Network Working Group J. Veizades
Request for Comments: 2165 @Home Network
Category: Standards Track E. Guttman
C. Perkins
Sun Microsystems
S. Kaplan
June 1997
Service Location Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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.
Table of Contents
1. Introduction 3
2. Terminology 3
2.1. Notation Conventions . . . . . . . . . . . . . . . . . . 5
2.2. Service Information and Predicate Representation . . . . 5
2.3. Specification Language . . . . . . . . . . . . . . . . . 6
3. Protocol Overview 6
3.1. Protocol Transactions . . . . . . . . . . . . . . . . . . 7
3.2. Schemes . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. The "service:" URL scheme . . . . . . . . . . . . 9
3.3. Standard Attribute Definitions . . . . . . . . . . . . . 9
3.4. Naming Authority . . . . . . . . . . . . . . . . . . . . 10
3.5. Interpretation of Service Location Replies . . . . . . . 10
3.6. Use of TCP, UDP and Multicast in Service Location . . . . 10
3.6.1. Multicast vs. Broadcast . . . . . . . . . . . . 11
3.6.2. Service-Specific Multicast Address . . . . . . . 11
3.7. Service Location Scaling, and Multicast Operating Modes . 12
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4. Service Location General Message Format 14
4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . . 15
4.2. URL Entries . . . . . . . . . . . . . . . . . . . . . . . 16
4.3. Authentication Blocks . . . . . . . . . . . . . . . . . . 17
4.4. URL Entry Lifetime . . . . . . . . . . . . . . . . . . . 19
5. Service Request Message Format 19
5.1. Service Request Usage . . . . . . . . . . . . . . . . . . 22
5.2. Directory Agent Discovery Request . . . . . . . . . . . . 23
5.3. Explanation of Terms of Predicate Grammar . . . . . . . . 24
5.4. Service Request Predicate Grammar . . . . . . . . . . . . 26
5.5. String Matching for Requests . . . . . . . . . . . . . . 27
6. Service Reply Message Format 28
7. Service Type Request Message Format 29
8. Service Type Reply Message Format 31
9. Service Registration Message Format 32
10. Service Acknowledgement Message Format 35
11. Service Deregister Message Format 37
12. Attribute Request Message Format 38
13. Attribute Reply Message Format 40
14. Directory Agent Advertisement Message Format 42
15. Directory Agents 43
15.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 43
15.2. Finding Directory Agents . . . . . . . . . . . . . . . . 43
16. Scope Discovery and Use 45
16.1. Protected Scopes . . . . . . . . . . . . . . . . . . . . 46
17. Language and Character Encoding Issues 47
17.1. Character Encoding and String Issues . . . . . . . . . . 48
17.1.1. Substitution of Character Escape Sequences . . . 49
17.2. Language-Independent Strings . . . . . . . . . . . . . . 49
18. Service Location Transactions 50
18.1. Service Location Connections . . . . . . . . . . . . . . 50
18.2. No Synchronous Assumption . . . . . . . . . . . . . . . . 51
18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . . 51
19. Security Considerations 51
20. String Formats used with Service Location Messages 52
20.1. Previous Responders' Address Specification . . . . . . . 53
20.2. Formal Definition of the "service:" Scheme . . . . . . . 53
20.2.1. Service Type String . . . . . . . . . . . . . . . 54
20.3. Attribute Information . . . . . . . . . . . . . . . . . . 54
20.4. Address Specification in Service Location . . . . . . . . 55
20.5. Attribute Value encoding rules . . . . . . . . . . . . . 55
21. Protocol Requirements 56
21.1. User Agent Requirements . . . . . . . . . . . . . . . . . 56
21.2. Service Agent Requirements . . . . . . . . . . . . . . . 58
21.3. Directory Agent Requirements . . . . . . . . . . . . . . 59
22. Configurable Parameters and Default Values 61
22.1. Service Agent: Use Predefined Directory Agent(s) . . . . 62
22.2. Time Out Intervals . . . . . . . . . . . . . . . . . . . 63
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23. Non-configurable Parameters 63
24. Acknowledgments 64
A. Appendix: Technical contents of ISO 639:1988 (E/F): "Code for
the representation of names of languages" 65
B. SLP Certificates 66
C. Example of deploying SLP security using MD5 and RSA 68
D. Example of use of SLP Certificates by mobile nodes 68
E. Appendix: For Further Reading 69
1. Introduction
Traditionally, users find services by using the name of a network
host (a human readable text string) which is an alias for a network
address. The Service Location Protocol eliminates the need for a
user to know the name of a network host supporting a service.
Rather, the user names the service and supplies a set of attributes
which describe the service. The Service Location Protocol allows the
user to bind this description to the network address of the service.
Service Location provides a dynamic configuration mechanism for
applications in local area networks. It 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.
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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.
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, called a "service
scheme", 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,
the Internet Assigned Numbers Authority.
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.
Alphanumeric
A character within the range 'a' to 'z', 'A' to 'Z', or
Scope A collection of services that make up a logical group.
See sections 3.7 and 16.
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Site Network
All the hosts accessible within the Agent's multicast
radius, which defaults to a value appropriate for
reaching all hosts within a site (see section 22). If
the site does not support multicast, the agent's site
network is restricted to a single subnet.
URL A Universal Resource Locator - see [6].
Address Specification
This is the network layer protocol dependent mechanism
for specifying an Agent. For Internet systems this is
part of a URL.
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 human readable. 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.
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2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification [8]. 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.
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, 11], 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 may be 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 which are unicasted using TCP or UDP. This
means that a Directory Agent must first be discovered, using DHCP,
the DA Discovery Multicast address, the multicast mechanism described
above, or manual configuration. See section 5.2.
