Hi, I’m Carrie Anne, and welcome to CrashCourse Computer Science!
(｡･∀･)ﾉﾞ嗨，我是 Carrie Anne \N 欢迎收看计算机科学速成课！

As we talked about last episode, your computer is connected to a large, distributed network,
上集讲到，你的计算机和一个巨大的分布式网络连在一起

called The Internet.
这个网络叫互联网

I know this because you’re watching a YouTube video,
你现在就在网上看视频呀

which is being streamed over that very internet.
你现在就在网上看视频呀

It’s arranged as an ever-enlarging web of interconnected devices.
互联网由无数互联设备组成，而且日益增多

For your computer to get this video,
计算机为了获取这个视频 \N 首先要连到局域网，也叫 LAN

the first connection is to your local area network, or LAN,
计算机为了获取这个视频 \N 首先要连到局域网，也叫 LAN

which might be every device in your house that’s connected to your wifi router.
你家 WIFI 路由器连着的所有设备，组成了局域网.

This then connects to a Wide Area Network, or WAN,
局域网再连到广域网，广域网也叫 WAN

which is likely to be a router run by your Internet Service Provider, or ISP，
WAN 的路由器一般属于你的"互联网服务提供商"，简称 ISP

companies like Comcast, AT&T or Verizon.
比如 Comcast，AT&T 和 Verizon 这样的公司

At first, this will be a regional router, like one for your neighborhood,
广域网里，先连到一个区域性路由器，这路由器可能覆盖一个街区。

and then that router connects to an even bigger WAN,
然后连到一个更大的 WAN，可能覆盖整个城市

maybe one for your whole city or town.
然后连到一个更大的 WAN，可能覆盖整个城市

There might be a couple more hops, but ultimately you’ll connect to the backbone of the internet
可能再跳几次，但最终会到达互联网主干

made up of gigantic routers with super high-bandwidth connections running between them.
互联网主干由一群超大型、带宽超高路由器组成

To request this video file from YouTube,
为了从 YouTube 获得这个视频，

a packet had to work its way up to the backbone,
数据包（packet）要先到互联网主干

travel along that for a bit, and then work its way back down to a YouTube server that had the file.
沿着主干到达有对应视频文件的 YouTube 服务器

That might be four hops up, two hops across the backbone,
数据包从你的计算机跳到 Youtube 服务器，可能要跳个10次，

and four hops down, for a total of ten hops.
先跳4次到互联网主干，2次穿过主干，\N主干出来可能再跳4次，然后到 Youtube 服务器

If you’re running Windows, Mac OS or Linux, you can see the route data takes to different
如果你在用 Windows, Mac OS 或 Linux系统，可以用 traceroute 来看跳了几次

places on the internet by using the traceroute program on your computer.
如果你在用 Windows, Mac OS 或 Linux系统，可以用 traceroute 来看跳了几次

Instructions in the Doobly Doo.
更多详情看视频描述（YouTube原视频下）

For us here at the Chad & Stacey Emigholz Studio in Indianapolis,
我们在"印第安纳波利斯"的 Chad&Stacy Emigholz 工作室，\N 访问加州的 DFTBA 服务器，

the route to the DFTBA server in California goes through 11 stops.
经历了11次中转

We start at 192.168.0.1 -- that's the IP address for my computer on our LAN.
从 192.168.0.1 出发，这是我的电脑在 局域网（LAN）里的 IP 地址

Then there’s the wifi router here at the studio,
然后到工作室的 WIFI 路由器

then a series of regional routers, then we get onto the backbone,
然后穿过一个个地区路由器，到达主干.

and then we start working back down to the computer hosting "DFTBA.com”,
然后从主干出来，又跳了几次，到达"DFTBA.com”的服务器

which has the IP address 104.24.109.186.
IP 地址是 104.24.109.186.

But how does a packet actually get there?
但数据包*到底*是怎么过去的？

What happens if a packet gets lost along the way?
如果传输时数据包被弄丢了，会发生什么？

If I type "DFTBA.com” into my web browser, how does it know the server’s address?
如果在浏览器里输 "DFTBA.com"，浏览器怎么知道服务器的地址多少？

These are our topics for today!
我们今天会讨论这些话题.

