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Versions: 00 01 02 03 04 05 07 08 09 10 11 12 13 14 15 RFC 2608

Internet Engineering Task Force                             Erik Guttman
INTERNET DRAFT                                           Charles Perkins
09 October 1998                                         Sun Microsystems
Updates:  RFC 2165                                         John Veizades
                                                           @Home Network
                                                             Michael Day
                                                                   Intel

                  Service Location Protocol, Version 2
                  draft-ietf-svrloc-protocol-v2-09.txt


Status of This Memo

   This document is a submission by the Service Location Working Group
   of the Internet Engineering Task Force (IETF).  Comments should be
   submitted to the srvloc@srvloc.org mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To view the entire list of current Internet-Drafts, please check
   the ``1id-abstracts.txt'' listing contained in the Internet-Drafts
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   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).


Abstract

   The Service Location Protocol provides a scalable framework for
   the discovery and selection of network services.  Using this
   protocol, computers using the Internet need little or no static
   configuration of network services for network based applications.
   This is especially important as computers become more portable, and
   users less tolerant or able to fulfill the demands of network system
   administration.









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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1
     1.1. Applicability Statement . . . . . . . . . . . . . . . . .    2
     1.2. Changes to the Service Location Protocol from v1 to v2  .    2

 2. Terminology                                                        3
     2.1. Notation Conventions  . . . . . . . . . . . . . . . . . .    3

 3. Protocol Overview                                                  4

 4. URLs used with Service Location                                    7
     4.1. Service: URLs . . . . . . . . . . . . . . . . . . . . . .    7
     4.2. Naming Authorities  . . . . . . . . . . . . . . . . . . .    8
     4.3. URL Entries . . . . . . . . . . . . . . . . . . . . . . .    9

 5. Service Attributes                                                 9

 6. Required Features                                                 11
     6.1. Use of Ports, UDP, and Multicast  . . . . . . . . . . . .   12
     6.2. Use of TCP  . . . . . . . . . . . . . . . . . . . . . . .   13
     6.3. Retransmission of SLP messages  . . . . . . . . . . . . .   13
     6.4. Strings in SLP messages . . . . . . . . . . . . . . . . .   14
           6.4.1. Scope Lists in SLP  . . . . . . . . . . . . . . .   15

 7. Errors                                                            15

 8. Required SLP Messages                                             16
     8.1. Service Request . . . . . . . . . . . . . . . . . . . . .   17
     8.2. Service Reply . . . . . . . . . . . . . . . . . . . . . .   20
     8.3. Service Registration  . . . . . . . . . . . . . . . . . .   20
     8.4. Service Acknowledgment  . . . . . . . . . . . . . . . . .   22
     8.5. Directory Agent Advertisement . . . . . . . . . . . . . .   22
     8.6. Service Agent Advertisement . . . . . . . . . . . . . . .   24

 9. Optional Features                                                 25
     9.1. Service Location Protocol Extensions  . . . . . . . . . .   25
     9.2. Authentication Blocks . . . . . . . . . . . . . . . . . .   26
           9.2.1. SLP Message Authentication Rules  . . . . . . . .   27
           9.2.2. DSA with SHA-1 in Authentication Blocks . . . . .   28
     9.3. Incremental Service Registration  . . . . . . . . . . . .   29
     9.4. Tag Lists . . . . . . . . . . . . . . . . . . . . . . . .   29

10. Optional SLP Messages                                             30



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    10.1. Service Type Request  . . . . . . . . . . . . . . . . . .   30
    10.2. Service Type Reply  . . . . . . . . . . . . . . . . . . .   31
    10.3. Attribute Request . . . . . . . . . . . . . . . . . . . .   31
    10.4. Attribute Reply . . . . . . . . . . . . . . . . . . . . .   32
    10.5. Attribute Request/Reply Examples  . . . . . . . . . . . .   33
    10.6. Service Deregistration  . . . . . . . . . . . . . . . . .   34

11. Scopes                                                            35
    11.1. Scope Rules . . . . . . . . . . . . . . . . . . . . . . .   36
    11.2. Administrative and User Selectable Scopes . . . . . . . .   36

12. Directory Agents                                                  37
    12.1. Directory Agent Rules . . . . . . . . . . . . . . . . . .   37
    12.2. Directory Agent Discovery . . . . . . . . . . . . . . . .   38
          12.2.1. Active DA Discovery . . . . . . . . . . . . . . .   38
          12.2.2. Passive DA Advertising  . . . . . . . . . . . . .   39
    12.3. Reliable Unicast to DAs . . . . . . . . . . . . . . . . .   39
    12.4. DA Scope Configuration  . . . . . . . . . . . . . . . . .   39
    12.5. DAs and Authentication Blocks . . . . . . . . . . . . . .   40

13. Protocol Timing Defaults                                          41

14. Optional Configuration                                            41

15. IANA Considerations                                               43

16. Internationalization Considerations                               44

17. Year 2000 Considerations                                          44

18. Security Considerations                                           45

 A. Appendix:  SLP Protocol Extensions                                46
     A.1. Required Attribute Missing Option . . . . . . . . . . . .   46

 B. Full Copyright Statement                                          49


1. Introduction

   The Service Location Protocol (SLP) provides a flexible and scalable
   framework for providing hosts with access to information about
   the existence, location, and configuration of networked services.
   Traditionally, users have had to find services by knowing the name of
   a network host (a human readable text string) which is an alias for a
   network address.  SLP eliminates the need for a user to know the name
   of a network host supporting a service.  Rather, the user supplies
   the desired type of service and a set of attributes which describe




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   the service.  Based on that description, the Service Location
   Protocol resolves the network address of the service for the user.

   SLP provides a dynamic configuration mechanism for applications in
   local area networks.  Applications are modeled as clients that need
   to find servers attached to any of the available networks within an
   enterprise.  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.

   This document specifies the Service Location Protocol (SLP) in
   two main parts.  The first describes the required features of the
   protocol.  The second describes the extended features of the protocol
   which are optional, and allow greater scalability.


1.1. Applicability Statement

   SLP is intended to function within networks under cooperative
   administrative control.  Such networks permit a policy to be
   implemented regarding security, multicast routing and organization
   of services and clients into groups which are not be feasible on the
   scale of the Internet as a whole.

   SLP has been designed to serve enterprise networks with shared
   services, and it may not necessarily scale for wide-area service
   discovery throughout the global Internet, or in networks where
   there are hundreds of thousands of clients or tens of thousands of
   services.


1.2. Changes to the Service Location Protocol from v1 to v2

   SLP version 2 (SLPv2) corrects race conditions present in SLPv1
   [ [19]].  In addition, authentication has been reworked to provide
   more flexibility and protection (especially for DA Advertisements).
   SLPv2 also changes the formats and definition of many flags and
   values and reduced the number of 'required features.'  SLPv2
   clarifies and changes the use of 'Scopes', eliminating support
   for 'unscoped directory agents' and 'unscoped requests'.  Other
   changes (such as Language and Character set handling) adopt practices
   recommended by the Internet Engineering Steering Group.

   Effort has been made to make SLPv2 operate the same whether DAs
   are present or not.  For this reason, a new message (the SAAdvert)
   has been added.  This allows UAs to discover scope information in
   the absence of administrative configuration and DAs.  This was not
   possible in SLPv1.



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   SLPv2 is incompatible in some respects with SLPv1.  If a DA which
   supports both SLPv1 and SLPv2 with the same scope is present,
   services advertised by SAs using either version of the protocol will
   be available to both SLPv1 and SLPv2 UAs.  SLPv1 DAs SHOULD be phased
   out and replace with SLPv2 DAs which support both versions of the
   protocol.

   SLPv1 allows services to be advertised and requested without a scope.
   Further, DAs can be configured without a scope.  This is incompatible
   with SLPv2 and presents scalability problems.  To facilitate this
   forward migration, SLPv1 agents MUST use scopes for all registrations
   and requests.  SLPv1 DAs MUST be configured with a scope list.  This
   constitutes a revision of RFC 2165 [19].


2. Terminology

      User Agent (UA)
                A process working on the user's behalf to establish
                contact with some service.  The UA retrieves service
                information from the Service Agents or Directory Agents.

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

      Directory Agent (DA)
                A process which collects service advertisements.  There
                can only be one DA present per given host.

      Service Type
                Each type of service has a unique Service Type string.

      Naming Authority
                The agency or group which catalogues given Service Types
                and Attributes.  The default Naming Authority is IANA.

      Scope     A set of services, typically making up a logical
                administrative group.

      URL       A Universal Resource Locator [8].


2.1. Notation Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119  [9].




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      Syntax        Syntax for string based protocols follow the
                    conventions defined for ABNF [11].

      Strings       All strings are encoded using the UTF8 [20]
                    transformation of the Unicode [6] character set and
                    are NOT null terminated when transmitted.  Strings
                    are preceded by a two byte length field.

      <string-list> A comma delimited list of strings with the
                    following syntax:

                       string-list = string / string `,' string-list

   In format diagrams, any field ending with a \ indicates a variable
   length field, given by a prior length field in the protocol.


3. Protocol Overview

   The Service Location Protocol supports a framework by which client
   applications are modeled as 'User Agents' and services are advertised
   by 'Service Agents.'  A third entity, called a 'Directory Agent'
   provides scalability to the protocol.

   The User Agent issues a 'Service Request' (SrvRqst) on behalf of the
   client application, specifying the characteristics of the service
   which the client requires.  The User Agent will receive a Service
   Reply (SrvRply) specifying the location of all services in the
   network which satisfy the request.

   The Service Location Protocol framework allows the User Agent to
   directly issue requests to Service Agents.  In this case the request
   is multicast.  Service Agents receiving a request for a service which
   they advertise unicast a reply containing the service's location.

      +------------+ ----Multicast SrvRqst----> +---------------+
      | User Agent |                            | Service Agent |
      +------------+ <----Unicast SrvRply------ +---------------+

   In larger networks, one or more Directory Agents are used.  The
   Directory Agent functions as a cache.  Service Agents send register
   messages (SrvReg) containing all the services they advertise to
   Directory Agents and receive acknowledgements in reply (SrvAck).
   These advertisements must be refreshed with the Directory Agent
   or they expire.  User Agents unicast requests to Directory Agents
   instead of Service Agents if any Directory Agents are known.






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 +-------+ -Unicast SrvRqst-> +-----------+ <-Unicast SrvReg- +--------+
 | User  |                    | Directory |                   |Service |
 | Agent |                    |   Agent   |                   | Agent  |
 +-------+ <-Unicast SrvRply- +-----------+ -Unicast SrvAck-> +--------+

   User and Service Agents discover Directory Agents two ways.  First,
   they issue a multicast Service Request for the 'Directory Agent'
   service when they start up.  Second, the Directory Agent sends
   an unsolicited advertisement infrequently, which the User and
   Service Agents listen for.  In either case the Agents receive a DA
   Advertisement (DAAdvert).

