fix the mtu reduction problem
fix the mtu reduction problem

wfxiang08/tcpcopy,eliteYang/tcpcopy,liviusATnetskope/second,horryq/tcpcopy,ycaihua/tcpcopy,caosiyang/tcpcopy,eliteYang/tcpcopy,left2right/tcpcopy,horryq/tcpcopy,u20024804/tcpcopy,caosiyang/tcpcopy,left2right/tcpcopy,sshling/tcpcopy,daodaoliang/tcpcopy,wfxiang08/tcpcopy,daodaoliang/tcpcopy,u20024804/tcpcopy,liviusATnetskope/second,sshling/tcpcopy,ycaihua/tcpcopy