Network Working Group                                           R. Khare
Internet-Draft                                 4K Associates / UC Irvine
Expires: December 21, 1999                                   S. Lawrence
                                                   Agranat Systems, Inc.
                                                           June 22, 1999

                    Upgrading to TLS Within HTTP/1.1

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as

   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
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 21, 1999.


   This memo applies the Upgrade mechanism in HTTP/1.1 to employ
   Transport Layer Security (TLS) over an existing TCP connection. This
   allows unsecured and secured traffic to share the same well known
   port (in this case, http: at 80 rather than https: at 443). This
   also enables "virtual hosting," by allowing a single HTTP + TLS
   server to disambiguate traffic intended for several hostnames at a
   single IP address.

   This memo also clarifies how to exploit the HTTP/1.1 Upgrade
   mechanism in general. It creates new IANA registries for public HTTP
   status codes, and public or private Upgrade product tokens.

   This memo also argues that 'https' is insufficient to discriminate
   between secure and non-secure URIs, and henceforth http: alone
   should be used. That is to say, both https: and port 443 could be
   safely deprecated upon deployment of this mechanism.

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Status Notes

   This memo is intended to proceed directly to Proposed Standard,
   since its functionality has been extensively debated, but not
   implemented, over the last two years. It is expected to update RFC

Copyright Notice

   Copyright (C) The Internet Society (1999). All Rights Reserved.

Table of Contents

   1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Client Requested Upgrade to HTTP over TLS  . . . . . . . . . .  4
   3.1 Requesting Upgrade When Unsecured Is Not Acceptable  . . . . .  4
   3.2 Requesting Upgrade When Unsecured Is Acceptable  . . . . . . .  4
   3.3 Server Acceptance of Upgrade Request . . . . . . . . . . . . .  5
   4.  Server Requested Upgrade to HTTP over TLS  . . . . . . . . . .  5
   4.1 Server Required Upgrade to HTTP over TLS . . . . . . . . . . .  5
   4.2 Server Advertised HTTP over TLS  . . . . . . . . . . . . . . .  6
   5.  HTTP Upgrade Usage Considerations  . . . . . . . . . . . . . .  6
   5.1 Upgrading across HTTP Proxies  . . . . . . . . . . . . . . . .  6
   6.  Rationale for the use of a 4xx (client error) response code  .  7
   7.  Rationale for the HTTP+TLS/1.0 Upgrade token . . . . . . . . .  7
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   8.1 HTTP Status Code Registry  . . . . . . . . . . . . . . . . . .  8
   8.2 HTTP Upgrade Token Registry  . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.1 Implications for the https: URI Scheme . . . . . . . . . . . .  9
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11

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1. Motivation

   The historical practice for deploying HTTP over SSL3[2] has
   distinguished the combination from HTTP alone by a unique URI scheme
   and the TCP port number. The scheme 'http' meant the HTTP protocol
   alone on port 80, while 'https' meant the HTTP protocol over SSL on
   port 443.  Other protocols have similarly requested (and in some
   cases were issued) a second well known port so that they can
   distinguish the secured and unsecured modes of operation in this way
   as well.  Taken to its extreme, this approach in effect cuts in half
   the number of available well known ports.

   At the Washington DC IETF meeting in December 1997, the Applications
   Area Directors, and the IESG broadly, reaffirmed that the practice
   of issuing parallel "secure" port numbers should be deprecated. The
   HTTP/1.1 Upgrade mechanism can indeed apply Transport Layer
   Security[5] to an HTTP connection, over the same port.

   In the nearly two years since, there has been broad acceptance of
   the concept behind this proposal, but little interest in
   implementing alternatives to port 443 for generic Web browsing.
   However, the Internet Printing Protocol[6], one of the first new
   application protocols built atop HTTP, has called for just such a
   mechanism in order to move forward in the IETF standards process.

   The Upgrade mechanism also solves the "virtual hosting" problem.
   Rather than allocating multiple IP addresses to a single host, an
   HTTP/1.1 server will use the Host: header to disambiguate the
   intended web service. As HTTP/1.1 usage has grown more prevalent,
   more ISPs are offering name-based virtual hosting, thus delaying IP
   address space exhaustion.

   TLS (and SSL) have been hobbled by the same limitation as earlier
   versions of HTTP: the initial handshake does not specify the
   intended hostname, relying exclusively on the IP address. Using a
   cleartext HTTP/1.1 Upgrade: preamble to the TLS handshake --
   choosing the certificates based on the initial Host: header -- will
   allow ISPs to provide secure name-based virtual hosting as well.

