the appropriate section if accepted.

   The endpoint refuses the stream prior to performing any application
   processing.  This server is not configured to provide a definitive
   response for the requested resource.  Clients receiving this error
   code SHOULD remove the endpoint from their cache of alternate

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   services, if present.

3.6.  Proposal: Discovery of TLS Support for http:// URIs

   NOTE: This section, if accepted, ought to be added as a new
   subsection of "Starting HTTP/2".

   A server wishing to advertise support for HTTP/2 over TLS for http://
   URIs MAY do so by including an Alt-Svc (Section 3.2 response header
   with the "h2t" protocol identifier.

   For example, a HTTP/1 connection could indicate support for HTTP/2 on
   port 443 for use with future http:// URI requests with this Alt-Svc

   HTTP/1.1 200 OK
   Alt-Svc: "h2t"=443

   The process for starting HTTP/2 over TLS for an http:// URI is the
   same as the connection process for an https:// URI, except that
   authentication of the TLS channel is not required.  The client MAY
   use either anonymous or authenticated ciphersuites.  If an
   authenticated ciphersuite is used, the client MAY ignore
   authentication failures.  This enables servers that only serve
   http:// URIs to use credentials that are not tied to a global PKI,
   such as self-signed certificates.  Clients MAY reserve the use of
   certain security sensitive optimizations, such as caching the
   existence of this successful connection, for authenticated

   Additionally, if a client has previously successfully connected to a
   given server over TLS, for example as part of an https:// request,
   then it MAY attempt to use TLS for requests certain http:// URIs.  To
   use this mechanism the server MUST have sent a non-zero
   SETTINGS_UNIVERSAL_SCHEMES setting to indicate support for this

   Eligible http:// URIs:

   1.  Contain the same host name as the URI accessed over TLS, and
   2.  Do not contain an explicit port number.

   For example, if the client has successfully made a request for the
   URI "", then it may attempt to use TLS to make
   a request for the URI "", but not for the URI
   "".  In particular, if a client has a TLS
   connection open to a server (for example, due to a past "https"
   request), then it may re-use that connection for "http" requests,

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   subject to the constraints above.

4.  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 alternate services;
   more specific ones might be necessary.

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 advertise
   alternate services, 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 3.1.1 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

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   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

   As a result, there is a requirement in Section 3.1.2 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-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

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              Negotiation Extension", draft-ietf-tls-applayerprotoneg-04
              (work in progress), January 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.

5.2.  Informative References

              Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
              "Hypertext Transfer Protocol version 2.0",
              draft-ietf-httpbis-http2-09 (work in progress),
              December 2013.

              Nottingham, M., "Opportunistic Encryption for HTTP URIs",
              draft-nottingham-http2-encryption-02 (work in progress),
              December 2013.

   [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
   Alternate-Protocol header in SPDY.

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Appendix B.  TODO

   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.
   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  Advice for setting headers: guidelines for servers that use the
      Alt-Svc header field.
   o  For the load balancing use case, it's necessary for clients to
      always flush altsvc cache upon a network change, but right now
      they're only required to examine the cache for suspicious entries.
      We should discuss whether this should be upgraded to always flush.

Authors' Addresses

   Mark Nottingham


   Patrick McManus


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