Internet Engineering Task Force (IETF) T. Lodderstedt, Ed.
Request for Comments: 9701 yes.com AG
Category: Standards Track V. Dzhuvinov
ISSN: 2070-1721 Connect2id Ltd.
November
December 2024
JSON Web Token (JWT) Response for OAuth Token Introspection
Abstract
This specification proposes an additional response secured by JSON
Web Token (JWT)
secured response for OAuth 2.0 Token Introspection.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9701.
Copyright Notice
Copyright (c) 2024 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
(https://trustee.ietf.org/license-info) in effect on the date of
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Requirements Notation
3. Resource Server Management
4. Requesting a JWT Response
5. JWT Response
6. Client Metadata
7. Authorization Server Metadata
8. Security Considerations
8.1. Cross-JWT Confusion
8.2. Token Data Leakage
9. Privacy Considerations
10. IANA Considerations
10.1. OAuth Dynamic Client Registration Metadata Registration
10.1.1. Registry Contents
10.2. OAuth Authorization Server Metadata Registration
10.2.1. Registry Contents
10.3. Media Type Registration
10.3.1. Registry Contents
10.4. JWT Claim Registration
10.4.1. Registry Contents
11. References
11.1. Normative References
11.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
"OAuth 2.0 Token Introspection" [RFC7662] specifies a method for a
protected resource to query an OAuth 2.0 authorization server to
determine the state of an access token and obtain data associated
with the access token. This enables deployments to implement opaque
access tokens in an interoperable way.
The introspection response, as specified in "OAuth 2.0 Token
Introspection" [RFC7662], is a plain JSON object. However, there are
use cases where the resource server requires stronger assurance that
the authorization server issued the token introspection response for
an access token, including cases where the authorization server
assumes liability for the content of the token introspection
response. An example is a resource server using verified personal
data to create certificates, which in turn are used to create
qualified electronic signatures.
In such use cases, it may be useful or even required to return a
signed JWT [RFC7519] as the introspection response. This
specification extends the token introspection endpoint with the
capability to return responses as JWTs.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Resource Server Management
The authorization server (AS) and the resource server (RS) maintain a
strong, two-way trust relationship. The resource server relies on
the authorization server to obtain authorization, user, and other
data as input to its access control decisions and service delivery.
The authorization server relies on the resource server to handle the
provided data appropriately.
In the context of this specification, the token introspection
endpoint is used to convey such security data and potentially also
privacy-sensitive data related to an access token.
In order to process the introspection requests in a secure and
privacy-preserving manner, the authorization server MUST be able to
identify, authenticate, and authorize resource servers.
The AS MAY additionally encrypt the token introspection response
JWTs. If encryption is used, the AS is provisioned with encryption
keys and algorithms for the RS.
The AS MUST be able to determine whether an RS is the audience for a
particular access token and what data it is entitled to receive;
otherwise, the RS is not authorized to obtain data for the access
token. The AS has the discretion of how to fulfill this requirement.
The AS could, for example, maintain a mapping between scope values
and RSes. RSs.
The requirements given above imply that the AS maintains credentials
and other configuration data for each RS.
One way is by utilizing dynamic client registration [RFC7591] and
treating every RS as an OAuth client. In this case, the AS is
assumed to at least maintain a "client_id" and a
"token_endpoint_auth_method" with complementary authentication method
metadata, such as "jwks" or "client_secret". In cases where the AS
needs to acquire consent to transmit data to an RS, the following
client metadata fields are recommended: "client_name", "client_uri",
"contacts", "tos_uri", and "policy_uri".
The AS MUST restrict the use of client credentials by an RS to the
calls it requires, e.g., the AS MAY restrict such a client to call
the token introspection endpoint only. How the AS implements this
restriction is beyond the scope of this specification.
This specification further introduces client metadata to manage the
configuration options required to sign and encrypt token
introspection response JWTs.
4. Requesting a JWT Response
An RS requests a JWT introspection response by sending an
introspection request with an Accept HTTP header field set to
"application/token-introspection+jwt".
The AS MUST authenticate the caller at the token introspection
endpoint. Authentication can utilize client authentication methods
or a separate access token that is issued to the RS and identifying it identifies
the RS as the subject.
