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draft-ietf-svrloc-protocol-15.txt
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Internet Engineering Task Force John Veizades
INTERNET DRAFT @Home Network
8 January 1997 Erik Guttman
Sun Microsystems
Charles Perkins
IBM Research
Scott Kaplan
Service Location Protocol
draft-ietf-svrloc-protocol-15.txt
Status of This Memo
This draft document is a product of the Service Location Working
Group of the Internet Engineering Task Force (IETF); it will be
submitted to the RFC editor as a standards document. Please respond
with comments to the srvloc@tgv.com mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Abstract
The Service Location Protocol provides a scalable framework for
the discovery and selection of network services. Using this
protocol, computers using the Internet no longer need so much 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 2
2. Terminology 2
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 4
2.2. Service Information and Predicate Representation . . . . 4
2.3. Specification Language . . . . . . . . . . . . . . . . . 4
3. Protocol Overview 5
3.1. Protocol Transactions . . . . . . . . . . . . . . . . . . 6
3.2. Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. The ``service:'' URL scheme . . . . . . . . . . 8
3.3. Standard Attribute Definitions . . . . . . . . . . . . . 8
3.4. Naming Authority . . . . . . . . . . . . . . . . . . . . 9
3.5. Interpretation of Service Location Replies . . . . . . . 9
3.6. Use of TCP, UDP and Multicast in Service Location . . . . 10
3.6.1. Multicast vs. Broadcast . . . . . . . . . . . . 10
3.6.2. Service-Specific Multicast Address . . . . . . . 11
3.7. Service Location Scaling, and Multicast Operating Modes . 11
4. Service Location General Message Format 13
4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . . 15
4.2. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 16
5. Service Request Message Format 16
5.1. Service Request Usage . . . . . . . . . . . . . . . . . . 18
5.2. Directory Agent Discovery Request . . . . . . . . . . . . 19
5.3. Explanation of Terms of Predicate Grammar . . . . . . . . 20
5.4. Service Request Predicate Grammar . . . . . . . . . . . . 22
5.5. String Matching for Requests . . . . . . . . . . . . . . 24
6. Service Reply Message Format 24
7. Service Type Request Message Format 25
8. Service Type Reply Message Format 27
9. Service Registration Message Format 28
10. Service Acknowledgement Message Format 31
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11. Service Deregister Message Format 32
12. Attribute Request Message Format 33
13. Attribute Reply Message Format 35
14. Directory Agent Advertisement Message Format 37
15. Directory Agents 38
15.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 38
15.2. Finding Directory Agents . . . . . . . . . . . . . . . . 38
16. Scope Discovery and Use 40
17. Language and Character Encoding Issues 41
17.1. Character Encoding and String Issues . . . . . . . . . . 42
17.1.1. Substitution of Character Escape Sequences . . . 43
17.2. Language Dialect . . . . . . . . . . . . . . . . . . . . 43
17.3. Language-Independent Strings . . . . . . . . . . . . . . 44
18. Service Location Transactions 44
18.1. Service Location Connections . . . . . . . . . . . . . . 44
18.2. No Synchronous Assumption . . . . . . . . . . . . . . . . 45
18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . . 45
19. Security Considerations 45
20. String Formats used with Service Location Messages 46
20.1. Previous Responders' Address Specification . . . . . . . 47
20.2. Formal Definition of the ``service:'' Scheme . . . . . . 47
20.2.1. Service Type String . . . . . . . . . . . . . . . 48
20.3. Attribute Information . . . . . . . . . . . . . . . . . . 48
20.4. Address Specification in Service Location . . . . . . . . 49
20.5. Attribute Value encoding rules . . . . . . . . . . . . . 49
21. Implementation Requirements 50
22. Configurable Parameters and Default Values 54
22.1. Service Agent: Use Predefined Directory Agent(s) . . . . 55
22.2. Time Out Intervals . . . . . . . . . . . . . . . . . . . 56
23. Non-configurable Parameters 56
24. Acknowledgments 57
A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for
the representation of names of languages" 58
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B. Appendix: For Further Reading 59
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 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 is not a global resolution
system for the entire Internet; rather it is intended to serve
enterprise networks with shared services. 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 acquire service
attributes and configuration. The User Agent 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 attributes and configuration.
Service Information
A collection of attributes and configuration information
associated with a single service. The Service Agents
advertise service information for a collection of service
instances.
Service
The service is a process or system providing a facility
to the network. The service itself is accessed using
a communication mechanism external to the the Service
Location Protocol.
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Directory Agent (DA)
A process which collects information from Service Agents
to provide a single repository of service information
in order to centralize it for efficient access by User
Agents. 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 including expected
attributes, values and protocol behavior.
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
interpreted as a boolean, integer or opaque value if it
takes specific forms (see section 20.5).
Predicate
A boolean expression of attributes, relations and logical
operators. The predicate is used to find services which
satisfy particular requirements. See section 5.3.
Scope
A collection of services that make up a logical group.
See sections 16 and 3.7.
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.
Address Specification
This is the network layer protocol dependent mechanism
for specifying an Agent. For Internet systems this is
part of a URL (Universal Resource Locator - see [7]).
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2.1. Notation Conventions
CAPS Strings which appear in all capital letters are protocol
literal. All string comparison is case insensitive,
however, (see section 5.5). 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.
| |
\ \ Message layouts with this notation indicate a variable
| | length field.
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 also human readable and well defined. Only
the datagram headers are encoded in a form which is not human
readable. Strings used in the Service Location Protocol are NOT
null-terminated.
Predicates are expressed in a simple boolean notation using keywords,
attributes, and logical connectives, as described in Section 5.4.
The logical connectives and subexpressions are presented in
prefix-order, so that the connective comes first and the expressions
it operates on follow afterwards.
2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
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SHOULD This word, or the adjective "recommended", means
that, in some circumstances, valid reasons may exist
to ignore this item, but the full implications must
be understood and carefully weighed before choosing
a different course. Unexpected results may result
otherwise.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST
be prepared to interoperate with another implementation
which does include the option.
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.
3. Protocol Overview
The basic operation in Service Location is that a client attempts
to discover the location of a Service. In smaller installations,
each service will be configured to respond individually to each
client. In larger installations, services will register their
services with one or more Directory Agents, and clients will
contact the Directory Agent to fulfill requests for Service Location
information. Clients may discover the whereabouts of a Directory
Agent by preconfiguration, DHCP [2, 10], or by issuing queries to the
Directory Agent Discovery multicast address.
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3.1. Protocol Transactions
The diagram below illustrates the relationships described below:
+---------------+ we want this info: +-----------+
| Application | - - - - - - - - - - - -> | Service |
+---------------+ +-----------+
/|\ | |
| +-------------+ |
| | |
\|/ \|/ \|/
+---------------+ +-----------+ +----------------+
| User Agent |<-------->| Service | | Service |
+---------------+ | Agent | | Agent which |
| +-----------+ | does not reply |
| | | to UA requests |
| \|/ +----------------+
| +-------------+ |
+------------------>| Directory |<----------+
| Agent |
+-------------+ ___________
/|\ / Many other\
+------------>| SA's |
\___________/
The following describes the operations a User Agent would employ
to find services on the site's network. The User Agent needs no
configuration to begin network interaction. The User Agent can
acquire information to construct predicates which describe the
services that match the user's needs. The User Agent may build on
the information received in earlier network requests to find the
Service Agents advertising service information.
A User Agent will operate two ways: If the User Agent has already
obtained the location of a Directory Agent, the User Agent will
unicast a request to it in order to resolve a particular request.
The Directory Agent will unicast a reply to the User Agent. The
User Agent will retry a request to a Directory Agent until it gets
a reply, so if the Directory Agent cannot service the request (say
it has no information) it must return an response with zero values,
possibly with an error code set.
If the User Agent does not have knowledge of a Directory Agent or if
there are no Directory Agents available on the site network, a second
mode of discovery is used. The User Agent multicasts a request to
the service-specific multicast address, to which the service it
wishes to locate will respond. All the Service Agents which are
listening to this multicast address will respond, provided they can
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satisfy the User Agent's request. A similar mechanism is used for
Directory Agent discovery; see section 5.2. Service Agents which
have no information for the User Agent MUST NOT respond.
When a User Agent wishes to obtain an enumeration of ALL services
which satisfy the query, a retransmission/convergence algorithm is
used. The User Agent resends the request, together with a list of
previous responders. Only those Service Agents which are not on
the list respond. Once there are no new responses to the request
the accumulation of responses is deemed complete. 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 3.7 should be used.
While the multicast/convergence model may be important for
discovering services (such as Directory Agents) it is the exception
rather than the rule. Once a User Agent knows of the location of a
Directory Agent, it will use a unicast request/response transaction.
The Service Agent SHOULD listen for multicast requests on the
service-specific multicast address, and MUST register with an
available Directory Agent. This Directory Agent will resolve
requests from User Agents as described above. This means that
a Directory Agent must first be discovered, using the multicast
mechanism described above.