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 lightweight
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, and will eventually expire.
Service Registrations need to be refreshed by the Service Agent
before their Lifetime runs out. If need be, Service Agents can
advertise signed URLs to prove that they are authorized to provide
the service.
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
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existing code. Moreover, it is hoped that browsers will be written
to take advantage of the similarity in locator format, so that a
client can dynamically formulate requests for services that are
resolved differently depending upon the circumstances.
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
Attributes and Keywords
Attribute Descriptions and interpretations
Service Types not registered with IANA will use their own Naming
Authority string. The registration process for new Service Types is
defined in [13].
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
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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. IANA stands for the Internet Assigned Numbers Authority.
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 5 and 9.
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
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 UDP
(connectionless) and TCP (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. Service Agents and User Agents
use ephemeral ports for transmitting information to the service
location port, which is 427.
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The Service Location discovery mechanisms typically multicast
messages to as many enterprise networks as needed to establish
service availability. 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
DHCP is available, or multicast is supported at the network layer.
To support this protocol when only network layer broadcast is
supported, the following procedures may be followed.
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
information that resides on different subnets. The Directory Agent
address can be dynamically configured with Agents using DHCP. The
address can also be determined by static configuration.
As dynamic discovery is not feasible in a broadcast environment with
multiple subnets and manual configuration is difficult, deploying DAs
to serve enterprises with 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
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, on the other hand,
the underlying hardware will not support the number of needed
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multicast addresses the Service Location General Multicast Address
MAY be used. Service Agents MUST listen on this multicast address as
well as the service-specific multicast addresses for the service
types they advertise.
Service-Specific Multicast Addresses are computed by calculating a
string hash on the Service Type string. The Service Type string MUST
first be converted to an ASCII string from whatever character set it
is represented in, so the hash will have well-defined results.
The string hash function is modified from a code fragment attributed
to Chris Torek:
/*
* SLPhash returns a hash value in the range 0-1023 for a
* string of single-byte characters, of specified length.
*/
unsigned long SLPhash (const char *pc, unsigned int length)
unsigned long h = 0;
while (length-- != 0) {
h *= 33;
h += *pc++;
}
return (0x3FF & h); /* round to a range of 0-1023 */
}
This value is added to the base range of Service Specific Discovery
Addresses, to be assigned by IANA. These will be 1024 contiguous
multicast addresses.
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|U|A|F| rsvd| Dialect | Language Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Char Encoding | XID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version This protocol document defines version 1 of the Service
Location protocol.
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.
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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.
U The 'URL Authentication Present' bit. See sections 4.2,
4.3, 9, and 11 for the use of this field.
A The 'Attribute Authentication Present' bit. See
sections 4.2, 4.3, and 13 for the use of this field.
F If the 'F' bit is set in a Service Acknowledgement, the
directory agent has registered the service as a new
entry, not as an updated entry.
rsvd MUST be zero.
Dialect Dialect tags will be used by future versions of the
Service Location Protocol to indicate a variant of
vocabulary used. This field is reserved and MUST be set
to 0 for compatibility with future versions of the
Service Location Protocol.
Language Code
Strings within the remainder of the message which follows
are to be interpreted in the language encoded (see
section 17 and appendix A) in this field.
Character Encoding
The characters making up strings within the remainder of
the message may be encoded in any standardized encoding
(see section 17.1).
Transaction Identifier (XID)
The XID (transaction ID) field allows the requester to
match replies to individual requests (see section 4.1).
Note that, whenever there is an Attribute Authentication
block, there will also be a URL Authentication block.
Thus, it is an error to have the 'A' bit set without also
having the 'U' bit set.
4.1. 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
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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.
Directory Agents use XID values in their DA Advertisements to
indicate their state (see section 15.2).
4.2. URL Entries
When URLs are registered, they have lifetimes and lengths, and may be
authenticated. These values are associated with the URL for the
duration of the registration. The association is known as a "URL-
entry", and has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Length of URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ URL \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) URL Authentication Block .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Lifetime The length of time that the registration is valid, in
the absence of later registrations or deregistration.
Length of URL
The length of the URL, measured in bytes and < 32768.
URL Authentication Block
(if present) A timestamped authenticator (section 4.3)
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The URL conforms to RFC 1738 [6]. If the 'U' bit is set in the
message header, the URL is followed by an URL Authentication Block.
If the scheme used in the URL does not have a standardized
representation, the minimal requirement is:
service:<srvtype>://<addr-spec>
"service" is the URL scheme of all Service Location Information
included in service registrations and service replies. Each URL
entry contains the service:<srvtype> scheme name. It may also
include an <addr-spec> except in the case of a reply to a Service
Type request (see section 7).
4.3. Authentication Blocks
Authentication blocks are used to authenticate service registrations
and deregistrations. URLs are registered along with an URL
Authentication block to retain the authentication information in the
URL entry for subsequent use by User Agents who receive a Service
Reply containing the URL entry. Service attributes are registered
along with an Attribute Authentication block. Both authentication
blocks have the format illustrated below.
If a service registration is accompanied by authentication which can
be validated by the DA, the DA MUST validate any subsequent service
deregistrations, so that unauthorized entities cannot invalidate such
registered services. Likewise, if a service registration is
accompanied by an Attribute Authentication block which can be
validated by the DA, the DA MUST validate any subsequent attribute
registrations, so that unauthorized entities cannot invalidate such
registered attributes.
To avoid replay attacks which use previously validated
deregistrations, the deregistration or attribute registration message
must contain a timestamp for use by the DA. To avoid replay attacks
which use previously validated registrations to nullify a valid
deregistration, registrations must also contain a timestamp.
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An authentication block has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Structure Descriptor | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Structured Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Timestamp A 64-bit value formatted as specified by the Network
Time Protocol (NTP) [16].
Block Structure Descriptor (BSD)
A value describing the structure of the Authenticator.
The only value currently defined is 1, for
Object-Identifier.
Length The length of the Authenticator
Structured Authenticator
An algorithm specification, and the authentication data
produced by the algorithm.
The Structured Authenticator contains a digital signature of the
information being authenticated. It contains sufficient information
to determine the algorithm to be used and the keys to be selected to
verify the digital signature.
The digital signature is computed over the following ordered stream
of data:
CHARACTER ENCODING OF URL (2 bytes in network byte order)
LIFETIME (2 bytes in network byte order)
LENGTH OF URL (2 bytes in network byte order)
URL (n bytes)
TIMESTAMP (8 bytes in SNTP format [16])
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When producing a URL Authentication block, the authentication data
produced by the algorithm identified within the Structured
Authenticator calculated over the following ordered stream of data:
ATTRIBUTE CHARACTER ENCODING (2 bytes in network byte order)
LENGTH OF ATTRIBUTES (2 bytes in network byte order)
ATTRIBUTES (n bytes)
TIMESTAMP (8 bytes in SNTP format [16])
Every Service Location Protocol entity (User Agent, Service Agent, or
Directory Agent) which is configured for use with protected scopes
SHOULD implement "md5WithRSAEncryption" [4] and be able to associate
it with BSD value == 1.
In the case where BSD value == 1 and the OID "md5WithRSAEncryption"
is selected, the Structured Authenticator will start with the ASN.1
Distinguished Encoding (DER) [9] for "md5WithRSAEncryption", which
has the as its value the bytes (MSB first in hex):
"30 0d 06 09 2a 86 48 86 f7 0d 01 01 04 05 00"
This is then immediately followed by an ASN.1 Distinguished Encoding
(as a "Bitstring") of the RSA encryption (using the Scope's private
key) of a bitstring consisting of the OID for "MD5" concatenated by
the MD5 [22] message digest computed over the fields above. The
exact construction of the MD5 OID and digest can be found in RFC 1423
[4].
4.4. 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.
Veizades, et. al. Standards Track [Page 19]
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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. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 TCP.
The <Previous Responders Addr Spec> is described in sections 7 and
20.1.
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.
Veizades, et. al. Standards Track [Page 20]
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- 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 a query which allows the selection of those service
instances which 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.
3. It must have the same scope. (If the scope of the request
as 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.
Veizades, et. al. Standards Track [Page 21]
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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 13). 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,
containing the name of the host managing the requested printer.
Files would be printed by spooling to that port on that host. The
word 'draft' refers to the name of the print queue the lpr server
supports.
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In the absence of a Directory Agent, the request above could be
multicast. In this case it would be sent to the Service Specific
Multicast Address for "service:printer" 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
"IANA" is assumed. We want to reach all the available directory
agents. If the scope were supplied, only DAs supporting that scope
would reply.
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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.
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.
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RFC 2165 Service Location Protocol June 1997
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.
degenerate expressions but they should be tolerated. They are
equivalent to <query-item>.
query-item:
A query item has the form:
'(' <attr-tag> <comp-op> <attr-val> ')'
or
'(' <keyword> ')'
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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
alphanumeric characters, '+', and '-' are allowed.
<na> ::= string representing the Naming Authority.
Only alphanumeric characters, '+',
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
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.)
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<keyword> ::= a class name of an attribute which will have
no values. This string has the same limits
as the <attr-tag>, except that 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 '/', ','
'=', '<', '>', or '*' 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.
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".
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RFC 2165 Service Location Protocol June 1997
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
registration, not using any language specific rules. The ordering is
strictly by the character value, i.e. "0" < "A" is true when the
character set is US-ASCII, since "0" has the value of 48 and "A" has
the value 65.
The special character '*' may precede or follow a string in order to
allow substring matching. If the '*' precedes a string, it matches
any attribute value which ends with the string. If the string ends
with a '*', it matches any attribute value which begins with the
string. Finally, if a string begins and ends with a '*', the string
will match any attribute value which contains the string.
Examples:
"bob*" matches "bob", "bobcat", and "bob and sue" "*bob" matches
"bob", "bigbob", and "sue and bob" "*bob*" matches "bob",
"bobcat", "bigbob", and "a bob I know"
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 | URL Entry count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry 1> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
\ . \
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| <URL Entry N> ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each Service Reply message is composed of a list of URL Entries.
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The Error Code may have one of the following values:
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 SrvRply is received
which cannot be parsed or the declared string lengths
overrun the message.
SCOPE_NOT_SUPPORTED
A DA will return this error if it receives a request
which has a scope not supported by the DA. An SA will not
return this error; it will simply not reply to the
multicast request.
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 in Section 4.2.
The URL entries in the reply have no delimiters between them, other
than the length fields. The URL length fields indicate where the URL
strings end. If the presence of an URL Authenticator block is
signalled by the 'U' bit, the length of the authenticator block is
determined by information within the block as discussed in section
4.3. A User Agent MAY use the authentication block to determine
whether the Service Agent advertising the URL is, in fact, authorized
to offer the indicated service. If, in a list of URL entries, some
of the URLs indicate services which are in protected scopes (see
section 16.1) while other URLs in the list indicate services which
are not in protected scopes, the latter must still have
Authentication Blocks, but the length of the authentcitor is shown as
zero, and no authentication need be done.
7. Service Type Request Message Format
The Service Type Request is used to determine all the types of
services supported on a network.
Veizades, et. al. Standards Track [Page 29]
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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 includes 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.
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>
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RFC 2165 Service Location Protocol June 1997
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.
