2.4 Electronic Mail P73

Three major components:

• user agents • mail servers • simple mail transfer

protocol: SMTP

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Mail Servers • mailbox( 邮箱 ) contains inco

ming messages (yet to be read) for user

• message queue( 邮件队列 ) of outgoing (to be sent) mail messages

• SMTP protocol between mail servers to send email messages– client: sending mail server– “server”: receiving mail se

rver

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

User Agent•a.k.a. “mail reader”•composing, editing, reading,saving mail messages•e.g., Foxmail, Outlook, Netscape Messenger•outgoing, incoming messages stored on server

P73-75

Electronic Mail: smtp [RFC 821] P76-77

• uses TCP to reliably transfer email msg from client to server, port 25

• three phases of transfer– Handshaking( 握手 ) – transfering messages– Closing connection

• direct transfer: sending server to receiving server• command/response interaction

– commands: ASCII text– response: status code and phrase

• messages must be in 7-bit ASCII

• The TCP connection is a direct connection between this two mail server.

• SMTP use persistent connection( 持久连接 ).

Bob’sMail

server

Alice’sMail

server

SMTP

Internetuser agent

user agent

Sample smtp interaction eg.telnet smtp.126.com 25<CR>

S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

smtp: final words

• smtp uses persistent connections• smtp requires that message (header & body)

be in 7-bit ASCII• smtp server uses CRLF.CRLF to determine

end of message

Comparison with http P77

similar:• both use persistent

connection

• both have ASCII command/response interaction, status codes

difference:

•http: pull protocol( 内拉式协议 )•smtp: push protocol( 外推式协议 )

•http: each object is encapsulated in its own response message without limited data type.•smtp: multiple objects message sent in one message

Mail message format

smtp: protocol for exchanging email msgs

RFC 822: standard for text message format:

• header lines( 首部行 ), e.g.,– To:– From:– Subject:

• body– the “message”, ASCII chara

cters only

header

body

blankLine

(CRLF)

Multipurpose Internet Mail Extensions P77-81

• MIME: ( 多用途因特网邮件扩展 ), RFC 2045, 2046 (P78)

• additional lines in msg header declare MIME content type

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

Content-Type: type/subtype; parameters

Text• example subtypes: plain, html

Image• example subtypes: jpeg, gif

Audio• exampe subtypes: basic (8-bit

u-law encoded), 32kadpcm (32 kbps formatcoding)

Video• example subtypes: mpeg, quicktime

Application• other data that must be processed

by reader before “viewable”• example subtypes: msword, msex

cel

Multipart

When a multimedia message contains more than one object,the message typically has: content type:

multipart/mix

类型名 / 子类型名 参数(P79)

Multipart TypeFrom: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart -- StartOfNextPart Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.-- StartOfNextPartContent-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data -- StartOfNextPart --

received messge

Received:from crepes.fr by hamburger.edu;12 Oct 98From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

Mail access protocols P81

• SMTP: delivery from sender to recipient• Mail access protocol( 邮件访问协议 ): 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 msgs on server

– HTTP: Hotmail , Yahoo! Mail, etc.

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server

POP3 protocol P82-83

• Used to transfer mail from the recipient’s mail server to the recipient’s user agent

• Underlying protocol is TCP,port 110• Three phases:authorization,transaction,update

user agent can be configured to be “download and delete” or “download and keep”

POP3 protocolauthorization phase• client commands:

– user: declare username– pass: password

• server responses– +OK– -ERR

transaction phase, client:• list: list message numbers• retr: retrieve message by nu

mber• dele: delete• quit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on

• SMTP: delivery from sender to recipient

• Mail access protocol: retrieval from server

– POP: user manages mail message in his host.Its simple.

– IMAP: user can manage mail messages on the remote mail server.Its complex.

