HTTP Basic Auth: The 'Basic' HTTP Authentication Scheme

Internet Engineering Task Force (IETF)                        J. Reschke
Request for Comments: 7617                                    greenbytes
Obsoletes: 2617                                           September 2015
Category: Standards Track
ISSN: 2070-1721


                 The 'Basic' HTTP Authentication Scheme

Abstract

   This document defines the "Basic" Hypertext Transfer Protocol (HTTP)
   authentication scheme, which transmits credentials as user-id/
   password pairs, encoded using Base64.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7617.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology and Notation  . . . . . . . . . . . . . . . .   3
   2.  The 'Basic' Authentication Scheme . . . . . . . . . . . . . .   3
     2.1.  The 'charset' auth-param  . . . . . . . . . . . . . . . .   5
     2.2.  Reusing Credentials . . . . . . . . . . . . . . . . . . .   7
   3.  Internationalization Considerations . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changes from RFC 2617  . . . . . . . . . . . . . . .  13
   Appendix B.  Deployment Considerations for the 'charset'
                Parameter  . . . . . . . . . . . . . . . . . . . . .  13
     B.1.  User Agents . . . . . . . . . . . . . . . . . . . . . . .  13
     B.2.  Servers . . . . . . . . . . . . . . . . . . . . . . . . .  13
     B.3.  Why not simply switch the default encoding to UTF-8?  . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document defines the "Basic" Hypertext Transfer Protocol (HTTP)
   authentication scheme, which transmits credentials as user-id/
   password pairs, encoded using Base64 (HTTP authentication schemes are
   defined in [RFC7235]).

   This scheme is not considered to be a secure method of user
   authentication unless used in conjunction with some external secure
   system such as TLS (Transport Layer Security, [RFC5246]), as the
   user-id and password are passed over the network as cleartext.

   The "Basic" scheme previously was defined in Section 2 of [RFC2617].
   This document updates the definition, and also addresses
   internationalization issues by introducing the 'charset'
   authentication parameter (Section 2.1).

   Other documents updating RFC 2617 are "Hypertext Transfer Protocol
   (HTTP/1.1): Authentication" ([RFC7235], defining the authentication
   framework), "HTTP Digest Access Authentication" ([RFC7616], updating
   the definition of the "Digest" authentication scheme), and "HTTP
   Authentication-Info and Proxy-Authentication-Info Response Header
   Fields" ([RFC7615]).  Taken together, these four documents obsolete
   RFC 2617.

1.1.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The terms "protection space" and "realm" are defined in Section 2.2
   of [RFC7235].

   The terms "(character) repertoire" and "character encoding scheme"
   are defined in Section 2 of [RFC6365].

2.  The 'Basic' Authentication Scheme

   The Basic authentication scheme is based on the model that the client
   needs to authenticate itself with a user-id and a password for each
   protection space ("realm").  The realm value is a free-form string
   that can only be compared for equality with other realms on that
   server.  The server will service the request only if it can validate
   the user-id and password for the protection space applying to the
   requested resource.

   The Basic authentication scheme utilizes the Authentication Framework
   as follows.

   In challenges:

   o  The scheme name is "Basic".

   o  The authentication parameter 'realm' is REQUIRED ([RFC7235],
      Section 2.2).

   o  The authentication parameter 'charset' is OPTIONAL (see
      Section 2.1).

   o  No other authentication parameters are defined -- unknown
      parameters MUST be ignored by recipients, and new parameters can
      only be defined by revising this specification.

   See also Section 4.1 of [RFC7235], which discusses the complexity of
   parsing challenges properly.

   Note that both scheme and parameter names are matched case-
   insensitively.

   For credentials, the "token68" syntax defined in Section 2.1 of
   [RFC7235] is used.  The value is computed based on user-id and
   password as defined below.

   Upon receipt of a request for a URI within the protection space that
   lacks credentials, the server can reply with a challenge using the
   401 (Unauthorized) status code ([RFC7235], Section 3.1) and the
   WWW-Authenticate header field ([RFC7235], Section 4.1).

   For instance:

      HTTP/1.1 401 Unauthorized
      Date: Mon, 04 Feb 2014 16:50:53 GMT
      WWW-Authenticate: Basic realm="WallyWorld"

   where "WallyWorld" is the string assigned by the server to identify
   the protection space.

   A proxy can respond with a similar challenge using the 407 (Proxy
   Authentication Required) status code ([RFC7235], Section 3.2) and the
   Proxy-Authenticate header field ([RFC7235], Section 4.3).

