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
draft-ietf-tls-http-upgrade-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on December 21, 1999.
Abstract
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
2616.
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
Host: bank.example.com
Upgrade: HTTP+TLS/1.0
Connection: Upgrade
The client MUST use the OPTIONS method if unsecured operation is
unacceptable.
3.2 Requesting Upgrade When Unsecured Is Acceptable
The client MAY offer to switch to secured operation during a clear
HTTP operation:
GET http://bank.example.com/acct_stat.html?749394889300 HTTP/1.1
Host: bank.example.com
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
section).
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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
response.
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
completed.
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
request.
[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
request.
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
HTTP/1.1[1].
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
registered.
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.
References
[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
1999.
[5] Dierks, T., Allen, C., "The TLS Protocol", RFC 2246, January
1999.
[6] Herriot, R., Butler, S., Moore, P., Turner, R., "Internet
Printing Protocol/1.0: Encoding and Transport", RFC 2565, April
1999.
[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
US
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
US
Phone: +1 978 461 0888
EMail: lawrence@agranat.com
URI: http://www.agranat.com/
Appendix A. Acknowledgments
Thanks to:
o Paul Hoffman for his work on the STARTTLS command extension for
ESMTP.
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
tools.
o Jim Whitehead, for sorting out the current range of available
HTTP status codes.
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