3rd Edition: Chapter 2

Transcript 3rd Edition: Chapter 2

Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

 2.6 P2P Applications
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
SMTP, POP3, IMAP
 2.5 DNS
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Vocabulary and Term
 Client-server paradigm
 客户-服务器范式
 Peer-to-Peer
 对等范式
paradigm
 WWW（World Wide Web）
 万维网
 HTTP（HyperText Transfer Protocol）
 超文本传输协议
 Persistent HTTP
 永久性/持续性HTTP
2: Application Layer
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Chapter 2: Application Layer
Our goals:
 conceptual,
implementation
aspects of network
application protocols
 transport-layer
service models
 client-server
paradigm
 peer-to-peer
paradigm
 learn about protocols
by examining popular
application-level
protocols





HTTP
FTP
SMTP / POP3 / IMAP
DNS
…
 programming network
applications
 socket API
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Some typical network applications
 e-mail
 web
 instant messaging
 remote login
 P2P file sharing
 multi-user network games
 voice over IP
 real-time video
conferencing
 grid computing
 … …
 … …
 streaming stored video
clips
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Creating a network application
write programs that



run on (different) end
systems
communicate over network
e.g., web server software
communicates with browser
software
little software written for
devices in network core


network core devices do
not run user applications
applications on end systems
allows for rapid application
development, propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
2: Application Layer
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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
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Application architectures
 Client-server
 Peer-to-peer (P2P)
 Hybrid of client-server and P2P
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C/S Service Infrastructure
request
Server
Client1
response
Client2
Client N
2: Application Layer
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Client-server architecture
server:



always-on host
permanent IP address
server farms for scaling
clients:


client/server


communicate with server
may be intermittently
connected
may have dynamic IP
addresses
do not communicate directly
with each other
2: Application Layer
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位于俄勒冈州(Oregon)的Google数据中心
Google公司比利时无冷却器数据中心
2: Application Layer
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Pure P2P architecture
 no always-on server
 arbitrary end systems
directly communicate peer-peer
 peers are intermittently
connected and change IP
addresses
 example: Gnutella
Highly scalable but
difficult to manage
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Hybrid of client-server and P2P
Skype
 voice-over-IP: typical P2P application
 centralized server: find address of remote party
 client-client connection: direct (not through server)
Instant messaging [QQ]
chatting between two users is P2P

centralized service: client presence detection
/location
• user registers its IP address with central server
when it comes online
• user contacts central server to find IP
addresses of buddies
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P2P：Sample applications
 File Sharing
 Media
 Storage
 Distributed computing
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Processes communicating
Process: program running
within a host.
 within the same host,
two processes
communicate using
inter-process
communication (defined
by OS).
 processes in different
hosts communicate by
exchanging messages
Client process: process
that initiates
communication
Server process: process
that waits to be
contacted
 Note: applications with
P2P architectures have
client processes &
server processes
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Sockets
 process sends/receives
messages to/from its
socket
 socket analogous to door
 sending process shoves
message out door

sending process relies on
transport infrastructure
on other side of door which
brings message to socket
at receiving process
host or
server
host or
server
process
controlled by
app developer
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
 API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
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Addressing processes
 to receive messages,
process must have
identifier
 host device has unique
32-bit IP address
 Q: does IP address of
host on which process
runs suffice for
identifying the process?
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Addressing processes
 to receive messages,
process must have
identifier
 host device has unique
32-bit IP address
 Q: does IP address of
host on which process
runs suffice for
identifying the
process?
 A: many processes can
be running on the same
host
 identifier includes both
IP address and port
numbers associated with
process on host.
 Example port numbers:


HTTP server: 80
Mail server: 25
 to send HTTP message
to gaia.cs.umass.edu web
server:


IP address: 128.119.245.12
Port number: 80
 more shortly…
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Application-layer protocol defines
 Types of messages
exchanged,

e.g., request, response
 Message syntax:
 what fields in messages &
how fields are delineated
 Message semantics:
 meaning of information in
fields
Public-domain protocols:
 defined in RFCs
 allows for
interoperability (互操作性)
 e.g., HTTP, SMTP
Proprietary protocols:
 e.g., Skype
 Rules for when and how
processes send &
respond to messages
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What transport service does an applications
need?
Data loss
 some applications (e.g., audio)
can tolerate some loss.
 other applications (e.g., file
transfer, telnet) require 100%
reliable data transfer.
Timing
 some applications (e.g.,
Internet telephony,
interactive games)
require low delay to be
“effective”
Bandwidth
 some applications (e.g.,
multimedia) require
minimum amount of
bandwidth to be
“effective”
 other applications
(“elastic applications ”)
make use of whatever
bandwidth they get
Security
secure service, encryption, data integrity, etc.
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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
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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,
 bandwidth guarantee
Q: why bother? Why is
there a UDP?
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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
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Chapter 2: Application layer
 2.1 Principles of
network applications


app architectures
app requirements
 2.2 Web and HTTP
 2.3 FTP
 2.6 P2P file sharing
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.4 Electronic Mail
 SMTP, POP3, IMAP
 2.5 DNS
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Web and HTTP
First some jargon (术语)
 Web page consists of objects
 Object can be HTML file, JPEG image, Java applet,
audio file,…
 Web page consists of base HTML-file which includes
several referenced objects (引用对象)
 Each object is addressable by a URL (Uniform
Resource Locators, 统一资源定位符)
 Example URL:
www.someschool.edu/someDept/pic.gif
host name
path name
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WEB & HTTP: Concepts
 Web
Multimedia
Text
Hyper
Media
Networked
HyperMedia
HyperText
 Homepage
Objects
 URLs (Uniform Resource Locators) （统一资源定位符）

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Client-Server (Browser-Server)
 Architecture
browser
Web Server
 Client Software
 1990:Tim. Berners-lee wrote the
first web client (browser-editor)
and server.
 1993: Mark Andreesen in US
“Mosaic” Netscape Navigator
 IE
 Mozilla (Netscape 6)
 Tecent
the Director of the
World Wide Web
Consortium
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HTTP overview
HyperText Transfer Protocol
 Web’s application layer protocol
 client/server model
client: browser that
requests, receives, “displays”
Web objects
 server: Web server sends
objects in response to
requests
 HTTP 1.0: RFC 1945
 HTTP 1.1: RFC 2068