As we discussed last episode, the internet is a huge distributed network
上集说过，互联网是一个巨型分布式网络 \N 会把数据拆成一个个数据包来传输

that sends data around as little packets.
上集说过，互联网是一个巨型分布式网络 \N 会把数据拆成一个个数据包来传输

If your data is big enough, like an email attachment,
如果要发的数据很大，比如邮件附件 \N 数据会被拆成多个小数据包

it might get broken up into many packets.
如果要发的数据很大，比如邮件附件 \N 数据会被拆成多个小数据包

For example, this video stream is arriving to your computer right now
举例，你现在看的这个视频 \N 就是一个个到达你电脑的数据包

as a series of packets, and not one gigantic file.
而不是一整个大文件发过来

Internet packets have to conform to a standard called the Internet Protocol, or IP.
数据包（packet）想在互联网上传输 \N 要符合"互联网协议"的标准，简称 IP

It’s a lot like sending physical mail through the postal system
就像邮寄手写信一样，邮寄是有标准的\N 每封信需要一个地址，而且地址必须是独特的

– every letter needs a unique and legible address written on it,
就像邮寄手写信一样，邮寄是有标准的\N 每封信需要一个地址，而且地址必须是独特的

and there are limits to the size and weight of packages.
并且大小和重量是有限制的

Violate this, and your letter won’t get through.
违反这些规定，信件就无法送达.

IP packets are very similar.
IP 数据包也是如此

However, IP is a very low level protocol
因为 IP 是一个非常底层的协议

– there isn’t much more than a destination address in a packet’s header
数据包的头部（或者说前面）只有目标地址

which is the metadata that’s stored in front of the data payload.
头部存 "关于数据的数据" \N 也叫 元数据(metadata)

This means that a packet can show up at a computer, but the computer may not know
这意味着当数据包到达对方电脑 \N 对方不知道把包交给哪个程序，是交给 Skype 还是使命召唤？

which application to give the data to; Skype or Call of Duty.
这意味着当数据包到达对方电脑 \N 对方不知道把包交给哪个程序，是交给 Skype 还是使命召唤？

For this reason, more advanced protocols were developed that sit on top of IP.
因此需要在 IP 之上，开发更高级的协议.

One of the simplest and most common is the User Datagram Protocol, or UDP.
这些协议里 \N 最简单最常见的叫"用户数据报协议"，简称 UDP

UDP has its own header, which sits inside the data payload.
UDP 也有头部，这个头部位于数据前面

Inside of the UDP header is some useful, extra information.
头部里包含有用的信息

One of them is a port number.
信息之一是端口号

Every program wanting to access the internet will
每个想访问网络的程序 \N 都要向操作系统申请一个端口号.

ask its host computer’s Operating System to be given a unique port.
每个想访问网络的程序 \N 都要向操作系统申请一个端口号.

Like Skype might ask for port number 3478.
比如 Skype 会申请端口 3478

When a packet arrives to the computer, the Operating System
当一个数据包到达时 \N 接收方的操作系统会读 UDP 头部，读里面的端口号

will look inside the UDP header and read the port number.
当一个数据包到达时 \N 接收方的操作系统会读 UDP 头部，读里面的端口号

Then, if it sees, for example, 3478, it will give the packet to Skype.
如果看到端口号是 3478，就把数据包交给 Skype

So to review, IP gets the packet to the right computer,
总结：\NIP 负责把数据包送到正确的计算机 \N UDP 负责把数据包送到正确的程序

but UDP gets the packet to the right program running on that computer.
总结：\NIP 负责把数据包送到正确的计算机 \N UDP 负责把数据包送到正确的程序

UDP headers also include something called a checksum,
UDP 头部里还有"校验和"，用于检查数据是否正确

which allows the data to be verified for correctness.
UDP 头部里还有"校验和"，用于检查数据是否正确

As the name suggests, it does this by checking the sum of the data.
正如"校验和"这个名字所暗示的 \N 检查方式是把数据求和来对比

Here’s a simplified version of how this works.
以下是个简单例子

Let's imagine the raw data in our UDP packet is
假设 UDP 数据包里 \N 原始数据是 89 111 33 32 58 41

89 111 33 32 58 and 41.
假设 UDP 数据包里 \N 原始数据是 89 111 33 32 58 41

Before the packet is sent, the transmitting computer calculates the checksum
在发送数据包前 \N 电脑会把所有数据加在一起，算出"校验和"

by adding all the data together: 89 plus 111 plus 33 and so on.
89+111+33+... 以此类推

In our example, this adds up to a checksum of 364.
得到 364，这就是"校验和".

