Download TCP

Chapter 2
Application Layer
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Computer Networking:
A Top Down Approach,
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2007
J.F Kurose and K.W. Ross, All Rights Reserved
2: Application Layer
1
第二章：应用层
 2.1 网络应用程序的设计
 2.6 P2P 应用程序
原则
 2.2 Web 与 HTTP
 2.3 FTP
 2.4 电子邮件
 2.7 使用TCP的套接字编程

 2.8使用UDP的套接字编程
SMTP, POP3, IMAP
 2.5 DNS
2: Application Layer
2
第二章：应用层
学习目标:
 网络应用协议的概念与
实现
 传输层服务模型
 client-server 范例
 peer-to-peer 范例
 通过探讨应用层协议了
解网络协议




HTTP
FTP
SMTP / POP3 / IMAP
DNS
 编程实现网络程序

套接字 API
2: Application Layer
3
网络应用程序举例
 电子邮件
 VOIP
 Web
 实时视频会议
 即时消息
 网格计算
 远程登录

 P2P 文件共享

 多用户网络游戏

 流媒体视频
2: Application Layer
4
创建一个网络应用程序
编写程序：



application
transport
network
data link
physical
运行在（不同的）端系统上
通过网络通信
e.g., Web 服务器软件与浏览
器软件之间进行通信
不需要为网络核心的设备编写
软件

网络核心的设备上不运行用
户程序

端系统上的应用程序可以快
速部署和传播
application
transport
network
data link
physical
application
transport
network
data link
physical
2: Application Layer
5
第二章：应用层
 2.1 网络应用程序的设计
 2.6 P2P 应用程序
原则
 2.2 Web 与 HTTP
 2.3 FTP
 2.4 电子邮件
 2.7 使用TCP的套接字编程

 2.8使用UDP的套接字编程
 2.9 构建一个 Web 服务器
SMTP, POP3, IMAP
 2.5 DNS
2: Application Layer
6
网络应用程序体系结构
 Client-Server
 Peer-to-peer (P2P)
 C/S 与 P2P 的混杂结构
2: Application Layer
7
Client-Server 结构
服务器:
 一直工作的主机
 永久 IP 地址
 可伸缩的服务器集群
客户端:

client/server



与服务器进行通信
可能只是间歇性的连接
可能使用的动态 IP
客户端之间不直接互相通信
2: Application Layer
8
纯 P2P 结构

没有一直工作的服务器
 任意端系统键直接通信
 对等端之间间歇的建立连peer-peer
接，IP地址是改变的
 example: Gnutella
高度的可伸缩性但是难于管
理
2: Application Layer
9
C/S 与 P2P 混合结构
Skype
 VOIP P2P 应用程序
 中心服务器：查找远端的地址
 客户端对客户端连接：直接的 (不通过服务器)
即时消息/即时通信
 两个用户之间聊天采用的是 P2P 结构
 中心化的服务：用于客户端的存在感知/位置
• 用户上线时向中心服务器注册它的IP地址
• 用户通过练习中心服务器获得伙伴的 IP 地址
2: Application Layer
10
进程通信
进程: 在主机上运行的一个
程序
 在同一个主机上，两个进
程使用进程间通信 (由
OS定义).
客户端进程: 发起通信的进
程
服务器进程: 等待连接的进
程
 不同主机上的进程通过交
换消息实现通信
 注意: P2P 结构的应用程
序也存在客户端进程和服
务器端进程
2: Application Layer
11
套接字
 进程通过使用套接字收发消
息
 套接字类似于门
host or
server
host or
server

发送进程将消息推出门外
process

发送进程依靠门另一边的传
输基础设施将消息送给接收
进程的套接字
socket
controlled by
app developer
process
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
 API: (1) 选择传输协议; (2) 规定一些参数
2: Application Layer
12
进程寻址
标识符 包括 IP 地址和与
 要接收消息，进程必须具

有标识符
 主机设备具有唯一的 32bit IP 地址
 Q: IP 地址是否足够标识
运行在其上的进程？
 A: 不能，同一个主机
上可能在运行多个进
程
主机上的进程关联的端口
号。
 端口号举例:


HTTP 服务器: 80
邮件服务器: 25
 向 gaia.cs.umass.edu 的
Web 服务器发送HTTP消
息：


IP address: 128.119.245.12
Port number: 80
2: Application Layer
13
应用层协议定义
 交换消息的类型:
 e.g., request, response
 消息语法:
 消息包括哪些数据域 & 数
据域如何设置
 消息语义
 数据域中信息的意义
公共协议:
 在 RFC 中定义
 考虑了互操作性
 e.g., HTTP, SMTP
专用协议:
 e.g., Skype
 进程何时、如何发送 & 回
应消息的规则
2: Application Layer
14
What transport service does an app need?
Data loss
 some apps (e.g., audio) can
tolerate some loss
 other apps (e.g., file
transfer, telnet) require
100% reliable data
transfer
Timing
 some apps (e.g.,
Internet telephony,
interactive games)
require low delay to be
“effective”
Bandwidth
 some apps (e.g.,
multimedia) require
minimum amount of
bandwidth to be
“effective”
 other apps (“elastic
apps”) make use of
whatever bandwidth
they get
2: Application Layer
15
Transport service requirements of common apps
Data loss
Bandwidth
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
no loss
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
instant messaging
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
2: Application Layer
16
Internet transport protocols services
TCP service:
 connection-oriented: setup