A Service Agent which does not respond to multicast requests will not
be useful in the absence of Directory Agents. Some Service Agents
may not include this functionality, if an especially light-weight
implementation is required.
If the service is to become unavailable, it should be deregistered
with the Directory Agent. The Directory Agent responds with an
acknowledgment to either a registration or deregistration. Service
Registrations include a Lifetime value and will eventually expire.
Service Registrations need to be refreshed by the Service Agent
before their Lifetime runs out.
3.2. Schemes
The Service Location Protocol, designed as a way for clients to
access resources on the network, is a natural application for
Universal Resource Locators (URLs). It is intended that by re-using
URL specification and technology from the World Wide Web, clients and
servers will be more flexible and able to be written using already
existing code. Moreover, it is hoped that browsers will be written
to take advantage of the similarity in locator format, so that a
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client can dynamically formulate requests for services that are
resolved differently depending upon the circumstances. There is
the possibility for beneficial interaction between Directory Agents
and Web Browsers which we wish to facilitate by means of compatible
locator format.
3.2.1. The ``service:'' URL scheme
The service URL scheme is used by Service Location. It is used to
specify a Service Location. Many Service Types will be named by
including a scheme name after the ``service:'' scheme name. Service
Types are used by SAs to register and deregister Services with DAs.
It is also used by SAs and DAs to return Service Replies to UAs. The
formal definition of the ``service:'' URL scheme is in section 20.2.
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.
Well known Service Types are registered with the IANA and templates
are available as RFCs. Private Service Types may also be supported.
3.3. Standard Attribute Definitions
Service Types used with the Service Location Protocol must describe
the following:
Service Type string of the service
Service-specific multicast address, if used
Attributes and Keywords
Attribute Descriptions and interpretations
Service Types note registered with IANA will use their own Naming
Authority string and, possibly, a service-specific multicast
address from the unassigned range. This is only an option for
a site-specific deployment, as there may be conflicts with this
multicast address somewhere, in some other site.
If a service-specific multicast address is not supplied by a
standards document registered with IANA, nor is a site specific
address being used, the Service Location General Multicast address
is the default. All Service Agents SHOULD listen to this address,
especially if they have not registered their service information
with any Directory Agent. The service-specific multicast address is
merely used for efficiency and is not strictly needed for correct
operation.
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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 User Agents.
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 which is implied in its definition. If a Service Type
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.
See section 20.2.1 for the format of a Service Type String as used in
the Service Location Protocol.
3.4. 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 9 and 5.
3.5. 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
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does not allow a User Agent 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. These configuration attributes will be used by
applications which interpret the Service Location Reply.
3.6. Use of TCP, UDP and Multicast in Service Location
The Service Location Protocol requires the implementation of
connectionless and a connection oriented transport protocols. The
latter is used for bulk transfer, only when necessary. Connections
are always initiated by an agent request or registration, not
by a replying Directory Agent. The Service Location discovery
mechanisms use possibly internetwork-wide multicast. The protocol
will operate in a broadcast environment with limitations detailed in
section 3.6.1.
3.6.1. Multicast vs. Broadcast
The Service Location Protocol was designed for use in networks where
multicast at the network layer is supported; in some instances
multicast may not be supported. To support this protocol in networks
where multicast is not supported the following modifications are
made to support the protocol in an environment where network layer
broadcast is supported.
3.6.1.1. Single Subnet
If a network is not connected to any other networks simple network
layer broadcasts will work in place of multicast.
Service Agents SHOULD and Directory Agents 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.
3.6.1.2. Multiple Subnets
The Directory Agent provides a central clearing house of information
for User Agents. If the network is designed so that a Directory
Agent address is statically configured with each User Agent
and Service Agent, the Directory Agent will act as a bridge for
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information that resides on different subnets. The Directory Agent
address can be dynamically configured with Agents using DHCP or
staticly configured, but Agents will not be able to discover DAs on
non-bridged subnets.
As dynamic discovery is not feasible in a broadcast environment with
multiple subnets and manual configuration is difficult, deploying
multiple DAs in multiple subnets will require use of multicast
discovery with multiple hops (i.e., TTL > 1 in the IP header).
3.6.2. Service-Specific Multicast Address
Each service type MAY have a unique multicast address which is
expected to be used for discovering services of that type. This
multicast address may be obtained from the naming authority (e.g.,
IANA). This mechanism is used so that the number of datagrams any one
service agent receives is minimized. The Service Location General
Multicast Address may be used to query for any service, though one
should use the service-specific multicast address if it exists.
If the site network does not support multicast then the query
should be broadcast to the Service Location port. If the underlying
hardware will not support the number of needed multicast addresses
the Service Location General Multicast Address may be used.
Service Agents listen on this multicast address as well as the
service-specific multicast addresses for the service types they
advertise.
3.7. Service Location Scaling, and Multicast Operating Modes
In a very small network, with few nodes, no DA is required. A User
Agent can detect services by multicasting requests. Service Agents
will then reply to them. Further, Service Agents which respond to
user requests must be used to make service information available.
This does not scale to environments with many hosts and services.
When scaling Service Location systems to intermediate sized networks,
a central repository (Directory Agent) may be added to reduce the
number of Service Location messages transmitted in the network
infrastructure. Since the central repository can respond to
all Service and Attribute Requests, fewer Service and Attribute
Replies will be needed; for the same reason, there is no need to
differentiate between Directory Agents.
A site may also grow to such a size that it is not feasible to
maintain only one central repository of service information. In this
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case more Directory Agents are needed. The services (and service
agents) advertised by the several Directory Agents are collected
together into logical groupings called "Scopes".
All Service Registrations that have a Scope must be registered with
all DAs (within the appropriate multicast radius) of that Scope which
have been or are subsequently discovered. Service Registrations
which have no Scope are only registered with unscoped DAs. User
Agents make requests of DAs whose Scope they are configured to use.
Service Agents MUST register with unscoped DAs even if they are
configured to specifically register with DAs which have a specific
Scope or set of Scopes. User Agents MAY query DAs without Scopes,
even if they are configured to use DAs with a certain Scope. This
is because any DA with no Scope will have all the available service
information.
Scoped User Agents SHOULD always use a DA which supports their
configured Scope when possible instead of an unscoped DA. This will
prevent the unscoped DAs from becoming overused and thus a scaling
problem.
It is possible to specially configure Service Agents to register
only with a specific set of DAs (see Section 22.1). In that case,
services may not be available to User Agents via all Directory
Agents, but some network administrators may deem this appropriate.
There are thus 3 distinct operating modes. The first requires no
administrative intervention. The second requires only that a DA be
run. The last requires that all DAs be configured to have Scope and
that a coherent strategy of assigning Scopes to services be followed.
Users must be instructed which Scopes are appropriate for them to
use. This administrative effort will allow users and applications to
subsequently dynamically discover services without assistance.
The first mode (no DAs) is intended for a LAN. The second mode
(using a DA or DAs, but not using Scopes) scales well to a group
of interconnected LANs with a limited number of hosts. The third
mode (with DAs and Scopes) allows the SLP protocol to be used in an
internetworked campus environment.
If Scoped DAs are used, they will not accept unscoped registrations
or requests. UAs which issue unscoped requests will discover only
unscoped services. They SHOULD use a Scope in their requests if
possible and SHOULD use a DA with their Scope in preference to an
unscoped DA. In a large campus environment it would be a bad idea to
have ANY unscoped DAs: They attract ALL registrations and will thus
present a scaling problem eventually.
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A subsequent protocol document will describe mechanisms for
supporting a service discovery protocol for the global Internet.
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| rsvd | Dialect | Language Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Char Encoding | XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version This protocol document defines version 1 of the Service
Location protocol.
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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 SrvReq 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 18 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 17) that is indicated by the
Service or Attribute Request.
rsvd MUST be zero.
Dialect Dialect tags are used in Service Location messages to
indicate a variant of vocabulary used.
Language Code
Strings within the remainder of the message which follows
are to be interpreted in the language encoded (see
appendix A) in this field. This header field MUST be
set. See also section 17.
Character Encoding
The characters making up strings within the remainder of
the message may be encoded in any standardized encoding
(see section 17.1). This header field MUST be set.
Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to
match replies to individual requests (see section 4.1).
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When URLs are registered, they have lengths and lifetimes. These
two values are associated with the URL for the duration of the
registration. The triplet (length, lifetime, URL) is known as a
"URL-entry", and has the following format when used in Service
Replies and Service Registrations:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The URL conforms to RFC 1738 [7]. If the scheme used in the URL does
not have a standardized representation, the minimal requirement is:
service:<srvtype>://<addr-spec>
The "SERVICE" string is the URL scheme of all Service Location
Information included in Service Registrations and Service Replies.
Each entry in the Reply will always have a <srvtype>. It may also
include an <addr-spec> except in the case of a reply to a Service
Type request (see section 7).
