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RFC 2165

Proposed STD
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Service Location Protocol

Part 1 of 3, p. 1 to 19
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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

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 4. Service Location General Message Format                           14
     4.1. Use of Transaction IDs (XIDs) . . . . . . . . . . . . . .   15
     4.2. URL Entries . . . . . . . . . . . . . . . . . . . . . . .   16
     4.3. Authentication Blocks . . . . . . . . . . . . . . . . . .   17
     4.4. URL Entry Lifetime  . . . . . . . . . . . . . . . . . . .   19
 5. Service Request Message Format                                    19
     5.1. Service Request Usage . . . . . . . . . . . . . . . . . .   22
     5.2. Directory Agent Discovery Request . . . . . . . . . . . .   23
     5.3. Explanation of Terms of Predicate Grammar . . . . . . . .   24
     5.4. Service Request Predicate Grammar . . . . . . . . . . . .   26
     5.5. String Matching for Requests  . . . . . . . . . . . . . .   27
 6. Service Reply Message Format                                      28
 7. Service Type Request Message Format                               29
 8. Service Type Reply Message Format                                 31
 9. Service Registration Message Format                               32
 10. Service Acknowledgement Message Format                           35
 11. Service Deregister Message Format                                37
 12. Attribute Request Message Format                                 38
 13. Attribute Reply Message Format                                   40
 14. Directory Agent Advertisement Message Format                     42
 15. Directory Agents                                                 43
    15.1. Introduction  . . . . . . . . . . . . . . . . . . . . . .   43
    15.2. Finding Directory Agents  . . . . . . . . . . . . . . . .   43
 16. Scope Discovery and Use                                          45
    16.1. Protected Scopes  . . . . . . . . . . . . . . . . . . . .   46
 17. Language and Character Encoding Issues                           47
    17.1. Character Encoding and String Issues  . . . . . . . . . .   48
          17.1.1. Substitution of Character Escape Sequences  . . .   49
    17.2. Language-Independent Strings  . . . . . . . . . . . . . .   49
 18. Service Location Transactions                                    50
    18.1. Service Location Connections  . . . . . . . . . . . . . .   50
    18.2. No Synchronous Assumption . . . . . . . . . . . . . . . .   51
    18.3. Idempotency . . . . . . . . . . . . . . . . . . . . . . .   51
 19. Security Considerations                                          51
 20. String Formats used with Service Location Messages               52
    20.1. Previous Responders' Address Specification  . . . . . . .   53
    20.2. Formal Definition of the "service:" Scheme  . . . . . . .   53
          20.2.1. Service Type String . . . . . . . . . . . . . . .   54
    20.3. Attribute Information . . . . . . . . . . . . . . . . . .   54
    20.4. Address Specification in Service Location . . . . . . . .   55
    20.5. Attribute Value encoding rules  . . . . . . . . . . . . .   55
 21. Protocol Requirements                                            56
    21.1. User Agent Requirements . . . . . . . . . . . . . . . . .   56
    21.2. Service Agent Requirements  . . . . . . . . . . . . . . .   58
    21.3. Directory Agent Requirements  . . . . . . . . . . . . . .   59
 22. Configurable Parameters and Default Values                       61
    22.1. Service Agent:  Use Predefined Directory Agent(s) . . . .   62
    22.2. Time Out Intervals  . . . . . . . . . . . . . . . . . . .   63

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 23. Non-configurable Parameters                                      63
 24. Acknowledgments                                                  64
 A. Appendix:  Technical contents of ISO 639:1988 (E/F): "Code for
   the representation of names of languages"                          65
 B. SLP Certificates                                                  66
 C. Example of deploying SLP security using MD5 and RSA               68
 D. Example of use of SLP Certificates by mobile nodes                68
 E. Appendix:  For Further Reading                                    69

1. Introduction

   Traditionally, users find services by using the name of a network
   host (a human readable text string) which is an alias for a network
   address.  The Service Location Protocol eliminates the need for a
   user to know the name of a network host supporting a service.
   Rather, the user names the service and supplies a set of attributes
   which describe the service.  The Service Location Protocol allows the
   user to bind this description to the network address of the service.

