Internet Engineering Task Force Erik Guttman
INTERNET DRAFT Charles Perkins
30 October 1997
Sun Microsystems
John Veizades
@Home Network
Michael Day
Intel
Service Location Protocol
draft-ietf-svrloc-protocol-v2-01.txt
Status of This Memo
This document is a submission by the Service Location Working Group
of the Internet Engineering Task Force (IETF). Comments should be
submitted to the srvloc@corp.home.net mailing list.
Distribution of this memo is unlimited.
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Abstract
The Service Location Protocol provides a scalable framework for
the discovery and selection of network services. Using this
protocol, computers using the Internet need little or no static
configuration of network services for network based applications.
This is especially important as computers become more portable, and
users less tolerant or able to fulfill the demands of network system
administration.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Terminology 2
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 3
2.2. Service Information and Predicate Representation . . . . 4
3. Protocol Overview 4
3.1. Protocol Operations . . . . . . . . . . . . . . . . . . . 5
3.2. Minimal SLP Implementation Requirements . . . . . . . . . 6
3.2.1. Minimal UA Requirements . . . . . . . . . . . . . 7
3.2.2. Minimal SA Requirements . . . . . . . . . . . . . 7
3.2.3. Peer-to-Peer Usage of SLP . . . . . . . . . . . . 8
3.3. Interactive Service Selection . . . . . . . . . . . . . . 8
3.4. Using SLP Directory Agents In Larger Environments . . . . 9
3.4.1. Directory Agents . . . . . . . . . . . . . . . . 9
3.4.2. Scopes . . . . . . . . . . . . . . . . . . . . . 10
3.4.3. Scaling Configuration Options . . . . . . . . . . 10
3.5. Introduction to Directory Agents . . . . . . . . . . . . 11
3.6. URLs used in the Service Location Protocol . . . . . . . 11
3.6.1. The ``service:'' URL scheme . . . . . . . . . . 12
3.7. Standard Attribute Definitions . . . . . . . . . . . . . 12
3.8. Naming Authority . . . . . . . . . . . . . . . . . . . . 13
3.9. Interpretation of Service Location Replies . . . . . . . 13
3.10. Transmission of SLP messages . . . . . . . . . . . . . . 13
3.10.1. Use of TCP . . . . . . . . . . . . . . . . . . . 14
3.10.2. Use of Multicast Addresses . . . . . . . . . . . 15
3.10.3. Multicast vs. Broadcast . . . . . . . . . . . . 15
4. Service Location General Message Format 16
4.1. Service Location Extension Options . . . . . . . . . . . 18
4.2. Retransmission and Transaction IDs (XIDs) . . . . . . . . 19
4.3. URL Entries . . . . . . . . . . . . . . . . . . . . . . . 20
4.4. Authentication Blocks . . . . . . . . . . . . . . . . . . 20
4.5. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 23
5. Service Location Protocol Requests 23
6. Service Request Message Format 24
6.1. Service Request Usage . . . . . . . . . . . . . . . . . . 25
6.2. Directory Agent Discovery Request . . . . . . . . . . . . 26
6.3. Explanation of Terms of Predicate Grammar . . . . . . . . 27
6.4. Service Request Predicates . . . . . . . . . . . . . . . 28
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6.5. String Matching for Requests . . . . . . . . . . . . . . 29
7. Service Reply Message Format 30
8. Service Type Request Message Format 31
9. Service Type Reply Message Format 32
10. Attribute Request Message Format
33
11. Attribute Reply Message Format
34
12. Directory Agent Advertisement Message Format
35
13. Service Registration Message Format
37
14. Service Acknowledgement Message Format
39
15. Service Deregister Message Format
41
16. Directory Agents
42
16.1. Finding Directory Agents . . . . . . . . . . . . . . . . 42
17. Scope Discovery and Use
43
17.1. Rules Governing Scopes . . . . . . . . . . . . . . . . . 44
17.1.1. Scope Strings in SLP Messages . . . . . . . . . . 45
17.1.2. Registration . . . . . . . . . . . . . . . . . . 46
17.1.3. Query Handling . . . . . . . . . . . . . . . . . 46
17.2. Protected Scopes . . . . . . . . . . . . . . . . . . . . 47
17.2.1. Protected Scope Rules . . . . . . . . . . . . . . 48
18. Language Internationalization Issues
49
18.1. Language Tags and Dialects . . . . . . . . . . . . . . . 49
18.2. Scope Strings are not Language Specific . . . . . . . . . 49
18.3. Declaring the language of registrations . . . . . . . . . 49
18.4. Translation of Attribute Strings . . . . . . . . . . . . 50
18.5. Declaring the language of a Request . . . . . . . . . . . 50
19. Substitution of Character Escape Sequences
51
19.1. Language-Independent Strings . . . . . . . . . . . . . . 51
20. String Formats used with Service Location Messages
53
20.1. Previous Responders' Address Specification . . . . . . . 53
20.1.1. Service Type String . . . . . . . . . . . . . . . 54
20.1.2. String List . . . . . . . . . . . . . . . . . . . 54
20.1.3. Select List . . . . . . . . . . . . . . . . . . . 54
20.2. Attribute Information . . . . . . . . . . . . . . . . . . 55
20.3. Address Specification in Service Location . . . . . . . . 55
20.4. Attribute Value encoding rules . . . . . . . . . . . . . 56
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21. Protocol Timing Rules
57
21.1. Active DA Discovery . . . . . . . . . . . . . . . . . . . 57
21.2. Passive DA Advertising . . . . . . . . . . . . . . . . . 57
21.3. Reliable Unicast to DAs . . . . . . . . . . . . . . . . . 58
21.4. Multicast/Convergence . . . . . . . . . . . . . . . . . . 58
22. Configurable Parameters and Default Values
58
22.1. Time Out Intervals . . . . . . . . . . . . . . . . . . . 59
22.2. Service Agent: Use Predefined Directory Agent(s) . . . . 61
23. Security Considerations
61
24. Protocol Requirements
62
24.1. Directory Agent Requirements . . . . . . . . . . . . . . 62
24.2. Service Agent Requirements . . . . . . . . . . . . . . . 63
24.3. User Agent Requirements . . . . . . . . . . . . . . . . . 63
24.4. Common Requirements for all SLP Agents . . . . . . . . . 63
25. Non-configurable Parameters
64
26. Acknowledgments
64
A. Version 2 Notes 65
B. SLP Certificates 70
C. Example of deploying SLP security using MD5 and RSA 72
D. Scaling and Deployment of the Service Location Protocol 72
E. Using SLP For Network and Systems Management 74
F. Full Copyright Statement 76
1. Introduction
Traditionally, users find services by using the name of a network
host (a human readable text string) which is an alias for a network
address. The Service Location Protocol eliminates the need for
a user to know the name of a network host supporting a service.
Rather, the user names the service and supplies a set of attributes
which describe the service. The Service Location Protocol allows the
user to bind this description to the network address of the service.
Service Location provides a dynamic configuration mechanism for
applications in local area networks. It has been designed to serve
enterprise networks with shared services. In its in its current
form, it may not scale for wide-area service discovery throughout the
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global Internet. Applications are modeled as clients that need to
find servers attached to the enterprise network at a possibly distant
location. For cases where there are many different clients and/or
services available, the protocol is adapted to make use of nearby
Directory Agents that offer a centralized repository for advertised
services.
2. Terminology
User Agent (UA)
A process working on the user's behalf to establish
contact with a useful service. The UA retrieves service
information from the Service Agents or Directory Agents.
Service Agent (SA)
A process working on the behalf of one or more services
to advertise service information.
Service Information
A collection of attributes and configuration information
associated with a single service. The SAs advertise
service information for a collection of service
instances.
Directory Agent (DA)
A process which collects information from SAs to provide
a single repository of service information in order to
centralize it for efficient access by UAs. There can
only be one DA present per given host.
Service Type
Each type of service has a unique Service Type string.
The Service Type defines a template, called a "service
scheme", including expected attributes, values and
protocol behavior.
IANA IANA stands for the Internet Assigned Numbers Authority.
Naming Authority
The agency or group which catalogues given Service Types
and Attributes. The default Naming Authority is IANA.
Keyword
A string describing a characteristic of a service.
Attribute
A (class, value-list) pair of strings describing a
characteristic of a service. The value string may be
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interpreted as a boolean, integer or opaque value if it
takes specific forms (see section 20.4).
Predicate
A boolean expression of attributes, relations and
logical operators. The predicate is used to find
services which satisfy particular requirements. See
section 6.3.
Alphanumeric
A character within the range 'a' to 'z', 'A' to 'Z', or
'0' to '9'.
Scope A collection or set of services that make up a logical
group. See section 17 and appendix D.
Site Network
All the hosts accessible within the Agent's multicast
radius, which defaults to a value appropriate for
reaching all hosts within a site (see section 22). If
the site does not support multicast, the agent's site
network is restricted to a single subnet.
URL A Universal Resource Locator - see [5].
Address Specification
This is the network layer protocol dependent mechanism
for specifying an Agent. This is part of a URL.
SLPv1 The version of Service Location Protocol specified in
RFC 2165 [19].
2.1. Notation Conventions
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 RFC 2119 [7].
Quoted Strings
Some strings are quoted in this document to indicate
they should be used literally. Single characters inside
apostrophes are included literally.
<> Values set off in this manner are fully described in
section 20. In general, all definitions of items in
messages are described in section 20 or immediately
following their first use.
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silently discard
The implementation discards the datagram without further
processing, and without indicating an error to the
sender. The implementation SHOULD provide the capability
of logging the error, including the contents of the
discarded datagram, and SHOULD record the event in a
statistics counter.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| A | B \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ C \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A is a 2 byte field. B is of *arbitrary length*, where the length
in bytes is typically indicated by A. C is of arbitrary length. A
string field in a SLP message is 'omitted' by setting the length of
the string to 0 and transmitting no character bytes.
Syntax for string based protocols will follow the conventions defined
for ABNF [9]. Terms in angular brackets are defined formally in
Section 20 or where they are introduced.
2.2. Service Information and Predicate Representation
Service information is represented in a text format. The goal is
that the format be human readable and transmissible via email. The
location of network services is encoded as a Universal Resource
Locator (URL) which is human readable. Strings used in the Service
Location Protocol are NOT null-terminated.
3. Protocol Overview
The Service Location Protocol (SLP) provides a flexible and scalable
framework for providing hosts with access to information about the
existence, location, and configuration of networked services.
SLP is a request-reply protocol; in a typical operation a User Agent
(UA) issues a request for service information and awaits one or more
replies containing the requested information. Depending on the
environment, replies will be sent to the UA by a Service Agent (SA),
a Directory Agent (DA), or by both.
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For smaller environments, SLP allows a simple peer-to-peer deployment
consisting only of UAs and SAs. For larger environments, SLP allows
the consolidation of service configuration data at one or more
Directory Agents (DAs).
DAs, in addition to consolidating service information, allow
information to be organized according to administrative, usage, or
type domains using "scopes." Section 3.4 contains information on
deploying SLP in larger environments using DAs and scopes.
SLP PDUs are normally transmitted in datagrams using UPD/IP. Requests
may be unicast, multicast, or broadcast. When a UA multicasts or
broadcasts a request, it will often recieve more than one reply.
Replies must be unicast. In cases where a SLP PDU is too large to
fit within a datagram, the UA, SA, or DA may unicast that PDU using
TCP.
SLP allows a host to "bootstrap" itself, beginning with no knowledge
of any services or SLP agents beyond its own UA. To bootstrap itself,
the host must multicast or broadcast its first request.
Hosts may also be preconfigured--statically or by using DHCP
options--to issue requests to specific SAs or DAs, hence avoiding
multicast or broadcast requests.
Certain conditions will influence the best strategy for deploying SLP
in specific environments. Centralizing service information using
DAs simplifies the process by which UAs obtain service information.
However, it may not be practical to centralize service-related
information that changes frequently. Also, specific environments may
have special policies regarding broadcasting or multicasting.
This document specifies a range of usage models for SLP, beginning
with a lightweight and simple minimal implementation for smaller or
constrained environments. SLP can be scaled upward from the minimal
implementation by deploying more richly featured UAs and SAs, and by
adding DAs.
3.1. Protocol Operations
The diagram below shows the objects that engage in the SLP and their
relationships with each other.
+--------------------+ we want this info: +-----------+
| Application |- - - - - - - - - - - - ->>| Service |
+--------------------+ +-----------+
^ |
/|\ |
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| |
| |
\|/ \|/
v v
+----------------+ +------------+
| User Agent(UA) | -------------------------->| Service |
+----------------+ | Agent (SA) |
^ +------------+
/|\ ^
| /|\
| |
| +-------------+ |
+------------------>| Directory |<----------+
| Agent (DA) |
+-------------+
^ ___________
/|\ / Many other\
+------------>| SA's |
\___________/
In the diagram above, the service contains information the
application requires in order to use the service. (If the
application knew this information, it could ask the service for it
directly; but it doesn't and it can't.)
The service publishes its location and configuration by providing
that information to the SA. When there is a DA present, the SA also
passes the service's configuration and location on to the DA.
The application obtains the service's location and configuration by
causing the UA to issue a service request to the SA. The UA awaits
the service reply from the SA; when it arrives the UA makes the
service information available to the application. The SLP includes
a predicate grammar that allows the application to form service
requests of varying specificity.
When one or more DAs are present, unless there is a very good reason
not to (see, for example, Appendix E) the UA will issue the service
request to a DA rather than to a SA. Information on using DAs is
contained in section 3.4.
3.2. Minimal SLP Implementation Requirements
This section lists the required characteristics of UAs and SAs,
which, together, comprise the absolute minimum functionality of a
compliant SLP implementation.
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Sections 3.3 and 3.4 discuss increasing degrees of greater SLP
functionality, and how a more functional and scalable implementation
of SLP can be deployed.
An implementation of SLP MUST, at a minimum, include a User Agent
(UA) or a Service Agent (SA). A host supporting SLP MAY host one
or more processes using UA and SA functions concurrently. An
implementation MAY support both UA and SA funtionality. In order to
deploy SLP, both UAs and SAs must be present on the network.
3.2.1. Minimal UA Requirements
A minimal UA implementation has the the following requirements:
- The UA must be able to send Service Requests (see Section 6) and
receive Service Replies (see Sections 7).
- The UA must be able to send Service Requests and receive DA
Advertisements, to discover DAs. The UA will do this initially
and periodically. (See Section 21.1).
- The UA must be able to handle replies which overflow a single
datagram (see Section 3.10.1).
- The UA must support a number of configurable parameters (see
Section 22). The UA will work fine without any additional
configuration, however.
- If the UA is configured to use a protected scope, it must
reject unauthenticated Service or Attribute Replies (see
Section 17.2.1).
For a complete list of UA characteristics see Section 24.3.
3.2.2. Minimal SA Requirements
A minimal SA implementation has the following requirements:
- The SA MUST perform both active and passive DA discovery (see
Section 21.1 and 21.2) unless it is specifically configured not
to perform DA Discovery (see Section 22.2).
- SAs must forward service registrations and deregistrations (see
Section 13 and 15).
