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Versions: 00 01 02 03 04 05 RFC 2817

Network Working Group                                           R. Khare
Internet-Draft                                 4K Associates / UC Irvine
Expires: July 5, 2000                                        S. Lawrence
                                                   Agranat Systems, Inc.
                                                         January 5, 2000

                    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
   material or to cite them other than as "work in progress."

     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 July 5, 2000.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.


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

   Since HTTP/1.1[1] defines Upgrade as a hop-by-hop mechanism, this
   memo also documents the HTTP CONNECT method for establishing
   end-to-end tunnels across HTTP proxies. Finally, this memo
   establishes new IANA registries for public HTTP status codes, as
   well as public or private Upgrade product tokens.

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   This memo does NOT affect the current definition of the 'https' URI
   scheme, which already defines a separate namespace
   (http://example.org/ and https://example.org/ are not equivalent).

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

Table of Contents

   1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.1 Requirements Terminology . . . . . . . . . . . . . . . . . . .  4
   3.  Client Requested Upgrade to HTTP over TLS  . . . . . . . . . .  4
   3.1 Optional Upgrade . . . . . . . . . . . . . . . . . . . . . . .  4
   3.2 Mandatory Upgrade  . . . . . . . . . . . . . . . . . . . . . .  4
   3.3 Server Acceptance of Upgrade Request . . . . . . . . . . . . .  5
   4.  Server Requested Upgrade to HTTP over TLS  . . . . . . . . . .  5
   4.1 Optional Advertisement . . . . . . . . . . . . . . . . . . . .  5
   4.2 Mandatory Advertisement  . . . . . . . . . . . . . . . . . . .  5
   5.  Upgrade across Proxies . . . . . . . . . . . . . . . . . . . .  6
   5.1 Implications of Hop By Hop Upgrade . . . . . . . . . . . . . .  6
   5.2 Requesting a Tunnel with CONNECT . . . . . . . . . . . . . . .  7
   5.3 Establishing a Tunnel with CONNECT . . . . . . . . . . . . . .  7
   6.  Rationale for the use of a 4xx (client error) Status Code  . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.1 HTTP Status Code Registry  . . . . . . . . . . . . . . . . . .  8
   7.2 HTTP Upgrade Token Registry  . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.1 Implications for the https: URI Scheme . . . . . . . . . . . . 10
   8.2 Security Considerations for CONNECT  . . . . . . . . . . . . . 10
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

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1. Motivation

   The historical practice of deploying HTTP over SSL3 [3] 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.  Parallel well-known port numbers have similarly been
   requested -- and in some cases, granted -- to distinguish between
   secured and unsecured use of other application protocols (e.g.
   snews, ftps). This approach effectively halves the number of
   available well known ports.

   At the Washington DC IETF meeting in December 1997, the Applications
   Area Directors and the IESG reaffirmed that the practice of issuing
   parallel "secure" port numbers should be deprecated. The HTTP/1.1
   Upgrade mechanism can apply Transport Layer Security[6] to an open
   HTTP connection.

   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. In
   fact, nothing in this memo affects the current interpretation of
   https: URIs. However, new application protocols built atop HTTP,
   such as the Internet Printing Protocol[7], call for just such a
   mechanism in order to move ahead 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

   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
   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

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   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.

   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 "TLS/1.0" Upgrade
   token, and a new HTTP Status Code, "426 Upgrade Required".

   Section 3 and Section 4 describe the operation of a directly
   connected client and server. Intermediate proxies must establish an
   end-to-end tunnel before applying those operations, as explained in
   Section 5.

2.1 Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
   "MAY" that appear in this document are to be interpreted as
   described in RFC 2119[11].

3. Client Requested Upgrade to HTTP over TLS

   When the client sends an HTTP/1.1 request with an Upgrade header
   field containing the token "TLS/1.0", it is requesting the server to
   complete the current HTTP/1.1 request after switching to TLS/1.0.

3.1 Optional Upgrade

   A client MAY offer to switch to secured operation during any clear
   HTTP request when an unsecured response would be acceptable:

         GET http://example.bank.com/acct_stat.html?749394889300 HTTP/1.1
         Host: example.bank.com
         Upgrade: 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

   Note that HTTP/1.1[1] specifies "the upgrade keyword MUST be
   supplied within a Connection header field (section 14.10) whenever
   Upgrade is present in an HTTP/1.1 message."

3.2 Mandatory Upgrade

   If an unsecured response would be unacceptable, a client MUST send
   an OPTIONS request first to complete the switch to TLS/1.0 (if

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          OPTIONS * HTTP/1.1
          Host: example.bank.com
          Upgrade: TLS/1.0
          Connection: Upgrade

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" and MUST include an Upgrade response header specifying the
   tokens of the protocol stack it is switching to:

         HTTP/1.1 101 Switching Protocols
         Upgrade: TLS/1.0, HTTP/1.1
         Connection: Upgrade

    Note that the protocol tokens listed in the Upgrade header of a 101
   Switching Protocols response specify an ordered 'bottom-up' stack.

