1-1 HTTP and Electronic Mail Services Objectives: TCP, UDP, HTTP protocols HTTP clients and...
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Transcript of 1-1 HTTP and Electronic Mail Services Objectives: TCP, UDP, HTTP protocols HTTP clients and...
1-1
HTTP and Electronic Mail Services
Objectives: TCP, UDP, HTTP protocols HTTP clients and servers Electronic mail
1-2
TCP/IP As A Protocol Suite
Application Telnet, FTP, Web, e-mail, etc.
Transport TCP, UDP
Network IP, ICMP, IGMP
Data Link +Physical device driver and interface card
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TCP Connection Management
Recall: TCP sender, receiver establish “connection” before exchanging data segments
initialize TCP variables: seq. #s buffers, flow control info
(e.g. RcvWindow) client: connection initiator Socket clientSocket = new
Socket("hostname","port
number"); server: contacted by client Socket connectionSocket =
welcomeSocket.accept();
Three way handshake:
Step 1: client host sends TCP SYN segment to server specifies initial seq # no data
Step 2: server host receives SYN, replies with SYNACK segment
server allocates buffers specifies server initial
seq. #Step 3: client receives SYNACK,
replies with ACK segment, which may contain data
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TCP Connection Establishment
client
SYN, seq=x
server
SYN+ACK, seq=y, ack=x+1
ACK, ack=y+1
CLOSED
LISTEN
SYN_SENTSYN_RCVD
Established
Passive open
SYN/SYN+ACK
ACK
Active open;SYN
SYN+ACK/ACK
Solid line for client
Dashed line for server
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TCP Connection Termination
client
FIN
server
ACK
ACK
FIN
closing
tim
ed w
ait
FIN_WAIT1
FIN_WAIT2
CLOSE_WAIT
LAST_ACK
CLOSED
TIME_WAIT
CLOSED
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Multiplexing/Demultiplexing
Use same communication channel between hosts for several logical communication processes
How does Mux/DeMux work? Sockets: doors between process & host UDP socket: (dest. IP, dest. Port) TCP socket: (src. IP, src. port, dest. IP, dest. Port)
TransportLayer
NetworkLayer
TransportLayer
NetworkLayer
HTTP
FTP
Telnet
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Connectionless demux
UDP socket identified by two-tuple: (dest IP address, dest port number)
When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number
IP datagrams with different source IP addresses and/or source port numbers are directed to the same socket
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Connection-oriented demux
TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets: each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
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Example Standardized Services
DNSFTP (your Assignment #1)
SCPPing
FingerTelnet, SSH
SMTPPOP IMAP
HTTP (your assignment #3)
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Web and HTTP
First some jargon 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 Each object is addressable by a URL Example URL:
www.someschool.edu/someDept/pic.gif
host name path name
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HTTP overview
HTTP: 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 runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
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Steps For Fetching URLs
The browser makes up an absolute URL if URL is relative The browser examines the protocol part and the host
part Browser uses DNS to resolve host name to IP address For HTTP protocol, the browser makes a TCP connection
to IP address and port 80 The port maybe :portno following the hostname in URL
The browser sends a GET request as in: GET /path/somefile.htm HTTP/1.0
The server sends the file and closes the connection. Browser renders file. If it is HTML file then any image
referenced by some <img> tag, then go back to step 4
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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 (application-layer 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
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
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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 HTTPSuppose user enters URL 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 and notifies the client
3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
time
(contains text, references to 10
jpeg images)
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Nonpersistent HTTP (cont.)
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
4. HTTP server closes TCP connection.
time
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Response time modeling
Definition of RTT: time to send a small packet to travel from client to server and back.
Response time: one RTT to initiate TCP
connection one RTT for HTTP request and
first few bytes of HTTP response to return
file transmission timetotal = 2RTT+transmit time
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
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Persistent HTTP
Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate
host resources for each TCP connection
but 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 are sent over the same 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)
GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
header lines
Carriage return, line feed
indicates end of message
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User-server state: cookies
Many major Web sites use cookies
Four components:1) cookie header line in
the HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host and managed by user’s browser
4) back-end database at Web site
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Cookies: keeping “state” (cont.)
client server
usual http request msgusual http response
+Set-cookie: 1678
usual http request msg
cookie: 1678usual http response
msg
usual http request msg
cookie: 1678usual http response msg
cookie-specificaction
cookie-spectificaction
servercreates ID
1678 for user
entry in backend
database
access
acce
ss
Cookie file
amazon: 1678ebay: 8734
Cookie file
ebay: 8734
Cookie file
amazon: 1678ebay: 8734
one week later:
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Cookies (continued)
What cookies can bring:
authorization shopping carts recommendations one-click shopping user session state
(Web e-mail)
Cookies and privacy: cookies permit sites to
learn a lot about you you may supply name
and e-mail to sites search engines use
redirection & cookies to learn yet more
advertising companies obtain info across sites
aside
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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 requestIf-modified-since:
<date> server: response contains
no object if cached copy is up-to-date: HTTP/1.0 304 Not
Modified
cache server
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0
304 Not Modified
object not
modified
HTTP request msgIf-modified-since:
<date>
HTTP responseHTTP/1.0 200 OK
<data>
object modified
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Electronic Mail
Three major components: user agents mail servers simple mail transfer protocol:
SMTP
User Agent a.k.a. “mail reader” composing, editing, reading mail
messages e.g., Eudora, Outlook, elm,
Netscape Messenger, Thunderbird
outgoing, incoming messages stored on server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
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Email using SMTP/IMAP/POP Email consists of two components
Simple Mail Transfer Protocol (SMTP) for email clients to send out emails (e.g. smtp.nd.edu, port 25)
Internet Message Access Protocol (IMAP) or Post Office Protocol (POP)
• email clients to receive your emails (e.g. imap.nd.edu, port 143)
You can use telnet to “talk” to these servers directly
E.g. type ‘telnet smtp.nd.edu 25’ and then type ‘help’
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Email using SMTP/IMAP/POP For SMTP, your client (say Outlook), connects
to smtp.nd.edu and then delivers an email destined for [email protected]
Smtp.nd.edu then locates the SMTP server responsible for AOL. These servers may delegate to other SMTP servers. Eventually it reaches [email protected]
Friend will use IMAP or POP to retrieve this email
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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 no middle server in-between!
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 (mostly)
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Mail message format
SMTP: protocol for exchanging email messages
RFC 2822: standard for text message format:
header lines, e.g., To: From: Subject:different from SMTP
commands! body
the “message”, ASCII characters only (mostly)
header
body
blankline
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Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content
type
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
multimedia datatype, subtype,
parameter declaration
method usedto encode data
MIME version
encoded data
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Mail access protocols
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 msgs on server
HTTP: Hotmail , Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP accessprotocol
receiver’s mail server
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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
number 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 bob S: +OK C: pass hungry S: +OK user successfully logged on
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Mailreader
Maildaemon
SMTP/TCP
Mail gateway
Maildaemon
SMTP/TCP
Mailreader
Maildaemon
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SMTP versus HTTP
SMTP uses persistent connections
SMTP requires message (header & body) to be in 7-bit ASCII
SMTP server uses CRLF.CRLF to determine end of message
Comparison with HTTP: HTTP: pull SMTP: push
both have ASCII command/response interaction, status codes
HTTP: each object encapsulated in its own response msg
SMTP: multiple objects sent in multipart msg