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Transcript of 2: Application Layer 1 Chapter 2: Application Layer Our goals: r conceptual, implementation aspects...
2: Application Layer 1
Chapter 2: Application LayerOur 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 P2P
programming network applications socket API
2: Application Layer 2
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 3
Some network apps
e-mail web instant messaging remote login P2P file sharing multi-user network
games streaming stored
video clips
voice over IP real-time video
conferencing social networking …
2: Application Layer 4
Creating a network app
write programs that run on (different) end
systems communicate over
network e.g., web server software
communicates with browser software
No need to write software for network-core devices Network-core devices do
not run user applications applications on end
systems allows for rapid app development, propagation
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
2: Application Layer 5
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 6
Application architectures
Client-server (C/S) Peer-to-peer (P2P) Hybrid of C/S and P2P
2: Application Layer 7
Client-server architecture
server: always-on host permanent IP address server farms for
scalingclients:
communicate with server may be intermittently
connected may have dynamic IP
addresses do not communicate
directly with each other
client/server
2: Application Layer 8
Pure P2P architecture
no always-on server arbitrary end systems
directly communicate peers are
intermittently connected and change IP addresses
Highly scalable but difficult to manage
peer-peer
2: Application Layer 9
Hybrid of client-server and P2PSkype
voice-over-IP P2P application centralized server: finding address of
remote party: client-client connection: direct (not through
server) Instant messaging
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
2: Application Layer 10
Processes communicating
Process: program running within a host.
within 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
2: Application Layer 11
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
process
TCP withbuffers,variables
socket
host orserver
process
TCP withbuffers,variables
socket
host orserver
Internet
controlledby OS
controlled byapp developer
API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later)
2: Application Layer 12
Addressing processes to receive messages,
process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host suffice for identifying the process?
2: Application Layer 13
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: No, many
processes can be running on 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…
2: Application Layer 14
App-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
Rules for when and how processes send & respond to messages
Public-domain protocols:
defined in RFCs allows for
interoperability e.g., HTTP, SMTPProprietary protocols: e.g., Skype
2: Application Layer 15
What transport service does an app need?
Data loss some apps (e.g., audio)
can tolerate some loss other apps (e.g., file
transfer, telnet) require 100% reliable data transfer
Timing some apps (e.g.,
Internet telephony, interactive games) require low delay to be “effective”
Throughput some apps (e.g.,
multimedia) require minimum amount of throughput to be “effective”
other apps (“elastic apps”) make use of whatever throughput they get
Security Encryption, data
integrity, …
2: Application Layer 16
Transport service requirements of common apps
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive gamesinstant messaging
Data loss
no lossno lossno lossloss-tolerant
loss-tolerantloss-tolerantno loss
Throughput
elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
2: Application Layer 17
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 throughput guarantees, security
UDP service: unreliable data transfer
between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security
Q: why bother? Why is there a UDP?
2: Application Layer 18
Internet apps: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]HTTP (eg Youtube), RTP [RFC 1889]SIP, RTP, proprietary(e.g., Skype)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
typically UDP
2: Application Layer 19
Chapter 2: Application layer
2.1 Principles of network applications app architectures app requirements
2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 20
Web/WWW 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
WWW Architectural Overview
(a) A Web page (b) The page reached by clicking on Department of Animal Psychology.
2: Application Layer 21
WWW Architectural Overview
The parts of the Web model.
2: Application Layer 22
HTML – HyperText Markup Language
(a) The HTML for a sample Web page. (b) The formatted page.
(b)
2: Application Layer 23
HTML
A selection of common HTML tags. some can have additional parameters.
2: Application Layer 24
Dynamic Web Documents
Steps in processing the information from an HTML form.
2: Application Layer 25
Dynamic Web Documents
A sample HTML page with embedded PHP.
2: Application Layer 26
2: Application Layer 27
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
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
2: Application Layer 28
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
aside
2: Application Layer 29
HTTP connections
Nonpersistent HTTP At most one object is
sent over a TCP connection.
Persistent HTTP Multiple objects can
be sent over single TCP connection between client and server.