The following is a non-normative example request, with the RS
authenticating with a private key JWT:
POST /introspect HTTP/1.1
Host: as.example.com
Accept: application/token-introspection+jwt
Content-Type: application/x-www-form-urlencoded
token=2YotnFZFEjr1zCsicMWpAA&
client_assertion_type=
urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer&
client_assertion=PHNhbWxwOl[...omitted for brevity...]ZT
5. JWT Response
The introspection endpoint responds with a JWT, setting the Content-
Type HTTP header field to "application/token-introspection+jwt" and
the JWT typ ("type") header parameter to "token-introspection+jwt".
The JWT MUST include the following top-level claims:
iss
MUST be set to the issuer URL of the authorization server.
aud
MUST identify the resource server receiving the token
introspection response.
iat
MUST be set to the time when the introspection response was
created by the authorization server
token_introspection
A JSON object containing the members of the token introspection
response, as specified in [RFC7662], Section 2.2. The separation
of the introspection response members into a dedicated JSON object
containing a JWT claim is intended to prevent conflict and
confusion with top-
level top-level JWT claims that may bear the same name.
If the access token is invalid, expired, revoked, or not intended
for the calling resource server (audience), the authorization
server MUST set the value of the active member in the
token_introspection claim to false and MUST NOT include other
members. Otherwise, the active member is set to true.
The AS SHOULD narrow down the scope value to the scopes relevant
to the particular RS.
As specified in Section 2.2 of [RFC7662], implementations MAY
extend the token introspection response with service-specific
claims. In the context of this specification, such claims will be
added as top-level members of the token_introspection claim.
Token introspection response parameter names intended to be used
across domains MUST be registered in the "OAuth Token
Introspection Response" registry [IANA.OAuth.Token.Introspection]
defined by [RFC7662].
When the AS acts as a provider of resource owner identity claims
to the RS, the AS determines, based on its RS-specific policy,
what identity claims to return in the token introspection
response. The AS MUST ensure the release of any privacy-sensitive
data is legally based (see Section 9).
Further content of the introspection response is determined by the
RS-specific policy at the AS.
The JWT MAY include other claims, including those from the "JSON Web
Token Claims" registry established by [RFC7519]. The JWT SHOULD NOT
include the sub and exp claims, as an additional measure to prevent
misuse of the JWT as an access token (see Section 8.1).
Note: Although the JWT format is widely used as an access token
format, the JWT returned in the introspection response is not an
alternative representation of the introspected access token and is
not intended to be used as an access token.
This specification registers the "application/token-
introspection+jwt" media type, which is used as the value of the typ
("type") header parameter of the JWT to indicate that the payload is
a token introspection response.
The JWT is cryptographically secured as specified in [RFC7519].
Depending on the specific resource server policy, the JWT is either
signed or signed and encrypted. If the JWT is signed and encrypted,
it MUST be a Nested JWT, as defined in JWT [RFC7519].
Note: An AS compliant with this specification MUST refuse to serve
introspection requests that don't authenticate the caller and return
an HTTP status code 400. This is done to ensure token data is
released to legitimate recipients only and prevent downgrading to
[RFC7662] behavior (see Section 8.2).
