wikipedia Transmission Control Protocol

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP.

TCP provides reliable, ordered, and error-checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network.

TCP is connection-oriented, and a connection between client and server is established before data can be sent. The server must be listening (passive(被动) open) for connection requests from clients before a connection is established. Three-way handshake (active(主动) open), retransmission, and error-detection adds to reliability but lengthens latency.

NOTE: 上面描述了TCP的特性

Major internet applications such as the World Wide Web, email, remote administration, and file transfer rely on TCP.

However, there are vulnerabilities(缺陷) to TCP including denial of service, connection hijacking, TCP veto, and reset attack.

Network function

NOTE: 本节讨论的是TCP的功能

The Transmission Control Protocol provides a communication service at an intermediate level between an application program and the Internet Protocol(即Internet Layer). It provides host-to-host connectivity at the transport layer of the Internet model.

NOTE: 上面这段话需要结合Network\Theory\Network-protocol-model.md中的"Internet protocol suite by layer"章节的内容来进行理解

Abstraction

NOTE: 本节标题的Abstraction有如下两个含义:

1) 底层对传输细节进行了封装，提供给上层abstraction

2) TCP network connection is an abstraction

An application does not need to know the particular mechanisms for sending data via a link to another host, such as the required IP fragmentation(分片) to accommodate the maximum transmission unit of the transmission medium. At the transport layer, TCP handles all handshaking and transmission details and presents an abstraction of the network connection to the application typically through a network socket interface.

NOTE: 上面这段话的最后一句描述了TCP connecting的本质，可以认为TCP network connection is an abstraction，关于TCP network connecting的本质，在./TCP-connection章节会进行说明。

At the lower levels of the protocol stack, due to network congestion, traffic load balancing, or unpredictable network behaviour, IP packets may be lost, duplicated, or delivered out of order. TCP detects these problems, requests re-transmission of lost data, rearranges out-of-order data and even helps minimize network congestion to reduce the occurrence of the other problems. If the data still remains undelivered, the source is notified of this failure. Once the TCP receiver has reassembled the sequence of octets originally transmitted, it passes them to the receiving application. Thus, TCP abstracts the application's communication from the underlying networking details.

Positive acknowledgement with re-transmission

NOTE: TCP采用的"The sender re-transmits a packet if the timer expires before receiving the acknowledgement"。

TCP is a reliable stream delivery service which guarantees that all bytes received will be identical and in the same order as those sent. Since packet transfer by many networks is not reliable, TCP achieves this using a technique known as positive acknowledgement with re-transmission. This requires the receiver to respond with an acknowledgement message as it receives the data. The sender keeps a record of each packet it sends and maintains a timer from when the packet was sent. The sender re-transmits a packet if the timer expires before receiving the acknowledgement. The timer is needed in case a packet gets lost or corrupted.[6]

TCP segment structure

Transmission Control Protocol accepts data from a data stream, divides it into chunks, and adds a TCP header creating a TCP segment. The TCP segment is then encapsulated into an Internet Protocol (IP) datagram, and exchanged with peers.[4]

NOTE: 关于TCP segment、IP datagram，参见Network\Theory\Network-protocol-model.md的“Unit of layer”章节。

A TCP segment consists of a segment header and a data section. The TCP header contains 10 mandatory fields, and an optional extension field (Options, pink background in table).

The data section follows the header. Its contents are the payload data carried for the application. The length of the data section is not specified in the TCP segment header. It can be calculated by subtracting the combined length of the TCP header and the encapsulating IP header from the total IP datagram length (specified in the IP header).

NOTE: 在IP header中指定了Total IP datagram length，所以:

Length of the TCP segment data section = Total IP datagram length - Length of the TCP header - Length of the IP header

NOTE: 下面逐行介绍各字段

Row1

field	explanation
Source port (16 bits)	Identifies the sending port.
Destination port (16 bits)	Identifies the receiving port.

NOTE: 需要注意的是在TCP协议的header中仅有port而无IP；IP是在IP协议中需要的；

Row2

Sequence number (32 bits)

Has a dual role:

1) If the SYN flag is set (1), then this is the initial sequence number. The sequence number of the actual first data byte and the acknowledged number in the corresponding ACK are then this sequence number plus 1.

2) If the SYN flag is clear (0), then this is the accumulated sequence number of the first data byte of this segment for the current session.