2. Introduction

   Either the client or server can use the HTTP/1.1[1] Upgrade
   mechanism (Section 14.42) to indicate that a TLS-secured connection
   is desired or necessary. This draft defines the "HTTP+TLS/1.0"
   Upgrade token and a new HTTP Reply Code, "426 Upgrade Required".

   TLS, a/k/a SSL (Secure Sockets Layer) establishes a private
   end-to-end connection, optionally including strong mutual
   authentication, using a variety of cryptosystems. Initially, a

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   handshake phase uses three subprotocols to set up a record layer,
   authenticate endpoints, set parameters, as well as report errors.
   Then, there is an ongoing layered record protocol that handles
   encryption, compression, and reassembly for the remainder of the
   connection. The latter is intended to be completely transparent. For
   example, there is no dependency between TLS's record markers and or
   certificates and HTTP/1.1's chunked encoding or authentication.
   This specification provides a procedure for either a client or
   server to request that this TLS handshake phase begin on an existing
   HTTP/1.1 connection.

3. Client Requested Upgrade to HTTP over TLS

   The client sends an HTTP/1.1 request with an Upgrade header field
   containing the token "HTTP+TLS/1.0".

3.1 Requesting Upgrade When Unsecured Is Not Acceptable

   To complete the switch to secured operation before sending any clear
   HTTP traffic, the client MAY use a method such as "OPTIONS*".

         OPTIONS * HTTP/1.1
         Upgrade: HTTP+TLS/1.0
         Connection: Upgrade

   The client MUST use the OPTIONS method if unsecured operation is

3.2 Requesting Upgrade When Unsecured Is Acceptable

   The client MAY offer to switch to secured operation during a clear
   HTTP operation:

        GET HTTP/1.1
        Upgrade: HTTP+TLS/1.0
        Connection: Upgrade

   In this case, the server MAY respond to the clear HTTP operation
   normally, OR switch to secured operation (as detailed in the next

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3.3 Server Acceptance of Upgrade Request

   As specified in HTTP/1.1[1], if the server is prepared to initiate
   the TLS handshake, it MUST send the intermediate "101 Switching
   Protocol" response specifying the upgrade tokens it is switching to:

       HTTP/1.1 101 Switching Protocols
       Upgrade: HTTP+TLS/1.0

   The TLS handshake bytes begin after the final CRNL of the HTTP

   If the TLS handshake completes, the server MUST continue with the
   response to the original request. Any TLS handshake failure MUST
   lead to disconnection, per the TLS error alert specification.

   In the 'required upgrade' case described in Section 3.1, the client
   will send the real request after the OPTIONS ("no-op") request has

4. Server Requested Upgrade to HTTP over TLS

   The Upgrade header field can be used in HTTP responses to advertise
   server policy.

4.1 Server Required Upgrade to HTTP over TLS

   A server can indicate that a request can not be fulfilled without
   TLS secured operation using the "426 Upgrade Required" status code
   [see Section 6 for the rationale for why this is not a 3xx redirect
   response].  The 426 response MUST include an Upgrade header field
   specifying the token for the required TLS version.

       HTTP/1.1 426 Upgrade Required
       Upgrade: HTTP+TLS/1.0

   The server cannot know whether or not the client is willing or able
   to Upgrade. The use of 426 means that the request has failed, as any
   4xx code would.  This has two important implications:
   1.  The server SHOULD include a message body in the 426 response
       which indicates in human readable form the reason for the error
       and describes any alternative courses which may be available to
       the user.
   2.  Neither the server nor the client can immediately begin the TLS
       handshake -- a new request must be made, whether over the same
       TCP connection or not.

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   If the client is capable of the protocol set specified by the server
   in the Upgrade header of a 426 response, it MAY begin a
   client-initiated sequence as specified in Section 3 to repeat the

   [Since the original request was presumably sent in the clear, the
   Section 3.2 method reduce the number of round-trips in this case]

4.2 Server Advertised HTTP over TLS

   As specified in [HTTP], the server MAY include an Upgrade header in
   any response to indicate a willingness to switch to any
   (combination) of the protocols listed.  Only a 101 or 426 response
   lists Upgrade tokens that MUST be used to successfully complete the

5. HTTP Upgrade Usage Considerations

   In the course of formalizing this mechanism, several principles of
   HTTP Upgrade usage have been clarified for future users.

   o  Servers MUST select at most one of the offered Upgrade tokens in
      the 101 Switching Protocols response.
   o  This implies that Upgrade tokens represent "bundles" of
      functionality. Skipping a sequential upgrade to X/1.0 then to
      Y/1.0 would require defining a joint XY/1.0 token, for example.
   o  This implies public Upgrade tokens should be managed by IANA,
      according to the process in [8].
   o  Reliable deployment of new protocol extensions requires a
      definitive failure error, "426 Upgrade Required" in this case.
      This is broadly useful for any Upgrade usage.