   To receive authorization, the client

   1.  obtains the user-id and password from the user,

   2.  constructs the user-pass by concatenating the user-id, a single
       colon (":") character, and the password,

   3.  encodes the user-pass into an octet sequence (see below for a
       discussion of character encoding schemes),

   4.  and obtains the basic-credentials by encoding this octet sequence
       using Base64 ([RFC4648], Section 4) into a sequence of US-ASCII
       characters ([RFC0020]).

   The original definition of this authentication scheme failed to
   specify the character encoding scheme used to convert the user-pass
   into an octet sequence.  In practice, most implementations chose
   either a locale-specific encoding such as ISO-8859-1 ([ISO-8859-1]),
   or UTF-8 ([RFC3629]).  For backwards compatibility reasons, this
   specification continues to leave the default encoding undefined, as
   long as it is compatible with US-ASCII (mapping any US-ASCII
   character to a single octet matching the US-ASCII character code).

   The user-id and password MUST NOT contain any control characters (see
   "CTL" in Appendix B.1 of [RFC5234]).

   Furthermore, a user-id containing a colon character is invalid, as
   the first colon in a user-pass string separates user-id and password
   from one another; text after the first colon is part of the password.
   User-ids containing colons cannot be encoded in user-pass strings.

   Note that many user agents produce user-pass strings without checking
   that user-ids supplied by users do not contain colons; recipients
   will then treat part of the username input as part of the password.

   If the user agent wishes to send the user-id "Aladdin" and password
   "open sesame", it would use the following header field:

      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

2.1.  The 'charset' auth-param

   In challenges, servers can use the 'charset' authentication parameter
   to indicate the character encoding scheme they expect the user agent
   to use when generating "user-pass" (a sequence of octets).  This
   information is purely advisory.

   The only allowed value is "UTF-8"; it is to be matched case-
   insensitively (see [RFC2978], Section 2.3).  It indicates that the
   server expects character data to be converted to Unicode
   Normalization Form C ("NFC"; see Section 3 of [RFC5198]) and to be
   encoded into octets using the UTF-8 character encoding scheme
   ([RFC3629]).

   For the user-id, recipients MUST support all characters defined in
   the "UsernameCasePreserved" profile defined in Section 3.3 of
   [RFC7613], with the exception of the colon (":") character.

   For the password, recipients MUST support all characters defined in
   the "OpaqueString" profile defined in Section 4.2 of [RFC7613].

   Other values are reserved for future use.

      Note: The 'charset' is only defined on challenges, as Basic
      authentication uses a single token for credentials ('token68'
      syntax); thus, the credentials syntax isn't extensible.

      Note: The name 'charset' has been chosen for consistency with
      Section 2.1.1 of [RFC2831].  A better name would have been
      'accept-charset', as it is not about the message it appears in,
      but the server's expectation.

   In the example below, the server prompts for authentication in the
   "foo" realm, using Basic authentication, with a preference for the
   UTF-8 character encoding scheme:

      WWW-Authenticate: Basic realm="foo", charset="UTF-8"

   Note that the parameter value can be either a token or a quoted
   string; in this case, the server chose to use the quoted-string
   notation.

   The user's name is "test", and the password is the string "123"
   followed by the Unicode character U+00A3 (POUND SIGN).  Using the
   character encoding scheme UTF-8, the user-pass becomes:

      't' 'e' 's' 't' ':' '1' '2' '3' pound
      74  65  73  74  3A  31  32  33  C2  A3

   Encoding this octet sequence in Base64 ([RFC4648], Section 4) yields:

      dGVzdDoxMjPCow==
   Thus, the Authorization header field would be:

      Authorization: Basic dGVzdDoxMjPCow==

   Or, for proxy authentication:

      Proxy-Authorization: Basic dGVzdDoxMjPCow==

2.2.  Reusing Credentials

   Given the absolute URI ([RFC3986], Section 4.3) of an authenticated
   request, the authentication scope of that request is obtained by
   removing all characters after the last slash ("/") character of the
   path component ("hier_part"; see [RFC3986], Section 3).  A client
   SHOULD assume that resources identified by URIs with a prefix-match
   of the authentication scope are also within the protection space
   specified by the realm value of that authenticated request.

   A client MAY preemptively send the corresponding Authorization header
   field with requests for resources in that space without receipt of
   another challenge from the server.  Similarly, when a client sends a
   request to a proxy, it MAY reuse a user-id and password in the Proxy-
   Authorization header field without receiving another challenge from
   the proxy server.

