Communication in Client-Server Systems

Sockets

• A socket is defined as an endpoint for communication

• Concatenation of IP address and port – a number included at start of message packet to differentiate network services on a host

• The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8

• Communication consists between a pair of sockets

• All ports below 1024 are well known, used for standard services

• Special IP address 127.0.0.1 (loopback) to refer to system on which process is running

Remote Procedure Calls (RPC)

• Remote procedure call (RPC) abstracts procedure calls between processes on networked systems
  • Again uses ports for service differentiation

• Stubs – client-side proxy for the actual procedure on the server

• The client-side stub locates the server and marshalls the parameters

• The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server

• On Windows, stub code compile from specification written in Microsoft Interface
  Definition Language (MIDL)

• Data representation handled via External Data Representation (XDL) format to account for different architectures
  • Big-endian and little-endian

• Remote communication has more failure scenarios than local

• Messages can be delivered exactly once rather than at most once

• OS typically provides a rendezvous (or matchmaker) service to connect client and server