2: Application Layer 30
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, notifying 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)
2: Application Layer 31
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
2: Application Layer 32
Non-Persistent HTTP: Response timeDefinition of RTT: time for 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
2: Application Layer 33
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
client sends requests as soon as it encounters a referenced object
as little as one RTT for all the referenced objects
2: Application Layer 34
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
2: Application Layer 35
HTTP request message: general format
2: Application Layer 36
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
2: Application Layer 37
Method types
HTTP/1.0 GET POST HEAD
asks server to leave requested object out of response
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
2: Application Layer 38
HTTP response message
HTTP/1.1 200 OK Connection closeDate: 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 ...
status line(protocol
status codestatus phrase)
header lines
data, e.g., requestedHTML file
2: Application Layer 39
HTTP response status 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
In first line in server->client response message.A few sample codes:
2: Application Layer 40
Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
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
telnet cis.poly.edu 80
2. Type in a GET HTTP request:
GET /~ross/ HTTP/1.1Host: cis.poly.edu
By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server
3. Look at response message sent by HTTP server!
2: Application Layer 41
User-server state: cookies
Many major Web sites use cookies
Four components:1) cookie header line of
HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host, managed by user’s browser
4) back-end database at Web site
Example: Susan always access
Internet always from PC visits specific e-
commerce site for first time
when initial HTTP requests arrives at site, site creates: unique ID entry in backend
database for ID
2: Application Layer 42
Cookies: keeping “state” (cont.)
client server
usual http response msg
usual http response msg
cookie file
one week later:
usual http request msg
cookie: 1678cookie-specificaction
access
ebay 8734usual http request
msgAmazon server
creates ID1678 for usercreate
entry
usual http response Set-cookie: 1678
ebay 8734amazon 1678
usual http request msg
cookie: 1678cookie-spectificaction
accessebay 8734amazon 1678
backenddatabase
2: Application Layer 43
Cookies (continued)
What cookies can bring: authorization shopping carts recommendations 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
aside
How to keep “state”: protocol endpoints: maintain
state at sender/receiver over multiple transactions
cookies: http messages carry state
2: Application Layer 44
Web caches (proxy 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
Goal: satisfy client request without involving origin server
client
Proxyserver
client
HTTP request
HTTP response
HTTP request HTTP request
origin server
origin server
HTTP response HTTP response
2: Application Layer 45
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)
2: Application Layer 46
Caching example
Assumptions average object size = 100,000
bits avg. 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% utilization on access link = 100% total delay = Internet delay +
access delay + LAN delay = 2 sec + minutes + milliseconds
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
2: Application Layer 47
Caching example (cont)
possible solution increase bandwidth of
access link to, say, 10 Mbps
consequence utilization on LAN = 15% utilization on access link =
15% Total delay = Internet delay +
access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade
originservers
public Internet
institutionalnetwork 10 Mbps LAN
10 Mbps access link
institutionalcache
2: Application Layer 48
Caching example (cont)
possible solution: install cache
suppose hit rate is 0.4consequence 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
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
2: Application Layer 49
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
2: Application Layer 50
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 51
FTP: the file transfer protocol
transfer file to/from remote host client/server model
client: side that initiates transfer (either to/from remote)
server: remote host ftp: RFC 959 ftp server: port 21
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
2: Application Layer 52
FTP: separate control, data connections
FTP client contacts FTP server at port 21, TCP is transport protocol
client authorized over control connection
client browses remote directory by sending commands over control connection.
when server receives file transfer command, server opens 2nd TCP connection (for file) to client
after transferring one file, server closes data connection.
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
server opens another TCP data connection to transfer another file.
control connection: “out of band”
FTP server maintains “state”: current directory, earlier authentication
2: Application Layer 53
FTP commands, responses
Sample commands: sent as ASCII text over
control channel USER username PASS password LIST return list of file in
current directory RETR filename retrieves
(gets) file STOR filename stores
(puts) file onto remote host
Sample return codes status code and phrase
(as in HTTP) 331 Username OK,
password required 125 data connection
already open; transfer starting
425 Can’t open data connection
452 Error writing file
[01] ftp nic.ddn.mil[02] connected to nic.ddn.mil[03] 220 nic FTP server (Sunos 4.1)ready.[04] Name: anonymous[05] 331 Guest login ok, send ident as password.[06] Password: [email protected][07] 230 Guest login ok, access restrictions apply.[08] ftp> cd rfc[09] 250 CWD command successful.[10] ftp> get rfc1261.txt nicinfo[11] 200 PORT command successful.[12] 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 54
Sample FTP interaction
2: Application Layer 55
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
Electronic Mail
Some smileys. They will not be on the final exam :-).
