RLP is used to help locate network services and works with a simple request/reply method to locate network services/resources.
RLP (Resource Location Protocol)
This protocol performs a variety of tasks to others, including self-generated services for managing personal information such as DNS, DHCP, and certain services of computers connecting and communicating over the IP network.
In short, RLP is designed to enable computers that can communicate in a particular or group of computers to automatically find other computers that provide and offer various services.
In this communication system, the protocol defines a set of request messages that can be sent by the calling computer and sends the generated messages using the UDP transport protocol.
A request message is sent to UDP port 39 of another host, in which case the request contains a description of one or more services sought.
The request with this service description includes hosts providing certain services and sending a reply message.
RLP uses the following question types to define these request messages:
1) Who Provides?
It usually makes a broadcast on the local network. The host performing one of the described services may respond to this request. Hosts that do not provide any of the specified services do not respond to this request.
2) Do You Provide?
With this request message, a request is sent directly to a specific host, ie the computer. As the first message type, this request may not use broadcast.
A host receiving this message responds regardless of whether it provides any of the specified services.
3) Who Anywhere Provides?
This type of message is also broadcast to a LAN. In this case, hosts that provide any service on the LAN or know other hosts that do this may respond to this message.
4) Does Anyone Provide?
In this type of message, only a request is sent to a specific host. In this case, a host providing or not providing any of the services should send a response regardless of whether it knows that service or not.
The message structure used by this protocol includes Type, Flags, Local Only, Message-ID, and Resource List fields.
The Type field defines the question or answer type.
Local Only and Flag indicate that only hosts with the same IP network address should respond or be included in the answer list.
Message-ID ensures that the responses received are matched with previously submitted requests.
The Resource List contains a description of the services and supporting hosts sought or provided.
Resources and services are defined in several fields, and the first description byte indicates the protocol number of the IP protocol the services are using. The next byte identifies the port typically used by the service.
Then self-generated random services are identified using the additional bytes.