   Note that since Upgrade was only defined in HTTP/1.1 (and above),
   upgraded protocols can assume persistent-connections by default.

5.1 Upgrading across HTTP Proxies

   As a hop-by-hop header, Upgrade must be negotiated between each pair
   of HTTP counterparties. As an end-to-end protocol, HTTP+TLS/1.0 is
   only applicable across tunnels. The HTTP CONNECT method explicitly
   constructed a tunnel, but it requires unique port numbers to
   disambiguate services.

   The following rules apply to relaying Upgrade requests:

   1.  Upon receipt of an Upgrade header field, a proxy server MUST
       either discard all the offers, or choose to forward only those
       it agrees to become a tunnel for.
   2.  Upon receipt of a "101 Switching Protocols" response, a proxy

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       server MUST become a tunnel, or report a more detailed proxy
       server error.

   Furthermore a caching proxy SHOULD not reply to a request with
   Upgrade tokens from its cache. Clients are still advised to
   explicitly include "Cache-control: no-cache" in this case.

   Note that these scenarios slightly complicate diagnosis of a
   426-status response. Since Upgrade: is a hop-by-hop header, a proxy
   may have removed the client's original Upgrade request, while the
   origin server continues to insist no offer was received.

6. Rationale for the use of a 4xx (client error) response code

   Reliable, interoperable negotiation of Upgrade features requires an
   unambiguous failure signal. The 426 Upgrade Required status code
   allows a server to definitively state the precise protocol
   extensions a given resource must be served with. Otherwise, there
   would be no solution in the Section 4.1 case.

   It might at first appear that the response should have been some
   form of redirection (a 3xx code), by analogy to an old-style
   redirection to an https: URI.  User agents that do not understand
   Upgrade: preclude this:

   Suppose that the code 3YZ had been assigned for "Upgrade Required";
   a user agent that did not recognize it would treat it as 300.  It
   would then properly look for a "Location" header in the response and
   attempt to repeat the request at the URL in that header field.
   Since it did not know to Upgrade to HTTP+TLS/1.0, it would at best
   fail again at the new URL.

7. Rationale for the HTTP+TLS/1.0 Upgrade token

   While TLS (and SSL) are properly ignorant of the syntax and
   semantics of encapsulated, encrypted traffic, it remains
   inappropriate to infer the protocol being secured by TCP port
   number. To reinforce the point that the upgraded protocol is now the
   composition of HTTP and TLS/1.0, we explicitly named the Upgrade
   token HTTP+TLS/1.0.

   Note that the version number in the product token refers to the
   version of TLS employed;  the version of HTTP to be used over TLS
   following the switch is calculated normally, viz. per the version
   compatibility rules of HTTP. [Note that while TLS is compatible with
   previous versions of SSL, they do not have TLS version numbers. If
   there were a backwards-compatible Upgrade, it might have specified
   HTTP+SSL/3.0 instead.]

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   Purely HTTP-compliant extensions such as IPP will reuse
   HTTP+TLS/1.0, while derivative works such as the Session Initiation
   Protocol are encouraged to define their own Upgrade mechanism and
   their own tokens.

8. IANA Considerations

   IANA shall create registries for two name spaces, as described in
   BCP 26[8]:
   o  HTTP Status Codes
   o  HTTP Upgrade Tokens

8.1 HTTP Status Code Registry

   The HTTP Status Code Registry defines the name space for the
   Status-Code token in the Status line of an HTTP response.  The
   initial values for this name space are those specified by
   1.  Draft Standard for HTTP/1.1[1]
   2.  Web Distributed Authoring and Versioning[3] [defines 420-424]
   3.  WebDAV Advanced Collections[4] (Work in Progress) [defines 425]
   4.  section Section 6 of this specification.[defines 426]

   Values to be added to this name space SHOULD be subject to review in
   the form of a standards track document within the IETF Applications
   Area.  Any such document SHOULD be traceable through statuses of
   either 'Obsoletes' or 'Updates' to the Draft Standard for

8.2 HTTP Upgrade Token Registry

   The HTTP Upgrade Token Registry defines the name space for product
   tokens used to identify protocols in the the Upgrade HTTP header
   field.  Each registered token should be associated with one or a set
   of specifications, and with contact information.