In UDP, the checksum value is stored in 16 bits.
UDP 中，\N"校验和"以 16 位形式存储 (就是16个0或1)

If the sum exceeds the maximum possible value, the upper-most bits overflw,
如果算出来的和，超过了 16 位能表示的最大值 \N 高位数会被扔掉，保留低位

and only the lower bits are used.
如果算出来的和，超过了 16 位能表示的最大值 \N 高位数会被扔掉，保留低位

Now, when the receiving computer gets this packet,
当接收方电脑收到这个数据包

it repeats the process, adding up all the data.
它会重复这个步骤 \N 把所有数据加在一起，89+111+33... 以此类推

89 plus 111 plus 33 and so on.
它会重复这个步骤 \N 把所有数据加在一起，89+111+33... 以此类推

If that sum is the same as the checksum sent in the header, all is well.
如果结果和头部中的校验和一致 \N 代表一切正常

But, if the numbers don’t match, you know that the data got corrupted
如果不一致，数据肯定坏掉了

at some point in transit, maybe because of a power fluctuation or faulty cable.
也许传输时碰到了功率波动，或电缆出故障了

Unfortunately, UDP doesn’t offer any mechanisms to fix the data, or request a new copy
不幸的是，UDP 不提供数据修复或数据重发的机制

receiving programs are alerted to the corruption, but typically just discard the packet.
接收方知道数据损坏后，一般只是扔掉.

Also, UDP provides no mechanisms to know if packets are getting through
而且，UDP 无法得知数据包是否到达.

a sending computer shoots the UDP packet off,
发送方发了之后，无法知道数据包是否到达目的地

but has no confirmation it ever gets to its destination successfully.
发送方发了之后，无法知道数据包是否到达目的地

Both of these properties sound pretty catastrophic, but some applications are ok with this,
这些特性听起来很糟糕，但是有些程序不在意这些问题

because UDP is also really simple and fast.
因为 UDP 又简单又快.

Skype, for example, which uses UDP for video chat, can handle corrupt or missing packets.
拿 Skype 举例 \N 它用 UDP 来做视频通话，能处理坏数据或缺失数据

That’s why sometimes if you’re on a bad internet connection,
所以网速慢的时候 Skype 卡卡的 \N 因为只有一部分数据包到了你的电脑

Skype gets all glitchy – only some of the UDP packets are making it to your computer.
所以网速慢的时候 Skype 卡卡的 \N 因为只有一部分数据包到了你的电脑

But this approach doesn’t work for many other types of data transmission.
但对于其他一些数据，这个方法不适用.

Like, it doesn’t really work if you send an email, and it shows up with the middle missing.
比如发邮件，\N 邮件不能只有开头和结尾 没有中间.

The whole message really needs to get there correctly!
邮件要完整到达收件方

When it "absolutely, positively needs to get there”,
如果"所有数据必须到达" \N 就用"传输控制协议"，简称 TCP

programs use the Transmission Control Protocol, or TCP,
如果"所有数据必须到达" \N 就用"传输控制协议"，简称 TCP

which like UDP, rides inside the data payload of IP packets.
TCP 和 UDP 一样，头部也在存数据前面

For this reason, people refer to this combination of protocols as TCP/IP.
因此，人们叫这个组合 TCP/IP

Like UDP, the TCP header contains a destination port and checksum.
就像 UDP ，TCP 头部也有"端口号"和"校验和"

But, it also contains fancier features, and we’ll focus on the key ones.
但 TCP 有更高级的功能，我们这里只介绍重要的几个

First off, TCP packets are given sequential numbers.
1. TCP 数据包有序号

So packet 15 is followed by packet 16, which is followed by 17, and so on...
15号之后是16号，16号之后是17号，以此类推 \N 发上百万个数据包也是有可能的.

for potentially millions of packets sent during that session.
15号之后是16号，16号之后是17号，以此类推 \N 发上百万个数据包也是有可能的.

