Introduction

I’ve been working with sockets since 2000, using Delphi 5.0 and some third-party libraries (Synapse). My very first socket application just copied files between many clients and one server. The client app checks a folder to see if files exist, asks the server where to copy the files in the network and, after copying the files, flags the database record indicating that a file has been moved. The server listens to the client connections, and both exchange XML messages indicating the state of each file copy. Synapse is a blocking socket implementation, and I needed a thread pooling mechanism that works like an HTTP server, because I couldn’t keep the connection open (one thread per connection). My solution was to use some IOCP functions to pool the client requests (code) and close the connection after the message exchange was terminated.

Now, using C#, I decided to write a socket server and client library that helps me to only have to think about the message exchange (the process) and let .NET do the hard job. So, I needed the following features:

• Asynchronous processing
• Some encryption and compression capabilities
• Encapsulate the socket, and encrypt the services in the interfaces and separate them from the host implementation

Socket Connection

The ISocketConnection is the base interface for the socket connections, and describes all the connection properties and the methods. The ConnectionID property defines a unique connection ID using a GUID string. The CustomData property defines a custom object that can be associated with the connection. The Header property is the socket service header used in each message that is encapsulated in a packet message. Only messages with a defined header will be accepted. The LocalEndPoint and RemoteEndPoint are the socket IP end points used in the connection. SocketHandle is the socket handle given by the OS.

The IClientSocketConnection and IServerSocketConnection inherit the ISocketConnection, and each one has special functions. The IClientSocketConnection can reconnect to the server using the BeginReconnect method, and the IServerSocketConnection can communicate with other connections in the server host using the BeginSendTo and BeginSendToAll methods, and can get the ConnectionId using the GetConnectionById method. Every connection knows the host, the encryption, the compression type, and can send, receive, and disconnect itself from the other part. This interface is used in the ISocketService interface to allow the user to interact with socket connections.

Internally, in the library implementation, all the connection interfaces are created using the base connection implementations: BaseSocketConnection, ClientSocketConnection, and ServerSocketConnection.

Socket Service

The ISocketService describes the connection events. These events are fired by the host, and have a ConnectionEventArgs argument which has an ISocketConnection that identifies the connection. In the OnReceived and OnSent events, a MessageEventArgs is passed, which has the sent or received array of bytes. In the OnDisconnected event, a DisconnectedEventArgs is passed; the Exception property indicates if the disconnection has been caused by an exception.

Here is an example of a ISocketService implementation:

The ISocketService implementation can be done in the same host assembly, or another assembly referenced by the host. This allows the user to separate the host implementation from the socket service, helping the administration in a server or a domain.

Connection Host

With the ISocketService created, you need to host the service and the service connections. Both the server and the client host have the same parent class, BaseSocketConnectionHost, which keeps a list of connections, encrypts and compresses the data buffers, enqueues the service requests and ensures that all data buffer has been sent or received, checks messages headers, and checks for idle connections. The CheckTimeoutTimer, periodically, at IdleCheckInterval, checks if the connections become idle, using the IdleTimeOutValue as the idle timeout. Header is the socket service header used by the host. HostType indicates if a host is a server or a client host. SocketBufferSize defines the size of the socket send and receive buffer. SocketService is the instance of ISocketService that drives the message exchange between the connections.

Encrypt and Compress

Every time you send and receive messages, the host checks if the data must be encrypted and/or compressed, and this work is made by the CryptUtils static class. The CreateSymmetricAlgoritm creates an ISymmetricAlgoritm based on the encryptType parameter. The DecryptData and DecryptDataForAuthenticate are used, respectively, to decrypt the received message and check the hash sign on the authenticate procedure. The EncryptData and EncryptDataForAuthenticate, respectively, encrypt the data to be sent and sign the authenticated message.

The encrypted data buffer is labelled with the service header and the data buffer length, becoming a packet buffer. This packet buffer is controlled by the MessageBuffer class that keeps information about the packet buffer offset, length, the remaining bytes, and the raw buffer.

Check Message Header

If the socket service uses some header, all the send and receive processes need to create a packet message indicating the header and the message length. This packet label is created using the following structure:

SocketServer Constructor and Methods

In the SocketServer constructor, the socketService parameter defines the ISocketService instance used by the server. The header parameters define the array of bytes used in the message header exchange. The socketBufferSize adjusts the socket buffer size. The messageBufferSize defines the maximum message size of the service. The idleCheckInterval indicates the interval for idle connections checking, in milliseconds. The idleTimeoutValue defines the timeout, in milliseconds, to be compared to each connection LastAction property.

To add SocketListener items in SocketServer, the method AddListener must be used. The localEndPoint parameter defines the local socket IP endpoint used to listen to connections. The encryptType and compressionType defines, respectively, the encryption and compression methods used in the new accepted connection. The cryptoService defines the ICryptoService used to authenticate the encryption method chosen. The backLog limits the listen queue of the OS socket to the defined number, and acceptThreads sets the calling number of the socket’s BeginAccept to increase the accepted performance.

The SocketClient and SocketConnector are the classes needed to create a socket client. SocketClient is derived from BaseSocketConnectionHost and, like SocketServer, manages ISocketConnections. The SocketConnector is derived from BaseSocketConnectionCreator, and it connects with the socket server and creates a new ISocketConnection to be used. A SocketClient can have as many SocketConnectors attached as required, each one connecting to a socket server, and they can be assigned to a local address and a local port to start the connection.

SocketClient Constructor and Methods