   The Draft Standard for HTTP/1.1[1] specifies that these tokens obey
   the production for 'product':

       product         = token ["/" product-version]
       product-version = token

   Registrations should be allowed on a First Come First Served basis
   as described in BCP 26[8]. These specifications need not be IETF
   documents or be subject to IESG review, but should obey the
   following rules:

   1.  The registration for a given token MUST NOT be changed once
   2.  The registry MUST NOT register a token whose 'product' component

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       is the same as that of an already registered token, unless the
       source of the authority for the registration is the same as the
       previous registry (if company XYZ, Inc. registered "XYZ/1.0",
       then no other entity should be allowed to register any token
       whose product component is "XYZ" without the consent of XYZ, Inc.

   An initial value in this namespace is defined in Section Section 7
   of this specification.

   It is NOT required that specifications for upgrade tokens be made
   publically available, but the contact information for the
   registration SHOULD be.

9. Security Considerations

   The potential for a man-in-the-middle attack (deleting the
   HTTP+TLS/1.0 upgrade token) remains the same as current, mixed
   http/https practice:
   o  Removing the Upgrade token is similar to rewriting web pages to
      change https:// links to http:// links.
   o  The risk is only present if the server is willing to vend that
      information over both a secure and an insecure channel in the
      first place.
   o  If the client knows for a fact that a server is TLS-compliant, it
      can insist on it by only connecting as https: (currently) or by
      only sending an Upgrade request with a no-op method like OPTIONS.
   o  Finally, as the https: specification warns, "users should
      carefully examine the certificate presented by the server to
      determine if it meets their expectations." -- there is no
      substitute for vigilance.

   Furthermore, for clients which do not actively try to invoke TLS,
   servers can use Upgrade: to advertise TLS compliance, too. Since
   TLS-compliance should be considered a feature of the server and not
   the resource at hand, it should be sufficient to send it once, and
   let clients cache that fact.

9.1 Implications for the https: URI Scheme

   This mechanism does not use the URI scheme name to indicate the
   protocol used. That is, any http: URI could be upgraded; and that
   https: URIs are no guarantee the server will upgrade.

   Instead, the choice of what security characteristics are required on
   the connection is left to the client and server.  This allows either
   party to use any information available in making this determination.
   For example, user agents may rely on user preference settings or
   information about the security of the network such as 'TLS required
   on all POST operations not on my local net or VPN', and servers may

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   resource access rules such as 'the form on this page must be served
   and submitted using TLS'.

   This also implies both parties have the option of fallback to a less
   secure mode of operation if either party cannot shift to TLS and
   such unsecured operation is acceptable to both and to the human
   user; this is not possible with the 'https' scheme.


   [1]  Fielding, R.T., et. al, , "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [2]  Rescorla, E.K., "HTTP Over TLS", Internet-Draft (Work In
        Progress), September 1998.

   [3]  Goland, Y.Y., Whitehead, E.J., et. al, , "Web Distributed
        Authoring and Versioning", RFC 2518, February 1999.

   [4]  Slein, J., Whitehead, E.J., et. al, , "WebDAV Advanced
        Collections Protocol", Internet-Draft (Work in Progress), June

   [5]  Dierks, T., Allen, C., "The TLS Protocol", RFC 2246, January

   [6]  Herriot, R., Butler, S., Moore, P., Turner, R., "Internet
        Printing Protocol/1.0: Encoding and Transport", RFC 2565, April

   [7]  Rose, M.T., "Writing I-Ds and RFCs using XML", April 1999.

   [8]  Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, October 1998.

Authors' Addresses

   Rohit Khare
   4K Associates / UC Irvine
   3207 Palo Verde
   Irvine, CA  92612

   Phone: +1 626 806 7574

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   Scott Lawrence
   Agranat Systems, Inc.
   5 Clocktower Place
   Suite 400
   Maynard, MA  01754

   Phone: +1 978 461 0888

Appendix A. Acknowledgments

   Thanks to:
   o  Paul Hoffman for his work on the STARTTLS command extension for
   o  Roy Fielding for assistance with the rationale behind Upgrade:
      and its interaction with OPTIONS.
   o  Eric Rescorla for his work on standardizing the existing https:
      practice to compare with.
   o  Marshall Rose, for the xml2rfc document type description and
   o  Jim Whitehead, for sorting out the current range of available
      HTTP status codes.

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