rfc2165.txt

SLP协议在linux下的实现。此版本为1.2.1版。官方网站为www.openslp.org

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RFC 2165               Service Location Protocol               June 1997


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

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

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

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

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

3.7. Service Location Scaling, and Multicast Operating Modes

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

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

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



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RFC 2165               Service Location Protocol               June 1997


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

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

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

   Scoped user agents SHOULD always use a DA which supports their
   configured scope when possible instead of an unscoped DA. This will
   prevent the unscoped DAs from becoming overused and thus a scaling
   problem.

   It is possible to specially configure Service Agents to register only
   with a specific set of DAs (see Section 22.1).  In that case,
   services may not be available to User Agents via all Directory
   Agents, but some network administrators may deem this appropriate.

   There are thus 3 distinct operating modes.  The first requires no
   administrative intervention.  The second requires only that a DA be
   run.  The last requires that all DAs be configured to have scope and
   that a coherent strategy of assigning scopes to services be followed.
   Users must be instructed which scopes are appropriate for them to
   use.  This administrative effort will allow users and applications to
   subsequently dynamically discover services without assistance.

   The first mode (no DAs) is intended for a LAN. The second mode (using
   a DA or DAs, but not using scopes) scales well to a group of
   interconnected LANs with a limited number of hosts.  The third mode
   (with DAs and scopes) allows the SLP protocol to be used in an
   internetworked campus environment.

   If scoped DAs are used, they will not accept unscoped registrations
   or requests.  UAs which issue unscoped requests will discover only
   unscoped services.  They SHOULD use a scope in their requests if
   possible and SHOULD use a DA with their scope in preference to an
   unscoped DA. In a large campus environment it would be a bad idea to
   have ANY unscoped DAs:  They attract ALL registrations and will thus
   present a scaling problem eventually.



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RFC 2165               Service Location Protocol               June 1997


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

4. Service Location General Message Format

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

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

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

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

               Message Type             Abbreviation     Function Value

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

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

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







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RFC 2165               Service Location Protocol               June 1997


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

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

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

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

      rsvd     MUST be zero.

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

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

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

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

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

4.1. Use of Transaction IDs (XIDs)

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



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RFC 2165               Service Location Protocol               June 1997


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

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

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

4.2. URL Entries

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

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

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

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

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







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RFC 2165               Service Location Protocol               June 1997


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

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

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

4.3. Authentication Blocks

   Authentication blocks are used to authenticate service registrations
   and deregistrations.  URLs are registered along with an URL
   Authentication block to retain the authentication information in the
   URL entry for subsequent use by User Agents who receive a Service