具有压缩功能的TCP的客户端服务器

Question

I have a program that work like a chat.
Client and server are connected with 2 TCP sockets, one for incoming messages another for outgoing messages.
Sometimes the messages can be very big (ex. 2 MByte of text) so I want to compress them before sending over the channel.
The problem is that I don't know how to find the start and end of compressed message.
Now I use two special characters to find start and end of message but with compression there can be errors.

There is maybe a type of compression that don't use some specific bytes?
I use C# to open and manage sockets so I need a compression that work under windows.

Answer 1

Append to start of message it length. After that you just need to read length, and after that get exactly count of bytes what you need.
It will looks like:

|length|data|..|..|length|data|..|..|..|

And more exactly

|3|26|125|36|4|12|45|16|34|

Where 3 and 4 are length.

Answer 2

You just need an escaping scheme.

• Send STX (Start of Transmission) at the start.
• Send ETX (End of Transmission) at the end.
• If an STX or ETX appears in the data, prefix it with ESC (escape).
• If an ESC appears in the data, prefix it with ESC.

At the receiver:

• The first byte should be STX, otherwise you have a bug. Discard it.
• After that, if a byte is ESC, discard it and accept the next byte, whatever it is.
• Otherwise, if the next byte is ETX, discard it and stop reading.

The problem with the length-word prefix suggested in another answer is that you can't know the length without doing the compression first, which costs time and space.