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 \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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 use this error.
The service type's name is provided in the <Service Type String>. If
the service type has a naming authority other than "IANA" it should
be returned following the service type string and a "." character.
See section 20.2.1 for the formal definition of this field. User
Agents calculate Service Specific Multicast addresses based on a hash
of the Service Type (see Section 3.6.2). This multicast address may
then be used for issuing Service and Attribute Requests directly to
SAs.
The following are examples of Service Type Strings which might be
found in Service Type Replies:
service:lpr://
service:http://
service:nfs://
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. If the service registration
in a protected scope 16.1, then the service MUST include both a URL
Authentication block and an Attribute Authentication block (see
section 4.3). In that case, the service agent MUST set both the 'U'
bit and the 'A' bit (see section 4).
Veizades, et. al. Standards Track [Page 32]
RFC 2165 Service Location Protocol June 1997
A Directory Agent must acknowledge each service registration request.
If authentication blocks are included, the Directory Agent MUST
verify the authentication before registering the service. This
requires obtaining key information, either by preconfiguration,
maintenance of a security association with the service agent, or
acquiring the appropriate certificate.
The format of a Service Registration is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Location header (function = SrvReg) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <URL-Entry> \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Attr List String | <attr-list> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ <attr-list>, Continued. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) Attribute Authentication Block ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The <URL-Entry> is defined at the end of Section 4.2. The <attr-
list> is defined in Section 20.3. The Attribute Authentication
Block, which is only present if the 'A' bit is set in the message
header, is defined in section 4.3.
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.
If a SA attempts to register a service with a DA and the registration
is larger than the site path MTU, then the DA will reply with a
SrvAck, with the error set to INVALID_REGISTRATION and the 'Overflow'
byte set.
Veizades, et. al. Standards Track [Page 33]
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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.
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)
Scoped registrations must contain the SCOPE attribute. Unscoped
registrations must be registered with all unscoped Directory Agents.
Registrations of a previously registered service are considered an
update. If such an attribute registration is performed in a
protected scope (see section 16.1), a new Attribute Authentication
block must also be included, and the 'A' bit set in the registration
message header.
The new registration's attributes replace the previous
registration's, but do not effect attributes which were included
previously and are not present in the update.
Veizades, et. al. Standards Track [Page 34]
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For example, suppose service:x://a.org has been registered with
attributes A=1, B=2, C=3. If a new registration comes for
service:x://a.org with attributes C=30, D=40, then the attributes for
the service after the update are A=1, B=2, C=30, D=40.
In the example above, the SCOPE is set to DEVELOPMENT (in English)
and ENTWICKLUNG (in German). Recall that all strings in a message
must be in one language, which is specified in the header. The
string SCOPE is *not* translated, as it is one of the reserved
strings in the Service Location Protocol (see section 17.2.)
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
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.
When the URL entry accompanying a registration also contains an
authentication block (section 4.3), the DA MUST perform the indicated
authentication, and subsequently indicate the results in the Service
Acknowledgement message.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Veizades, et. al. Standards Track [Page 35]
RFC 2165 Service Location Protocol June 1997
The Error Code may have one of the following values:
0 Success
PROTOCOL_PARSE_ERROR
A DA returns this error when the SrvReg or SrvDereg is
received which cannot be parsed or the declared string
lengths overrun the message.
INVALID_REGISTRATION
A DA returns this error when a SrvReg or SrvDeReg is
invalid. For instance, an invalid URL, unknown or
malformed attributes, or deregistering an unregistered
service all cause this error to be reported.
SCOPE_NOT_SUPPORTED
A DA which is configured to have a scope will return this
error if it receives a SrvReq which is set to have a
scope which it does not support.
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_ABSENT
If DA has been configured to require an authentication
for any service registered in the requested scope, and
there are no authentication blocks in the registration,
the DA will return this error.
AUTHENTICATION_FAILED
If the registration contains an authentication block
which fails to match the correct result as calculated
(see section 4.3) over the URL or attribute data to be
authenticated, the DA will return this error.
If the Directory Agent accpets a Service Registration, and already
has an existing entry, it updates the existing entry with the new
lifetime information and possibly new attributes and new attribute
values. Otherwise, if the registration is acceptable (including all
necessary authentication checks) the Directory Agent creates a new
entry, and sets the 'F' bit in the Service Acknowledgement returned
to the Service Agent.
Veizades, et. al. Standards Track [Page 36]
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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. \
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) authentication block .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 message. 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
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
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RFC 2165 Service Location Protocol June 1997
If the service was originally registered with a URL entry containing
a URL authentication block, then the Service Deregistration message
header MUST have the 'U' bit set, and the URL entry is then followed
by the authentication block, with the authenticator calculated over
the URL data, the timestamp, and the length of the authenticator as
explained in section 4.3. In this calculation, the lifetime of the
URL data is considered to be zero, no matter what the current value
for the remaining lifetime of the registered URL.
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, then the reply includes all
attributes and all values for that Service Type. The reply includes
only those attributes for which services exist and are advertised by
the DA or SA which received the Attribute Request. Since different
instances of a given service can, and very likely will, have
different values for the attributes defined by the Service Type, the
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 been registered for
that URL or Service Type, the attribute or keyword information is
simply not returned.
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RFC 2165 Service Location Protocol June 1997
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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RFC 2165 Service Location Protocol June 1997
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
If followed by an '*', it matches all class names
which begin with the partial tag. If preceded by
a partial tag. If both preceded and followed by
'*' it matches all class names which contain the
partial tag.
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
Veizades, et. al. Standards Track [Page 40]
RFC 2165 Service Location Protocol June 1997
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 DA or SA returns this error when a AttrRqst is received
which cannot be parsed or the declared string lengths
overrun the message.
