Sockets and data tranferring theory

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C#

TCP/IP

sockets

Greetings, i'm new in Sockets, and i have a some theoretical questions regarding Client <-> Server communication.

Question is about message type and format.

1. If i will use a string with delimeters as a message, for all client-server operations, is it a good practice for a high load project?
Example:
"CMSG_AUTH:Login:Password"

2. I've heard, that somebody using opcodes as server return command. Should i use string and opcodes in tandem? And how? Which design patterns would be good to learn, for a internal server architecture?
(I know, it's a stupid question, but i really interested in server development, and it's my dream to write good, scalable server application). I don't want to use WCF, i want to make it from scratch, just to understand how it works.

2. Which books will be good to read reagarding current subject?

Thank you!