4.1. Use of Transaction IDs (XIDs)
Retransmission is used to ensure reliable transactions in the
Service Location Protocol. If a User Agent or Service Agent sends
a message and fails to receive an expected response, the message
will be sent again. Retransmission of the same Service Location
datagram 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_INTERVAL_0.) Any registration or deregistration at
a Directory Agent, 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 back to zero and continue incrementing from there.
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Directory Agents use XID values in their DA Advertisements to
indicate their state (see section 15.2).
4.2. 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. User Agents 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_INTERVAL_1. 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 Request Message Format
The Service Request is used to obtain URLs from a Directory Agent or
Service Agents.
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 = SrvReq) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|length of prev resp list string|<Previous Responders Addr Spec>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of predicate string | Service Request <predicate> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Service Request <predicate>, contd. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responders Addr Spec> is described in sections 7
and 20.1.
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After a User Agent restarts (say, after rebooting of a system,
loading of the network kernel), Service Requests should be delayed
for some random time uniformly distributed within a one second
interval centered about a configured delay value (by default,
CONFIG_INTERVAL_4).
The Service Request allows the User Agent to specify the Service Type
of the service and a Predicate in a specific language. The general
form of a Service Request is shown below:
<srvtype>[.<na>]/[<scope>]/[<where>]/
The punctuation is necessary even where the fields are omitted.
- The <srvtype> refers to the Service Type. For each type of
service available, there is a unique Service type name string.
See section 20.2.1.
- The <na> is the Naming Authority. This string determines the
semantic interpretation of the attribute information in the
<where> part of the Service Request.
- The <scope> is a string used to restrict the range of the query.
Scope is determined administratively, at a given site. It is
not necessarily related to network topology (see Section 16).
Leaving this field out means that the request can be satisfied
only by unscoped service advertisements.
- The <where> string is the Where Clause of the request. It
contains query which specify which service instances the User
Agent is interested in. The query includes attributes, boolean
operators and relations. (See section 5.3.)
In the case of a multicast Service request, 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. The request is multicast repeatedly (with a recommended
wait interval of CONFIG_INTERVAL_2) until there are no new responses,
or a certain time (CONFIG_INTERVAL_3) has elapsed. Different timing
values are applied to a Service Request used for Directory Agent
Discovery, see Section 5.2.
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.
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3. It must have the same Scope. (If the <scope> of the request
was omitted, the request will only match services which were
registered with no Scope. Note that a Scoped request WILL match
all unscoped Services).
4. The conditions specified in the Where Clause must match the
attributes and keywords registered for the service.
5.1. Service Request Usage
The User Agent may form Service Requests using preconfigured
knowledge of a Service Type's attributes. It may also issue
Attribute Requests to obtain the attribute values for a Service Type
before issuing Service Requests (see Section 12). Having obtained
the attributes which describe a particular kind of service from an
Attribute Request, (or using configured knowledge of a service's
attributes,) the User Agent can build a predicate that describes the
service needs of the user.
Service Requests may be sent directly to a Directory Agent. 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:
lpr//(& (PAGES PER MINUTE==12)
(UNRESTRICTED_ACCESS)
(LOCATION==12TH FLOOR))/
Suppose there is no such printer. The Directory Agent responds
with a Service Reply with 0 in the number of responses and no reply
values.
The User Agent then tries a less restrictive query to find a printer,
using the 12th floor as "where" criteria.
lpr//(LOCATION==12TH FLOOR)/
In this case, there is now only one reply:
Returned URL: service:lpr://igore.wco.ftp.com:515/draft
The Address Specification for the printer is: igore.wco.ftp.com:515.
This is the location of the printer. Files would be printed by
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spooling to that port on that host. The word 'draft' refers to the
name of the print queue the lpr server supports.
In the absence of a Directory Agent, the request above could
be multicast. In this case it would be sent to the printer
Multicast Address and not to the Directory Agent. Service Agents
that can satisfy the predicate will reply. Service Agents which
cannot support the character set of the request MUST return
CHARSET_NOT_UNDERSTOOD in the SrvRply. In all other circumstances,
Service Agents which cannot satisfy the reply do not send any reply
at all.
The only way a User Agent can be sure there are no services which
match the query is by retrying the request (CONFIG_INTERVAL_8). If
no response comes, the User Agent gives up and assumes there are no
such printers.
Another form of query is a simpler 'join' query. Its syntax has no
parentheses or logical operators. Each term is conjoined (AND-ed
together.) Rewriting the initial query provides an example:
lpr//PAGES PER MINUTE==12,
UNRESTRICTED_ACCESS,
LOCATION==12TH FLOOR/
5.2. Directory Agent Discovery Request
Normally a Service Request returns a Service Reply. The sole
exception to this is a Service Request for the Service Type
"directory-agent". This Service Request is answered with a DA
Advertisement.
Without configured knowledge of a Directory Agent (DA), a User Agent
or Service Agent uses a Service Request to discover a DA. (See
section 15.1 for mechanisms by which a client may be configured to
have knowledge of a DA.) Such a Service Request used for Directory
Agent Discovery includes a predicate of the form:
directory-agent///
This query is always sent to the Directory Agent Discovery multicast
address. The Service Type of a Directory Agent is "directory-agent",
hence it is the Service Type used in the request. No Scope is
included in the request, so all Directory Agents will reply. This is
the only request which omits a Scope which all Directory Agents MUST
respond to. Normally, a Directory Agent with a Scope ONLY responds
to requests with that Scope. No Naming Authority is included, so
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"IANA" is assumed. We want to reach all the available directory
agents. If the Scope were supplied, only DAs supporting that Scope
would reply.
DA Advertisement Replies may arrive from different sources, similar
in form to:
URL returned: service:directory-agent://slp-resolver.catch22.com
Scope returned: ACCOUNTING
URL returned: service:directory-agent://204.182.15.66
Scope returned: JANITORIAL SERVICES
The DA Advertisement format is defined in Section 14.
If the goal is merely to discover any Directory Agent, the first
reply will do. If the goal, however, is to discover all reachable
DAs, the request must be retransmitted after an interval (the
recommended time is CONFIG_INTERVAL_5). This retransmitted request
will include a list of DAs which have already responded. See
sections 7 and 20.1. Directory Agents which receive the request will
only respond if they are not on this list. After there are no new
replies, all DAs are presumed to have been discovered.
If a DA fails to respond after CONFIG_INTERVAL_6 seconds, the UA or
Service Agent should use a different DA. DA addresses may be cached
from previous discovery attempts, preconfigured, or by use of DHCP
(see section 15.2). If no such DA responds, DA discovery should be
used to find a new DA. Only after CONFIG_INTERVAL_7 seconds should
it be assumed that no DA exists and multicast based Service Requests
should be used.
5.3. 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 terms used in the grammar
are as follows:
predicate:
Placed in a Service Request, this is interpreted by a Service
Agent or Directory Agent to determine what information to
return.
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scope:
If this is absent in a Service Request, the request will match
only services registered without a Scope. If it is present,
only services registered under that Scope or are unscoped will
match the request.
where-clause:
This determines which services the request matches. An empty
where-clause will match all services. The request will be
limited to services which have the specified Service Type, so
the where-clause is not the sole factor in picking out which
services match the request.
where-list:
The where-list is a logical expression. It can be a single
expression, a disjunction or a conjunction. A single
expression must apply for the where-clause to match. A
disjunction matches if any expression in the OR list matches.
A conjunction matches only if all elements in the AND list
match.
Note that there is no logical negation operator: This is
because there is no notion of returning "everything except"
what matches a given criteria.
A where-list can be nested and complex. For example, the
following requires that three subexpressions must all be true:
(& (| <query-item> <query-item>)
<query-item>
(& <query-item> <query-item> <query-item>)
)
Notice that white space, tabs or carriage returns can be added
anywhere outside query-items. Each list has 2 or more items in
it, and lists can be nested. Services which fulfill the entire
logical expression match the where-clause.
'(' '|' <query-item> ')' and '(' '&' <query-item> ')' are
degenerate expressions but they should be tolerated. They are
equivalent to <query-item>.
query-item:
A query item has the form:
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'(' <attr-tag> <comp-op> <attr-val> ')'
or
'(' <keyword> ')'
Examples of this would be:
(SOME ATTRIBUTE == SOME VALUE)
(RESERVED)
(QUEUE LENGTH <= 234)
query-join:
The query-join is a comma delimited list of conditions which
the service must satisfy in order to match the query. The
items are considered to be logically conjoined. Thus the
query-join:
ATTR1=VALUE1, KEYWORD1, KEYWORD2, ATTR2>=34
is equivalent to the where-list:
(& (ATTR1=VALUE1) (KEYWORD1) (KEYWORD2) (ATTR2>=34))
The query-join cannot be mixed with a where-list. It is
provided as a convenient mechanism to provide a statement of
necessary conditions without building a logical expression.
5.4. Service Request Predicate Grammar
Service Requests can precisely describe the services they need by
including a Predicate the body of the Request. This Predicate must
be constructed according to the grammar below.