   Service Location provides a dynamic configuration mechanism for
   applications in local area networks.  It is not a global resolution
   system for the entire Internet; rather it is intended to serve
   enterprise networks with shared services.  Applications are modeled
   as clients that need to find servers attached to the enterprise
   network at a possibly distant location.  For cases where there are
   many different clients and/or services available, the protocol is
   adapted to make use of nearby Directory Agents that offer a
   centralized repository for advertised services.

2. Terminology

      User Agent (UA)
                A process working on the user's behalf to acquire
                service attributes and configuration.  The User Agent
                retrieves service information from the Service Agents or
                Directory Agents.

      Service Agent (SA)
                A process working on the behalf of one or more services
                to advertise service attributes and configuration.

      Service Information
                A collection of attributes and configuration information
                associated with a single service.  The Service Agents
                advertise service information for a collection of
                service instances.

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      Service   The service is a process or system providing a facility
                to the network.  The service itself is accessed using a
                communication mechanism external to the the Service
                Location Protocol.

      Directory Agent (DA)
                A process which collects information from Service Agents
                to provide a single repository of service information in
                order to centralize it for efficient access by User
                Agents.  There can only be one DA present per given

      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.

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      Site Network
                All the hosts accessible within the Agent's multicast
                radius, which defaults to a value appropriate for
                reaching all hosts within a site (see section 22).  If
                the site does not support multicast, the agent's site
                network is restricted to a single subnet.

      URL       A Universal Resource Locator - see [6].

      Address Specification
                This is the network layer protocol dependent mechanism
                for specifying an Agent.  For Internet systems this is
                part of a URL.

2.1. Notation Conventions

      CAPS   Strings which appear in all capital letters are protocol
             literal.  All string comparison is case insensitive,
             however, (see section 5.5).  Some strings are quoted in
             this document to indicate they should be used literally.
             Single characters inside apostrophes are included

      <>     Values set off in this manner are fully described in
             section 20.  In general, all definitions of items in
             messages are described in section 20 or immediately
             following their first use.

      |  |
      \  \   Message layouts with this notation indicate a variable
      |  |   length field.

2.2. Service Information and Predicate Representation

   Service information is represented in a text format.  The goal is
   that the format be human readable and transmissible via email.  The
   location of network services is encoded as a Universal Resource
   Locator (URL) which is human readable.  Only the datagram headers are
   encoded in a form which is not human readable.  Strings used in the
   Service Location Protocol are NOT null-terminated.

   Predicates are expressed in a simple boolean notation using keywords,
   attributes, and logical connectives, as described in Section 5.4.

   The logical connectives and subexpressions are presented in prefix-
   order, so that the connective comes first and the expressions it
   operates on follow afterwards.

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2.3. Specification Language

   In this document, several words are used to signify the requirements
   of the specification [8].  These words are often capitalized.

      MUST       This word, or the adjective "required", means that
                 the definition is an absolute requirement of the

      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.

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3.1. Protocol Transactions

   The diagram below illustrates the relationships described below:

      +---------------+   we want this info:     +-----------+
      |  Application  | - - - - - - - - - - - -> |  Service  |
      +---------------+                          +-----------+
           /|\                                      |     |
            |                         +-------------+     |
            |                         |                   |
           \|/                       \|/                 \|/
      +---------------+          +-----------+      +----------------+
      |   User Agent  |<-------->|  Service  |      |    Service     |
      +---------------+          |   Agent   |      | Agent which    |
            |                    +-----------+      | does not reply |
            |                         |             | to UA requests |
            |                        \|/            +----------------+
            |                   +-------------+           |
            +------------------>|  Directory  |<----------+
                                |    Agent    |
                                +-------------+      ___________
                                     /|\            / Many other\
                                      +------------>|   SA's    |

   The following describes the operations a User Agent would employ to
   find services on the site's network.  The User Agent needs no
   configuration to begin network interaction.  The User Agent can
   acquire information to construct predicates which describe the
   services that match the user's needs.  The User Agent may build on
   the information received in earlier network requests to find the
   Service Agents advertising service information.