- SAs must reply to Service Requests (see Section 6).
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- SAs must reply to UDP requests which overflow so that a UA can
retransmit using TCP (see Section 3.10.1).
- SAs must be configurable with a number of parameters (see
Section 22).
- If the SA is configured to work with protected scopes, it MUST
generate and include signatures in all Service Reply, Service
Register, and Service Deregister messages it sends. (See
Section 17.2.1).
For a complete list of SA characteristics, see Section 24.2.
3.2.3. Peer-to-Peer Usage of SLP
A peer-to-peer usage of SLP is possible by having stations host both
UA and SA functionality. Each station is therefore a peer, and peers
make requests and provide responses directly to each other. This
usage allows a lightweight implementation of SLP that works well in
small environments or on smaller broadcast networks.
DAs are not required under a peer-to-peer usage of SLP. Instead of
discovering centralized service information by issuing requests to
DAs, SLP peers can discover services by multicasting or broadcasting
service requests to other peers. Multicasting and broadcasting
require usage of the convergence algorithm described in Section 21.4.
3.3. Interactive Service Selection
By supporting the Service Type Request and the Attribute Request SLP
can enable interactive service discovery and Internationalization.
With interactive service selection, the user can discover all types
of services present on a network. He or she can select a service
type and discover all the attributes supported on the network by that
type of service. The user can then form a query for specific service
attributes; if such a query fails, the user can form a less specific
attribute query, and so on.
Internationalization requires service attributes to be stored in
multiple languages, thus making it possible for users from different
communities to share resources in a more natural manner. However,
internationalization of SLP service attributes requires support for
the Attribute Request.
For interactivity and internationalization, UAs SHOULD send Service
Type Requests and Attribute Requests by unicasting them over UDP (and
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TCP in case of overflow); and by multicasting or broadcasting (if
configured to do so) them over UDP.
SAs SHOULD respond to respond to Service Type Requests and Attribute
Requests by unicasting the appropriate responses over UDP (and TCP in
the case of overflow).
If the SA supports Service Type Requests or Attribute Requests,
when it receives an incorrect Service Type or Attribute Request,
it MUST return an error code to the requester. Error codes are in
Section 25.
3.4. Using SLP Directory Agents In Larger Environments
Through the deployment of DAs and the usage of scopes, SLP can scale
up to larger environments.
3.4.1. Directory Agents
DAs are consolidated stores of service location, configuration, and
attribute information. They exist to enhance the performance and
upward scalability of SLP. When DAs are present, SAs MUST register
all of their service information with the DAs.
When DAs are present, UAs SHOULD unicast requests directly to a DA
(when scoping rules allow), hence avoiding using the multicast and
convergence algorithm to obtain service information. This decreases
network utilization and increases the speed at which UAs can obtain
service information.
A single DA can support many UAs. Moreover, many DAs can coreside on
a network, which makes load balancing and redundancy possible. DAs
reduce the load on SAs, which makes simpler implementations of SAs
possible.
UAs can discover DAs using static configuration, DHCP options,
or by multicasting (or broadcasting) Service Requests using the
convergence algorithm in Section 21.4. n a large scale case, the
configuration of UAs might be through SRV records in the DNS or TXT
records pointing to a DA URL (see [16])
When DAs are deployed, are subject to the requirements listed in
Section 24.1.
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Scopes exist to make SLP easier to use and to administer in larger
environments, and require the presence of DAs. Scopes are a logical
mechanism that allows the grouping of services into different sets.
Services can be grouped according to organizational structure,
physical location, type, and so on. Some scopes can be protected
(subject to authentication of service information).
Scoped DAs accept registrations and requests that are within their
scope. For protected scopes, this means that the registrations and
requests are signed using the scope's Public Key. DAs that are not
scoped accept registrations and requests from any agent.
SAs and UAs must use scopes when they exist. UAs use scopes to focus
their requests to specific groups of services. SAs use scopes to
advertise (register) their service information as part of a specific
group.
Unscoped services are discoverable and usable by anyone.
In the special case of DA Discovery, requests may be sent to DAs with
scoping turned off (see Section 6.2).
UAs may issue requests to SAs with scoping turned off (see
Section 17.1.3). In this case, SAs will ignore scoping when replying
to the request.
All SLP agents can discover scopes by observing DA Advertisements, by
using DHCP options, or by static configuration.
Scoping rules are detailed in Section 17.1.
3.4.3. Scaling Configuration Options
By using specific configuration settings, SLP agents can work better
in larger environments.
The most effective mechanisms SLP offers for scaling up are DAs and
scopes. When DAs and scopes are present on the network, further
gains can be had by doing the following:
- Configuring UAs and SAs to have a predefined scope. These agents
can then perform DA discovery and make requests using their
scope. This will limit the number of replies.
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- Turning off DA discovery and using DHCP options to gain DA and
scoping information. This will limit the amount of bandwidth
consumed by DA discovery.
- Adjusting the TTL for multicast requests downward to limit
the scope of multicast requests. This focuses the multicast
convergence algorithm on smaller subnetworks, which decreases
the number of responses and increases the performance of service
location.
3.5. Introduction to Directory Agents
A DA acts on behalf of many SAs. It acquires information from them
and acts as a soft state cache. It is a single point of contact to
supply that information to UAs.
The queries that a UA multicasts to SAs (in an environment without a
DA) are the same as queries that the UA might unicast to a DA. A UA
may cache information about the presence of alternate DAs to use in
case a selected DA fails.
Aside from enhancing the scalability of the protocol (see Section D),
running multiple DAs provides robustness of operation. The DAs may
have replicated service information which remain accessible even when
one of the DAs fail. DAs, in the future, may use mechanisms outside
of this protocol to coordinate the maintenance of a distributed
database of Service Location information, and thus scale to larger
administrative domains.
Each SA MUST register with all DAs they are configured to use.
UAs may choose among DAs they are configured to use. UAs and
SAs determine which DAs to use based on scope rules described in
Section 17.1.
Locally, DA consistency is guaranteed using mechanisms in the
protocol. There isn't any DA to DA protocol yet. Rather, passive
detection of DAs by SAs ensures that eventually service information
will be registered consistently between equivalent DAs. Invalid
data will age out of the DA caches leaving only transient stale
registrations even in the case of a failure of a SA.
3.6. URLs used in the Service Location Protocol
The Service Location Protocol uses URLs to indicate the location of
services. URLs are used in a variety of Service Location Messages:
SAs send them to register and deregister service advertisements,
UAs obtain them in Service Replies and may send them in Attribute
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Requests. Any URL which conforms to standard URL syntax conventions
(see RFC 1738 [5]) may be used in these messages.
Service: URLs are useful for transmitting a service's location to a
client application. Other standard URL schemes may also be used for
this purpose.
3.6.1. The ``service:''
URL scheme
The service URL scheme is used specifically to communicate a Service
Location. Many Service Types will be named by including a standard
network service name after the ``service:'' scheme name.
The format of the information which follows the ``service:'' scheme
should as closely as possible follow the URL structure and semantics
as formalized by the IETF standardization process. See [12].
Well known Service Types are registered with the IANA and templates
are available as RFCs. Private Service Types may also be supported.
3.7. Standard Attribute Definitions
Service Types used with the Service Location Protocol must describe
the following:
Service Type string of the service
Attributes and Keywords
Attribute Descriptions and interpretations
Service Types not specified by documents maintained with IANA
will use their own Naming Authority string. The procedure for
standardizing new Service Types is defined in [12].
Services which advertise a particular Service Type must support the
complete set of standardized attributes. They may support additional
attributes, beyond the standardized set. Unrecognized attributes
MUST be ignored by UAs.
Service Type names which begin with "x-" are guaranteed not to
conflict with any officially registered Service Type names. It
is suggested that this prefix be used for experimental or private
Service Type names. Similarly, attribute names which begin with "x-"
are guaranteed not to be used for any officially registered attribute
names.
A service of a given Service Type should accept the networking
protocol if one is implied in its definition. If a Service Type
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can accept multiple protocols, configuration information SHOULD
be included in the Service Type attribute information. This
configuration information will enable an application to use the
results of a Service Request and Attribute Request to directly
connect to a service. The format of a Service Type string is
described in Section 20.1.1.
3.8. Naming Authority
The Naming Authority of a service defines the meaning of the
Service Types and attributes registered with and provided by Service
Location. The Naming Authority itself is a string which uniquely
identifies an organization. If no string is provided IANA is the
default.
Naming Authorities may define Service Types which are experimental,
proprietary or for private use. The procedure to use is to create
a 'unique' Naming Authority string and then specify the Standard
Attribute Definitions as described above. This Naming Authority
will accompany registration and queries, as described in Sections 6
and 13.
Service Types SHOULD be registered with IANA to allow for
Internet-wide interoperability.
3.9. Interpretation of Service Location Replies
Replies should be considered to be valid at the time of delivery.
The service may, however, fail or change between the time of the
reply and the moment an application seeks to make use of the service.
The application making use of Service Location MUST be prepared for
the possibility that the service information provided is either stale
or incomplete. In the case where the service information provided
does not allow a UA to connect to a service as desired, the Service
Request and/or Attribute Request may be resubmitted.
Service specific configuration information (such as which protocol
to use) should be included as attribute information in Service
Registrations.
3.10. Transmission of SLP messages
UAs MUST be able to issue requests to DAs using UDP and SAs using
multicast/convergence. UAs will use TCP in the case of overflow as
described below. UAs MAY be able to issue UDP requests to SAs, but
this is not normally necessary.
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SAs MUST be able to respond to UDP, TCP and multicast requests.
DAs MUST be able to respond to UDP and TCP requests, as well as
multicast DA Discovery SrvRqsts.
See Section 21 for timing and retransmission rules on how these
messages are exchanged.
The Service Location Protocol requires the implementation of UDP
(connectionless) and TCP (connection oriented) transport protocols.
The latter is used for bulk transfer only when necessary. The only
two reasons to use TCP are:
First, a registration or deregistration may be too large to fit into
a datagram. If no MTU information is available for the route, assume
that the MTU is 1400 which is enough to accommodate a IPv6 header and
UDP header in an ethernet frame. This value is configurable (see
Section 22). In this case a connection must be set up from a SA to a
DA. UAs may establish a TCP connection with a DA (not an SA) to send
requests which do not fit in a datagram to DAs.
Second, if the reply to a UA's request overflows a datagram, the
DA or SA truncates the reply to fit in one datagram and sets the
'overflow' bit in the SLP header. A UA which receives such a reply
MAY open a TCP connection with the DA or SA and retransmit the
request. It MAY also attempt to make use of the truncated reply or
reformulate a more restrictive request which will result in a smaller
reply.
DAs and SAs MUST respond to connection requests; SAs whose
registration data can overflow a datagram must be able to use TCP to
send the registration.
A TCP connection initiated by an Agent may be used for a single
transaction. It may also be used for multiple transactions. Since
there are length fields in the message headers, the Agents may send
multiple requests along a connection and read the return stream for
acknowledgments and replies.
The initiating agent is responsible for closing the TCP connection.
The DA should wait at least CONFIG_CLOSE_CONN seconds before closing
an idle connection. DAs and SAs SHOULD eventually close idle
connections to ensure robust operation, even when the agent which
opened a connection neglects to close it.
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SAs and UAs use ephemeral ports for transmitting information to the
service location port, which is 427.
3.10.2. Use of Multicast Addresses
SrvTypeRqst messages are sent to the Service Location General
Multicast Address. SrvRqst messages used for DA Discovery are sent
to the Directory Agent Discovery Multicast Address.
SAs must join multicast groups depending on which services they
advertise (called Service-Specific multicast addresses). They also
must join the Service Location General Multicast Address.
UAs send multicast SrvRqst or AttrRqst messages to the Service-
Specific multicast group corresponding to the service type of the
request.
Service-Specific Multicast addresses are computed by calculating a
string hash on the service type string. The service type string is
defined in Section 20.1.1. This string will always fall inside the
ASCII range of the UTF8 [21] encoding due to its definition.
The multicast group they join is determined by the string hash
function given below:
#define RANGE_SIZE 0x7f
/*
* SLPhash returns a hash value in the range 0-RANGE_SIZE for
* a string of single-byte characters, of specified length.
*/
unsigned long SLPhash (const char *pc, unsigned int length) {
unsigned long h = 0;
while (length-- != 0) {
h *= 33;
h += *pc++;
}
return (RANGE_SIZE & h); /* round to fit in range of addresses */
}
This value is added to the base range of Service Specific Discovery
Addresses, to be assigned by IANA. These will be 128 contiguous
multicast addresses from the administrative local multicast range.
3.10.3. Multicast vs.
Broadcast
The Service Location Protocol was designed for use in networks
where DHCP is available, or multicast is supported at the network
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layer. To support this protocol when only network layer broadcast is
supported, the following procedures may be followed.
In isolated networks, broadcasts will work in place of multicast.
SAs SHOULD and DAs MUST listen for broadcast Service Location request
messages to the Service Location port. This allows UAs which lack
multicast capabilities to still make use of Service Location on a
single subnet.
3.10.3.2. Multiple Subnets
In larger enterprises, a DA can be used to provide a central clearing
house of information for UAs. The DA address can be dynamically
configured with Agents using DHCP. The address can also be determined
by static configuration. Using multicast DA discovery in enterprises
with multiple subnets will require use of multicast discovery with
multiple hops (i.e., TTL > 1 in the IP header). Note that the
setting of the TTL in multicast packets sometimes must be interpreted
according to conventional scoping agreements rather than strictly as
the number of hops.
4. Service Location General Message Format
The following header is used in all of the message descriptions below
and is abbreviated by using "Service Location header =" followed by
the function being used.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Function | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|M|U|A|F|R|S| reserved | Language Tag Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next option, offset | XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Language Tag (ASCII string) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The header for SLP v2 has changed from SLP v1 in the following ways.
Dialect is no longer needed, as it is included in the Language Tag.
The Language Code field is now interpreted as the length of the
Language Tag which follows the header. Finally, since all characters
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will be encoded in UTF-8, the Char Encoding field of SLP v1 is no
longer necessary. This field is now used as an offset to the Next
Option.
Version This protocol document defines version 2 of the Service
Location protocol. A SLP implementation MAY support
version 1 [RFC2165]. If a SLP message indicates it
is sent using version 1 and this is not supported, a
PROTOCOL_V1_REJECTED error is returned.
Function Service Location datagrams can be identified as to their
operation by the function field. The following are the
defined operations:
Message Type Abbreviation Function Value
Service Request SrvRqst 1
Service Reply SrvRply 2
Service Registration SrvReg 3
Service Deregister SrvDeReg 4
Service Acknowledge SrvAck 5
Attribute Request AttrRqst 6
Attribute Reply AttrRply 7
DA Advertisement DAAdvert 8
Service Type Request SrvTypeRqst 9
Service Type Reply SrvTypeRply 10
Length The number of bytes in the message, including the Service
Location Header.
O The 'Overflow' bit. See Section 3.10 for the use of this
field.
M The 'Monolingual' bit. Requests with this bit set
indicate the User Agent will only accept responses in
the language (see Section 18) that is indicated by the
Service or Attribute Request.