2: Application Layer 56
2: Application Layer 57
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,
Mozilla Thunderbird outgoing, incoming
messages stored on server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
The User Agent
Envelopes and messages. (a) Paper mail. (b) Electronic mail.2: Application Layer 58
2: Application Layer 59
Electronic Mail: mail servers
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
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
2: Application Layer 60
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 61
Scenario: Alice sends message to Bob1) 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
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
useragent
mailserver
mailserver user
agent
1
2 3 4 56
2: Application Layer 62
Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... 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
2: Application Layer 63
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 64
SMTP: final words
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
2: Application Layer 65
Mail message format
SMTP: protocol for exchanging email msgs
RFC 822: standard for text message format:
header lines, e.g., To: From: Subject:different from SMTP
commands! body
the “message”, ASCII characters only
header
body
blankline
Message Formats – RFC 822
RFC 822 header fields related to message transport.
2: Application Layer 66
Message Formats – RFC 822 (2)
Some fields used in the RFC 822 message header.
2: Application Layer 67
MIME – Multipurpose Internet Mail Extensions
Problems with international languages:• Languages with accents
(French, German).• Languages in non-Latin alphabets
(Hebrew, Russian).• Languages without alphabets
(Chinese, Japanese).• Messages not containing text at all
(audio or images).
2: Application Layer 68
2: Application Layer 69
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
MIME
RFC 822 headers added by MIME.
2: Application Layer 70
MIME
The MIME types and subtypes defined in RFC 2045.
2: Application Layer 71
MIME
A multipart message containing enriched and audio alternatives.2: Application Layer 72
2: Application Layer 73
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: gmail, Hotmail, Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP accessprotocol
receiver’s mail server
2: Application Layer 74
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
2: Application Layer 75
POP3 (more) and IMAPMore about POP3 Previous example
uses “download and delete” mode.
Bob cannot re-read e-mail 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
Comparison of POP3 and IMAP
2: Application Layer 76
2: Application Layer 77
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 78
DNS: Domain Name System
People: many identifiers: SSN, name, passport #
Internet hosts, routers: IP address (32 bit) -
used for addressing datagrams
“name”, e.g., ww.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”
The DNS Name Space
A portion of the Internet domain name space.
2: Application Layer 79
2: Application Layer 80
DNS
Why not centralize DNS? single point of failure traffic volume distant centralized
database maintenance
doesn’t scale!
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
2: Application Layer 81
Root DNS Servers
com DNS servers org DNS servers edu DNS servers
poly.eduDNS servers
umass.eduDNS servers
yahoo.comDNS servers
amazon.comDNS servers
pbs.orgDNS servers
Distributed, Hierarchical Database
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 82
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
13 root name servers worldwideb USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)
i Autonomica, Stockholm (plus 28 other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul, Paris, SF)
a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)
2: Application Layer 83
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 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 84
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 85
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
23
4
5
6
authoritative DNS serverdns.cs.umass.edu
78
TLD DNS server
DNS name resolution example
Host at cis.poly.edu 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”
2: Application Layer 86
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
2
45
6
authoritative DNS serverdns.cs.umass.edu
7
8
TLD DNS server
3recursive query: puts burden of
name resolution on contacted name server
heavy load?
DNS name resolution example
2: Application Layer 87
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 88
DNS records
DNS: distributed db storing resource records (RR)
Type=NS name is domain (e.g.
foo.com) value is hostname of
authoritative name server for this domain
RR format: (name, value, type, ttl)
Type=A name is hostname value is IP address
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 89
DNS protocol, messagesDNS 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 90
DNS protocol, messages
Name, type fields for a query
RRs in responseto query
records forauthoritative servers
additional “helpful”info that may be used
2: Application Layer 91
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 92
Chapter 2: Application layer
2.1 Principles of network applications app architectures app requirements
2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
What is P2P?
