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RFC 2165 - Service Location Protocol

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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.


   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

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

 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

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.

      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

      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.

                A string describing a characteristic of a service.

                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).

                A boolean expression of attributes, relations and
                logical operators.  The predicate is used to find
                services which satisfy particular requirements.  See
                section 5.3.

                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.

      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

      <>     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.

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

      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

      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.

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

   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

   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

   A service of a given Service Type should accept the networking
   protocol which is implied in its definition.  If a Service Type can
   accept multiple protocols, configuration information SHOULD be

   included in the Service Type attribute information.  This
   configuration information will enable an application to use the
   results of a Service Request and Attribute Request to directly
   connect to a service.

   See section 20.2.1 for the format of a Service Type String as used in
   the Service Location Protocol.

3.4. Naming Authority

   The Naming Authority of a service defines the meaning of the Service
   Types and attributes registered with and provided by Service
   Location.  The Naming Authority itself is a string which uniquely
   identifies an organization.  If no string is provided IANA is the
   default.  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.

   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. 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. 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

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

   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

   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

   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

   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.

   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

      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

   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)

   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" 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.

   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

      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])

   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. URL Entry Lifetime

   The Lifetime field is set to the number of seconds the reply can be
   cached by any agent.  A value of 0 means the information must not be
   cached.  User Agents MAY cache service information, but if they do,
   they must provide a way for applications to flush this cached
   information and issue the request directly onto the network.

   Services should be registered with DAs with a Lifetime, the suggested
   value being CONFIG_INTERVAL_1.  The service must be reregistered
   before this interval elapses, or the service advertisement will no
   longer be available.  Thus, services which vanish and fail to
   deregister eventually become automatically deregistered.

5. Service Request Message Format

   The Service Request is used to obtain URLs from a Directory Agent or
   Service Agents.

   The format of the Service Request is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |         Service Location header (function = SrvReq)           |
     |length of prev resp list string|<Previous Responders Addr Spec>|
     |                                                               |
     \                  <Previous Responders Addr Spec>              \
     |                                                               |
     |  length of predicate string   |  Service Request <predicate>  |
     |                                                               |
     \               Service Request <predicate>, contd.             \
     |                                                               |

   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

   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,

   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:


   The punctuation is necessary even where the fields are omitted.

    -  The <srvtype> refers to the Service Type.  For each type of
       service available, there is a unique Service type name string.
       See section 20.2.1.

    -  The <na> is the Naming Authority.  This string determines the
       semantic interpretation of the attribute information in the
       <where> part of the Service Request.

    -  The <scope> is a string used to restrict the range of the query.
       Scope is determined administratively, at a given site.  It is not
       necessarily related to network topology (see Section 16).
       Leaving this field out means that the request can be satisfied
       only by unscoped service advertisements.

    -  The <where> string is the Where Clause of the request.  It
       contains 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.

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)
               (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

   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

   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,
           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

   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:


   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.

   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:// Scope

   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:


         Placed in a Service Request, this is interpreted by a Service
         Agent or Directory Agent to determine what information to


         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.


         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.


         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

         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>)

         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>.


         A query item has the form:

               '(' <attr-tag> <comp-op> <attr-val> ')'


               '(' <keyword> ')'

         Examples of this would be:

            (QUEUE LENGTH <= 234)


         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

               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.)

   <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> |

   <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 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

   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.


        "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.

   The Error Code may have one of the following values:

      0        Success

               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.

               A SA or DA returns this error when a SrvRply is received
               which cannot be parsed or the declared string lengths
               overrun the message.

               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.

               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.

   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:


   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

   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              \
     |                                                               |

   The Error Code may have one of the following values:

      0        Success

               A SA or DA returns this error when a SrvTypeRqst is
               received which cannot be parsed.

               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

               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

   The following are examples of Service Type Strings which might be
   found in Service Type Replies:


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).

   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

   If a SA attempts to register a service with a DA and the registration
   is larger than the site path MTU, then the DA will reply with a
   SrvAck, with the error set to INVALID_REGISTRATION and the 'Overflow'
   byte set.

   An example of Service Registration information is:

      Lifetime (seconds):   16-bit unsigned integer
      URL (at least):       service:<srvtype>://<addr-spec>
      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

      Lifetime:   10800
      URL:        service:lpr://igore.wco.ftp.com:515/draft
      Attributes: (SCOPE=DEVELOPMENT),
                  (PAPER COLOR=WHITE),
                  (PAPER SIZE=LETTER),
                  (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

      Lifetime:   10800
      URL:        service:lpr://igore.wco.ftp.com:515/draft
      Attributes: (SCOPE=ENTWICKLUNG),
                  (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.

