Does TCP guarantee packet transmission time

That sounds like UDP hole punching

Let A and B be the two hosts, each in its own private network; N1 and N2 are the two NAT devices; S is a public server with a well-known globally reachable IP address.

1. A and B each begin a UDP conversation with S; the NAT devices N1 and N2 create UDP translation states and assign temporary external port numbers
2. S relays these port numbers back to A and B
3. A and B contact each others' NAT devices directly on the translated ports; the NAT devices use the previously created translation states and send the packets to A and B

In this example, you are S. Your opponents are A and B. When you are kicked from the game, your opponents can continue playing because they had negotiated a connection to each other when they first connected to you.

TCP is a stream protocol, usually on top of IP (IPv4 or 6).It doesn't cover file transmission. Instead, this is typically delegated to other protocols such as FTP, HTTP, HTTPS, NFS, CIFS or rsync.

You are apparently removing the IP and TCP headers from the data, but not the metadata added by the other protocol(s) you're using. Since you haven't told us what those are, we can't specifically address them, but in general you'll have to implement a protocol handler for that.

Note that with advanced protocols such as HTTPS, you obviously and by design cannot recover a file from only the server side packets.