   For example, given an authenticated request to:

      http://example.com/docs/index.html

   requests to the URIs below could use the known credentials:

      http://example.com/docs/
      http://example.com/docs/test.doc
      http://example.com/docs/?page=1

   while the URIs

      http://example.com/other/
      https://example.com/docs/

   would be considered to be outside the authentication scope.

   Note that a URI can be part of multiple authentication scopes (such
   as "http://example.com/" and "http://example.com/docs/").  This
   specification does not define which of these should be treated with
   higher priority.

3.  Internationalization Considerations

   User-ids or passwords containing characters outside the US-ASCII
   character repertoire will cause interoperability issues, unless both
   communication partners agree on what character encoding scheme is to
   be used.  Servers can use the new 'charset' parameter (Section 2.1)
   to indicate a preference of "UTF-8", increasing the probability that
   clients will switch to that encoding.

   The 'realm' parameter carries data that can be considered textual;
   however, [RFC7235] does not define a way to reliably transport non-
   US-ASCII characters.  This is a known issue that would need to be
   addressed in a revision to that specification.

4.  Security Considerations

   The Basic authentication scheme is not a secure method of user
   authentication, nor does it in any way protect the entity, which is
   transmitted in cleartext across the physical network used as the
   carrier.  HTTP does not prevent the addition of enhancements (such as
   schemes to use one-time passwords) to Basic authentication.

   The most serious flaw of Basic authentication is that it results in
   the cleartext transmission of the user's password over the physical
   network.  Many other authentication schemes address this problem.

   Because Basic authentication involves the cleartext transmission of
   passwords, it SHOULD NOT be used (without enhancements such as HTTPS
   [RFC2818]) to protect sensitive or valuable information.

   A common use of Basic authentication is for identification purposes
   -- requiring the user to provide a user-id and password as a means of
   identification, for example, for purposes of gathering accurate usage
   statistics on a server.  When used in this way it is tempting to
   think that there is no danger in its use if illicit access to the
   protected documents is not a major concern.  This is only correct if
   the server issues both user-id and password to the users and, in
   particular, does not allow the user to choose his or her own
   password.  The danger arises because naive users frequently reuse a
   single password to avoid the task of maintaining multiple passwords.

   If a server permits users to select their own passwords, then the
   threat is not only unauthorized access to documents on the server but
   also unauthorized access to any other resources on other systems that
   the user protects with the same password.  Furthermore, in the
   server's password database, many of the passwords may also be users'
   passwords for other sites.  The owner or administrator of such a
   system could therefore expose all users of the system to the risk of
   unauthorized access to all those other sites if this information is
   not maintained in a secure fashion.  This raises both security and
   privacy concerns ([RFC6973]).  If the same user-id and password
   combination is in use to access other accounts, such as an email or
   health portal account, personal information could be exposed.

   Basic authentication is also vulnerable to spoofing by counterfeit
   servers.  If a user can be led to believe that she is connecting to a
   host containing information protected by Basic authentication when,
   in fact, she is connecting to a hostile server or gateway, then the
   attacker can request a password, store it for later use, and feign an
   error.  Server implementers ought to guard against this sort of
   counterfeiting; in particular, software components that can take over
   control over the message framing on an existing connection need to be
   used carefully or not at all (for instance: NPH ("Non-Parsed Header")
   scripts as described in Section 5 of [RFC3875]).

   Servers and proxies implementing Basic authentication need to store
   user passwords in some form in order to authenticate a request.
   These passwords ought to be stored in such a way that a leak of the
   password data doesn't make them trivially recoverable.  This is
   especially important when users are allowed to set their own
   passwords, since users are known to choose weak passwords and to
   reuse them across authentication realms.  While a full discussion of
   good password hashing techniques is beyond the scope of this
   document, server operators ought to make an effort to minimize risks
   to their users in the event of a password data leak.  For example,
   servers ought to avoid storing user passwords in plaintext or as
   unsalted digests.  For more discussion about modern password hashing
   techniques, see the "Password Hashing Competition"
   (<https://password-hashing.net>).

   The use of the UTF-8 character encoding scheme and of normalization
   introduces additional security considerations; see Section 10 of
   [RFC3629] and Section 6 of [RFC5198] for more information.

5.  IANA Considerations

   IANA maintains the "Hypertext Transfer Protocol (HTTP) Authentication
   Scheme Registry" ([RFC7235]) at <http://www.iana.org/assignments/
   http-authschemes>.

   The entry for the "Basic" authentication scheme has been updated to
   reference this specification.