<predicate> ::= <srvtype>['.'<na>]'/'<scope>'/'<where>'/'
<srvtype> ::= string representing type of service. Only
'a' to 'z', 'A' to 'Z', '+' and '-' are allowed.
<na> ::= string representing the Naming Authority.
Only characters from 'a' to 'z', 'A' to 'z',
'+' and '-' are allowed. If this field is
omitted then "IANA" is assumed.
<scope> ::= string representing the directory agent scope.
'/', ',' (comma) and ':' are not allowed in
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this string. The scopes "LOCAL" and "REMOTE"
are reserved.
<attr-tag> ::= class name of an attribute of a given Service
Type. This tag cannot include the following
characters: '(', ')', ',', '=', '!', '>',
'<', '/', '*', except where escaped (see 17.1.)
<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>. In addition white space
internal to the keyword is illegal.
<where> ::= <where-any> |
<where-list> |
<query-join>
<where-any> ::=
That is NOTHING or white space.
<where-list> ::= '(' '&' <where-list> <query-list> ')' |
'(' '|' <where-list> <query-list> ')' |
'(' <keyword> ')'
'(' <attr-tag> <comp-op> <attr-val> ')'
<query-list> ::= <where-list> |
<where-list> <query-list>
<query-join> ::= <keyword> |
<join-item> |
<query-join> ',' <keyword> |
<query-join> ',' <join-item>
<join-item> ::= <attr-tag> <comp-op> <attr-val>
<comp-op> ::= "!=" | "==" | '<' | "<=" | '>' | ">="
<attr-val> ::= any string (see Section 20.5 for the ways
in which attr-vals are interpreted.)
Value strings may not contain '/', ','
'=', '<', '>', except where escaped (see 17.1.).
'(' and ')' may be used in attribute values
for the purpose of encoding a binary values.
Binary encodings (See 20.5) may
include the above reserved characters.
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5.5. String Matching for Requests
All strings are case insensitive, with respect to string matching on
queries. All preceding or trailing blanks should not be considered
for a match, but blanks internal to a string are relevant.
For example, " Some String " matches "SOME STRING",
but not "some string".
String matching may only be performed over the same character sets.
If a request cannot be satisfied due to a lack of support for the
character set of the request a CHARSET_NOT_UNDERSTOOD error is
returned.
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.
String matching is done after escape sequences have been substituted.
See sections 17, 5.3, 17.1.
6. 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 | number of replies returned |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry>-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry>-N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each Service Reply message is composed of a list of URL Entries.
The Error Code may have one of the following values:
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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 and the request arrived
with the Monolingual bit set. See Section 17.
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvReq is received
which cannot be parsed.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a Scope will return this
error if it receives a request which is set to have a
Scope which it does not support. An SA will not return
this error, it will simply not reply to the multicast
request.
CHARSET_NOT_UNDERSTOOD
If the DA or SA receives a request or registration in a
character set which it does not support, it will return
this error.
Each <URL Entry> in the list has the form defined at the end of
Section 4.
The URLs in the reply have no delimiters between them, other than the
length fields. The length fields indicate where the strings end.
7. Service Type Request Message Format
The Service Type Request is used to determine all the types of
services supported on a network.
The request should be sent directly to a DA (though it may also be
sent to the Service Location General Multicast Address), in order
to find out all services available on the site network (which are
advertised by Directory Agents and Service Agents.) If no DA is
available, a User Agent MAY issue more than one request to insure
that all replies have been received. In each subsequent request, a
User Agent included those Service Types that it is aware of. When no
new replies arrive within CONFIG_INTERVAL_3 from a request, the User
Agent can presume that it has acquired a complete set of available
Service Types.
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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>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of naming authority | <Naming Authority String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Naming Authority String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Scope String | <Scope String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that the <Previous Responders Addr Spec> is a comma delimited
list. (See section 20.1.) The 'length of prev responder list' field
indicates the length of the comma delimited list string. A previous
responder list with 3 elements takes this form:
<addr-spec>,<addr-spec>,<addr-spec>
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.
The Scope String Field, if included, will limit replies to Service
Types which have the specified Scope or are unscoped. If this field
is omitted, all Service Types (from the specified Naming Authority)
are returned.
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8. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item>-1 \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type Item>-N \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of a Service Type Item 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of Service Type String | <Service Type String> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Service Type String>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of addr spec string | <addr spec> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <addr spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A SA or DA returns this error when a SrvTypeRqst is
received which cannot be parsed.
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SCOPE_NOT_SUPPORTED
A DA which is configured to have a Scope will return this
error if it receives a SrvTypeRqst which is set to have a
Scope which it does not support. An SA will not return
this error, it will simply silently discard the multicast
request.
CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvTypeRqst in a character set which
it does not support, it MUST this error.
The service type's name is provided in the <Service Type String>.
See section 20.2.1 for the formal definition of this field. The
<addr spec> format is described in 20.4. This field provides
the service specific multicast address. If the service specific
multicast address is omitted, the General Service Location Multicast
Address is assumed. User Agents may then use this multicast address
for issuing Service and Attribute Requests directly to SAs.
Example Service Type Replies might be:
Multicast Address Service Type String
224.0.3.10 service:lpr://
224.0.3.24 service:http://
224.0.3.115 service:nfs://
NOTE: These multicast addresses are examples only, the official
numbers have not yet been assigned.
9. Service Registration Message Format
After a Service Agent has found a Directory Agent, it begins to
register its advertised services one at a time. A Service Agent
must wait for some random time uniformly distributed within the
range specified by CONFIG_INTERVAL_11 before registering again.
Registration is done using the Service Registration message
specifying all attributes for a service. A Directory Agent must
acknowledge each service registration request.
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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 Attr List String | <attr-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <URL-Entry> is defined at the end of Section 4. The <attr-list>
is defined in Section 20.3.
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 Service Agent MUST use a connection oriented protocol
to register itself with the DA. When a Service Agent registers the
same attribute class more than once for a service instance, the
Directory Agent 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.
An example of Service Registration information is:
Lifetime (seconds): 16-bit unsigned integer
URL (at least): service:<srvtype>://<addr-spec>
Attributes (if any): (ATTR1=VALUE),KEYWORD,(ATTR2 = VAL1, VAL2)
In order to offer continuously advertised services, Service Agents
should start the reregistration process before the Lifetime they used
in the registration expires.
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An example of a service registration (valid for 3 hours) is as
follows:
Lifetime: 10800
URL: service:lpr://igore.wco.ftp.com:515/draft
Attributes: (SCOPE=DEVELOPMENT),
(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", "service", and "SCOPE" 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
Attributes: (SCOPE=ENTWICKLUNG),
(PAPIERFARBE=WEISS),
(PAPIERFORMAT=BRIEF),
UNBEGRENTZTER_ZUGANG,
(DRUECKERSPRACHE=POSTSCRIPT,HPGCL),
(STANDORT=11 ETAGE)
Registrations must contain an Attribute of SCOPE unless they are
unscoped and then they must be registered with all unscoped Directory
Agents.
Registrations of a previously registered service are considered an
update. 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:
service:x://a.org has been registered with attributes A=1, B=2, C=3.
A new registration comes for service:x://a.org with attributes C=30,
D=40. 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 17.3.)
The Directory Agent may return a server error in the acknowledgment.
This error is carried in the Error Codes field of the service
location message header. A Directory Agent MUST decline to register
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a service if it is specified with an unsupported Scope. In this case
a SCOPE_NOT_SUPPORTED error is returned in the SrvAck. A Directory
Agent MUST NOT accept Service Registrations which have an unsupported
Scope unless it is an unscoped Directory Agent, in which case it MUST
accept all Service Registrations.
An unscoped Service Registration will match all requests. A request
which specifies a certain scope will therefore return services which
have that Scope and services which are unscoped. It is strongly
suggested that one should use Scopes in all registrations or none.
See Sections 16 and 3.7 for details.
10. 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
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 could
not be parsed.
INVALID_REGISTRATION
A DA returns this error when a SrvReg is invalid (it
parses badly or is poorly formed in some way.)
SCOPE_NOT_SUPPORTED
A DA which is configured to have a Scope will return this
error if it receives a SrvReq which is set to have a
Scope which it does not support.
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CHARSET_NOT_UNDERSTOOD
If the DA receives a SrvReg or SrvDereg in a character
set which it does not support, it will return this error.
AUTHENTICATION_FAILED
If the DA uses IP Security Authentication and the
SA sending a SrvReg or SrvDereg message fails to be
authenticated, the DA will return this error.
11. Service Deregister Message Format
When a service is no longer available for use, the Service Agent must
deregister itself from Directory Agents that it has been registered
with. A service uses the following PDU to deregister itself.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL of Service to Deregister, contd. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <tag spec> string | <tag spec> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <tag spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Service Agent should retry this operation if there is no response
from the Directory Agent. The Directory Agent acknowledges this
operation with a service acknowledgment. Once the Service Agent
receives an acknowledgment indicating success, it can assume that the
service is no longer advertised by the Directory Agent. The Error
Code in the Acknowledgment of the Service Deregistration may have the
same values as described in section 10.