The following is a non-normative example response (with line breaks
for display purposes only):
HTTP/1.1 200 OK
Content-Type: application/token-introspection+jwt
eyJraWQiOiJ3RzZEIiwidHlwIjoidG9rZW4taW50cm9zcGVjdGlvbitqd3QiLCJhbGc
iOiJSUzI1NiJ9.eyJpc3MiOiJodHRwczovL2FzLmV4YW1wbGUuY29tLyIsImF1ZCI6I
mh0dHBzOi8vcnMuZXhhbXBsZS5jb20vcmVzb3VyY2UiLCJpYXQiOjE1MTQ3OTc4OTIs
InRva2VuX2ludHJvc3BlY3Rpb24iOnsiYWN0aXZlIjp0cnVlLCJpc3MiOiJodHRwczo
vL2FzLmV4YW1wbGUuY29tLyIsImF1ZCI6Imh0dHBzOi8vcnMuZXhhbXBsZS5jb20vcm
Vzb3VyY2UiLCJpYXQiOjE1MTQ3OTc4MjIsImV4cCI6MTUxNDc5Nzk0MiwiY2xpZW50X
2lkIjoicGFpQjJnb28wYSIsInNjb3BlIjoicmVhZCB3cml0ZSBkb2xwaGluIiwic3Vi
IjoiWjVPM3VwUEM4OFFyQWp4MDBkaXMiLCJiaXJ0aGRhdGUiOiIxOTgyLTAyLTAxIiw
iZ2l2ZW5fbmFtZSI6IkpvaG4iLCJmYW1pbHlfbmFtZSI6IkRvZSIsImp0aSI6InQxRm
9DQ2FaZDRYdjRPUkpVV1ZVZVRaZnNLaFczMENRQ3JXRERqd1h5NncifX0.przJMU5Gh
mNzvwtt1Sr-xa9xTkpiAg5IshbQsRiRVP_7eGR1GHYrNwQh84kxOkHCyje2g5WSRcYo
sGEVIiC-eoPJJ-qBwqwSlgx9JEeCDw2W5DjrblOI_N0Jvsq_dUeOyoWVMqlOydOBhKN
Y0smBrI4NZvEExucOm9WUJXMuJtvq1gBes-0go5j4TEv9sOP9uu81gqWTr_LOo6pgT0
tFFyZfWC4kbXPXiQ2YT6mxCiQRRNM-l9cBdF6Jx6IOrsfFhBuYdYQ_mlL19HgDDOFal
eyqmru6lKlASOsaE8dmLSeKcX91FbG79FKN8un24iwIDCbKT9xlUFl54xWVShNDFA
The example response JWT header contains the following JSON document:
{
"typ": "token-introspection+jwt",
"alg": "RS256",
"kid": "wG6D"
}
The example response JWT payload contains the following JSON
document:
{
"iss":"https://as.example.com/",
"aud":"https://rs.example.com/resource",
"iat":1514797892,
"token_introspection":
{
"active":true,
"iss":"https://as.example.com/",
"aud":"https://rs.example.com/resource",
"iat":1514797822,
"exp":1514797942,
"client_id":"paiB2goo0a",
"scope":"read write dolphin",
"sub":"Z5O3upPC88QrAjx00dis",
"birthdate":"1982-02-01",
"given_name":"John",
"family_name":"Doe",
"jti":"t1FoCCaZd4Xv4ORJUWVUeTZfsKhW30CQCrWDDjwXy6w"
}
}
6. Client Metadata
The authorization server determines the algorithm to secure the JWT
for a particular introspection response. This decision can be based
on registered metadata parameters for the resource server, supplied
via dynamic client registration [RFC7591] with the resource server
acting as a client, as specified below.
The parameter names follow the pattern established by OpenID Connect
Dynamic Client Registration [OpenID.Registration] for configuring
signing and encryption algorithms for JWT responses at the UserInfo
endpoint.
The following client metadata parameters are introduced by this
specification:
introspection_signed_response_alg
OPTIONAL. "JSON Web Signature (JWS)" [RFC7515] algorithm (alg
value), as defined in "JSON Web Algorithms (JWA)" [RFC7518], for
signing introspection responses. If this is specified, the
response will be signed using JWS and the configured algorithm.
The default, if omitted, is RS256.
introspection_encrypted_response_alg
OPTIONAL. "JSON Web Encryption (JWE)" [RFC7516] algorithm (alg
value), as defined in JWA [RFC7518], for content key encryption.
If this is specified, the response will be encrypted using JWE and
the configured content encryption algorithm
(introspection_encrypted_response_enc). The default, if omitted,
is that no encryption is performed. If both signing and
encryption are requested, the response will be signed then
encrypted, with the result being a Nested JWT, as defined in JWT
[RFC7519].
introspection_encrypted_response_enc
OPTIONAL. JWE [RFC7516] algorithm (enc value), as defined in JWA
[RFC7518], for content encryption of introspection responses. The
default, if omitted, is A128CBC-HS256. Note: This parameter MUST
NOT be specified without setting
introspection_encrypted_response_alg.
Resource servers may register their public encryption keys using the
jwks_uri or jwks metadata parameters.
7. Authorization Server Metadata
Authorization servers SHOULD publish the supported algorithms for
signing and encrypting the JWT of an introspection response by
utilizing "OAuth 2.0 Authorization Server Metadata" [RFC8414]
parameters. Resource servers use this data to parametrize their
client registration requests.