U The 'URL Authentication Present' bit. See Sections 4.3,
4.4, 13, and 15 for the use of this field.
A The 'Attribute Authentication Present' bit. See
Sections 4.3, 4.4, and 11 for the use of this field.
F If the 'Fresh bit' is set by the SA when it makes
a Service Registration if it is to be considered
'fresh'. If this bit is not present, the registration is
considered to be an update.
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R The 'Multicast request' bit. This is set by the UA when
it multicasts or broadcasts a request.
S The 'Specific Scoping' bit. This is set by Service
Location Protocol entities wishing to exclude services
not assigned to any scope, or by services unwilling
to provide service to user agents not specifying the
particular scope.
reserved MUST be zero.
Language Tag Length
Strings within the remainder of the message which follows
are to be interpreted in the language specified (see
Section 18) in the bytes following the header. The
Language Tag is defined by RFC 1766 [2].
Next Option Offset
Options are added after the regular payload in the SLP
packet. If no options are present, then this field is
set to 0.
Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to
match replies to individual requests (see Section 4.2).
Note that, whenever there is an Attribute Authentication block, there
will also be a URL Authentication block. Thus, it is an error to
have the 'A' bit set without also having the 'U' bit set.
4.1. Service Location Extension Options
A service location extension option must be specified by a standards
track document. The option may be defined to accompany any or
all Service Location Messages. A conforming SLP implementation
MUST be able to ignore Service Location Extension Options it does
not recognize. It may be the case that future Options will be
defined and standardized which will become requirements of SLP
implementations. These documents will have to proceed as Standards
Track specifications. Otherwise, support of all extension options is
elective.
The format of a Service Location Extension Option is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Extension ID | Offset to next Option |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Extension Contents \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Option Extension ID is defined by a Standards document which also
defines the contents of the extension. The offset to next option
is 0 if there is no option following or is set to the length of the
Extension contents.
4.2. Retransmission and Transaction IDs (XIDs)
Retransmission is used by UAs and SAs to ensure reliable exchange of
unicast messages with DAs. If a UA or SA sends a message to a DA
fails to receive a response, the message will be sent again. The
message is retried up to 3 times
Multicast requests are retransmitted, but according to different
rules. See Section 21 for the details of the algorithm.
Replies received using the multicast convergence algorithm are
accumulated until convergence has been detected.
A list of previous responders is sent. This list will prevent those
in the list from responding, to be sure that responses from other
sources are not drowned out.
Retransmission of the same message should not contain an updated XID.
It is quite possible the original request reached the DA or SA, but
reply failed to reach the requester. Using the same XID allows the
DA or SA to cache its reply to the original request and then send it
again, should a duplicate request arrive. This cached information
should only be held very briefly (CONFIG_KEEP_RPLY is recommended.)
Any registration or deregistration at a DA, or change of service
information at a SA should flush this cache so that the information
returned to the client is always valid.
The requester creates the XID from an initial random seed and
increments it by one for each request it makes. The XIDs will
eventually wrap and continue incrementing from there. Requests are
never sent with an XID of 0.
An unsolicited DAAdvert has an XID of 0.
Requests all include XIDs which will match the XIDs of the replies.
The replies may be returned in any order. A UA or SA may have
multiple outstanding requests.
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4.3. URL Entries
When URLs are registered, they have lifetimes and lengths, and may
be authenticated. These values are associated with the URL for
the duration of the registration. The association is known as a
"URL-entry", and has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Lifetime |# of URL auths |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| URL length | URL (variable length) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| URL Auth. blocks (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|# of Attr auths| Attr Auth. blocks (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved MUST be zero. This field is reserved for longer
lifetimes used by a wide area service location
protocol.
Lifetime The length of time that the registration is valid, in
the absence of later registrations or deregistration.
Length of URL
The length of the URL, measured in bytes and < 32768.
URL Authentication Block
(if present) A timestamped authentication block
(Section 4.4)
If the 'U' bit is set in the message header, the URL is followed by
an URL Authentication Block. If the scheme used in the URL does not
have a standardized representation, the minimal requirement is:
service:<srvtype>://<addr-spec>
"service" is the URL scheme used for denoting a service access point,
(see [12] for the formal definition.) Other URLs besides service:
scheme URLs may be transmitted in Service Location Messages.
4.4. Authentication Blocks
Authentication blocks are used to authenticate service registrations
and deregistrations. URLs are registered along with an URL
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Authentication block to retain the authentication information in
the URL entry for subsequent use by UAs who receive a Service Reply
containing the URL entry. Service attributes are registered along
with an Attribute Authentication block. Both authentication blocks
have the format illustrated below.
If a service registration is accompanied by authentication which can
be validated by the DA, the DA MUST validate any subsequent service
deregistrations, so that unauthorized entities cannot invalidate
such registered services. Likewise, if a service registration
is accompanied by an Attribute Authentication block which can be
validated by the DA, the DA MUST validate any subsequent attribute
registrations, so that unauthorized entities cannot invalidate such
registered attributes.
To avoid replay attacks which use previously validated
deregistrations, the deregistration or attribute registration
message must contain a timestamp for use by the DA. To avoid replay
attacks which use previously validated registrations to nullify a
valid deregistration, registrations must also contain a timestamp.
A single Authentication Block is returned with an AttrRply and
SrvRply. A SrvReg may include multiple authentication blocks if
more the service is to be registered in more than one protected
scope, or if more than one cryptographic algorithm is supported by
the Service Location Protocol deployment. A DAAdvert will include
one Authentication Block per protected scope that the DA supports.
Unsolicited DAAdverts (see Section 12) will always be made in using
the default cryptographic algorithm (see below). DAAdverts sent as a
reply to a SrvRqst will include only one Authenticator Block.
An authentication block has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protected Scope String Length | Protected Scope String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Structure Descriptor | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Structured Authentication Block ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Timestamp A 64-bit value formatted as specified by the Network
Time Protocol (NTP) [15].
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Block Structure Descriptor (BSD)
A value describing the structure of the Authentication
Block. The only value currently defined is 1, for
Object-Identifier.
Length The length of the Authentication Block
Structured Authentication Block
An algorithm specification, and the authentication data
produced by the algorithm.
The Structured Authentication Block contains a digital signature
of the information being authenticated. It contains sufficient
information to determine the algorithm to be used and the keys to be
selected to verify the digital signature.
The digital signature is computed over the following ordered stream
of data:
LIFETIME (2 bytes in network byte order)
LENGTH OF URL (2 bytes in network byte order)
URL (n bytes)
TIMESTAMP (8 bytes in SNTP format [15])
LENGTH OF SCOPE STRING (2 bytes)
SCOPE STRING (n bytes)
When producing a URL Authentication block, the authentication
data produced by the algorithm identified within the Structured
Authentication Block calculated over the following ordered stream of
data:
LENGTH OF ATTRIBUTES (2 bytes in network byte order)
ATTRIBUTES (n bytes)
TIMESTAMP (8 bytes in SNTP format [15])
LENGTH OF SCOPE STRING (2 bytes)
SCOPE STRING (n bytes)
Every Service Location Protocol entity (UA, SA, or DA) which
is configured for use with protected scopes MUST implement
"md5WithRSAEncryption" [4] and be able to associate it with BSD value
== 1. A conforming SLP implementation MAY implement other digital
signature systems.
In the case where BSD == 1 and OID == "md5WithRSAEncryption" is
selected, the Structured Authentication Block will start with the
ASN.1 Distinguished Encoding (DER) [8] for "md5WithRSAEncryption",
which has the as its value the bytes (MSB first in hex):
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"30 0d 06 09 2a 86 48 86 f7 0d 01 01 04 05 00"
This is then immediately followed by an ASN.1 Distinguished Encoding
(as a "Bitstring") of the RSA encryption (using the Scope's private
key) of a bitstring consisting of the OID for "MD5" concatenated by
the MD5 [18] message digest computed over the fields above. The
exact construction of the MD5 OID and digest can be found in RFC
1423 [4].
4.5. URL Entry Lifetime
The Lifetime field is set to the number of seconds the reply can be
cached by any agent. A value of 0 means the information must not be
cached. UAs MAY cache service information, but if they do, they must
provide a way for applications to flush this cached information and
issue the request directly onto the network.
Services should be registered with DAs with a Lifetime, the suggested
value being CONFIG_LIFETIME. The service must be reregistered before
this interval elapses, or the service advertisement will no longer
be available. Thus, services which vanish and fail to deregister
eventually become automatically deregistered.
5. Service Location Protocol Requests
SLP includes SrvRqst, AttrRqst and SrvTypeRqst messages. All UAs
MUST be able to issue SrvRqst messages and SHOULD be able to issue
AttrRqst and SrvTypeRqst messages. SAs issue only SrvRqst messages,
and only for the purpose of DA discovery.
SAs MUST be able to respond to SrvRqsts and SHOULD be able to respond
to AttrRqsts and SrvTypeRqsts.
UAs MUST use scoped DAs in preference to unscoped DAs and any DA
in preference to multicasting to SAs. See Section 17.1 for rules
concerning the use of scopes.
Multicast and broadcast requests MUST set the 'R' bit in the header.
This will ease implementation of combined DA and SA services by
making it apparent whether an arriving datagram was unicast or
multicast. Multicast (or broadcast) service requests MUST be ignored
by DAs.
Replies to SLP requests include a 2 byte error code. If the error
code indicates failure the rest of the message SHOULD be omitted.
The error codes which may be returned are:
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0 Success
LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received
from a UA which is in a language for which there is no
registered Service Information which can satisfy it, or
the request arrived with the Monolingual bit set and no
registration is available in the specified language. See
Section 18. Note: SrvTypeRqst messages do not contain a
Language Tag, so they never elicit this error.
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvRply is received
which cannot be parsed or the declared string lengths
overrun the message.
SCOPE_NOT_SUPPORTED
A DA will return this error if it receives a request
which has a scope not supported by the DA. An SA will
not return this error; it will simply not reply to the
multicast request.
6. Service Request Message Format
The Service Request is used by UAs to obtain URLs from a DA or SAs.
The format of the Service Request is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string| Previous Responders Addr Spec \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <scope-list> | <scope-list> String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request <predicate> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responders Addr Spec> is described in Sections 8
and 20.1.
See Section 17.1 for the interpretation of the scope-list field.
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The predicate allows the UA to request the Service Type and specific
required attributes of a services in a specific language. The
minimal form of a Service Request Predicate is shown below:
"(service-type=" <srvtype> ["."<na>] ")"
Service Requests MAY include a scope term and a 'where clause'. The
syntax of the query language is described in Section 6.4. The form
of a Service Request is:
"(&(service-type=" <srvtype> ["."<na>] ")" <where> ")"
where:
- The <srvtype> refers to the Service Type. For each type
of service available, there is a unique Service type name
string. This term MUST be part of every Service Request. See
Section 20.1.1.
- The <na> is the Naming Authority. This term is OPTIONAL. See
Section 20.1.1.
- The <where> string contains a set of query terms which will
indicate those service instances which the User Agent is
interested in. This clause includes attributes, boolean
operators and relations. (See Section 6.3.)
In order for a request to succeed in matching registered information,
the following conditions must be met:
1. The result must have the same Service Type as the request.
2. It must have the same Naming Authority.
3. It must satisfy the scoping rules for requests (see
Section 17.1).
4. The conditions specified in the Where Clause must match the
attributes and keywords registered with the service.
6.1. Service Request Usage
The UA forms SrvRqsts using standard or conventionally known Service
Type attributes. It MAY also issue AttrRqsts to obtain the attribute
values for a Service Type before issuing SrvRqsts (see Section 11).
Having obtained the attributes which describe a particular kind
of service from an AttrRqsts, or using configured knowledge of a
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service's attributes, the UA can build a predicate that describes the
service needs of the user.
Suppose a printer supporting the lpr protocol is needed on the 12th
floor which has UNRESTRICTED_ACCESS and prints 12 pages per minute.
Suppose further that a Attribute Request indicates that there is a
printer on the 12th floor, a printer that prints 12 pages per minute,
and a printer that offers UNRESTRICTED_ACCESS. To check whether they
are same printer, issue the following request:
(& (SERVICE-TYPE=LPR)(PAGES PER MINUTE=12)
(UNRESTRICTED_ACCESS=*)
(LOCATION=12th FLOOR))
Suppose there is no such printer. The DA responds with a SrvRply
with 0 in the number of responses and no reply values. A SA silently
discards the request.
The UA might then try a less restrictive query to find a printer,
using only the 12th floor as "where" criteria.
(&(SERVICE-TYPE=LPR)(LOCATION=12th FLOOR))
In this case, there might be the reply:
Returned URL: service:lpr://igore.wco.ftp.com:515/draft
The Address Specification for the printer is: igore.wco.ftp.com:515,
containing the name of the host managing the requested printer.
Files would be printed by spooling to that port on that host. The
word 'draft' refers to the name of the print queue the lpr server
supports.
6.2. Directory Agent Discovery Request
Normally a SrvRqst returns a Service Reply. The sole exception to
this is a SrvRqst for the Service Type "directory-agent". This
SrvRqst is answered with a DAAdvert.
UAs and SAs which lack preconfigured or DHCP configured knowledge of
a DA MUST multicast a Service Request to the DA Discovery Multicast
Address. For the details of the multicast convergence algorithm see
Section 21. The predicate included in this request is:
(service-type=directory-agent)
The <scope-list> in the DA discovery request may include only the
string "*" in their <scope-list>. In this case, all DAs respond.
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If it includes any scope strings, only those DAs which support the
indicated scope reply, or those which support unscoped requests. A
DA discovery request which has the 'S' bit set in the message header
will not get responses from unscoped DAs.
Normally, a DA will not respond to multicast requests, nor will
it respond to requests which are improperly scoped. This request
is a special case, if a properly scoped DA Discovery request is
received from the DA Discovery multicast group a DA MUST respond. If
the request specifies a scope, the DA MUST NOT respond unless the
scope request matches its scope. If a DA has no scope, it will only
respond if there is no scope term or the request does not have the
'S' bit set.
DA Advertisement Replies may arrive from different sources, similar
in form to:
URL returned: service:directory-agent://slp-resolver.big.org
Scope returned: NONE (ie. the length field is set to 0 for the
scope string.)
URL returned: service:directory-agent://204.182.15.66
Scope returned: JANITORIAL SERVICES,ADMIN
URL returned: service:directory-agent://204.182.15.66
Scope returned: Legal Department ('S' bit set)
The first DA supports only unscoped advertisements. The second
supports advertisements which are unscoped, or are in the JANITORIAL
SERVICES or ADMIN scope. The DA in the last example supports
only the LEGAL DEPARTMENT scope, and explicitly NOT unscoped
advertisements.
The DA Advertisement format is defined in Section 12.
If the goal is merely to discover any DA, the first DA Advert which
is received will do. If the goal, however, is to discover all
reachable DAs, the multicast convergence algorithm must be used, see
Section 21.
6.3. Explanation of Terms of Predicate Grammar
A predicate has a simple structure, which depends on parentheses,
commas and slashes to delimit the elements. Examples of proper usage
are given throughout this document.