These sequence numbers allow a receiving computer to put the packets into the correct order,
序号使接收方可以把数据包排成正确顺序，即使到达时间不同.

even if they arrive at different times across the network.
序号使接收方可以把数据包排成正确顺序，即使到达时间不同.

So if an email comes in all scrambled, the TCP implementation in your computer’s operating
哪怕到达顺序是乱的，TCP 协议也能把顺序排对

system will piece it all together correctly.
哪怕到达顺序是乱的，TCP 协议也能把顺序排对

Second, TCP requires that once a computer has correctly received a packet
2. TCP 要求接收方的电脑收到数据包 \N 并且"校验和"检查无误后（数据没有损坏）\N 给发送方发一个确认码，代表收到了

– and the data passes the checksum – that it send back an acknowledgement,
2. TCP 要求接收方的电脑收到数据包 \N 并且"校验和"检查无误后（数据没有损坏）\N 给发送方发一个确认码，代表收到了

or "ACK” as the cool kids say, to the sending computer.
"确认码" 简称 ACK \N 得知上一个数据包成功抵达后，发送方会发下一个数据包

Knowing the packet made it successfully, the sender can now transmit the next packet.
"确认码" 简称 ACK \N 得知上一个数据包成功抵达后，发送方会发下一个数据包

But this time, let’s say, it waits, and doesn’t get an acknowledgement packet back.
假设这次发出去之后，没收到确认码 \N 那么肯定哪里错了

Something must be wrong. If enough time elapses,
如果过了一定时间还没收到确认码 \N 发送方会再发一次

the sender will go ahead and just retransmit the same packet.
如果过了一定时间还没收到确认码 \N 发送方会再发一次

It’s worth noting here  that the original packet might have actually gotten there,
注意 数据包可能的确到了

but the acknowledgment is just really delayed.
只是确认码延误了很久，或传输中丢失了

Or perhaps it was the acknowledgment that was lost.
只是确认码延误了很久，或传输中丢失了

Either way, it doesn’t matter, because the receiver has those sequence numbers,
但这不碍事 因为收件方有序列号

and if a duplicate packet arrives, it can be discarded.
如果收到重复的数据包就删掉

Also, TCP isn’t limited to a back and forth conversation – it can send many packets,
还有，TCP 不是只能一个包一个包发

and have many outstanding ACKs, which increases bandwidth significantly, since you aren’t
可以同时发多个数据包，收多个确认码 \N 这大大增加了效率，不用浪费时间等确认码

wasting time waiting for acknowledgment packets to return.
可以同时发多个数据包，收多个确认码 \N 这大大增加了效率，不用浪费时间等确认码

Interestingly, the success rate of ACKs, and also the round trip time
有趣的是，确认码的成功率和来回时间 \N 可以推测网络的拥堵程度

between sending and acknowledging, can be used to infer network congestion.
有趣的是，确认码的成功率和来回时间 \N 可以推测网络的拥堵程度

TCP uses this information to adjust how aggressively it sends packets –
TCP 用这个信息，调整同时发包数量，解决拥堵问题

a mechanism for congestion control.
TCP 用这个信息，调整同时发包数量，解决拥堵问题

So, basically, TCP can handle out-of-order packet delivery, dropped packets
简单说，TCP 可以处理乱序和丢失数据包，丢了就重发.

