How to verify data integrity between client-server connection?

Question

I recently developed an online multiplayer game and it's working fine. There's a central server handling TCP connections with clients, and they both receive and send data.
Is there a way to verify that the data sent hasn't been modified before the receiver reads it?
Does a TCP connection handle it in some way? If not what is the best way (not asking for code) to implement it?
By now I came up with those ideas:

• Modify the data sent adding some sort of verify-value, such the packet length.
• When a packet is received, ask the server to send it again and verify if they are equal.

I searched for a common used solution but couldn't find much.
Encryption should be my last option.

Edit
For a simple way to achieve this:

1. MD5
   
   • Wikipedia
     
   • Example
2. SHA1
   
   • Wikipedia
     
   • Example
3. SSL
   
   • Wikipedia
     
   • Example

Answer 1

Agreed with Bart, you may want to look at using a Hash method for data verification.