The Service Deregister Information sent to the Directory Agent has
the following form:
service:<srvtype>://<addr-spec>
Attribute tags (if any): ATTR1,KEYWORD,ATTR2
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This will deregister the specified attributes from the service
information from the Directory Agent. If 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
12. 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, and 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 User Agent 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 include a 'select clause'. This may be used to
limit 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 ben registered for that
URL or Service Type, the attribute or keyword information is simply
not returned.
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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>|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Previous Responders Addr Spec>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of URL | URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <Scope> | <Scope> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length of <select-list> | <select-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <select-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <Previous Responder Address List> functions exactly as introduced
in Section 7. See also Section 20.1.
The URL can take two forms: Either it is simply a Service
Type, such as "service:http:", or it can be a URL, such as
"service:lpr://igore.wco.ftp.com:515/draft". In the former case, all
attributes and the full range of values for each attribute for the
Service Type is returned. In the latter case, only the attributes
for the service whose URL is defined are returned.
The Scope String is provided so that Attribute Requests for Service
Types can be made so that only the Attribute information pertaining
to a specific Scope will be returned. This field is ignored in the
case when a full URL is sent in the Attribute Request. The rules for
encoding of the Scope String are given in Section 5.4.
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The select list takes the form:
<select-list> ::= <select-item> |
<select-item> ',' <select-list>
<select-item> ::= <keyword> | <attr-tag> | <partial-tag> '*'
<partial-tag> ::= the partial class name of an attribute
followed by an '*' matches all class names
which begin with the characters preceding
the '*'
For definitions of <attr-tag> and <keyword> see 5.4.
An example of a select-list following the printer example is:
PAGES PER MINUTE, UNRESTRICTED_ACCESS, LOCATION
If sent to a Directory Agent, the number of previous responders is
zero and there are no Previous Responder Address Specification.
These fields are only used for repeated multicasting, exactly as for
the Service Request.
13. 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> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code may have the following values:
0 Success
LANGUAGE_NOT_SUPPORTED
A SA or DA returns this when a request is received
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from a UA which is in a language for which there is no
registered Service Information and the request arrived
with the Monolingual bit set. See Section 17.
PROTOCOL_PARSE_ERROR
A DA or SA returns this error when the AttrRqst could not
be parsed.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a Scope will return this
error if it receives an AttrRqst which is set to have
a Scope which it does not support. SAs will silently
discard multicast AttrRqst messages for Scopes they do
not support.
CHARSET_NOT_UNDERSTOOD
If the DA receives an AttrRqst in a character set which
it does not support, it will return this error. SAs will
silently discard multicast AttrRqst messages which arrive
using character sets they do not support.
The <attr-list> (attribute list) has the same form as the attribute
list in a Service Registration, see Section 20.3 for a formal
definition of this field.
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),
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14. Directory Agent Advertisement Message Format
Directory Agent 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of <Scope-list> | <Scope-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <Scope-list>, continued \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code is set when a DA Advertisement is returned as the
result of a Service Request. It will always be set to 0 in the case
of an unsolicited DA Advertisement. The Error Code may take the
values specified in Section 6.
The URL corresponds to the Directory Agent's location. The
<Scope-list> is a comma delimited list of Scopes which the DA
supports, in the following format:
<Scope-list> ::= <Scope> | <Scope-list> ',' <Scope>
<Scope> ::= String representing a scope
See Section 5.4 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
The Directory Agent can be reached at the Address Specification
returned, and supports the SCOPE called "ADMIN".
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15. Directory Agents
15.1. Introduction
A Directory Agent acts on behalf of many Service Agents. It acquires
information from them and acts as a single point of contact to supply
that information to User Agents.
The queries that a User Agent multicasts to Service Agents (in an
environment without a Directory Agent) are the same as queries that
the User Agent might unicast to a Directory Agent. A User Agent may
cache information about the presence of alternate Directory Agents to
use in case a selected Directory Agent fails.
Aside from enhancing the scalability of the protocol (see
section 3.7), running multiple DAs provides robustness of operation.
The DAs have replicated service information which remain accessible
even when one of the DAs fail. Directory Agents, 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 enterprise networks or larger administrative domains.
Each Service Agent must register with all DAs they are configured to
use. UAs may choose among DAs they are configured to use.
Locally, Directory Agent consistency is guaranteed using mechanisms
in the protocol. There isn't any Directory to Directory Agent
protocol yet. Rather, passive detection of DAs by SAs ensures that
eventually service information will be registered consistently
between DAs. Invalid data will age out of the Directory Agents
leaving only transient stale registrations even in the case of a
failure of a Service Agent.
15.2. Finding Directory Agents
A User or Service Agent 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 [2, 10] may use it to obtain a
Directory Agent's address. A DHCP option will be assigned for this
purpose. It has not yet been, at the time this document was written.
The third way to discover DAs is dynamically. This occurs actively
by sending out a Directory Agent Discovery request (see Section 5.2).
Lastly, the agent may be informed passively as follows:
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When a Directory Agent first comes on-line it sends an unsolicited DA
Advertisement to the Service Location general multicast address. If
a DA supports a particular Scope or set of Scopes these are placed in
the reply. The class for this attribute is 'SCOPE'.
Every CONFIG_INTERVAL_9 a Directory Agent will send an unsolicited
DA Advertisement again. This will ensure that eventually it will be
discovered by all applications which are concerned.
When a Directory Agent first comes up it begins with 0 as its XID,
and increments this by one each time it sends an unsolicited DA
Advertisement. When the counter wraps, it should go from 0xFFFF to
0x0100, not 0.
If the Directory Agent has stored all of the service information in
a nonvolatile store, it should initially set the XID to 0x100, as it
is not coming up 'stateless.' If it stores service registrations in
memory only, it will restart without any state. It should indicate
this by resetting its XID to 0.
All Service Agents which receive the unsolicited DA Advertisement
should examine its XID. If the Directory Agent has never before
been heard from or if the XID is less than it was previously and
less than 256, the Service Agent 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 Directory Agent, after waiting a random interval
CONFIG_INTERVAL_10.
When a Service Agent or User Agent first comes on-line it must issue
a Directory Agent Discovery Request unless it is using static or DHCP
configuration, as described in 5.2.
A Service Agent registers information with ALL newly discovered
Directory Agents when either of the above two events take place.
When Scopes are being used, a Service Agent SHOULD choose a set of
Scopes to be advertised in and need only register with Directory
Agents that support the Scopes in which they wish to be registered.
Services MUST be registered with DAs that support their Scope and
those which have no Scope, unless specifically configured not to do
so (see section 22.1.)
Once a User Agent becomes aware of a Directory Agent it will unicast
its queries there. In the event that more than one Directory Agent
is detected, it will select one to communicate with. When Scopes
are supported, the User Agent will direct its queries to different
Directory Agents depending on which Scopes are appropriate domains
for the query to be answered in.
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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.
16. 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 of the system can be presented with these organizational
elements as a top level selection, before services within this domain
are sought.
A site network that has grown beyond a size that can be reasonably
serviced by a few DAs can use the Scope mechanism. DAs have the
attribute class "SCOPE". The values for this attribute are a list
of strings that represent the administrative areas for which this
Directory Agent is an authority. 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 administrator 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.
Services with the attribute SCOPE should only be registered with DAs
which support the same Scope or DAs which have no Scope.
Directory Agents advertise their available Scopes. A Service Agent
may then choose a scope in which to register, and SHOULD register
with all Directory Agents in that Scope, as well as all DAs which
have no Scope. Failure to be comprehensive in registration according
to this rule will mean that the service advertisement may not be
discoverable by all User Agents.
A Directory Agent which has a Scope will send replies to Directory
Agent Discovery requests with the scope information included. Note
that the directory-agent Service Type is registered with the IANA
naming authority (which is automatically selected by leaving the
Naming Authority field empty.)
The query:
directory-agent/MATH DEPT//
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Could receive the following DA Advertisement:
Returned URL: service:directory-agent://diragent.blah.edu
Returned SCOPE: MATH DEPT
The same Directory Agent if it had no Scope value would reply:
Returned URL: service:directory-agent://diragent.void.com
Returned SCOPE:
If a Directory Agent supported more than one Scope it would reply as:
Returned URL: service:directory-agent://srv.domain.org
Returned SCOPE: MATH DEPT,ENGLISH DEPT,CS DEPT
A DA which has no Scope will reply to any Directory Agent Discovery
Request.
Being a member of a Scope means that an agent SHOULD use those
Directory Agents that support its Scope. User Agents send all
requests to DAs which support the indicated Scope. Services are
registered with the DA(s) in their Scope. For a UA to find a service
that is registered in a particular Scope it must send requests to a
DA which supports the indicated Scope. There is no limitation on
Scope membership built into the protocol; that is to say, a User
Agent or Service Agent may be a member of more than one Scope.
