[펌]TCP/IP 헤더 내 맘대로...tcp[13] == 2 의 의미는 무엇일까?

http://www.packetinside.com/2010/10/tcpip-%ED%97%A4%EB%8D%94-%EB%82%B4-%EB%A7%98%EB%8C%80%EB%A1%9Ctcp13-2-%EC%9D%98-%EC%9D%98%EB%AF%B8%EB%8A%94-%EB%AC%B4%EC%97%87%EC%9D%BC%EA%B9%8C.html