– including retransmission – and even throttle its transmission rate according to available bandwidth.
还可以根据拥挤情况自动调整传输率

Pretty awesome!
相当厉害！

You might wonder why anyone would use UDP when TCP has all those nifty features.
你可能会奇怪，既然 TCP 那么厉害，还有人用 UDP 吗？

The single biggest downside are all those acknowledgment packets
TCP 最大的缺点是 \N 那些"确认码"数据包把数量翻了一倍

– it doubles the number of messages on the network,
TCP 最大的缺点是 \N 那些"确认码"数据包把数量翻了一倍

and yet, you're not transmitting any more data.
但并没有传输更多信息

That overhead, including associated delays, is sometimes not worth the improved robustness,
有时候这种代价是不值得的 \N 特别是对时间要求很高的程序，比如在线射击游戏

especially for time-critical applications, like Multiplayer First Person Shooters.
有时候这种代价是不值得的 \N 特别是对时间要求很高的程序，比如在线射击游戏

And if it’s you getting lag-fragged you’ll definitely agree!
如果你玩游戏很卡，你也会觉得这样不值!

When your computer wants to make a connection to a website, you need two things
当计算机访问一个网站时 \N 需要两个东西：1.IP地址 2.端口号

- an IP address and a port.
当计算机访问一个网站时 \N 需要两个东西：1.IP地址 2.端口号

Like port 80, at 172.217.7.238.
例如 172.217.7.238 的 80 端口 \N 这是谷歌的 IP 地址和端口号

This example is the IP address and port for the Google web server.
例如 172.217.7.238 的 80 端口 \N 这是谷歌的 IP 地址和端口号

In fact, you can enter this into your browser’s address bar, like so,
事实上，你可以输到浏览器里，然后你会进入谷歌首页

and you’ll end up on the google homepage.
事实上，你可以输到浏览器里，然后你会进入谷歌首页

This gets you to the right destination,
有了这两个东西就能访问正确的网站 \N 但记一长串数字很讨厌

but remembering that long string of digits would be really annoying.
有了这两个东西就能访问正确的网站 \N 但记一长串数字很讨厌

It’s much easier to remember: google.com.
google.com 比一长串数字好记

So the internet has a special service that maps these domain names to addresses.
所以互联网有个特殊服务 \N 负责把域名和 IP 地址一一对应

It’s like the phone book for the internet.
就像专为互联网的电话簿 \N 它叫"域名系统"，简称 DNS

And it’s called the Domain Name System, or DNS for short.
就像专为互联网的电话簿 \N 它叫"域名系统"，简称 DNS

You can probably guess how it works.
它的运作原理你可能猜到了

When you type something like "youtube.com” into your web browser,
在浏览器里输 youtube.com \N 浏览器会去问 DNS 服务器，它的 IP 地址是多少

it goes and asks a DNS server – usually one provided by your ISP – to lookup the address.
一般 DNS 服务器 \N 是互联网供应商提供的

DNS consults its huge registry, and replies with the address... if one exists.
DNS 会查表，如果域名存在，就返回对应 IP 地址.

In fact, if you try mashing your keyboard, adding ".com”, and then hit enter in your
如果你乱敲键盘加个.com 然后按回车

browser, you’ll likely be presented with an error that says DNS failed.
你很可能会看到 DNS 错误

That’s because that site doesn’t exist, so DNS couldn’t give your browser an address.
因为那个网站不存在，所以 DNS 无法返回给你一个地址

But, if DNS returns a valid address, which it should for "YouTube.com”, then your
如果你输的是有效地址，比如 youtube.com \N DNS 按理会返回一个地址

browser shoots off a request over TCP for the website’s data.
然后浏览器会给这个 IP 地址 \N 发 TCP 请求

There’s over 300 million registered domain names, so to make out DNS Lookup a little
如今有三千万个注册域名，所以为了更好管理

more manageable, it’s not stored as one gigantically long list,
DNS 不是存成一个超长超长的列表，而是存成树状结构

but rather in a tree data structure.
DNS 不是存成一个超长超长的列表，而是存成树状结构

What are called Top Level Domains, or TLDs, are at the very top.
顶级域名（简称 TLD）在最顶部，比如 .com 和 .gov

These are huge categories like .com and .gov.
顶级域名（简称 TLD）在最顶部，比如 .com 和 .gov

Then, there are lower level domains that sit below that, called second level domains; Examples
下一层是二级域名，比如 .com 下面有 \N google.com 和 dftba.com

under .com include google.com and dftba.com.
下一层是二级域名，比如 .com 下面有 \N google.com 和 dftba.com

