009A-TCP基础

TCP基础
帧-类型
ip-协议
传输层-端口(应用)
TCP不能分片，ip允许分片
TCP中的MSS(不包含ip头部和tcp头部)最大段尺寸 ,TCP能承载的最大数据

MTU

MTU(1500巨型帧)，属于二层以上数据大小
只有IP允许分片，TCP不能分片，二层帧也不能分片

<Huawei>dis in g0/0/0
GigabitEthernet0/0/0 current state : UP
Line protocol current state : DOWN
Description:HUAWEI, AR Series, GigabitEthernet0/0/0 Interface
Route Port,The Maximum Transmit Unit is 1500          			    #MTU
Internet protocol processing : disabled
 ---- More ----

[R1-GigabitEthernet0/0/0]ping ?
  STRING<1-255>  IP address or hostname of a remote system
  -a             Select source IP address, the default is the IP address of the 
                 output interface
  -c             Specify the number of echo requests to be sent, the default is 
                 5
  -d             Specify the SO_DEBUG option on the socket being used
  -f             Set Don't Fragment flag in packet (IPv4-only)
  -s             Specify the number of data bytes to be sent, the default is    
                 56bytes
  ---- More ----                                          
[R1-GigabitEthernet0/0/0]ping -f -s 1500 202.100.1.2   #ping不允许分片，大小1500，-s指的是IP承载的数据大小(不包含IP的头部)
  PING 202.100.1.2: 1501  data bytes, press CTRL_C to break
    Request time out
    Request time out
    Request time out
    Request time out
    Request time out

  --- 202.100.1.2 ping statistics ---
    5 packet(s) transmitted
    0 packet(s) received
    100.00% packet loss

[R1-GigabitEthernet0/0/0]ping  -s 1500 202.100.1.2  #ping允许分片
  PING 202.100.1.2: 1500  data bytes, press CTRL_C to break
    Reply from 202.100.1.2: bytes=1500 Sequence=1 ttl=255 time=100 ms
    Reply from 202.100.1.2: bytes=1500 Sequence=2 ttl=255 time=30 ms
    Reply from 202.100.1.2: bytes=1500 Sequence=3 ttl=255 time=30 ms
    Reply from 202.100.1.2: bytes=1500 Sequence=4 ttl=255 time=30 ms
    Reply from 202.100.1.2: bytes=1500 Sequence=5 ttl=255 time=40 ms

  --- 202.100.1.2 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 30/46/100 ms

在AR1的g0/0/0口抓取ping -s 1500 202.100.1.2的报文

ICMP消息封装格式，如下图

[R1-GigabitEthernet0/0/0]ping -f -s 1472 202.100.1.2
  PING 202.100.1.2: 1472  data bytes, press CTRL_C to break
    Reply from 202.100.1.2: bytes=1472 Sequence=1 ttl=255 time=20 ms
    Reply from 202.100.1.2: bytes=1472 Sequence=2 ttl=255 time=20 ms
    Reply from 202.100.1.2: bytes=1472 Sequence=3 ttl=255 time=10 ms
    Reply from 202.100.1.2: bytes=1472 Sequence=4 ttl=255 time=20 ms
    Reply from 202.100.1.2: bytes=1472 Sequence=5 ttl=255 time=20 ms

  --- 202.100.1.2 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 10/18/20 ms

MSS

head length=固定长度20+options的12=32

TCP

TCP头部

TCP的三次握手和4次挥手

• TCP通过三次握手建立可靠连接
  小写的ack是acknowledge number
  三次建立过程解释，以打电话比喻，张三(a)想和李四(b)打电话
  1、a问b能听到？
  2、b回答我可以听到你说话，你能听到我说话吗？
  3、a回答我可以听到
  双方互相确认可以听到，于是可以通信了

• 主机在关闭连接之前，要确认收到来自对方的ACK
  4次挥手
  1、a向b说，我要和你分手
  2、b回复说我知道了
  3、b经过一番考虑(b把自己未处理的事情处理后)，向a说，我也要和你分手，请你确认
  4、a回复说，我确认，分手吧