PC running
Explorer
Server
running
Apache Web
server
Mac running
Navigator
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HTTP overview (continued)
Uses TCP:
 client initiates TCP
connection (creates socket)
to server, port 80
 server accepts TCP
connection from client
 HTTP messages (applicationlayer protocol messages)
exchanged between browser
(HTTP client) and Web
server (HTTP server)
 TCP connection closed
HTTP is “stateless”
 server maintains no
information about
past client requests
aside
Protocols that maintain
“state” are complex!
 past history (state) must
be maintained
 if server/client crashes,
their views of “state” may
be inconsistent, must be
reconciled （一致）
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HTTP connections
Nonpersistent HTTP
 At most one object is
sent over a TCP
connection.
 HTTP/1.0 uses
nonpersistent HTTP
Persistent HTTP
 Multiple objects can be
sent over single TCP
connection between client
and server.
 HTTP/1.1 uses persistent
connections in default
mode
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Nonpersistent HTTP
Suppose user enters URL
(contains text,
references to 10
jpeg images)
www.someSchool.edu/someDepartment/home.index
1a. HTTP client initiates TCP
connection to HTTP server
(process) at
www.someSchool.edu on port 80
2. HTTP client sends HTTP
request message (containing
URL) into TCP connection
socket.
Message indicates that client
wants object
someDepartment/home.index
1b. HTTP server at host
www.someSchool.edu waiting
for TCP connection at port 80.
“accepts” connection, notifying
client
3. HTTP server receives request
message, forms response
message containing requested
object, and sends message
into its socket
time
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Nonpersistent HTTP (cont.)
4. HTTP server closes TCP
5. HTTP client receives response
connection.
message containing html file,
displays html. Parsing html
file, finds 10 referenced jpeg
objects
time 6. Steps 1-5 repeated for each
of 10 jpeg objects
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Non-Persistent HTTP: Response time
Definition of RTT: time to send a
small packet to travel from
client to server and back.
Response time:
initiate TCP
 one RTT (round trip time,往返 connection
时延) to initiate TCP connection
RTT
request
 one RTT for HTTP request and
file
first few bytes of HTTP
RTT
response to return
 file transmission time
file
total = 2RTT+transmit time
received
time
time to
transmit
file
time
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Persistent HTTP
Nonpersistent HTTP issues:
 requires 2 RTTs per object
 OS overhead for each TCP
connection
 browsers often open parallel
TCP connections to fetch
referenced objects
Persistent HTTP
 server leaves connection
open after sending response
 subsequent HTTP messages
between same client/server
sent over open connection
Persistent without pipelining:
 client issues new request
only when previous
response has been received
 one RTT for each
referenced object
Persistent with pipelining:
 default in HTTP/1.1
 client sends requests as
soon as it encounters a
referenced object
 as little as one RTT for all
the referenced objects
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HTTP request message
 two types of HTTP messages: request, response
 HTTP request message:
 ASCII (human-readable format)
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（回车）,
(extra carriage return, line feed)
line feed（换行）
indicates end
of message
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HTTP request message: general format
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Uploading form input
Post method:
 Web page often
includes form input
 Input is uploaded to
server in entity body
URL method:
 Uses GET method
 Input is uploaded in
URL field of request
line:
www.somesite.com/animalsearch?monkeys&banana
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Method types
HTTP/1.0
 GET
 POST
 HEAD

asks server to leave
requested object out of
response (许可响应请求
的对象)
HTTP/1.1
 GET, POST, HEAD
 PUT

uploads file in entity
body to path specified
in URL field
 DELETE
 deletes file specified in
the URL field
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HTTP response message
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 ...
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HTTP response status codes
In first line in server->client response message.
A few sample codes:
200 OK

request succeeded, requested object later in this message
301 Moved Permanently

requested object moved, new location specified later in
this message (Location:)
400 Bad Request

request message not understood by server
404 Not Found

requested document not found on this server
505 HTTP Version Not Supported
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Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet cis.poly.edu 80
Opens TCP connection to port 80
(default HTTP server port) at cis.poly.edu.
Anything typed in sent
to port 80 at cis.poly.edu
2. Type in a GET HTTP request:
GET /~ross/ HTTP/1.1
Host: cis.poly.edu
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to HTTP server
3. Look at response message sent by HTTP server!
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User-server state: cookies
Example:
 Susan always access
Internet always from PC
 visits specific e1) cookie header line of
HTTP response message
commerce site for first
2) cookie header line in
time
HTTP request message
 when initial HTTP
3) cookie file kept on
user’s host, managed by
requests arrives at site,
user’s browser
site creates:
4) back-end database at
 unique ID
Web site
 entry in backend
database for ID
Many major Web sites
use cookies
Four components:
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Cookies: keeping “state” (cont.)
client
ebay 8734
cookie file
ebay 8734
amazon 1678
server
usual http request msg
usual http response
Set-cookie: 1678
usual http request msg
cookie: 1678
one week later:
ebay 8734
amazon 1678
usual http response msg
usual http request msg
cookie: 1678
usual http response msg
Amazon server
creates ID
1678 for user create
entry
cookiespecific
action
access
access
backend
database
cookiespectific
action
2: Application Layer
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Cookies (continued)
What cookies can bring:
 authorization
 shopping carts
 recommendations
 user session state
(Web e-mail)
aside
Cookies and privacy:
 cookies permit sites to
learn a lot about you
 you may supply name
and e-mail to sites
How to keep “state”:
 protocol endpoints: maintain state
at sender/receiver over multiple
transactions
 cookies: http messages carry state
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Web caches (proxy server)
Goal: satisfy client request without involving origin server
 user sets browser:
Web accesses via
cache
 browser sends all
HTTP requests to
cache