Membership is open to all, unless some external authorization
mechanism is added to limit access.
17. Language and Character Encoding Issues
All Service Registrations declare the language in which the strings
in the service attributes are written by specifying the appropriate
code 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.
If a Service is fully deregistered (the URL is given in the Service
Deregister request, without any attribute information) then the
Service needs to be deregistered only once. This will effectively
deregister the service in all languages it has been registered in.
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 Service Agent.
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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.
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.
All Service Requests specify a requested language in the message
header. The Directory Agent or Service Agent 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 can be sent in the default
language (which is English.) If the 'monolingual bit' flag in the
header is set and the requested language is not supported, a SrvRply
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
should be matched against the same dialect. If that is not possible,
it MAY be matched against any dialect of the same language.
17.1. Character Encoding and String Issues
Values for character encoding can be found in the Internet Assigned
Numbers Authority's (IANA) database
(http://www.isi.edu/in-notes/iana/assignments/character-sets), and
have the values referred by the MIBEnum value.
The encoding will determine the interpretation of all character data
which follows the Service Location Protocol header. There is no way
to mix ASCII and UNICODE, for example. All responses must be in the
character set of the request or use US-ASCII. If a request is sent
to a DA or SA or a registration is sent to a DA, which is unable to
manipulate or store the character set of the incoming message, the
request will fail. The SA or DA returns a CHARSET_NOT_UNDERSTOOD
error in a SrvAck message in this case. Requests using US-ASCII will
never fail for this reason, since all SAs and DAs must be able to
accept this character set.
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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 delimiter characters must not be used as 'data text.'
17.1.1. Substitution of Character Escape Sequences
The Service Location Protocol has an 'escape mechanism' which
is consistent with HTTP 2.0 [6] and SGML [13]. If the character
sequence "&#" is followed by one or more digits, followed by
a semicolon ';' the entire sequence is interpreted as a single
character. The digits are interpreted as a decimal value in the
character set of the request, as specified by the header. Thus, in
US-ASCII , would be interpreted as a comma. Substitution of
these escape strings must be done in all <attr-list> and strings
present in SrvReq and AttrRqst messages. Only numerical character
references are accepted, not 'Entity References,' as defined in HTML.
These escape values should only be used to provide a mechanism for
including reserved characters in attribute tag and value strings.
The interpretation of these escape values is different than in
HTML in one respect: In HTML the escape values are considered to
be in the ISO Latin-1 character set. In Service Location they
are interpreted in the character set defined in the header of the
message.
This escape mechanism allows characters like commas to be included in
attribute tags and values, which would otherwise be illegal as the
comma is a protocol delimiter.
Attribute tags and values of different languages are considered to be
mutually unintelligible. A query in one language SHOULD use service
information registered in that language.
17.2. Language Dialect
Dialect tags are used in Service Location messages to indicate a
variant of vocabulary used. If one service is registered in more
than one dialect, a DA or SA SHOULD return the one with the same
dialect tag as in the query, but MAY choose to return any registered
service that matches the criteria.
Dialects (unlike languages) are assumed to be mutually intelligible,
but may have variations in spelling. Since string matching is used,
it is advantageous in some cases to register service information in
multiple dialects.
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Dialect tags will be assigned as enumerated values to correspond to
the official dialects registered with the IANA. There are as of this
writing no enumerated dialect values; they will be created as needed.
17.3. Language-Independent Strings
Some strings, such as Service Type names, have standard definitions.
These strings should be considered as tokens and not as words in a
language to be translated.
Reserved String Section Definition
--------------- ------- --------------------------------------
SCOPE 3, 15 Used to limit the matching of requests.
SERVICE 9, 6 The URL scheme of all Service Location
information registered with a DA or
returned from a Service Request.
<srvtype> 20.2.1 Used in all service registrations
and replies.
domain names 20.4 A fully qualified domain name, used
in registrations and replies.
IANA 3.3 The default naming authority.
LOCAL 16 Reserved.
REMOTE 16 Reserved.
TRUE 20.5 Boolean true.
FALSE 20.5 Boolean false.
18. Service Location Transactions
18.1. Service Location Connections
When a Service Location Request or Attribute Request results in a
UDP reply from a Service or Directory Agent that will overflow a
datagram, the User Agent can open a connection to the Agent and
reissue the request over the connection. The reply will be returned
with the overflow bit set (see section 4). The reply will contain as
much data as will fit into a single datagram. If no MTU information
is available for the route, assume that the MTU is 1400; this value
is configurable (see section 22).
When a request results in overflowed data that cannot be correctly
parsed (say, because of duplicate or dropped IP datagrams), a
User Agent that wishes to reliably obtain the overflowed data must
establish a TCP connection with the Directory Agent or Service Agent
with the data. The request is sent again with a new XID. The reply
is returned over the connection stream.
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When registration data exceeds one datagram in length, the Service
Registration should be made by establishing a connection with a
Directory Agent and sending the registration over the connection
stream.
Directory Agents and Service Agents must respond to connection
requests; services whose registration data can overflow a datagram
must be able to use TCP to send the registration. User Agents
should be able to make Service and Attribute Requests using TCP. If
they fail to implement this, they must be able to interpret partial
replies and/or reissue requests with more selective criteria to
reduce the size of the replies.
A 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_INTERVAL_12 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.
18.2. No Synchronous Assumption
There is no requirement that one transaction complete before a
given host begins another. An agent may have multiple outstanding
transactions, initiated either using UDP or TCP.
18.3. Idempotency
All Service Location actions are idempotent. Of course registration
and deregistration will change the state of a DA, but repeating these
actions will have exactly the same effect each time.
19. Security Considerations
The Service Location Protocol does not provide authentication,
integrity or 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. Authentication and integrity
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are functionally equivalent in the context of this protocol.
Authentication is generally needed with this protocol.
An adversary can 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 consider the security
risks of deploying this protocol prior to deploying it.
The security risks in this protocol can be significantly reduced or
eliminated by using the IP Authentication Header [5, 3] with all
Service Location messages. It is recommended that sites use the
IP Authentication header with all Service Location messages. For
the security considerations listed above, it is recommended that
all nodes which implement Service Location also implement the IP
Authentication Header.
Sites requiring confidentiality should implement the IP Encapsulating
Security Payload (ESP) [4] to provide confidentiality for Service
Location messages.
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 deploy the IP Authentication Header if some
certificate information can be preconfigured with the end systems
before they use Service Location.
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 SrvReq
Service Request <predicate> 5.4 SrvReq
URL 20.2 SrvReg,
SrvDereg,
SrvRply
<attr-list> 20.3 SrvReg,
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SrvRply,
AttrRply
<Service Registration Information> 9 SrvReg
<Service Deregister Information> 11 SrvDereg
<Service Type String> 20.2.1 AttrRqst
20.1. Previous Responders' Address Specification
The previous responders' Address Specification is specified as
<Previous Responders' Address Specification> ::=
<addr-spec>, |
<addr-spec>, <Previous Responders' Address Specification>
i.e., a list separated and terminated by commas with no intervening
white space. The Address Specification is the address of the
Directory Agent or Service Agent which supplied the previous
response. The format for Address Specifications in Service Location
is defined in section 20.4. The comma delimiter is required between
each <addr-spec>. 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
20.2. Formal Definition of the ``service:'' Scheme
A URL with a ``service:'' scheme is used in the SrvReg, SrvDereg,
SrvRply and AttrRqst messages in Service Location. URLs are defined
in RFC 1738 [7]. A URL with the ``service:'' scheme must contain at
least:
<url> ::= service:<srvtype>://<addr-spec>
where:
service the URL scheme for Service Location, to return
Replies.
<srvtype> a string; Service Types may be standardized
by developing a specification for the "service
type"-specific part and registering it with IANA.
See sections 20.2.1 and 3.3.
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<addr-spec> the service access point of the service. It is the
network address or domain name where the service can
be accessed. See section 20.4.
The ``service:'' scheme may be followed by any legal URL. The
'minimal' service URL provides a service type and an access point for
a particular service. The protocol used to access the service at
the given service access <addr-spec> may be implicit in the Service
Type name. If this is not the case, the Service Type MUST be defined
in such a way that attribute information will include all necessary
configuration and protocol information. A User Agent MUST therefore
be able to use either a ``service:'' URL alone or a ``service:''
URL in conjunction with service attributes to make use of a service.
20.2.1. Service Type String
The Service Type is a string describing the type of service. These
strings may only be comprised of 'a' through 'z', '+' and '-'. Upper
case is considered equivalent to lower case in Service Type names.
If the Service Type name is followed by a '.' and a string (which
has the same limitations) the 'suffix' is considered to be the Naming
Authority of the service. If the Naming Authority is omitted, IANA
is assumed to be the Naming Authority.
