[Ethernet] 如何理解tcp协议三次握手 [复制链接]

shenjq

军衔等级：

上士

发表于 2007-3-30 16:43:00 |显示全部楼层

Tcp协议的三次握手如果能深刻理解,并举一反三的话,那么对于帮助学习tcp/ip协议栈有很大好处.下面来说说三次握手,tcp/ip详解卷1第18章中提到三次握手的概念: 1) 请求端（通常称为客户）发送一个S Y N段指明客户打算连接的服务器的端口，以及初始序号（I S N）。这个S Y N段为报文段1。2) 服务器发回包含服务器的初始序号的S Y N报文段（报文段2）作为应答。同时，将确认序号设置为客户的I S N加1以对客户的S Y N报文段进行确认。一个S Y N将占用一个序号。3) 客户必须将确认序号设置为服务器的I S N加1以对服务器的S Y N报文段进行确认（报文段3）。这三个报文段完成连接的建立。这个过程也称为三次握（three-wayhandshake）。根据这个定义,大家有以下几个问题需要明确:1,报文段1如何确定客户端初始序号ISN, 同理报文段2如何确定服务器端初始序号ISN?2,tcp通过三次握手机制保证发送和接受的同步,保证数据的可靠传送.3,和可靠性相关的还有以下三个概念很重要:未连接队列：在三次握手协议中，服务器维护一个未连接队列，该队列为每个客户端的SYN包开设一个条目，该条目表明服务器已收到SYN包，并向客户发出确认，正在等待客户的确认包。这些条目所标识的连接在服务器处于Syn_RECV状态，当服务器收到客户的确认包时，删除该条目，服务器进入ESTABLISHED状态。Backlog参数：表示未连接队列的最大容纳数目。
     SYN-ACK 重传次数 服务器发送完SYN－ACK包，如果未收到客户确认包，服务器进行首次重传，等待一段时间仍未收到客户确认包，进行第二次重传，如果重传次数超过系统规定的最大重传次数，系统将该连接信息从半连接队列中删除。注意，每次重传等待的时间不一定相同。
     半连接存活时间：是指半连接队列的条目存活的最长时间，也即服务从收到SYN包到确认这个报文无效的最长时间，该时间值是所有重传请求包的最长等待时间总和。有时大家也称半连接存活时间为Timeout时间、SYN_RECV存活时间。附:TCP首部0                                16                            32<table class="MsoNormalTable" cellspacing="0" cellpadding="0" width="528" border="1" style="BORDER-RIGHT: medium none; BORDER-TOP: medium none; MARGIN: auto auto auto 32.4pt; BORDER-LEFT: medium none; WIDTH: 396pt; BORDER-BOTTOM: medium none; BORDER-COLLAPSE: collapse; mso-border-alt: solid windowtext .5pt; mso-padding-alt: 0cm 5.4pt 0cm 5.4pt; mso-border-insideh: .5pt solid windowtext; mso-border-insidev: .5pt solid windowtext;"><tbody><tr style="HEIGHT: 21pt; mso-yfti-irow: 0; mso-yfti-firstrow: yes;"><td width="264" colspan="3" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: windowtext 1pt solid; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 198pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt;">16位源端口号</td><td width="264" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: windowtext 1pt solid; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; WIDTH: 198pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt;">16目的端口号</td></tr><tr style="HEIGHT: 21pt; mso-yfti-irow: 1;"><td width="528" colspan="4" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 396pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">32位序号</td></tr><tr style="HEIGHT: 21pt; mso-yfti-irow: 2;"><td width="528" colspan="4" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 396pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">32位确认序号</td></tr><tr style="HEIGHT: 21pt; mso-yfti-irow: 3;"><td width="66" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 49.5pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">4位首部长度</td><td width="99" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; WIDTH: 74.25pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">保留(6位)</td><td width="99" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; WIDTH: 74.25pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">6位标志bit</td><td width="264" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; WIDTH: 198pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">16位窗口大小</td></tr><tr style="HEIGHT: 21pt; mso-yfti-irow: 4;"><td width="264" colspan="3" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 198pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">16位检验和</td><td width="264" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: #ece9d8; WIDTH: 198pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">16位紧急指针</td></tr><tr style="HEIGHT: 21pt; mso-yfti-irow: 5;"><td width="528" colspan="4" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 396pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 21pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">选项</td></tr><tr style="HEIGHT: 42.5pt; mso-yfti-irow: 6; mso-yfti-lastrow: yes;"><td width="528" colspan="4" style="BORDER-RIGHT: windowtext 1pt solid; PADDING-RIGHT: 5.4pt; BORDER-TOP: #ece9d8; PADDING-LEFT: 5.4pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext 1pt solid; WIDTH: 396pt; PADDING-TOP: 0cm; BORDER-BOTTOM: windowtext 1pt solid; HEIGHT: 42.5pt; BACKGROUND-COLOR: transparent; mso-border-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt;">数据</td></tr></tbody></table>