Application Layer 8-1 Chapter 2: Application Layer Our goals: r conceptual / implementation aspects...

Application Layer 8-1

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 application-layer protocols by looking at examples: HTTP (web) FTP (file

transfer) DNS (web

addressing)

interface to transport layer the socket concept


Creating network applications

Write programs that run on different end systems

and communicate over a

network. e.g., Web: Web server

software communicates with browser software

No app. software written for devices in network core Network core devices do not

function at application layer This design allows for rapid

app development

application

transportnetworkdata linkphysical

application


application



App-layer protocol defines Types of messages

exchanged, e.g. request & response messages

Syntax of message types: what fields in messages & where fields are located

Semantics of the fields, i.e. meaning of information in fields

Rules for when and how processes send & respond to messages

2TYPES OF PROTOCOLS: Public-domain protocols: defined in RFCs enable interoperability eg: HTTP, SMTP, SIP

Proprietary protocols: owned by a company eg: Skype, Messenger


Client / Server concepts

Hardware view Process / interaction view Client-Server architecture Peer-to-Peer architecture


Client Hosts /Server Hosts

server host : always-on host permanent IP address server farms for scaling

client host (=workstation): located next to user may be intermittently

connected may have dynamic IP

addressNote: Both client and server are called hosts


Process: program running within a host.

processes in different hosts communicate by exchanging messages

when processes cooperate for a common purpose, there must be one of them that initiates the common work Sends the first message

Client process: process that initiates communication

Server process: process that waits to be contacted by client

Note: Client process may run on a client host or on a server

host Server process may run on a client host (see Peer to

Peer architecture)

Client Process/Server Process


Process Interaction Scenarioserver process:

waits all the time for work its IP address must be

known to the client process before interaction starts

client process: initiates interaction with

server process when needs its help

if server process agrees : the two processes

exchange messages the messages enable

them to cooperate to get the work done

Note: Server host usually runs several server processes in

parallel

Client process 1

Client process 2

Server proc. 1Server proc. 2


Client-Server architectureserver process:

runs on server host only the server host’s IP

address is publicly known

client process: runs in most cases on

client host (personal computers)

in some applications, client process may run on a server computer

• e,g, in e-mail application, the mail server may run a mail client process


Peer to Peer (P2P) architecture

most of the data in such application exchanged between client hosts

one of the client hosts (say A) must initiate the interaction with the other one (say B). Thus: A runs a client process B runs a server process

• note: B is a client host In order to start, A must know B’s

IP address, which is usually not publicly advertised. therefore P2P interaction is

usually preceded by a preliminary stage which uses the Client-Server (C-S) architecture

datainitializatio

n

C-S

C-SP2P

P2P


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 or low delay variation 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


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

Bandwidth

elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, a few secsyes, 100’s msecyes and no


Web and HTTP

First some jargon (שפה מקצועית) 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


HTTP overviewHTTP: hypertext

transfer protocol Web’s application layer

protocol client/server model

client : browser that requests, receives and “displays” Web objects

server : Web server that sends objects in response to requests

HTTP 1.0 : RFC 1945 HTTP 1.1 : RFC 2616

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

Note: Different H/W and S/W use the same protocol


HTTP overview (continued)HTTP uses TCP: client initiates TCP

connection (creates socket) to server port 80 (WKP)

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” HTTP keeps

no information about past client requests BUT, cookies allow

Web server to run stateful applications

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 connections

Nonpersistent HTTP At most one object is

sent over a TCP connection.

HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP Multiple objects can

be sent over single TCP connection between client and server.

HTTP/1.1 uses persistent connections in default mode


Nonpersistent HTTPSuppose user enters URL of a page :

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

(page contains text and references to10 jpeg images)

Client Server

www.someSchool.edu/someDepartment/home.index

time


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. after sending the object, HTTP server closes TCP connection.

time

requestfile

RTT

time time


Response time modelingDefinition of RTT: time to

send a small packet to travel from client to server and back.

Response time: one RTT to initiate TCP

connection one RTT for HTTP

request and first few bytes of HTTP response to return

file transmission time

total = 2 RTT + transmit time

initiate TCPconnection

RTT

filereceived

time to transmit file


Persistent HTTPNonpersistent HTTP issues: 2 RTT overhead per object

Extra Delay OS must work and allocate

host resources 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 are sent over same connection

Persistent without pipelining: client issues new request

only when previous response has been received

one RTT overhead for each referenced object

Qn: What time is saved?Persistent with pipelining: default in HTTP/1.1 client sends requests as

soon as it encounters a referenced object

only one RTT for all the referenced objects

Qn: Can you see an advantage of Persistent?