        +---------------+ --Multicast SrvRqst-> +-----------+
        |    User or    | <--Unicast DAAdvert-- | Directory |
        | Service Agent |                       |   Agent   |
        +---------------+ <-Multicast DAAdvert- +-----------+

   Services are grouped together using 'scopes'.  These are strings
   which identify services which are administratively identified.  A
   scope could indicate a location, administrative grouping, proximity
   in a network topology or some other category.  Service Agents and
   Directory Agents are always assigned a scope string.

   A User Agent is normally assigned a scope string (in which case the
   User Agent will only be able to discover that particular grouping
   of services).  This allows a network administrator to 'provision'
   services to users.  Alternatively, the User Agent may be configured
   with no scope at all.  In that case, it will discover all available
   scopes and allow the client application to issue requests for any
   service available on the network.

   +---------+   Multicast  +-----------+   Unicast   +-----------+
   | Service | <--SrvRqst-- |   User    | --SrvRqst-> | Directory |
   |  Agent  |              |   Agent   |             |   Agent   |
   | Scope=X |   Unicast    | Scope=X,Y |   Unicast   |  Scope=Y  |
   +---------+ --SrvRply--> +-----------+ <-SrvRply-- +-----------+

   In the above illustration, the User Agent is configured with scopes X
   and Y. If a service is sought in scope X, the request is multicast.
   If it is sought in scope Y, the request is unicast to the DA.
   Finally, if the request is to be made in both scopes, the request
   must be both unicast and multicast.

   Service Agents and User Agents may verify digital signatures provided
   with DAAdverts.  User Agents and Directory Agents may verify service
   information registered by Service Agents.  The keying material to
   use to verify digital signatures is identified using a SLP Security
   Parameter Index, or SLP SPI.



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   Every host configured to generate a digital signature includes the
   SLP SPI used to verify it in the Authentication Block it transmits.
   Every host which can verify a digital signature must be configured
   with keying material and other parameters corresponding with the SLP
   SPI such that it can perform verifying calculations.

   SAs MUST accept multicast service requests and DAAdverts and unicast
   service requests.

   The features described up to this point are required to implement.
   A minimum implementation consists of a User Agent, Service Agent or
   both.

   There are several optional features in the protocol.

  Service Type Request   A request for all types of service on the
                         network.  This allows generic service browsers
                         to be built.

  Service Type Reply     A reply to a Service Type Request.

  Attribute Request      A request for attributes of a given type of
                         service or attributes of a given service.

  Attribute Reply        A reply to an Attribute Request.

  Service Deregister     A request to deregister a service or some
                         attributes of a service.

  Service Update         A subsequent SrvRqst to an advertisement.
                         This allows individual dynamic attributes to
                         be updated.

  SA Advertisement       In the absense of Directory Agents, a User
                         agent may request Service Agents in order
                         to discover their scope configuration.  The
                         User Agent may use these scopes in requests.

   In the absense of Multicast support, Broadcast MAY be used.  The
   location of DAs may be staticly configured, discovered using SLP as
   described above, or configured using DHCP. If a message is too large,
   it may be unicast using TCP.

   A SLPv2 implementation SHOULD support SLPv1 [19].  This support
   includes:

    1. SLPv2 DAs are deployed, phasing out SLPv1 DAs.





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    2. Unscoped SLPv1 requests are considered to be of DEFAULT scope.
       SLPv1 UAs MUST be reconfigured to have a scope if possible.

    3. There is no way for an SLPv2 DA to behave as an unscoped SLPv1
       DA. SLPv1 SAs MUST be reconfigured to have a scope if possible.

    4. SLPv2 DAs answer SLPv1 requests with SLPv1 replies and SLPv2
       requests with SLPv2 replies.

    5. SLPv2 DAs use registrations from SLPv1 and SLPv2 in the same
       way.  That is, incoming requests from agents using either version
       of the protocol will be matched against this common set of
       registered services.

    6. SLPv2 registrations which use Language Tags which are greater
       than 2 characters long will be inaccessible to SLPv1 UAs.

    7. SLPv2 DAs MUST return only service type strings in SrvTypeRply
       messages which conform to SLPv1 service type string syntax.

   SLPv1 UAs will not receive replies from SLPv2 SAs and SLPv2 UAs will
   not receive replies from SLPv1 SAs.  In order to interoperate UAs and
   SAs of different versions require a SLPv2 DA to be present on the
   network which supports both protocols.


4. URLs used with Service Location

   A Service URL indicates the location of a service.  This URL may be
   of the service: scheme [13] (reviewed in section 4.1), or any other
   URL scheme conforming to the URI standard [8], except that URLs
   without address specifications SHOULD NOT be advertised by SLP. The
   service type for an 'generic' URL is its scheme name.  For example,
   the service type string for "http://www.srvloc.org" would be "http".

   Reserved characters in URLs follow the rules in [8].


4.1. Service: URLs

   Service URL syntax and semantics are defined in [13].  Any network
   service may be encoded in a Service URL.

   This section provides an introduction to Service URLs and an example
   showing a simple application of them, representing standard network
   services.

   A Service URL may be of the form:




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      "service:"<srvtype>"://"<addrspec>

   The Service Type of this service: URL is defined to be the string up
   to (but not including) the final `:'  before <addrspec>, the address
   specification.

   <addrspec> is a hostname (which should be used if possible) or
   dotted decimal notation for a hostname, followed by an optional `:'
   and port number.

   A service: scheme URL may be formed with any standard protocol
   name by concatenating "service:" and the reserved port [1]
   name.  For example, "service:tftp://myhost" would indicate a
   tftp service.  An tftp service on a nonstandard port could be
   "service:tftp://bad.glad.org:8080".

   Service Types SHOULD be defined by a "service template" [13], which
   provides expected attributes, values and protocol behavior.  An
   abstract service type (also described in [13]) has the form

      "service:<abstract-type>:<concrete-type>".

   The service type string "service:<abstract-type>" matches all
   services of that abstract type.  If the concrete type is included
   also, only these services match the request.  For example:  a
   SrvRqst or AttrRqst which specifies "service:printer" as the
   Service Type will match the URL service:printer:lpr://hostname
   and service:printer:http://hostname.  If the requests specified
   "service:printer:http" they would match only the latter URL.

   An optional substring MAY follow the last `.'  character in the
   <srvtype> (or <abstract-type> in the case of an abstract service
   type URL). This substring is the Naming Authority, as described in
   Section 9.6.  Service types with different Naming Authorities are
   quite distinct.  In other words, service:x.one and service:x.two
   are different service types, as are service:abstract.one:y and
   service:abstract.two:y.


4.2. Naming Authorities

   A Naming Authority MAY optionally be included as part of the Service
   Type string.  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 typically a string
   which uniquely identifies an organization.  IANA is the implied
   Naming Authority when no string is appended.  "IANA" itself MUST NOT
   be included explicitly.




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   Naming Authorities may define Service Types which are experimental,
   proprietary or for private use.  Using a Naming Authority, one
   may either simply ignore attributes upon registration or create a
   local-use only set of attributes for one's site.  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 8.1 and 8.3.  Service Types SHOULD be registered with IANA
   to allow for Internet-wide interoperability.


4.3. URL Entries

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Reserved    |          Lifetime             |   URL Length  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |URL len, contd.|            URL (variable length)              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |# of URL auths |            Auth. blocks (if any)              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   SLP stores URLs in protocol elements called URL Entries, which
   associate a length, a lifetime, and possibly authentication
   information along with the URL. URL Entries, defined as shown above,
   are used in Service Replies and Service Registrations.


5. Service Attributes

   A service advertisement is often accompanied by Service Attributes.
   These attributes are used by UAs in Service Requests to select
   appropriate services.

   The allowable attributes which may be used are typically specified
   by a Service Template  [13] for a particular service type.  Services
   which are advertised according to a standard template MUST register
   all service attributes which the standard template requires.  URLs
   with schemes other than "service:" MAY be registered with attributes.
   Non-standard attribute names SHOULD begin with "x-", because no
   standard attribute name will ever have those initial characters.

   An attribute list is a string encoding of the attributes of a
   service.  The following ABNF [11] grammar defines attribute lists:

   attr-list = attribute / attribute `,' attr-list
   attribute = `(' attr-tag `=' attr-val-list `)' / attr-tag
   attr-val-list = attr-val / attr-val `,' attr-val-list



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   attr-tag = 1*safe-tag
   attr-val = intval / strval / boolval / opaque
   intval = [-]1*DIGIT
   strval = 1*safe-val
   boolval = "true" / "false"
   opaque = "\FF" 1*escape-val
   safe-val = ; Any character except reserved.
   safe-tag = ; Any character except reserved, star and bad-tag.
   reserved = `(' / `)' / `,' / `\' / `!'  / `<' / `=' / `>' / `~' / CTL
   escape-val = `\' HEXDIG HEXDIG
   bad-tag = CR / LF / HTAB / `_'
    star = `*'

   The <attr-list>, if present, MUST be scanned prior to evaluation for
   all occurrences of the escape character `\'.  Reserved characters
   MUST be escaped (other characters MUST NOT be escaped).  All escaped
   characters must be restored to their value before attempting string
   matching.  For Opaque values, escaped characters are not converted -
   they are interpreted as bytes.

      Boolean      Strings which have the form "true" or "false" can
                   only take one value and may only be compared with
                   '='.  Booleans are case insensitive when compared.

      Integer      Strings which take the form [-] 1*<digit> and fall
                   in the range "-2147483648" to "2147483647" are
                   considered to be Integers.  These are compared using
                   integer comparison.

      String       All other Strings are matched using strict lexical
                   ordering (see Section 6.4).

      Opaque       Opaque values are sequences of bytes.  These are
                   distinguished from Strings since they begin with
                   the sequence "\FF".  This, unescaped, is an illegal
                   UTF8 encoding, indicating that what follows is a
                   sequence of bytes expressed in escape notation which
                   constitute the binary value.  For example, a '0' byte
                   is encoded "\FF\00".

   A string which contains escaped values other than from the reserved
   set of characters is illegal.  If such a string is included in an
   <attr-list>, <tag-list> or search filter, the SA or DA which
   receives it MUST return a PARSE_ERROR to the message.

   A keyword has only an <attr-tag>, and no values.  Attributes can
   have one or multiple values.  All values are expressed as strings.





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   When values have been advertised by a SA or are registered in a
   DA, they can take on implicit typing rules for matching incoming
   requests.

   Stored values must be consistent, i.e., x=4,true,sue,\ff\00\00 is
   disallowed.  A DA or SA receiving such an <attr-list> MUST return an
   INVALID_REGISTRATION error.


6. Required Features

   This section defines the minimal implementation requirements for
   SAs and UAs as well as their interaction with DAs.  A DA is not
   required for SLP to function, but if it is present, the UA and SA
   MUST interact with it as defined below.

