Application Layer Protocols

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Application Layer Protocols

• NETS3303/3603
• Week 12

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Lesson Outcomes

• How to define an application layer protocol?
• Understand the design of request/response
  protocols
• Remote login
• File transfer protocols
• Email protocols
• HTTP
• P2P File-sharing

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Some network apps

• E-mail
• Web
• Instant messaging
• Remote login
• P2P file sharing
• Multi-user network games
• Streaming stored video clips

• Internet telephone
• Real-time video conference
• Massive parallel computing

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Internet apps application, transport protocols
Application layer protocol SMTP RFC
2821 Telnet RFC 854 HTTP RFC 2616 FTP RFC
959 proprietary (e.g. RealNetworks) proprietary (
e.g., Dialpad)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia Internet
telephony
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App-layer protocol defines

• Types of messages exchanged, eg, request
  response messages
• Syntax of message types what fields in messages
  how fields are delineated
• Semantics of the fields, ie, meaning of
  information in fields
• Timing - rules for when and how processes send
  respond to messages

• Public-domain protocols
• defined in RFCs
• allows for interoperability
• eg, HTTP, SMTP
• Proprietary protocols
• eg, KaZaA, Skype

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Remote Login(TELNET and SSH)
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Remote Interaction

• Devised when computers used (ASCII) terminals
• Terminal abstraction extended to remote access
  over a network

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Client-Server Interaction

• Client
• Invoked by user
• Forms connection to remote server
• Passes keystrokes from users keyboard to server
  and displays output from server on users screen
• Server
• Accepts connection over the network
• Passes incoming characters to OS as if they were
  typed on a local keyboard
• Sends output over connection to client

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TELNET

• Standard protocol for remote terminal access over
  TCP
• Allows a user to log into a computer remotely
• Passes keystrokes directly to remote machine as
  if coming from local keyboard
• Defines network virtual terminal that provides
  standard interface
• NVT describes system independent encoding
• TELNET client and server map NVT into local
  computers representation
• Mechanism that allows client and server to
  negotiate options (e.g., character set)

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Illustration Of How NVTAccommodates Heterogeneity
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Secure Remote Login (ssh)

• Alternative to TELNET
• Can be used as a transport layer protocol with
  service authentication
• User authentication protocol
• Connection protocol
• Multiplexes multiple transfers
• Uses encryption for privacy

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ApplicationsFile Transfer And Access(FTP, TFTP)
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On-Line File Sharing

• Always a popular application
• Two basic paradigms
• Whole-file copying (gets a local copy)
• On-line access

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File Transfer

• Whole file copying
• Client
• Contacts server
• Specifies file
• Specifies transfer direction
• Server
• Maintains set of files on local disk
• Waits for contact
• Honours request from client

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File Transfer Protocol (FTP)

• Major TCP/IP protocol for whole-file copying
• Uses TCP for transport
• FTP client contacts FTP server at port 21
• Features
• Interactive access
• Format specification (ASCII or EBCDIC)
• Authentication control (login and password)

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FTP Process Model

• Separate processes handle
• Interaction with user
• Individual transfer requests
• Data transfer connections created dynamically
  when needed
• The control connection persists throughout a
  session

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Control Connection Vs. Data Connection

• For data transfer, client side becomes server and
  server side becomes client
• Client
• Creates process to handle data transfer
• Allocates port and sends number to server over
  control connection
• Process waits for contact
• Server
• Receives request
• Creates process to handle data transfer
• Process contacts client-side

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Out-of-band Control

• Control connection out of band
• FTP server maintains state
• current directory, earlier authentication
• What special relationship is required between FTP
  and NAT??

