SLP ( Service Location Protocol )

• Protocol Transactions
  The diagram below illustrates the relationships described below:




      +---------------+   we want this info:     +-----------+

      |  Application  | - - - - - - - - - - - -> |  Service  |

      +---------------+                          +-----------+

           /|                                      |     |

            |                         +-------------+     |

            |                         |                   |

           |/                       |/                 |/

      +---------------+          +-----------+      +----------------+

      |   User Agent  |<-------->|  Service  |      |    Service     |

      +---------------+          |   Agent   |      | Agent which    |

            |                    +-----------+      | does not reply |

            |                         |             | to UA requests |

            |                        |/            +----------------+

            |                   +-------------+           |

            +------------------>|  Directory  |<----------+

                                |    Agent    |

                                +-------------+      ___________

                                     /|            / Many other

                                      +------------>|   SA's    |

                                                    ___________/


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

A User Agent will operate two ways:

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



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



• Schemes
  The Service Location Protocol, designed as a way for clients to access resources on the network, is a natural application for Universal Resource Locators (URLs). It is intended that by re-using URL specification and technology from the World Wide Web, clients and servers will be more flexible and able to be written using already existing code. Moreover, it is hoped that browsers will be written to take advantage of the similarity in locator format, so that a client can dynamically formulate requests for services that are resolved differently depending upon the circumstances.

• Version
  The current version is version 1



• Function
  The function field describes the operation of the Service location datagram. The following message types exist:




Function	Message Type
1	Service Request
2	Service Reply
3	Service Registration
4	Service Deregister
5	Service Acknowledge
6	Attribute Request
7	Attribute Reply
8	DA Advertisement
9	Service Type Request
10	Service Type Reply




• Length
  Number of bytes in the message including the Service location header.



• O
  The overflow bit.



• M
  The monolingual bit.



• U
  RL Authentication bit present.



• A
  Attribute authentication bit present.



• F
  If the F bit is set in a Service Acknowledgement, the directory agent has registered the service as a new entry.



• Rsvd
  These bits are reserved and must have a value of 0.



• Dialect
  To be use by future versions of the SLP. Must be set to zero.



• Language Code
  The language encoded in this field indicates the language in which the remainder of the message should be interpreted.



• Character Encoding
  The characters making up strings within the remainder of this message may be encoded in any standardized encoding.



• Transaction Identifier (XID)
  This field allows the requester to match replies to individual requests. Note that, whenever there is an Attribute Authentication block, there will also be a URL Authentication block.

• Version
  Must be set to 2. Version of Service Location Protocol.



• Function
  Same as in version 1.



• Length
  The total number of bytes in the message including the SLP header.



• O, Overflow
  Set when the length of a message exceeds what can fit into a datagram.



• F, Fresh
  If set and the function is a Service Acknowledgement, the directory agent has registered the service as a new entry, not as an updated entry.



• R, Request multicast
  Set when multicasting or broadcasting requests.



• Reserved
  Must be cleared to zero.



• Next Ext Offset
  Cleared 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.



• XID, Transaction Identifier
  This field allows the requester to match replies to individual requests. Note that, whenever there is an Attribute Authentication block, there will also be a URL Authentication block.

■ Parent layer
■ Child layer

TCP/UDP

SLP