   As specified in  HTTP/1.1[1], Section 10.1.2: "The server will
   switch protocols to those defined by the response's Upgrade header
   field immediately after the empty line which terminates the 101

   Once the TLS handshake completes successfully, 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.

4. Server Requested Upgrade to HTTP over TLS

   The Upgrade response header field advertises possible protocol
   upgrades a server MAY accept. In conjunction with the "426 Upgrade
   Required" status code, a server can advertise the exact protocol
   upgrade(s) that a client MUST accept to complete the request.

4.1 Optional Advertisement

   As specified in HTTP/1.1[1], the server MAY include an Upgrade
   header in any response other than 101 or 426 to indicate a
   willingness to switch to any (combination) of the protocols listed.

4.2 Mandatory Advertisement

   A server MAY indicate that a client request can not be completed
   without TLS using the "426 Upgrade Required" status code, which MUST
   include an an Upgrade header field specifying the token of the
   required TLS version.

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         HTTP/1.1 426 Upgrade Required
         Upgrade: TLS/1.0, HTTP/1.1
         Connection: Upgrade

   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

   Note that even if a client is willing to use TLS, it must use the
   operations in Section 3 to proceed; the TLS handshake cannot begin
   immediately after the 426 response.

5. Upgrade across Proxies

   As a hop-by-hop header, Upgrade is negotiated between each pair of
   HTTP counterparties.  If a User Agent sends a request with an
   Upgrade header to a proxy, it is requesting a change to the protocol
   between itself and the proxy, not an end-to-end change.

   Since TLS, in particular, requires end-to-end connectivity to
   provide authentication and prevent man-in-the-middle attacks, this
   memo specifies the CONNECT method to establish a tunnel across

   Once a tunnel is established, any of the operations in Section 3 can
   be used to establish a TLS connection.

5.1 Implications of Hop By Hop Upgrade

   If an origin server receives an Upgrade header from a proxy and
   responds with a 101 Switching Protocols response, it is changing the
   protocol only on the connection between the proxy and itself.
   Similarly, a proxy might return a 101 response to its client to
   change the protocol on that connection independently of the
   protocols it is using to communicate toward the origin server.

   These scenarios also complicate diagnosis of a 426 response.  Since
   Upgrade is a hop-by-hop header, a proxy that does not recognize 426
   might remove the accompanying Upgrade header and prevent the client
   from determining the required protocol switch.  If a client receives
   a 426 status without an accompanying Upgrade header, it will need to
   request an end to end tunnel connection as described in Section 5.2
   and repeat the request in order to obtain the required upgrade

   This hop-by-hop definition of Upgrade was a deliberate choice.  It
   allows for incremental deployment on either side of proxies, and for
   optimized protocols between cascaded proxies without the knowledge

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   of the parties that are not a part of the change.

5.2 Requesting a Tunnel with CONNECT

   A CONNECT method requests that a proxy establish a tunnel connection
   on its behalf. The Request-URI portion of the Request-Line is always
   an 'authority' as defined by URI Generic Syntax[2], which is to say
   the host name and port number destination of the requested
   connection separated by a colon:

        CONNECT server.example.com:80 HTTP/1.1
        Host: server.example.com:80

   Other HTTP mechanisms can be used normally with the CONNECT method
   -- except end-to-end protocol Upgrade requests, of course, since the
   tunnel must be established first.

    For example, proxy authentication might be used to establish the
   authority to create a tunnel:

        CONNECT server.example.com:80 HTTP/1.1
        Host: server.example.com:80
        Proxy-Authorization: basic aGVsbG86d29ybGQ=

   Like any other pipelined HTTP/1.1 request, data to be tunneled may
   be sent immediately after the blank line. The usual caveats also
   apply: data may be discarded if the eventual response is negative,
   and the connection may be reset with no response if more than one
   TCP segment is outstanding.

5.3 Establishing a Tunnel with CONNECT

   Any successful (2xx) response to a CONNECT request indicates that
   the proxy has established a connection to the requested host and
   port, and has switched to tunneling the current connection to that
   server connection.

   It may be the case that the proxy itself can only reach the
   requested origin server through another proxy.  In this case, the
   first proxy SHOULD make a CONNECT request of that next proxy,
   requesting a tunnel to the authority.  A proxy MUST NOT respond with
   any 2xx status code unless it has either a direct or tunnel
   connection established to the authority.

   An origin server which receives a CONNECT request for itself MAY
   respond with a 2xx status code to indicate that a connection is

   If at any point either one of the peers gets disconnected, any

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   outstanding data that came from that peer will be passed to the
   other one, and after that also the other connection will be
   terminated by the proxy. If there is outstanding data to that peer
   undelivered, that data will be discarded.

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

   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 a 3xx code 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 incorporate the TLS layer, it would at
   best fail again at the new URL.