– Electronic Mail based on Web(HTTP):

user agent is browser

useragent

sender’s mail server

useragent

HTTP SMTP HTTP

receiver’s mail server

2.5 DNS: Domain Name System P84

People: many identifiers:– ID, name, Passport #

Internet hosts, routers:– IP address (32 bit) - used for addressing datagrams

– hose name( 主机名 ), e.g., gaia.cs.umass.edu - used by humans

– domain name( 域名 ):eg.www.cnu.edu.cn

DNS: Domain Name System P85-86

Domain Name System:• distributed database implemented in hierarchy of many name s

ervers• application-layer protocol in hosts, routers, name servers to co

mmunicate to resolve( 解析 ) names (address/name translation)• underlying protocol is UDP,port 53• function:resolve between domain name and IP address

other services: P86• Host aliasing ( 别名 ) • Mail server aliasing• Load distributing

DNS: Domain Name System

domain name:hierarchical structurethird level dn .second level dn. top level dn

root

top level

shop com org edu gov cn uk ….

cnu pku bnu moe tsinghua

ie www ftp

www ftpeg. www.cnu.edu.cn

ftp.ie.cnu.edu.cn

Q: How to map between IP addresses and name ?

Second level

DNS name servers

problems with centralized DNS in Internet? P87• single point of failure( 单点故障 )

• traffic volume( 巨大访问量 )

• distant centralized database• maintenance• no server has all name-to-IP address mappings

doesn’t scale!( 不可扩展 )

local name servers( 局部名称服务器 ):–each ISP, company has local (default) name server–host DNS query first goes to local name server

root name servers( 根名称服务器 ):– contacted by local name server that can not resolve name–if it hasn’t name mapping, contacts authoritative name server–gets mapping–returns mapping to local name server–~ dozen root name servers worldwide

authoritative name server( 授权名称服务器 ):–every host has its domain name registered in authoritative name server: stores that host’s IP address, name–can perform name/address translation for that host’s name

Distributed DNS in Internet:

some name server maybe authoritative name server and local name server

DNS: Root name servers

Simple DNS example P86-91

host surf.eurecom.fr wants IP address of gaia.cs.umass.edu

1. Contacts its local DNS server, dns.eurecom.fr

2. dns.eurecom.fr contacts root name server, if necessary

3. root name server contacts authoritative name server, dns.umass.edu, if necessary

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

1

23

4

5

6

three query msg and three reply msg

DNS exampleRoot name server:• may not know author

atiative name server• may know intermedi

ate name server: who to contact to find authoritative name server

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

four query msg and four reply msg

DNS: recursive queries

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

recursive query:• puts burden of name

resolution on contacted name server

• heavy load?

( 递归查询 )

DNS: iterated queries

iterated query:

• contacted server replies with name of server to contact

• “I don’t know this name, but ask this server”

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query

both iterated and recursive quiries

( 迭代查询 )

DNS records P91

DNS: distributed db storing resource records( 资源记录 ) (RR)

• Type=NS– name is domain (e.g. umass.edu ,foo.com)– value is IP address of authoritative name server for this domain– eg.(www.cnu.edu.cn, dns.edu.cn, NS,TTL)

RR format: (name, value, type,ttl)

• Type=A– name is hostname

– value is IP address

– eg.(www.cnu.edu.cn,202.204.220.53,A,TTL)

DNS recordsDNS: distributed db storing resource records( 资源记录 ) (RR)

RR format: (name, value, type,ttl)

• Type=CNAME– name is an alias name for some “cannonical”( 规范主机名 ) (t

he real) name

– value is cannonical name

– eg.(www.cnu.edu.cn,relay1.bar.cnu.edu.cn,CNAME,TTL)

• Type=MX– value is hostname of mail server associated with “cannonic

al” name

– eg.(hotmail.com,relay1.west-coast.hotmail.com,MX,TTL)

a company can use same alias name on its mail server and Web server .why?

