The Service Location Protocol, Version 2 [1] defines a number of
features which are extensible. This document clarifies exactly which
mechanisms can be used to that end (Sections 3-5) and which cannot
(Section 6). This document updates [1], specifying conventions that
ensure the protocol extension mechanisms in the SLPv2 specification
will not possibly have ambiguous interpretations.
This specification introduces only one new protocol element, the
Vendor Opaque Extension. This Extension makes it possible for a
vendor to extend SLP independently, once the vendor has registered
itself with IANA and obtained an Enterprise Number. This is useful
for vendor-specific applications.
Vendor extensions to standard protocols come at a cost.
- Vendor extensions occur without review from the community.
They may not make good engineering sense in the context of the
protocol they extend, and the engineers responsible may
discover this too late.
- Vendor extensions preclude interoperation with compliant but
non-extended implementations. There is a real danger of
incompatibility if different implementations support different
feature sets.
- By extending SLPv2 privately, ubiquitous automatic
configuration is impossible, which is the primary benefit of a
standard service discovery framework.
For these reasons, registration of service templates with IANA is
strongly encouraged! This process is easy and has proved to be rapid
(taking less than 2 weeks in most cases).
In this document, the key words "MAY", "MUST", "MUST NOT",
"optional", "recommended", "SHOULD", and "SHOULD NOT", are to be
interpreted as described in [2].
Service Location Protocol terminology is defined in [1]. IANA
registration terminology is defined in [5].
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RFC 3224 Vendor Extensions for Service January 2002
Enterprise Numbers are used to distinguish different vendors in IETF
protocols. Vendor Extensions to SLPv2 SHOULD use these values to
avoid any possibility of a name space collision. Each vendor is
responsible for ensuring that vendor extensions under their own
authority are non-conflicting.
IANA maintains a repository of all 'SMI Network Management Private
Enterprise Codes,' whose prefix is
iso.org.dod.internet.private.enterprise (1.3.6.1.4.1). The number
which follows is unique and may be registered by an on-line form [3].
The complete up-to-date list of Enterprise Numbers is maintained by
IANA [3].
Naming Authorities are defined by SLPv2 [1] as an agency or group
which catalogues Service Types and attributes.
A Service Type is a string representing a service which can be
discovered by SLPv2. Attributes may be associated with a particular
Service Type which is advertised by SLPv2.
Service Type strings and service attributes may be registered with
IANA by creating a Service Template [4]. The template is included in
an internet draft and an email message is sent to srvloc-
list@iana.org requesting that the template be included in the Service
Template registry. In this case, the naming authority for the
service type is IANA.
It is also possible for a Vendor to create their own naming
authority. In this case, any service type or attribute may be used.
SLPv2 allows arbitrary naming authorities to coexist. To use an
explicit naming authority, a vendor simply employs their Enterprise
Number as a naming authority. For example, for the following
(fictitious) Enterprise Number
9999 Acme, Inc. Erik Guttman femur@example.com
the Naming Authority string to use would be "9999". A service: URL
which used this Naming Authority to advertise a Roadrunner Detector
service could look like
service:roadrunner-detector.9999://example.com:9341
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RFC 3224 Vendor Extensions for Service January 2002
Service types which are defined under a naming authority based on an
Enterprise Number are guaranteed not to conflict with other service
type strings which mean something entirely different. That is also
true of attributes defined for service types defined under a naming
authority.
To create a safe naming authority with no possibility of name
collisions, a vendor SHOULD use their Enterprise Number as a naming
authority.
SLPv2 [1] suggests that
Non-standard attribute names SHOULD begin with "x-", because no
standard attribute name will ever have those initial characters.
It is possible that two non-standard attributes will conflict that
both use the "x-" prefix notation. For that reason, vendors SHOULD
use "x-" followed by their Enterprise Number followed by a "-" to
guarantee that the non-standard attribute name's interpretation is
not ambiguous.
For example, Acme, Inc.'s Enterprise Number is 9999. Say the Service
Template for NetHive (a fictitious game) was:
------------------------------------------------------------
template-type=NetHive
template-version=1.0
template-description=
The popular NetHive game.
template-url-syntax=
url-path = ; There is no path for a NetHive service URL.
features= string M O
# The list of optional features the NetHive server supports.
secure session, fast mode
current-users= string M
# The list of users currently playing
------------------------------------------------------------
Acme's server advertises a feature which is not on the list of
standard features, "x-9999-cheat-mode". Only an Acme client would
request this attribute to discover servers, since it is not standard.
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RFC 3224 Vendor Extensions for Service January 2002
SLPv2 [1] defines a protocol extensibility mechanism. SLPv2
Extensions are added at the end of a message and have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension ID | Next Extension Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset, contd.| Extension Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Extension Data depends on the Extension ID. Refer
to [4] for a full description of different mechanisms available for
registration of values with IANA.
SLPv2 may be extended in any of three ways.
