Based on the needs of different systems, several communication protocols have been proposed for use in RPC which are mentioned below:

i.The Request Protocol

• This protocol is also called as R (request) protocol.
• It is used in RPC when a called procedure has nothing to return as a result of execution and the requirement of client confirmation about procedure execution is not needed.
• As no acknowledgement or reply message is involved, only single message is transmitted from client to server.
• The client proceeds after the request message is sent as there is no reply message.
• This protocol provides May-be call semantics and does not need retransmission of request messages.
• RPC that uses the R protocol is known as asynchronous RPC which helps to improve the combined performance of the client and server. This is done because the client does not ait for a reply and server does not need to send a reply.
• For an asynchronous RPC, the RPCRuntime does not retry a request in case of communication failure. TCP is better alternative then UDP since no retransmission is required and it is connection oriented.
• Asynchronous RPC with unreliable transport protocol are generally used in implementing periodic update services. Distributed system window is one of its applications.

ii.The Request/Reply protocol

• This protocol is also known as RR(request/reply) protocol.
• It is useful for designing systems which involve simple RPCs.
• In a simple RPC all the arguments and result fit in a single packet buffer while the call duration and intervals between calls are short.
• This protocol is based on the idea of using implicit acknowledgement to eliminate explicit acknowledgement messages.
• In this protocol a server reply is considered as an ACK for a clients request and a subsequent call from a client is considered as ACK of the client’s previous call.
• Timeout-and-retires technique is used with RR protocol for failure handling. Retransmission of request message is done when there is no response.
• RR protocol and timeout technique provides at-least-once call semantics on if duplicate requests are not filtered out.
• Exactly once semantics are supported by servers using reply cache which stores replies.

iii.The Request/Reply/Acknowledgement-Reply Protocol

• This protocol is also known as RRA (request/reply/acknowledge-reply) protocol.
• RR protocol implements exactly once semantics which requires storage of a lot of information in the server cache and can lead to loss of replies that have not been delivered.
• To overcome the limitations or RR protocol, RSA protocol is used.
• In this clients acknowledge the receipt of reply messages and the server deletes information from its cache only after it receives an acknowledgement from client.
• Sometimes the reply acknowledgement message may get lost therefore RRA protocol needs a unique ordered message identifiers. This keeps a track of the acknowledgement series sent.