required between client and
server processes
reliable transport between
sending and receiving process
flow control: sender won’t
overwhelm receiver
congestion control: throttle
sender when network
overloaded
does not provide: timing,
minimum bandwidth
guarantees
UDP service:
 unreliable data transfer
between sending and
receiving process
 does not provide:
connection setup,
reliability, flow control,
congestion control, timing,
or bandwidth guarantee
Q: why bother? Why is
there a UDP?
2: Application Layer
17
Internet apps: application, transport protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
Application
layer protocol
Underlying
transport protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
proprietary
(e.g. RealNetworks)
proprietary
(e.g., Vonage,Dialpad)
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
2: Application Layer
18
Chapter 2: Application layer
 2.1 Principles of
network applications


app architectures
app requirements
 2.2 Web and HTTP
 2.4 Electronic Mail
 SMTP, POP3, IMAP
 2.6 P2P file sharing
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.5 DNS
2: Application Layer
19
Web 与 HTTP
一些术语
 Web 页由一系列对象构成
 对象可以是 HTML 文件、 JPEG 图片、 Java 小程序
、音频文件…
 Web 页由包含数个引用对象的基本 HTML文件构成
 每个对象可通过 URL 寻址
 URL 示例:
www.someschool.edu/someDept/pic.gif
主机名称
路径名称
2: Application Layer
20
HTTP 概述
HTTP: 超文本传输协议（
hypertext transfer
protocol）
 Web 的应用层协议
PC running
Explorer
 C/S 模型

客户端: 浏览器，请求、
接收、“显示” Web 对
象
 服务器: 发送对象响应请
求
 HTTP 1.0: RFC 1945
 HTTP 1.1: RFC 2068
Server
running
Apache Web
server
Mac running
Navigator
2: Application Layer
21
HTTP 概述
使用 TCP:
HTTP 是“无状态”的
 客户端发起到服务器的 TCP 连
 服务器不维护过去的客
接 (创建套接字)，端口号：80
 服务器接受来自客户端的 TCP
连接
 HTTP 消息 (应用层协议消息)
在浏览器 (HTTP 客户端) 与
Web 服务器 (HTTP 服务器端)
之间进行交换
 TCP 连接关闭
户端请求的信息
aside
维护“状态”的协议比较复杂
 过去的历史（状态）必须维
护
 如果服务器/客户端崩溃，它
们对于“状态”的认知将是
不一致的 ，必须进行一致性
重建
2: Application Layer
22
HTTP 连接
非持久 HTTP
 一个 TCP 连接至多发送
一个对象
 HTTP/1.0 使用 非持久
HTTP
持久 HTTP
 多个对象可经单个 TCP
连接在客户端和服务器
之间进行传递
 HTTP/1.1 在默认模式下
使用持久连接
2: Application Layer
23
HTTP 请求消息
 两种类型的 HTTP 消息: request, response
 HTTP 消息:
 ASCII (人能读懂的格式)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-agent: Mozilla/4.0
header Connection: close
lines Accept-language:fr
Carriage return,
line feed
indicates end
of message
(extra carriage return, line feed)
2: Application Layer
24
HTTP request message: general format
2: Application Layer
25
HTTP 响应消息
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
HTML file
HTTP/1.1 200 OK
Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
2: Application Layer
26
FTP: 文件传输协议
user
at host
FTP
FTP
user
client
interface
file transfer
FTP
server
remote file
system
local file
system
 传输文件到/从远端主机
 C/S 模型
client: 发起传输的一端 (到或者从远端)
 server: 远端主机
 ftp: RFC 959
 ftp server: port 21

2: Application Layer
27
FTP: 分离的控制与数据连接
 FTP 客户端通过 21 端口连




TCP control connection
port 21
接 FTP 服务器，使用的传输
协议是 TCP
TCP data connection
FTP
FTP
port 20
客户端经由控制连接进行验 client
server
证
客户端经由控制连接通过发  传输另一个文件时，服务器将
打开另一个 TCP 连接
送命令来浏览远端目录
服务器接收到文件传输命令  控制连接: “out of band”
是，建立到客户端的第二个  FTP 服务器维护“状态”:当前
目录、先前进行过的验证
TCP 连接
传输完一个文件后，服务器
关闭数据连接
2: Application Layer
28
FTP 命令、响应
命令举例:
返回码举例：
 通过控制通道发送 ASCII 文
 状态码与短语 (与HTTP中相
本
 USER username
 PASS password