Service Types developed for in-house or experimental use may have any
name and attribute semantics provided that they do not conflict with
the standardized Service Types. The Service Type's Service specific
Multicast Address used should taken from the range of experimental
multicast addresses reserved by the IANA.
20.3. 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>) | <keyword>
<attr-val-list> ::= <attr-val> | <attr-val>, <attr-val-list>
An <attr-list> must be scanned prior to evaluation for all
occurrences of the string "&#" followed by one or more digit followed
by ';'. See Section 17.1 and specifically Section 17.1.1.
A keyword has only an <attr-tag>, and no values.
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A comma cannot appear in an <attr-val>, as the comma is used as the
multiple value delimiter. Examples of an <attr-list> are:
(SCOPE=ADMINISTRATION)
(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 can take
on the values red, white and blue. There are several other examples
of replies throughout the document.
20.4. 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
It is preferable to use a fully qualified domain name wherever
possible as renumbering of host addresses will make IP addresses
invalid over time.
When no Domain Name Server is available SAs and DAs must use dotted
decimal conventions for IP addresses.
Generally, just the host domain name (or address) is sufficient to
return. When there is a non-standard port for the protocol, that
should be returned as well. Some applications may make use of the
<user>:<password>@ syntax, but its use is not encouraged in this
context until mechanisms are established to maintain confidentiality.
Address specification in Service Location is consistent with standard
URL format [7].
20.5. Attribute Value encoding rules
Attribute values, and attribute tags are CASE INSENSITIVE for
purposes of lexical comparison.
Attribute values can have be any string with the exception of '(',
')', '=', '>', '<', '/' and ',' (the comma) except in the case
described below where opaque values are encoded. These characters
may be included using the character value escape mechanism described
in section 17.1.
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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 (i.e. in French or Telugu, Boolean
TRUE is "TRUE", as well as in English.) Boolean attributes can
take only one value.
- Integer values are expressed as a sequence of numbers. The range
of allowable values, for this 32 bit quantity, is "-2147483648"
to "2147483647". Note: No other form of numeric representation
is interpreted as such, save 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> ::= number of bytes of the original data
<radix-64-data> ::= radix-64 encoding of the original data
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 1521 [8], MIME Part One, Section 5.2
Base64 Content Transfer Encoding, page 21.
21. Implementation Requirements
A User Agent MAY:
- Provide a way for the application to configure the default DA, so
that it can be used without needing to find it each initially.
- Be able to request the address of a DA from DHCP, if configured
to do so.
- Ignore any unauthenticated Service Reply or Attribute Reply.
- Be able to issue requests in any language or character set
provided that it can switch to the default language and character
set if the request can not be serviced by DAs and SAs at the
site.
A User Agent SHOULD:
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- Use a Scope in all requests, if possible.
- Issue requests to scoped DAs if the UA has been configured with a
scope.
- Listen on the Service Location General Multicast address for
unsolicited DA Advertisements. This will increase the set
of Directory Agents available to it for making replies. See
Section 15.2.
If the UA does not listen fro DA Advertisements, new DAs will not
be passively detected. A UA which does not have a configured DA
and has not yet discovered one and is not listening for unsolicited
DA Advertisements will remain ignorant of DAs. It may then do
a DA discovery before each query performed or it may simply use
multicasted queries to Service Agents.
A User Agent MUST:
- Be able to unicast requests and receive replies from a DA.
Transactions should be made reliable by using retransmission
of the request if the reply does not arrive within a timeout
interval.
- Be able to detect DAs using a Directory Agent Discovery request
issued when the UA starts up.
- Be able to send requests to a multicast address. If the
multicast address is not known, the UA must be able to use a
Service Type query to obtain the multicast address for the
Service Type of the request.
- Be able to handle numerous replies after a multicast request.
The implementation may be configurable so it will either return
the first reply, all replies until a timeout or keep trying till
the results converge.
- Ignore any unauthenticated Service Reply or Attribute Reply when
an appropriate IPSec Security Association for that Reply exists.
- Use the IP Authentication Header or IP Encapsulating Payload in
all Service Location messages, whenever an appropriate IPSec
Security Association exists.
- Be able to issue requests using the US-ASCII character set.
A Service Agent MAY be able to:
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- Get the address of a local Directory Agent by way of DHCP.
- Accept requests in non-US-ASCII character encodings. This is
encouraged, especially for UNICODE [1] and UTF-8 [17] encodings.
- Register services with a DA in non-US-ASCII character encodings.
This is encouraged, especially for UNICODE [1] and UTF-8 [17]
encodings.
A Service Agent SHOULD be able to:
- Listen to the service-specific multicast address of the service
it is advertising. The incoming requests should be filtered:
If the Address Specification of the SA is in the Previous
Responders Address Specification list, the SA SHOULD NOT respond.
Otherwise, a response to the multicast query SHOULD be unicast to
the UA which sent the request.
- Listen for and respond to broadcast requests and TCP connection
requests, to the Service Location port.
- Listen to the Service Location General Multicast address for
queries (e.g., Service Type Requests). If the query can be
replied to by the Service Agent, the Service Agent must do
so. It MUST check first to make sure it is not on the list of
'previous responders.'
A Service Agent MUST be able to:
- Listen to the Service Location General Multicast address for
unsolicited DA Advertisements. If one is detected, and the DA
has the right Scope, (or has no Scope), all services which are
currently being advertised MUST be registered with the DA (unless
configured to only use a single DA (see section 22.1), or the
DA has already been detected, subject to certain rules (see
section 15.2)).
- Unicast registrations and deregistrations to a DA. Transactions
should be made reliable by using retransmission of the request if
the reply does not arrive within a timeout interval.
- Be able to detect DAs using a Directory Agent Discovery request
issued when the SA starts up (unless configured to only use a
single DA, see section 22.1.)
- Use the IP Authentication Header or IP Encapsulating Payload in
all Service Location messages, whenever an appropriate IPSec
Security Association exists.
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- Be able to register service information with a DA using US-ASCII
character encoding. It must also be able to reply to requests
from UAs which use US-ASCII character encoding.
- Reregister with a DA before the Lifetime of registered service
information elapses.
A Directory Agent MAY:
- Ignore any unauthenticated Service Registration or Service
Deregistration.
- Accept registrations and requests in non-US-ASCII character
encodings. This is encouraged, especially for UNICODE [1] and
UTF-8 [17] encodings.
A Directory Agent MUST be able to:
- Send an unsolicited DA Advertisements to the Service Location
General Multicast address on startup and repeat it periodically.
This reply has an XID which is incremented by one each time. If
the DA starts with state, it initializes the XID to 0x0100. If
it starts up stateless, it initializes the XID to 0x0000.
- Listen on the Directory Agent Discovery Multicast Address for
Directory agent discovery requests. Filter these requests if the
Previous Responder Address Specification list includes the DA's
Address Specification.
- Listen for broadcast requests to the Service Location port.
- Listen on the TCP and UDP Service Location Ports for unicast
requests, registrations and deregistrations and service them.
- Provide a way in which Scope information can be used to configure
the Directory Agent.
- Age out the services which have been registered so that when
the service registration's Lifetime expires, the service
advertisement is withdrawn.
- When a Directory Agent has been configured with a Scope, it MUST
refuse all requests and registrations which do not have this
Scope. The DA replies with a SCOPE_NOT_SUPPORTED error. There
is one exception: All DAs MUST respond to DA discovery requests
which have no Scope.
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- When a Directory Agent has been configured without a Scope, it
MUST accept ALL registrations and requests.
- Ignore any unauthenticated Service Location messages when an
appropriate IPSec Security Association exists for that request.
- Use the IP Authentication and IP Encapsulating Security Payload
in Service Location messages whenever an appropriate IPSec
Security Association exists.
- Accept requests and registrations in US-ASCII.
NOTE: Service Agents and User Agents use ephemeral ports for
transmitting information to the service location port.
22. Configurable Parameters and Default Values
There are several configuration parameters for Service Location.
The protocol will work fine if only default values are used. Due
to the nature of the protocol, it may be deployed in many different
environments. The configuration options parameters will allow an
implementation of Service Location to be useful in a variety of
different scenarios.
Multicast vs. Broadcast
All Service Location entities must use multicast by
default. The ability to use broadcast messages must
be configurable. 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. The value for the
site's multicast TTL may be obtained from DHCP. The DHCP
option has not yet been assigned.
Directory Agent Address
The Directory Agent 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 15.2. The default value should be "no default
address." In this case the UA or SA must do a Directory
Agent Discovery query.
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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.
Default Path MTU
The default path MTU is assumed to be 1400. This value
may be too large for the infrastructure of some sites.
For this reason this value MUST be configurable for all
SAs and DAs.
If a UA issues a request which will result in a
reply which is too large, the SA or DA will return an
abbreviated response (in a datagram the size of the
site's MTU) which has the 'Overflow' bit flag set.
The UA must then issue the request again using a tcp
connection.