   A minimal implementation may consist of either a UA or SA or both.
   The only required features of a UA are that it can issue SrvRqsts
   according to the rules below and interpret DAAdverts, SAAdverts and
   SrvRply messages.  The UA MUST issue requests to DAs as they are
   discovered.  An SA MUST reply to appropriate SrvRqsts with SrvRply or
   SAAdvert messages.  The SA MUST also register with DAs as they are
   discovered.

   UAs perform discovery by issuing Service Request messages.  SrvRqst
   messages are issued, using UDP, following these prioritized rules:

    1. A UA issues a request to a DA which it has been configured with
       by DHCP.

    2. A UA issues requests to DAs which it has been statically
       configured with.

    3. A UA uses multicast/convergence SrvRqsts to discover DAs, then
       uses that set of DAs.  A UA that does not know of any DAs SHOULD
       retry DA discovery once every CONFIG_DA_FIND seconds.

    4. A UA with no knowledge of DAs sends requests using multicast
       convergence to SAs.  SAs unicast replies to UAs according to the
       multicast convergence algorithm.

   UAs and SAs are configured with a list of scopes to use according to
   these prioritized rules:

    1. With DHCP.

    2. With static configuration.  The static configuration may be
       explicitely set to NO SCOPE for UAs, if the User Selectable Scope
       model is used.  See section 11.2.



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    3. In the absense of configuration, the agent's scope is "DEFAULT".

   A UA MUST issue requests with one or more of the scopes it has been
   configured to use.

   A UA which has been statically configured with NO SCOPE LIST will use
   DA or SA discovery to determine its scope list dynamically.  In this
   case it uses an empty scope list to discover DAs and possibly SAs.
   Then it uses the scope list it obtains from DAAdverts and possibly
   SAAdverts in subsequent requests.

   The SA MUST register all its services with any DA it discovers, if
   the DA advertises any of the scopes it has been configured with.  A
   SA obtains information about DAs as a UA does.  In addition, the SA
   MUST listen for multicast unsolicited DAAdverts.  The SA registers
   by sending SrvReg messages to DAs, which reply with SrvReg messages
   to indicate success.  SAs register in ALL the scopes they were
   configured to use.


6.1. Use of Ports, UDP, and Multicast

   DAs MUST accept unicast requests and multicast directory
   agent discovery service requests (for the service type
   "service:directory-agent").

   The Service Location Protocol uses multicast by default.  The
   reserved listening port for SLP is 427.  This is the destination
   port for all SLP messages.  SLP messages MAY be transmitted on an
   ephemeral port.  Replies and acknowledgements are sent to the port
   from which the request was issued.  The default maximum transmission
   unit for UDP messages is 1400 bytes.

   If a SLP message does not fit into a UDP datagram it MUST be
   truncated to fit, and the OVERFLOW flag is set in the reply message.
   A UA which receives a truncated message MAY open a TCP connection
   (see section 6.2) with the DA or SA and retransmit the request, using
   the same XID. It MAY also attempt to make use of the truncated reply
   or reformulate a more restrictive request which will result in a
   smaller reply.

   SLP Requests messages are multicast to The Administratively Scoped
   SLP Multicast [15] address, which is 239.255.255.253.  The default
   TTL to use for multicast is 255.

   In isolated networks, broadcasts will work in place of multicast.
   To that end, SAs SHOULD and DAs MUST listen for broadcast Service
   Location messages at port 427.  This allows UAs which do not support
   multicast the use of Service Location on isolated networks.



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   Setting multicast TTL to less than 255 (the default) limits the range
   of SLP discovery in a network, and localizes service information in
   the network.


6.2. Use of TCP

   A SrvReg or SrvDeReg may be too large to fit into a datagram.  To
   send such large SLP messages, a TCP (unicast) connection MUST be
   established.

   To avoid the need to implement TCP, one MUST insure that:

    -  UAs never issue requests larger than the Path MTU. SAs can omit
       TCP support only if they never have to receive unicast requests
       longer than the path MTU.

    -  UAs can accept replies with the 'OVERFLOW' flag set, and make use
       of the first result included, or reformulate the request.

    -  Ensure that a SA can send a SrvRply, SrvReg, or SrvDeReg in
       a single datagram.  This means limiting the size of URLs,
       the number of attributes and the number of authenticators
       transmitted.

   DAs MUST be able to respond to UDP and TCP requests, as well as
   multicast DA Discovery SrvRqsts.  SAs MUST be able to respond to TCP
   unless the SA will NEVER receive a request or send a reply which will
   exceed a datagram in size (e.g., some embedded systems).

   A TCP connection MAY be used for a single SLP transaction, or for
   multiple transactions.  Since there are length fields in the message
   headers, SLP Agents can send multiple requests along a connection and
   read the return stream for acknowledgments and replies.

   The initiating agent SHOULD close the TCP connection.  The DA SHOULD
   wait at least CONFIG_CLOSE_CONN seconds before closing an idle
   connection.  DAs and SAs SHOULD close an idle TCP connection after
   CONFIG_CLOSE_CONN seconds to ensure robust operation, even when the
   initiating agent neglects to close it.  See Section 13 for timing
   rules.


6.3. Retransmission of SLP messages

   Requests to SAs are multicast repeatedly (with a recommended wait
   interval of CONFIG_MC_RETRY) until there are no new responses, or
   CONFIG_MC_MAX seconds have elapsed.  DA discovery requests use
   different timing for repeated requests, CONFIG_DA_RETRY.



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   Multicast requests SHOULD be reissued over 15 seconds (say 3 times
   total) until a result has been obtained.  UAs need only wait till
   they obtain the first reply which matches their request.  Unicast
   requests (SrvReg or SrvRqst) to a DA should be retried until either
   a response (which might be an error) has been obtained, or for 5
   seconds.

   When SLP SrvRqst, SrvTypeRqst, and AttrRqst messages are multicast,
   they contain a <PRList> of previous responders.  Initially the
   <PRList> is empty.  The message SHOULD be retransmitted until the
   <PRList> causes no further responses to be elicited or the previous
   responder list and the request will not fit into a single datagram.
   Retransmission is not required if the requesting agent is prepared to
   use the 'first reply' instead of 'as many replies as possible within
   a bounded time interval.'

   Any DA or SA which sees its address in the <PRList> MUST NOT respond
   to the request.

   UAs which retransmit a request use the same XID. This allows a 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.  XIDs SHOULD be randomly chosen to avoid
   duplicate XIDs in requests if UAs restart frequently.


6.4. Strings in SLP messages

   The escape character is a backslash (UTF8 0x5c) followed by the
   two hexadecimal digits of the escaped character.  Only reserved
   characters are escaped.  For example, a comma (UTF8 0x29) is escaped
   as `\29', and a backslash `\' is escaped as `\5c'.  String lists used
   in SLP define the comma to be the delimiter between list elements, so
   commas in data strings must be escaped in this manner.  Backslashes
   are the escape character so they also must always be escaped when
   included in a string literally.

   String comparison for order and equality in SLP MUST be case
   insensitive inside the 0x00-0x7F subrange of UTF8 (which corresponds
   to ASCII character encoding).  Case insensitivity SHOULD be supported
   throughout the entire UTF8 encoded Unicode [6] character set.

   White space (SPACE, CR, LF, TAB) internal to a string value is folded
   to a single SPACE character for the sake of string comparisons.
   White space preceding or following a string value is ignored for
   the purposes of string comparison.  For example, "  Some String  "
   matches "SOME    STRING".





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   String comparisons (using comparison operators such as `<=' or `>=')
   are done using lexical ordering in UTF8 encoded characters, not using
   any language specific rules.

   The reserved character `*' may precede, follow or be internal to a
   string value in order to indicate substring matching.  The query
   including this character matches any character sequence which
   conforms to the letters which are not wildcarded.


6.4.1. Scope Lists in SLP

   Scope Lists in SLPv2 have the following grammar:

   scope-list = scope-val / scope-val `,' scope-list
   scope-val = 1*safe
    safe = ; Any character except reserved.
   reserved = `(' / `)' / `,' / `\' / `!'  / `<' / `=' / `>' / `~' / CTL
         / `;' / `*' / `+'
   escape-val = `\' HEXDIG HEXDIG

   Scopes which include any reserved characters must replace the escaped
   character with the the escaped-val format.


7. Errors

   If the Error Code in a SLP reply message is nonzero, the rest of
   the message MAY be truncated.  No data is necessarily transmitted
   or should be expected after the header and the error code, except
   possibly for some optional extensions to clarify the error, for
   example as in section A.1.

   Errors are only returned for unicast requests.  Multicast requests
   are silently discarded if they result in an error.

   LANGUAGE_NOT_SUPPORTED = 1: There is data for the service type in
         the scope in the AttrRqst or SrvRqst, but not in the requested
         language.
   PARSE_ERROR = 2: The message fails to obey SLP syntax.
   INVALID_REGISTRATION = 3: The SrvReg has problems -- e.g., a zero
         lifetime or an omitted Language Tag.
   SCOPE_NOT_SUPPORTED = 4: The SLP message did not include a scope in
         its <scope-list> supported by the SA or DA.
   AUTHENTICATION_UNKNOWN = 5: The DA or SA receives a request for an
         unsupported SLP SPI.
   AUTHENTICATION_ABSENT = 6: The DA expected URL and ATTR
         authentication in the SrvReg and did not receive it.




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   AUTHENTICATION_FAILED = 7: The DA detected an authentication error in
         an Authentication block.
   VER_NOT_SUPPORTED = 9: Unsupported version number in message header.
   INTERNAL_ERROR = 10: The DA (or SA) is too sick to respond.
   DA_BUSY_NOW = 11: UA or SA SHOULD retry, using exponential back off.
   OPTION_NOT_UNDERSTOOD = 12: The DA (or SA) received an unknown option
         from the mandatory range (see section 9.1).
   INVALID_UPDATE = 13: The DA received a SrvReg without FRESH set, for
         an unregistered service or with inconsistent Service Types.
   MSG_NOT_SUPPORTED = 14: The SA received an AttrRqst or SrvTypeRqst
         and does not support it.
   INVALID_LIFETIME = 15: The SA registered a service with a lifetime
         the DA has disallowed in its DAAdvert.


8. Required SLP Messages

   All length fields in SLP messages are in network byte order.  Where
   'tuples' are defined, these are sequences of bytes, in the precise
   order listed, in network byte order.

   SLP messages all begin with the following header:

      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-ID  |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Length, contd.|O|F|R|       reserved          |Next Ext Offset|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Extension Offset, contd.|              XID              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Language Tag Length      |         Language Tag          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Message Type             Abbreviation     Function-ID

          Service Request          SrvRqst              1
          Service Reply            SrvRply              2
          Service Registration     SrvReg               3
          Service Deregister       SrvDeReg             4
          Service Acknowledge      SrvAck               5
          Attribute Request        AttrRqst             6
          Attribute Reply          AttrRply             7
          DA Advertisement         DAAdvert             8
          Service Type Request     SrvTypeRqst          9
          Service Type Reply       SrvTypeRply          10
          SA Advertisement         SAAdvert             11




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   SAs and UAs MUST support SrvRqst, SrvRply and DAAdvert.  SAs MUST
   also support SrvReg, SAAdvert and SrvAck.  For UAs and SAs, support
   for other messages are OPTIONAL.