   A User Agent will operate two ways:  If the User Agent has already
   obtained the location of a Directory Agent, the User Agent will
   unicast a request to it in order to resolve a particular request.
   The Directory Agent will unicast a reply to the User Agent.  The User
   Agent will retry a request to a Directory Agent until it gets a
   reply, so if the Directory Agent cannot service the request (say it
   has no information) it must return an response with zero values,
   possibly with an error code set.

   If the User Agent does not have knowledge of a Directory Agent or if
   there are no Directory Agents available on the site network, a second
   mode of discovery may be used.  The User Agent multicasts a request
   to the service-specific multicast address, to which the service it
   wishes to locate will respond.  All the Service Agents which are
   listening to this multicast address will respond, provided they can

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   satisfy the User Agent's request.  A similar mechanism is used for
   Directory Agent discovery; see section 5.2.  Service Agents which
   have no information for the User Agent MUST NOT respond.

   When a User Agent wishes to obtain an enumeration of ALL services
   which satisfy the query, a retransmission/convergence algorithm is
   used.  The User Agent resends the request, together with a list of
   previous responders.  Only those Service Agents which are not on the
   list respond.  Once there are no new responses to the request the
   accumulation of responses is deemed complete.  Depending on the
   length of the request, around 60 previous responders may be listed in
   a single datagram.  If there are more responders than this, the
   scaling mechanisms described in section 3.7 should be used.

   While the multicast/convergence model may be important for
   discovering services (such as Directory Agents) it is the exception
   rather than the rule.  Once a User Agent knows of the location of a
   Directory Agent, it will use a unicast request/response transaction.

   The Service Agent SHOULD listen for multicast requests on the
   service-specific multicast address, and MUST register with an
   available Directory Agent.  This Directory Agent will resolve
   requests from User Agents which are unicasted using TCP or UDP. This
   means that a Directory Agent must first be discovered, using DHCP,
   the DA Discovery Multicast address, the multicast mechanism described
   above, or manual configuration.  See section 5.2.

   A Service Agent which does not respond to multicast requests will not
   be useful in the absence of Directory Agents.  Some Service Agents
   may not include this functionality, if an especially lightweight
   implementation is required.

   If the service is to become unavailable, it should be deregistered
   with the Directory Agent.  The Directory Agent responds with an
   acknowledgment to either a registration or deregistration.  Service
   Registrations include a lifetime, and will eventually expire.
   Service Registrations need to be refreshed by the Service Agent
   before their Lifetime runs out.  If need be, Service Agents can
   advertise signed URLs to prove that they are authorized to provide
   the service.

3.2. Schemes

   The Service Location Protocol, designed as a way for clients to
   access resources on the network, is a natural application for
   Universal Resource Locators (URLs).  It is intended that by re-using
   URL specification and technology from the World Wide Web, clients and
   servers will be more flexible and able to be written using already

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   existing code.  Moreover, it is hoped that browsers will be written
   to take advantage of the similarity in locator format, so that a
   client can dynamically formulate requests for services that are
   resolved differently depending upon the circumstances.

3.2.1. The "service:"  URL scheme

   The service URL scheme is used by Service Location.  It is used to
   specify a Service Location.  Many Service Types will be named by
   including a scheme name after the "service:"  scheme name.  Service
   Types are used by SAs to register and deregister Services with DAs.
   It is also used by SAs and DAs to return Service Replies to UAs.  The
   formal definition of the "service:" URL scheme is in section 20.2.
   The format of the information which follows the "service:"  scheme
   should as closely as possible follow the URL structure and semantics
   as formalized by the IETF standardization process.

   Well known Service Types are registered with the IANA and templates
   are available as RFCs.  Private Service Types may also be supported.

3.3. Standard Attribute Definitions

   Service Types used with the Service Location Protocol must describe
   the following:

         Service Type string of the service
         Attributes and Keywords
         Attribute Descriptions and interpretations

   Service Types not registered with IANA will use their own Naming
   Authority string.  The registration process for new Service Types is
   defined in [13].

   Services which advertise a particular Service Type must support the
   complete set of standardized attributes.  They may support additional
   attributes, beyond the standardized set.  Unrecognized attributes
   MUST be ignored by User Agents.