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),
(QUEUES=LEGAL,LETTER,ENVELOPE,LETTER HEAD)
If the message header has the 'A' bit set, the Attribute Reply will
have an Attribute Authentication block set. In this case, the
Attribute Authenticator must be returned with the entire list of
attributes, exactly as it was registered by an SA in a protected
scope. In this case, the URL was registered in a protected scope and
the UA included a URL but not a select clause. If the AttrRqst
specifies that only certain attributes are to be returned, the DA
does not (typically cannot) compute a new Authenticator so it simply
returns the attributes without an authenticator block.
Veizades, et. al. Standards Track [Page 41]
RFC 2165 Service Location Protocol June 1997
A UA which wishes to obtain authenticated attributes for a service in
a protected scope MUST therefore must include a particular URL and no
select list with the AttrRqst.
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
Veizades, et. al. Standards Track [Page 42]
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The Directory Agent can be reached at the Address Specification
returned, and supports the SCOPE called "ADMIN".
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
may 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, 11] may use it to obtain a
Directory Agent's address. DHCP options 78 and 79 have been assigned
for this purpose [21].
The third way to discover DAs is dynamically. This is done by
sending out a Directory Agent Discovery request (see Section 5.2).
Veizades, et. al. Standards Track [Page 43]
RFC 2165 Service Location Protocol June 1997
Lastly, the agent may be informed passively as follows:
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. 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.)
Veizades, et. al. Standards Track [Page 44]
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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.
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 configured. 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
available to all User Agents.
Veizades, et. al. Standards Track [Page 45]
RFC 2165 Service Location Protocol June 1997
A Directory Agent which has a scope will return advertisements in
response to Directory Agent Discovery requests with the scope
information included. Note that the "service:directory-agent" scheme
is registered with the IANA naming authority (which is automatically
selected by leaving the Naming Authority field empty.)
The query:
directory-agent/MATH DEPT//
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.
16.1. Protected Scopes
Scope membership MAY also define the security access and
authorization for services in the scope; such scopes are called
protected scopes. If a User Agent wishes to be sure that Service
Agents are authorized to provide the service they advertise, then the
User Agent should request services from a protected scope which has
been configured to have the necessary authentication mechanism and
keys distributed to the Service Agents within the scope. A directory
Veizades, et. al. Standards Track [Page 46]
RFC 2165 Service Location Protocol June 1997
agent distributing URLs for services in a protected scope will reject
any registrations or deregistrations for service agents which cannot
provide cryptographically strong authentication to prove their
authorization to provide the services.
For instance, if a campus registrar wishes to find a working printer
to produce student grade information for mailing, the registrar would
require the printing user agent to transmit the printable output only
to those printing Service Agents which have been registered in the
appropriate protected scope. Notice that each service agent is,
under normal circumstances, validated two times: once when
registering with the directory agent, and once when the user agent
validates the URL received with the Service Reply. This protects
against the possibilities of malicious Directory Agents as well as
malicious Service Agents.
Note that services in protected scopes provide separate
authentication for their URL entry, and for their attributes. This
follows naturally from the needs of the protocol operation. User
Agents which specify a service type and attributes needed for service
in that service type will not receive attribute information from the
directory agent; they will only receive the appropriate URL entries.
Only the information returned needs to be authenticated.
User agents which receive attribute information for a particular URL
(see section 12), on the other hand, need to authenticate the
attributes when they are returned (see section 13). In this case,
there may be much more data to authenticate, but this operation is
also performed much less often, usually only while the user is
browsing the available network resources.
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.
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If, on the other hand, attribute information is included in the
Service Deregistration request, a separate Service Deregistration of
selected attributes must be undertaken in each language in which
service information has been provided to the DA by a Service Agent.
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 IANA's 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
Veizades, et. al. Standards Track [Page 48]
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never fail for this reason, since all SAs and DAs must be able to
accept this character set.
Certain characters are illegal in certain contexts of the protocol.
Since the protocol is largely character string based, in some
contexts characters are used as protocol delimiters. In these cases
the delimiting characters must not be used as 'data text.'
17.1.1. Substitution of Character Escape Sequences
The Service Location Protocol has an 'escape mechanism' which is
consistent with HTTP 2.0 [5] and SGML [15]. 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-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.
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Reserved String Section xDefinition
--------------- ------- --------------------------------------
SCOPE 3, 15 Used to limit the matching of requests.
SERVICE 6, 9 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. When the request is sent again with a new XID, the
reply is returned over the connection.
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.
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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 with the same XID will have exactly the same effect each
time. Repeating a registration with a new XID has the effect of
extending the lifetime of the registration.
19. Security Considerations
The Service Location Protocol provides for authentication of Service
Agents as part of the scope mechanism, and consequently, integrity of
the data received as part of such registrations. Service Location
does not provide confidentiality. Because the objective of this
protocol is to advertise services to a community of users,
confidentiality might not generally be needed when this protocol is
used in non-sensitive environments. Specialized schemes might be
able to provide confidentiality, if needed in the future. Sites
requiring confidentiality should implement the IP Encapsulating
Security Payload (ESP) [3] to provide confidentiality for Service
Location messages.
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Using unprotected scopes, an adversary might easily use this protocol
to advertise services on servers controlled by the adversary and
thereby gain access to users' private information. Further, an
adversary using this protocol will find it much easier to engage in
selective denial of service attacks. Sites that are in potentially
hostile environments (e.g. are directly connected to the Internet)
should consider the advantages of distributing keys associated with
protected scopes prior to deploying the sensitive directory agents or
service agents.