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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
• 331 Username OK, password required
• 125 data connection already open transfer
  starting
• 425 Cant open data connection
• 452 Error writing file

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Secure File Transfer Protocols

• Secure Sockets Layer FTP (SSL-FTP)
• Uses secure sockets layer technology
• All transfers are confidential
• Secure File Transfer Program (sftp)
• Almost nothing in common with FTP
• Uses ssh tunnel
• Secure Copy (scp)
• Derivative of Unix remote copy (rcp)
• Uses ssh tunnel

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Trivial File Transfer Protocol (TFTP)

• Alternative to FTP
• Whole-file copying
• Not as much functionality as FTP
• Code is much smaller
• Intended for use on Local Area Network
• Runs over UDP
• Diskless machine can use to obtain image at
  bootstrap

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TFTP Packet Types
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TFTP Retransmission

• Symmetric
• both sides implement timeout and retransmission
• Network File System (NFS)
• A protocol for on-line file access, not copying

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ApplicationsElectronic Mail(SMTP, POP, IMAP,
MIME)
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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, Thurderbird, pine
• outgoing, incoming messages stored on server

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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

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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
• 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

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Scenario Alice sends message to Bob

• 4) SMTP client sends Alices message over the TCP
  connection
• 5) Bobs mail server places the message in Bobs
  mailbox
• 6) Bob invokes his user agent to read message

• 1) Alice uses UA to compose message and to
  bob_at_someschool.edu
• 2) Alices UA sends message to her mail server
  message placed in message queue
• 3) Client side of SMTP opens TCP connection with
  Bobs mail server

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Sample SMTP interaction
S 220 hamburger.edu SMTP Ready C HELO
crepes.fr S 250 Hello crepes.fr, pleased
to meet you C MAIL FROM ltalice_at_crepes.frgt
S 250 alice_at_crepes.fr... Sender ok
C RCPT TO ltbob_at_hamburger.edugt S 250
bob_at_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
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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)

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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

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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
blank line
body
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Multipurpose Internet Mail Extension (MIME)

• Permits nontextual data to be sent in email
• Graphics image
• Voice or video clip
• Sender
• Encodes binary item into printable characters
• Places in email message for transfer
• Receiver
• Receives email message containing encoded item
• Decodes message to extract original binary value

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MIME multimedia extensions

• multimedia mail extension - RFC 2045, 2056
• additional lines in msg header declare MIME
  content type and encoding

MIME version
method used to encode data
multimedia data type, subtype, parameter
declaration
encoded data
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Mail access protocols
SMTP
access protocol
receivers mail server

• SMTP delivery/storage to receivers server
• Mail access protocol retrieval from server
• POP Post Office Protocol RFC 1939
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
• more features (more complex)
• manipulation of stored msgs on server
• HTTP Hotmail , Yahoo! Mail, etc.

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POP3 protocol
S OK POP3 server ready C user bob S OK
C pass hungry S OK user successfully logged
on

• 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 ltmessage 1
contentsgt S . C dele 1 C retr
2 S ltmessage 1 contentsgt S .
C dele 2 C quit S OK POP3 server
signing off
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POP3 (more) and IMAP

• More 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

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World Wide Web(HTTP)
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World Wide Web

• Distributed hypermedia paradigm
• various forms of information, as data, text,
  graphics, video, and audio, are linked together
  by hyperlinks
• Major service on the Internet
• Use surpassed file transfer in 1995

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Web Page Identifier

• Known as Uniform Resource Locator (URL)
• Encodes
• Access protocol to use
• Domain name of server
• Protocol port number (optional)
• Path through servers file system (optional)
• Parameters (optional)
• Query (optional)
• Format
• http // hostname port / path parameters
  ? query

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Web Standards

• Separate standards for
• Representation
• Transfer

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Representation

• HyperText Markup Language (HTML)
• Document contains text plus embedded links and
  formatting
• HTML gives guidelines for display, not details
• Consequence two browsers may choose to display
  same document differently

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Transfer

• Used between browser and web server
• Protocol is HyperText Transfer Protocol (HTTP)
• Runs over TCP

HTTP vs SMTP Pull vs push paradigm
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HTTP Characteristics