   Service Type names which begin with "x-" are guaranteed not to
   conflict with any officially registered Service Type names.  It is
   suggested that this prefix be used for experimental or private
   Service Type names.  Similarly, attribute names which begin with "x-"
   are guaranteed not to be used for any officially registered attribute

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

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

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

3.4. Naming Authority

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

   Naming Authorities may define Service Types which are experimental,
   proprietary or for private use.  The procedure to use is to create a
   'unique' Naming Authority string and then specify the Standard
   Attribute Definitions as described above.  This Naming Authority will
   accompany registration and queries, as described in sections 5 and 9.

3.5. Interpretation of Service Location Replies

   Replies should be considered to be valid at the time of delivery.
   The service may, however, fail or change between the time of the
   reply and the moment an application seeks to make use of the service.
   The application making use of Service Location MUST be prepared for
   the possibility that the service information provided is either stale
   or incomplete.  In the case where the service information provided
   does not allow a User Agent to connect to a service as desired, the
   Service Request and/or Attribute Request may be resubmitted.

   Service specific configuration information (such as which protocol to
   use) should be included as attribute information in Service
   Registrations.  These configuration attributes will be used by
   applications which interpret the Service Location Reply.

3.6. Use of TCP, UDP and Multicast in Service Location

   The Service Location Protocol requires the implementation of UDP
   (connectionless) and TCP (connection oriented) transport protocols.
   The latter is used for bulk transfer, only when necessary.
   Connections are always initiated by an agent request or registration,
   not by a replying Directory Agent.  Service Agents and User Agents
   use ephemeral ports for transmitting information to the service
   location port, which is 427.

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   The Service Location discovery mechanisms typically multicast
   messages to as many enterprise networks as needed to establish
   service availability.  The protocol will operate in a broadcast
   environment with limitations detailed in section 3.6.1.

3.6.1. Multicast vs.  Broadcast

   The Service Location Protocol was designed for use in networks where
   DHCP is available, or multicast is supported at the network layer.
   To support this protocol when only network layer broadcast is
   supported, the following procedures may be followed. 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

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   multicast addresses the Service Location General Multicast Address
   MAY be used.  Service Agents MUST listen on this multicast address as
   well as the service-specific multicast addresses for the service
   types they advertise.

   Service-Specific Multicast Addresses are computed by calculating a
   string hash on the Service Type string.  The Service Type string MUST
   first be converted to an ASCII string from whatever character set it
   is represented in, so the hash will have well-defined results.

   The string hash function is modified from a code fragment attributed
   to Chris Torek:

         *  SLPhash returns a hash value in the range 0-1023 for a
         *  string of single-byte characters, of specified length.
        unsigned long SLPhash (const char *pc, unsigned int length)
            unsigned long h = 0;
    while (length-- != 0) {
                h *= 33;
                h += *pc++;
            return (0x3FF & h);  /* round to a range of 0-1023 */

   This value is added to the base range of Service Specific Discovery
   Addresses, to be assigned by IANA. These will be 1024 contiguous
   multicast addresses.

3.7. Service Location Scaling, and Multicast Operating Modes

   In a very small network, with few nodes, no DA is required.  A user
   agent can detect services by multicasting requests.  Service Agents
   will then reply to them.  Further, Service Agents which respond to
   user requests must be used to make service information available.
   This does not scale to environments with many hosts and services.

   When scaling Service Location systems to intermediate sized networks,
   a central repository (Directory Agent) may be added to reduce the
   number of Service Location messages transmitted in the network
   infrastructure.  Since the central repository can respond to all
   Service and Attribute Requests, fewer Service and Attribute Replies
   will be needed; for the same reason, there is no need to
   differentiate between Directory Agents.

   A site may also grow to such a size that it is not feasible to
   maintain only one central repository of service information.  In this

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   case more Directory Agents are needed.  The services (and service
   agents) advertised by the several Directory Agents are collected
   together into logical groupings called "Scopes".

   All Service Registrations that have a scope must be registered with
   all DAs (within the appropriate multicast radius) of that scope which
   have been or are subsequently discovered.  Service Registrations
   which have no scope are only registered with unscoped DAs.  User
   Agents make requests of DAs whose scope they are configured to use.