HTTP request message

two types of HTTP messages: request, response both use ASCII format (human-readable format)

HTTP request message (example):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


HTTP request message: general format

(Optional)


Uploading form input

Using the POST method: Input is uploaded to

server in entity body of the POST message

Using GET with parameters: Uses GET method Input is uploaded as part

of the URL field of request line:

www.somesite.com/animalsearch?monkeys&banana

Web page often includes form input

The input can be uploaded to the server in two ways:


Methods

HTTP/1.0 GET POST HEAD

asks server to leave requested object out of response

Sends only header fields

Used for debugging

HTTP/1.1 GET, POST, HEAD PUT

uploads file in the entity body to the path specified in URL field

DELETE deletes file specified

in the URL field


HTTP response message

HTTP/1.1 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821Connection: close Content-Type: text/html data data data data data ...

status line

header lines

message body*

cr-lf

(Closing TCP connection after data transmitted)

status phraseprotocol status code

* message body = data e.g., requested HTML file


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 of the response message.A few examples of status codes:


User-server state: cookiesMany major Web sites use cookies

Four components:1) the HTTP server sends

response message with a ‘Set-Cookie:’ header

2) user’s browser puts the cookie header content in cookie file on user’s host, labeled with server name

3) next request message to this server will contain Cookie: header with the same value

4) cookie points to database record at server site

Example: Susan accesses

Internet always from same PC

She visits a specific e-commerce site for first time

When initial HTTP requests arrives at site, site creates a unique ID and an entry in backend database for that ID

The ID is the cookie between them


Cookies: keeping “state” (cont.)

client server

http request to ebay

usual http response +

Set-cookie: 1678

next http request +cookie: 1678

usual http response msg

http request to ebay +

cookie: 1678usual http response msg

cookie-specificaction

cookie-specificaction

servercreates ID

1678 for user

entry in backend

database

access

acce

ss

Cookie file

ebay : 1678

amazon : 8734

Cookie file

amazon : 8734

Cookie file

ebay : 1678amazon : 8734

one week later:


Cookies (continued)

What cookies can bring:

user session state (e.g. in Web e-mail)

user identification on given workstation

shopping carts recommendations

Cookies and privacy: cookies permit sites

to learn a lot about you

you may supply name and e-mail to sites

advertising companies obtain info across sites from inserted banner addso info collected by

the advertizing server

aside


Web cache (proxy server)

user sets up browser to access Web via proxy

browser sends all HTTP requests to proxy IF object in the

proxy’s cache: proxy returns object

ELSE proxy requests object from origin server, then returns object to client

keeps file in its cache

Goal: satisfy client request without involving origin server

client

ProxyServer

(web cache)

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server


More about Web caching proxy acts as HTTP

client and server Typically proxy is

installed by ISP (Organization, ISP)

Caching advantages Reduce response time for

client request. Reduce traffic on an

institution’s access link. Internet dense with

proxies enables “poor” content providers to effectively deliver content

Qn :what is the main problem with caching as described?

Caching rules Origin server may :

forbid caching allow caching by private

cache only (privacy) Origin server sends:

Last Modified date/time Expiration time or Max. Age

Proxy should respect age and other restrictions


Conditional GET Goal: don’t send object if

proxy has up-to-date cached version used if cacheability

conditions or data are missing

proxy specifies date of cached copy in HTTP request header:

server sends short response if cached copyis up-to-date:

(see Case 1 on this slide)

HTTP/1.0 304 Not Modified(contains no object)

If-modified-since: <date>

proxy server

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0

304 Not Modified

object not

modified

Case 1

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0 200 OK

<data>

object modified

proxy serverCase 2


FTP: the file transfer protocol (RFC 959)

transfer file to/from remote host remote management of a file system

client/server model client (user’s workstation) initiates the connection server (file server) responds to client’s commands

• mainly files transfers to or from client ftp Well Known Port for control traffic : 21 ftp Well Known Port for data traffic : 20

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host


FTP: separate control, data connections

FTP client contacts FTP server by TCP at port 21, sets up control connection

client obtains authorization over control connection

client browses remote directory by sending commands over control connection.

before a file transfer, client asks server to open a data-connection to a client port N (from server port 20)

client sends command:RETR or STOR (see next slide)

After sending/receiving one file, data connection is closed

FTPclient

FTPserver

TCP control connection, server

port 21

TCP data connection,server port 20

Server opens a second TCP data connection to transfer another file.