Similarly, if a SA attempts to register a service with a
DA and the registration is larger than the site path MTU
the DA will reply with a SrvAck, with the error set to
INVALID_REGISTRATION and the 'Overflow' byte set.
22.1. Service Agent: Use Predefined Directory Agent(s)
A Service Agent's default configuration is to do passive and active
DA discovery and to register with all DAs which are properly scoped.
A Service Agent 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 Service Agent 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.
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22.2. 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_INTERVAL_0 4.1 1 minute Cache replies by XID.
CONFIG_INTERVAL_1 4.2 10800 seconds registration Lifetime,
(ie. 3 hours)after which ad expires
CONFIG_INTERVAL_2 5 each second, Retry multicast query
backing off until no new values
gradually arrive.
CONFIG_INTERVAL_3 5 15 seconds Max time to wait for a
complete multicast query
response (all values.)
CONFIG_INTERVAL_4 9 3 seconds Wait to register on
reboot.
CONFIG_INTERVAL_5 5.2 3 seconds Retransmit DA discovery,
try it 3 times.
CONFIG_INTERVAL_6 5.2 5 seconds Give up on requests sent
to a DA.
CONFIG_INTERVAL_7 5.2 15 seconds Give up on DA discovery
CONFIG_INTERVAL_8 5.1 15 seconds Give up on requests
sent to SAs.
CONFIG_INTERVAL_9 15.2 3 hours DA Heartbeat, so that SAs
will
passively detect new DAs.
CONFIG_INTERVAL_10 15.2 1-3 seconds Wait to register services
on passive DA discovery.
CONFIG_INTERVAL_11 9 1-3 seconds Wait to register services
on active DA discovery.
CONFIG_INTERVAL_12 18.1 5 minutes DAs and SAs close idle
connections.
A note on CONFIG_INTERVAL_9: While it might seem advantageous to
have frequent heartbeats, this poses a significant risk of generating
a lot of overhead traffic. This value should be kept high to prevent
routine protocol operations from using any significant bandwidth.
23. Non-configurable Parameters
IP Port number for unicast requests to Directory Agents:
UDP and TCP Port Number: 427
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Multicast Addresses
Service Location General Multicast Address: 224.0.1.22
Directory Agent Discovery Multicast Address: 224.0.1.35
Further service-specific multicast address will be assigned for
specific types of service through the IANA.
Error Codes:
No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
CHARSET_NOT_UNDERSTOOD 5
AUHENTICATION_FAILED 6
24. Acknowledgments
This protocol owes some of the original ideas to other service
location protocols found in many other networking protocols. Leo
McLaughlin and Mike Ritter (Metricom) provided much input into early
version of this document. Thanks also to Steve Deering (Xerox) for
providing his insight into distributed multicast protocols. Harry
Harjono and Charlie Perkins supplied the basis for the URL based
wire protocol in their Resource Discovery Protocol. Thanks also to
Peerlogic, Inc. for supporting this work.
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A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for the
representation of names of languages"
Two-letter lower-case symbols are used. The Registration Authority
for ISO 639 [12] is Infoterm, Osterreiches Normungsinstitut (ON),
Postfach 130, A-1021 Vienna, Austria. Contains additions from ISO
639/RA Newsletter No.1/1989
aa Afar ga Irish mg Malagasy
ab Abkhazian gd Scots Gaelic mi Maori
af Afrikaans gl Galician mk Macedonian
am Amharic gn Guarani ml Malayalam
ar Arabic gu Gujarati mn Mongolian
as Assamese mo Moldavian
ay Aymara ha Hausa mr Marathi
az Azerbaijani he Hebrew ms Malay
hi Hindi mt Maltese
ba Bashkir hr Croatian my Burmese
be Byelorussian hu Hungarian
bg Bulgarian hy Armenian na Nauru
bh Bihari ne Nepali
bi Bislama ia Interlingua nl Dutch
bn Bengali; Bangla in Indonesian no Norwegian
bo Tibetan ie Interlingue
br Breton ik Inupiak oc Occitan
is Icelandic om (Afan) Oromo
ca Catalan it Italian or Oriya
co Corsican ja Japanese
cs Czech jw Javanese pa Punjabi
cy Welsh pl Polish
ka Georgian ps Pashto, Pushto
da Danish kk Kazakh pt Portuguese
de German kl Greenlandic
dz Bhutani km Cambodian qu Quechua
rw Kinyarwanda
el Greek kn Kannada rm Rhaeto-Romance
en English ko Korean rn Kirundi
eo Esperanto ks Kashmiri ro Romanian
es Spanish ku Kurdish ru Russian
et Estonian ky Kirghiz
eu Basque
la Latin
fa Persian ln Lingala
fi Finnish lo Laothian
fj Fiji lt Lithuanian
fo Faeroese lv Latvian, Lettish
fr French
fy Frisian
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sa Sanskrit ta Tamil ug Uigar
sd Sindhi te Telugu uk Ukrainian
sg Sangro tg Tajik ur Urdu
sh Serbo-Croatian th Thai uz Uzbek
si Singhalese ti Tigrinya
sk Slovak tk Turkmen vi Vietnamese
sl Slovenian tl Tagalog vo Volapuk
sm Samoan tn Setswana
sn Shona to Tonga wo Wolof
so Somali tr Turkish
sq Albanian ts Tsonga xh Xhosa
sr Serbian tt Tatar
ss Siswati tw Twi yi Yiddish
st Sesotho yo Yoruba
su Sundanese
sv Swedish za Zhuang
sw Swahili zh Chinese
zu Zulu
B. Appendix: For Further Reading
Three related resource discovery protocols are NBP and ZIP
which are part of the AppleTalk protocol family [11], the Legato
Resource Administration Platform [18], and the Xerox Clearinghouse
system [16]. Domain names and representation of addresses are
used extensively in the Service Location Protocol. The references
for these are RFCs 1034 and 1035 [14, 15]. An example of service
discovery protocol for a specific service is Router Discovery [9].
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References
[1] Unicode Technical Report #4. The unicode standard, version 1.1
(volumes 1 and 2). Technical Report (ISBN 0-201-56788-1) and
(ISBN 0-201-60845-6), Unicode Consortium, 1994.
[2] S. Alexander and R. Droms. DHCP Options and BOOTP Vendor
Extensions. RFC 1533, October 1993.
[3] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[4] R. Atkinson. IP Encapsulating Security Payload. RFC 1827,
August 1995.
[5] R. Atkinson. Security Architecture for the Internet Protocol.
RFC 1825, August 1995.
[6] T. Berners-Lee and D. Connolly. Hypertext Markup Language -
2.0. RFC 1866, November 1995.
[7] T. Berners-Lee, L. Masinter, and M. McCahill. Uniform Resource
Locators (URL). RFC 1738, December 1994.
[8] N. Borenstein and N. Freed. MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies. RFC 1521, September
1993.
[9] Stephen E. Deering, editor. ICMP Router Discovery Messages.
RFC 1256, September 1991.
[10] Ralph Droms. Dynamic Host Configuration Protocol. RFC 1541,
October 1993.
[11] S. Gursharan, R. Andrews, and A. Oppenheimer. Inside AppleTalk.
Addison-Wesley, 1990.
[12] Geneva ISO. Code for the representation of names of languages.
ISO 639:1988 (E/F), 1988.
[13] Geneva ISO 8879. Information Processing -- Text and Office
Systems - Standard Generalized Markup Language (SGML).
<URL:http://www.iso.ch/cate/d16387.html>, 1986.
[14] P. Mockapetris. Domain Names - Concepts and Facilities. RFC
1034, November 1987.
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[15] P. Mockapetris. DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION. RFC 1035, November 1987.
[16] D. Oppen and Y. Dalal. The clearinghouse: A decentralized
agent for locating named objects in a distributed environment.
Technical Report Tech. Rep. OPD-78103, Xerox Office Products
Division, 1981.
[17] X/Open Preliminary Specification. File System Safe UCS
Transformation Format (FSS_UTF). Technical Report Document
Number: P316, X/Open Company Ltd., 1994.
[18] Legato Systems. The Legato Resource Administration Platform.
Legato Systems, 1991.
Authors' Addresses
Questions about this memo can be directed to:
John Veizades Erik Guttman
@Home Network Sun Microsystems
385 Ravendale Dr. Gaisbergstr. 6
Mountain View, CA 94043 69115 Heidelberg Germany
Phone: +1 415 944 7332 Phone: +49 6221 601649
Fax: +1 415 944 8500 Fax: +49 6221 161019
Email: veizades@home.com Email: Erik.Guttman@eng.sun.com
Charles Perkins Scott Kaplan
IBM Corporation
P.O. Box 704 346 Fair Oaks St.
Yorktown Heights NY 10598 San Francisco, CA 94110
Phone: +1 914 784 7350 Phone: +1 415 285 4526
Fax: +1 914 784 6205
EMail: perk@watson.ibm.com Email: scott@catch22.com
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