1

Application LayerGoals: conceptual +

implementation aspects of network application protocols client server

paradigm service models

learn about protocols by examining popular application-level protocols

More goals specific protocols:

http – our focus ftp smtp pop dns: will be discussed

later

programming network applications socket programming

• Done!

2

Applications and application-layer protocols

Application: communicating, distributed processes running in network hosts

in “user space” exchange messages to

implement app e.g., email, file transfer,

the WebApplication-layer protocols

one “piece” of an app define messages

exchanged by apps and actions taken

use services provided by lower layer protocols

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

3

Network applications: some jargon

A process is a program that is running within a host.

Within the same host, two processes communicate with IPC defined by the OS.

Processes running in different hosts communicate with an application-layer protocol

A user agent is an interface between the user and the network application. Web:browser E-mail: mail reader streaming

audio/video: media player

4

Client-server paradigm

Typical network app has two pieces: client and server

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

Client: initiates contact with server

(“speaks first”) typically requests service

from server, for Web, client is

implemented in browser; for e-mail, in mail reader, e.g., outlook

Server: provides requested service

to client e.g., Web server sends

requested Web page, mail server delivers e-mail

request

reply

5

Application-layer protocols (cont).

API: application programming interface

defines interface between application and transport layer

socket: Internet API two processes

communicate by sending data into socket, reading data out of socket

Q: how does a process “identify” the other process with which it wants to communicate? IP address of host

running other process “port number” -

allows receiving host to determine to which local process the message should be delivered

… refer to socket prorgamming

6

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”

Bandwidth some apps (e.g.,

multimedia) require minimum amount of bandwidth to be “effective”

other apps (“elastic apps”) make use of whatever bandwidth they get

7

Transport service requirements of common apps

Application

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive games

financial apps

Data loss

no lossno lossloss-tolerantloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no

8

Services provided by Internet transport protocols

TCP service: connection-oriented: setup

required between client, server

reliable transport between sending and receiving process

flow control: sender won’t overwhelm receiver

congestion control: throttle sender when network overloaded

does not providing: timing, minimum bandwidth guarantees

UDP service: unreliable data transfer

between sending and receiving process

does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

Q: why bother? Why is there a UDP?

9

Internet apps: their protocols and transport protocols

Application

e-mailremote terminal access

Web file transfer

streaming multimedia

remote file serverInternet telephony

Applicationlayer protocol

smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]proprietary(e.g. RealNetworks)NSFproprietary(e.g., Vocaltec)

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP

10

The Web: some jargon

Web page: consists of “objects” addressed by a URL

Most Web pages consist of: base HTML page, and several referenced

objects.

URL has two components: host name and path name:

User agent for Web is called a browser: MS Internet Explorer Netscape

Communicator Firefox

Server for Web is called Web server: Apache (public

domain) MS Internet

Information Server

www.someSchool.edu/someDept/pic.gif

11

The Web: the http protocol

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

http1.0: RFC 1945 http1.1: RFC 2068 (1997),

2616 (1999)

PC runningExplorer

Server runningWeb

server

Mac runningNavigator

http request

http re

quest

http response

http re

sponse

12

The http protocol: more

http: TCP transport service:

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

HTTP Usage

HTTP is the protocol that supports communication between web browsers and web servers.

A “Web Server” is a HTTP server

Most clients/servers today speak version 1.1, but 1.0 is also in use.

13

From the RFC

“HTTP is an application-level protocol with the lightness and speed necessary for distributed, hypermedia information systems.”

14

15

http 1.0 exampleSuppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server.

2. http client sends http request message (containing URL) into TCP connection socket

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 (someDepartment/home.index), sends message into socket

time

(contains text, references to 10

jpeg images)

16

http example (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

17

Non-persistent and persistent connections

Non-persistent HTTP/1.0 server parses

request, responds, and closes TCP connection

2 RTTs to fetch each object

Each object transfer suffers from slow start

Persistent default for HTTP/1.1 on same TCP

connection: server, parses request, responds, parses new request,..