object in cache: cache
returns object
else cache requests
object from origin
server, then returns
object to client
origin
server
client
client
Proxy
server
origin
server
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How to configure a Proxy?
2: Application Layer
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More about Web caching
 cache acts as both
client and server
 typically cache is
installed by ISP
(university, company,
residential ISP)
Why Web caching?
 reduce response time
for client request
 reduce traffic on an
institution’s access
link.
 Internet dense with
caches: enables “poor”
content providers to
effectively deliver
content (but so does
P2P file sharing)
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Caching example
origin
servers
Assumptions
 average object size = 1Mbps
average request rate from
institution’s browsers to origin
servers = 15/sec
 delay from institutional router
to any origin server and back
to router = 2 sec
Consequences
 utilization on LAN = 15%
public
Internet
15 Mbps
access link
institutional
network
100 Mbps LAN
 utilization on access link = 100%
 total delay
= Internet delay +
access delay + LAN delay
= 2 sec + minutes + milliseconds
No institutional
cache
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Caching example (cont.)
Origin servers
possible solution
 increase bandwidth of access
link to, say, 100 Mbps
consequence
public
Internet
 utilization on LAN = 15%
 utilization on access link = 15%
= Internet delay +
access delay + LAN delay
= 2 sec + msecs + msecs
 often a costly upgrade
100 Mbps
access link
 Total delay
institutional
network
100 Mbps LAN
No Institutional cache
2: Application Layer
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Caching example (cont.)
origin
servers
possible solution: install cache
 suppose hit rate is 0.4
consequence
public
Internet
 40% requests will be satisfied almost
immediately
 60% requests satisfied by origin
server
 utilization of access link reduced to
60%, resulting in negligible delays
(say 10 msec)
 total avg delay = Internet delay +
access delay + LAN delay =
.6*(2.01) secs + .4*milliseconds < 1.4
secs
15 Mbps
access link
institutional
network
100 Mbps LAN
Institutional
cache
2: Application Layer
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Conditional GET
 Goal: don’t send object if
cache has up-to-date cached
version
 cache: specify date of
cached copy in HTTP request
If-modified-since:
<date>
 server: response contains no
object if cached copy is upto-date:
HTTP/1.0 304 Not
Modified
server
cache
HTTP request msg
If-modified-since:
<date>
HTTP response
object
not
modified
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since:
<date>
HTTP response
object
modified
HTTP/1.0 200 OK
<data>
2: Application Layer
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Hyperlink between files
2: Application Layer
51
① file 2-8.htm
<BODY> <CENTER>
<A href=“ddd5.aspx”> 上一页 </A>     <A href=“ddd6.aspx”> 下一页 </A>
<HR size=4 color="#FF00FF">
<MARQUEE BGCOLOR="#FFFFFF" width=80% behavior="alternate" loop=-1>
<FONT size=6 color="#FF0000">链接同一目录内的文件</FONT>
</MARQUEE></CENTER>
<P><SPACE type="horizontal" size=10>
  随着<A href="2-8-1.htm">Internet</A>的普及，人们已不只满足于仅通过
<A href=“2-8-2.htm">WWW</A>浏览器在网上查询和浏览信息，开始试着设计自己的Web
网页。本书就是介绍使用<A href="2-8-2.htm">HTML语言</A>设计Web网页的基本方法和
技术。
<HR size=4 color="#FF00FF"> </BODY>
② 链接的目标文件 2-8-1.htm
<HTML> <HEAD> </HEAD>
<BODY>
<EM> Internet入门 </EM>
<HR>     Internet是当前世界上最大的计算机网络。Internet是冷战时代
美国军方开发使用的网络，而后作为学术间使用，近年来Internet迅速拓展，成为拥有一亿
多用户的世界性网络。
<HR> <A href=“2-8.htm”> 返回 </A>
</BODY> </HTML>
2: Application Layer
52
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
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Vocabulary and Term
 FTP (File Transfer Protocol
 文件传输协议
)
 Control Connection/Data Connection

控制连接/数据连接
2: Application Layer
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FTP: the file transfer protocol
user
at host
FTP
FTP
user
client
interface
file transfer
local file
system
FTP
server
remote file
system
 transfer file to/from remote host
ftp://ftp.gimp.org
 client/server model
 client: side that initiates transfer (either to/from remote)
 server: remote host
 ftp: RFC 959
 ftp server: keep user’s state (stateless?)
2: Application Layer
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Client
 FTP Now
 SmartFTP
 CuteFTP
 Resume from breakpoint (断点续传)
 Directory Operation
 Smart Keep Alive
 Automatic Update
 CuteFTP Pro
 SSL or SSH2
 Directory Synchronization (目录同步)
 Multiple Protocols （FTP、SFTP、HTTP、HTTPS）
 Simultaneous Multiple Sites
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Server
 Super Ftp Server
 WS-FTP Server
 Quick Easy FTP Server
 FTP Serv-U
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FTP: separate control, data connections
 FTP client contacts FTP server




TCP control connection
port 21
at port 21, TCP is transport
protocol
TCP data connection
FTP
FTP
port 20
client authorized over control
client
server
connection
client browses remote
 server opens another TCP
directory by sending commands
data connection to transfer
over control connection.
another file.
when server receives file
 control connection: “out of
transfer command, server
band”
opens 2nd TCP connection (for
 FTP server maintains “state”:
file) to client
current directory, earlier
after transferring one file,
authentication
server closes data connection.
2: Application Layer
58
控制连接和数据连接
FTP 客户
用户
用户接口
FTP 服务器
文件
系统
协议
解释器
控制连接
协议
解释器
数据传
输进程
数据连接
数据传
输进程
文件
系统
控制连接
Two connections: ports
 control connection

Well-known port: 21
 data connection
 Well-known port 20
- However on the Server Side, and temporary
port No. at Client Side
2: Application Layer
61
FTP客户在控制连接上发送PORT
命令
FTP 服务器
FTP 客户
端口 1173
控制连接
PORT 140,252,13,34,4,150 \r \n
端口 1174
(被动打开)
140.252.13.34
端口 21
FTP服务器主动建立数据连接
FTP 客户
端口 1173
端口 1174
(被动打开)
140.252.13.34
FTP 服务器
控制连接
端口 21
SYN 到 140.252.13.34
端口 1174
端口 20
(主动打开)
FTP Commands
FTP [host]
Example： %ftp ftp.nudt.edu.cn
username：test -login with real name
password：******
or：
username：anonymous
password：[email protected]
2: Application Layer
64
文件传输
（1）Help commands


List all ftp commands
• ftp>?
List the explanantion of a command
• ftp> help open
（2）Connection commands





ftp> open host
ftp> close
ftp> disconnect
ftp> bye
ftp> quit
（3）Commands on directory



ftp> pwd
ftp> cd
ftp> ls [remote-dir] [local-file]
2: Application Layer
66
（4）File transfer commands
－upload file
• ftp> put local-file [remote-file]
• ftp> mput local-files
－download file
• ftp> get [local-file] remote-file
• ftp> mget remote-files
（5）Commands on file type
• ftp> ascii
• ftp> binary
（6） Others
rename、delete，mdelete，size，…
2: Application Layer
67
文件传送举例
Anonymous FTP
[01]
[02]
[03]
[04]
[05]
[06]
[07]
[08]
[09]
[10]
[11]
[12]
ftp nic.ddn.mil
connected to nic.ddn.mil
220 nic FTP server (Sunos 4.1)ready.
Name: anonymous
331 Guest login ok, send ident as password.
Password: [email protected]
230 Guest login ok, access restrictions apply.
ftp> cd rfc
250 CWD command successful.
ftp> get rfc1261.txt nicinfo
200 PORT command successful.
150 ASCII data connection for rfc1261.txt
(128.36.12.27,1401) (4318 bytes).
[13] 226 ASCII Transfer complete.
local: nicinfo remote: rfc1261.txt
4488 bytes received in 15 seconds (0.3 Kbytes/s).
[14] ftp> quit
[15] 221 Goodbye.
2: Application Layer
69
FTP commands, responses
Sample commands:
 sent as ASCII text over
control channel
 USER username
 PASS password
 LIST return list of file in
Sample return codes
 status code and phrase (as in


current directory
 RETR filename retrieves
(gets) file
 STOR filename stores
(puts) file onto remote
host


HTTP)
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
70
T (Trivial) FTP
 Its purpose is often used for router
bootstrap;
 Adopted in two nodes communication, one
with no disk;
 Transmission Protocol: UDP
 RFC 1350
2: Application Layer
71
TFTP报文类型
文件读写
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
74
Vocabulary and Term
 EMAIL（Electronic Mail）
 电子邮件
 SMTP（Simple Mail Transfer Protocol）
 简单邮件传输协议
 POP3（Post Office Protocol Version 3）
 邮局协议-版本3
 IMAP4（Internet Mail Access Protocol
Version 4）

互联网邮件访问协议-版本4
 Web Mail
2: Application Layer
75
Email History-1
[[email protected]]
 1970s: Invention
 In
the early 1970's, Engineer from BBN
Company Ray Tomlinson chose the "commercial
at" symbol to combine the user and host names,
and "user@host" is the standard for email
addressing .
 Larry Roberts of IPTO wrote RD in one threeday weekend to sort and order email headers
by subject and date in their Inbox, and to read,
save, and delete messages in the order they
wished.
2: Application Layer
76
Email History-2
 1980s: Development
Mid-term in 1980s: Spread with the PC；
 1988: Steve Dorner developed Eudora- the
first mail management with GUI( Graphics User
Interface);
 1989: Lotus (莲花)Company released 35000
Lotus Notes email systems.

2: Application Layer
77
Email History-3
 1990s：
prevail（鼎盛）
Mid-term in 1990s: Spread quickly with the
WWW
 1996：Microsoft Outlook 1.0

1998： Microsoft Acquires （收购） Hotmail
 Success of Hotmail led to Email Service by
Yahoo！, netscape，Exicite ， Lycos, [Sina,
Sohu,…] Portal sites (门户网站) and
companies.

2: Application Layer
78
Email History-4
 New Century
2011: 40th anniversary
 1995: Hotmail (free email)
 2004 :Google’s GMail has Giga-byte Mailbox 
New Competitions
… … …
 Email plays more important role in our life and
its security problems become prominent.

2: Application Layer
79
Features of Email
 Asynchronous Communication Media
between People

people send and read messages when it is
convenient for them, not at the same time
 Comprehensive Media
 include text, images, sound and even video.
 To communicate ideas, advertisement, file
sharing,…
2: Application Layer
80
Electronic Mail
outgoing
message queue
Three major components:
 user agents
 mail servers
 SMTP (Simple Mail Transfer
user mailbox
user
agent
mail
server
SMTP
Protocol)
User Agent
SMTP
 a.k.a. “mail reader”
 composing, editing, reading
mail
mail messages
server
 e.g., Eudora, Outlook, elm,
Mozilla Thunderbird
 outgoing, incoming messages
stored on server
user
SMTP
user
agent
mail
server
user
agent
user
agent
user
agent
agent
2: Application Layer
81
Electronic Mail: mail servers
user
agent
Mail Servers
 mailbox contains incoming
messages for user
 message queue of outgoing
(to be sent) mail messages
 SMTP protocol between mail
servers to send email
messages
 client: sending mail
server
 “server”: receiving mail
server
mail
server
SMTP
SMTP
mail
server
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
user
agent
2: Application Layer
82
Electronic Mail: SMTP [RFC 2821]
 uses TCP to reliably transfer email message from client
to server, port 25
 direct transfer: sending server to receiving server
 three phases of transfer
 handshaking (greeting)
 transfer of messages
 closure
 command/response interaction
 commands: ASCII text
 response: status code and phrase
 messages must be in 7-bit ASCII
2: Application Layer
83
Scenario: Alice sends message to Bob
1) Alice uses UA to compose
message and “to”
[email protected]
2) Alice’s UA sends message
to her mail server; message
placed in message queue
3) Client side of SMTP opens
TCP connection with Bob’s
mail server
1
user
agent
2
mail
server
3
4) SMTP client sends Alice’s
message over the TCP
connection
5) Bob’s mail server places the
message in Bob’s mailbox
6) Bob invokes his user agent
to read message
mail
server
4
5
6
user
agent
2: Application Layer
84
Sample SMTP interaction
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
S:
C:
S:
220 hamburger.edu
HELO crepes.fr
250 Hello crepes.fr, pleased to meet you
MAIL FROM: <[email protected]>
250 [email protected]... Sender ok
RCPT TO: <[email protected]>
250 [email protected] ... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 hamburger.edu closing connection
2: Application Layer
85
Try SMTP interaction for yourself:
 telnet servername 25
 see 220 reply from server
 enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands above lets you send email without using
email client (reader)
2: Application Layer
86
SMTP: Comparison with HTTP:
 SMTP uses persistent
connections
 SMTP requires
message (header &
body) to be in 7-bit
ASCII, however no
such requirement in
http
 SMTP server uses
CRLF. to determine
end of message
 HTTP: pull protocol
 SMTP: push protocol
 both have ASCII
command/response
interaction, status
codes
 HTTP: each object
encapsulated in its own
response message,
SMTP: multiple
objects sent in
multipart message
2: Application Layer
87
Mail message format
SMTP: protocol for
exchanging email
messages
RFC 822: standard for
text message format:
 header lines, e.g.,



header
blank
line
body
To:
From:
Subject:
 body
 the “message”, ASCII
characters only
2: Application Layer
88
MIME
MIME
 RFC 2045


(MIME) Part One:
Format of Internet Message Bodies
 RFC 2046


(MIME) Part Two:
Media Types
 MIME
 designed to be fully compatible with existing electronic
mail protocols – SMTP, POP and IMAP.
 not just restricted to email, it is now used in HTTP to
deliver audio, video, etc.
2: Application Layer
90
Message format: multimedia extensions
 MIME: multimedia mail extension, RFC 2045, 2056
 additional lines in message header declare MIME content type
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
encoded data
From: [email protected]
To: [email protected]
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
.........................
......base64 encoded data
2: Application Layer
91
MIME数据类型及子类型（1/2）
类型
子类型
说明
文本（text）
普通（plaintext）
无格式的文本
格式文本（richtext）
格式化的文本
混合（mixed）
正文中包含按序排列的
不同类型数据
并行（parallel）
同上，但无序
JPEG
JPEG 图像
GIF
GIF 图像
音频（audio）
基本
8kHz 的单声道话音编码
视频（vedio）
MPEG
MPEG 视频
应用（applicaion）
PostScript
Adobe PostScript 格式
多部分（multipart）
图像（image）
MIME内容-传送-编码
类型
说明
7bits
NVT ASCII 字符和短行
8bits
非 ASCII 字符和短行
二进制
非 ASCII 字符和长度不受限的行
Base64
6 比特数据块被编码成 8 比特的 ASCII
字符
引用可打印
非 ASCII 被编码成等号后面跟一个
ASCII 字符
Base64编码
Base64编码表
quoted-printable encoding
MIME邮件举例
MIME-Version：1.1
Content-Type：multipart/mixed；boundary=“-------417CBA32DF8945BE748”
From：Alice [email protected]
To：[email protected]
Subject：hello
Date：Mon. 18 Sep 2006 19:40:19 -0400
-------417CBA32DF8945BE748
Content-Type：text/plain；charset=us-ascii
Content-Transfer-Encoding：7bit
Bob，
Here’s something I collect.
Alice
-------417CBA32DF8945BE748
Content-Type：image/jepg
Content-Transfer-Encoding：base64
.
. jpeg 图象的 base64 编码
.
Mail access protocols
user
agent
SMTP
SMTP
sender’s mail
server
access
protocol
user
agent
receiver’s mail
server
 SMTP: delivery/storage to receiver’s server
 Mail access protocol: retrieval from server



POP: Post Office Protocol [RFC 1939]
• authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730]
• more features (more complex)
• manipulation of stored messages on server
HTTP: Gmail, Hotmail, Yahoo! Mail, etc.
2: Application Layer
98
POP3 protocol
authorization phase
 client commands:
user: declare username
 pass: password
 server responses
 +OK
 -ERR

transaction phase, client:
 list: list message numbers
 retr: retrieve message by No.
 dele: delete
 quit:
S:
C:
S:
C:
S:
+OK POP3 server ready
user bob
+OK
pass hungry
+OK user successfully logged
C:
S:
S:
S:
C:
S:
S:
C:
C:
S:
S:
C:
C:
S:
list
1 498
2 912
.
retr 1
<message 1 contents>
.
dele 1
retr 2
<message 1 contents>
.
dele 2
quit
+OK POP3 server signing off
2: Application Layer
on
99
POP3 (more) and IMAP
More about POP3
 Previous example uses
“download and delete”
mode.
 Bob cannot re-read email if he changes
client
 “Download-and-keep”:
copies of messages on
different clients
 POP3 is stateless
across sessions
IMAP
 Keep all messages in
one place: the server
 Allows user to
organize messages in
folders
 IMAP keeps user state
across sessions:

names of folders and
mappings between
message IDs and folder
name
2: Application Layer
100
IMAP: Why?
 Using POP3
 downloaded his messages to the local machine
 he can create mail folders and move the
downloaded messages into the folders.
 delete messages, move messages across
folders, and search for messages
 BUT For nomadic(到处游动的) user
 who would prefer to maintain a folder hierarchy
on a remote server that can be accessed by
from any computer
2: Application Layer
101
IMAP
 IMAP uses TCP on port 143
 Allows user to organize messages in folders
 Operate system mailbox in server like user
mailbox in client
 Can obtain components of message
 A wireless pr low-speed user can only fetch
text part of the mail in MIME format, while
keeping the video/image parts in the server
2: Application Layer
102
WebMail: email with http
Feng Xiang
Cai Kaiyu
Foxmail
Hotmail
Receiver
Receiver
Receiver
User
Agent
User
UserAgent
Agent
Originator
Originator
Originator
UserAgent
Agent
User
User
Agent
http
http
Mail
server
Mail.gfkd.mtn
Mail.xd.mtn
SMTP
Mail
Server
2: Application Layer
103
The Problems in Mail System
 SPAM
 Mail Phishing
 Mail Virus/Worm
 Privacy /confidentiality
2: Application Layer
104
2: Application Layer
105
SPAM (垃圾邮件)
 UBE (Unsolicited Bulk Email)
 UCE (Unsolicited Commercial Email)
 Junk Mail
 Spam Filtering
2: Application Layer
106
Mail Phishing
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=iso-8859-1" =
http-equiv=Content-Type>
<META content="MSHTML 5.00.2920.0" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>If the message will not displayed automatically,<br>
follow the link to read the delivered message.<br><br>
Received message is available at:<br>
<a href=cid:031401Mfdab4$3f3dL780$73387018@57W81fa70Re height=0
width=0>www.cernet.edu.cn/inbox/dhx/read.php?sessionid-17370</a>
<iframe
src=cid:031401Mfdab4$3f3dL780$73387018@57W81fa70Re height=0 width=0></iframe>
<DIV> </DIV></BODY></HTML>
2: Application Layer
107
Mail Virus and Worm
 爱虫（ 2000-2-14）、
 nimda（2001-9-19）、
 求职信（2001-10-26）、
 中文版求职信（2002-年5-10）、
 怪物（2002-10-02）、
 sobig（2003-1-11）、
 爱情后门（2003-2-25）、
 小邮差（2003-8-04）、
 斯文（2003-9-19）、
 MyDoom (SCO炸弹)（2004-1-27）
 Netsky及其变种（2003-今）
2: Application Layer
108
Privacy & Confidentiality
 Authentication (认证)
 PEM （ Privacy Enhancement for Internet
Electronic Mail ）Standards
 Attachment Security
 PGP Software （the whole mail security）
2: Application Layer
109
Authentication (认证)
2: Application Layer
110
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
111
DNS: Domain Name System
People: many identifiers:

SSN, name, passport #
Internet hosts, routers:


IP address (32 bit) used for addressing
datagrams
“name”, e.g.,
www.yahoo.com
- used by humans
Q: map between IP
addresses and name ?
Domain Name System:
 distributed database
implemented in hierarchy of
many name servers
 application-layer protocol
host, routers, name servers to
communicate to resolve names
(address/name translation)
 note: core Internet
function, implemented as
application-layer protocol
 complexity at network’s
“edge”
2: Application Layer
112
DNS
DNS services
 hostname to IP
address translation
 host aliasing

Canonical, alias names
 mail server aliasing
 load distribution
 replicated Web
servers: set of IP
addresses for one
canonical name
Why not centralize DNS?
 single point of failure
 traffic volume
 distant centralized
database
 maintenance
doesn’t scale!
2: Application Layer
113
Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
yahoo.com
amazon.com
DNS servers DNS servers
org DNS servers
pbs.org
DNS servers
edu DNS servers
poly.edu
umass.edu
DNS serversDNS servers
Client wants IP for www.amazon.com; 1st approx:
 client queries a root server to find com DNS server
 client queries com DNS server to get amazon.com
DNS server
 client queries amazon.com DNS server to get IP
address for www.amazon.com
2: Application Layer
114
DNS: Root name servers
 contacted by local name server that can not resolve name
 root name server:



contacts authoritative name server if name mapping not known
gets mapping
returns mapping to local name server
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also LA)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 21 locations)
e NASA Mt View, CA
f Internet Software C. Palo Alto,
k RIPE London (also 16 other locations)
i Autonomica, Stockholm (plus
28 other locations)
m WIDE Tokyo (also Seoul,
Paris, SF)
CA (and 36 other locations)
13 root name
servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
2: Application Layer
115
TLD and Authoritative Servers
 Top-level domain (TLD) servers:
 responsible for com, org, net, edu, etc, and all
top-level country domains uk, fr, ca, jp.
 Network Solutions maintains servers for com TLD
 Educause maintains servers for edu TLD
 Authoritative DNS servers:
 organization’s DNS servers, providing
authoritative hostname to IP mappings for
organization’s servers (e.g., Web, mail).
 can be maintained by organization or service
provider
2: Application Layer
116
Local Name Server
 does not strictly belong to hierarchy
 each ISP (residential ISP, company,
university) has one.

also called “default name server”
 when host makes DNS query, query is sent
to its local DNS server

acts as proxy, forwards query into hierarchy
2: Application Layer
117
DNS name
resolution example
root DNS server
2
 Host at cis.poly.edu
3
wants IP address for
gaia.cs.umass.edu
iterated query:
 contacted server
replies with name of
server to contact
 “I don’t know this
name, but ask this
server”
TLD DNS server
4
5
local DNS server
dns.poly.edu
1
8
requesting host
7
6
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer
118
DNS name
resolution example
root DNS server
2
recursive query:
3
7
6
TLD DNS server
 puts burden of name
resolution on contacted
name server
local DNS server
dns.poly.edu
 heavy load?
1
5
4
8
requesting host
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer
119
The Combination of Recursion and Iteration
Root DNS Server
dns.com
③
⑤
Local DNS server
dns.y.abc.com
④
Local DNS server
Local DNS server
⑥
②
dns.abc.com
⑦
dns.xyz.com
⑧
IP(t.y.abc.com)
= (198.54.23.15)
t.y.abc.com
①
IP(t.y.abc.com)=?
m.xyz.com
2: Application Layer
120
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name,
 Type=A
 name is hostname
 value is IP address
 Type=NS
 name is domain (e.g.
foo.com)
 value is hostname of
authoritative name
server for this domain
value, type, ttl)
 Type=CNAME
 name is alias name for some
“canonical” (the real) name
www.ibm.com is really
servereast.backup2.ibm.com

value is canonical name
 Type=MX
 value is name of mailserver
associated with name
2: Application Layer
121
CNAME
 假设Yahoo的Web网站的规范名CNAME是
www.yahoo.com.cn
 但是很多人喜欢用cn.yahoo.com、yahoo.com.cn去
访问Yahoo网站，这个时候该怎么处理呢？
 这就要用到CNAME项
Yahoo的DNS配置
< cn.yahoo.com，www.yahoo.com.cn，CNAME，
IN>
< yahoo.com.cn，www.yahoo.com.cn，CNAME，
IN>
<www.yahoo.com.cn，10.1.1.1，IP，IN>
MX
 [email protected]－原来的
 [email protected]－现在的
 国防科大邮件服务器名字

mail.nudt.edu.cn－原来的

webmail.nudt.edu.cn－现在的
 只要将国防科大DNS上的配置成
< mail.nudt.edu.cn，172.16.100.181，IP，IN>
改为
< nudt.edu.cn，webmail. nudt.edu.cn，MX，IN>
< webmail.nudt.edu.cn，172.16.100.181，IP，IN>
DNS protocol, messages
DNS protocol : query and reply messages, both with
same message format
msg header
 identification: 16 bit #
for query, reply to query
uses same #
 flags:
 query or reply
 recursion desired
 recursion available
 reply is authoritative
2: Application Layer
125
DNS protocol, messages
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used
2: Application Layer
126
Inserting records into DNS
 example: new startup “Network Utopia”
 register name networkuptopia.com at DNS registrar
(e.g., Network Solutions)


provide names, IP addresses of authoritative name server
(primary and secondary)
registrar inserts two RRs into com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
 create authoritative server Type A record for
www.networkuptopia.com; Type MX record for
networkutopia.com
 How do people get IP address of your Web site?
2: Application Layer
127
Review
DNS (Domain Name Service)
 DNS is an infrastructural service by the
Internet like routing and data forwarding
 Mapping between domain name and IP
address
http://www.google.com
DNS
216.239.57.99
2: Application Layer
128
Domain Space
Root
Top Level Domain Name
… coop info biz aero com net org edu gov mil int cn uk …
Authoritative Domain Name
cctv… ibm hp
mot
Local Domain Name
… hk js sh bj org net gov edu com ac
Local Domain Name
mail
Local Domain Name
…
tsinghua pku fudan sjtu seu
mail csnetl ep
…
2: Application Layer
129
DNS: caching and updating records
 once (any) name server learns mapping, it caches
mapping
 cache entries timeout (disappear) after some
time
 TLD servers typically cached in local name
servers
• Thus root name servers not often visited
 update/notify mechanisms under design by IETF
 RFC 2136

http://www.ietf.org/html.charters/dnsind-charter.html
2: Application Layer
130
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
 2.5 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
131
P2P file sharing
Example
 Alice runs P2P client
application on her
notebook computer
 intermittently
connects to Internet;
gets new IP address
for each connection
 asks for “Hey Jude”
 application displays
other peers that have
copy of Hey Jude.
 Alice chooses one of
the peers, Bob.
 file is copied from
Bob’s PC to Alice’s
notebook: HTTP
 while Alice downloads,
other users uploading
from Alice.
 Alice’s peer is both a
Web client and a
transient Web server.
All peers are servers =
highly scalable!
2: Application Layer
132
Content for the Section
 1.Overview of Peer-to-Peer Applications
1.1 What is P2P ?
 1.2 Why is P2P so popular?
 1.3 Who are P2P applications?

 2.How does P2P work?
 2.1 Centralized: Napster
 2.2 Fully Distributed :Gnutella
 2.3 Hierarchical: KaZaA
 2.4 Bit Torrent (BT) [Supplementary,补充的]
 3.Where to go:
Issues and Future Directions
2: Application Layer
133
Vocabulary and Term
 Peer

对等体
 P2P (Peer-to-Peer) Application/ Network

对等应用、对等网络
 Centralized/Distributed/Hierarchical

集中的、分布的、层次的
 Free Riding

免费搭车
2: Application Layer
134
Client-Server Mode
F bits
d4
upload rate us
Internet
d3
d1
d2
Server becomes bottleneck
(CPU/Bandwidth)
2: Application Layer
135
Peer-to-Peer Mode
Different Hosts Download from Multiple Sources
F bits
D
d4
u4
upload rate us
A
Internet
d3
d1
u1
d2
u2
Pieces of one file can be
downloaded from Multiple Sources
u3
C
B
2: Application Layer
136
Why is P2P so Popular?
 P2P networks generate more traffic than any
other internet application
 2/3 of all bandwidth on some backbones
2: Application Layer
137
Killer Application
 1st generation: “raw” Internet
 Killer Application： EMAIL
 2nd generation: the Web
Killer Application： WWW
 Client –Server Mode

 3rd generation: P2P Network
Killer Application： P2P Applications
 Peer-to-Peer Mode: Every PC acts as active participant
to provide service (file sharing, BLOG/PODCAST）
 Peer-to-Peer Mode: “I work for everyone, and everyone
for me”(我为人人，人人为我)

2: Application Layer
138
Advantages of P2P model
 High Performance（高性能）
 Help find the shared resources
 Speed up the file distribution
 Reliability (可靠性)
 Client-server mode: The whole network will depend on
the highly loaded server to function properly
 There is zero reliance on centralized serviced or
resources for operations
 Scalability （扩展性）
 In client-server mode: there is a higher demand for
computing power, storage space, and bandwidth
associated with the server-side.
2: Application Layer
139
Who are P2P Applications？
Video Streaming
Advanced
Applications
VOIP
PPLive
Skype
Downloading
BitTorrent
File Sharing
Basic
Applications
Napster
2001
2003
2004
2005
2: Application Layer
140
Who are P2P Applications？
 File Share
KuGoo（酷狗），PPGou
 P2P Download Accelerator(加速器)
2: Application Layer
141
Comparing Client-server, P2P architectures
Question : How much time distribute file
initially at one server to N other computers?
us: server upload
bandwidth
Server
us
File, size F
dN
uN
u1
d1
u2
ui: client/peer i
upload bandwidth
d2
di: client/peer i
download bandwidth
Network (with
abundant bandwidth)
2: Application Layer
142
Client-server: file distribution time
 server sequentially
sends N copies:

Server
F
us
F*N/us time
 client i takes F/di
time to download
dN
u1 d1 u2
d2
Network (with
abundant bandwidth)
uN
Time to distribute F
to N clients using = dcs = max {F*N/us, F/min(di) }
i
client/server approach
increases linearly in N
(for large N)
2: Application Layer
143
P2P: file distribution time
Server
 server must send one
F
u1 d1 u2
d2
copy: F/us time
us
 client i takes F/di time
Network (with
dN
to download
abundant bandwidth)
uN
 F*N bits must be
downloaded (aggregate)
 fastest possible upload rate (assuming
all nodes sending file chunks to same
peer): us + Sui
i=1,N
dP2P = max { F/us, F/min(di) , F*N/(us + Sui) }
i
i=1,N
2: Application Layer
144
File distribution time
2: Application Layer
145
P2P Case Study: BitTorrent
 BitTorrent history
2002: B. Cohen debuted (发布) BitTorrent
 BitTorrent motivation
 Focused on efficient fetching, not searching
 Distribute same file to many peers
 Single publisher, many downloaders

2: Application Layer
146
2: Application Layer
147
BitTorrent
 P2P file distribution
tracker: tracks peers
participating in torrent
torrent: group of
peers exchanging
chunks of a file
obtain list
of peers
trading
chunks
peer
2: Application Layer
148
BitTorrent
 file divided into 256KB chunks.
 peer joining torrent:
has no chunks, but will accumulate them over time
 registers with tracker to get list of peers,
connects to subset of peers (“neighbors”)
 while downloading, peer uploads chunks to other
peers.
 peers may come and go
 once peer has entire file, it may (selfishly) leave or
(altruistically) remain

2: Application Layer
149
BitTorrent
Sending Chunks: tit-for-tat
Pulling Chunks
 Alice sends chunks to
 at any given time,
four neighbors currently
different peers have
sending her chunks at the
different subsets of file
highest rate
chunks
 re-evaluate top 4
 periodically, a peer
every 10 secs
(Alice) asks each
neighbor for list of
 every 30 secs: randomly
chunks that they have.
select another peer,
starts sending chunks
 Alice issues requests for
her missing chunks
 newly chosen peer may
join top 4
 rarest first(最少优先)
2: Application Layer
150
BitTorrent: Strengths
 BitTorrent builds a network for every file that is
being distributed
 Downside of BitTorrent: No searching possible
Websites with “link collections” and search
capabilities exist
 Works quite well
 Download a bit slow in the beginning, but speeds
up considerably as peer
 gets more and more chunks(参与者越多，系统越
优化)

2: Application Layer
151
2.How does file-sharing P2P application work?
 Approaches to Search for the file

Central directory (Napster) [Centralized]

Fully Distributed (Gnutella) [the other extreme：另一极端]

Hierarchical (KazaA) [compromise:折衷]
2: Application Layer
152
P2P: centralized directory
original “Napster” design
1) when peer connects, it
informs central server:


Bob
centralized
directory server
1
peers
1
IP address
content
2) Alice queries for “Hey
Jude”
3) Alice requests file from
Bob
3
1
2
1
Alice
2: Application Layer
153
P2P: problems with centralized directory
 single point of failure
 performance bottleneck
 copyright infringement:
“target” of lawsuit is
obvious
file transfer is
decentralized, but
locating content is
highly centralized
2: Application Layer
154
Centralized: Napster
 Napster history: the rise
 January 1999: Napster version 1.0
 May 1999: company founded
 2000: 80 million users
Shawn Fanning
 Napster history: the fall
 Mid 2001: out of business due to Northeastern freshman(18)
lawsuits（诉讼）against copyright
 Napster history: the resurrection(复苏)
 2003: Napster reconstituted as a pay service
 2007: still lots of file sharing going on
2: Application Layer
155
Napster Technology: Directory Service
 User installing the software
 Client1 contacts Napster server
 Provides a list of music files it will share
 … and Napster’s central server updates the directory
 Client2 searches a music by title or performer
 Napster identifies online clients with the file
 … and provides IP addresses
 Client2 requests the file from the chosen supplier
 Supplier transmits the file to the client
 Both client and supplier report status to Napster
2: Application Layer
156
P2P: problems with centralized directory
file transfer is decentralized(分散的), but locating content
is highly centralized
 Single point of failure
 System has single points of entry; one fails could
bring whole system down
 Performance bottleneck
 Broken links, out of date (过期的) information
 Copyright infringement （版权侵害)
 So, later P2P systems were more distributed

Gnutella went to the other extreme…
2: Application Layer
157
Fully Distributed :Gnutella
 Gnutella history



2000: J. Frankel & T. Pepper released Gnutella
2001: protocol enhancements, e.g., “ultrapeers”
[Modern Gnutella]
Peer flooding (泛洪) Query to all its neighbors
2: Application Layer
158
Query flooding: Gnutella
 fully distributed
 no central server
 public domain protocol
 开放网络协议
 many Gnutella clients
implementing protocol
overlay network: graph
 edge between peer X
and Y if there’s a TCP
connection
 all active peers and
edges form overlay net
 edge: virtual (not
physical) link
 given peer typically
connected with < 10
overlay neighbors
2: Application Layer
159
Gnutella: protocol
 Query message
sent over existing TCP
connections
 peers forward
Query message
 QueryHit
sent over
reverse
Query
path
File transfer:
HTTP
Query
QueryHit
QueryHit
Scalability:
limited scope
flooding
2: Application Layer
160
Partial Map of the Gnutella Network
July 2000
2: Application Layer
161
Gnutella: Peer joining
joining peer Alice must find another peer in
Gnutella network: use list of candidate peers
2. Alice sequentially attempts TCP connections with
candidate peers until connection setup with Bob
3. Flooding: Alice sends Ping message to Bob; Bob
forwards Ping message to his overlay neighbors
(who then forward to their neighbors….)
 peers receiving Ping message respond to Alice
with Pong message
4. Alice receives many Pong messages, and can then
setup additional TCP connections
Peer leaving: see homework problem!
1.
2: Application Layer
162
Gnutella: Pros and Cons
 Advantages
Fully decentralized
 Search cost is distributed

 Disadvantages
Search scope may be quite large
 Search time may be quite long
 High overhead, and nodes come and go often

2: Application Layer
163
Hierarchical Overlay
 between centralized
index, query flooding
approaches
 each peer is either a
group leader or assigned
to a group leader.