Service Location is useful as a bootstrap protocol. It may be used
in environments in which no preconfiguration is possible. In such
situations, a certain amount of "blind faith" is required: Without
any prior configuration it is impossible to use any of the security
mechanisms described above. Service Location will make use of the
mechanisms provided by the Security Area of the IETF for key
distribution as they become available. At this point it would only
be possible to gain the benefits associated with the use of protected
scopes if some cryptographic information can be preconfigured with
the end systems before they use Service Location. For User Agents,
this could be as simple as supplying the public key of a Certificate
Authority. See Appendix B.
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,
SrvRply,
AttrRply
<Service Registration Information> 9 SrvReg
<Service Deregister Information> 11 SrvDereg
<Service Type String> 20.2.1 AttrRqst
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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 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 [6]. 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.
<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 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
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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 alphanumeric characters, '+', and
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.
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 17.1.1.
A keyword has only an <attr-tag>, and no values.
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.
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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
When no Domain Name Server is available, SAs and DAs must use dotted
decimal conventions for IP addresses. Otherwise, it is preferable to
use a fully qualified domain name wherever possible as renumbering of
host addresses will make IP addresses invalid over time.
Generally, just the host domain name (or address) is returned. When
there is a non-standard port for the protocol, that should be
returned as well. 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 [6].
20.5. Attribute Value encoding rules
Attribute values, and attribute tags are CASE INSENSITIVE for
purposes of lexical comparison.
Attribute values are strings containing any characters 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 17.1.1.
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.
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- Integer values are expressed as a sequence of numbers. The
range of allowable values for integers is "-2147483648" to
"2147483647". No other form of numeric representation is
interpreted as such except integers. For example, hexadecimal
numbers such as "0x342" are not interpreted as integers, but as
strings.
- Opaque values (i.e. binary values) are expressed in radix-64
notation. The syntax is:
<opaque-val> ::= (<len>:<radix-64-data>)
<len> ::= number of bytes of the original data
<radix-64-data> ::= radix-64 encoding of the original data
<len> is a 16-bit binary number. 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 [7],
MIME Part One, Section 5.2 Base64 Content Transfer Encoding, page
21.
21. Protocol Requirements
In this section are listed various protocol requirements for User
Agents, Service Agents, and Directory Agents.
21.1. User Agent 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.
- 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.
- Require an authentication block in any URL entry returned as
part of a Service Request, before making use of the advertised
service.
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A User Agent SHOULD:
- Try to contact DHCP to obtain the address of a DA.
- 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 requests. See
Section 15.2.
- Be able to be configured to require an authentication block in
any received URL entry advertised as belonging to a protected
scope, before making use of the service.
If the UA does not listen for 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 multicast
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. Service
Specific Multicast addresses are computed based on a hash of the
Service Type. See Section 3.6.2.
- 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.
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- Whenever it obtains its IP address from DHCP in the first place,
also attempt to obtain scope information, and the address of a
DA, from DHCP.
- 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.
- If configured to use a protected scope, be able to use
"md5WithRSAEncryption" [4] to verify the signed data.
21.2. Service Agent Requirements
A Service Agent MAY be able to:
- 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 [24] encodings.
- Register services with a DA in non-US-ASCII character encodings.
This is encouraged, especially for UNICODE [1] and UTF-8 [24]
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.
- Be configurable to calculate authentication blocks and thereby
be enabled to register in protected scopes. This requires that the
service agent be configured to possess the necessary keys to
calculate the authenticator.
A Service Agent MUST be able to:
- 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.
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It MUST check first to make sure it is not on the list of
'previous responders.'
- 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)).
- Whenever it obtains its IP address from DHCP in the first place,
also attempt to obtain scope information, and the address of a
DA, from DHCP.
- 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.
- 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.
- If configured to use a protected scope, be able to use
"md5WithRSAEncryption" [4] to produce the signed data.
21.3. Directory Agent Requirements
A Directory Agent MAY:
- Accept registrations and requests in non-US-ASCII character
encodings. This is encouraged, especially for UNICODE [1] and
UTF-8 [24] encodings.
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A Directory Agent SHOULD:
- Be able to configure certain scopes as protected scopes, so that
registrations within those scopes require the calculation of
cryptographically strong authenticators. This requires that the
DA be able to possess the keys needed for the authentication, or
that the DA be able to acquire a certificate generated by a
trusted Certificate Authority [23], before completing Service
Registrations for protected scopes.
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.
- Ignore any unauthenticated Service Registration or Service
Deregistration from an entity with which it maintains a security
association.
- 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.
- Expire registrations when the service registration's lifetime
expires.
- 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.
- When a Directory Agent has been configured without a scope, it
MUST accept ALL registrations and requests.
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- 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.
- If configured with a protected scope, be able to authenticate (at
least by using "md5WithRSAEncryption" [4]) Service Registrations
advertising services purporting to belong to such configured
protected scopes.
22. Configurable Parameters and Default Values
There are several configuration parameters for Service Location.
Default values are chosen to allow protocol operation without the
need for selection of these configuration parameters, but other
values may be selected by the site administrator. The configurable
parameters will allow an implementation of Service Location to be
more useful in a variety of scenarios.
Multicast vs. Broadcast
All Service Location entities must use multicast by
default. The ability to use broadcast messages must be
configurable for UAs and SAs. Broadcast messages are to
be used in environments where not all Service Location
entities have hardware or software which supports
multicast.
Multicast Radius
Multicast requests should be sent to all subnets in a
site. The default multicast radius for a site is 32.
This value must be configurable. The value for the
site's multicast TTL may be obtained from DHCP using an
option which is currently unassigned.
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 use of DHCP, with "no
default address" used if DHCP does not respond. In this
case the UA or SA must do a Directory Agent Discovery
query.