   Service Agents MUST register with unscoped DAs even if they are
   configured to specifically register with DAs which have a specific
   scope or set of scopes.  User Agents MAY query DAs without scopes,
   even if they are configured to use DAs with a certain scope.  This is
   because any DA with no scope will have all the available service

   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.

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   A subsequent protocol document will describe mechanisms for
   supporting a service discovery protocol for the global Internet.

4. Service Location General Message Format

   The following header is used in all of the message descriptions below
   and is abbreviated by using "Service Location header =" followed by
   the function being used.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |    Version    |    Function   |            Length             |
     |O|M|U|A|F| rsvd|    Dialect    |        Language Code          |
     |        Char Encoding          |              XID              |

      Version  This protocol document defines version 1 of the Service
               Location protocol.

      Function Service Location datagrams can be identified as to their
               operation by the function field.  The following are the
               defined operations:

               Message Type             Abbreviation     Function Value

               Service Request          SrvReq               1
               Service Reply            SrvRply              2
               Service Registration     SrvReg               3
               Service Deregister       SrvDereg             4
               Service Acknowledge      SrvAck               5
               Attribute Request        AttrRqst             6
               Attribute Reply          AttrRply             7
               DA Advertisement         DAAdvert             8
               Service Type Request     SrvTypeRqst          9
               Service Type Reply       SrvTypeRply          10

      Length   The number of bytes in the message, including the Service
               Location Header.

      O        The 'Overflow' bit.  See Section 18 for the use of this

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      M        The 'Monolingual' bit.  Requests with this bit set
               indicate the User Agent will only accept responses in the
               language (see section 17) that is indicated by the
               Service or Attribute Request.

      U        The 'URL Authentication Present' bit.  See sections 4.2,
               4.3, 9, and 11 for the use of this field.

      A        The 'Attribute Authentication Present' bit.  See
               sections 4.2, 4.3, and 13 for the use of this field.

      F        If the 'F' bit is set in a Service Acknowledgement, the
               directory agent has registered the service as a new
               entry, not as an updated entry.

      rsvd     MUST be zero.

      Dialect  Dialect tags will be used by future versions of the
               Service Location Protocol to indicate a variant of
               vocabulary used.  This field is reserved and MUST be set
               to 0 for compatibility with future versions of the
               Service Location Protocol.

      Language Code
               Strings within the remainder of the message which follows
               are to be interpreted in the language encoded (see
               section 17 and appendix A) in this field.

      Character Encoding
               The characters making up strings within the remainder of
               the message may be encoded in any standardized encoding
               (see section 17.1).

      Transaction Identifier (XID)
               The XID (transaction ID) field allows the requester to
               match replies to individual requests (see section 4.1).

               Note that, whenever there is an Attribute Authentication
               block, there will also be a URL Authentication block.
               Thus, it is an error to have the 'A' bit set without also
               having the 'U' bit set.

4.1. Use of Transaction IDs (XIDs)

   Retransmission is used to ensure reliable transactions in the Service
   Location Protocol.  If a User Agent or Service Agent sends a message
   and fails to receive an expected response, the message will be sent
   again.  Retransmission of the same Service Location datagram should

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   not contain an updated XID. It is quite possible the original request
   reached the DA or SA, but reply failed to reach the requester.  Using
   the same XID allows the DA or SA to cache its reply to the original
   request and then send it again, should a duplicate request arrive.
   This cached information should only be held very briefly
   (CONFIG_INTERVAL_0.)  Any registration or deregistration at a
   Directory Agent, or change of service information at a SA should
   flush this cache so that the information returned to the client is
   always valid.

   The requester creates the XID from an initial random seed and
   increments it by one for each request it makes.  The XIDs will
   eventually wrap back to zero and continue incrementing from there.

   Directory Agents use XID values in their DA Advertisements to
   indicate their state (see section 15.2).

4.2. URL Entries

   When URLs are registered, they have lifetimes and lengths, and may be
   authenticated.  These values are associated with the URL for the
   duration of the registration.  The association is known as a "URL-
   entry", and has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |           Lifetime            |        Length of URL          |
     |                                                               |
     \                              URL                              \
     |                                                               |
     |              (if present) URL Authentication Block .....