     - Length is the length of the entire SLP message, header included.
     - The flags are:  OVERFLOW (0x80) is set when a message's length
       exceeds what can fit into a datagram.  FRESH (0x40) is set on
       every new SrvReg.  REQUEST MCAST (0x20) is set when multicasting
       or broadcasting requests.  Reserved bits MUST be 0.
     - Next Extension Offset is set to 0 unless extensions are used.
       The first extension begins at 'offset' bytes, from the message's
       beginning.  It is placed after the SLP message data.  See
       Section 9.1 for how to interpret unrecognized options.
     - XID is set to a unique value for each unique request.  If the
       request is retransmitted, the same XID is used.  Replies set
       the XID to the same value as the xid in the request.  Only
       unsolicited DAAdverts are sent with an XID of 0.
     - Lang Tag Length is the length in bytes of the Language Tag field.
     - Language Tag conforms to [7].  The Language Tag in a reply MUST
       be the same as the Language Tag in the request.  This field must
       be encoded 1*8ALPHA ["-" 1*8ALPHA].

   If an option is specified, and not included in the message, the
   receiver MUST respond with a PARSE_ERROR.


8.1. Service Request

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Service Location header (function = SrvRqst = 1)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      length of <PRList>       |        <PRList> String        \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   length of <service-type>    |    <service-type> String      \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    length of <scope-list>     |     <scope-list> String       \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  length of predicate string   |  Service Request <predicate>  \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  length of <SLP SPI> string   |       <SLP SPI> String        \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In order for a Service to match a SrvRqst, it must belong to at least
   one requested scope, support the requested service type, and match
   the predicate.  If the predicate is present, the language of the
   request (ignoring the dialect part of the Language Tag) must match
   the advertised service.



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   <PRList> is the Previous Responder List.  This <string-list>
   contains either fully qualified domain names or dotted decimal
   notation IP (v4) addresses, and is iteratively multicast to obtain
   all possible results (see Section 6.3).  UAs SHOULD implement this
   discovery algorithm.  SAs MUST use this to discover all available DAs
   in their scope, if they are not already configured with DA addresses
   by some other means.  A SA silently drops all requests which include
   the SA's address in the <PRList>.  Once a <PRList> plus the request
   exceeds the path MTU, multicast convergence stops.  This algorithm
   is not intended to find all instances; it finds 'enough' to provide
   useful results.

   The <scope-list> is a <string-list> of configured scope names.  SAs
   and DAs which have been configured with any of the scopes in this
   list will respond.  DAs and SAs MUST reply to unicast requests with a
   SCOPE_NOT_SUPPORTED error if the <scope-list> is omitted or fails to
   include a scope they support (see Section 11).  The only exceptions
   to this are described in Section 11.2.

   The <service-type> string is discussed in Section 4.  Normally,
   a SrvRqst elicits a SrvRply.  There are two exceptions:  If
   the <service-type> is set to "service:directory-agent", DAs
   respond to the SrvRqst with a DAAdvert (see Section 8.5.)  If
   set to "service:service-agent", SAs respond with a SAAdvert (see
   Section 8.6.)  If this field is omitted, a PARSE_ERROR is returned -
   as this field is REQUIRED.

   The <predicate> is a LDAPv3 search filter [14].  This field is
   OPTIONAL. Services may be discovered simply by type and scope.
   Otherwise, services are discovered which satisfy the <predicate>.
   If present, it is compared to each registered service.  If the
   attribute in the filter has been registered with multiple values, the
   filter is compared to each value and the results are ORed together,
   i.e., "(x=3)" matches a registration of (x=1,2,3); "(!(Y=0))"
   matches (y=0,1) since Y can be nonzero.  Note the matching is case
   insensitive.  Keywords (i.e., attributes without values) are matched
   with a "presence" filter, as in "(keyword=*)".

   An incoming request term MUST have the same type as the attribute
   in a registration in order to match.  Thus, "(x=33)" will not
   match 'x=true', etc.  while "(y=foo)" will match 'y=FOO'.
   "(|(x=33)(y=foo))" will be satisfied, even though "(x=33)" cannot be
   satisfied, because of the `|' (boolean disjunction).

   Wildcard matching MUST be done with the '=' filter.  In any other
   case, a PARSE_ERROR is returned.  Request terms which include
   wildcards are interpreted to be Strings.  That is, (x=34*) would
   match 'x=34foo', but not 'x=3432' since the first value is a String
   while the second value is an Integer; Strings don't match Integers.



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   Examples of Predicates follow.  <t> indicates the service type of
   the SrvRqst, <s> gives the <scope-list> and <p> is the predicate
   string.

      <t>=service:http  <s>=DEFAULT  <p>=  (empty string)
               This is a minimal request string.  It matches all http
               services advertised with the default scope.

      <t>=service:pop3  <s>=SALES,DEFAULT  <p>=(user=wump)
               This is a request for all pop3 services available in
               the SALES or DEFAULT scope which serve mail to the user
               `wump'.

      <t>=service:backup  <s>=BLDG 32  <p>=(&(q<=3)(speed>=1000))
               This returns the backup service which has a queue length
               less than 3 and a speed greater than 1000.  It will
               return this only for services registered with the BLDG 32
               scope.

      <t>=service:directory-agent  <s>=DEFAULT  <p>=
               This returns DAAdverts for all DAs in the DEFAULT scope.

   DAs are discovered by sending a SrvRqst with the service type set
   to "service:directory-agent".  If a predicate is included in the
   SrvRqst, the DA SHOULD respond only if the predicate can be satisfied
   with the DA's attributes.  The <scope-list> SHOULD contain all
   scopes configured for the service.  The DA responds to this SrvRqst
   if the <scope-list> or <SLP SPI> string has been omitted.

   The <SLP SPI> string indicates a SLP SPI that the requester has
   been configured with.  If this string is omitted, the responder
   does not include any Authentication Blocks in its reply.  If it is
   included, the responder MUST return a reply which has an associated
   authentication block with the SLP SPI in the SrvRqst.  If no replies
   may be returned because the SLP SPI is not supported, the responder
   returns an AUTHENTICATION_UNKNOWN error.
















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8.2. Service Reply

      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 = 2)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Error Code             |        URL Entry count        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       <URL Entry 1>          ...       <URL Entry N>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The service reply contains one or more URL entries (see Section 4.3)
   that satisfy a SrvRqst.  If the reply overflows, the UA MAY simply
   use the first URL Entry in the list.  A URL obtained by SLP may
   not be cached longer than Lifetime seconds, unless there is a URL
   Authenticator block present.  In that case, the cache lifetime
   is indicated by the Timestamp in the URL Authenticator (see
   Section 9.2).

   An authentication block is returned in the URL Entries, including
   the SLP SPI in the SrvRqst.  If no SLP SPI was included in the
   request, no Authentication Blocks are returned in the reply.  URL
   Authentication Blocks are defined in Section 9.2.1.

   If a SrvRply is sent by UDP, a URL Entry MUST NOT be included unless
   it fits entirely without truncation.


8.3. Service Registration

      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 = 3)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          <URL-Entry>                          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | length of service type string |        <service-type>         \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     length of <scope-list>    |         <scope-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  length of attr-list string   |          <attr-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |# of AttrAuths |(if present) Attribute Authentication Blocks...\
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The <entry> is a URL Entry (see section 4.3).  The Lifetime defines
   how long a DA can cache the registration.  SAs SHOULD reregister
   before this lifetime expires (but SHOULD NOT more often than once
   per second).  The Lifetime MAY be set to any value between 0 and
   0xffff (maximum, around 18 hours).  Long-lived registrations remain
   stale longer if the service fails and the SA does not deregister the
   service.

   The <service-type> defines the service type of the URL to be
   registered, regardless of the scheme of the URL. The <scope-list>
   MUST be contain the names of all scopes configured for the SA, which
   the DA it is registering with supports.  The default value for the
   <scope-list> is "DEFAULT" (see Section 11).

   The SA MUST register consistently with all DAs.  If a SA is
   configured with scopes X and Y and there are three DAs, whose scopes
   are "X", "Y" and "X,Y" respectively, the SA will register the with
   all three DAs in their respective scopes.  All future updates and
   deregistrations of the service must be sent to the same set of DAs in
   the same scopes the service was initially registered in.

   The <attr-list>, if present, specifies the attributes and values to
   be associated with the URL by the DA (see Section 5).

   A SA configured with the ability to sign service registrations MUST
   sign each of the URLs and Attribute Lists using each of the keys it
   is configured to use, and the DA it is registering with accepts.
   (The SA MUST aquire DAAdverts for all DAs it will register with
   to obtain the DA's SLP SPI list and attributes, as described in
   Section 8.5).  The SA supplies a SLP SPI in each authentication block
   indicating the SLP SPI configuration required to verify the digital
   signature.  The format of the digital signatures used is defined in
   section 9.2.1.

   Subsequent registrations of previously registered services MUST
   contain the same list of SLP SPIs as previous ones or else DAs will
   reject them, replying with an AUTHENTICATION_ABSENT error.

   A registration with the FRESH flag set will replace *entirely* any
   previous registration for the same URL in the same language.  If
   the FRESH flag is not set, the registration is an "incremental"
   registration (see Section 9.3).










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8.4. Service Acknowledgment

      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 = 4)      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Error Code           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A DA returns a SrvAck to an SA after a SrvReg.  It carries only a two
   byte Error Code (see Section 7).


8.5. Directory Agent Advertisement

      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 = 8)      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Error Code           |  DA Stateless Boot Timestamp  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |DA Stateless Boot Time,, contd.|         Length of URL         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                              URL                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Length of <scope-list>    |         <scope-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Length of <attr-list>     |          <attr-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Length of <SLP SPI List>   |     <SLP SPI List> String     \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | # Auth Blocks |         Authentication block (if any)         \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Error Code is set to 0 unless the DAAdvert is being returned due
   to a SrvRqst without the MCAST REQEST flag set.  In this case it
   returns the same errors a SrvRply would.

   The <scope-list> of the SrvRqst must either be omitted or include
   a scope which the DA supports.  The DA Stateless Boot Timestamp
   indicates the state of the DA (see section 12.1).

   The DA MAY include a list of its attributes in the DAAdvert.  This
   list SHOULD be kept short, as the DAAdvert must fit into a datagram
   in order to be multicast.





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   A potential scaling problem occurs in SLPv2 if SAs choose too low a
   Lifetime.  In this case, an onerous amount of reregistration occurs
   as more services are deployed.  SLPv2 allows DAs to control SAs
   frequency of registration.  A DA MAY reissue a DAAdvert with a new
   set of attributes at any time, to change the reregistration behavior
   of SAs.  These apply only to subsequent registrations; existing
   service registrations with the DA retain their registered lifetimes.

