TCP Segment and Connection on Linux

I have a Java program using Thrift that open() and close() to remote server.

Before program runs:
[ricky@pc009 ~]$ netstat -natu
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 127.0.0.1:3350          0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:6010            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:6011            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:3389            0.0.0.0:*               LISTEN
tcp        0      0 10.1.10.112:22          10.1.10.111:62012       ESTABLISHED
tcp        0      0 10.1.10.112:22          10.1.10.111:62060       ESTABLISHED
tcp        0      0 10.1.10.112:22          10.1.10.111:62013       ESTABLISHED
tcp6       0      0 :::22                   :::*                    LISTEN
tcp6       0      0 ::1:631                 :::*                    LISTEN
tcp6       0      0 ::1:25                  :::*                    LISTEN
tcp6       0      0 :::6010                 :::*                    LISTEN
tcp6       0      0 :::6011                 :::*                    LISTEN
udp        0      0 0.0.0.0:55270           0.0.0.0:*
udp        0      0 0.0.0.0:68              0.0.0.0:*
udp        0      0 127.0.0.1:323           0.0.0.0:*
udp        0      0 0.0.0.0:49957           0.0.0.0:*
udp        0      0 0.0.0.0:5353            0.0.0.0:*
udp6       0      0 ::1:323                 :::*
udp6       0      0 :::51781                :::*

After program runs:

tcp6       0      0 10.1.10.112:57094       27.111.195.209:7039     TIME_WAIT

tcp6       0      0 10.1.10.112:56920       27.111.195.209:7039     TIME_WAIT

tcp6       0      0 10.1.10.112:56746       27.111.195.209:7039     TIME_WAIT

tcp6       0      0 10.1.10.112:57003       27.111.195.209:7039     TIME_WAIT

The number of TCP connections increases at around 100 connections per second, and it becomes stable at 7000.

[ricky@pc009 ~]$ date && netstat -natu | wc -l

Thu Feb  9 15:40:26 HKT 2017

24

[ricky@pc009 ~]$ date && netstat -natu | wc -l

Thu Feb  9 15:40:31 HKT 2017

190

[ricky@pc009 ~]$ date && netstat -natu | wc -l

Thu Feb  9 15:40:32 HKT 2017

297

[ricky@pc009 ~]$ date && netstat -natu | wc -l

Thu Feb  9 15:41:02 HKT 2017

2179

[ricky@pc009 ~]$ date && netstat -natu | wc -l
Thu Feb  9 15:43:38 HKT 2017
6414
[ricky@pc009 ~]$ date && netstat -natu | wc -l
Thu Feb  9 15:47:39 HKT 2017
6821

What is the default ipv4 params in Centos 7.2? [ricky@pc009 ~]$ sysctl net.ipv4.ip_local_port_range
net.ipv4.ip_local_port_range = 32768    61000
[ricky@pc009 ~]$ sysctl net.ipv4.tcp_fin_timeout
net.ipv4.tcp_fin_timeout = 60

Max connection is (61000 - 32768) / 60 = 470 socket connections per second.

What is the TCP segment header?
20 Bytes, or 5 32-bit words, or 5 4-byte words, or 5 words.
Source port has 2 bytes max to 65535.
Destination port has 2 bytes max to 65535.
Sequence Number 4 bytes max 4,294,967,295
Acknowledgement Number 4 bytes max 4,294,967,295
Data offset has 4 bits, min 5 for 20 bytes header, max 15 for 60 bytes header
Reserved 3 bits
Flags 9 bits, including NS, CWR, ECE, URG urgent pointer, ACK acknowledge, PSH push, RST reset connection, SYN synchronize sequence number, FIN finish with last package.
Window Size 2 bytes max 65535, receiving window size
Checksum 2 bytes max 65535 for both header and data
Urgent Pointer 2 bytes max 65535

What is TCP 3-way handshake?
1st: Machine A sends a packet with (SYN=1, SN=123456) so that Machine B knows it is a request of creating a session by reading SYN=1, and it enters SYN_SEND state.
2nd: B sends a packet with (SYN=1, ACK=1, SN=666666, AN=123457) to A indicating that it has confirmed the request of connection by (SYN=1, ACK=1), and it enters SYN_RECV.
3rd: A reads the 2nd package and verify that AN is corrects as the expecting 123457 and ACK is set to 1. If so, A replies B a confirmation by sending a packet with (ACK=1, AN=666667). A enters ESTABLISHED after packet sent. B enters ESTABLISHED after packet received.

Or in short it exchanges the sequence numbers in both direction

1. A SYN(SN=1111), SYN_SEND
2. B SYNACK(AN=1112,SN=5555), SYN_RECV
3. A ACK(AN=5556), ESTABLISHED