简介
TCP协议是传输层重要的协议，TCP是面向连接、字节流和提供可靠传输。要使用TCP连接的双方必须先建立连接，然后才能开始数据的读写。TCP是全双工的，所以双发的内核都需要一定的资源保存TCP连接的状态和连接上的数据。在完成数据交换之后，通信双方都必须断开连接已释放系统资源。
头部结构
头部结构字段：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |U|A|P|R|S|F| |
| Offset| Reserved |R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Source Port: 16 bits
- Destination Port: 16 bits
- Sequence Number: 32 bits
- Acknowledgment Number: 32 bits
- Data Offset: 4 bits
- Reserved: 6 bits
- Control Bits: 6 bits
- Window: 16 bits
- Checksum: 16 bits
- Urgent Pointer: 16 bits
- Options: variable
- Padding: variable
Sequence Number
Acknowledgment Number
Data Offset
Control Bits
Window
Checksum
检验和是填充一个伪包头加上TCP报文求和算出。
伪包头结构字段：
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+- Source Address: 32 bits
- Destination Address: 32 bits
- zero: 8 bits
- PTCL: 8 bits
- TCP Length: 16 bits
Urgent Pointer
Options
当前定义的选项字段
| Kind | Length | Meaning |
|---|---|---|
| 0 | - | End of option list. |
| 1 | - | No-Operation. |
| 2 | 4 | Maximum Segment Size. |
连接
TCP 连接状态机
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+各个状态意义
- LISTEN - represents waiting for a connection request from any remote
TCP and port. - SYN-SENT - represents waiting for a matching connection request
after having sent a connection request. - SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request. - ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer phase
of the connection. - FIN-WAIT-1 - represents waiting for a connection termination request
from the remote TCP, or an acknowledgment of the connection
termination request previously sent. - FIN-WAIT-2 - represents waiting for a connection termination request
from the remote TCP. - CLOSE-WAIT - represents waiting for a connection termination request
from the local user. - CLOSING - represents waiting for a connection termination request
acknowledgment from the remote TCP. - LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP
(which includes an acknowledgment of its connection termination
request). - TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request. - CLOSED - represents no connection state at all.