   If the DAAdvert includes the attribute "min-lifetime" it MUST be set
   to a single Integer value.  If this attribute is present SAs MUST NOT
   set registered service lifetimes to be shorter than this value (in
   seconds).  Further, SAs MUST NOT refresh the advertisement less often
   than 80% of this interval.  If the DAAdvert includes the attribute
   "max-lifetime", it MUST be set to a single Integer value.  SAs MUST
   NOT set registered service lifetimes to be longer than this value
   (in seconds).  If a registered lifetime is below the min-lifetime
   or above the max-lifetime advertised by the DA it will reject the
   registration and return a INVALID_LIFETIME error in the SrvAck.

   The URL is "service:directory-agent://"<addr> of the DA, where
   <addr> is the dotted decimal numeric address of the DA. The
   <scope-list> of the DA MUST NOT be NULL.

   The SLP SPI List is the list of SPIs that the DA is capable of
   verifying.  SAs MUST NOT register services with authentication
   blocks for those SLP SPIs which are not on the list.  DAs will
   reject service registrations which they cannot verify, returning an
   AUTHENTICATION_UNKNOWN error.

   The format of DAAdvert signatures is defined in Section 9.2.1.

   The SLP SPI which is used to verify the DAAdvert is included in
   the Authentication Block.  When DAAdverts are multicast, they may
   have to transmit multiple DAAdvert Authentication Blocks.  If the
   DA is configured to be able to generate signatures for more than
   one SPI, the DA MUST include one Authentication Block for each SPI.
   If all these Authentication Blocks do not fit in a single datagram
   (to multicast or broadcast) the DA MUST send separate DAAdverts so
   that Authentication Blocks for all the SPIs the DA is capable of
   generating are sent.

   If the DAAdvert is being sent in response to a SrvRqst, the DAAdvert
   contains only the authentication block with the SLP SPI in the
   SrvRqst, if the DA is configured to be able to produce digital
   signatures using that SLP SPI. If the SrvRqst is sent to the DA
   without the MCAST RQST flag set and an unsupported SLP SPI is
   included, the DA replies with a DAAdvert with the Error Code set to
   an AUTHENTICATION_UNKNOWN error.




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   UAs SHOULD be configured with SLP SPIs that will allow them to
   verify DA Advertisements.  If the UA is configured with SLP SPIs and
   receives a DAAdvert which fails to be verified using one of them, the
   UA MUST discard it.


8.6. Service Agent Advertisement

   User Agents MUST NOT solicit SA Advertisements if they have been
   configured to use a particular DA, if they have been configured
   with a <scope-list> or if DAs have been discovered.  UAs solicit
   SA Advertisements only when they are explicitly configured to use
   User Selectable scopes (see Section 11.2) in order to discover the
   scopes that SAs support.  This allows UAs without scope configuration
   to make use of either DAs or SAs without any functional difference
   except performance.

   A SA MAY be configured with attributes, and SHOULD support the
   attribute 'service-type' whose value is all the service types
   of services represented by the SA. SAs MUST NOT respond if the
   SrvRqst predicate is not satisfied.  For example, only SAs offering
   'nfs' services SHOULD respond with a SAAdvert to a SrvRqst for
   service type "service:service-agent" which includes a predicate
   "(service-type=nfs)".

      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 = SAAdvert = 11)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Length of URL         |              URL              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Length of <scope-list>    |         <scope-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Length of <attr-list>     |          <attr-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | # auth blocks |        authentication block (if any)          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The SA responds only to multicast SA discovery requests which either
   include no <scope-list> or a scope which they are configured to use.

   The SAAdvert MAY include a list of attributes the SA supports.  This
   attribute list SHOULD be kept short so that the SAAdvert will not
   exceed the path MTU in size.

   The URL is "service:service-agent://"<addr> of the SA, where <addr>
   is the dotted decimal numeric address of the SA. The <scope-list> of
   the SA MUST NOT be null.



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   The SAAdvert contains one SAAdvert Authentication block for each SLP
   SPI the SA can produce Authentication Blocks for.  If the UA can
   verify the Authentication Block of the SAAdvert, and the SAAdvert
   fails to be verified, the UA MUST discard it.


9. Optional Features

   The features described in this section are not mandatory.  Some are
   useful for interactive use of SLP (where a user rather than a program
   will select services, using a browsing interface for example) and for
   scalability of SLP to larger networks.


9.1. Service Location Protocol Extensions

   The format of a Service Location Extension 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Extension ID          |       Next Extension Offset   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Offset, contd.|                Extension Data                 \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Extension IDs are assigned in the following way:

   0x0000-0x3FFF Standardized.  Optional to implement.  Ignore if
         unrecognized.
   0x4000-0x7FFF Standardized.  Mandatory to implement.  A UA or SA
         which receives this option in a reply and does not understand
         it MUST silently discard the reply.  A DA or SA which receives
         this option in a request and does not understand it MUST return
         an OPTION_NOT_UNDERSTOOD error.
   0x8000-0x8FFF For private use (not standardized).  Optional to
         implement.  Ignore if unrecognized.
   0x9000-0xFFFF Reserved.

   The three byte offset to next extension indicates the position of the
   next extension as offset from the beginning of the SLP message.

   The offset value is 0 if there are no extensions following the
   current extension.

   If the offset is 0, the length of the current Extension Data is
   determined by subtracting total length of the SLP message as given in
   the SLP message header minus the offset of the current extension.




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   Extensions defined in this document are in Section A.  See section 15
   for procedures that are required when specifying new SLP extensions.


9.2. Authentication Blocks

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Block Structure Descriptor   |  Authentication Block Length  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Timestamp                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     SLP SPI String Length     |         SLP SPI String        \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Structured Authentication Block ...              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Authentication blocks are returned with certain SLP messages to
   verify that the contents have not been modified, and have been
   transmitted by an authorized agent.  The authentication data
   (contained in the Structured Authentication Block) is typically
   case sensitive.  Even though SLP registration data (e.g., attribute
   values) are typically are not case sensitive, the case of the
   registration data has to be preserved by the registering DA so that
   UAs will be able to verify the data used for calculating digital
   signature data.

   The Block Structure Descriptor (BSD) identifies the format of the
   Authenticator which follows.  BSDs 0x0000-0x7FFF will be maintained
   by IANA. BSDs 0x8000-0x8FFF are for private use.

   The Authentication Block Length is the length of the entire block,
   starting with the BSD.

   The Timestamp is the time that the authenticator expires (to prevent
   replay attacks.)  The Timestamp is a 32-bit unsigned fixed-point
   number of *minutes* relative to 0h on 1 January 1900.  SAs use this
   value to indicate when the validity of the digital signature expires.
   This timestamp will wrap back to 0 in the year 10,066.

   The SLP Security Parameters Index (SPI) string identifies the key
   length, algorithm parameters and keying material to be used by agents
   to verify the signature data in the Structured Authentication Block.
   The SLP SPI string has the following grammar:

   slp-spi = 1*safe
    safe = ; Any UTF8 character except reserved.
   reserved = `,' / `\' / `=' / CTL / `;'



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   escape-val = `\' HEXDIG HEXDIG

   Reserved characters in SLP SPI strings must be escaped using the same
   convention as used throughout SLPv2.

   SLP SPIs deployed in a site MUST be unique.  An SLP SPI used for
   BSD=0x0002 must not be the same as used for some other BSD.

   All SLP agents MUST implement DSA [18] (BSD=0x0002).  SAs MUST
   register services with DSA authentication blocks, and they
   MAY register them with other authentication blocks using other
   algorithms.  SAs MUST use DSA authentication blocks in SrvDeReg
   messages and DAs MUST use DSA authentication blocks in unsolicited
   DAAdverts.


9.2.1. SLP Message Authentication Rules

   The sections below define how to calculate the value to apply to the
   algorithm identified by the BSD value.  The components listed are
   used as if they were a contiguous single byte aligned buffer in the
   order given.

      URL
          16-bit Length of SLP SPI String, SLP SPI String.
          16-bit Length of URL, URL,
          32-bit Timestamp.

      Attribute List
          16-bit Length of SLP SPI String, SLP SPI String,
          16-bit length of <attr-list>, <attr-list>,
          32-bit Timestamp.

      DAAdvert
          16-bit Length of SLP SPI String, SLP SPI String,
          32-bit DA Stateless Boot Timestamp,
          16-bit Length of URL, URL,
          16-bit Length of <attr-list>, <attr-list>,
          16-bit Length of DA's <scope-list>, DA's <scope-list>,
          16-bit Length of DA's <SLP SPI List>, DA's <SLP SPI List>,
          32-bit Timestamp.

          The first SLP SPI is the SLP SPI in the Authentication
          Block.  This SLP SPI indicates the keying material and other
          parameters to use to verify the DAAdvert.  The SLP SPI List is
          the list of SLP SPIs the DA itself supports, and is able to
          verify.





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      SAAdvert
          16-bit Length of SLP SPI String, SLP SPI String,
          16-bit Length of URL, URL,
          16-bit Length of <attr-list>, <attr-list>,
          16-bit Length of <scope-list>, <scope-list>,
          32-bit Timestamp.


9.2.2. DSA with SHA-1 in Authentication Blocks

   BSD=0x0002 is defined to be DSA with SHA-1.  The signature
   calculation is defined by [18].  The signature format conforms to
   that in the X.509 v3 certificate:

    1. The signature algorithm identifier (an OID)
    2. The signature value (an octet string)
    3. The certificate path.

   All data is represented in ASN.1 encoding:

        id-dsa-with-sha1 ID  ::=  {
                        iso(1) member-body(2) us(840) x9-57 (10040)
                        x9cm(4) 3 }

   i.e., the ASN.1 encoding of 1.2.840.10040.4.3 followed immediately
   by:

        Dss-Sig-Value  ::=  SEQUENCE  {
                        r       INTEGER,
                        s       INTEGER  }

   i.e., the binary ASN.1 encoding of r and s computed using DSA
   and SHA-1.  This is followed by a certificate path, as defined by
   X.509 [10], [2], [3], [4], [5].

   Authentication Blocks for BSD=0x0002 have the following format.  In
   the future, BSDs may be assigned which have different formats.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   ASN.1 encoded DSA signature                 \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The ASN.1 encoded DSA signature is defined in Section 9.2.2.







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9.3. Incremental Service Registration

   Incremental registrations update attribute values for a previously
   registered service.  Incrmental service registrations are useful when
   only a single attribute has changed, for instance.  In an incremental
   registration, the FRESH flag in the SrvReg header is NOT set.

   The new registration's attributes replace the previous
   registration's, but do not affect 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 an incremental 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.

   Incremental registrations MUST NOT be performed for services
   registered with Authentication Blocks.  These must be registered
   with ALL attributes, with the FRESH flag in the SrvReg header
   set.  DAs which receive such registration messages return an
   AUTHENTICATION_FAILED error.