The predicate uses the same syntax as a LDAP search filter [13].
This means that a SLP service request could be handled by a LDAP
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server. The reverse is not true: SLP uses a greatly simplified
attribute typing system. Thus a SA or DA cannot interpret an
arbitrary LDAP query. There are some restrictions and assumptions
which are necessary in order to interpret SrvRqst predicates in the
context of SLP.
- A query for keywords uses a 'present' filter type. Thus, to
query for services including the keyword 'X', the filter would be
"X=*".
- Degenerate queries such as "(&" <filter> ")" and "(|" <filter>
")" must be tolerated. They are equivalent to <filter>.
- LDAPv2 defines several data types [14]. The only data types
which are supported in SLP are String (equivalent to a Case
Ignore String), Opaque (equivalent to a Case Exact String, since
the binary values is encoded as a radix64 value). Integer and
Boolean values are identically represented in SLP and LDAPv2.
6.4. Service Request Predicates
The Service Request Predicate follows the grammar of the LDAPv2
string search filter [13].
The Predicate MUST contain a simple term which defines a service type
for the query. It MAY contain a simple term which defines the scope
for the query (see Section 17.1.)
Restrictions which are not implied by the grammar are:
- LDAPv2 string text filters [13] and string based attribute
encodings [14] are all ASCII based. For UTF8 character encoding,
which SLP supports, either the LDAPv3 filters (and UTF8) must
be used or the <a> production in RFC 1778, Section 2, must be
expanded to include the entire range of allowed alphanumerics
excluding those in the <p> (protected) rule.
- <approx> filters, ``=~'', are interpreted as <equal> filters
``=''. For strings the <approx> filter MAY be implemented
so that it returns true for the service with the 'greatest
possible' match of the string sequence. For example: (x=~ABCD)
applied to services S1 with x=AAAA, S2 with x=ABBBBB and S3 with
x=ABCCCC would return S3 since it matches the longest sequence of
characters in the request.
- <starval> terms are always interpreted as string values and may
only be used with <equal> filters, with the two exceptions
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- <greater> filters applied to ``*'' as the value will return true
only for the item whose value is the MAXIMUM for the attribute.
This only applies to an Integer attribute. Otherwise, it returns
a PROTOCOL_PARSE_ERROR.
- <less> filters applied to ``*'' as the value will return
true only for the item(s) whose value is the MINIMUM for
the attribute. This only applies to an Integer attribute.
Otherwise, it returns a PROTOTOL_PARSE_ERROR.
The following are examples of query predicates in Service Requests.
When the 'S' is present, it means that the 'S' bit is set in
the message header. See Section 17.1 for how details on how the
<scope-list> works.
"(service-type=http)" <scope-list>= none
This is a minimal request string. It matches all
http services.
"(service-type=lpr)" <scope-list>="SALES"
This request is for all lpr services either
universally available or available in the SALES
scope.
"(service-type=lpr)" <scope-list>="SALES", 'S'
This request is for all lpr services which have
been explicitly configured to reside within in
the SALES scope.
"(&(service-type=pop3)(user=wump))" <scope-list>="ADMIN"
This is a request for all pop3 services available
in the ADMIN scope (or are unscoped) and which
serve mail to the user 'wump'.
"(&(service-type=backup)(qlength<=*))" <scope-list>="BLDG 32", 'S'
This returns the backup service which has
the shortest queue-length. (This assumes that
the queue length is an integer based attribute).
It will return this only for services registered
with the BLDG 32 scope (not unscoped services.)
6.5. String Matching for Requests
All strings are case insensitive, with respect to string matching
on queries. All preceding or trailing blanks which are not escaped
should not be considered for a match. White space (SPACE, CR, LF,
TAB) internal to a string value is folded to a single SPACE character
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for the sake of string comparisons.
For example, " Some String " matches "SOME STRING".
String comparisons (using comparison operators such as '<' or
'>=') are done using lexical ordering in the character set of the
registration, not using any language specific rules. The ordering
is strictly by the character value, i.e. "0" < "A" is true when the
character set is US-ASCII, since "0" has the value of 48 and "A" has
the value 65.
The special character '*' may precede, follow or be internal to a
string value in order to indicate substring matching. The query
including this character matches any character sequence which
conforms to the letters which are not wildcarded.
Examples:
"bob*" matches "bob", "bobcat", and "bob and sue"
"*bob" matches "bob", "bigbob", and "sue and bob"
"*bob*" matches "bob", "bobcat", "bigbob", and "a bob I know"
"b*b" matches "bob" and "big dreams no grub"
String matching is done after escape sequences have been substituted.
See Sections 18, 6.3, 19.
7. Service Reply Message Format
The format of the Service Reply Message is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | URL Entry count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry 1> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry 2> ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ . . . \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry N> ... \
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each Service Reply message is is sent in response to a Service
Request message. The XID field in the Service Reply is copied from
the XID field in the Service Request message.
A successful Service Reply message is composed of a list of URL
Entries, and an error code indicating the successful status of the
request. If an error is encountered during the processing of the
Service Request, the Error Code is set appropriately, according to
the available errors in 25. If the error code indicates failure the
rest of the message SHOULD be omitted. In addition to the errors
listed in section 5, the EXPECTED_ATTRIBUTE_MISSING error may be
returned. A DA or SA with access to the scheme definition for a
service: URL [12] MAY return this error (along with the matching
URLs) if the Service Request query does NOT contain a value for an
"expected" attribute (i.e., an attribute marked with the 'X' flag in
the scheme template). A SA will not return this error upon receiving
a multicast request; it will silently discard the multicast request
instead.
Each <URL Entry> in the list has the form defined in Section 4.3.
If the presence of an URL Authentication block is signaled by the
'U' bit, the length of the authentication block is determined by
information within the block as discussed in Section 4.4. The URL
Authentication block will include the authentication block calculated
using the algorithm specified in the SrvRqst if possible. If not,
the Authentication block calculated using the default algorithm is
supplied.
A UA MAY use the authentication block to determine whether the SA
advertising the URL is, in fact, authorized to offer the indicated
service.
If, in a list of URL entries, some of the URLs indicate services
which are in protected scopes (see Section 17.2) while other URLs in
the list indicate services which are not in protected scopes, the
latter must still have Authentication Blocks, but the length of the
authentication block is shown as zero, and no authentication need be
done.
8. Service Type Request Message Format
The Service Type Request is used to determine all the types of
services supported on a network.
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The format of a Service Type Request is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of prev resp string |<Previous Responders Addr Spec>\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of naming authority | <Naming Authority String> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <scope-list> | <scope-list> String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responders Addr Spec> is a comma delimited list. See
Section 20.1.
The Naming Authority, if included, will limit the replies to Service
Type Requests to Service Types which have the specified Naming
Authority. If this field is omitted (i.e., the length field is
zero), the default Naming Authority ("IANA") is assumed. If the
length field is -1, service types from all naming authorities are
requested.
See Section 17.1 for the interpretation of the scope-list field.
9. Service Type Reply Message Format
The Service Type Reply has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvTypeRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | number of service types |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Service Type String | <Service Type String-1> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ . . . \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Service Type String | <Service Type String-N> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The service type's name is provided in the <Service Type String>. If
the service type has a naming authority other than "IANA" it MUST be
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returned following the service type string and a "." character. See
Section 20.1.1 for the formal definition of this field.
The following are examples of Service Type Strings which might be
found in Service Type Replies:
service:lpr:
service:http:
nfs:
10. Attribute Request Message Format
The Attribute Request is used to obtain attribute information. The
UA supplies a request and the appropriate attribute information is
returned.
If the UA supplies only a Service Type, then the reply includes
all attributes and all values for that Service Type. The reply
includes only those attributes for which services exist and are
advertised by the DA or SA which received the Attribute Request.
Since different instances of a given service can, and very likely
will, have different values for the attributes defined by the Service
Type, the UA must form a union of all attributes returned by all
service Agents. The Attribute information will be used to form
Service Requests.
If the UA supplies a URL, the reply will contain service information
corresponding to that URL.
Attribute Requests MAY include a 'select clause'. This limits the
amount of information returned. If the select clause is empty,
all information is returned. Otherwise, the UA supplies a comma
delimited list of attribute tags and keywords. If the attribute
or keyword is defined for a service, it will be returned in the
Attribute Reply, along with all registered values for that attribute.
If the attribute selected has not been registered for that URL or
Service Type, the attribute or keyword information is simply not
returned.
The Attribute Request message has 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRqst) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| length of URL | URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <scope-list> | <scope-list> string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of <Select-List> string | <Select-List> string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responder Address List> functions exactly as introduced
in Section 8. See also Section 20.1.
The URL field can take two forms: Either it is simply a Service
Type (see Section 20.1.1), such as "service:http:" or "nfs:". All
attributes and the full range of values for each attribute of all
services of the given Service Type is returned.
The URL field may also be a full URL, such as
"service:lpr://igore.wco.ftp.com:515/draft" or "ftp://max.net/znoo".
In this, only the attributes for the service of the specified URL is
defined are returned.
See Section 17.1 for the interpretation of the scope-list field.
The select list takes the form of a <Select-List>, see 20. The
items on the list are attribute tags or keywords, which can be either
complete tags or include '*' characters for wildcard string matching.
An example of a select-list following the printer example is:
"PAGES PER MINUTE,UNRESTRICTED_ACCESS,LOCATION"
11. Attribute Reply Message Format
An Attribute Reply Message takes the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = AttrRply) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | length of <attr-list> string \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ <attr-list> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ Attribute Authentication Block (if any) \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The <attr-list> (attribute list) has the same form as the attribute
list in a Service Registration, see Section 20.2 for a formal
definition of this field.
If the AttrRqst included a Service Type in the URL field, all
attribute results must be coallated. For example, a SA receives
an AttrRqst and there are three services of the specified type
(in the requested scope, etc.). The first service supports the
attribute "(A=1,2)", the second service, "(A=3,4)" and the third,
"(A=5,6)". The SA returns the attribute "A" in the AttrRply as
"(A=5,2,3,1,4,6)". (Note that the SA does not need to sort the
return values in any way.)
An Attribute Request for "lpr:" might elicit the following reply
(UNRESTRICTED_ACCESS is a keyword):
(PAPER COLOR=WHITE,BLUE),
(PAPER SIZE=LEGAL,LETTER,ENVELOPE,TRACTOR FEED),
UNRESTRICTED_ACCESS,
(PAGES PER MINUTE=1,3,12),
(LOCATION=12th, NEAR ARUNA'S OFFICE),
(QUEUES=LEGAL,LETTER,ENVELOPE,LETTER HEAD)
If the message header has the 'A' bit set, the Attribute Reply will
have an Attribute Authentication block set. In this case, the
Attribute Authentication Block must be returned with the entire list
of attributes, exactly as it was registered by an SA in a protected
scope. In this case, the URL was registered in a protected scope
and the UA included a URL but not a select clause. If the AttrRqst
specifies that only certain attributes are to be returned, the DA
does not (typically cannot) compute a new Authentication so it simply
returns the attributes without an authentication block.
The SA or DA returns the Attribute Authentication block. A UA
which wishes to obtain authenticated attributes for a service in a
protected scope MUST therefore must include a particular URL and no
select list with the AttrRqst.
12. Directory Agent Advertisement Message Format
DA Advertisement Messages 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = DAAdvert) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | DAAdvert Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of URL | URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of <scope-list> | <scope-list> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Auth Blocks | authentication block 1 ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ autentication block 1 (continued) ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ autentication block N (continued) ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code is set when a DA Advertisement is returned as the
result of a Service Request, as specified in Section 5. The Error
Code will always be set to 0 in the case of an unsolicited DA
Advertisement.
The DAAdvert Sequence Number is 0 when the DA comes up initially
and increases by one each time the DAAdvert is sent unsolicited.
DAs which store service advertisements in a nonvolatile store will
set their initial sequence number to 256. The sequence number will
be used by SAs to determine if they must reregister services when
a DA is discovered. See Section 16.1 which describes unsolicited
DAAdverts and how SAs respond to them.
The URL corresponds to the DA's location.
There MUST be at least one authentication block for each protected
scope. There MAY be more than one authentication block for each
protected scope if more than one authentication algorithm (identified
by the BSD field) is used.
See Section 17.1 for the definition of the <scope-list>.
If the 'U' bit in the SLP header is set, an authentication block
is included to allow the SA and UA to ascertain whether the DA
Advertisement is valid. See 4.4. See Section 6.4 for the lexical
rules regarding <Scope>.
DA Advertisements sent in reply to a Directory Agent Discovery
Request has the same format as the unsolicited DA Advertisement, for
example:
URL: service:directory-agent://SLP-RESOLVER.CATCH22.COM
SCOPE List: ADMIN
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The DA can be reached at the Address Specification returned, and
supports the SCOPE called "ADMIN".
13. Service Registration Message Format
After a SA has found a DA, due to either active DA Discovery (see
Section 21.1) or passive DA Discovery (see Section 21.2) it begins
to register its advertised services one at a time. A SA must wait
for some random time uniformly distributed within the range specified
by CONFIG_REG_ACTIVE seconds before registering again. Registration
is done using the Service Registration message specifying all
attributes for a service. If the service registration in a protected
scope 17.2, then the service MUST include both a URL Authentication
block and an Attribute Authentication block (see Section 4.4). In
that case, the service agent MUST set both the 'U' bit and the 'A'
bit (see Section 4).
A DA must acknowledge each service registration request. If
authentication blocks are included, the DA MUST verify the
authentication before registering the service. This requires
obtaining key information, either by preconfiguration, maintenance
of a security association with the service agent, or acquiring the
appropriate certificate.
The format of a Service Registration is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ <URL-Entry> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of <scope-list> | <scope-list> String \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attr List String | <attr-list> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ (if present) Attribute Authentication Block ... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <URL-Entry> is defined at the end of Section 4.3. The
<attr-list> is defined in Section 20.2. The Attribute Authentication
Block, which is only present if the 'A' bit is set in the message
header, is defined in Section 4.4.
The <scope-list> is defined in Section 17.1.
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Service registration may use a connectionless protocol (e.g. UDP),
or a connection oriented protocol (e.g. TCP). If the registration
operation may contain more information than can be sent in one
datagram, the SA MUST use a connection oriented protocol to register
itself with the DA.
When a SA registers the same attribute class more than once for a
service instance, the DA overwrites the all the values associated
with that attribute class for that service instance. Separate
registrations must be made for each language that the service is to
be advertised in.
If a SA attempts to register a service with a DA and the registration
is larger than the site path MTU, then the DA will reply with a
SrvAck, with the error set to INVALID_REGISTRATION and the 'Overflow'
byte set.
An example of Service Registration information is:
Lifetime (seconds): 16-bit unsigned integer
URL (at least): service:<srvtype>://<addr-spec>
scope-list: omitted (ie. no items, 0 bytes)
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)
In order to offer continuously advertised services, SAs should start
the reregistration process before the Lifetime they used in the
registration expires.