NOTE: SYN flag is clear (0)表示是在*data transfer phase* ？

Row3

Acknowledgment number (32 bits)

If the ACK flag is set then the value of this field is the next sequence number that the sender of the ACK is expecting. This acknowledges receipt（收到） of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.

NOTE: Sequence number 和 Acknowledgment number是TCP实现在”4.4 Data transfer“中介绍的“Reliable transmission” 特性的关键。

关于sequence number和acknowledgment number，在Network\Theory\TCP\TCP-SEQ-number-and-ACK-number.md中会进行专门介绍。

Row4

Data offset (4 bits)

Specifies the size of the TCP header in 32-bit words (4 byte). The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.

Reserved (3 bits)

For future use and should be set to zero.

Flags (9 bits) (aka Control bits)

Contains 9 1-bit flags

NS ECN-nonce - concealment（隐藏） protection (experimental: see RFC 3540).

CWR Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism (added to header by RFC 3168).

ECE ECN-Echo has a dual role, depending on the value of the SYN flag. It indicates:

If the SYN flag is set (1), that the TCP peer is ECN capable.
If the SYN flag is clear (0), that a packet with Congestion Experienced flag set (ECN=11) in the IP header was received during normal transmission (added to header by RFC 3168). This serves as an indication of network congestion (or impending congestion) to the TCP sender.

URG indicates that the Urgent pointer field is significant

ACK indicates that the Acknowledgment field is significant. All packets after the initial SYN packet sent by the client should have this flag set.

PSH Push function. Asks to push the buffered data to the receiving application.

RST Reset the connection

SYN Synchronize sequence numbers. Only the first packet sent from each end should have this flag set. Some other flags and fields change meaning based on this flag, and some are only valid when it is set, and others when it is clear.

NOTE: 标识开始传输

FIN Last packet from sender.

NOTE: 标识完成了传输

NOTE: 在packetlife Understanding TCP Sequence and Acknowledgment Numbers 中对重要字段的解释如下:

SYN - (Synchronize) Initiates a connection

FIN - (Final) Cleanly terminates a connection

ACK - Acknowledges received data

Window size (16 bits)

The size of the receive window, which specifies the number of window size units (by default, bytes) (beyond the segment identified by the sequence number in the acknowledgment field) that the sender of this segment is currently willing to receive (see Flow control and Window Scaling).

Row5

Checksum (16 bits)

The 16-bit checksum field is used for error-checking of the header, the Payload and a Pseudo-Header. The Pseudo-Header consists of the Source IP Address, the Destination IP Address, the protocol number for the TCP-Protocol (0x0006) and the length of the TCP-Headers including Payload (in Bytes).

Urgent pointer (16 bits)

if the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte.

Row6~Row20

Options (Variable 0–320 bits, divisible by 32)

Padding

The TCP header padding is used to ensure that the TCP header ends, and data begins, on a 32 bit boundary. The padding is composed of zeros.[12]

4 Protocol operation

TCP protocol operations may be divided into three phases.

phase	explanation
connection establishment	Connections must be properly established in a multi-step handshake process (connection establishment) before entering the data transfer phase.
data transfer
connection termination	After data transmission is completed, the connection termination closes(关闭) established virtual circuits and releases all allocated resources.

NOTE: 本节基于此来对TCP protocol进行描述、组织内容。

A TCP connection is managed by an operating system through a programming interface that represents the local end-point for communications, the Internet socket. During the lifetime of a TCP connection the local end-point undergoes a series of state changes:[13]

LISTEN

(server) represents waiting for a connection request from any remote TCP and port.

SYN-SENT

(client) represents waiting for a matching connection request after having sent a connection request.

SYN-RECEIVED

(server) represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.

ESTABLISHED

(both server and client) 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

(both server and client) represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.

FIN-WAIT-2

(both server and client) represents waiting for a connection termination request from the remote TCP.

CLOSE-WAIT

(both server and client) represents waiting for a connection termination request from the local user.

CLOSING

(both server and client) represents waiting for a connection termination request acknowledgment from the remote TCP.

LAST-ACK

(both server and client) 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

(either server or client) represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. [According to RFC 793 a connection can stay in TIME-WAIT for a maximum of four minutes known as two MSL (maximum segment lifetime).]