(1) Anyone may request the designated expert for SLP to register a
new extension ID with IANA. Send requests to the
svrloc-list@iana.org.
It is recommended that an internet draft specifying this
extension be published, with the intention of publishing the
document as an Informational RFC. This way others can use the
extension as well. This is not a 'vendor extension' - rather
this is the preferred way of extending the protocol in a vendor
neutral manner.
If no specification is published and the extension is intended
for vendor specific use only - the 'Vendor Extension' option
below probably makes more sense than assigning an extension ID.
(2) An experimental extension may be done using the range 0x8000 to
0x8FFF. There is always the risk, however, that another vendor
will use the same ID, since these IDs are not registered.
(3) A Vendor Extension may be used. This extension allows a Vendor
to define their own extensions which are guaranteed to have a
unique interpretation. It is OPTIONAL to implement.
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RFC 3224 Vendor Extensions for Service January 2002
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension ID = 0x0003 | Next Extension Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset, contd.| Enterprise Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ent. #, contd.| Extension Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Enterprise Number is included in the Extension as a 4 byte
unsigned integer value. The Extension Data following is guaranteed
to have an unambiguous interpretation determined by the vendor.
The Acme Corporation, whose Enterprise Number is 9999, can define an
extension to SLP. In this example, Acme creates one such extension
to create an application level access control to service information.
This would allow replies to be sent only to clients who could
authenticate themselves.
The engineers at Acme give the Extension Data the following form:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ACME Ext ID = 1| Client ID Length | Client ID ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ACME Ext ID: The ACME engineers decided to define the first byte of
their extension data as an extension ID field. In the future, ACME
may decide to define more than this extension. Since there is 8 bits
in the ID field, ACME can define up to 256 different extensions. If
ACME were to omit this field and begin directly with their 'Extension
for UA Authentication', they would only be able to define one ACME
specific SLP extension. For the 'Extension for UA Authentication,'
the ACME Extension ID is set to 1. This ID has to be managed within
ACME, to make sure that each new extension they invent has a unique
ID assigned to it.
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RFC 3224 Vendor Extensions for Service January 2002
Client ID Length: This declares how many bytes of Client ID data
follow.
Client ID: The Acme application user ID.
Timestamp: # of seconds since January 1, 2000, 0:00 GMT.
Authenticator: a 16 byte MD5 digest [6] calculated on the following
data fields, concatenated together
- UA request bytes, including the header, but not any extensions.
- UA SECRET PASS PHRASE
- Acme UA Authentication Extension - Client ID
- Acme UA Authentication Extension - Timestamp
The SA or DA which receives this extension and supports this
extension will check if it (1) recognizes the Client ID, (2) has an
associated SECRET PASS PHRASE for it, (3) whether upon calculating an
MD5 digest over the same data as listed above it arrives at the same
Authenticator value as included in the extension. If all 3 of these
steps succeed, the UA has been authenticated.
Note this example is for explanatory purposes only. It would not
work well in practice. It requires a shared secret be configured in
SAs and DAs, for every UA. Furthermore, the UA secret pass phrase
would be susceptible to a dictionary attack.
Modification or extension of any feature of SLPv2 whatsoever, aside
from those listed in Sections 3-5 of this document, requires a
standards action as defined in [1].
Terminology and procedures for IETF Actions related to registration
of IDs with IANA are defined in [5]. Existing SLPv2 extensions
assignments are registered with IANA [3].
This document clarifies procedures described in other documents [1]
[4]. The Vendor Opaque Extension ID has already been registered [3].
No additional IANA action is required for publication of this
document.
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RFC 3224 Vendor Extensions for Service January 2002
Vendor extensions may introduce additional security considerations
into SLP.
This memo describes mechanisms which are standardized elsewhere [1]
[4]. The only protocol mechanism described in this document (see
Section 5 above) is no less secure than 'private use' extensions
defined in SLPv2 [1].
The example in Section 5.2 above shows how Vendor Opaque Extensions
can be used to include an access control mechanism to SLP so that SAs
can enforce an access control policy using an authentication
mechanism. This is merely an example and protocol details were
intentionally not provided. A vendor could, however, create a
mechanism similar to this one and provide additional security
services to SLPv2 in the manner indicated in the example.
Acknowledgements
I thank the IESG, for their usual persistence and attention to
detail.
References
[1] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC 2608, July 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] http://www.iana.org/numbers.html
[4] Guttman, E., Perkins, C. and J. Kempf, "Service Templates and
URLs", RFC 2609, July 1999.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[6] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
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RFC 3224 Vendor Extensions for Service January 2002
Author's Address
Erik Guttman
Sun Microsystems
Eichhoelzelstr. 7
74915 Waibstadt
Germany
Phone: +49 7263 911 701
Messages: +49 6221 356 202
EMail: erik.guttman@sun.com
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RFC 3224 Vendor Extensions for Service January 2002
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