An example of a service registration (valid for 3 hours) is as
follows:
Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
scope-list: DEVELOPMENT
Attributes: (PAPER COLOR=WHITE),
(PAPER SIZE=LETTER),
UNRESTRICTED_ACCESS,
(LANGUAGE=POSTSCRIPT, HPGCL),
(LOCATION=12 FLOOR)
The same registration could be done again, as shown below, in German;
however, note that "lpr" and "service" are reserved terms and will
remain in the language they were originally registered (English).
Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
scope-list: ENTWICKLUNG
Attributes: (PAPIERFARBE=WEISS),
(PAPIERFORMAT=BRIEF),
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UNBEGRENTZTER_ZUGANG,
(DRUECKERSPRACHE=POSTSCRIPT,HPGCL),
(STANDORT=11 ETAGE)
Scoped registrations must contain the SCOPE attribute. Unscoped
registrations must be registered with all unscoped DAs.
Registrations of a previously registered service are considered
an update. If such an attribute registration is performed in a
protected scope (see Section 17.2), a new Attribute Authentication
block must also be included, and the 'A' bit set in the registration
message header.
The new registration's attributes replace the previous
registration's, but do not effect attributes which were
included previously and are not present in the update.
For example, suppose service:x://a.org has been registered
with attributes A=1, B=2, C=3. If a new registration comes for
service:x://a.org with attributes C=30, D=40, then the attributes for
the service after the update are A=1, B=2, C=30, D=40.
In the example above, the SCOPE is set to DEVELOPMENT (in English)
and ENTWICKLUNG (in German). Recall that all strings in a message
must be in one language, which is specified in the header. The
string SCOPE is *not* translated, as it is one of the reserved
strings in the Service Location Protocol (see Section 19.1.)
The DA may return a server error in the acknowledgment. This error
is carried in the Error Codes field of the service location message
header. The SrvReg may result in the error codes described in
Section 14.
There are various rules concerning scopes: Which DAs a SA MUST
register with and which DAs accept which registrations. See
Section 17.1.
When the URL entry accompanying a registration also contains an
authentication block (Section 4.4), the DA MUST perform the indicated
authentication, and subsequently indicate the results in the Service
Acknowledgement message.
14. Service Acknowledgement Message Format
A Service Acknowledgement is sent as the result of a DA receiving
and processing a Service Registration or Service Deregistration. An
acknowledgment indicating success must have the error code set to
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zero. Once a DA acknowledges a service registration it makes the
information available to clients.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvAck) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A DA returns this error when the SrvReg or SrvDeReg is
received which cannot be parsed or the declared string
lengths overrun the message.
INVALID_REGISTRATION
A DA returns this error when a SrvReg or SrvDeReg is
invalid. For instance, an invalid URL, unknown or
malformed attributes, or deregistering an unregistered
service all cause this error to be reported.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvRqst which is set to have a
scope which it does not support.
AUTHENTICATION_ABSENT
If DA has been configured to require an authentication
for any service registered in the requested scope, and
there are no authentication blocks in the registration,
the DA will return this error.
AUTHENTICATION_FAILED
If the registration contains an authentication block
which fails to match the correct result as calculated
(see Section 4.4) over the URL or attribute data to be
authenticated, the DA will return this error.
INTERNAL_ERROR
A DA will return this if it cannot satisfy a request due
to an internal failure. This might occur, for example,
if the DA could not allocate sufficient resources. The
SA MUST assume its request was not satisfied.
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If the DA accepts a Service Registration, and already has an
existing entry, it updates the existing entry with the new lifetime
information and possibly new attributes and new attribute values.
Otherwise, if the registration is acceptable (including all necessary
authentication checks) the DA creates a new entry, and sets the 'F'
bit in the Service Acknowledgement returned to the SA.
15. Service Deregister Message Format
When a service is no longer available for use, the SA must deregister
itself from DAs that it has been registered with. A service uses the
following PDU to deregister itself from all scopes the service was
registered in.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvDeReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ (if present) authentication block ..... \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <tag spec> string | <tag spec> \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SA should retry this operation if there is no response from the
DA. The DA acknowledges this operation with a Service Acknowledgment
message. Once the Service Agent receives an acknowledgment
indicating success, it can assume that the service is no longer
advertised by the DA. The Error Code in the Acknowledgment of the
Service Deregistration may have the same values as described in
Section 14.
The Service Deregister Information sent to the directory agent has
the following form:
service:<srvtype>://<addr-spec>
Attribute tags (if any): ATTR1,KEYWORD,ATTR2
This will deregister the specified attributes from the service
information from the directory agent. If the Language Tag is omitted
or no attribute tags are included, the entire service information is
deregistered in every language and every scope it was registered in.
To deregister the printer from the preceding example, use:
service:lpr://igore.wco.ftp.com:515/draft
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If the service was originally registered with a URL entry containing
a URL authentication block, then the Service Deregistration
message header MUST have the 'U' bit set, and the URL entry is then
followed by the authentication block, with the authentication block
calculated over the URL data, the timestamp, and the length of the
authentication as explained in Section 4.4. In this calculation, the
lifetime of the URL data is considered to be zero, no matter what the
current value for the remaining lifetime of the registered URL.
16. Directory Agents
16.1. Finding Directory Agents
A UA or SA may be statically configured to use a particular DA. This
is discouraged unless the application resides on a network where any
form of multicast or broadcast is impossible.
Alternatively, a host which uses DHCP [11, 1] may use it to obtain
a DA's address. DHCP options 78 and 79 have been assigned for this
purpose [17]. SLP Agents SHOULD be configurable using DHCP.
The third way to discover DAs is dynamically. This is done by
sending out a Directory Agent Discovery request (see Section 6.2).
Lastly, the agent may be informed passively as follows:
When a DA first comes on-line, and periodically, it sends an
unsolicited DA Advertisement to the Service Location general
multicast address. See Section 21 for details. If a DA supports a
particular scope or set of scopes these are placed in its DAAdvert.
When a SA or UA first comes on-line it must issue a Directory Agent
Discovery Request unless it is using static or DHCP configuration, as
described in 6.2.
A SA registers information with ALL newly discovered DAs when either
of the above two events take place.
Once a UA becomes aware of a DA it will unicast its queries there.
In the event that more than one DA is detected, it will select one to
communicate with.
The protocol will cause all DAs (of the same scope) to eventually
obtain consistent information. Thus one DA should be as good as any
other for obtaining service information. There may be temporary
inconsistencies between DAs.
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17. Scope Discovery and Use
The scope mechanism in the Service Location Protocol enhances its
scalability. The primary use of scopes is to provide the capability
to organize a site network along administrative lines. A set of
services can be assigned to a given department of an organization,
to a certain building or geographical area or for a certain purpose.
The users in a department can be configured to request services from
a particular scope. A scope is not an attribute of the service
instance, because it is completely independent of the characteristics
of the service instance.
Services in a scope effectively offer their service only to users
that indicate that particular scope in their user requests. Just
as sets can be non-disjoint, services can belong to several scopes.
Users, on the other hand, are not viewed as belonging to particular
scopes, but instead as having access to services in one or more
scopes. This access can be acquired in several ways, as further
detailed below.
Services that do not belong to any scope effectively offer their
service to any user agent. Since a user agent selects its desired
service by specifying the desired attributes of that service, any
service satisfying those attributes will satisfy the needs of the
user agent. Thus, a user agent typically does not care about which
service agent provides the needed service. Cases in which this is
not true require the user agent to specify that only services from
the specific scope are desired, but such cases always arise from
administrative considerations, and not from the specific values
assigned to service attributes. The 'S' bit is supplied by user
agents requiring the exclusion of services not assigned to the
specific requested scope or scopes.
A site network that has grown beyond a size that can be reasonably
serviced by a few DAs can use the scope mechanism. DAs may be
configured with a scope list, representing the administrative
categories which they serve. The semantics and language of the
strings used to describe the scope are almost entirely the choice of
the administrative entity of the particular domain in which these
scopes exist. The values of SCOPE should be configurable, so the
system administration Block can set its value. The scopes "LOCAL"
and "REMOTE" are reserved and SHOULD NOT be used. Use of these
reserved values is to be defined in a future protocol document.
DAs advertise their available scopes, together with a
"service:directory-agent:" URL indicating its location.
These advertisements are sent periodically starting as soon as the
DA first becomes available. The UA and SA may also solicit these DA
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advertisements when they first come on line. Thus, UAs and SAs are
consistently informed of available scopes provided by scoped DAs.
UAs may select or be configured with one or more scopes to use. This
selection may be automated through the use of DHCP or it may be done
by the individual client application or service programmatically.
UAs send all requests to DAs which support the scope or scopes they
select. A SA is configured with a scope (or scopes) in which to
register. It MUST register with all DAs in that scope that it can
discover. Failure to be comprehensive in registration according
to this rule may mean that the service advertisement may not be
available to all UAs.
17.1. Rules Governing Scopes
For the sake of illustration, imagine that a service is a candy M&M,
and that scopes divide the candy up into sets all containing the same
color. Users want candy, but some users are more particular than
others, and some users are constrained by their administration to
only eat candy of a particular color.
Rules for User Agents, for various combinations of settings of the
'S' bit, and contents of the <scope-list>:
<'S'=on, scope-list=""> I want colorless M&Ms.
<'S'=on, scope-list="green"> I want green M&Ms.
<'S'=off, scope-list="green"> I want green M&Ms, or else
colorless M&Ms.
<'S'=off, scope-list=""> I want colorless M&Ms.
Rules for Service Agents, for various combinations of settings of the
'S' bit, and contents of the <scope-list>:
<'S'=on, scope-list=""> I am a colorless M&M.
<'S'=on, scope-list="green"> I am a green M&M, and always appear
colorful.
<'S'=off, scope-list="green"> I am a green M&M, but I'm happy to
appear colorless.
<'S'=off, scope-list=""> I am a colorless M&M.
Rules for Directory Agents, for various combinations of settings of
the 'S' bit, and contents of the <scope-list>:
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<'S'=on, scope-list=""> I am a jar for colorless M&Ms.
<'S'=on, scope-list="green"> I am a jar for green M&Ms only.
<'S'=off, scope-list="green"> I am a jar for green M&Ms, but I'm
happy to accept colorless.
<'S'=off, scope-list=""> I am a jar for colorless M&Ms.
The intention motivating this algorithm for satisfying scoped service
requests is to allow a smooth transition between administration of
unscoped service domains into scoped service domains. Notice that if
a DA is installed and configured to offer scoped service, that user
agents sending requests to that DA will typically be able to find
the existing unscoped services until the services are configured for
scope membership. This should enable system administrators to become
more familiar with the scope model without the need for "flag days"
or discontinuous changeovers of services.
If it is intended that all user agents be configured to request
services from particular scopes, then while the user agents receive
the necessary scope configuration information they will work as
before in the unscoped administration until the time finally arrives
when all user agents are configured with scopes. After that time,
all service agents can be configured to register their services with
the 'S' bit set, and the user agents will all continue to work as
before.
17.1.1. Scope Strings in SLP Messages
Scope strings are found in several service location messages. The
<scope-list> takes the form of a <String -List > (see Section 20) of
scope names. The only characters which are reserved in scope strings
are commas ',', asterisk '*' and the backslash character '\'.
If this scope list is omitted, the message is said to be 'not
scoped'.
will explicitly exclude unscoped requests, as will be explained
below. The scope list may include '*' when multicasting requests to
SAs, which specifies that scopes will be ignored for the purposes
of handling the request. The list may also include any other valid
scope string.
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17.1.2. Registration
Services which are registered with a non-null scope list are
considered 'scoped'. A service registration which has no scope
string is considered unscoped. That means it will be registered with
all DAs which do not explicitly refuse unscoped registrations. A DA
does this by setting the 'S' bit in its DAAdvert.
A DA may be configured to be unscoped or scoped. An unscoped DA
accepts only unscoped registrations. SAs MUST register all unscoped
services with an unscoped DA. By default, a scoped DA will accept
unscoped registrations. A DA MUST be configurable to deny unscoped
registrations.
Scopes determine which requests a DA will accept. A DA MUST decline
to register a service if it is specified with an unsupported scope.
In this case a SCOPE_NOT_SUPPORTED error is returned in the SrvAck.
SAs MUST register scoped services with every DA they are aware of,
that supports the service's scopes.
DAs return a SrvAck in response to a SrvReg or SrvDeReg as defined in
Section 13 and 15.
SA registration with DAs occurs with built in delays, depending
on whether the DA was discovered actively (see Section 21.1) or
passively (see Section 21.2).
17.1.3. Query Handling
In every case, the rules specified in Section 17.1 apply.
UAs MUST use scoped DAs in preference to unscoped DAs, and unscoped
DAs in preference to multicasting requests to SAs. UAs MUST use
scoped requests when making requests of scoped DAs. UAs SHOULD use
requests with no scope when multicasting requests to SAs. UAs MUST
use requests with no scope when sending a request to an unscoped DA.
17.1.3.1. DA Query Handling
An unscoped DA accepts only unscoped requests. It will send the
corresponding reply with a SCOPE_NOT_SUPPORTED error if a scoped
service registration is received.
A scoped DA MUST accept only requests which have the scope of the DA
or where the request is unscoped (ie. the <scope-list> is omitted).
The DA will respond with information which matches the request in
the scope of the request AND unscoped service information. If the
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request has the 'S' bit set in the message header, the DA will
respond only with scoped service information.
If a scoped DA receives a request which does not include a supported
scope in its <scope-list>, or includes the string "*" the DA replies
with a SCOPE_NOT_SUPPORTED error in the reply corresponding to the
request. This rule does not apply DA Discovery requests, which are
described in Section 6.2.
DAs will return a reply with 0 results if they support the request
(ie. do not return an error) and they have no matches to the
request.
17.1.3.2. SA Query Handling
SAs apply the same rules for matching requests except that if there
is no match they will drop the request if there are no matches. They
will never reply with a SCOPE_NOT_SUPPORTED error.
SAs which receive a request where the <scope-list> is set to the
string "*" does not apply scope matching rules to the request.
This may mean that a SrvRply or AttrRply will not include digital
signatures for services which registered in a protected scope. In
order to receive these digital signatures in the reply, the UA MUST
specify the protected scope explicitely in the Service Request.
See 17.2.
17.2. Protected Scopes
Scope membership MAY also define the security access and
authorization for services in the scope; such scopes are called
protected scopes. If a UA wishes to be sure that SAs are authorized
to provide the service they advertise, then the UA should request
services from a protected scope which has been configured to have the
necessary authentication mechanism and keys distributed to the SAs
within the scope. A directory agent distributing URLs for services
in a protected scope will reject any registrations or deregistrations
for service agents which cannot provide cryptographically strong
authentication to prove their authorization to provide the services.
For instance, if a campus registrar wishes to find a working printer
to produce student grade information for mailing, the registrar would
require the printing user agent to transmit the printable output only
to those printing SAs which have been registered in the appropriate
protected scope. Notice that each service agent is, under normal
circumstances, validated two times: once when registering with
the directory agent, and once when the user agent validates the
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URL received with the Service Reply. This protects against the
possibilities of malicious DAs as well as malicious SAs.