   If the FRESH flag is not set and the DA does not have a prior
   registration for the service, the incremental registration fails with
   error code INVALID_UPDATE.

   The SA MUST use the same <scope-list> in an update message as was
   used in the prior registration.  If this is not done, the DA returns
   a SCOPE_NOT_SUPPORTED error.  In order to change the scope of a
   service advertisement it MUST be deregistered first and reregistered
   with a new <scope-list>.

   The SA MUST use the same <service-type> in an update message as was
   used in a prior registration of the same URL. If this is not done,
   the DA returns an INVALID_UPDATE error.


9.4. Tag Lists

   Tag lists are used in SrvDeReg and AttrReq messages.  The syntax of a
   <tag-list> item is:

   tag-filter = simple-tag / substring
   simple-tag = 1*filt-char
   substring = [initial] any [final]
   initial = 1*filt-char
     any = `*' *(filt-char `*')
   final = 1*filt-char




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   filt-char = Any character excluding <reserved> and <bad-tag> (see
         grammar in Section 5).

   Wild card characters in a <tag-list> item match arbitrary sequences
   of characters.  For instance "*bob*" matches "some bob I know",
   "bigbob", "bobby" and "bob".


10. Optional SLP Messages

   The additional requests provide features for user interaction and for
   efficient updating of service advertisements with dynamic attributes.


10.1. Service Type Request

   The Service Type Request (SrvTypeRqst) allows a UA to discover all
   types of service on a network.  This is useful for general purpose
   service browsers.

      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 = 9)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        length of PRList       |        <PRList> String        \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   length of Naming Authority  |   <Naming Authority String>   \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     length of <scope-list>    |      <scope-list> String      \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The <PRList> list and <scope-list> are interpreted as in
   Section 8.1.

   The Naming Authority string, if present in the request, will
   limit the reply to Service Type strings with the specified Naming
   Authority.  If the Naming Authority string is absent, the IANA
   registered service types will be returned.  If the length of the
   Naming Authority is set to 0xFFFF, the Naming Authority string is
   omitted and ALL Service Types are returned, regardless of Naming
   Authority.










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10.2. Service Type Reply

      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 = 10)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Error Code          |    length of <srvType-list>   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       <srvtype--list>                         \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The service-type Strings (as described in Section 4.1) are provided
   in <srvtype-list>, which is a <string-list>.

   If a service type has a Naming Authority other than IANA it MUST be
   returned following the service type string and a `.'  character.
   Service types with the IANA Naming Authority do not include a Naming
   Authority string.


10.3. Attribute Request

   The Attribute Request (AttrRqst) allows a UA to discover attributes
   of a given service (by supplying its URL) or for an entire service
   type.  The latter feature allows the UA to construct a query for an
   available service by selecting desired features.  The UA may request
   that all attributes are returned, or only a subset of them.

      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 = 6)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       length of PRList        |        <PRList> String        \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         length of URL         |              URL              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    length of <scope-list>     |      <scope-list> string      \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  length of <tag-list> string  |       <tag-list> string       \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   length of <SLP SPI> string  |        <SLP SPI> string       \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The <PRList>, <scope-list> and <SLP SLP> string are interpreted as
   in Section 8.1.





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   The URL field can take two forms.  It can simply be a Service Type
   (see Section 4.1), such as "http" or "service:tftp".  In this case,
   all attributes and the full range of values for each attribute of all
   services of the given Service Type is returned.

   The URL field may alternatively be a full URL, such as
   "service:printer:lpr://igore.wco.ftp.com:515/draft" or
   "nfs://max.net/znoo".  In this, only the registered attributes for
   the specified URL are returned.

   The <tag-list> field is a <string-list> of attribute tags, as
   defined in Section 9.4 which indicates the attributes to return
   in the AttrRply.  If <tag-list> is omitted, all attributes are
   returned.  <tag-list> MUST be omitted and a full URL MUST be
   included when attributes when a SLP SLP List string is included,
   otherwise the DA will reply with an AUTHENTICATION_FAILED error.


10.4. Attribute Reply

      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 = 7)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Error Code            |      length of <attr-list>    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         <attr-list>                           \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |# of AttrAuths |  Attribute Authentication Block (if present)  \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the <attr-list> and the Authentication Block is as
   specified for SrvReg (see Section 9.2.1).

   Attribute replies SHOULD be returned with the original case of the
   string registration intact, as they are likely to be human readable.
   In the case where the AttrRqst was by service type, all attributes
   defined for the service type, and all their values are returned.

   Although white space is folded for string matching, attribute
   tags and values MUST be returned with their original white space
   preserved.

   Only one copy of each attribute tag or String value should be
   returned, arbitrarily choosing one version (with respect to upper
   and lower case and white space internal to the strings):  Duplicate
   attributes and values SHOULD be removed.  An arbitrary version of the




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   string value and tag name is chosen for the merge.  For example:
   "(A=a a,b)" merged with "(a=A   A,B)" may yield "(a=a a,B)".


10.5. Attribute Request/Reply Examples

   Suppose that printer services have been registered as follows:

   Registered Service:
     URL        = service:printer:lpr://igore.wco.ftp.com/draft
     scope-list = Development
     Lang. Tag  = en
     Attributes = (Name=Igore),(Description=For developers only),
                  (Protocol=LPR),(location-description=12th floor),
                  (Operator=James Dornan \3cdornan@monster\3e),
                  (media-size=na-letter),(resolution=res-600),x-OK

     URL        = service:printer:lpr://igore.wco.ftp.com/draft
     scope-list = Entwicklung
     Lang. Tag  = de
     Attributes = (Name=Igore),(Beschreibung=Nur fuer Entwickler),
                  (Protocol=LPR),(Standort-beschreibung=13te Etage),
                  (Techniker=James Dornan \3cdornan@monster\3e),
                  (Format=na-letter),(Resolution=res-600),x-OK

     URL        = service:printer:http://not.wco.ftp.com/cgi-bin/pub-prn
     scope-list = Development
     Lang. Tag  = en
     Attributes = (Name=Not),(Description=Experimental IPP printer),
                  (Protocol=http),(location-description=QA bench),
                  (media-size=na-letter),(resolution=other),x-BUSY

   Notice the first printer, "Igore" is registered in both English and
   German.  The `<' and `>' characters in the Operator attribute value
   which are part of the Email address had to be escaped, as they are
   reserved characters for values.

   The string "PROTOCOL" is 'literal' so it is not translated to
   different languages, see [13].

   The attribute Request:

     URL        = service:printer:lpr://igore.wco.ftp.com/draft
    scope-list = Entwicklung
     Lang. Tag  = de
     tag-list   = Resolution,St*

   receives the Attribute Reply:




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     (Standort-beschreibung=13te Etage),(Resolution=res-600)

   The attribute Request:

     URL        = service:printer
     scope-list = Development
     Lang. Tag  = en
     tag-list   = x-*,resolution,protocol

   receives an Attribute Reply containing:

     (protocols=http,LPR),(resolution=res-600,other),x-OK,x-BUSY

   The first request is by service instance and returns the requested
   values, in German.  The second request is by abstract service type
   (see Section 4) and returns values from both "Igore" and "Not".

   An attribute Authentication Block is returned if an authentication
   block with the SLP SPI in the AttrRqst can be returned.  Note that
   the <attr-list> returned from a DA with an Authentication Block MUST
   be identical to the <attr-list> registered by a SA, in order for the
   authentication verification calculations to be possible.

   A SA or DA only returns an Attribute Authentication Block if the
   AttrRqst included a full URL in the request and no tag list.

   If an SLP SPI is specified in a request which does not have the MCAST
   RQST flag set, and the SA or DA cannot return an Authentication Block
   with that SLP SPI, an AUTHENTICATION_UNKNOWN error is returned.  The
   # of Attr Auths field is set to 0 if there no Authentication Block is
   included, or 1 if an Authentication Block follows.


10.6. Service Deregistration

   A DA deletes a service registration when its Lifetime expires.
   Services SHOULD be deregistered when they are no longer available,
   rather than leaving the registrations to time out.














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      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 = 5)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Length of <scope-list>     |         <scope-list>          \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           URL Entry                           \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Length of <tag-list>     |            <tag-list>         \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The <scope-list> is a <string-list> (see section 2.1).

   The SA MUST retry if there is no response from the DA, see Section
   12.3.  The DA acknowledges a SrvDeReg with a SrvAck.  Once the SA
   receives an acknowledgment indicating success, the service and/or
   attributes are no longer advertised by the DA. The DA deregisters
   the service or service attributes from every scope specified in the
   SrvDeReg which it was previously registered in.

   The SA MUST deregister all services with the same scope list used to
   register the service with a DA. If this is not done in the SrvDeReg
   message, the DA returns a SCOPE_NOT_SUPPORTED error.  The Lifetime
   field in the URL Entry is ignored for the purposes of the SrvDeReg.

   The <tag-list> is a <string-list> of attribute tags to deregister
   as defined in Section 9.4.  If no <tag-list> is present, the
   SrvDeReg deregisters the service in all languages it has been
   registered in.  If the <tag-list> is present, the SrvDeReg
   deregisters the attributes whose tags are listed in the tag spec.
   Services registered with Authentication Blocks MUST NOT include
   a <tag-list> in a SrvDeReg message:  A DA will respond with an
   AUTHENTICATION_FAILED error in this case.

   If the service to be deregistered was registered with an
   authentication block or blocks, a URL authentication block for
   each of the SLP SPIs registered must be included in the SrvDeReg.
   Otherwise, the DA returns an AUTHENTICATION_ABSENT error is
   returned.  If the message fails to be verified by the DA, an
   AUTHENTICATION_FAILED error is returned by the DA.


11. Scopes

   Scopes are sets of services.  The primary use of Scopes is to provide
   the ability to create administrative groupings of services.  A set
   of services may be assigned a scope by network administrators.  A



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   client seeking services is configured to use one or more scopes.  The
   user will only discover those services which have been configured
   for him or her to use.  By configuring UAs and SAs with scopes,
   administrators may provision services.  Scopes strings are case
   insensitive.  The default SCOPE string is "DEFAULT".

   Scopes are the primary means an administrator has to scale SLP
   deployments to larger networks.  When DAs with NON-DEFAULT scopes are
   present on the network, further gains can be had by configuring UAs
   and SAs to have a predefined non-default scope.  These agents can
   then perform DA discovery and make requests using their scope.  This
   will limit the number of replies.


11.1. Scope Rules

   SLP messages which fail to contain a scope that the receiving Agent
   is configured to use are dropped (if the request was multicast) or a
   SCOPE_NOT_SUPPORTED error is returned (if the request was unicast).
   Every SrvRqst (except for DA and SA discovery requests), SrvReg,
   AttrRqst, SrvTypeRqst, DAAdvert, and SAAdvert message MUST include a
   <scope-list>.