      Lifetime   The length of time that the registration is valid, in
               the absence of later registrations or deregistration.

      Length of URL
               The length of the URL, measured in bytes and < 32768.

      URL Authentication Block
               (if present) A timestamped authenticator (section 4.3)

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   The URL conforms to RFC 1738 [6].  If the 'U' bit is set in the
   message header, the URL is followed by an URL Authentication Block.
   If the scheme used in the URL does not have a standardized
   representation, the minimal requirement is:


   "service" is the URL scheme of all Service Location Information
   included in service registrations and service replies.  Each URL
   entry contains the service:<srvtype> scheme name.  It may also
   include an <addr-spec> except in the case of a reply to a Service
   Type request (see section 7).

4.3. Authentication Blocks

   Authentication blocks are used to authenticate service registrations
   and deregistrations.  URLs are registered along with an URL
   Authentication block to retain the authentication information in the
   URL entry for subsequent use by User Agents who receive a Service
   Reply containing the URL entry.  Service attributes are registered
   along with an Attribute Authentication block.  Both authentication
   blocks have the format illustrated below.

   If a service registration is accompanied by authentication which can
   be validated by the DA, the DA MUST validate any subsequent service
   deregistrations, so that unauthorized entities cannot invalidate such
   registered services.  Likewise, if a service registration is
   accompanied by an Attribute Authentication block which can be
   validated by the DA, the DA MUST validate any subsequent attribute
   registrations, so that unauthorized entities cannot invalidate such
   registered attributes.

   To avoid replay attacks which use previously validated
   deregistrations, the deregistration or attribute registration message
   must contain a timestamp for use by the DA. To avoid replay attacks
   which use previously validated registrations to nullify a valid
   deregistration, registrations must also contain a timestamp.

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   An authentication block has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                                                               |
     +                           Timestamp                           +
     |                                                               |
     |  Block Structure Descriptor   |            Length             |
     |            Structured Authenticator ...

      Timestamp A 64-bit value formatted as specified by the Network
               Time Protocol (NTP) [16].

      Block Structure Descriptor (BSD)
               A value describing the structure of the Authenticator.
               The only value currently defined is 1, for

      Length   The length of the Authenticator

      Structured Authenticator
               An algorithm specification, and the authentication data
               produced by the algorithm.

   The Structured Authenticator contains a digital signature of the
   information being authenticated.  It contains sufficient information
   to determine the algorithm to be used and the keys to be selected to
   verify the digital signature.

   The digital signature is computed over the following ordered stream
   of data:

       CHARACTER ENCODING OF URL   (2 bytes in network byte order)
       LIFETIME                    (2 bytes in network byte order)
       LENGTH OF URL               (2 bytes in network byte order)
       URL                         (n bytes)
       TIMESTAMP                   (8 bytes in SNTP format [16])

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   When producing a URL Authentication block, the authentication data
   produced by the algorithm identified within the Structured
   Authenticator calculated over the following ordered stream of data:

       ATTRIBUTE CHARACTER ENCODING   (2 bytes in network byte order)
       LENGTH OF ATTRIBUTES           (2 bytes in network byte order)
       ATTRIBUTES                     (n bytes)
       TIMESTAMP                      (8 bytes in SNTP format [16])

   Every Service Location Protocol entity (User Agent, Service Agent, or
   Directory Agent) which is configured for use with protected scopes
   SHOULD implement "md5WithRSAEncryption" [4] and be able to associate
   it with BSD value == 1.

   In the case where BSD value == 1 and the OID "md5WithRSAEncryption"
   is selected, the Structured Authenticator will start with the ASN.1
   Distinguished Encoding (DER) [9] for "md5WithRSAEncryption", which
   has the as its value the bytes (MSB first in hex):

      "30 0d 06 09 2a 86 48 86 f7 0d 01 01 04 05 00"

   This is then immediately followed by an ASN.1 Distinguished Encoding
   (as a "Bitstring") of the RSA encryption (using the Scope's private
   key) of a bitstring consisting of the OID for "MD5" concatenated by
   the MD5 [22] message digest computed over the fields above.  The
   exact construction of the MD5 OID and digest can be found in RFC 1423

4.4. 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.

(page 19 continued on part 2)

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