school of information technologies application layer protocols nets3303/3603 week 12

School of Information Technologies

Application Layer Protocols

NETS3303/3603

Week 12


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


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]proprietary(e.g. RealNetworks)proprietary(e.g., Dialpad)

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

typically UDP


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


Remote Login(TELNET and SSH)


Remote Interaction

• Devised when computers used (ASCII) terminals

• Terminal abstraction extended to remote access over a network


Client-Server Interaction• Client

– Invoked by user– Forms connection to remote server– Passes keystrokes from user’s keyboard to server and

displays output from server on user’s 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


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 computer’s

representation

• Mechanism that allows client and server to negotiate options (e.g., character set)


Illustration Of How NVTAccommodates Heterogeneity


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


Applications:File Transfer And Access

(FTP, TFTP)


On-Line File Sharing

• Always a popular application

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


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

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

user at host


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)


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

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20


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


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?


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


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


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


TFTP Packet Types


TFTP Retransmission

• Symmetric – both sides implement timeout and

retransmission

• Network File System (NFS)– A protocol for on-line file access, not copying


Applications:Electronic Mail

(SMTP, POP, IMAP, MIME)


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

• outgoing, incoming messages stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


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


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


Scenario: Alice sends message to Bob

1) Alice uses UA to compose message and “to” [email protected]

2) Alice’s UA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

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


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: <[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


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)


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


Mail message formatSMTP: 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


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


MIME: multimedia extensions

• multimedia mail extension - RFC 2045, 2056

• additional lines in msg header declare MIME content type and encoding

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


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


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


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


World Wide Web(HTTP)


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


Web Page Identifier• Known as Uniform Resource Locator (URL)• Encodes

– Access protocol to use– Domain name of server– Protocol port number (optional)– Path through server’s file system (optional)– Parameters (optional)– Query (optional)

• Format– http: // hostname [: port] / path [; parameters] [? query]


Web Standards

• Separate standards for– Representation– Transfer


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


Transfer

• Used between browser and web server

• Protocol is HyperText Transfer Protocol (HTTP)

• Runs over TCP

HTTP vs SMTP:Pull vs push paradigmHTTP vs SMTP:

Pull vs push paradigm


HTTP Characteristics

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


HTTP Operation

• Browser sends 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 permittedGET /current_students/ HTTP/1.1


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


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 version 1.1 permits connection to persist across multiple requests


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


Headers And Length Encoding

• HTTP headers use same syntax as email headers– Lines of text followed by blank line

– Lines of text have form keyword:information

• For persistent connection, header specifies length (in octets) of data item that follows

Header Meaning Content-Length Size of item in octetsContent-Type Type of itemContent-Encoding Encoding used for itemContent-Language Language(s) used in item


Example Of HeaderContent-Length: 34Content-Language: englishContent-Encoding: ascii

<HTML> A trivial example. </HTML>

• Note: if length is not known in advance, server can inform browser that connection will close following transfer

Connection: close

Blank line!


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 08:00:01 GMT

– Avoids transfer if item older than 28 May 2007


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)


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)


P2P file sharing(Gnutella)


P2P file sharingExample• Alice runs P2P client

application on her notebook computer

• Intermittently connects to Internet; gets new IP address for each connection

• Asks for “Hey Jude”• Application displays other

peers that have copy of Hey Jude.

• Alice chooses one of the peers, Bob.

• File is copied from Bob’s PC to Alice’s notebook: HTTP

• While Alice downloads, other users download from Alice

• Alice’s peer is both a Web client and a transient Web server

All peers are servers = highly scalable!


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 there’s a TCP connection

• all active peers and edges is overlay net

• Edge is not a physical link• Given peer will typically

be connected with < 10 overlay neighbors


Gnutella: protocol

Query

QueryHit

Query

Query

QueryHit

Query

Query

QueryHit

File transfer:HTTP

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

Scalability:limited scopeflooding


Gnutella: Peer joining

1. Joining peer X must find some other peer in Gnutella network: use list of candidate peers

2. X sequentially attempts to make TCP with peers on list until connection setup with Y

3. X sends Ping message to Y; Y forwards Ping message.

4. All peers receiving Ping message respond with Pong message

5. X receives many Pong messages. It can then setup additional TCP connections


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!

school of information technologies application layer protocols nets3303/3603 week 12

Documents

client slide

remote server

network slide

udp slide

ssh slide

remote access

privacy slide

tftp slide