TCP 协议理解

参考文档：http://tools.ietf.org/html/rfc793

3.1. Header Format

Control Bits: 6 bits (from left to right):

URG: Urgent Pointer field significant

ACK: Acknowledgment field significant

PSH: Push Function

RST: Reset the connection

SYN: Synchronize sequence numbers

FIN: No more data from sender

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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                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
用wireshark抓包
Flags: 0x10 (ACK) ==16，2的4次方，第五位
Flags: 0x18 (PSH, ACK) == 16+8,第五位，第4位
Flags: 0x12 (SYN, ACK)==16+2，第五位，第二位

3.2. Terminology

TCB: Transmission Control Block,some variables being stored in a connection record.

Send Sequence Variables

SND.UNA - send unacknowledged

SND.NXT - send next

SND.WND - send window

SND.UP - send urgent pointer

SND.WL1 - segment sequence number used for last window update

SND.WL2 - segment acknowledgment number used for last window

update

ISS - initial send sequence number

Receive Sequence Variables

RCV.NXT - receive next

RCV.WND - receive window

RCV.UP - receive urgent pointer

IRS - initial receive sequence number

Current Segment Variables

SEG.SEQ - segment sequence number

SEG.ACK - segment acknowledgment number

SEG.LEN - segment length

SEG.WND - segment window

SEG.UP - segment urgent pointer

SEG.PRC - segment precedence value

9+1 states during its lifetime:

LISTEN, SYN-SENT, SYN-RECEIVED,ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,TIME-WAIT, and the fictional state CLOSED.

3.3. Sequence Numbers

SND.UNA < SEG.ACK =< SND.NXT

RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

and so on...

3.4. Establishing a connection

1, the data must be buffered at the receiver until the connection reaches the ESTABLISHED state

TCP A TCP B

1. CLOSED LISTEN

2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED

3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED

5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED

server每收到一个TCP数据包，都要回送一个ACK，接受失败的会[rst,ack]

client发送数据时，也要发送<PSH><ACK>（PSH的为data,ACK是上一次的）

2, Simultaneous initiation is only slightly more complex，双方都进行init connection，这种情况稍微复杂些。

3, Half-Open Connections

4, re-Open Connections

3.5. Closing a Connection

The user who CLOSEs may continue to RECEIVE until he is told that the other side has CLOSED also

TCP A TCP B

1. ESTABLISHED ESTABLISHED

2. (Close)

FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT

3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT

4. (Close)

TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK

5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED

6. (2 MSL)

CLOSED

三次握手，c->s[SYN],s->c[SYN,ACK],c->s[ACK]

四次分手，c->s[FIN],s->c[ACK],s->c[FIN],c->s[ACK]，因为server接收到fin时，还可以把未发送完的数据发送完，所以拆开了[FIN],[ACK]

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