   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           |

   The Error Code may have one of the following values:

      0        Success

               A DA returns this error when the SrvReg or SrvDereg is
               received which cannot be parsed or the declared string
               lengths overrun the message.

               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.

               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.

               If the DA receives a SrvReg or SrvDereg in a character
               set which it does not support, it will return this error.

               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.

               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.

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:

        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:


   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

   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.

   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.

   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:


   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

               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.

               A DA or SA returns this error when a AttrRqst is received
               which cannot be parsed or the declared string lengths
               overrun the message.

               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.

               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):

         (PAGES PER MINUTE=1,3,12),

   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.

   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

      URL:        service:directory-agent://SLP-RESOLVER.CATCH22.COM
      SCOPE List: ADMIN

   The Directory Agent can be reached at the Address Specification
   returned, and supports the SCOPE called "ADMIN".

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).

   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

   When a Service Agent or User Agent first comes on-line it must issue
   a Directory Agent Discovery Request unless it is using static or DHCP
   configuration, as described in 5.2.

   A Service Agent registers information with ALL newly discovered
   Directory Agents when either of the above two events take place.
   When scopes are being used, a Service Agent SHOULD choose a set of
   scopes to be advertised in and need only register with Directory
   Agents that support the scopes in which they wish to be registered.
   Services MUST be registered with DAs that support their scope and
   those which have no scope, unless specifically configured not to do
   so (see section 22.1.)

   Once a User Agent becomes aware of a Directory Agent it will unicast
   its queries there.  In the event that more than one Directory Agent
   is detected, it will select one to communicate with.  When scopes are
   supported, the User Agent will direct its queries to different
   Directory Agents depending on which scopes are appropriate domains
   for the query to be answered in.

   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.

   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

   A DA which has no scope will reply to any Directory Agent Discovery

   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

   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.

   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
   and have the values referred by the MIBEnum value.

   The encoding will determine the interpretation of all character data
   which follows the Service Location Protocol header.  There is no way
   to mix ASCII and UNICODE, for example.  All responses must be in the
   character set of the request, or use US-ASCII. If a request is sent
   to a DA or SA or a registration is sent to a DA, which is unable to
   manipulate or store the character set of the incoming message, the
   request will fail.  The SA or DA returns a CHARSET_NOT_UNDERSTOOD
   error in a SrvAck message in this case.  Requests using US-ASCII will

   never fail for this reason, since all SAs and DAs must be able to
   accept this character set.

   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 &#44; 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

   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.

    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

   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.

   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,
      <attr-list>                           20.3             SrvReg,
      <Service Registration Information>    9                SrvReg
      <Service Deregister Information>      11               SrvDereg
      <Service Type String>                 20.2.1           AttrRqst

20.1. Previous Responders' Address Specification

   The previous responders' Address Specification is specified as

      <Previous Responders' Address Specification> ::=
             <addr-spec> |
             <addr-spec>, <Previous Responders' Address Specification>

   i.e., a list separated 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.



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

   <url> ::= service:<srvtype>://<addr-spec>


      service       the URL scheme for Service Location, to return

      <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

   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:


   The third example has three attributes in the list.  Color can take
   on the values red, white and blue.  There are several other examples
   of replies throughout the document.

20.4. Address Specification in Service Location

   The address specification used in Service Location is:

     <addr-spec> ::= [<user>:<password>@]<host>[:<port>]

     <host>      ::= Fully qualified domain name |
                     dotted decimal IP address notation

   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.

    -  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

    -  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. 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

    -  Require an authentication block in any URL entry returned as
       part of a Service Request, before making use of the advertised

   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

    -  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

    -  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.

    -  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]

   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.

       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

    -  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.

   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

    -  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

    -  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.

    -  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 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

      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.

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
   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

   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

   Multicast Addresses

         Service Location General Multicast Address:
         Directory Agent Discovery Multicast Address:

   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
         PROTOCOL_PARSE_ERROR       2
         SCOPE_NOT_SUPPORTED        4

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.

  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

    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

   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

      "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

   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

   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

   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].


   [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.

   [15] ISO 8879, Geneva.  Information Processing -- Text and Office
        Systems - Standard Generalized Markup Language (SGML).
        <URL:http://www.iso.ch/cate/d16387.html>, 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.

        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.

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


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