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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.
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.
Keys for Protected Scopes
If the local administration designates certain scopes as
"protected scopes", the agents making use of those scopes
have to be able to acquire keys to authenticate data sent
by services along with their advertised URLs for services
within the protected scope. For instance, service agents
would use a private key to produce authentication data.
By default, service agents use "md5WithRSAEncryption" [4]
to produce the signed data, to be be included with
service registrations and deregistrations (see appendix
B, 4.3). This authentication data could be verified by
user agents and directory agents that possess the
corresponding public key.
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 4.4 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
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
A range of 1024 contiguous multicast addresses for use as Service
Specific Discovery Multicast Addresses will be assigned by IANA.
Error Codes:
No Error 0
LANGUAGE_NOT_SUPPORTED 1
PROTOCOL_PARSE_ERROR 2
INVALID_REGISTRATION 3
SCOPE_NOT_SUPPORTED 4
CHARSET_NOT_UNDERSTOOD 5
AUTHENTICATION_ABSENT 6
AUTHENTICATION_FAILED 7
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. Lastly, thanks to Jeff
Schiller for his help in shaping the security architecture specified
in this document.
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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 [14] is Infoterm, Osterreiches Normungsinstitut (ON),
Postfach 130, A-1021 Vienna, Austria. Contains additions from ISO
639/RA Newsletter No.1/1989. See also RFC 1766.
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. SLP Certificates
Certificates may be used in SLP in order to distribute the public
keys of trusted protected scopes. Assuming public keys, this
appendix discusses the use of such certificates in the Service
Location Protocol.
Possession of the private key of a protected scope is equivalent to
being a trusted SA. The trustworthiness of the protected scope
depends upon all of these private keys being held by trusted hosts,
and used only for legitimate service registrations and
deregistrations.
With access to the proper Certificate Authority (CA), DAs and UAs do
not need (in advance) hold public keys which correspond to these
protected scopes. They do require the public key of the CA. The CA
produces certificates using its unique private key. This private key
is not shared with any other system, and must remain secure. The
certificates declare that a given protected scope has a given public
key, as well as the expiration date of the certificate.
The ASCII (mail-safe) string format for the certificate is the
following list of tag and value pairs:
"certificate-alg=" 1*ASN1CHAR CRLF
"scope-charset=" 1*DIGIT CRLF
"scope=" 1*RADIX-64-CHAR CRLF
"timestamp=" 16HEXDIGIT CRLF
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"public-key=" 1*RADIX-64-CHAR CRLF
"cert-digest=" 1*RADIX-64-CHAR CRLF
ASN1CHAR = DIGIT | '.'
HEXDIGIT = DIGIT | 'a'..'f' | 'A'..'F'
RADIX-64-CHAR = DIGIT | 'a'..'z' | 'A'..'Z' | '+' | '/' | '='
The radix-64 notation is described in RFC 1521 [7]. Spaces are
ignored in the computation of the binary value corresponding to a
Radix-64 string. If the value for scope, public-key or cert-digest
is greater than 72 characters, the Radix-64 notation may be broken up
on to separate lines. The continuation lines must be preceded by one
or more spaces. Only the tags listed above may start in the first
column of the certificate string. This removes ambiguity in parsing
the Radix-64 values (since the tags consist of legal Radix-64
values.)
The certificate-alg is the ASN.1 string for the Object Identifier
value of the algorithm used to produce the "cert-digest". The
scope-charset is a decimal representation of the MIBEnum value for
the character set in which the scope is represented.
The radix-64 encoding of the scope string will allow the ASCII
rendering of a scope string any character set.
The 8 byte NTP format timestamp is represented as 16 hex digits.
This timestamp is the time at which the certificate will expire.
The format for the public key will depend on the type of cryptosystem
used, which is identified by the certificate-alg. When the CA
generated the certificate holding the public key being obtained, it
used the message digest algorithm identified by certificate-alg to
calculate a digest D on the string encoding of the certificate,
excepting the cert-digest. The CA then encrypted this value using
the CA's private key to produce the cert-digest, which is included in
the certificate.
The CA generates the certificate off-line. The mechanism to
distibute certificates is not specified in the Service Location
Protocol, but may be in the future. The CA specifies the algorithms
to use for message digest and public key decryption. The DA or SA
need only obtain the certificate, have a preconfigured public key for
the CA and support the algorithm specified in the certificate-alg in
order to obtain certified new public keys for protected scopes.
The DA or UA may confirm the certificate by calculating the message
digest D, using the message digest algorithm identified by the
certificate-alg. The input to the message digest algorithm is the
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string encoding of the certificate, excepting the cert-digest. The
cert-digest is decrypted using the CA's public key to produce D'. If
D is the same as D', the certificate is legitimate. The public-key
for the protected scope may be used until the expiration date
indicated by the certificate timestamp.
The certificate may be distributed along untrusted channels, such as
email or through file transfer, as it must be verified anyhow. The
CA's public key must be delivered using a trusted channel.
C. Example of deploying SLP security using MD5 and RSA
In our site, we have a protected scope "CONTROLLED". We generate a
private key - public key pair for the scope, using RSA. The private
key is maintained on a secret key ring by all SAs in the protected
scope. The public key is available to all DAs which support the
protected scope and to all UAs which will use it.
In order to register or deregister a URL, the data required to be
authenticated (as described in section 4.3) is digestified using MD5
[22] to create a digital signature, then encrypted by RSA with the
protected scope's private key. The output of RSA is used in the
authenticator data field of the authenticator block.