The following parameters are introduced by this specification:
introspection_signing_alg_values_supported
OPTIONAL. JSON array containing a list of the JWS [RFC7515]
signing algorithms (alg values), as defined in JWA [RFC7518],
supported by the introspection endpoint to sign the response.
introspection_encryption_alg_values_supported
OPTIONAL. JSON array containing a list of the JWE [RFC7516]
encryption algorithms (alg values), as defined in JWA [RFC7518],
supported by the introspection endpoint to encrypt the content
encryption key for introspection responses (content key
encryption).
introspection_encryption_enc_values_supported
OPTIONAL. JSON array containing a list of the JWE [RFC7516]
encryption algorithms (enc values), as defined in JWA [RFC7518],
supported by the introspection endpoint to encrypt the response
(content encryption).
8. Security Considerations
8.1. Cross-JWT Confusion
The iss and potentially the aud claim of a token introspection JWT
can resemble those of a JWT-encoded access token. An attacker could
try to exploit this and pass a JWT token introspection response as an
access token to the resource server. The typ ("type") JWT header
"token-introspection+jwt" and the encapsulation of the token
introspection members, such as sub and scope in the
token_introspection claim, are intended to prevent such substitution
attacks. Resource servers MUST therefore check the typ JWT header
value of received JWT-encoded access tokens and ensure all minimally
required claims for a valid access token are present.
Resource servers MUST additionally apply the countermeasures against
access token replay, as described in [RFC9700], Section 3.2. [RFC9700].
JWT confusion and other attacks involving JWTs are discussed in
[RFC8725].
8.2. Token Data Leakage
The authorization server MUST use Transport Layer Security (TLS) 1.2
(or higher), per BCP 195 [RFC7525], [RFC9325], in order to prevent token data
leakage.
Section 2.1 of [RFC7662] permits requests to the introspection
endpoint to be authorized with an access token that doesn't identify
the caller. To prevent introspection of tokens by parties that are
not the intended consumer, the authorization server MUST require all
requests to the token introspection endpoint to be authenticated.
9. Privacy Considerations
The token introspection response can be used to transfer personal
identifiable information (PII) from the AS to the RS. The AS MUST
conform to legal and jurisdictional constraints for the data transfer
before any data is released to a particular RS. The details and
determining of these constraints vary by jurisdiction and are outside
the scope of this document.
A commonly found way to establish the legal basis for releasing PII
is by explicit user consent gathered from the resource owner by the
AS during the authorization flow.
It is also possible that the legal basis is established out of band,
for example, in an explicit contract or by the client gathering the
resource owner's consent.
If the AS and the RS belong to the same legal entity (1st party
scenario), there is potentially no need for an explicit user consent,
but the terms of service and policy of the respective service
provider MUST be enforced at all times.
In any case, the AS MUST ensure that the scope of the legal basis is
enforced throughout the whole process. The AS MUST retain the scope
of the legal basis with the access token, e.g., in the scope value,
it MUST authenticate the RS, and the AS MUST determine the data an RS
is allowed to receive based on the RS's identity and suitable token
data, e.g., the scope value.
Implementers should be aware that a token introspection request lets
the AS know when the client (and potentially the user) is accessing
the RS, which is also an indication of when the user is using the
client. If this implication is not acceptable, implementers MUST use
other means to relay access token data, for example, by directly
transferring the data needed by the RS within the access token.
10. IANA Considerations
10.1. OAuth Dynamic Client Registration Metadata Registration
The following client metadata definitions have been registered in the
IANA "OAuth Dynamic Client Registration Metadata" registry
[IANA.OAuth.Parameters] established by [RFC7591]:
10.1.1. Registry Contents
Client Metadata Name: introspection_signed_response_alg
Client Metadata Description: String value indicating the client's
desired introspection response signing algorithm
Change Controller: IETF
Reference: Section 6 of RFC 9701
Client Metadata Name: introspection_encrypted_response_alg
Client Metadata Description: String value specifying the desired
introspection response content key encryption algorithm (alg
value)
Change Controller: IETF
Reference: Section 6 of RFC 9701
Client Metadata Name: introspection_encrypted_response_enc
Client Metadata Description: String value specifying the desired
introspection response content encryption algorithm (enc value)
Change Controller: IETF
Reference: Section 6 of RFC 9701
10.2. OAuth Authorization Server Metadata Registration
The following values have been registered in the IANA "OAuth
Authorization Server Metadata" registry [IANA.OAuth.Parameters]
established by [RFC8414].
