Chapter 8 Application Layer based on: Computer Networking: A Top Down Approach, 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. A note

Chapter 8Application Layer

based on:

Computer Networking: A Top Down Approach, 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form,

that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2010J.F Kurose and K.W. Ross, All Rights Reserved

Application 8-1

Application Layer 8-2

Chapter 8: 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) DNS (web

addressing)

mention briefly other application protocols


Creating network applications

Write programs that run on different end

systems and communicate over a

network. e.g., Web server software

talks to 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

Semantics: list of needed fields per msg & meaning of each field

Syntax of each message type: where fields are located, how they are formatted

Rules for when and how processes send & respond to messages

2TYPES OF PROTOCOLS: Public-domain protocols: defined in RFCs

(mostly) enable interoperability e.g: 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 Host /Server Host

server host : always-on host serves remote users 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 server host or a client host

(see Peer to Peer architecture)

Client Process/Server Process

Note: Server host usually runs several server processes in

parallel

server process: waits all the time for work its IP address must be

known to the client process at interaction start. WHY?

client process: initiates interaction with

server process when it needs server’s help

if server process agrees : they exchange messages the messages enable

them to cooperate in order to get the work done

Does client address have to be known in advance? Why?Application Layer 8-8

Process Interaction ScenarioClient 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 on client host

(usually) or on a server host

example: in e-mail appl. the sender mail server runs a client process, while the receiving MS runs a server process

Pure P2P architecture

no always-on server arbitrary end systems

(usually workstations)directly communicate

peers are intermittently connected and change IP addresses

highly scalable but difficult to manage

peer-peer

Application 8-10

Hybrid of client-server and P2PSkype

voice-over-IP P2P application centralized server: finding

address of remote party: client-client connection: direct

voice communicationInstant messaging

chatting btw. two users is P2P centralized service: client

presence detection/location• user registers its IP address

with central server when it comes online

• initiating user contacts central server to find IP addresses of buddies

Application 8-11

datainitializati

on

C-S

C-SP2P

P2P

What transport service does an app need?

Data loss some apps (e.g., file

transfer, telnet) require 100% reliable data transfer

other apps (e.g., audio) can tolerate some loss

Timing some apps (e.g.,

Internet telephony, interactive games) require low delay to be “effective”

Throughput some apps (e.g.,

multimedia) require ≥ minimum amount of throughput to be “effective”

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

Security encryption, data

integrity, …

Application 8-12


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 msecsometimes

#14

HTTP

Application Layer


Web and HTTP

First a review 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 applicationsProtocols 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

note


HTTP connectionsNonpersistent HTTP At most one object

sent over a TCP connection.

is default in HTTP/1.0 use Keep Alive header

to force Persistent mode

Disadvantages:1. wastes 1 RTT per

message2. adds overhead packets3. stays most of the time in

Slow Start Cong. Ctrl Partial help:

use parallel connections

Persistent HTTP Multiple objects sent

over single TCP connection between client and server. Last request contains the

Close Connection header is default in HTTP/1.1Persistent with Pipelining Send requests for

referenced objects one after the other, without waiting for response to previous requests

saves another RTT per object

two types of HTTP messages: request, response

all msgs in ASCII (human-readable) format HTTP request message example:

GET tau.ac.il/index.html HTTP/1.1\r\nHost: www-net.cs.umass.edu\r\nUser-Agent: Firefox/3.6.10\r\nAccept: text/html,application/xhtml+xml\r\nAccept-Language: en-us,en;q=0.5\r\nAccept-Encoding: gzip,deflate\r\nAccept-Charset: ISO-8859-1,utf-8;q=0.7\r\nConnection: Close\r\n\r\n

carriage return, line feed at startof line indicatesend of header lines

HTTP request message

request line

header lines

Application 8-19

carriage return character

line-feed charactermethod (other methods: POST, HEAD, …)

HTTP request message: general format

Application 8-20

requestline

headerlines

body

Uploading form inputPOST method: browser sends to server data that was input by

user into a page form data appended in entity body of message

Such input can be also sent using GET in this case the data is loaded onto the URL

field of the request line appears after a “?” sign; example of such URL:

www.somesite.com/animalsearch?monkeys&banana

Application 8-21


Methods

HTTP/1.0 GET POST HEAD

asks server to leave requested object out of response

Used for:• debugging• troubleshooting

HTTP/1.1 GET, POST, HEAD PUT

uploads file in the entity body to the server to be saved path specified in the path stated in the URL field

DELETE deletes file specified in

the URL field

HTTP response message

status line

header lines

data, e.g., requestedHTML file

Application 8-23

HTTP/1.1 200 OK\r\nDate: Sun, 26 Sep 2010 20:09:20 GMT\r\nServer: Apache/2.0.52 (CentOS)\r\nLast-Modified: Tue, 30 Oct 2007 17:00:02 GMT\

r\nETag: "17dc6-a5c-bf716880"\r\nAccept-Ranges: bytes\r\nContent-Length: 2652\r\nConnection: Close\r\nContent-Type: text/html;charset=ISO-8859-1\r\n\r\n

status code status phraseprotocol

entity body


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”: header line)

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:

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 cis.poly.edu.anything typed is sent to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

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!

Application 8-25

(or use wireshark!)


User-server “state”: cookiesMany Web sites use cookies

Four components:1) the HTTP server sends

response message with a ‘Set-Cookie:’ header

2) user’s browser puts the ‘Set-Cookie:’ header content ( “cookie”), labeled with server name, in the cookie file on user’s host,

3) next request message to this server will contain Cookie: header containingthe relevant cookie

4) cookie points to database record at server site (or holds client data for server)

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

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msg

cookie: 1678cookie-specificaction

access

ebay 8734usual http request

msgAmazon server

creates ID1678 for usercreate

entry

usual http response Set-cookie: 1678

ebay 8734amazon 1678

usual http request msg

cookie: 1678cookie-specificaction

accessebay 8734amazon 1678

backenddatabase

Application 8-27


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 youo you may supply name and

e-mail to sites advertising companies

obtain info across sites from inserted banner addso info collected by the

advertizing servero there are regulatory

restrictions on releasing it

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

and keeps file in its cache

Note: user’s browser has a private cache on its host.

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 or user’s organization.

Caching advantages Reduces response time

for client request. Reduces 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. time to hold object

Proxy should respect age and other restrictions

Caching example (1) assumptions average object size =

100,000 bits average request rate from

institution’s browsers to origin servers = 15/sec

delay from institutional router to any origin server and back to router = 2 sec

consequences utilization on LAN = 15% utilization on access link =

100% total delay = Internet delay +

access link delay + LAN delay = 2 sec + minutes +

milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Application 8-31

Caching example (2)

possible solution increase bandwidth of

access link to, say, 10 Mbpsconsequence utilization on LAN = 15% utilization on access link =

15% Total delay = Internet

delay + access delay + LAN delay

= 2 sec + msecs + msecs often a costly upgrade

originservers

public Internet


10 Mbps access link

institutionalcache

Application 8-32

Caching example (3)

possible solution: install cache locally consequence suppose hit rate is 0.4

40% requests will be satisfied almost immediately

60% requests satisfied by origin server

utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)

total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet


1.5 Mbps access link

institutionalcache

Application 8-33


Conditional GET Don’t send object if

cached version is up-to-date used if proxy has object

but cacheability conditions are missing or not satisfied

proxy specifies date of cached copy in HTTP request header:

server sends short response if cached copy is 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


Legacy Internet ApplicationsTelnet Remote terminal Enables user to enter line

commands to remote host

Remote host returns display to user.

characters sent one by one and echoed on display

sets char. encoding rules

Client-Server Email Mail client on user’s host

e.g. Outlook User mailbox on his Server Msg prepared on client, sent

to origin user’s Mail Server (MS) may use SMTP

MS sends to dest. MS using SMTP (a PUSH protocol)

dest. MS stores msg in mailbox

User collects msg using PULL protocol (POP3 or IMAP)

Webmail Client functionality is on

Server User has an HTTP GUI

terminal

FTP File transfer and remote

control of a file server Uses Telnet encoding

ruleso as do most other

applications

#36

DNS

Application Layer


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 database

implemented in hierarchy of many name servers (NS’s)

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

infrastructure function, implemented as an application-layer protocol

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


DNS

Why not centralize DNS? difficult to manage and

maintain single point of failure traffic volume distant centralized

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

addr. (by sending message to UDP port 53)

It then queries the com NS to get the NS for amazon.com It then queries the amazon.com NS to get IP address for

www.amazon.com

(*) The addresses of root servers are known to all name servers

(**) One possible scenario


The DNS Database architecture:Authoritative NS’s, Zones, TLD’s

Root NS’s: hold addresses of all Top Level Domain NSs

Top-level domains : domains with single word name generic TLD’s: com, org, net, edu, etc,

• e.g. Network Solutions maintains servers for com TLD top-level country domains: uk, fr, ca, jp, il, etc.

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 \ υ { subdomains that have their own

zones } Example:

tau.ac.il zone includes all university servers except dep”ts that have their own zones (e.g. cs.tau.ac.il & math.tau.ac.il )


DNS Name Resolution: Resolver and Local Name Server

Each host has a Resolver = a function that resides in the host and nows how to communicate with the DNS system

Each ISP (residential ISP, company, university) has a “Local name server” (it was called “Client’s NS” above) Also called “default name server”

When application needs to translate a DNS name, it calls the resolver on its computer thus, resolver acts as a kind of ‘proxy’ for the application

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

here again the LNS gives a proxy type service to the resolver LNS finds the answer in its cache or by querying other

name servers and sends answer to Resolver Resolver supplies answer to the application

requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

78

TLD DNS server

DNS name resolution example

host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated query: contacted server

replies with name of server to contact

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

There are 2 iterated queries in the example

Application 8-42

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

3recursive query: puts burden of name

resolution on contacted name server

heavy load?Note: actual scenario may

involve a mix of the two modes.

root name servers are usually iterative

NS may act iteratively on one type of queries, recursively on another

DNS name resolution example

Application 8-43


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: administrator downloads

a file the others copy the zone from the main one main NS notifies them 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=A name is hostname value is IP address

Type=CNAME name is alias name of

server value is the canonical

(“real”) name of that servere.g., www.ibm.com has real

name:servereast.backup2.ibm.com Type=MX name is domain, e.g.

gmail.com value is hostname of mail

server

DNS protocol, messages

DNS protocol : query and reply messages, both with same message format

msg header identification: 16 bit

query # ; reply to query uses same #

flags: query or reply recursion desired recursion available reply is authoritative

Application 8-46

DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

Application 8-47

Inserting records into DNS example: new domain (startup) “Network Utopia” register name networkuptopia.com at DNS registrar

(e.g., Network Solutions) provide names, IP addresses of authoritative name servers

(primary and secondary) of this domain registrar inserts two RRs into com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)(networkutopia.com, dns2.networkutopia.com, NS)(dns2.networkutopia.com, 212.212.212.5, A)(networkutopia.com, nili.networkutopia.com, MX)(www.networkutopia.com, a1.networkutopia.com, CNAME)etc.

What does each of these RRs define? What RRs are missing?

Application 8-48

#49

Peer to Peer Applications

Application Layer

Pure P2P architecture no always-on server arbitrary end systems

directly communicate peers are intermittently

connected and change IP addresses

No background server to initialize the system

Pure P2P appl. are rare.we discuss three topics:

file distribution searching for information case Study: Skype

peer-peer

Application 8-50

File Distribution: Server-Client vs P2P

Question : How much time it takes to distribute file from one server to N peers?

us

u2d1 d2u1

uN

dN

Server

Network (with abundant bandwidth)

File, size F

us: server upload bandwidth

ui: peer i upload bandwidth

di: peer i download bandwidth

Application 8-51

upload : sending to Networkdownload : receiving from Network

= dcs ≥ max {NF/us +F/max(di) ,

F/us +F/min(di) }

Time to distribute F to N clients by client/server

i

File distribution time: server-client

us

u2d1 d2u1

uN

dN

Server


F server sequentially

sends N copies: NF/us time

client i takes F/di

time to download

increases linearly in N(for large N)

Application 8-52

equal under best server scheduling arrangement

i

File distribution time: P2P

us

u2d1 d2u1

uN

dN

Server


F server must send one

copy: F/us time client i takes F/di time

to download NF bits must be

downloaded (aggregate) fastest possible upload rate: us + Sui

dP2P ≥ max { F/us, F/min(di) , NF/(us + Sui) }i i=1

Application 8-53

N

independent of N if the ui are all of same order of magnitude

equal under best data distribution arrangement

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

N

Min

imu

m D

istr

ibut

ion

Tim

e P2P

Client-Server

Server-client vs. P2P: example

Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us

Application 8-54

File distribution: 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

Application 8-55

BitTorrent (1)

file divided into 256KB chunks. peer joining torrent:

has no chunks, but will accumulate them over time

registers with tracker to get list of peers, connects to a chosen subset of peers (“neighbors”)

while downloading, peer uploads chunks to other peers.

peers may come and go once peer has entire file, it may (selfishly) leave

(“leech”) or (cooperatively) remain (“seed”)Application 8-56

BitTorrent (2)

Pulling Chunks at any given time,

different peers have different subsets of file chunks

periodically, a peer (Alice) asks each neighbor for list of chunks that they have.

then sends requests for her missing chunks rarest first

Sending Chunks: tit-for-tat Alice sends chunks to

four neighbors currently sending to her chunks at the highest rate Alice re-evaluates her

“top 4” list every 10 sec each 30 sec she

randomly selects another peer, starts sending chunks “optimistically unchoke”

he may be better than current neighbors

newly chosen peer may join top 4, if sends fast to her

Application 8-57

BitTorrent: Tit-for-tat

(1) Alice “optimistically unchokes” Bob(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates(3) Bob becomes one of Alice’s top-four providers

“optimistic unchoke” procedurehelps find better trading partners & get file faster!Application 8-58

Distributed Hash Table (DHT)

DHT: distributed P2P database database has (key, value) pairs; e.g:

key: Soc.Security number; value: human name

key: content title; value: IP address peers query DB with key

DB returns values that match the key peers can also insert (key, value) pairs

Application 8-59

DHT Identifiers

assign to each peer an integer identifier in range [0,2n-1]. Each identifier can be represented by n bits.

require each key to also be an integer in [0,2n-1].

since original key may be a string, we build aninteger key by applying an n-bit hash function h to the original key. e.g., key = h(“Led Zeppelin IV”) = 231567935 this is why they call it a distributed “hash” tableApplication 8-60

How to assign keys to peers? central issue:

assigning (key, value) pairs to peers that will store them.

rule: assign key to the peer that has the closest ID.

convention in our presentation: “closest” is the closest from above or equal.

e.g.,: n=4; peers: 1,3,4,5,8,10,12,15; key = 13, then successor peer = 15 key = 17, then successor peer = 1

o the numbers are arranged in a ring (cyclic order)

Application 8-61

1

3

4

5

810

12

15

Circular DHT (1)

each peer only aware of immediate successor and predecessor and communicates only with them.

this forms the “overlay network”Application 8-62

Circular DHT (2)

0001

0011

0100

0101

10001010

1100

1111

Who’s resp

for key 1110 ?I am

O(N) messageson avg required to resolve query in overlay network, when there are N peers 1110

1110

1110

1110

1110

1110

Application 8-63

Circular DHT with Shortcuts

each peer keeps track of IP addresses of predecessor, successor and shortcuts (call all of them “neighbors”).

reduced from 6 to 2 messages. possible to design shortcuts so O(log N) neighbors

and O(log N) messages per query

1

3

4

5

810

12

15

Who’s resp for key 1110?

Application 8-64

Peer Churn

Example: peer 5 abruptly leaves Peer 4 detects; makes 8 its immediate successor;

asks 8 who its immediate successor is; makes 8’s immediate successor (10) its second successor. How?

Who else needs to update its neighbor list? What happens with the database info held by

peer 5? What if peer 13 wants to join?

1

3

4

5

810

12

15

To handle peer churn, require each peer to know the IP address of its two successors.

Each peer periodically pings its two successors to see if they are still alive.