   A UA MUST unicast its SLP messages to a DA which supports the desired
   scope, in preference to multicasting a request to SAs.  A UA MAY
   multicast the request if no DA is available in the scope it is
   configured to use.


11.2. Administrative and User Selectable Scopes

   All requests and services are scoped.  The two exceptions are
   SrvRqsts for "service:directory-agent" and "service:service-agent".
   These MAY have a zero-length <scope-list> when used to enable the
   user to make scope selections.  In this case UAs obtain their scope
   list from DAAdverts (or if DAs are not available, from SAAdverts.)

   Otherwise, if SAs and UAs are to use any scope other than the default
   (i.e., "DEFAULT"), the UAs and SAs are configured with lists of
   scopes to use by system administrators, perhaps automatically by way
   of DHCP option 78 or 79.  Such administrative scoping allows services
   to be provisioned, so that users will only see services they are
   intended to see.

   User configurable scopes allow a user to discover any service, but
   require them to do their own selection of scope.  This is similar
   to the way AppleTalk [12] and LanManager [17] networking allow user
   selection of AppleTalk Zone or Windows Workgroups.




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   Note that the two configuration choices are not compatible.  One
   model allows administrators control over service provision.  The
   other delegates this to users (who may not be prepared to do any
   configuration of their system).


12. Directory Agents

   DAs cache service location and attribute information.  They exist to
   enhance the performance and scalability of SLP. Multiple DAs provide
   further scalability and robustness of operation, since they can each
   store service information for the same SAs, in case one of the DAs
   fails.

   For use in networks with multiple subnets, a DA provides a
   centralized store for service information.  The DA address can be
   dynamically configured with UAs and SAs using DHCP, or by using
   static configuration.

   SAs configured to use DAs with DHCP or static configuration MUST
   unicast a SrvRqst to the DA with the scope list omitted and the
   service type set to "service:directory-agent".  The DA will return
   a DAAdvert with its attributes, SLP SPI list, and other parameters
   which are essential for proper SA to DA communication.

   Passive detection of DAs by SAs enables services to be advertised
   consistently among DAs of the same scope.  Advertisements expire if
   not renewed, leaving only transient stale registrations in DAs, even
   in the case of a failure of a SA.

   A single DA can support many UAs.  UAs send the same requests to DAs
   that they would send to SAs and expect the same results.  DAs reduce
   the load on SAs, making simpler implementations of SAs possible.

   UAs be prepared for the possibility that the service information they
   obtain from DAs is stale.


12.1. Directory Agent Rules

   When DAs are present, each SA MUST register its services with DAs
   that support one or more of its scope(s).

   UAs MUST unicast requests directly to a DA (when scoping rules
   allow), hence avoiding using the multicast convergence algorithm, to
   obtain service information.  This decreases network utilization and
   increases the speed at which UAs can obtain service information.





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   DAs MUST flush service advertisements once their lifetime expires or
   their URL Authentication Block "Timestamp" of expiration is past.

   DAAdverts MUST include DA Stateless Boot Timestamp, in the same
   format as the Authentication Block (see Section 9.2).  The Timestamp
   in the Authentication Block indicates the time at which all previous
   registrations were lost (i.e., the last stateless reboot).  The
   Timestamp is set to 0 in a DAAdvert to notify UAs and SAs that the DA
   is going down.

   DAs which receive a multicast SrvRqst for the service type
   "service:directory-agent" MUST silently discard it if the
   <scope-list> is (a) not omitted and (b) does not include a scope
   they are configured to use.  Otherwise the DA MUST respond with a
   DAAdvert.

   DAs MUST respond to AttrRqst and SrvTypeRqst messages (these are
   OPTIONAL only for SAs, not DAs.)


12.2. Directory Agent Discovery

   UAs can discover DAs using static configuration, DHCP options 78 and
   79, or by multicasting (or broadcasting) Service Requests using the
   convergence algorithm in Section 6.3.

   See Section 6 regarding unsolicited DAAdverts.  Section 12.2.2
   describes how SAs may reduce the number of times they must reregister
   with DAs in response to unsolicited DAAdverts.

   DAs MUST send unsolicited DAAdverts once per CONFIG_DA_BEAT. An
   unsolicited DAAdvert has an XID of 0.  SAs MUST listen for DAAdverts,
   passively, as described in Section 8.5.  UAs SHOULD do this.

   A URL with the scheme "service:directory-agent" indicates
   the DA's location as defined in Section 8.5.  For example:
   "service:directory-agent://foobawooba.org".

   The following sections suggest timing algorithms which enhance the
   scalability of SLP.


12.2.1. Active DA Discovery

   After a UA or SA restarts, its initial DA discovery request SHOULD
   be delayed for some random time uniformly distributed from 0 to
   CONFIG_START_WAIT seconds.





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   The UA or SA sends the DA Discovery request using a SrvRqst, as
   described in Section 8.1.  DA Discovery requests MUST include a
   Previous Responder List.  SrvRqsts for Active DA Discovery SHOULD NOT
   be sent more than once per CONFIG_DA_FIND seconds.

   After discoverying a new DA, a SA MUST wait a random time between 0
   and CONFIG_REG_ACTIVE seconds before registering their services.


12.2.2. Passive DA Advertising

   A DA MUST multicast (or broadcast) an unsolicited DAAdvert every
   CONFIG_DA_BEAT seconds.  CONFIG_DA_BEAT SHOULD be specified to
   prevent DAAdverts from using more than 1% of the available bandwidth.

   All UAs and SAs which receive the unsolicited DAAdvert SHOULD examine
   its DA stateless Boot Timestamp.  If it is set to 0, the DA is going
   down and no further messages should be sent to it.

   If a SA detects a DA it has never encountered (with a nonzero
   timestamp,) the SA must register with it.  SAs MUST examine the
   DAAdvert's timestamp to determine if the DA has had a stateless
   reboot since the SA last registered with it.  If so it registers
   with the DA. SAs MUST wait a random interval between 0 and
   CONFIG_REG_PASSIVE before beginning DA registration.


12.3. Reliable Unicast to DAs

   If a DA fails to respond to a unicast UDP message in CONFIG_DA_RETRY
   seconds, the message should be retried.  If a DA fails to respond
   after CONFIG_DA_MAX seconds, the SA should consider the DA to have
   gone down.  The UA should use a different DA. If no such DA responds,
   DA discovery should be used to find a new DA. If no DA is available,
   multicast is used.


12.4. DA Scope Configuration

   By default, DAs are configured with the "DEFAULT" scope.
   Administrators may add other configured scopes, in order to support
   UAs and SAs in non default scopes.  The default configuration MUST
   NOT be removed from the DA unless:

    -  There are other DAs which support the "DEFAULT" scope, or

    -  All UAs and SAs have been configured with non-default scopes.





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   Non-default scopes can be phased-in as the SLP deployment grows.
   Default scopes should be phased out only when the non-default scopes
   are universally configured.

   If a DA and SA are coresident on a host (quite possibly implemented
   by the same process), configuration of the host is considerably
   simplified if the SA supports only scopes also supported by the DA.
   That is, the SA SHOULD NOT advertise services in any scopes which are
   not supported by the coresident DA. This means that incoming requests
   can be answered by a single data store; the SA and DA registrations
   do not need to be kept separately.


12.5. DAs and Authentication Blocks

   DAs are not configured to sign service registrations or attribute
   lists.  They simply cache services registered by Service Agents.  DAs
   MUST NOT accept registrations including authentication blocks for SLP
   SPIs which it is not configured with, see Section 8.5.

   A DA protects registrations which are made with authentication blocks
   using SLP SPIs it is configured to use.  If a SrvReg updates or a
   SrvDeReg deregisters a previously registered URL, the message must
   include an Authentication Block with the corresponding SLP SPI, see
   Section 8.3 and Section 10.6.

   Example:

   A DA is configured to be able to verify Authentication Blocks with
   SLP SPIs "X,Y", that is X and Y.

   An SA registers a service with an Authentication Block with SPI "Z".
   The DA stores the registration, but discards the Authentication
   Block.  If a UA requests a service with an SLP SPI string "Z", the DA
   will respond with an AUTHENTICATION_UNKNOWN error.

   An SA registers a service S with Authentication Blocks including SLP
   SPIs "X" and "Y".  If a UA requests a service with an SLP SPI string
   "X" the DA will be able to return S (if the service type, language,
   scope and predicate of the SrvRqst match S) The DA will also return
   the Authentication Block with SLP SPI set to "X".  If the DA receives
   a SrvDereg message for S, the SrvDereg message must include two
   URL Authentication Blocks, one each for SPIs "X" and "Y".  If the
   DA receives a subsequent SrvReg for S, it too must include both
   Authentication Blocks.







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13. Protocol Timing Defaults

Interval name        Section  Default Value  Meaning
-------------------  -------  -------------  ------------------------
CONFIG_MC_RETRY      6.3      each second,   Retry multicast query
                              backing off    until no new values
                              gradually      arrive.
CONFIG_MC_MAX        6.3      15 seconds     Max time to wait for a
                                             complete multicast query
                                             response (all values.)
CONFIG_START_WAIT    12.2.1   3 seconds      Wait to perform DA
                                             discovery on reboot.
CONFIG_DA_RETRY      12.3     2 seconds      Retransmit DA discovery,
                                             try it 3 times.
CONFIG_DA_MAX        12.3     6 seconds      Give up on requests sent
                                             to a DA.
CONFIG_DA_BEAT       12.2.2   3 hours        DA Heartbeat, so that SAs
                                             passively detect new DAs.
CONFIG_DA_FIND       12.3     900 seconds    Minimum interval to wait
                                             before repeating Active
                                             DA discovery.
CONFIG_REG_PASSIVE   12.2     1-3 seconds    Wait to register services
                                             on passive DA discovery.
CONFIG_REG_ACTIVE    8.3      1-3 seconds    Wait to register services
                                             on active DA discovery.
CONFIG_CLOSE_CONN    6.2      5 minutes      DAs and SAs close idle
                                             connections.




14. Optional Configuration

      Broadcast Only
               Any SLP agent SHOULD be configurable to use broadcast
               only.  See Sections 6.1 and 12.2.

      Predefined DA
               A UA or SA SHOULD be configurable to use a predefined DA.

      No DA Discovery
               The UA or SA SHOULD be configurable to ONLY use
               predefined and DHCP-configured DAs and perform no active
               or passive DA discovery.

      Multicast TTL
               The default multicast TTL is 32.  Agents SHOULD be
               configurable to use other values.  A lower value will
               focus the multicast convergence algorithm on smaller



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               subnetworks, decreasing the number of responses and
               increases the performance of service location.  This
               may result in UAs obtaining different results for the
               identical requests depending on where they are connected
               to the network.

      Timing Values
               Time values other than the default MAY be configurable.
               See Section 13.