The DA or UA discovers the appropriate method for verifying the
authentication by looking inside the authentication block. Suppose
that the "md5WithRSAEncryption" [4] algorithm has to be used to
verify the signed data. The DA or UA calculates the message digest
of the URL Entry by using md5, exactly as the SA did. The
authenticator block is decrypted using the public key for the
"CONTROLLED" scope, which is stored in the public key ring of the UA
or DA under the name "CONTROLLED". If the digest calculated by the
UA or DA matches that of the SA, the URL Entry has been validated.
D. Example of use of SLP Certificates by mobile nodes
Say a mobile node needs to make use of protected scopes. The mobile
node is first preconfigured by adding a single public key to its
public key ring: We will call it the CA-Key. This key will be used
to obtain SLP certificates in the format described in Appendix B.
The corresponding private key will be used by the CA to create the
certificates in the necessary format.
The CA might be operated by a system administrator using a computer
which is not connected to any networks. The certificate's duration
will depend on the policy of the site. The duration, scope, and
public key for the protected scope, are used as input to 'md5sum'.
This sum is then encrypted with RSA using the CA's private key. The
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radix 64 encoding of this is added to the mail-safe string based
certificate encoding defined in Appendix B.
The certificate, say for the protected scope "CONTROLLED" could be
made available to the mobile node. For example, it might be on a web
page. The mobile node could then process the certificate in order to
obtain the public key for the CONTROLLED scope. There is still no
reason to *trust* this key is really the one to use (as in Appendix
C). To trust it, calculate the md5 checksum of the ascii encoded
certificate, excluding the cert-digest. Next, decrypt the cert-
digest using the CA's public key and RSA. If the cert-digest matches
the output of MD5, the certificate may be trusted (until it expires).
The mobile node requires only one key (CA-key) in order to obtain
others dynamically and make use of protected scopes. Notice that we
do not define any method for access control by arbitrary UAs to SAs
in protected scopes.
E. Appendix: For Further Reading
Three related resource discovery protocols are NBP and ZIP which are
part of the AppleTalk protocol family [12], the Legato Resource
Administration Platform [25], and the Xerox Clearinghouse system
[20]. Domain names and representation of addresses are used
extensively in the Service Location Protocol. The references for
these are RFCs 1034 and 1035 [17, 18]. Example of a discovery
protocol for routers include Router Discovery [10] and Neighbor
Discovery [19].
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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] Alexander, S. and R. Droms. DHCP Options and BOOTP Vendor
Extensions. RFC 2131, March 1997.
[3] Atkinson, R. IP Encapsulating Security Payload. RFC 1827,
August 1995.
[4] Balenson, D. Privacy Enhancement for Internet Electronic
Mail: Part III: Algorithms, Modes, and Identifiers. RFC 1423,
February 1993.
[5] Berners-Lee, T. and D. Connolly. Hypertext Markup Language -
2.0. RFC 1866, November 1995.
[6] Berners-Lee, T., L. Masinter, and M. McCahill. Uniform Resource
Locators (URL). RFC 1738, December 1994.
[7] Borenstein, N. and N. Freed. MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies. RFC 2045, November 1996.
[8] Bradner, Scott. Key words for use in RFCs to Indicate
Requirement Levels. BCP 14, RFC 2119, March 1997.
[9] CCITT. Specification of the Abstract Syntax Notation One
(ASN.1). Recommendation X.208, 1988.
[10] Deering, Stephen E., editor. ICMP Router Discovery Messages.
RFC 1256, September 1991.
[11] Droms, Ralph. Dynamic Host Configuration Protocol. RFC 2131,
March 1997.
[12] Gursharan, S., R. Andrews, and A. Oppenheimer. Inside
AppleTalk. Addison-Wesley, 1990.
[13] Guttman, E. The service: URL scheme, November 1996.
Work In Progress.
[14] Geneva ISO. Code for the representation of names of languages.
ISO 639:1988 (E/F), 1988.
Veizades, et. al. Standards Track [Page 70]
RFC 2165 Service Location Protocol June 1997
[15] ISO 8879, Geneva. Information Processing -- Text and Office
Systems - Standard Generalized Markup Language (SGML).
, 1986.
[16] Mills, D. Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI. RFC 2030, October 1996.
[17] Mockapetris, P. Domain Names - Concepts and Facilities. STD 13,
RFC 1034, November 1987.
[18] Mockapetris, P. DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION. STD 13, RFC 1035, November 1987.
[19] Narten, T., E. Nordmark, and W. Simpson. Neighbor Discovery for
IP version 6 (IPv6). RFC 1970, August 1996.
[20] Oppen, D. 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.
[21] Perkins, C. DHCP Options for Service Location Protocol, August
1996. Work In Progress.
[22] Rivest, Ronald. The MD5 Message-Digest Algorithm. RFC 1321,
April 1992.
[23] Schneier, Bruce. Applied Cryptography: Protocols, Algorithms,
and Source Code in C. John Wiley, New York, NY, USA, 1994.
[24] X/Open Preliminary Specification. File System Safe UCS
Transformation Format (FSS_UTF). Technical Report Document
Number: P316, X/Open Company Ltd., 1994.
[25] Legato Systems. The Legato Resource Administration Platform.
Legato Systems, 1991.
Veizades, et. al. Standards Track [Page 71]
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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: +1 415 336 6697
Fax: +1 415 944 8500
Email: veizades@home.com Email: Erik.Guttman@eng.sun.com
Charles E. Perkins Scott Kaplan
Sun Microsystems
2550 Garcia Avenue 346 Fair Oaks St.
Mountain View, CA 94043 San Francisco, CA 94110
Phone: +1 415 336 7153 Phone: +1 415 285 4526
Fax: +1 415 336 0670
EMail: cperkins@Corp.sun.com Email: scott@catch22.com
Veizades, et. al. Standards Track [Page 72]