Client sends requests for all referenced objects as soon as it receives base HTML.

Fewer RTTs and less slow start.But most 1.0 browsers use

parallel TCP connections.

A Typical HTTP Session

User types “www.seattleu.edu/index.html” into a browser Browser translates www.seattleu.edu into an IP address

and tries to open a TCP connection with port 80 of that address

Once a connection is established, the browser sends the following byte stream:

GET /index.html HTTP/1.1 HOST: www.seattleu.edu” (plus an empty line below)

The host responds with a set of headers indicating which protocol is actually being used,

whether or not the file requested was found, how many bytes are contained in that file, and what kind of information is contained in the file ("MIME" type)

a blank line to indicate the end of the headers the contents of the page

18

A Typical HTTP Session

If the browser finds images embedded in the page, it starts a separate request for each image …

The TCP connection is kept alive a bit longer, then closes if no further requests are received from the browser

19

Request - Response

HTTP has a simple structure: client sends a request server returns a reply.

HTTP can support multiple request-reply exchanges over a single TCP connection.

20

Well Known Address

The “well known” TCP port for HTTP servers is port 80.

Other ports can be used as well...

21

HTTP Versions

The original version now goes by the name “HTTP Version 0.9” HTTP 0.9 was used for many years.

Starting with HTTP 1.0 the version number is part of every request. tells the server what version the client can

talk (what options are supported, etc).

22

HTTP 1.0+ Request

Lines of text (ASCII).

Lines end with CRLF “\r\n”

First line is called “Request-Line”

Request-LineHeaders...

Content...blank lineblank line

23

Request Line

Method URI HTTP-Version\r\n

The request line contains 3 tokens (words).

space characters “ “ separate the tokens.

Newline (\n) seems to work by itself (but the protocol requires CRLF)

24

HTTP Request Format:

Method URI HttpVersion

Method Description

OPTIONS capabilities of resource/server

GET retrieve resource

HEADretrieve headers for resource, metadata

POST submit data to server & retrieve result

PUT replace/insert resource on server

DELETE remove resource from server

TRACE trace request route through Web25

Common Usage

GET, HEAD and POST are supported everywhere .

HTTP 1.1 servers often support PUT, DELETE, OPTIONS & TRACE.

26

URI: Universal Resource Identifier

URIs defined in RFC 2396.

Absolute URI: scheme://hostname[:port]/pathhttp://www.cs.rpi.edu:80/blah/foo

Relative URI: /path/blah/foo

No server mentioned27

URI Usage When dealing with a HTTP 1.1 server,

only a path is used (no scheme or hostname). HTTP 1.1 servers are required to be capable

of handling an absolute URI, but there are still some out there that won’t…

When dealing with a proxy HTTP server, an absolute URI is used. client has to tell the proxy where to get the

document! more on proxy servers in a bit….

28

HTTP Version Number

“HTTP/1.0” or “HTTP/1.1”

HTTP 0.9 did not include a version number in a request line.

If a server gets a request line with no HTTP version number, it assumes 0.9

29

The Header Lines

After the Request-Line come a number (possibly zero) of HTTP header lines.

Each header line contains an attribute name followed by a “:” followed by a space and the attribute value.

The Name and Value are just text.