“the sharing of computer resources and services by direct exchange of information”
2: Application Layer 93
What is P2P?
“P2P is a class of applications that take advantage of resources – storage, cycles, content, human presence – available at the edges of the Internet. Because accessing these decentralized resources means operating in an environment of unstable and unpredictable IP addresses P2P nodes must operate outside the DNS system and have significant, or total autonomy from central servers”
2: Application Layer 94
What is P2P?
“A distributed network architecture may be called a P2P network if the participants share a part of their own resources. These shared resources are necessary to provide the service offered by the network. The participants of such a network are both resource providers and resource consumers”
2: Application Layer 95
What is P2P?
Various definitions seem to agree on sharing of resources direct communication between equals
(peers) no centralized control
2: Application Layer 96
What is a peer?
“…an entity with capabilities similar to other entities in the system.”
“…a network entity with which one performs peering operations.”
“…a participant that acts as both client and server.”
2: Application Layer 97
Client/Server Architecture
Well known, powerful, reliable server is a data source
Clients request data from server
Very successful model WWW (HTTP), FTP,
Web services, etc.
Server
Client
Client Client
Client
Internet
* Figure from http://project-iris.net/talks/dht-toronto-03.ppt2: Application Layer 98
Client/Server Limitations
Scalability is hard to achieve Presents a single point of failure Requires administration Unused resources at the network edge
P2P systems try to address these limitations
2: Application Layer 99
P2P Architecture
All nodes are both clients and servers Provide and consume
data Any node can initiate
a connection
No centralized data source “The ultimate form of
democracy on the Internet”
“The ultimate threat to copy-right protection on the Internet”
* Content from http://project-iris.net/talks/dht-toronto-03.ppt
Node
Node
Node Node
Node
Internet
2: Application Layer 100
P2P Network Characteristics
Clients are also servers and routers Nodes contribute content, storage, memory, CPU
Nodes are autonomous (no administrative authority)
Network is dynamic: nodes enter and leave the network “frequently”
Nodes collaborate directly with each other (not through well-known servers)
Nodes have widely varying capabilities
2: Application Layer 101
P2P Goals and Benefits Efficient use of resources
Unused bandwidth, storage, processing power at the “edge of the network”
Scalability No central information, communication and computation bottleneck Aggregate resources grow naturally with utilization
Reliability Replicas Geographic distribution No single point of failure
Ease of administration Nodes self-organize Built-in fault tolerance, replication, and load balancing Increased autonomy
Anonymity – Privacy not easy in a centralized system
Dynamism highly dynamic environment ad-hoc communication and collaboration
2: Application Layer 102
P2P Applications
File sharing (Napster, Gnutella, Kazaa, …)
Multiplayer games (Unreal Tournament, DOOM, …)
Collaborative applications (ICQ, shared whiteboard, …)
Distributed computation (Seti@home, …)
Ad-hoc networks (…)
…
2: Application Layer 103
P2P Goals/Benefits
Cost sharing Resource aggregation Improved scalability/reliability Increased autonomy Anonymity/privacy Dynamism Ad-hoc communication
2: Application Layer 104
P2P Challenges
Decentralization Scalability and Performance Anonymity Fairness Dynamism Security Transparency Fault Resilience and Robustness
2: Application Layer 105
Popular file sharing P2P Systems
Napster, Gnutella, Kazaa, Freenet
Large scale sharing of files. User A makes files (music, video, etc.) on
their computer available to others User B connects to the network, searches
for files and downloads files directly from user A
Issues of copyright infringement
2: Application Layer 106
Research Areas
Peer discovery and group management Data placement and searching Reliable and efficient file exchange Security/privacy/anonymity/trust
2: Application Layer 107
Design Concerns Group Management
Per-node state Load balancing Fault tolerance/resiliency
Search Bandwidth usage Time to locate item Success rate Fault tolerance/resiliency
2: Application Layer 108
Approaches
Centralized Unstructured Structured (Distributed Hash Tables)
2: Application Layer 109
Centralized index
original “Napster” design