Note that services in protected scopes provide separate
authentication for their URL entry, and for their attributes. This
is so that the URLs in SrvRply message can be authenticated (see
Section 7). This is the common case. In interactive mode, a user
may wish to obtain the set of all attributes a service supports using
an AttrRqst. The AttrRply for this request, made in a protected
scope, will include a digital signature over the attributes (see
Section 11).
17.2.1. Protected Scope Rules
Protected Scopes make use of structured Authenticator Blocks. These
are described in Section 4.4.
DAs and SAs are configured for use in a protected scope by
configuring them with the Public and Private keys of the protected
scope. UAs are configured for use in a protected scope by
configuring them with the Public key of the protected scope. See
Section22.
- SAs which have been configured with a protected scope MUST
include Authentication Blocks with all SrvReg and SrvDeReg
messages sent to DAs.
- SAs which have been configured with a protected scope MUST
include URL Authentication Blocks in SrvRply messages and
Attribute Authentication Blocks in AttrRply messages if the
corresponding SrvRqst or AttrRqst included the protected scope
explicitely in the <scope-list> in the request.
- SAs must reject DAAdverts they receive for a protected scope
which do not contain an Authenticator Block which the SA can
verify.
- DAs which have been configured with a protected scope must
verify the signatures of SrvReg and SrvDeReg messages from
SAs. The DA returns a SrvAck reply. If the signature fails,
the DA returns an AUTHENTICATION_FAILED error. If the BSD of
the Authentication Block is not supported by the DA, the DA
returns an AUTHENTICATION_ALGO_UNKNOWN error. If the SA fails
to include an authenticator where it should, the DA returns an
AUTHENTICATION_ABSENT error.
- DAs which have been configured with a Public Key for a protected
scope MUST include URL Authentication Blocks in SrvRply messages
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and Attribute Authentication Blocks in AttrRply messages. They
must also include Authentication Blocks in their DAAdvert
messages (see Section 12.)
- If the UA is configured with a protected scope, and if the UA
receives a SrvRply, AttrRply or DAAdvert from that protected
scope, the UA MUST verify the reply or advertisement before
accepting it.
18. Language Internationalization Issues
18.1. Language Tags and Dialects
Service location messages often include a Language Tag in the header.
A SrvDeReg message MAY omit a Language Tag if the service is to be
deregistered in ALL languages it has been registered in. If the
Language Tag is omitted, the Language Tag length in the header is set
to 0.
If the Language Tag used includes a dialect, the dialect is to be
used the following way: Requests which can be fulfilled by matching
a language and dialect will be preferred to those which match only
the language portion. Otherwise, dialects have no effect on matching
requests.
18.2. Scope Strings are not Language Specific
A scope string, which is used to configure DAs and which are used
by both UAs and SAs for various operations (see Section 17.1) is
NOT language specific. Thus, the language declared in a SrvRqst
used for DA discovery has no effect on whether the request predicate
matches the scopes of the DA or not. The DAAdvert does not include a
Language Tag as the scope strings are not language specific.
18.3. Declaring the language of registrations
All Service Registrations declare the language in which the strings
in the service attributes are written by specifying a Language Tag
in the message header. For each language the Service advertises a
separate registration takes place. Each of these registrations uses
the same URL to indicate that they refer to the same service.
A Service is fully deregistered if the URL is given in the Service
Deregister message without any attribute information or Language Tag
sent in the message.
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If, on the other hand, attribute information is included in the
Service Deregistration request, a separate Service Deregistration
of selected attributes must be undertaken in each language in which
service information has been provided to the DA by a SA.
Service Registrations in different languages are mutually
unintelligible. They share no information except for their service
type and URL with which they were registered. No attempt is made
to match queries with "language independence." Instead, queries are
handled using string matching against registrations in the same
language as the query.
18.4. Translation of Attribute Strings
Service Types which are standardized will have definitions for
all attributes and value strings. Official translations to other
languages of the attribute tags and values may be created and
submitted as part of the standard; this is not feasible for all
languages. For those languages which are not defined as part of the
Service Type, a best effort translation of the standard definitions
of the Service type's attribute strings MAY be used.
18.5. Declaring the language of a Request
Service and Attribute requests specify a requested language in the
message header. The DA or SA will respond in the same language as
the request, if it has a registration in the same language as the
request.
If this language is not supported, and the Monolingual bit is not
specified, a reply MAY be sent in the default language (which is
English) on a best effort basis.
In the case where there are services registered in the type
requested in a SrvRqst (or AttrRqst), but not in a language
which the service information supplies, the following two rules
apply: If the 'monolingual bit' flag in the header is set, a
SrvRply (or AttrRply) is returned with the error field set to
LANGUAGE_NOT_SUPPORTED. A SrvRqst may include a predicate (or an
AttrRqst may contain a tag list) in an unsupported language. If any
of the attribute strings are not language literal (see Section 19.1),
the SrvRply (or AttrRply) is returned with the error field set to
LANGUAGE_NOT_SUPPORTED.
If a query is in a supported language on a SA or DA, but has a
different dialect than the available service information, the query
MUST be serviced on a best-effort basis. If possible, the query
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should be matched against the same dialect. If that is not possible,
it MAY be matched against any dialect of the same language.
When there are several replies returned in one message and the reply
includes attributes which were registered with separate dialects, the
dialect portion of the Language Tag in the reply is dropped. This
could occurs primarily with an attribute reply.
19. Substitution of Character Escape Sequences
Certain characters are illegal in certain contexts of the protocol.
Since the protocol is largely character string based, in some
contexts characters are used as protocol delimiters. In these cases
the delimiting characters must not be used as 'data text.'
Characters which are reserved as part of the encoding of attribute
tags or values MUST be escaped if they are included in Attribute
Lists or Predicates. Any character in these strings MAY be escaped.
The escape mechanism uses the reserved backslash character '\' (ASCII
0x5c). This character is followed by two hexadecimal digits of the
escaped character. Note that some characters are multibyte, using
UTF8 encoding.
Examples:
Character to encode Unicode UTF8 SLP Escaped value
------------------- ------- --------- -----------------
',' 0x0029 0x0029 \29
e accent aigue 0x0039 0xc3a9 \c3\a9
not equals glyph 0x2260 0xe289a0 \e2\89\a0
Characters in URLs which are reserved according to the URL syntax
specification must be escaped using the URL escape character
convention. In this case a 'percent' is followed by the hex encoding
of the escaped character exactly as defined above for the 'slash'
character. The current URL syntax specification limits all encoded
characters to a subset of the ASCII character set. The update to
this defines a way to transmit UTF8 encoded characters outside of the
ASCII range.
19.1. Language-Independent Strings
Some strings, such as Service Type names, have standard definitions.
These reserved strings should be considered as tokens and not as
words in a language to be translated. They are case-independent.
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Reserved String Section xDefinition
--------------- ------- --------------------------------------
SCOPE 3, 16 Used to limit the matching of requests.
SERVICE 7, 13 The URL scheme of all Service Location
information registered with a DA or
returned from a Service Request.
<srvtype> 20.1.1 Used in all service registrations
and replies.
domain names 20.3 A fully qualified domain name, used
in registrations and replies.
IANA 3.7 The default naming authority.
LOCAL 17 Reserved.
REMOTE 17 Reserved.
TRUE 20.4 Boolean true.
FALSE 20.4 Boolean false.
SERVICE-TYPE 6 Standard term in predicates.
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20. String Formats used with Service Location Messages
The following section supplies formal definitions for fields and
protocol elements introduced in the sections indicated.
Protocol Element Defined in Used in
----------------------------------- ------------ ------------
<Previous Responders' Addr Spec> 20.1 SrvRqst
Service Request <predicate> 6.4 SrvRqst
URL [12] SrvReg,
SrvDeReg,
SrvRply
<attr-list> 20.2 SrvReg,
SrvRply,
AttrRply
<Service Registration Information> 13 SrvReg
<Service Deregister Information> 15 SrvDeReg
<Service Type String> 20.1.1 AttrRqst
<String-List> DAAdvert,
SrvTypeRply
attr vals
<Select-List> AttrRqst,
SrvDeReg
20.1. Previous Responders' Address Specification
The previous responders' Address Specification is specified as a
<String-List> of <addr-spec> terms.
The Address Specification is the address of the DA or SA
which supplied the previous response. The format for Address
Specifications in Service Location is defined in Section 20.3. The
use of dotted decimal IP address notation should only be used in
environments which have no Domain Name Service.
Example:
RESOLVO.NEATO.ORG,128.127.203.63
SAs or DAs which are listed in a Previous Responders list in a
multicast Service Location request will silently discard the request.
Depending on the length of the request, around 60 previous responders
may be listed in a single datagram. If there are more responders
than this, the scaling mechanisms described in Section D should be
used.
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20.1.1. Service Type String
The Service Type is a string describing the type of service. These
strings may only be comprised of alphanumeric characters, '+', and
'-'. Upper case is considered equivalent to lower case in Service
Type names.
The Service Type of a service: scheme URL is encoded as the string
up to and including the last ":". This includes the Naming Authority
string.
URL Service Type String
----------------------------------- ------------------
service:lpr://biglaser.dog.com service:lpr:
service:wp:http://dir.cat.org service:wp:
service:game.cs312://morb.lug.edu service:game.cs312:
ftp://ftpserv.mouse.gov ftp:
20.1.2. String List
String lists transmitted by the service location protocol are items
with a ',' delimiter.
str-list = str-item / str-item ',' str-list
The definition of <str-item> depends on the string list. For a
DAAdvert, the <str-item> is a scope attribute value (see <strval>
in Section 20.2). The other important case is described in
Section 20.1, which may be included with SrvRqst, AttrRqst and
SrvTypeRqst messages.
20.1.3. Select List
Select Lists are String List used in AttrRqst and SrvDeReg messages.
The <str-item> is a <tag-filter> with the following syntax:
tag-filter = simple-tag / substring
simple-tag = 1*filt-char
substring = [initial] any [final]
initial = 1*filt-char
any = '*' *(filt-char '*')
final = 1*filt-char
filt-char = Any character excluding <reserved> and <bad-tag> See
Section 20.2.
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20.2. Attribute Information
The <attr-list> is returned in the Attribute Reply if the Attribute
Request does not result in an empty result.
attr-list = attribute / attribute ',' attr-list
attribute = '(' attr-tag '=' attr-val-list ')' / attr-tag
attr-val-list = attr-val / attr-val ',' attr-val-list
attr-tag = 1*safe-tag
attr-val = intval / strval / boolval / opaque
intval = [-]1*DIGIT
strval = 1*safe-val
boolval = "TRUE" / "FALSE"
opaque = "(" 1*DIGIT ":" 1*r64chars ")"
safe-val = Any character except reserved.
safe-tag = Any character except reserved, star and bad-tag.
reserved = '(' / ')' / ',' / '\' / '!' / '<' / '=' / '>' / 'tilde'
bad-tag = CR / LF / HT
star = '*'
r64chars = DIGIT / ALPHA / '=' / '/' / '-'
An <attr-list> must be scanned prior to evaluation for all
occurrences of the escape character '\'. Reserved characters in
attributes placed in attribute lists must be escaped. All escaped
characters must be restored to their value before used for string
matching. See Section 19.
A keyword has only an <attr-tag>, and no values.
(OPERATOR=Joe Javaman)
(COLOR=RED\, WHITE\, BLUE)
(DELAY=10 MINS), BUSY, (LATEST BUILD=10-5-95), (PRIORITY=L,M,H)
The third example has three attributes in the list. Color takes
the value "RED, WHITE, BLUE" (the commas in the value string are
escaped). Priority, on the other hand, takes the three values "L",
"M", and "H". There are several other examples of attribute lists
throughout the document.
20.3. Address Specification in Service Location
The address specification used in Service Location is:
<addr-spec> ::= [<user> ':' <password> '@' ] <host> [ ':' <port>]
<host> ::= Fully qualified domain name /
dotted decimal IP address notation
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When no Domain Name Server is available, SAs and DAs must use dotted
decimal conventions for IP addresses. Otherwise, it is preferable to
use a fully qualified domain name wherever possible as renumbering of
host addresses will make IP addresses invalid over time.
Generally, just the host domain name (or address) is returned.
When there is a non-standard port for the protocol, that should be
returned as well. Passwords SHOULD NOT be sent with SLP messages
(as cleartext) unless an IPSec ESP is used for delivery of the SLP
message.
Address specification in Service Location is consistent with standard
URL format [5].
20.4. Attribute Value encoding rules
Attribute values, and attribute tags are CASE INSENSITIVE for
purposes of lexical comparison.
The syntax of attribute values is described in Section 20.2.
While an attribute can take any value, there are three types
of values which differentiate themselves from general strings:
Booleans, Integers and Opaque values.
- Boolean values are either "TRUE" or "FALSE". This is the case
regardless of the language strings are encoded in. Boolean
attributes can take only one value.
- Integer values are expressed as a sequence of DIGITs. The
range of allowable values for integers is "-2147483648" to
"2147483647". No other form of numeric representation is
interpreted as such except integers. For example, hexadecimal
numbers such as "0x342" are not interpreted as integers, but as
strings.
- Opaque values (i.e. binary values) are expressed in radix-64
notation. The syntax is:
<opaque-val> ::= "(" <len> ":" <radix-64-data> ")"
<len> ::= 1*DIGIT
<radix-64-data> ::= radix-64 encoding of the original data
<len> is a 16-bit binary number expressed as a decimal string.
For example, if the opaque value is 34 bytes long before encoding
the <len> field would be the string "34".
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Radix-64 encodes every 3 bytes of binary data into 4 bytes of
ASCII data which is in the range of characters which are fully
printable and transferable by mail. For a formal definition of
the Radix-64 format see RFC 2045 [6], Section 6.8.
The Opaque value encoding includes otherwise reserved
characters in attribute values: '(', ')'. It may include
another reserved character: '='. These characters
are escaped in SrvRqst predicates. For example
"(&(service-type=thing)(att=\(3:AAAA\)))" is a query for
a service of type "thing" which has an attribute "att" with a
value of a 3 zero bytes.
21. Protocol Timing Rules
There are four protocol actions which require timing rules. Active
DA Discovery, Passive DA Advertising, reliable unicast requests to a
DA and multicast/convergence.
21.1. Active DA Discovery
After a UA or SA restarts (say, after rebooting of a system or
loading of the network kernel), their initial DA discovery request
should be delayed for some random time uniformly distributed from 0
to CONFIG_START_WAIT seconds.
The UA or SA sends the DA Discovery request using a SrvRqst, as
described in Section 6.2 and receives a DAAdvert as a reply, as
described in Section 12.
DA Discovery requests should be sent according to CONFIG_DA_RETRY.
The second and third transmission of the DA Discovery request MUST
include a previous responders list of all DAs which have responded.
See Section 20.1.
SAs which discover DAs actively must wait CONFIG_REG_ACTIVE seconds
before registering their services.
21.2. Passive DA Advertising
Upon startup, then every CONFIG_DA_BEAT seconds a DA will multicast
(or broadcast) an unsolicited DAAdvert. This will ensure that
eventually it will be discovered by all applications which are
concerned.