NOTE: 第一句话的含义是：本端**等待足够的时间以确保remote TCP(即通信的**对端)接收到了由**本端**发送给对端的acknowledgment（这个acknowledgment是对对端发送过来的connection termination request的ACK)

在文章Network\Theory\TCP\TCP-connection-termination.md 的 “2MSL wait” 段中对MSL、TIME-WAIT进行了说明。

CLOSED

(both server and client) represents no connection state at all.

4.1 Connection establishment

To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:

1) SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment's sequence number to a random value A.

2) SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.

NOTE: SYN-ACK其实可以看做是同时发送SYN和ACK

3) ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A+1, and the acknowledgement number is set to one more than the received sequence number i.e. B+1.

NOTE: 为什么是plus 1？在Network\Theory\TCP\TCP-SEQ-number-and-ACK-number.md的cnblogs TCP 中的Sequence Number中对这个问题进行解释

At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.

NOTE: full-duplex，每个direction都有对应的connection parameter: sequence number。

NOTE: 在文章packetlife Understanding TCP Sequence and Acknowledgment Numbers 中展示了如何使用wireshark来观察connection establishment的过程，这篇文章收录在Network\Theory\TCP\TCP-SEQ-number-and-ACK-number.md中。

4.2 Connection termination

NOTE: 在文章Network\Theory\TCP\TCP-connection-termination.md中对connection termination进行了补充，其中解释本段中很多没有说明清楚的问题:

RST、half-duplex close sequence

half-open、half-close

2MSL wait

The connection termination phase uses a four-way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges with an ACK. Therefore, a typical tear-down requires a pair of FIN and ACK segments from each TCP endpoint( four-way handshake). After the side that(引导定语从句) sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this prevents confusion due to delayed packets being delivered during subsequent connections.

NOTE: 最后一段话的意思是：在这段时间内，这个port是不能够用于新的connection的，这样做的原因是：如果允许，可能导致delayed packet被后续在这个port上的connection进行传输；

NOTE: why connection establishment使用three-way handshake，而connection termination使用four-wary handshake？我想这其中的差别在于传输buffer中的数据；

A connection can be "half-open", in which case one side has terminated its end, but the other has not. The side that has terminated can no longer send any data into the connection, but the other side can. The terminating side should continue reading the data until the other side terminates as well.

3-way handshake termination

It is also possible to terminate the connection by a 3-way handshake, when host A sends a FIN and host B replies with a FIN & ACK (merely combines 2 steps into one) and host A replies with an ACK.[14]

half-duplex close sequence

Some host TCP stacks may implement a half-duplex close sequence, as Linux or HP-UX do. If such a host actively closes a connection but still has not read all the incoming data the stack already received from the link, this host sends a RST instead of a FIN (Section 4.2.2.13 in RFC 1122). This allows a TCP application to be sure the remote application has read all the data the former sent—waiting the FIN from the remote side, when it actively closes the connection. But the remote TCP stack cannot distinguish between a Connection Aborting RST and Data Loss RST. Both cause the remote stack to lose all the data received.

NOTE:

一、"If such a host actively closes a connection but still has not read all the incoming data the stack already received from the link, this host sends a RST instead of a FIN (Section 4.2.2.13 in RFC 1122)."

这段话的意思是: 如果没有将buffer中的data全部读完，那么在调用close的时候，发送的是RST而不是FIN

Some application protocols using the TCP open/close handshaking for the application protocol open/close handshaking may find the RST problem on active close. As an example:

s = connect(remote);
send(s, data);
close(s);

For a program flow like above, a TCP/IP stack like that described above does not guarantee that all the data arrives to the other application if unread data has arrived at this end.

4.3 Resource usage

NOTE: port是一种resource

4.4 Data transfer

NOTE: TCP数据传输的核心特性

The Transmission Control Protocol differs in several key features from the User Datagram Protocol:

features	注解
Ordered data transfer	在下面的“Reliable transmission”会进行详细介绍
Retransmission of lost packets	简单来说就是“补漏”
Error-free data transfer	无错传输
Flow control	流控
Congestion control	拥塞控制

NOTE: 下面将上述Ordered data transfer、Retransmission of lost packets、Error-free data transfer统称为Reliable transmission。

Reliable transmission

NOTE: TCP是否会等收到ACK后，才传下一个segment？

Reliability is achieved by the sender detecting lost data and retransmitting it. TCP uses two primary techniques to identify loss: