Chapter 2 Application Layerqianzh/comp562-2009/notes/Chapter2_2009.pdf · Chapter 2: Application Layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic

Chapter 2Application Layer

Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

A note on the use of these ppt slides:The notes used in this course are substantially based on powerpoint slides developed and copyrighted by J.F. Kurose and K.W. Ross, 2007

2: Application Layer 1

Chapter 2: Application Layer

2.1 Principles of network applications2.2 Web and HTTP2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP2.9 Building a Web server


Application Architectures

Client‐server

Peer‐to‐peer (P2P)

Hybrid of client‐server and P2P


clientclient

client

Processes CommunicatingProcess: program running within a host.Within same host, two processes communicate using inter‐process communication (defined by OS)Processes in different

Client process: process that initiates communication

Server process: process that waits to be contacted

Note: applications with P2P architectures have client processes & server processes

hosts communicate by exchanging messages


Addressing Processes

Identifier includes both IP address and port numbersassociated with process on hostExample port numbers:

HTTP server: 80Mail server: 25

To send HTTP message to www.cs.ust.hk web server:

IP address: 143.89.40.4Port number: 80

More on this later

To receive messages, process must have identifierHost device has unique 32‐bit IP addressQ: does IP address of host on which process runs suffice for identifying the process?Answer: NO, many processes can be running on same host


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 RFCsAllows for interoperabilitye.g., HTTP, SMTP

Proprietary protocols:e.g., KaZaA


What Transport Service does an App Need?

Data lossSome apps (e.g., audio, video) can tolerate some lossOther apps (e.g., file transfer, telnet) require 100% reliable data transfer

BandwidthSome apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”Other apps (“elastic apps”) make use of whatever bandwidth they get (Best‐Efforts)

TimingSome apps (e.g., Internet telephony, interactive games) require low delay to be “effective”



2.1 Principles of network applications

app architecturesapp requirements

2.2 Web and HTTP2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P file sharing2.7 Socket programming with TCP2.8 Socket programming with UDP


HTTP Overview

HTTP: hypertext transfer protocolWeb’s application layer protocolClient/server model

Client: browser that requests, receives, “displays”Web objectsServer:web server sends objects in response to requests

HTTP 1.0: RFC 1945HTTP 1.1: RFC 2068

HTTP request

HTTP request

HTTP response

HTTP response

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator


HTTP Overview (cont.)

HTTP is “stateless”Server maintains no

Uses TCP:Client initiates TCP connection (creates socket) to server, port 80Server accepts TCP connection from clientHTTP messages (application‐layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)TCP connection closed

information about past client requests


Protocols that maintain “state”are complex!Past history (state) must be maintainedIf server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

HTTP Connections

Nonpersistent HTTP Persistent HTTPAt most one object is sent over a TCP connectionEach TCP connection is closed after the server sends the objectHTTP/1.0 uses nonpersistent HTTP

Multiple objects can be sent over single TCP connection between client and serverHTTP/1.1 uses persistent connections in default mode


Persistent HTTP

Nonpersistent HTTP issues:Requires 2 RTTs per objectOS overhead for each TCP connectionBrowsers often open parallel TCP connections to fetch referenced objects

Persistent HTTPServer leaves connection open after sending responseSubsequent HTTP messages between same client/server sent over open connection

Persistent without pipelining:Client issues new request only when previous response has been receivedOne RTT for each referenced object

Persistent with pipelining:Default in HTTP/1.1Client sends requests as soon as it encounters a referenced objectAs little as one RTT for all the referenced objects


User-Server State: Cookies

Why need cookies?HTTP server is stateless

Simplify server design and handle thousands of simultaneous TCP connections

It is often desirable to identify usersRestrict user accessServe content as a function of the user identity

Cookies [RFC 2109]Allow sites to keep track of users


User-Server State: Cookies

Many major Web sites use cookies

Four components:

Example:Susan access Internet always from same PCShe visits a specific e‐commerce site for first timeWhen initial HTTP requests

1) Cookie header line of HTTP responsemessage

2) Cookie header line in HTTP requestmessage

3) Cookie file kept on user’s host, managed by user’s browser

4) Back‐end database at Web site

arrives at site, site creates a unique ID and creates an entry in backend database for ID


Cookies: Keeping “State” (cont.)

client serverusual http request msg

usual http response +Set-cookie: 1678

usual http request msgcookie: 1678

usual http response msg

usual http request msgcookie: 1678

usual http response msg

servercreates ID

1678 for user

Cookie file

ebay: 8734

entry in backend database

access

access

Cookie file

amazon: 1678ebay: 8734

cookie-specificaction

one week later:

Cookie file

amazon: 1678ebay: 8734

cookie-spectific

action


Cookies (continued)What cookies can bring:

AuthorizationShopping cartsRecommendationsUser session state (Web e‐mail)

162: Application Layer

Cookies and privacy:Cookies permit sites to learn a lot about youYou may supply name and e‐mail to sites

aside

How to keep “state”:Protocol endpoints: maintain state at sender/receiver over multiple transactionsCookies: http messages carry state

Web Caches (Proxy Server)

User sets browser: Web accesses via cacheBrowser 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

clientorigin server

origin server

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response


More about Web Caching

Why Web caching?Cache acts as both client and serverTypically cache is installed by

Reduce response time for client request.Reduce traffic on an institution’s access link.Internet dense with caches:

ISP (university, company, residential ISP)

enables “poor” content providers to effectively deliver content (but so does P2P file sharing)


Conditional GET

Goal: don’t send object if cache has up‐to‐date cached versionCache: 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

The copy of an object residing in the cache may be stale 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



2.1 Principles of network applications2.2 Web and HTTP2.3 FTP2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS



FTP: the File Transfer Protocol

Transfer file to/from remote hostClient/server model

Client: side that initiates transfer (either to/from remote)Server: remote host

Ftp: RFC 959Ftp server: port 21

file transferFTP

serverFTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host


FTP: Separate Control, Data Connections

FTP client contacts FTP server at port 21, specifying TCP as transport protocolClient obtains authorization over control connectionClient browses remote directory by sending commands over control connection.When server receives file transfer command, server opens 2nd TCP connection (for file) to clientAfter transferring one file, server closes data connection.


TCP control connectionport 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

FTPclient

FTPserver



SMTP, POP3, IMAP

2.5 DNS



Electronic Mail

Three major components:User agents Mail servers Simple mail transfer protocol: SMTP

User Agenta.k.a. “mail reader”Composing, editing, reading mail messagese.g., Eudora, Outlook, elm, Netscape MessengerOutgoing, incoming messages stored on server


user mailbox

outgoing message queue

useragent

useragent

useragent

useragent

useragent

useragent

mailserver

mailserver

mailserver

SMTP

SMTP

SMTP

Electronic Mail: Mail Servers

Mail ServersMailbox contains incoming messages for userMessage queue of outgoing (to be sent) mail messagesSMTP protocol between mail servers to send email messages

Client: sending mail serverServer: receiving mail server


useragent

useragent

useragent

useragent

useragent

useragent

mailserver

mailserver

mailserver

SMTP

SMTP

SMTP

user mailbox

outgoing message queue

Scenario: Alice sends Message to Bob

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

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

mailserver

mailserver user

agent

1

2 3 4 56

useragent


Mail Access Protocols

SMTP: delivery/storage to receiver’s serverMail 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.

SMTP SMTP accessprotocol

sender’s mail server

useragent

receiver’s mail server

useragent




SMTP, POP3, IMAP

2.5 DNS



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 databaseimplemented in hierarchy of many name serversApplicationlayer protocol host, routers, name servers to communicate to resolve names (address/name translation)

Note: core Internet function, implemented as application‐layer protocolComplexity at network’s “edge”


Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:Client queries a root server to find com DNS server (top‐level domain)Client queries com DNS server to get amazon.com DNS server (authoritative)Client queries amazon.com DNS server to get IP

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

address for www.amazon.com


DNS: Root Name ServersContacted by local name server that can not resolve nameRoot name server:

Contacts authoritative name server if name mapping not knownGets mappingReturns mapping to local name server


13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 3 other locations)

k RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

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 TLDEducause for edu TLD

Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail)

Can be maintained by organization or service provider


Local Name Server

Does not strictly belong to hierarchyEach ISP (residential ISP, company, university) has oneAlso called “default name server”

When a host makes a DNS query, query is sent to its local DNS serverActs as a proxy, forwards query into hierarchy


Example root DNS server

Host at cis.poly.eduwants IP address for gaia.cs.umass.edu

local DNS serverdns.poly.edu

23

4

5

6781

requesting hostcis.poly.edu

gaia.cs.umass.edu

authoritative DNS serverdns.cs.umass.edu

TLD DNS server

Recursive query

Iterated query


Recursive QueriesRecursive query:

Puts burden of name resolution on contacted name serverHeavy load?

Iterated query:Contacted server replies with name of server to contact“I don’t know this name, but ask this server” requesting host

cis.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 query


Chapter 2: Application layer

2.1 Principles of network applications

app architecturesapp requirements

2.2 Web and HTTP2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS



P2P: Centralized Directory

Original “Napster” design1) When peer connects, it informs central server:

IP addressContent

2) Alice queries for “Hey Jude”3) Alice requests file from Bob

centralizeddirectory server

peers

Alice

Bob

1

1

1

12

3


P2P: Problems with Centralized Directory

Single point of failurePerformance bottleneckCopyright infringement

File transfer is decentralized, but locating content is highly centralized


Query Flooding: Gnutella

Fully distributedNo central server

Public domain protocolMany Gnutella clients implementing protocol

Overlay network: graphEdge between peer X and Y if there’s a TCP connectionAll active peers and edges is overlay netEdge is not a physical linkGiven peer will typically be connected with < 10 overlay neighbors


Gnutella: Protocol


Query

QueryHit

Query

Query

QueryHit

Query

Query

QueryH

it

File transfer:HTTPQuery message

sent over existing TCPconnections

Peers forwardQuery message

QueryHit sentover reversepath

Scalability:limited scope flooding

Hierarchical Overlay

Between centralized index, query flooding approachesEach peer is either a group leader or assigned to a group leader.

TCP connection between peer and its group leader.TCP connections between some pairs of group leaders.

Group leader tracks content ordinary peer

group-leader peer

neighoring relationshipsin overlay network

in its children


Comparing Client-Server, P2P Architectures

Question : How much time distribute file initially at one server to N other computers?

us: server upload bandwidth


us

u2d1 d2u1

Serverui: client/peer i upload bandwidth

42

uN

dN Network (with abundant bandwidth)

di: client/peer i download bandwidthFile, size F

Comparing Client-Server, P2P Architectures

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

N

Min

imum

Dis

tribu

tion

Tim

e P2PClient-Server

P2P Case Study: 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

BitTorrent (1)

File divided into 256KB chunksPeer joining torrent:

Has no chunks, but will accumulate themover timeRegisters with tracker to get list of peers, connects to subset of peers (“neighbors”)

While downloading, peer uploads chunks to other peersPeers may come and goOnce peer has entire file, it may (selfishly) leave or (altruistically) remain


BitTorrent (2)Pulling ChunksAt any given time, different peers have different subsets of file chunksPeriodically, a peer (Alice) asks each neighbor for list of chunks that they have.Alice issues requests for her missing chunks

rarest first

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

P2P Case Study: Skype

P2P (pc‐to‐pc, pc‐to‐phone, phone‐to‐pc) Voice‐Over‐IP (VoIP) application

also IMProprietary application‐layer protocol (inferred via reverse engineering) Hierarchical overlay

Skype clients (SC)

Supernode(SN)

Skype login server


Skype: Making a Call

User starts SkypeSC registers with SN

list of bootstrap SNs

SC logs in (authenticate)

Skype login server

Call: SC contacts SN will callee ID

SN contacts other SNs(unknown protocol, maybe flooding) to find addr of callee; returns addr to SC


SC directly contacts callee, over TCP48

Chapter 2: Summary

Typical request/reply message exchange:

Client requests info or serviceServer responds with data, status code

Message formats:Headers: fields giving info about dataData: info being communicated

Most importantly: learned about protocols


Important themes: Control vs. data msgs

In‐band, out‐of‐bandCentralized vs. decentralized Stateless vs. statefulReliable vs. unreliable msg transfer “Complexity at network edge”

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Documents

Chapter 2 Application Layerqianzh/comp562-2009/notes/Chapter2_2009.pdf · Chapter 2: Application Layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic