man in the middle – Tool for injecting data in existing tcp connection

I would like to perform arp spoofing on an existing tcp connection between a server and a client and perform a mitm attack. In addition to altering existing packets, I would like to be able to inject my own packets into the connection without disturbing the legitimate flow of packets. As I understand, this would require handling seq and ack numbers that are out of sync between the client and the server, as the server will receive more packets than the client sent. Is there a good tool for this?