30

Headers

Request Headers provide information to the server about the client what kind of client what kind of content will be accepted who is making the request

There can be 0 headers (HTTP 1.0) HTTP 1.1 requires a Host: header

31

HTTP Headers

Accept: Indicates which data formats are acceptable. Accept: text/html, text/plain

Content-Language: Language of the content Content-Language: en

Content-Length: Size of message body Content-Length: 1234

Content-Type: MIME type of content body Content-Type: text/html

Date: Date of request/response Date: Tue, 15 Nov 1994 08:12:31 GMT

Expires: When content is no longer valid Expires: Tue, 15 Nov 1994 08:12:31 GMT

Host: Machine that request is directed to Host: www.cs.uct.ac.za

Location: Redirection to a different resource Location: http://myserver.org/

Retry-After: Indicates that client must try again in future Retry-After: 120

32

Example HTTP Headers

Accept: text/html

Host: www.seattleu.edu

From: zhuy@gmail.com

User-Agent: Mozilla/4.0

Referer: http://foo.com/blah

33

End of the Headers

Each header ends with a CRLF ( \r\n ) The end of the header section is marked

with a blank line. just CRLF

For GET and HEAD requests, the end of the headers is the end of the request!

34

35

http message format: request

two types of http messages: request, response http request message:

ASCII (human-readable format)

GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

36

http request message: general format

Entity body is empty for “GET”, but not for “POST”

POST A POST request includes some content

(some data) after the headers (after the blank line).

There is no format for the data (just raw bytes).

A POST request must include a Content-Length line in the headers:

Content-length: 267

37

Example GET Request

GET /testanswers.html HTTP/1.1

Accept: */*

Host: www.seattleu.edu

User-Agent: Internet Explorer

From: cheater@cheaters.org

Referer: http://foo.com/There is a blank line here!

38

Example POST RequestExample POST Request

POST /changegrade.cgi HTTP/1.1POST /changegrade.cgi HTTP/1.1

Accept: */*Accept: */*

Host: www.seattleu.eduHost: www.seattleu.edu

User-Agent: SecretAgent V2.3User-Agent: SecretAgent V2.3

Content-Length: 35Content-Length: 35

Referer: http://monte.seattleu.edu/blahReferer: http://monte.seattleu.edu/blah

stuid=6660182722&item=test1&grade=99stuid=6660182722&item=test1&grade=99

39

Typical Method Usage

GET used to retrieve an HTML document.

HEAD used to find out if a document has changed.

POST used to submit a form.

40

41

http message format: respone

HTTP/1.0 200 OK Date: 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

HTTP Response Format:

HTTP Version Status Code Reason

Status Reason Description

200 OK Successful request

206 Partial Content Successful request for partial content

301Moved Permanently

Resource has been relocated

304 Not ModifiedConditional GET but resource has not changed

400 Bad Request Request not understood

403 Forbidden Access to resource not allowed

404 Not Found URI/resource not found on server

500Internal Server Error

Unexpected error42

43

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:

Response Headers

Provide the client with information about the returned entity (document). what kind of document how big the document is how the document is encoded when the document was last modified

Response headers end with blank line

44

Response Header Examples

Date: Wed, 30 Jan 2002 12:48:17 EST

Server: Apache/1.17

Content-Type: text/html

Content-Length: 1756

Content-Encoding: gzip

45

Content

Content can be anything (sequence of raw bytes).

Content-Length header is required for any response that includes content.

Content-Type header also required.

46

Single Request/Reply

The client sends a complete request. The server sends back the entire reply. The server closes it’s socket.

If the client needs another document it must open a new connection.

This was the default for HTTP 1.0

47

Persistent Connections

HTTP 1.1 supports persistent connections (this is the default).

Multiple requests can be handled over a single TCP connection.

The Connection: header is used to exchange information about persistence (HTTP/1.1)

1.0 Clients used a Keep-alive: header

48

49

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 www.amazon.comAnything typed in sent to port 80 at www.eurecom.fr

telnet www.google.com 80

2. Type in a GET http request:

GET /index.html HTTP/1.0 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!

50

User-server interaction: authentication

Authentication goal: control access to server documents

stateless: client must present authorization in each request

authorization: typically name, password authorization: header

line in request if no authorization

presented, server refuses access, sendsWWW authenticate:

header line in response

client server

usual http request msg401: authorization req.

WWW authenticate:

usual http request msg

+ Authorization:lineusual http response

msg

usual http request msg

+ Authorization:lineusual http response

msg

time

Browser caches name & password sothat user does not have to repeatedly enter it.