Then, there are even lower level domains, called subdomains,
再下一层叫子域名，\N 比如 images.google.com, store.dftba.com

like images.google.com, store.dftba.com.
再下一层叫子域名，\N 比如 images.google.com, store.dftba.com

And this tree is absolutely HUGE!
这个树超！级！大！

Like I said, more than 300 million domain names, and that's just second level domain
我前面说的"三千万个域名"只是二级域名 \N 不是所有子域名

names, not all the sub domains.
我前面说的"三千万个域名"只是二级域名 \N 不是所有子域名

For this reason, this data is distributed across many DNS servers,
因此，这些数据散布在很多 DNS 服务器上

which are authorities for different parts of the tree.
不同服务器负责树的不同部分

Okay, I know you’ve been waiting for it...
好了 我知道你肯定在等这个梗：

We’ve reached a new level of abstraction!
我们到了一层新抽象！

Over the past two episodes, we’ve worked up from electrical signals on wires,
过去两集里 \N 我们讲了线路里的电信号，以及无线网络里的无线信号

or radio signals transmitted through the air in the case of wireless networks.
过去两集里 \N 我们讲了线路里的电信号，以及无线网络里的无线信号

This is called the Physical Layer.
这些叫"物理层"

MAC addresses, collision detection,
而"数据链路层"负责操控"物理层"，\N 数据链路层有：媒体访问控制地址（MAC），碰撞检测，

exponential backoff and similar low level protocols that
而"数据链路层"负责操控"物理层"，\N 数据链路层有：媒体访问控制地址（MAC），碰撞检测，

mediate access to the physical layer are part of the Data Link Layer.
指数退避，以及其他一些底层协议

Above this is the Network Layer,
再上一层是"网络层"

which is where all the switching and routing technologies that we discussed operate.
负责各种报文交换和路由

And today, we mostly covered the Transport layer, protocols like UDP and TCP,
而今天，我们讲了"传输层"里一大部分， 比如 UDP 和 TCP 这些协议,

which are responsible for point to point data transfer between computers,
负责在计算机之间进行点到点的传输

and also things like error detection and recovery when possible.
而且还会检测和修复错误

We’ve also grazed the Session Layer –
我们还讲了一点点"会话层"

where protocols like TCP and UDP are used to open a connection,
"会话层"会使用 TCP 和 UDP 来创建连接，传递信息，然后关掉连接

pass information back and forth, and then close the connection when finished
"会话层"会使用 TCP 和 UDP 来创建连接，传递信息，然后关掉连接

– what’s called a session.
这一整套叫"会话"

This is exactly what happens when you, for example, do a DNS Lookup, or request a webpage.
查询 DNS 或看网页时，就会发生这一套流程

These are the bottom five layers of the Open System Interconnection (OSI) model,
这是 开放式系统互联通信参考模型(OSI) 的底下5层

a conceptual framework for compartmentalizing all these different network processes.
这个概念性框架 把网络通信划分成多层

Each level has different things to worry about and solve,
每一层处理各自的问题

and it would be impossible to build one huge networking implementation.
如果不分层 \N 直接从上到下捏在一起实现网络通信，是完全不可能的

As we’ve talked about all series, abstraction allows computer scientists and engineers to
抽象使得科学家和工程师能分工同时改进多个层 \N 不被整体复杂度难倒.

be improving all these different levels of the stack simultaneously,
抽象使得科学家和工程师能分工同时改进多个层 \N 不被整体复杂度难倒.

without being overwhelmed by the full complexity.
抽象使得科学家和工程师能分工同时改进多个层 \N 不被整体复杂度难倒.

And amazingly, we’re not quite done yet
而且惊人的是！我们还没讲完呢！

The OSI model has two more layers, the Presentation Layer and the Application Layer,
OSI 模型还有两层，"表示层"和"应用程序层"

which include things like web browsers, Skype,
其中有浏览器，Skype，HTML解码，在线看电影等

HTML decoding, streaming movies and more.
其中有浏览器，Skype，HTML解码，在线看电影等

Which we’ll talk about next week.  See you then.
我们下周说，到时见