• Application level
• Request / response paradigm
• Stateless
• Permits bi-directional transfer
• Offers capability negotiation
• Support for caching
• Support for intermediaries (proxy)

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HTTP Operation

• Defines how client requests web pages and how
  server transfers web pages to client
• Browser sends HTTP requests to which server
  replies
• Typical request GET used to fetch document
• Example
• GET http//www.it.usyd.edu/current_students/
  HTTP/1.1
• Relative URL also permitted
• GET /current_students/ HTTP/1.1

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Error Messages

• HTTP includes set of error responses
• Server can format error as HTML message for user
  or use internal form and allow browser to format
  message

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Persistent Connections

• HTTP version 1.0 uses one TCP connection per
  transfer
• Browser forms TCP connection to server
• Browser sends GET request
• Server returns header describing item
• Server returns item
• Server closes connection
• HTTP/1.1 permits connection to persist across
  multiple requests
• One connection with certain timeout interval

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HTTP Headers

• HTTP uses MIME-like headers to carry meta
  information
• Both browsers and servers send headers that allow
  them to negotiate agreement on the document
  representation and encoding to be used

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Headers And Length Encoding

• HTTP headers use same syntax as email headers
• Lines of text followed by blank line
• Lines of text have form keywordinformation
• For persistent connection, header specifies
  length (in octets) of data item that follows

Header Meaning
Content-Length Size of item in
octets Content-Type Type of item Content-Encodin
g Encoding used for item Content-Language
Language(s) used in item
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Example Of Header

• Content-Length 34
• Content-Language english
• Content-Encoding ascii
• ltHTMLgt A trivial example. lt/HTMLgt
• Note if length is not known in advance, server
  can inform browser that connection will close
  following transfer
• Connection close

Blank line!
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Conditional Request

• Allows browser to check cached copy for freshness
• Eliminates useless latency
• Sends If-Modified-Since in header of GET request
• Example
• If-Modified-Since Wed, 28 May 2007 080001 GMT
• Avoids transfer if item older than 28 May 2007

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Proxy Servers

• Browser can be configured to contact proxy
• Permits caching for entire organization
• Server can specify maximum number of proxies
  along path (including none)

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Caching Of Web Pages

• Caching essential to efficiency
• Server specifies
• Whether page can be cached
• Maximum time page can be kept
• Intermediate caches and browser cache web pages
• Browser can specify maximum age of page (forces
  intermediate caches to revalidate)

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P2P file sharing(Gnutella)
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P2P file sharing

• Alice chooses one of the peers, Bob.
• File is copied from Bobs PC to Alices notebook
  HTTP
• While Alice downloads, other users download from
  Alice
• Alices peer is both a Web client and a transient
  Web server
• All peers are servers highly scalable!

• Example
• Alice runs P2P client application on her notebook
  computer
• Intermittently connects to Internet gets new IP
  address for each connection
• Asks for Hey Jude
• Application displays other peers that have copy
  of Hey Jude.

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Query flooding Gnutella

• fully distributed
• no central server
• public domain protocol
• many Gnutella clients implementing protocol

• overlay network graph
• edge between peer X and Y if theres a TCP
  connection
• all active peers and edges is overlay net
• Edge is not a physical link
• Given peer will typically be connected with lt 10
  overlay neighbors

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Gnutella protocol
File transfer HTTP

• Query messagesent over existing TCPconnections
• peers forwardQuery message
• QueryHit sent over reversepath

Scalability limited scopeflooding
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Gnutella Peer joining

• Joining peer X must find some other peer in
  Gnutella network use list of candidate peers
• X sequentially attempts to make TCP with peers on
  list until connection setup with Y
• X sends Ping message to Y Y forwards Ping
  message.
• All peers receiving Ping message respond with
  Pong message
• X receives many Pong messages. It can then setup
  additional TCP connections

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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

• control vs. data msgs
• in-band, out-of-band
• centralized vs. decentralized
• stateless vs. stateful
• The End!