Basic asynchronous TCP/IP chat app, developed live at https://www.youtube.com/playlist?list=PLIebvSMVr_dehKSoq6vuAW0BGEM6QnDlS

There is no such thing as a "simple" TCP/IP application, but this is pretty close. This repository is a minimal correct implementation.

Each socket connection is two unidirectional streams of bytes. A minimal correct implementation consists of:

• Message framing, to translate the streams of bytes to streams of messages.
• Error handling:
  • Always Be Reading - this lets you detect errors as quickly as possible.
  • Asynchronous APIs - so we don't need multiple threads per socket.
  • Close both streams for a connection at the same time - so that both sides can detect errors as long as data is still being transferred.
• Keepalives, to detect the half-open scenario.

• PipelineSocket - kind of like NetworkStream but using System.IO.Pipelines instead of Stream. PipelineSocket provides two unidirectional pipelines of bytes.
• ChatConnection:
  • Serializes chat protocol messages to/from bytes. ChatConnection provides two unidirectional channels of messages.
  • Handles keepalives:
    • Periodically sends keepalive messages.
    • Ignores incoming keepalive messages.
  • Handles request/response logic:
    • Has a collection of outstanding requests.
    • Handles response messages by completing their matching request.

• Security
  • Encryption. This protocol is a plain-text protocol.
  • Server/client authentication.
  • Misbehaving clients/servers. This app assumes correctly formed packets.
  • Denial of service. E.g., really large length prefixes, sending data too slowly, etc.
  • Reflection attacks. E.g., broadcast huge message at a high rate.
• Higher-level state machine. E.g., when connection is lost, retry connection after some timeout.
  • Consider a Desired State pattern: the user indicates the desired state, and your application performs whatever connection and messages are necessary to achieve that state.

• TCP/IP Illustrated, Volume 1
• My TCP/IP .NET FAQ
• (Unmanaged) WinSock Programmer's FAQ

• Process Monitor to trace socket APIs called by an application.
• TcpView to show the state of TCP/IP sockets.
• Clumsy for simulating network black holes.
• Wireshark for monitoring network traffic.
  • Template for custom TCP protocol extension for Wireshark to teach Wireshark how to understand your TCP/IP protocol.