Advance Computer Networks Lecture#06 Instructor: Engr. Muhammad Mateen Yaqoob.

Advance Computer NetworksLecture#06

Instructor:Engr. Muhammad Mateen Yaqoob

Mateen Yaqoob

Department of Computer Science

DNS: domain name system

people: many identifiers: SSN, name,

passport #Internet hosts, routers:

IP address (32 bit) - used for addressing datagrams

“name”, e.g., www.yahoo.com - used by humans

Q: how to map between IP address and name, and vice versa ?

Domain Name System: distributed database

implemented in hierarchy of many name servers

application-layer protocol: hosts, name servers communicate to resolve names (address/name translation) note: core Internet

function, implemented as application-layer protocol

complexity at network’s “edge”

Mateen Yaqoob


DNS: services, structure why not centralize

DNS? single point of failure traffic volume distant centralized

database maintenance

DNS services hostname to IP

address translation host aliasing

canonical, alias names

mail server aliasing load distribution

replicated Web servers: many IP addresses correspond to one name

A: doesn’t scale!

Mateen Yaqoob


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

DNS: a distributed, hierarchical database

client wants IP for www.amazon.com; 1st approx: client queries root server to find com DNS server client queries .com DNS server to get amazon.com

DNS server client queries amazon.com DNS server to get IP

address for www.amazon.com

… …

Mateen Yaqoob


Pure P2P architecture no always-on server arbitrary end systems

directly communicate peers are

intermittently connected and change IP addresses

examples: file distribution

(BitTorrent) Streaming (KanKan) VoIP (Skype)

Mateen Yaqoob


File distribution: client-server vs P2P

Question: how much time to distribute file (size F) from one server to N peers? peer upload/download capacity is limited

resource

us

uN

dN

server

network (with abundant bandwidth)

file, size F

us: server upload capacity

ui: peer i upload capacity

di: peer i download capacityu2 d2

u1 d1

di

ui

Mateen Yaqoob


File distribution time: client-server server transmission:

must sequentially send (upload) N file copies: time to send one copy:

F/us

time to send N copies: NF/us

increases linearly in N

time to distribute F to N clients using

client-server approach Dc-s > max{NF/us,,F/dmin}

client: each client must download file copy dmin = min client download

rate min client download time:

F/dmin

us

network

di

ui

F

Mateen Yaqoob


File distribution time: P2P server transmission:

must upload at least one copy time to send one copy:

F/us

time to distribute F to N clients using

P2P approach

us

network

di

ui

F

DP2P > max{F/us,,F/dmin,,NF/(us + Sui)}

client: each client must download file copy min client download time:

F/dmin clients: as aggregate must download NF bits max upload rate (limting max download

rate) is us + Sui

… but so does this, as each peer brings service capacityincreases linearly in N …

Mateen Yaqoob


P2P file distribution: BitTorrent

tracker: tracks peers participating in torrent

torrent: group of peers exchanging chunks of a file

Alice arrives …

file divided into 256Kb chunks peers in torrent send/receive file chunks

… obtains listof peers from tracker… and begins exchanging file chunks with peers in torrent

Mateen Yaqoob


peer joining torrent: has no chunks, but will

accumulate them over time from other peers

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

P2P file distribution: BitTorrent

while downloading, peer uploads chunks to other peers

peer may change peers with whom it exchanges chunks

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

or (altruistically) remain in torrent

Mateen Yaqoob


Socket programming

goal: learn how to build client/server applications that communicate using sockets

socket: door between application process and end-end-transport protocol

Internet

controlledby OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket

Mateen Yaqoob


Socket programming

Two socket types for two transport services:

UDP: unreliable datagram TCP: reliable, byte stream-oriented

Application Example:1. Client reads a line of characters (data)

from its keyboard and sends the data to the server.

2. The server receives the data and converts characters to uppercase.

3. The server sends the modified data to the client.

4. The client receives the modified data and displays the line on its screen.

Mateen Yaqoob


Socket programming with UDPUDP: no “connection” between client &

server no handshaking before sending data sender explicitly attaches IP destination

address and port # to each packet rcvr extracts sender IP address and port#

from received packet

UDP: transmitted data may be lost or received out-of-order

Application viewpoint: UDP provides unreliable transfer of groups

of bytes (“datagrams”) between client and server

Client/server socket interaction: UDP

closeclientSocket

read datagram fromclientSocket

create socket:clientSocket =DatagramSocket()

Create datagram with server IP andport=x; send datagram viaclientSocket

create socket, port= x:serverSocket =DatagramSocket()

read datagram fromserverSocket

write reply toserverSocketspecifying client address,port number

Application 2-14

server client

Mateen Yaqoob


Mateen Yaqoob


Socket programming with TCPclient must contact

server server process must

first be running server must have

created socket (door) that welcomes client’s contact

client contacts server by:

Creating TCP socket, specifying IP address, port number of server process

when client creates socket: client TCP establishes connection to server TCP

when contacted by client, server TCP creates new socket for server process to communicate with that particular client allows server to talk

with multiple clients source port numbers

used to distinguish clients

TCP provides reliable, in-orderbyte-stream transfer (“pipe”) between client and server

application viewpoint:

Mateen Yaqoob


Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =serverSocket.accept()

create socket,port=x, for incoming request:

welcomeSocket=serverSocket ()

create socket,connect to hostid, port=xclientSocket = socket()

server (running on hostid) client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

Mateen Yaqoob


Transport Layerour goals: understand

principles behind transport layer services: multiplexing,

demultiplexing reliable data

transfer flow control congestion

control

learn about Internet transport layer protocols: UDP: connectionless

transport TCP: connection-

oriented reliable transport

TCP congestion control

Mateen Yaqoob


Transport services and protocols provide logical

communication between app processes running on different hosts

transport protocols run in end systems send side: breaks

app messages into segments, passes to network layer

rcv side: reassembles segments into messages, passes to app layer

more than one transport protocol available to apps Internet: TCP and

UDP

applicationtransportnetworkdata linkphysical

logical end-end transport


Mateen Yaqoob


Transport vs. network layer network layer:

logical communication between hosts

transport layer: logical communication between processes relies on,

enhances, network layer services

12 kids in Ann’s house sending letters to 12 kids in Bill’s house:

hosts = houses processes = kids app messages =

letters in envelopes transport protocol =

Ann and Bill who demux to in-house siblings

network-layer protocol = postal service

household analogy:

Mateen Yaqoob


Internet transport-layer protocols reliable, in-order

delivery (TCP) congestion control flow control connection setup

unreliable, unordered delivery: UDP no-frills extension

of “best-effort” IP services not

available: delay guarantees bandwidth

guarantees



networkdata linkphysical







logical end-end transport

Advance Computer Networks Lecture#06 Instructor: Engr. Muhammad Mateen Yaqoob.

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