TCP connection between
peer and its group leader.
TCP connections between
some pairs of group leaders.
 group leader tracks
content in its children
ordinary peer
group-leader peer
neighoring relationships
in overlay network
2: Application Layer
164
Exploiting heterogeneity
 Each peer is either a group
leader （super-node）or
ordinary node.


TCP connection between peer
and its group leader.
TCP connections between
some pairs of group leaders.
 Group leader tracks the
content in all its children.
2: Application Layer
165
Smart query flooding
 Join（加入）
 on start, the client contacts a super-node
 Publish（发布）
 client sends list of files to its super-node
 Search（搜索）
 send query to super-node, and the super-nodes flood
queries among themselves
 Fetch（获取）
 get file directly from peer(s); can fetch from multiple
peers at once
2: Application Layer
166
P2P Case study: Skype
Skype clients (SC)
 P2P (pc-to-pc, pc-to-
phone, phone-to-pc)
Voice-Over-IP (VoIP)
Skype
application
login server
 also IM
 proprietary
application-layer
protocol (inferred via
reverse engineering)
 hierarchical overlay
Supernode
(SN)
2: Application Layer
167
Skype: making a call
 User starts Skype
 SC registers with SN
 list of bootstrap SNs
 SC logs in
Skype
login server
(authenticate)
 Call: SC contacts SN will
callee ID

SN contacts other SNs
(unknown protocol, maybe
flooding) to find addr of
callee; returns addr to SC
 SC directly contacts callee, overTCP
2: Application Layer
168
Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

 2.6 P2P file sharing
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
SMTP, POP3, IMAP
 2.5 DNS
2: Application Layer
169
Socket programming
Goal: learn how to build client/server application that
communicate using sockets
Socket API
 introduced in BSD4.1 UNIX,
1981
 explicitly created, used,
released by apps
 client/server paradigm
 two types of transport
service via socket API:
 unreliable datagram
 reliable, byte streamoriented
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
170
Socket系统调用
1、socket( )
2、bind( )
3、connect( )
4、listen( ) &accept( )
5、send( ) & sendto( )
6、read( ) & recvfrom( )
1、 socket( )
 功能

创建socket
 格式
sockid＝socket（af，type，protocol）
 参数
 AF(Address Family)
 Type
 Protocol
 返回结果
 成功， sockid返回一个大于0的整数，即socket号
 失败， sockid返回-1，同时返回一个出错代码errno
2、bind( )
 功能

指定本地IP地址和本地端口号
 调用格式

bind（sockid，localaddr，addrlen）
 调用参数

sockid，已获得的socket号

localaddr，本地IP地址和端口号

addrlen，以字节为单位的本地地址长度
 返回结果
 成功，返回一个大于0的整数
 失败，返回-1，同时返回一个出错代码errno
3、connect( )
 功能

建立传输层连接
 调用格式

connect（sockid，destaddr，addrlen ）
 调用参数

sockid，本地socket号。

destaddr，是一个指向服务器的socket地址指针

addr1en，服务器socket地址长度
 返回结果

connect( ) 调用要等到客户和服务器之间TCP连接建立
完后才返回大于0的值，否则将返回-1以及出错代码。
4、listen( )和accept( )
 面向连接的服务器进程一般在某个众所周知的端
口上接收客户进程的连接请求。
 面向连接的服务器进程通过listen( )和accept( )两
个系统调用来接收并处理客户进程的连接请求。

1isten( )系统调用表明服务器进程愿意接收客户进程的
连接请求。

accept( )系统调用用于服务器进程处理客户进程的连接
请求。
1isten( )
 调用格式

listen（sockid，quelen)
 调用参数

sockid，本地socket号

quelen，连接建立请求队列长度
• listen( )系统调用以此参数限制连接请求的排队个数，通
常允许的连接请求排队长度最大值为5。
accept( )
 调用格式

newsock = accept(sockid，clientaddr，addrlen)
 调用参数

sockid，本地socket号。

clientaddr，指向客户socket地址指针
• 初始值为空，当accept( )调用成功后，客户进程的socket地址
被填入该参数中。

addrlen，客户socket地址长度
• 初始值为0，当accept( )调用成功后，客户进程socket地址长
度填入到该参数中。
newsock
 accept( )调用除将客户进程的socket地址及地址
长度放入clientaddr所指的地址结构和addr1en所
指单元外，还将返回一个新的socket号newsock。
 新的socket号newsock用于服务器fork出来子进程
与客户进程通信。
 服务器进程继续利用原来的sockid处理客户进程的
连接请求。
 当newsock的值小于0时，表明accept( )调用出错
。
5、write( )与sendto( )
 功能

write( ) 用于面向连接的数据发送，面向连接的数据发送
系统调用中不必指定接收方socket地址。

sendto( ) 用于无连接的数据发送，无连接的数据发送系
统调用必须明确指定接收方的socket地址。
 调用格式

write（sockid，buff，bufflen）

sendto（sockid，buff，bufflen，dstaddr，addrlen）
6、read( )和recvfrom( )
 功能

read ( )用于面向连接的数据接收

recvfrom()用于无连接的数据接收
 调用格式

read（sockid，buff，bufflen）

recvfrom（sockid，buff，bufflen，suraddr，
addrlen）
客户/服务器流程图
 使用TCP协议的客户/服务器流程图
 使用UDP协议的客户/服务器流程图
使用TCP的客户/服务器流程图
使用UDP的客户/服务器流程图
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
184
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
185
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
186
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
187
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)
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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());
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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();
}
}
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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()));
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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
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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
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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
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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
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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
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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();
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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();
}
}
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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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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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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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201
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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202
Creativity
 TextMultimedia/HyperTextHyperMedi
a Networked HyperMedia
 Non-persistent HTTP Persistent without
pipelining Persistent with pipelining
 C/S FTP with “resume” function
NetAnts at Clients Side
Multiple Server  Peer-to-Peer
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203
Homework
 Reviews：1，2，3，6，11
 Problems：7, 8
 Discussion：8
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204