6.  References

6.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <http://www.rfc-editor.org/info/rfc2978>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <http://www.rfc-editor.org/info/rfc5198>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <http://www.rfc-editor.org/info/rfc6365>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <http://www.rfc-editor.org/info/rfc7613>.

6.2.  Informative References

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/IEC
              8859-1:1998, 1998.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <http://www.rfc-editor.org/info/rfc2617>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC2831]  Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, DOI 10.17487/RFC2831, May 2000,
              <http://www.rfc-editor.org/info/rfc2831>.

   [RFC3875]  Robinson, D. and K. Coar, "The Common Gateway Interface
              (CGI) Version 1.1", RFC 3875, DOI 10.17487/RFC3875,
              October 2004, <http://www.rfc-editor.org/info/rfc3875>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <http://www.rfc-editor.org/info/rfc6973>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <http://www.rfc-editor.org/info/rfc7615>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <http://www.rfc-editor.org/info/rfc7616>.

Appendix A.  Changes from RFC 2617

   The scheme definition has been rewritten to be consistent with newer
   specifications such as [RFC7235].

   The new authentication parameter 'charset' has been added.  It is
   purely advisory, so existing implementations do not need to change,
   unless they want to take advantage of the additional information that
   previously wasn't available.

Appendix B.  Deployment Considerations for the 'charset' Parameter

B.1.  User Agents

   User agents not implementing 'charset' will continue to work as
   before, ignoring the new parameter.

   User agents that already default to the UTF-8 encoding implement
   'charset' by definition.

   Other user agents can keep their default behavior and switch to UTF-8
   when seeing the new parameter.

B.2.  Servers

   Servers that do not support non-US-ASCII characters in credentials do
   not require any changes to support 'charset'.

   Servers that need to support non-US-ASCII characters, but cannot use
   the UTF-8 character encoding scheme will not be affected; they will
   continue to function as well or as badly as before.

   Finally, servers that need to support non-US-ASCII characters and can
   use the UTF-8 character encoding scheme can opt in by specifying the
   'charset' parameter in the authentication challenge.  Clients that do
   understand the 'charset' parameter will then start to use UTF-8,
   while other clients will continue to send credentials in their
   default encoding, broken credentials, or no credentials at all.
   Until all clients are upgraded to support UTF-8, servers are likely
   to see both UTF-8 and "legacy" encodings in requests.  When
   processing as UTF-8 fails (due to a failure to decode as UTF-8 or a
   mismatch of user-id/password), a server might try a fallback to the
   previously supported legacy encoding in order to accommodate these
   legacy clients.  Note that implicit retries need to be done
   carefully; for instance, some subsystems might detect repeated login
   failures and treat them as a potential credentials-guessing attack.

B.3.  Why not simply switch the default encoding to UTF-8?

   There are sites in use today that default to a local character
   encoding scheme, such as ISO-8859-1 ([ISO-8859-1]), and expect user
   agents to use that encoding.  Authentication on these sites will stop
   working if the user agent switches to a different encoding, such as
   UTF-8.

   Note that sites might even inspect the User-Agent header field
   ([RFC7231], Section 5.5.3) to decide which character encoding scheme
   to expect from the client.  Therefore, they might support UTF-8 for
   some user agents, but default to something else for others.  User
   agents in the latter group will have to continue to do what they do
   today until the majority of these servers have been upgraded to
   always use UTF-8.

Acknowledgements

   This specification takes over the definition of the "Basic" HTTP
   Authentication Scheme, previously defined in RFC 2617.  We thank John
   Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott
   D. Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart for
   their work on that specification, from which significant amounts of
   text were borrowed.  See Section 6 of [RFC2617] for further
   acknowledgements.

   The internationalization problem with respect to the character
   encoding scheme used for user-pass was reported as a Mozilla bug back
   in the year 2000 (see
   <https://bugzilla.mozilla.org/show_bug.cgi?id=41489> and also the
   more recent <https://bugzilla.mozilla.org/show_bug.cgi?id=656213>).
   It was Andrew Clover's idea to address it using a new auth-param.

   We also thank the members of the HTTPAUTH Working Group and other
   reviewers, namely, Stephen Farrell, Roy Fielding, Daniel Kahn
   Gillmor, Tony Hansen, Bjoern Hoehrmann, Kari Hurtta, Amos Jeffries,
   Benjamin Kaduk, Michael Koeller, Eric Lawrence, Barry Leiba, James
   Manger, Alexey Melnikov, Kathleen Moriarty, Juergen Schoenwaelder,
   Yaron Sheffer, Meral Shirazipour, Michael Sweet, and Martin Thomson
   for feedback on this revision.

Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Email: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/