Unit 1: The Internet and IP

image.png image.png
image.png image.png
image.png

You perform a traceroute from a computer in San Francisco to a computer located in New York and get the following, fictional output:

1 a.example.com (208.64.252.229) 0.363 ms 0.427 ms 0.458 ms

2 b.example.com (208.64.248.17) 0.433 ms 0.439 ms 0.491 ms

3 c.example.com (171.64.255.144) 90.742 ms 90.641 ms 90.683 ms

4 newyork.example.com (171.67.76.65) 91.535 ms 91.509 ms 91.556 ms

Your friend, Kristen, reads the output and says that the (geographic) distance between b.example.com and c.example.com must be very large because there was a long delay between b and c.

You perform the traceroute again, this time from a different source in San Francisco and to a different destination in New York, this time getting the following output:

1 d.example.com (108.64.252.229) 0.632 ms 0.627 ms 0.658 ms

2 e.example.com (108.64.248.17) 0.743 ms 0.739 ms 0.791 ms

3 f.example.com (71.64.255.144) 92.742 ms 92.641 ms 92.683 ms

4 g.example.com (71.67.76.65) 193.335 ms 193.109 ms 193.001 ms

5 newyork2.example.com (71.67.76.65) 195.201 ms 195.109 ms 195.813 ms

Reading this output, your friend, Ewen, says that servers f.example.com and g.example.com must also be geographically distant.

Are Kristen and/or Ewen right?

[图片上传失败...(image-686917-1520684413267)]

TCP的三次握手是怎么进行的了：发送端发送一个SYN=1，ACK=0标志的数据包给接收端，请求进行连接，这是第一次握手；接收端收到请求并且允许连接的话，就会发送一个SYN=1，ACK=1标志的数据包给发送端，告诉它，可以通讯了，并且让发送端发送一个确认数据包，这是第二次握手；最后，发送端发送一个SYN=0，ACK=1的数据包给接收端，告诉它连接已被确认，这就是第三次握手。之后，一个TCP连接建立，开始通讯。

SYN是TCP建立连接时包内设置的标记，例如（C是客户机，S是服务器。）：

C: SYN

S: SYN, ACK

C: ACK

就建立了一条连接。

[图片上传失败...(image-f54ff-1520684413266)]

ACK是一般的响应标记，一般报文都会有。

FIN是TCP试图关闭连接时包内设置的标记，例如（C是客户机，S是服务器。）：

C: FIN, ACK

S: ACK

S: FIN, ACK

C: ACK

就关闭了一条连接。

[图片上传失败...(image-cebb12-1520684413266)]