Control connection: “out of band”

FTP server maintains “state” throughout the session: current directory, authentication state


FTP commands, responsesSample commands:(sent as ASCII text over control

channel) USER username PASS password LIST - list the files in

current directory CWD – change working dir. PORT N – set up data

connect. to my port N RETR <filename> -

download a file from server to client

STOR <filename> - upload a file from client to server

Sample return codes(status code and phrase:

same 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

226 File transfer OK, closing connectionNote:in Windows FTP user

writes GET for RETR PUT for STOR


DNS: Domain Name System

People: many identifiers: SSN, name, passport

#Internet hosts, routers:

IP address (32 bit) - used for addressing datagrams (packets) – good for routers

“name”, e.g., www.yahoo.com – good for human use

Q: map between name and IP addresses ?

Domain Name System:1. distributed name database

implemented in hierarchy of many name servers

2. application-layer protocol host, routers, name servers communicate to resolve names (name-to-address translation) (WKP: UDP 53) Note: Internet

infrastructure (תשתית) function, implemented as application-layer protocol

all the work at network’s “edge”, not in the core


DNS

Why not centralize DNS? single point of failure traffic volume distant centralized

database maintenance difficult doesn’t scale!

DNS services Hostname to IP

address translation Host aliasing

canonical name = main name of computer

alias = additional name

Finding server names e.g hostname of mail

server for domain

Load distribution Replicated Web servers:

set of IP addresses for one canonical name


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 (NS) wants IP address for www.amazon.com : Client queries a root server to find com DNS server Client queries com DNS server to get amazon.com DNS

(name-) server (by sending message to UDP port 53) Client queries amazon.com DNS server to get IP address for

www.amazon.com

(*) There are 13 root servers world-wide; their addresses are known to all name servers


TLD, Authoritative Servers, Zones

Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp, il Network Solutions maintains servers for com TLD Educause does the same for edu TLD

Authoritative DNS server: organization’s DNS server, providing authoritative hostname to IP mappings for organization’s servers It is responsible for a “zone” (or several zones) Zone = Domain – { subdomains that have their own zones }

Example: ns.tau.ac.il zone includes all natural science faculty except dep”ts

that have their own zones (e.g. cs.ns.tau.ac.il & math.ns,tau.ac.il )


Resolver and Local Name Server

Each host has a Resolver = Host software dealing with DNS

Each ISP (residential ISP, company, university) has a “Local name server” Also called “default name server”

When application needs to translate a DNS name, it calls the resolver on its computer

Resolver sends a DNS query to its local name server (LNS)

LNS finds the answer in its cache or by querying other name servers and sends answer to Resolver

Resolver supplies answer to the application


gaia.cs.umass.edu

authoritative DNS serverdns.cs.umass.edu

resolver atcis.poly.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

678

TLD DNS server

DNS operation modes Name server can

operate in two modes: iterative recursive (next slide)

iterated query contacted server replies

with name of server to contact

“I don’t know this name, but ask this server”

the example on this slide shows LNS querying several iterative NS’s

Example: Host at cis.poly.edu wants IP address for gaia.cs.umass.edu


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

3

Recursive queriesrecursive query: contacted server asks

other NS’s the same query and returns final answer to the originator of query

puts burden of name resolution on contacted name server

a query may also involve mix of the two modes.

Note: root name servers are usually iterative

Example: same example with all NS’s operating in recursive mode


DNS: caching and mirroring once (any) name server learns mapping, it

caches mapping cache entries timeout (disappear) after

some time (each response includes a TTL) TLD servers typically cached in all local

name servers• Thus root name servers less often visited

Usually zone database is duplicated on several authoritative name servers (mirroring) the main one is updated by the administrator by a file the others copy the zone from the main one main NS notifies the others when update available


DNS records

DNS: distributed db storing resource records (RR)

Type=NS name is domain (e.g.

foo.com) value is IP address of

authoritative name server for this domain

RR format: (name, value, type, ttl)

Type=Aname is hostnamevalue is IP address

Type=CNAMEname is alias name for

some “canonical” (the real) name

www.ibm.com is really servereast.backup2.ibm.com

value is canonical name Type=MX name is domain, e.g.

gmail.com value is hostname of mail

server

Application Layer 8-1 Chapter 2: Application Layer Our goals: r conceptual / implementation aspects...

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