Capturing packets question

[Stuff_]

 

I'm writing a program to replay packets from captures. I'm only sending packets with a specific IP and capturing those replies. 

 

My question is, all i care about are the seq/ack numbers from the TCP header. I'm using pcap_activate() with provides me the ability to select a "snaplen." 

 

How far into the packet do I need to read to get only the seq and ack numbers? Everything else can be truncated and discarded. 

 

Any help would be appreciated. 

 

I'm using c, and libdnet if it matters.