1) when peer connects, it informs central server: IP address content
2) Alice queries for “Hey Jude”
3) Alice requests file from Bob
centralizeddirectory server
peers
Alice
Bob
1
1
1
12
3
2: Application Layer 110
Centralized modelBob Alice
JaneJudy
file transfer is decentralized, but locating content is highly centralized
2: Application Layer 111
Centralized Benefits:
Low per-node state Limited bandwidth usage Short location time High success rate Fault tolerant
Drawbacks: Single point of failure Limited scale Possibly unbalanced load
copyright infringement
Bob Alice
JaneJudy
2: Application Layer 112
Napster
program for sharing files over the Internet a “disruptive” application/technology? history:
5/99: Shawn Fanning (freshman, Northeasten U.) founds Napster Online music service
12/99: first lawsuit 3/00: 25% UWisc traffic Napster 2000: est. 60M users 2/01: US Circuit Court of
Appeals: Napster knew users violating copyright laws
7/01: # simultaneous online users:Napster 160K, Gnutella: 40K, Morpheus: 300K
2: Application Layer 113
Napster: how does it work
Application-level, client-server protocol over point-to-point TCP
Four steps: Connect to Napster server Upload your list of files (push) to server. Give server keywords to search the full list with. Select “best” of correct answers. (pings)
2: Application Layer 114
Napster
napster.com
users
File list is uploaded
1.
2: Application Layer 115
Napster
napster.com
user
Requestand
results
User requests search at server.
2.
2: Application Layer 116
Napster
napster.com
user
pingspings
User pings hosts that apparently have data.
Looks for best transfer rate.
3.
2: Application Layer 117
Napster
napster.com
user
Retrievesfile
User retrieves file
4.
2: Application Layer 118
Napster
Central Napster server Can ensure correct results Fast search
Bottleneck for scalability Single point of failure Susceptible to denial of service
• Malicious users• Lawsuits, legislation
Hybrid P2P system – “all peers are equal but some are more equal than others” Search is centralized File transfer is direct (peer-to-peer)
2: Application Layer 119
Unstructured
fully distributed no central server
used by Gnutella Each peer indexes
the files it makes available for sharing (and no other files)
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 120
Gnutella: Query flooding
Query
QueryHit
Query
Query
QueryHit
Query
Query
QueryHit
File transfer:HTTP
Query messagesent over existing TCPconnections peers forwardQuery message QueryHit sent over reversepath
2: Application Layer 121
Gnutella: Peer joining
1. 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 message4. Alice receives many Pong messages, and can
then setup additional TCP connections
2: Application Layer 122
Unstructured
Bob
Alice
Jane
Judy
Carl
Scalability:limited scopeflooding
2: Application Layer 123
Unstructured
Gnutella model Benefits:
Limited per-node state Fault tolerant
Drawbacks: High bandwidth usage Long time to locate item No guarantee on success
rate Possibly unbalanced load
Bob
Alice
Jane
Judy
Carl
2: Application Layer 124
Gnutella
Searching by flooding: If you don’t have the file
you want, query 7 of your neighbors.
If they don’t have it, they contact 7 of their neighbors, for a maximum hop count of 10.
Requests are flooded, but there is no tree structure.
No looping but packets may be received twice.
Reverse path forwarding
* Figure from http://computer.howstuffworks.com/file-sharing.htm
2: Application Layer 125
Gnutella
fool.* ?
TTL = 2
2: Application Layer 126
Gnutella
TTL = 1
TTL = 1
IPX:fool.her
fool.herX
TTL = 1
2: Application Layer 127
Gnutella
fool.you
fool.meY
IPY:fool.me fool.you2: Application Layer 128
Gnutella
IPY:fool.me fool.you 2: Application Layer 129
Gnutella: strengths and weaknesses pros:
flexibility in query processing complete decentralization simplicity fault tolerance/self-organization
cons: severe scalability problems susceptible to attacks
Pure P2P system
2: Application Layer 130
Gnutella: initial problems and fixes 2000: avg size of reachable network only 400-
800 hosts. Why so small? modem users: not enough bandwidth to provide
search routing capabilities: routing black holes
Fix: create peer hierarchy based on capabilities previously: all peers identical, most modem black
holes preferential connection:
• favors routing to well-connected peers• favors reply to clients that themselves serve large
number of files: prevent freeloading
2: Application Layer 131
Structured
FreeNet, Chord, CAN, Tapestry, Pastry model
001 012
212
305
332
212 ?