10.2.1. Registry Contents
Metadata Name: introspection_signing_alg_values_supported
Metadata Description: JSON array containing a list of algorithms
supported by the authorization server for introspection response
signing
Change Controller: IETF
Reference: Section 7 of RFC 9701
Metadata Name: introspection_encryption_alg_values_supported
Metadata Description: JSON array containing a list of algorithms
supported by the authorization server for introspection response
content key encryption (alg value)
Change Controller: IETF
Reference: Section 7 of RFC 9701
Metadata Name: introspection_encryption_enc_values_supported
Metadata Description: JSON array containing a list of algorithms
supported by the authorization server for introspection response
content encryption (enc value)
Change Controller: IETF
Reference: Section 7 of RFC 9701
10.3. Media Type Registration
The "application/token-introspection+jwt" media type has been
registered in the "Media Types" registry [IANA.MediaTypes] in the
manner described in [RFC6838]. It can be used to indicate that the
content is a token introspection response in JWT format.
10.3.1. Registry Contents
Type name: application
Subtype name: token-introspection+jwt
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary. A token introspection response is
a JWT; JWT values are encoded as a series of base64url-encoded
values (with trailing '=' characters removed), some of which may
be the empty string, separated by period ('.') characters.
Security considerations: see Section 8 of RFC 9701
Interoperability considerations: N/A
Published specification: Section 4 of RFC 9701
Applications that use this media type: applications that produce and
consume OAuth Token Introspection Responses in JWT format
Fragment identifier considerations: N/A
Additional information:
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information:
Torsten Lodderstedt (torsten@lodderstedt.net)
Intended usage: COMMON
Restrictions on usage: none
Author: Torsten Lodderstedt (torsten@lodderstedt.net)
Change controller: IETF
Provisional registration? No
10.4. JWT Claim Registration
The "token_introspection" claim has been registered in the "JSON Web
Token (JWT)" registry [IANA.JWT] in the manner described in
[RFC7519].
10.4.1. Registry Contents
Claim Name: token_introspection
Claim Description: Token introspection response
Change Controller: IETF
Reference: Section 5 of RFC 9701
11. References
11.1. Normative References
[IANA.JWT] IANA, "JSON Web Token (JWT) Claims",
<https://www.iana.org/assignments/jwt>.
[IANA.MediaTypes]
IANA, "Media Types",
<http://www.iana.org/assignments/media-types>.
[IANA.OAuth.Token.Introspection]
IANA, "OAuth Token Introspection Response",
<https://www.iana.org/assignments/oauth-parameters>.
[OpenID.Registration]
Sakimura, N., Bradley, J., and M. Jones, "OpenID Connect
Dynamic Client Registration 1.0 incorporating errata set
1", November 2014, <https://openid.net/specs/openid-
connect-registration-1_0.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<https://www.rfc-editor.org/info/rfc7516>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7525, DOI 10.17487/RFC7525, May 2015,
<https://www.rfc-editor.org/info/rfc7525>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8725] Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
Current Practices", BCP 225, RFC 8725,
DOI 10.17487/RFC8725, February 2020,
<https://www.rfc-editor.org/info/rfc8725>.
[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[RFC9700] Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"OAuth 2.0 Security Best
"Best Current Practice", Practice for OAuth 2.0 Security", BCP 240,
RFC 9700, DOI 10.17487/RFC9700, November December 2024,
<https://www.rfc-editor.org/info/rfc9700>.
11.2. Informative References
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>.
Acknowledgements
We would like to thank Petteri Stenius, Neil Madden, Filip Skokan,
Tony Nadalin, Remco Schaar, Justin Richer, Takahiko Kawasaki,
Benjamin Kaduk, Robert Wilton, and Roman Danyliw for their valuable
feedback.
Authors' Addresses
Torsten Lodderstedt (editor)
yes.com AG
Email: torsten@lodderstedt.net
Vladimir Dzhuvinov
Connect2id Ltd.
Email: vladimir@connect2id.com