返回当前目录的文
件列表

 RETR filename 获取文件

 LIST
 STOR filename 将文件存
储到远端主机

识)
331 Username OK,
password required
125 data connection
already open;
transfer starting
425 Can’t open data
connection
452 Error writing
file
2: Application Layer
29
套接字编程
Goal: learn how to build client/server application that
communicate using sockets
Socket API
 1981年在 BSD4.1 UNIX 中引
入
 由应用程序显式创建、使用和
释放
 C/S 模型
 使用套接字 API 的两种传输服
务:
 不可靠数据报
 面向字节流的可靠服务
socket
a host-local,
application-created,
OS-controlled interface
(a “door”) into which
application process can
both send and
receive messages to/from
another application
process
2: Application Layer
30
Socket-programming using TCP
Socket: a door between application process and endend-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one
process to another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
host or
server
internet
socket
TCP with
buffers,
variables
controlled by
application
developer
controlled by
operating
system
host or
server
2: Application Layer
31
Socket programming with TCP
Client must contact server
 server process must first
be running
 server must have created
socket (door) that
welcomes client’s contact
Client contacts server by:
 creating client-local TCP
socket
 specifying IP address, port
number of server process
 When client creates
socket: client TCP
establishes connection to
server TCP
 When contacted by client,
server TCP creates new
socket for server process to
communicate with client
 allows server to talk with
multiple clients
 source port numbers
used to distinguish
clients (more in Chap 3)
application viewpoint
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
2: Application Layer
32
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP
wait for incoming
connection request connection
connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
2: Application Layer
33
Stream jargon
keyboard
monitor
output
stream
inFromServer
Client
Process
process
input
stream
outToServer
characters that flow into
or out of a process.
 An input stream is
attached to some input
source for the process,
e.g., keyboard or socket.
 An output stream is
attached to an output
source, e.g., monitor or
socket.
inFromUser
 A stream is a sequence of
input
stream
client
TCP
clientSocket
socket
to network
TCP
socket
from network
2: Application Layer
34
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input (inFromUser
stream) , sends to server via
socket (outToServer
stream)
2) server reads line from socket
3) server converts line to
uppercase, sends back to
client
4) client reads, prints modified
line from socket
(inFromServer stream)
2: Application Layer
35
Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
2: Application Layer
36
Example: Java client (TCP), cont.
Create
input stream
attached to socket
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
Send line
to server
outToServer.writeBytes(sentence + '\n');
Read line
from server
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer
37
Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
2: Application Layer
38
Example: Java server (TCP), cont
Create output
stream, attached
to socket
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
Read in line
from socket
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
Write out line
to socket
outToClient.writeBytes(capitalizedSentence);
}
}
}
End of while loop,
loop back and wait for
another client connection
2: Application Layer
39
Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

SMTP, POP3, IMAP
 2.5 DNS
 2.6 P2P file sharing
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.9 Building a Web
server
2: Application Layer
40
Socket programming with UDP
UDP: no “connection” between
client and server
 no handshaking
 sender explicitly attaches
IP address and port of
destination to each packet
 server must extract IP
address, port of sender
from received packet
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
UDP: transmitted data may be
received out of order, or
lost
2: Application Layer
41
Client/server socket interaction: UDP
Server (running on hostid)
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port number
Client
create socket,
clientSocket =
DatagramSocket()
Create, address (hostid, port=x,
send datagram request
using clientSocket
read reply from
clientSocket
close
clientSocket
2: Application Layer
42
Example: Java client (UDP)
input
stream
Client
process
monitor
inFromUser
keyboard
Process
Input: receives
packet (recall
thatTCP received
“byte stream”)
UDP
packet
receivePacket
packet (recall
that TCP sent
“byte stream”)
sendPacket
Output: sends
client
UDP
clientSocket
socket
to network
UDP
packet
UDP
socket
from network
2: Application Layer
43
Example: Java client (UDP)
import java.io.*;
import java.net.*;
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
2: Application Layer
44
Example: Java client (UDP), cont.
Create datagram
with data-to-send,
length, IP addr, port
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
Send datagram
to server
clientSocket.send(sendPacket);
Read datagram
from server
clientSocket.receive(receivePacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer
45
Example: Java server (UDP)
import java.io.*;
import java.net.*;
Create
datagram socket
at port 9876
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
Create space for
received datagram
Receive
datagram
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
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46
Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
Get IP addr
port #, of
sender
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
Create datagram
to send to client
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
Write out
datagram
to socket
serverSocket.send(sendPacket);
}
}
}
End of while loop,
loop back and wait for
another datagram
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47
Chapter 2: Summary
our study of network apps now complete!
 application architectures
 client-server
 P2P
 hybrid
 application service
requirements:

reliability, bandwidth,
delay
 specific protocols:
 HTTP
 FTP
 SMTP, POP, IMAP
 DNS
 P2P: BitTorrent, Skype
 socket programming
 Internet transport
service model


connection-oriented,
reliable: TCP
unreliable, datagrams: UDP
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48
Chapter 2: Summary
Most importantly: learned about protocols
 typical request/reply
message exchange:


client requests info or
service
server responds with
data, status code
 message formats:
 headers: fields giving
info about data
 data: info being
communicated
Important themes:
 control vs. data msgs
 in-band, out-of-band
 centralized vs.
decentralized
 stateless vs. stateful
 reliable vs. unreliable
msg transfer
 “complexity at network
edge”
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49