      Scopes
               A UA MAY be configurable to support User Selectable
               scopes by omitting all predefined scopes.  See
               Section 11.2.  A UA or SA MUST be configurable to use
               specific scopes by default.  Additionally, a UA or SA
               MUST be configurable to use specific scopes for requests
               for and registrations of specific service types.  The
               scope or scopes of a DA MUST be configurable.  The
               default value for a DA is to have the scope "DEFAULT" if
               not otherwise configured.

      DHCP Configuration
               DHCP options 78 and 79 may be used to configure SLP. If
               DA locations are configured using DHCP, these SHOULD
               be used in preference to DAs discovered actively or
               passively.  One or more of the scopes configured using
               DHCP MUST be used in requests.  The entire configured
               <scope-list> MUST be used in registration and DA
               configuration messages.

      Service Template
               UAs and SAs MAY be configured by using Service Templates.
               Besides simplifying the specification of attribute
               values, this also allows them to enforce the inclusion
               of 'required' attributes in SrvRqst, SrvReg and SrvDeReg
               messages.  DAs MAY be configured with templates to
               allow them to WARN UAs and SAs in these cases.  See
               Section 10.4.

      SLP SPI for service discovery
               Agents SHOULD be configurable to support SLP SPIs using
               the following parameters:  BSD=2 (DSA with SHA-1) and
               a public key identified by the SLP SPI String.  In
               the future, when a Public Key Infrastructure exists,
               SLP Agents may be able to obtain public keys and
               cryptographic parameters corresponding to the names used
               in SLP SLP Strings.





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               Note that if the SLP SPI string chosen is identical
               to a scope string, it is effectively the same as a
               Protected Scope in SLPv1.  Namely, every SA advertising
               in that scope would be configured with the same Private
               Key.  Every DA and UA of that scope would be configured
               with the appropriate Public Key to verify signatures
               produced by those SAs.  This is a convenient way to
               configure SLP deployments in the absense of a Public Key
               Infrastructure.  Currently, it would be too difficult to
               manage the keying of UAs and DAs if each SA had its own
               key.

      SLP SPI for Directory Agent discovery
               Agents SHOULD be configurable to support SLP SPIs as
               above, to be used when discovering DAs.  This SPI SHOULD
               be sent in SrvRqsts to discover DAs and be used to verify
               multicast DAAdvert messages.

      SA and DA Private Key
               SAs and DAs which can generate digital signatures require
               a Private Key and a corresponding SLP SPI indentifier
               to include in the Authentication Block.  The SLP SPI
               identifies the Public Key to use to verify the digital
               signature in the Authentication Block.


15. IANA Considerations

   Further Block Structured Descriptor (BSD) values may be standardized
   in the future by submitting a document which describes:

      -     The data format of the Structured Authenticator block.

      -     Which cryptographic algorithm to use (including a reference
            to a technical specification of the algorithm.)

      -     The format of any keying material required for
            preconfiguring UAs, DAs and SAs.  Also include any
            considerations regarding key distribution.

      -     Security considerations to alert others to the strengths and
            weaknesses of the approach.

   The IANA will assign Cryptographic BSD numbers (from the range 0x0003
   to 0x7FFF) on a first come, first served basis.  These numbers are
   assigned when an RFC (of any status) is issued defining the SLP BSD
   and its use.





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   New function-IDs, in the range 12-255, may be standardized by the
   method of IETF Consensus [16].  Similarly, new extensions with types
   in the range 3-65535 may be standardized by the method of IETF
   Consensus.  Specification and Expert Review is required for the
   assignment of new error numbers in the range of 15-65535.

   Protocol elements used with Service Location Protocol may also
   require IANA registration actions.  SLP is used in conjunction with
   "service:" URLs and service templates [13].  These are standardized
   by the method of a Designated Expert and a mailing list (see [13].)


16. Internationalization Considerations

   SLP messages support the use of multiple languages by providing a
   Language Tag field in the common message header (see Section 8).

   Services MAY be registered in multiple languages.  This provides
   attributes so that users with different language skills may select
   services interactively.

   A service which is registered in multiple languages may be queried in
   multiple languages.  The language of the SrvRqst or AttrRqst is used
   to satisfy the request.  If the requested language is not supported,
   a LANGUAGE_NOT_SUPPORTED error is returned.  SrvRply and AttrRply
   messages are always in the same language of the request.

   A DA or SA MAY be configured with translations of Service Templates
   [13] for the same service type.  This will allow the DA or SA to
   translate a request (say in Italian) to the language of the service
   advertisement (say in English) and then translate the reply back to
   Italian.  Similarly, a UA MAY use templates to translate outgoing
   requests and incoming replies.

   The dialect field in the Language Tag MAY be used:  Requests which
   can be fulfilled by matching a language and dialect will be preferred
   to those which match only the language portion.  Otherwise, dialects
   have no effect on matching requests.


17. Year 2000 Considerations

   SLPv2 uses a Timestamp field in some messages.  This value is a
   32-bit unsigned fixed-point number of *minutes* relative to 0h on 1
   January 1900.  This timestamp will wrap back to 0 in the year 10,066.







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18. Security Considerations

   SLP provides for authentication of service URLs and service
   attributes.  This provides UAs and DAs with knowledge of the
   integrity of service URLs and attributes included in SLP messages.
   The only systems which can generate digital signatures are those
   which have been configured by administrators in advance.  Agents
   which verify signed data may assume it is 'trustworthy' inasmuch as
   administrators have ensured the cryptographic keying of SAs and DAs
   reflects 'trustworthiness.'

   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.

   If Agents are not configgured to generate Authentication Blocks and
   Agents are not configured to verify them, 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 SLP SPIs prior to deploying the sensitive directory
   agents or service agents.

   SLP 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.  SLP 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 Authentication Blocks if
   cryptographic information and SLP SPIs can be preconfigured with the
   end systems before they use SLP.

   SLPv2 enables a number of security policies with the mechanisms it
   includes.  A SLPv2 UA could, for instance, reject any SLP message
   which did not carry an authentication block which it could verify.
   This is not the only policy which is possible to implement.






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A. Appendix:  SLP Protocol Extensions

A.1. Required Attribute Missing Option

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Extension Type = 0x0001    |        Extension Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Template IDVer Length    |     Template IDVer String     \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Required Attr <tag-list> Length|    Required Attr <tag-list>   \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Required attributes and the format of the IDVer string are defined
   by [13].

   If a SA or DA receives a SrvRqst or a SrvReg which fails to include
   a Required Attribute for the requested Service Type (according
   to the service template), it MAY return the Required Attribute
   Extension in addition to the reply corresponding to the message.  The
   sender SHOULD reissue the message with a search filter including
   the attributes listed in the returned Required Attribute Extension.
   Similarly, the Required Attribute Extension may be returned in
   response to a SrvDereg message that contains a required attribute
   tag.

   The Template IDVer String is the name and version number string of
   the service template which defines the given attribute as required.
   It SHOULD be included, but can be omitted if a given SA or DA has
   been individually configured to have 'required attributes.'

   The Required Attribute <tag-list> MUST NOT include wild cards.


Acknowledgments

   This document incorporates ideas from work on several discovery
   protocols, including RDP by Perkins and Harjono, and PDS by
   Michael Day.  John Veizades was working group chair for much of
   the development of Service Location Protocol, instrumental in its
   standardization, and the lead author of SLPv1 [19].


References

    [1] Port numbers, July 1997.
        ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers.




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    [2] ISO/IEC JTC1/SC 21.  Certificate Extensions.  Draft Amendment
        DAM 4 to ISO/IEC 9594-2, December 1996.

    [3] ISO/IEC JTC1/SC 21.  Certificate Extensions.  Draft Amendment
        DAM 2 to ISO/IEC 9594-6, December 1996.

    [4] ISO/IEC JTC1/SC 21.  Certificate Extensions.  Draft Amendment
        DAM 1 to ISO/IEC 9594-7, December 1996.

    [5] ISO/IEC JTC1/SC 21.  Certificate Extensions.  Draft Amendment
        DAM 1 to ISO/IEC 9594-8, December 1996.

    [6] Unicode Technical Report #8.  The unicode standard, version 2.1.
        Technical report, The Unicode Consortium, 1998.

    [7] H. Alvestrand.  Tags for the Identification of Languages.  RFC
        1766, March 1995.

    [8] T. Berners-Lee, R. Fielding, and L. Masinter.  Uniform Resource
        Identifiers (URI): Generic Syntax.  RFC 2396, August 1998.

    [9] S. Bradner.  Key Words for Use in RFCs to Indicate Requirement
        Levels.  RFC 2119, March 1997.

   [10] CCITT.  The Directory Authentication Framework.  Recommendation
        X.509, 1988.

   [11] D. Crocker and P. Overell.  Augmented BNF for Syntax
        Specifications:  ABNF.  RFC 2234, November 1997.

   [12] S. Gursharan, R. Andrews, and A. Oppenheimer.  Inside AppleTalk.
        Addison-Wesley, 1990.

   [13] E. Guttman, C. Perkins, and J. Kempf.  Service Templates and
        service:  Schemes.  draft-ietf-svrloc-service-scheme-05.txt,
        November 1997.  (work in progress).

   [14] T. Howes.  The string representation of LDAP search filters.
        RFC 2254, December 1997.

   [15] D. Meyer.  Administratively Scoped IP Multicast.  RFC 2365, July
        1998.

   [16] Thomas Narten and Harald Tveit Alvestrand.  Guidelines
        for Writing an IANA Considerations Section in RFCs.
        draft-iesg-iana-considerations-04.txt, May 1998.  (work in
        progress).





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   [17] Microsoft Networks.  SMB File Sharing Protocol Extensions 3.0,
        Document Version 1.09, November 1989.

   [18] National Institute of Standards and Technology.  Digital
        signature standard.  Technical Report NIST FIPS PUB 186, U.S.
        Department of Commerce, May 1994.

   [19] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan.  Service
        Location Protocol.  RFC 2165, July 1997.

   [20] F. Yergeau.  UTF-8, a transformation format of ISO 10646.  RFC
        2279, January 1998.








































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B. Full Copyright Statement

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However,
   this document itself may not be modified in any way, such as by
   removing the copyright notice or references to the Internet Society
   or other Internet organizations, except as needed for the purpose
   of developing Internet standards in which case the procedures
   for copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."


Authors' Addresses

             Erik Guttman              Charles Perkins
             Sun Microsystems          Sun Microsystems
             Bahnstr. 2                901 San Antonio Road
             74915 Waibstadt           Palo Alto, CA 94040
             Germany                   USA

   Phone:    +49 7263 911 701          +1 650 786 6464
   Email:    Erik.Guttman@sun.com      cperkins@sun.com

             John Veizades             Michael Day
             @Home Network             Intel
             385 Ravendale Dr.         734 E. Utah Valley Dr., Ste. 300
             Mountain View, CA 94043   American Fork, Utah, 84003
             USA                       USA

   Phone:    +1 650 569 5243           +1 801 763 2341
   Email:    veizades@home.net         Michael_Day@ccm.ut.intel.com




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