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client wait 30s befor send SYN to ftp server on the passive port

hi, i have runed in a ftp problem, ask for help. ftp client: Oracle Database 11.2.0.3, IBM AIX 6100-07-03 OS on IBM Power7 ftp server: vsFTPd v2.2.2 , CentOS 6.6 x64 on x86 server

i run a Oracle PL/SQL procedure for transfer the BLOB file from database to the ftp server that at same subnet network. the PL/SQL procedure is copy from https://oracle-base.com/articles/misc/ftp-from-plsql. when the PL/SQL procedure is running, client wait 30s to establish ftp passive connection EVERY TIME !

i has trace the Oracle database when running the ftp procedure, found some 'TCP Socket (Kgas)' Waits. Oracle don't think the waits is from database but the network: https://docs.oracle.com/database/121/REFRN/GUID-203ACA60-9912-4493-9B79-EA4CDE89A78D.htm#REFRN00642

i has check the parameter of network on ftp client and server. rfc1323 or net.ipv4.tcp_timestamps is enabled on both. ftp client and server are connect to the same network switch(H3C S7510 core switch).

the tcpdump file from ftp client and server at the same time is here: https://c-t.work/s/84534f77f2b947

thanks.