51

User-server interaction: cookies

server sends “cookie” to client in response mstSet-cookie: 1678453

client presents cookie in later requestscookie: 1678453

server matches presented-cookie with server-stored info (Database) authentication remembering user

preferences, previous choices

client server

usual http request msgusual http response

+Set-cookie: #

usual http request msg

cookie: #usual http response

msg

usual http request msg

cookie: #usual http response msg

cookie-spectificaction

cookie-spectificaction

Sometimes it is deirable for servers to identify users!!! We need states!!!

52

User-server interaction: conditional GET

Goal: don’t send object if client has up-to-date stored (cached) version

client: specify date of cached copy in http requestIf-modified-since:

<date> server: response contains

no object if cached copy up-to-date: HTTP/1.0 304 Not

Modified

client 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/1.1 200 OK

…

<data>

object modified

Handle cached objects: stale or fresh?

53

Web Caches (proxy server)

user sets browser: Web accesses via web cache

client sends all http requests to web cache if object at web cache,

web cache immediately returns object in http response

else requests object from origin server, then returns http response to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

http request

http re

quest

http response

http re

sponse

http re

quest

http re

sponse

http requesthttp response

origin server

origin server

54

Why Web Caching?

Assume: cache is “close” to client (e.g., in same network)

smaller response time: cache “closer” to client

decrease traffic to distant servers link out of

institutional/local ISP network often bottleneck

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

55

Discussion

Why do we need web proxy or web caching?

If we use http/1.0 to request an object from a Web server, how many RTTs would it take?

How about http/1.1? How do we implement a web proxy?

56

How do we implement a Web proxy?

Based on TCP, using HTTP 1.0/1.1 Refer to http1.0: RFC 1945 and http1.1: RFC

2068 for HTTP protocols Two basic functionalities

Relay requests and responses for clients and web servers

Web caching• Cache hit, check the freshness of objects• Cache miss, get it from a server and cache it

Support http 1.0 and 1.1 ?

57

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

58

ftp: separate control, data connections

ftp client contacts ftp server at port 21, specifying TCP as transport protocol

two parallel TCP connections opened: control: exchange

commands, responses between client, server.

“out of band control” data: file data to/from

server ftp server maintains

“state”: current directory, earlier authentication

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

59

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

60

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

61

Electronic Mail: mail servers

Mail Servers mailbox contains

incoming messages (yet to be read) 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

62

Electronic Mail: smtp [RFC 821]

uses tcp to reliably transfer email msg 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 Binary -> ascii -> binary: painful!

63

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

64

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)

65

smtp: final words

smtp uses persistent connections

smtp requires that message (header & body) be in 7-bit ascii

certain character strings are not permitted in message (e.g., CRLF.CRLF). Thus message has to be encoded (usually into either base-64 or quoted printable)

smtp server uses CRLF.CRLF to determine end of message

Comparison with http http: pull email: push

both have ASCII command/response interaction, status codes

http: each object is encapsulated in its own response message

smtp: multiple objects message sent in a multipart message

66

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

67

Message format: multimedia extensions

MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type

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

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

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

68

MIME typesContent-Type: type/subtype; parameters

Text example subtypes:

plain, html

Image example subtypes: jpeg,

gif

Audio exampe subtypes: basic

(8-bit mu-law encoded), 32kadpcm (32 kbps coding)

Video example subtypes: mpeg,

quicktime

Application other data that must be

processed by reader before “viewable”

example subtypes: msword, octet-stream

69

Multipart TypeFrom: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.--98766789Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data --98766789--

70

Mail access protocols

SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939] POP3 version 3 • authorization (agent <-->server) and download

IMAP: Internet Mail Access Protocol [RFC 1730]• more features (more complex)• manipulation of stored msgs on server

HTTP: Hotmail , Yahoo! Mail, etc.

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server

71

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