DNS: caching and updating records

• once (any) name server learns mapping( 映射 ), it caches mapping– cache entries timeout (disappear) after some ti

me• update/notify mechanisms under design by IETF

– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html

DNS messageDNS protocol : query( 查询 ) and reply ( 响应 ) messages, both with same message format

msg header• identification: 16 bit # for

query, reply to query ,uses same #

• flags:

– query or reply

– reply is authoritative

– recursion desired

– recursion available

P92

DNS protocol, messages

Host name, type fields for a query

RRs in reponseto query

records for otherauthoritative servers

additional “helpful”info that may be used

P92

Socket programming

Socket API• introduced in BSD4.1 UNIX, 1981• explicitly created, used, released b

y apps • client/server mode • two types of transport service via s

ocket API: – unreliable datagram(UDP) – reliable, byte stream-oriented

(TCP)

a host-local, application-created/own

ed, OS-controlled interface (a “door”) into which

application process can both send and

receive messages to/from another (remote

or local) application

process

socket

Goal: learn how to build client/server application that communicate using sockets

Socket-programming using TCPSocket: a door between application process and end-end-

transport protocol (UCP or TCP)TCP service: reliable,in-order transfer of bytes from one

process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

Socket programming with TCP

Server waits to be contacted:

• 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

Socket programming with TCP

Example client-server app:• client reads line from standar

d input (inFromUser stream) , sends to server via socket (outToServer stream)

• server reads line from socket• server converts line to upper

case, sends back to client• client reads, prints modified

line from socket (inFromServer stream)

outT

oSer

ver

to network from network

inFr

omS

erve

r

inFr

omU

ser

keyboard monitor

Process

clientSocket

inputstream

inputstream

outputstream

TCPsocket

Input stream: sequence of bytesinto process

output stream: sequence of bytes out of process

Clientprocess

client TCP socket

Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

Server (running on hostid) Client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

Example: Java client (TCP)

import java.io.*; import java.net.*; class TCPClient {

public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create client socket,

connect to server

Createoutput stream

attached to socket

Example: Java client (TCP), cont.

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server

Example: Java server (TCP)

import java.io.*; import java.net.*;

class TCPServer {

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()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contact

by client

Create inputstream, attached

to socket

Example: Java server (TCP), cont

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); } } }

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection

Socket programming with UDP

UDP: no “connection” between client and server

• no handshaking• sender explicitly attaches IP

address and port of destination

• server must extract IP address, port of sender from received datagram

UDP: transmitted data may be received out of order, or lost

application viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server

Client/server socket interaction: UDP

closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port umber

Example: Java client (UDP)

sendP

ack

et

to network from network

rece

iveP

ack

et

inF

rom

Use

r

keyboard monitor

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sends packet (TCP sent “byte stream”)

Input: receives packet (TCP received “byte stream”)

Clientprocess

client UDP socket

Example: Java client (UDP)

import java.io.*; import java.net.*; 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();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS

Example: Java client (UDP), cont.

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }

}

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

Example: Java server (UDP)

import java.io.*; import java.net.*; 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) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receivedatagra

m

Example: Java server (UDP), cont

String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }

}

Get IP addrport #, of

sender

Write out datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client

Socket programming: references

C-language tutorial (audio/slides): • “Unix Network Programming” (J. Kurose),http://manic.cs.umass.edu/~amldemo/courseware/intro.

Java-tutorials:• “All About Sockets” (Sun tutorial), http://www.javaworl

d.com/javaworld/jw-12-1996/jw-12-sockets.html• “Socket Programming in Java: a tutorial,” http://www.ja

vaworld.com/javaworld/jw-12-1996/jw-12-sockets.html

Part 2: Summary

• application service requirements:– reliability, bandwidth, delay

• client-server paradigm• Internet transport service

model– connection-oriented, reliable:

TCP– unreliable, datagrams: UDP

Our study of network apps now complete!

• specific protocols:– http– ftp– smtp, pop3– dns

• socket programming– client/server implementati

on– using tcp, udp sockets

Part 2: Summary

• 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

Most importantly: learned about protocols

• control vs. data msgs– in-band, out-of-band

• centralized vs. decentralized • stateless vs. stateful• reliable vs. unreliable msg t

ransfer • “complexity at network edg

e”• security: authentication