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All SAs which receive the unsolicited DAAdvert should examine its
Sequence Number. If the DA has never before been heard from or if
the Sequence Number is less than it was previously and less than 256,
the SA should assume the DA does not have its service registration,
even if it once did. If this is the case and the DA has the proper
scope, the SA should register all service information with the DA,
after waiting a random interval CONFIG_REG_PASSIVE.
While it might seem advantageous to have frequent heartbeats, this
generates traffic throughout the network in proportion to the number
of DAs. Thus, CONFIG_DA_BEAT should be specified with a value in
seconds long enough to prevent routine protocol operations from using
any significant bandwidth.
21.3. Reliable Unicast to DAs
If a DA fails to respond to a unicast UDP message in CONFIG_DA_RETRY
seconds, the message should be retried. If a DA fails to respond
after CONFIG_DA_MAX seconds, the UA or SA should use a different DA.
DA addresses may be cached from previous discovery attempts,
preconfigured, or by use of DHCP (see Section 16.1).
If no such DA responds, DA discovery should be used to find a new DA.
Care should be taken not to do Active DA Discovery more than once per
CONFIG_DA_FIND seconds.
21.4. Multicast/Convergence
This algorithm is used by UAs to send requests to SAs using the
Service Location General Multicast Address. The algorithm is used by
both UAs and SAs to send DA Discovery requests to DAs using the DA
Discovery Multicast Address.
Requests to SAs are multicast repeatedly (with a recommended wait
interval of CONFIG_MC_RETRY) until there are no new responses, or
CONFIG_MC_MAX seconds have elapsed. DA discovery requests use
different timing for repeated requests, CONFIG_DA_RETRY.
Repeated requests include a previous responder list, see
Section 20.1.
22. Configurable Parameters and Default Values
There are several configuration parameters for Service Location.
Default values are chosen to allow protocol operation without the
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need for selection of these configuration parameters, but other
values may be selected by the site administrator. The configurable
parameters will allow an implementation of Service Location to be
more useful in a variety of scenarios.
22.1. Time Out Intervals
These values should be configurable in case the site deploying
Service Location has special requirements (such as very slow links.)
Interval name Section Default Value Meaning
========================================================================
CONFIG_KEEP_RPLY 4.2 1 minute Cache replies by XID.
CONFIG_LIFETIME 4.5 10800 seconds registration Lifetime,
(3 hours) after which ad expires
CONFIG_MC_RETRY 4.2 each second, Retry multicast query
backing off until no new values
gradually arrive.
CONFIG_MC_MAX 6 15 seconds Max time to wait for a
complete multicast query
response (all values.)
CONFIG_START_WAIT 13 3 seconds Wait to perform DA
discovery on reboot.
CONFIG_DA_RETRY 6.2 2 seconds Retransmit DA discovery,
try it 3 times.
CONFIG_DA_MAX 6.2 6 seconds Give up on requests sent
to a DA.
CONFIG_DA_BEAT 16.1 3 hours DA Heartbeat, so that SAs
passively detect new DAs.
CONFIG_DA_FIND 6.2 900 seconds Minimum interval to wait
before repeating Active
DA discovery.
CONFIG_REG_PASSIVE 16.1 1-3 seconds Wait to register services
on passive DA discovery.
CONFIG_REG_ACTIVE 13 1-3 seconds Wait to register services
on active DA discovery.
CONFIG_CLOSE_CONN 3.10 5 minutes DAs and SAs close idle
connections.
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Multicast vs. Broadcast
All Service Location entities must use multicast by
default. The ability to use broadcast messages must
be configurable for UAs and SAs. Broadcast messages
are to be used in environments where not all Service
Location entities have hardware or software which
supports multicast.
Multicast Radius
Multicast requests should be sent to all subnets in a
site. The default multicast radius for a site is 32.
This value MUST be configurable.
Directory Agent Address
The DA address discovery mechanism must be
configurable. There are three possibilities for this
configuration: A default address, no default address
and the use of DHCP to locate a DA as described in
Section 16.1. The default value should be use of
DHCP, with "no default address" used if DHCP does
not respond. In this case the UA or SA must do a
Directory Agent Discovery query.
Directory Agent Scope Assignment
The scope or scopes of a DA must be configurable.
The default value for a DA is to have no scope if not
otherwise configured.
Path MTU The default path MTU is assumed to be 1400. This
value MUST be configurable for all SAs and DAs.
Keys for Protected Scopes
If the local administration designates certain
scopes as "protected scopes", the agents making
use of those scopes MUST able to acquire keys to
authenticate data sent by services along with their
advertised URLs for services within the protected
scope. SAs and DAs require private keys for the
scope, where all entities require public keys for
the scope. See Section 17.2.1 which defines how
these keys are used. By default, service agents use
"md5WithRSAEncryption" [4] to produce the signed
data, to be be included with service registrations
and deregistrations (see appendix B, 4.4). This
authentication data could be verified by user agents
and directory agents that possess the corresponding
public key.
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22.2. Service Agent:
Use Predefined Directory Agent(s)
A SA's default configuration is to do passive and active DA discovery
and to register with all DAs which are properly scoped.
A SA SHOULD be configurable to allow a special mode of operation:
They will use only preconfigured DAs. This means they will *NOT*
actively or passively detect DAs.
If a SA is configured this way, knowledge of the DA must come through
another channel, either static configuration or by the use of DHCP.
The availability of the Service information will not be consistent
between DAs. The mechanisms which achieve eventual consistency
between DAs are ignored by the SA, so their service information will
not be distributed. This leaves the SA open to failure if the DA
they are configured to use fails.
23. Security Considerations
The Service Location Protocol provides for authentication of Service
Information registered by SAs and DA Advertisements. This allows
UAs to obtain information about services with confidence that is
reasonably complete and accurate.
Service Location does not provide confidentiality. Because the
objective of this protocol is to advertise services to a community
of users, confidentiality might not generally be needed when this
protocol is used in non-sensitive environments. Specialized schemes
might be able to provide confidentiality, if needed in the future.
Sites requiring confidentiality should implement the IP Encapsulating
Security Payload (ESP) [3] to provide confidentiality for Service
Location messages.
Using unprotected scopes, an adversary might easily use this protocol
to advertise services on servers controlled by the adversary and
thereby gain access to users' private information. Further, an
adversary using this protocol will find it much easier to engage in
selective denial of service attacks. Sites that are in potentially
hostile environments (e.g. are directly connected to the Internet)
SHOULD use protected scopes.
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Service Location is useful as a bootstrap protocol. It may be used
in environments in which no preconfiguration is possible. In such
situations, a certain amount of "blind faith" is required: Without
any prior configuration it is impossible to use any of the security
mechanisms described above. Service Location will make use of
the mechanisms provided by the Security Area of the IETF for key
distribution as they become available. At this point it would only
be possible to gain the benefits associated with the use of protected
scopes if some cryptographic information can be preconfigured with
the end systems before they use Service Location. For UAs, this
could be as simple as supplying the public key of a Certificate
Authority. See Appendix B.
24. Protocol Requirements
In this section are listed various protocol requirements for UAs,
SAs, and DAs.
24.1. Directory Agent Requirements
Protocol Feature Section Requirement Level
========================================================================
Announce using multicast on startup. 21.2 MUST
Announce using multicast, regularly. 21.2 MUST
Reply to unicast requests from UAs. 17.1.3.1 MUST
Reply to multicast DADiscovery requests. 6.2 MUST
Support TCP when overflow occurs. 3.10.1 MUST
Accept de/registration from SAs. 17.1.2 MUST
Age out expired service registrations. 4.5 MUST
Reply to message using DA scoping rules. 17.1.3.1 MUST
Obey language rules for de/reg, requests. 18 MUST
Announce and verify using pro. scopes. 17.2.1 MUST
Be able to ignore unrecognized options. 4.1 MUST
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24.2. Service Agent Requirements
Protocol Feature Section Requirement Level
========================================================================
Perform Active DA discovery on start up. 21.1 MUST
Perform Passive DA discovery always. 21.2 MUST
Forward Registrations discovered DAs. 17.1.2 MUST
Forward Deregistrations to DAs if needed. 17.1.2 MUST
Age out expired service registrations. 4.5 MUST
Obey pro. scope rules: in all messages. 17.2.1 MUST
Obey scope rules: forwarding to DAs. 17.1 MUST
Obey scope rules: responding to UAs. 17.1 MUST
Obey language rules on reply or register. 18 MUST
Support TCP for overflowed SLP messages. 3.10.1 MUST
Respond to multicast & unicast SrvRqst. 3.10 MUST
Also respond to AttrRqst and SrvTypeRqst. 3.10 SHOULD
Be able to ignore unrecognized options. 4.1 MUST
24.3. User Agent Requirements
Protocol Feature Section Requirement Level
========================================================================
Perform Active DA discovery on start up. 21.1 MUST
Perform Passive DA discovery always. 21.2 SHOULD
Obey rules: preferring DAs to SAs, etc. 5 MUST
Obey scope rules: where to send requests. 17.1 MUST
Obey pro. scope rules: reject bad info. 17.2.1 MUST
Support TCP for overflowed SLP messages. 3.10.1 MUST
Be able to ignore unrecognized options. 4.1 MUST
Send SrvRqst to SAs using multicast. 21.4 MUST
Send SrvRqst to DAs using unicast. 21.3 MUST
Send SrvTypeRqst and AttrRqst messages. 5 SHOULD
Send requests to SAs using unicast. 5 MAY
24.4. Common Requirements for all SLP Agents
Protocol Feature Section Requirement Level
========================================================================
Static Configurability. 22 MUST
DHCP Configurability. 16.1 SHOULD
Support protected scope configuration. 22 SHOULD
For protected scopes, support md5/RSA. RFC 1423[4] MUST
Support UTF-8 character encoding. 4 MUST
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25. Non-configurable Parameters
IP Port number for unicast requests to DAs:
UDP and TCP Port Number: 427
Multicast Addresses
Service Location General Multicast Address: 224.0.1.22
Directory Agent Discovery Multicast Address: 224.0.1.35
A range of 1024 contiguous multicast addresses for use as Service
Specific Discovery Multicast Addresses will be assigned by IANA.
Error Codes:
No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
AUTHENTICATION_ABSENT 6
AUTHENTICATION_FAILED 7
AUTHENTICATION_ALGO_UNKNOWN 8
PROTOCOL_V1_REJECTED 9
EXPECTED_ATTRIBUTE_MISSING 64
INTERNAL_ERROR 128
26. Acknowledgments
Early work on this protocol was done by Leo McLaughlin, Mike Ritter
and Scott Kaplan. Charlie Perkins and Harry Harjono's Resource
Discovery Protocol was merged with the Service Location Protocol.
Jeff Schiller assisted in shaping the security architecture specified
in this document. This protocol has benefited from the techniques,
focus and minimal requirements of the Ping Discovery Service (PDS)
from Intel in its recent revision.
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A. Version 2 Notes
This document revises the Service Location Protocol as specified in
RFC 2165. There are some open issues which are resolved here. Some
of these are protocol corrections (which result in a change in the on
the protocol as it is exchanged over the wire.) Some of the changes
to the document are merely clarifications and document restructuring.
Changes since draft-ietf-svrloc-protocol-v2-00.txt:
- URL length was 1 byte, now it is 2.
- Deregistration without language tag or attr tags fully
deregisters a service.
- We clarified the monolingual bit, so that it is clear that
LANGUAGE_NOT_SUPPORTED is returned only when there are services
available, but not in an available language.
- We added a new error: INTERNAL_ERROR, for the case where a DA or
SA cannot respond.
- We restored the header to as close a form to the v1 version as
possible and noted why it could not be the same.
- We clarified the 'S' bit.
- Michael Day added as an editor/author.
- PDS added to the acknowledgement section.
- ISOC copyright header added.
- We added 'service-type' to the list of reserved, non-translatable
strings.
- We fixed the bibliography.
- We added a section on minimal UA and SA requirements.
- We rewrote the introduction and added an appendix on SLP and
management.
- We updated the requirements tables at the end.
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Changes between RFC 2165 and version-00 of this document:
Clarifications:
SrvTypeRqst definition
SrvTypeRqst does not include a list of previously received
service types as was erroneously indicated.
Opaque value syntax
The ``(`` and ``)'' characters enclosing an opaque value are
to be taken literally. The length of the opaque value is to
be encoded as a decimal number.
Scopes
Are NOT language specific strings. All scope rules have
been coalesced into a single section. The rules for when to
use unscoped requests and registrations have been clarified
as well as the consequences of receiving them and the
behavior of unscoped DAs.
Requirements
The requirements section has been redone in a simpler matrix
style, with references to the protocol section, reflecting
*all* protocol requirements.
Wild Card values in search filters
Wild card values are only intended to be used for string
MATCHING not string COMPARISONS.
Service Type Reply strings
The definition of the reply strings needs to be clarified
and allow for arbitrary URL schemes.
Security considerations
The security section has been updated to reflect the changes
made to the certificate format section and the ability to
sign DAAdverts.
AttrRqst select-list length
The length of the select-list is the length of the
select-list string, which is the common convention in SLP.
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Modifications:
Crypto Algorithm field in Service requests
The UA requires a way to specify which crypto algorithm
it can use, if it is not the default, so the DA or SA can
determine what to use.
Version Number
Since the wire protocol has changed, the version number of
the protocol takes the version number 2.
Character Encoding
Character encodings are all be in UTF8 [21]. This conforms
to current IESG recommendations and simplifies the protocol.
Service Request Predicates
Service Request Predicate grammar have been replaced by the
request grammar used by LDAP string search filters. Some
conventions are necessary to ensure proper structure of
requests. The simple 'list format' queries is no longer
supported.
Character escapes in string encoding
To be consistent with the LDAP string search filters syntax
and to simplify the encoding rules in SLP, all character
escapes in SLP string based messages reserve the backslash
character as an escape for reserved characters. Character
escapes in URLs conform to standard URL syntax conventions.
Fresh Bit Handling
The Fresh bit is transmitted by SAs to DAs to indicate
freshness, not returned by DAs to SAs in the acknowledgement
of registration. This allowed for a race condition.
URL handling
SLPv1 only supports service: URLs. It now supports
arbitrary URLs provided that they follow standard URL
syntax. This support modifies SrvRply, AttrRqst, SrvReg,
DAAdvert and SrvDeReg.
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Further Modifications:
DAAdvert format
The DAAdvert message contains new information: It indicates
whether it was sent as the result of a service request or
at the DA's own initiative. It can also carry a digital
signature of the DAAdvert. The DA may also mark some of
the scopes it advertises in as 'protected' and provide a
pointer to the protected scope's certificate. DAAdverts no
longer override the XID in the header, but instead carry a
'DAAdvert sequence number'.
Request Flagging
The header indicates whether a request was multicast or
unicast. This aids interpretation of the request by SAs
and DAs implemented on platforms which receive all datagram
requests from the same socket and cannot distinguish between
the 'destination' address of the datagram they received.
String Interpretation
Strings are interpreted in line with other string based
query protocols: Interior white space is folded to a single
white space rather than interpreted literally for string
matching.
Language Tagging
The reserved Dialect field is used for inclusion of RFC 1766
language tags (or whatever obsoletes them.) This is in
accord with the new guidelines for internationalization.
SLP Certificate format
The certificate format needs an additional field indicating
the name of the CA. There also needs to be text which
encourages the use of standard certificate formats if
possible (X.509, etc.)
Requirements
The requirement that SAs handle SrvTypeRqst and AttrRqst
has been relaxed from a SHOULD to a MAY. The requirement
that a UA be able to issue a SrvTypeRqst and AttrRqst has
been relaxed from a MUST to a SHOULD. The requirement that
UAs be able to use multicast/convergence has been relaxed
from a MUST to a SHOULD (provided that DHCP is used for DA
discovery!) This allows conformant implementations to be
much more lightweight.
Service Location Extension Options
These options allow for the protocol to be enhanced without
requiring the main protocol specification standardization to
be derailed.
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Open Issues:
Service Specific Multicast Addresses
Use of Service Specific Discovery Multicast Addresses as
defined in RFC 2165 is deprecated until a multicast address
range allocation mechanism is defined using Administratively
Scoped Multicast Addresses. We need to assign the range of
service specific multicast addresses for use by UAs and SAs.
Lifetime may be too short
It may be that timeouts for soft state are too short. We
may want to make the URL entry's Lifetime field be longer
than 2 bytes worth of seconds.
Allow the tag list in SrvDeReg to include wildcards
This would allow for deletions of attributes with patterns,
such as all entries with a common prefix. This facilitates
efficient name deletion based on a hierarchical structure.
Include a MIN, MAX and BEST operator for queries
MIN and MAX would be used for integer query terms - to allow
the service with the smallest or largest value for a given
attribute to be returned. BEST would be a string match
which would return the service(s) for which the longest
substring match could be found.
Allow increasing ring multicast discovery
This would allow the 'nearest' services to be discovered.
Use multicast based de/registration of services
This would eliminate a scaling issue in the protocol but
would require the use of MTCP-like techniques so that
SAs could moderate their refresh intervals so that SLP
registration take only a fixed amount of network bandwidth.
LDAP search filters
Should LDAPv2 filters [13, 20, 14] be used, since they rely
on Draft Standard protocols? Or, should SLP use the new
LDAPv3 filters (which have not yet gone to PS)? The latter
handle internationalization far better!
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B. SLP Certificates
Certificates may be used in SLP in order to distribute the public
keys of trusted protected scopes. This section defines a very
simple format for certificates which provides only the most basic
functionality required for deploying protected scopes. If a public
key infrastructure has already been deployed, for instance using
X.509 certificates, these SHOULD be used instead. Implementation of
SLP Certificates is entirely OPTIONAL.
Possession of the private key of a protected scope is equivalent
to being a trusted SA. The trustworthiness of the protected scope
depends upon all of these private keys being held by trusted
hosts, and used only for legitimate service registrations and
deregistrations.
With access to the proper Certificate Authority (CA), DAs and UAs
do not need (in advance) hold public keys which correspond to these
protected scopes. They do require the public key of the CA. The CA
produces certificates using its unique private key. This private key
is not shared with any other system, and must remain secure. The
certificates declare that a given protected scope has a given public
key, as well as the expiration date of the certificate.
The ASCII (mail-safe) string format for the certificate is the
following list of tag and value pairs:
"certificate-auth=" 1*ALPHANUMERIC CRLF
"certificate-alg=" 1*ASN1CHAR CRLF
"scope-charset=" 1*DIGIT CRLF
"scope=" 1*RADIX-64-CHAR CRLF
"timestamp=" 16HEXDIGIT CRLF
"public-key=" 1*RADIX-64-CHAR CRLF
"cert-digest=" 1*RADIX-64-CHAR CRLF
ASN1CHAR = DIGIT | '.'
HEXDIGIT = DIGIT | 'a'..'f' | 'A'..'F'
RADIX-64-CHAR = DIGIT | 'a'..'z' | 'A'..'Z' | '+' | '/' | '='
The radix-64 notation is described in RFC 2045 [6]. Spaces are
ignored in the computation of the binary value corresponding to a
Radix-64 string. If the value for scope, public-key or cert-digest
is greater than 72 characters, the Radix-64 notation may be broken
up on to separate lines. The continuation lines must be preceded
by one or more spaces. Only the tags listed above may start in the
first column of the certificate string. This removes ambiguity in
parsing the Radix-64 values (since the tags consist of legal Radix-64
values.)
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The certificate-auth identifies the authority who created the
certificate. This enables a very unsophisticated mechanism for
enabling the use of multiple certificate authorities: An end
system may be configured with public keys for multiple certificate
authorities. In each case the public key will be identified by
a 'certificate-auth' string as part of the configuration. When
certificates are obtained, they may use the 'certificate-auth' field
to determine which public key to use to verify the certificate's
validity.
The certificate-alg is the ASN.1 string for the Object Identifier
value of the algorithm used to produce the "cert-digest". The
scope-charset is a decimal representation of the MIBEnum value for
the character set in which the scope is represented.
The radix-64 encoding of the scope string will allow the ASCII
rendering of a scope string any character set.
The 8 byte NTP format timestamp is represented as 16 hex digits.
This timestamp is the time at which the certificate will expire.
The format for the public key will depend on the type of cryptosystem
used, which is identified by the certificate-alg. When the CA
generated the certificate holding the public key being obtained,
it used the message digest algorithm identified by certificate-alg
to calculate a digest D on the string encoding of the certificate,
excepting the cert-digest. The CA then encrypted this value using
the CA's private key to produce the cert-digest, which is included in
the certificate.
The CA generates the certificate off-line. The mechanism to
distribute certificates is not specified in the Service Location
Protocol, but may be in the future. The CA specifies the algorithms
to use for message digest and public key decryption. The DA or SA
need only obtain the certificate, have a preconfigured public key for
the CA and support the algorithm specified in the certificate-alg in
order to obtain certified new public keys for protected scopes.
The DA or UA may confirm the certificate by calculating the message
digest D, using the message digest algorithm identified by the
certificate-alg. The input to the message digest algorithm is the
string encoding of the certificate, excepting the cert-digest.
The cert-digest is decrypted using the CA's public key to produce
D'. If D is the same as D', the certificate is legitimate. The
public-key for the protected scope may be used until the expiration
date indicated by the certificate timestamp.
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The certificate may be distributed along untrusted channels, such as
email or through file transfer, as it must be verified anyhow. The
CA's public key must be delivered using a trusted channel.
C. Example of deploying SLP security using MD5 and RSA
In our site, we have a protected scope "CONTROLLED". We generate a
private key - public key pair for the scope, using RSA. The private
key is maintained on a secret key ring by all SAs in the protected
scope. The public key is available to all DAs which support the
protected scope and to all UAs which will use it.
In order to register or deregister a URL, the data required to be
authenticated (as described in Section 4.4) is digestified using
MD5 [18] to create a digital signature, then encrypted by RSA with
the protected scope's private key. The output of RSA is used in the
authentication data field of the authentication block.
The DA or UA discovers the appropriate method for verifying the
authentication by looking inside the authentication block. Suppose
that the "md5WithRSAEncryption" [4] algorithm has to be used
to verify the signed data. The DA or UA calculates the message
digest of the URL Entry by using md5, exactly as the SA did. The
authentication block is decrypted using the public key for the
"CONTROLLED" scope, which is stored in the public key ring of the UA
or DA under the name "CONTROLLED". If the digest calculated by the
UA or DA matches that of the SA, the URL Entry has been validated.
D. Scaling and Deployment of the Service Location Protocol
The Service Location Protocol is meant to be deployable in a variety
of network configurations. The protocol is ideal for 'plug and play'
networks, which have the minimum of services offered. It also will
scale to sites, with many routed networks, even over wide-area low
bandwidth links.
The key to SLP's scalability is how it is configured in the default
case and how it can be configured for use in larger deployments.
The default configuration for SLP requires it to discover DAs and if
possible, to use them. This ensures that even in a larger deployment
SLP will not use multicast or broadcast more than necessary. Thus
SLP will not require much network bandwidth for its operation.
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If there are no DAs, as would be the case in the simplest
configuration, UAs will multicast (or broadcast) their requests to
SAs. This allows a network of client applications and services to be
set up with no configuration; no DNS, no DHCP, and so on. This is a
useful feature for extremely small networks, such as in a conference
room.
DAs are provided for scalability. The protocol works identically
whether DAs are present or not (UAs issue the same requests to either
SAs or DAs, and get the same replies.) The only difference is
efficiency and scalability.
The scoping feature of SLP provides further scalability. Services
which are scoped will be attracted to the subset of DAs which support
their scope. UAs will go to the DAs which supply service information
in the scope they are configured to use, or the scope which the user
selects if the UA is not configured with a scope.
In some cases multicast cannot be used to discover DAs. In others,
administration Blocks may decide that they would prefer to control
the way DAs are used. Here, DHCP may be used to configure UAs and
SAs with the location of DAs. This is particularly useful for
configuring hosts which are networked via slow wide-area links. It
would be a waste of scarce network bandwidth to use a multicast
routing protocol to distribute DA advertisements over such a link.
There is no guarantee that a DA of a particular scope will contain
the same service information as another DA in the same scope. This
consistency is only achieved when all SAs which advertise services
in that scope are aware of the same set of DAs. This may not be
possible, if multicast routing does not pervade the entire site or if
DHCP does not uniformly configure SAs with the same scope to use the
same DAs.
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Indeed, it may not even be desirable to allow such pervasive use
of multicast nor even be required for useful 'best effort' service
provision for there to be a consistent view of services from all DAs
of the same scope. This is a decision which network administrators
of SLP in large sites need to consider. The more consistency
they wish to achieve between DAs, the more complicated their DHCP
configuration will be, or the more SLP based multicast traffic they
will have in their networks.
E. Using SLP For Network and Systems Management
When deploying SLP for purposes of network or systems management,
there are some important points to consider. These include ease of
administration, having a low impact on the network, and providing SLP
support for the entire range of systems on the network. (Note that
using SLP to discover managed systems will only find managed systems
that support SLP.)
Network and systems management applications need to continue working
when other network components are broken. Further, management
applications frequently need to obtain information that has a
very short lifetime. These considerations motivate some of the
recommendations that are listed below.
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The Service Location Protocol can provide a number of applications
for network and systems management. For example:
- A manager can use SLP to discover managed systems without
performing brute-force scanning of the network.
- A manager can select manageable systems based on specific
attributes, such as support for a specific management protocol,
agent, instrumentation group, and software distribution format.
- A managed system can use SLP to find its manager(s), to which the
managed system can then send traps and issue trouble tickets.
- A managed system can use SLP to publish basic information about
itself, such as its operating system vendor, type and version;
its MAC address(es); operator information; management agent(s)
hosted by that system, and so on.
- A manager can periodically search for new hosts using SLP.
A good strategy to follow when using SLP for manageability includes
the following points:
- Peer-to-peer implementation. Each managed system, along with
each management application, implements UA and SA functionality.
The system advertises its management services. These service
types are defined elsewhere, for example [10].
- Bootstrap capability. Each managed system, along with each
management application can begin functioning with no knowledge
of exisiting manageability services beyond a knowledge of its
own UA. For example, a management application can, with no
foreknowledge, use SLP to build a list of manageable stations and
their basic capabilities.
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This bootstrap capability requires the use of multicast or
broadcast with convergence. Multicasting and broadcasting should
be avoided when possible. However, bootstrap capability is a
key aspect of systems management. (What happens when the DA is
broken and needs to be managed?) Therefore, managers and managed
stations should use DAs for discovery when they exist, but must
also have the ability to perform multicast or broadcast with
convergence.
- DAs for support of mobile and occasionally connected stations, as
well as for scalability. DAs provide a good way to support the
discovery of mobile and occasionally connected systems because
they can act as SLP proxies for such systems, which may be
connected via a slow link. In addition, DAs provide scalability
to SLP.
F. Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However,
this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Internet Society
or other Internet organizations, except as needed for the purpose
of developing Internet standards in which case the procedures
for copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
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References
[1] S. Alexander and R. Droms. DHCP Options and BOOTP Vendor
Extensions. RFC 2132, March 1997.
[2] H. Alvestrand. Tags for the Identification of Languages. RFC
1766, March 1995.
[3] R. Atkinson. IP Encapsulating Security Payload. RFC 1827,
August 1995.
[4] D. Balenson. Privacy Enhancement for Internet Electronic
Mail: Part III: Algorithms, Modes, and Identifiers. RFC 1423,
February 1993.
[5] T. Berners-Lee, L. Masinter, and M. McCahill. Uniform Resource
Locators (URL). RFC 1738, December 1994.
[6] N. Borenstein and N. Freed. Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies.
RFC 2045, November 1996.
[7] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[8] CCITT. Specification of the Abstract Syntax Notation One
(ASN.1). Recommendation X.208, 1988.
[9] D. Crocker and P. Overell. Augmented BNF for Syntax
Specifications: ABNF. RFC 2234, November 1997.
[10] M. Day. Manageability service: Schemes. November 1997.
draft-ietf-svrloc-sysman-00.txt (work in progress).
[11] R. Droms. Dynamic Host Configuration Protocol. RFC 2131, March
1997.
[12] E. Guttman, C. Perkins, and J. Kempf. Service Templates and
service: Schemes. draft-ietf-svrloc-service-scheme-05.txt,
November 1997. (work in progress).
[13] T. Howes. A String Representation of LDAP Search Filters. RFC
1960, June 1996.
[14] T. Howes, S. Kille, W. Yeong, and C. Robbins. The String
Representation of Standard Attribute Syntaxes. RFC 1778, March
1995.
[15] D. Mills. Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI. RFC 2030, October 1996.
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[16] R. Moats and M. Hamilton. Finding Stuff (How to discover
services). draft-ietf-svrloc-discovery-05.txt, October 1997.
(work in progress).
[17] C. Perkins. DHCP Options for Service Location Protocol, May
1997. draft-ietf-dhc-slp-02.txt (work in progress).
[18] Ronald L. Rivest. The MD5 Message-Digest Algorithm. RFC 1321,
April 1992.
[19] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. RFC 2165, July 1997.
[20] W. Yeong, T. Howes, and S. Kille. Lightweight Directory Access
Protocol. RFC 1777, March 1995.
[21] F. Yergeau. UTF-8, a transformation format of unicode and ISO
10646. RFC 2044, October 1996.
Authors' Addresses
Questions about this memo can be directed to:
Erik Guttman Charles Perkins
Sun Microsystems Sun Microsystems
Bahnstr. 2 901 San Antonio Road
74915 Waibstadt Palo Alto, CA 94040
Germany USA
Phone: +49 7263 911701 +1 650 786 6464
Fax: +1 650 786 6445
Email: Erik.Guttman@sun.com cperkins@sun.com
John Veizades Michael Day
@Home Network Intel
385 Ravendale Dr.
Mountain View, CA 94043
USA
Phone: +1 650 569 5243 +1 801 763 2341
Fax:
Email: veizades@home.net Michael_Day@ccm.ut.intel.com
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