7. IANA Considerations

   IANA shall create registries for two name spaces, as described in
   BCP 26[10]:
   o  HTTP Status Codes
   o  HTTP Upgrade Tokens

7.1 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[4] [defines 420-424]
   3.  WebDAV Advanced Collections[5] (Work in Progress) [defines 425]
   4.  Section 6 [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

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7.2 HTTP Upgrade Token Registry

   The HTTP Upgrade Token Registry defines the name space for product
   tokens used to identify protocols in 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[10]. These specifications need not be IETF
   documents or be subject to IESG review, but should obey the
   following rules:

   1.  A token, once registered, stays registered forever.
   2.  The registration MUST name a responsible party for the
   3.  The registration MUST name a point of contact.
   4.  The registration MAY name the documentation required for the
   5.  The responsible party MAY change the registration at any time.
       The IANA will keep a record of all such changes, and make them
       available upon request.
   6.  The responsible party for the first registration of a "product"
       token MUST approve later registrations of a "version" token
       together with that "product" token before they can be registered.
   7.  If absolutely required, the IESG MAY reassign the responsibility
       for a token. This will normally only be used in the case when a
       responsible party cannot be contacted.

   This specification defines the protocol token "TLS/1.0" as the
   identifier for the protocol specified by The TLS Protocol[6].

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

8. Security Considerations

   The potential for a man-in-the-middle attack (deleting the Upgrade
   header) remains the same as current, mixed http/https practice:
   o  Removing the Upgrade header 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 such
      information over both a secure and an insecure channel in the

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      first place.
   o  If the client knows for a fact that a server is TLS-compliant, it
      can insist on it 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."

   Furthermore, for clients that do not explicitly try to invoke TLS,
   servers can use the Upgrade header in any response other than 101 or
   426 to advertise TLS compliance. 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

8.1 Implications for the https: URI Scheme

   While nothing in this memo affects the definition of the 'https' URI
   scheme, widespread adoption of this mechanism for HyperText content
   could use 'http' to identify both secure and non-secure resources.

   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 servers may apply
   resource access rules such as 'the FORM on this page must be served
   and submitted using TLS'.

8.2 Security Considerations for CONNECT

   A generic TCP tunnel is fraught with security risks. First, such
   authorization should be limited to a small number of known ports.
   The Upgrade: mechanism defined here only requires onward tunneling
   at port 80. Second, since tunneled data is opaque to the proxy,
   there are additional risks to tunneling to other well-known or
   reserved ports. A putative HTTP client CONNECTing to port 25 could
   relay spam via SMTP, for example.


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

   [2]  Berners-Lee, T., Fielding, R.T. and L. Masinter, "URI Generic
        Syntax", RFC 2396, August 1998.

   [3]  Rescorla, E.K., "HTTP Over TLS",  Internet-Draft (Work In

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        Progress) (Non-Normative Background Information)
        draft-ietf-tls-https-02, September 1998.

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

   [5]  Slein, J., Whitehead, E.J. and  et. al, "WebDAV Advanced
        Collections Protocol",  Internet-Draft (Work In Progress)
        (Non-Normative Background Information)
        draft-ietf-webdav-collection-protocol-04, June 1999.

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

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

   [8]  Luotonen, A., "Tunneling TCP based protocols through Web proxy
        servers",  Internet-Draft (Work In Progress) (Non-Normative
        Historical Information; Also available in: Luotonen, Ari. Web
        Proxy Servers, Prentice-Hall, 1997 ISBN:0136806120)
        draft-luotonen-web-proxy-tunneling-01, August 1998.

   [9]  Rose, M.T., "Writing I-Ds and RFCs using XML", RFC 2629, June

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

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

Authors' Addresses

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

   Phone: +1 626 806 7574
   EMail: rohit@4K-associates.com
   URI:   http://www.4K-associates.com/

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

   Phone: +1 978 461 0888
   EMail: lawrence@agranat.com
   URI:   http://www.agranat.com/

Appendix A. Acknowledgments

   The CONNECT method was originally described in an Internet-Draft
   titled Tunneling TCP based protocols through Web proxy servers[8] by
   Ari Luotonen of Netscape Communications Corporation.  It was widely
   implemented by HTTP proxies, but was never made a part of any IETF
   Standards Track document. The method name CONNECT was reserved, but
   not defined in [1].

   The definition provided here is derived directly from that earlier
   draft, with some editorial changes and conformance to the stylistic
   conventions since established in other HTTP specifications.

   Additional 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.
   o  Henrik Frystyk Nielsen, whose work on the Mandatory extension
      mechanism pointed out a hop-by-hop Upgrade still requires
   o  Harald Alvestrand for improvements to the token registration

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Full Copyright Statement

   Copyright (C) The Internet Society (2000). All Rights Reserved.

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   kind, provided that the above copyright notice and this paragraph
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   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


   Funding for the RFC editor function is currently provided by the
   Internet Society.

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