Application 8-65

P2P Case study: Skype

inherently P2P: pairs of users communicate.

proprietary application-layer protocol (inferred via reverse engineering)

hierarchical overlay with Supernodes (SNs)

Index maps usernames to IP addresses; distributed over SNs

Skype clients (SC)

Supernode (SN)

Skype login server

Application 8-66

Note: SNs are hosts with non-NAT addresses, so any host can contact them

Peers as relays problem when both

Alice and Bob are behind “NATs”. NAT prevents an

outside peer from initiating a call to insider peer

solution: using Alice’s and Bob’s

SNs, relay host is chosen

each peer initiates a TCP session with relay.

peers can now transfer their realtime data through NATs via relay

Note: relay is another Skype user

Application 8-67

EXTRA SLIDES

68Application Layer


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 HTTP

Nonpersistent 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 NonPersistent?


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 connection

Note:in Windows FTP user

writes GET for RETR PUT for STOR

אפקה תשע"א ס"א


Electronic MailFour major components: user agents mail servers SMTP protocol btw servers POP3/ IMAP protocols to

retrieve mail by user agent

User Agent a.k.a. “mail reader” composing, editing,

reading mail messages e.g., Outlook, Eudora, elm,

Netscape Messenger sends outgoing messages

to server collects incoming

messages from server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP



Electronic Mail: mail serversMail Servers mailbox contains

incoming messages for user

message queue of outgoing (to be sent) mail messages

SMTP protocol used between mail servers to send email messages SMTP client:

the sending mail server

SMTP server: the receiving mail server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

Note:Some user agents use SMTP to send mail to their mail server



Electronic Mail: SMTP [RFC 821, 2821]

uses TCP to reliably transfer email messages from client to server SMTP server port 25 (WKP)

direct transfer: sending-server to receiving-server three phases of transfer

handshaking (greeting) transfer of messages closing connection

command/response interaction commands: ASCII text response: status code (number) and phrase

messages must be in 7-bit US-ASCII both the SMTP handshake and the message itself



Scenario: Alice sends message to Bob1) Alice uses UA (*) to

compose message with “to” field: [email protected]

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

3) Alice’s mail server (=SMTP client) opens TCP connection to 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 uses his user agent to retrieve message from mailbox and reads it

* UA=User agent (e.g. Outlook)

useragent

mailserver

mailserver user

agent

1

2 3 4 56



Sample SMTP interaction <client sets up a TCP connection to server port 25> S: 220 hamburger.edu 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? Mail headers skipped here C: How about pickles? (see a full mail example C: . on slide 73) S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection



SMTP: final words

SMTP uses persistent connections SMTP requires message (header & body) to be

in 7-bit ASCII SMTP server uses CRLF.CRLF

(single dot in a line)to signal end of message

Comparison with HTTP: HTTP: (mainly) PULL SMTP: always PUSH both have ASCII

command/response interaction, status codes

HTTP: a page may contain pictures and other objects each object in own message

SMTP: originally only ASCII text with MIME format, can

include any type of objects

objects sent inside msg.



Original Email msg format [RFC 822]

SMTP: protocol for transferring email msgs

Original email msg FORMAT:RFC 822: standard for text

message format: header lines, e.g.,

To: From: Subject:different from SMTP

commands ! body

the “message” text, ASCII characters only

header

body

blankline



MIME format: multimedia extensions MIME: Multimedia Mail Extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type and encoding method, this enables multimedia content

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,

parameters

method usedto encode data (*)

MIME version

encoded data(*) data always encoded into 7-bit ASCII



Mail access protocols

SMTP: mail delivery to receiver’s server – PUSH Mail access protocols: retrieval from server to agent – PULL

they require authentication of agent by server POP: Post Office Protocol [RFC 1939]

• download all messages and keep at your workstation IMAP: Internet Mail Access Protocol [RFC 1730]

• enables organizing mail folders on server• you can read mail conveniently from any workstation

HTTP: (e.g. gmail) - display tool for agent located at the server

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”: can get 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

Documents

Chapter 8 Application Layer based on: Computer Networking: A Top Down Approach, 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. A note