212 ?
2: Application Layer 132
Structured
FreeNet, Chord, CAN, Tapestry, Pastry model
Benefits: Manageable per-node state Manageable bandwidth
usage and time to locate item
Guaranteed success
Drawbacks: Possibly unbalanced load Harder to support fault
tolerance
001 012
212
305
332
212 ?
212 ?
2: Application Layer 133
Unstructured vs Structured P2P
The systems we described do not offer any guarantees about their performance (or even correctness)
Structured P2P Scalable guarantees on numbers of hops to
answer a query Maintain all other P2P properties (load balance,
self-organization, dynamic nature)
Approach: Distributed Hash Tables (DHT)
2: Application Layer 134
Improvements: SuperPeers
KaZaA model Hybrid centralized and unstructured Advantages and disadvantages?
2: Application Layer 135
Hierarchical Overlay
between centralized index, query flooding approaches
each peer is either a super node or assigned to a super node TCP connection between
peer and its super node. TCP connections between
some pairs of super nodes.
Super node tracks content in its children
ordinary peer
group-leader peer
neighoring re la tionshipsin overlay network
2: Application Layer 136
Kazaa (Fasttrack network)
Hybrid of centralized Napster and decentralized Gnutella hybrid P2P system
Super-peers act as local search hubs Each super-peer is similar to a Napster server for a small
portion of the network Super-peers are automatically chosen by the system based
on their capacities (storage, bandwidth, etc.) and availability (connection time)
Users upload their list of files to a super-peer Super-peers periodically exchange file lists You send queries to a super-peer for files of interest
2: Application Layer 137
File Distribution: Server-Client vs P2PQuestion : How much time to distribute file
from one server to N peers?
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
File, size F
us: server upload bandwidth
ui: peer i upload bandwidth
di: peer i download bandwidth
2: Application Layer 138
File distribution time: server-client
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F server
sequentially sends N copies: NF/us time
client i takes F/di
time to download
increases linearly in N(for large N)
= dcs = max { NF/us, F/min(di) }i
Time to distribute F to N clients using
client/server approach
2: Application Layer 139
File distribution time: P2P
us
u2d1 d2u1
uN
dN
Server
Network (with abundant bandwidth)
F server must send one
copy: F/us time
client i takes F/di time to download
NF bits must be downloaded (aggregate) fastest possible upload rate: us + ui
dP2P = max { F/us, F/min(di) , NF/(us + ui) }i
2: Application Layer 140
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Min
imu
m D
istr
ibut
ion
Tim
e P2P
Client-Server
Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
2: Application Layer 141
File distribution: BitTorrent Efficient content distribution system using file
swarming. Usually does not perform all the functions of a typical p2p system, like searching.
CacheLogic estimated (around 2003 or so) that BitTorrent Traffic accounts for roughly 35% of all traffic on the Internet.
Author: Bram Cohen
2: Application Layer 142
File distribution: BitTorrent
tracker: tracks peers participating in torrent
torrent: group of peers exchanging chunks of a file
obtain listof peers
trading chunks
peer
P2P file distribution
2: Application Layer 143
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) remain2: Application Layer 144
BT: File sharing
To share a file or group of files, a peer first creates a .torrent file, a small file that contains: metadata about the files to be shared, and Information about the tracker, the computer that
coordinates the file distribution.
Peers first obtain a .torrent file, and then connect to the specified tracker, which tells them from which other peers to download the pieces of the file.
2: Application Layer 145
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
TrackerWeb Server
.torr
ent
2: Application Layer 146
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Get-announce
Web Server
2: Application Layer 147
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Response-peer list
Web Server
2: Application Layer 148
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
Shake-hand
Web Server
Shake-hand
2: Application Layer 149
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
pieces
pieces
Web Server
2: Application Layer 150
Overall Architecture
Web page with link to .torrent
A
B
C
Peer
[Leech]
Downloader
“US”
Peer
[Seed]
Peer
[Leech]
Tracker
piecespieces
pieces
Web Server
2: Application Layer 151
BT: The .torrent file
The URL of the tracker Pieces <hash1, hash 2,…, hash n> Piece length Name of the file Length of the file
2: Application Layer 152
BT: The Tracker
IP address, port, peer id State information (Completed or
Downloading) Returns a random list of peers
2: Application Layer 153
BT: Pulling Chunks
at any given time, different peers have different subsets of file chunks
periodically, a peer (Alice) asks each neighbor for list of chunks that they have.
Alice sends requests for her missing chunks
2: Application Layer 154
BT: Pieces and Sub-Pieces
A piece is broken into sub-pieces ... typically 16KB in size
Until a piece is assembled, only download the sub-pieces of that piece only
This policy lets pieces assemble quickly
2: Application Layer 155
BT: Pipelining
When transferring data over TCP, always have
several requests pending at once, to avoid a
delay between pieces being sent. At any point in
time, some number, typically 5, are requested
simultaneously.
Every time a piece or a sub-piece arrives, a new
request is sent out.
2: Application Layer 156
BT: Piece Selection
The order in which pieces are selected by different peers is critical for good performance
If an inefficient policy is used, then peers may end up in a situation where each has all identical set of easily available pieces, and none of the missing ones.
If the original seed is prematurely taken down, then the file cannot be completely downloaded! What are “good policies?”
2: Application Layer 157
BT: Chunk Selection
Strict Priority First Priority
Rarest First General rule
Random First Piece Special case, at the beginning
Endgame Mode Special case
2: Application Layer 158
Random First Piece
Initially, a peer has nothing to trade Important to get a complete piece ASAP Select a random piece of the file and
download it
2: Application Layer 159
Rarest Piece First
Determine the pieces that are most rare
among your peers, and download those
first.
This ensures that the most commonly
available pieces are left till the end to
download.
2: Application Layer 160
BT: Sending Chunks
tit-for-tat Alice sends chunks to four neighbors currently
sending her chunks at the highest rate re-evaluate top 4 every 10 secs
every 30 secs: randomly select another peer, starts sending chunks newly chosen peer may join top 4 “optimistically unchoke”
2: Application Layer 161
BT: Choking
Choking is a temporary refusal to upload. It is
one of BitTorrent’s most powerful idea to deal
with free riders (those who only download but
never upload).
Tit-for-tat strategy is based on game-theoretic
concepts.
2: Application Layer 162
BitTorrent: Tit-for-tat(1) Alice “optimistically unchokes” Bob
(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates(3) Bob becomes one of Alice’s top-four providers
With higher upload rate, can find better trading partners & get file faster!
2: Application Layer 163
Optimistic unchoking
A BitTorrent peer has a single “optimistic unchoke” to which it uploads regardless of the current download rate from it. This peer rotates every 30s
Reasons: To discover currently unused connections
are better than the ones being used To provide minimal service to new peers
2: Application Layer 164
Upload-Only mode
Once download is complete, a peer has no download rates to use for comparison nor has any need to use them. The question is, which nodes to upload to?
Policy: Upload to those with the best upload rate. This ensures that pieces get replicated faster, and new seeders are created fast
2: Application Layer 165
Questions about BT
Which features contribute to the efficiency of BitTorrent?
What is the effect of bandwidth constraints?
Is the Rarest First policy really necessary?
Must nodes perform seeding after downloading is complete?
How serious is the Last Piece Problem?
Does the incentive mechanism affect the performance
much?
2: Application Layer 166
Trackerless torrents
BitTorrent also supports "trackerless" torrents,
featuring a DHT implementation that allows
the client to download torrents that have been
created without using a BitTorrent tracker.
2: Application Layer 167
P2P: searching for information
File sharing (eg e-mule)
Index dynamically tracks the locations of files that peers share.
Peers need to tell index what they have.
Peers search index to determine where files can be found
Instant messaging Index maps user
names to locations. When user starts IM
application, it needs to inform index of its location
Peers search index to determine IP address of user.
Index in P2P system: maps information to peer location(location = IP address & port number).
2: Application Layer 168
P2P Case study: Skype
inherently P2P: pairs of users communicate.
proprietary application-layer protocol (inferred via reverse engineering)
hierarchical overlay with SNs
Index maps usernames to IP addresses; distributed over SNs
Skype clients (SC)
Supernode (SN)
Skype login server
2: Application Layer 169
Peers as relays
Problem when both Alice and Bob are behind “NATs”. NAT prevents an
outside peer from initiating a call to insider peer
Solution: Using Alice’s and Bob’s
SNs, Relay is chosen Each peer initiates
session with relay. Peers can now
communicate through NATs via relay
2: Application Layer 170
P2P Review
Two key functions of P2P systems Sharing content Finding content
Sharing content Direct transfer between peers
• All systems do this Structured vs. unstructured placement of data Automatic replication of data
Finding content Centralized (Napster) Decentralized (Gnutella) Probabilistic guarantees (DHTs)
2: Application Layer 171
Issues with P2P
Free Riding (Free Loading) Two types of free riding
• Downloading but not sharing any data• Not sharing any interesting data
On Gnutella• 15% of users contribute 94% of content• 63% of users never responded to a query
– Didn’t have “interesting” data
No ranking: what is a trusted source?
2: Application Layer 172
2: Application Layer 173
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 174
Socket programming
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 stream-
oriented
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
socket
Goal: learn how to build client/server application that communicate using sockets
2: Application Layer 175
Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UCP or TCP)
TCP service: reliable 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
2: Application Layer 176
Socket programming with TCPClient 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)
TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server
application viewpoint
2: Application Layer 177
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
2: Application Layer 178ou
tToS
erve
r
to network from network
inFr
omS
erve
r
inFr
omU
ser
keyboard monitor
Process
clientSocket
inputstream
inputstream
outputstream
TCPsocket
Clientprocess
client TCP socket
Stream jargon
A stream is a sequence of 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.
2: Application Layer 179
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 socket3) server converts line to
uppercase, sends back to client
4) client reads, prints modified line from socket (inFromServer stream)
2: Application Layer 180
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
2: Application Layer 181
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
2: Application Layer 182
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
2: Application Layer 183
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
2: Application Layer 184
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 applications 2.7 Socket
programming with TCP 2.8 Socket
programming with UDP
2: Application Layer 185
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
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
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Client/server socket interaction: UDP
Server (running on hostid)
closeclientSocket
read datagram fromclientSocket
create socket,clientSocket = DatagramSocket()
Client
Create datagram with server IP andport=x; send datagram via clientSocket
create socket,port= x.serverSocket = DatagramSocket()
read datagram fromserverSocket
write reply toserverSocketspecifying client address,port number
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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 (recallthat TCP sent “byte stream”)
Input: receives packet (recall thatTCP received “byte stream”)
Clientprocess
client UDP socket
2: Application Layer 188
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
2: Application Layer 189
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
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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
2: Application Layer 191
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
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Chapter 2: Summary
application architectures client-server P2P hybrid
application service requirements: reliability, bandwidth, delay
Internet transport service model connection-oriented, reliable:
TCP unreliable, datagrams: UDP
our study of network apps now complete!
specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent
socket programming
2: Application Layer 193
Chapter 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
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”
Chapter 2: 1rt Homework
1. R28 2. P24 3. How to solve the problem of free-riding in P2P
applications? Describe as many solutions as possible.
Hand in at once by Monitors ONE week later.No print! Hand-writing with you ID and name.
2: Application Layer 194
Review QuestionsSee the textbook (P.204-) R1, R3, R11, R12, R22, R27 P9
Web, FTP, Email, DNS 等在设计上有哪些缺陷 ? 如何解决(或避免) P2P 应用中的版权问题 ?
2: Application Layer 195
After-class exercise
To learn: Socket programming in C
2: Application Layer 196