Lecture 4b

Data Communication I Lecture 4a

presented by Werner Wild, CEO Evolution Innsbruck, San Francisco, Zurich Contact: [email protected]

Today’s Lecture Plan 

Globe, Review TCP , UDP

Resolver Discovery Services  URN

resolution breaks into 2 steps:

– finding the appropriate resolver (RDS), and – then resolving the URN. kris RDS fer: r nd o e b u n: b o t z o. n l i ng l c . e nz:w elecom : e on r n .t u : ph n k r s u 2. A nd

Client

i 13. F . Fin

d ur n:ph one :nz: wel ling 4. + t on: 64bu b 4-5 end 55orfe 555 r:kr URN 5 is

urn.telecom.co.nz

Resolver

Alternative Architectures  Locating

resources is a more difficult problem if those resources are mobile.  This includes people, hardware, data and programs.  Alternative architectures must be designed to locate mobile resources. – Globe – Nomad

Globe  The

Globe system details a location service that is designed to: – scale globally (load distribution), and – handle mobility (via locality of reference).

 Distributed

Object Model:

– One ‘object’ is split over n hosts. – Object is a composite -> unit of identity.  Developed

at Virje University in Holland.

Globe Quad Tree (Load Balancing)  

The Globe location service divides the world geographically using a quad tree. When load increases above a load threshold in a region, the region is further divided.

Near empty regions are (physically) merged with neighbors. But still exist logically.

How it works (Insertion). A rooted hierarchical tree.  Insertions start at leaf:

Ref A, A Ref B, ... root Ref



– bottom up,

– to the root, 

Root must contain all references.

Ref B, Ref A

Ref B

Ref A

Ref B

B

Ref A

A

How it works (Location). 

Searches also start at local leaf, – geographic locality -> root is not traversed, – worst-case is to root

Uses heuristics to organize the nodes.  B finds A  A finds C

root Ref A, Ref B, ...

Ref B, Ref A

?



Ref A

?

B

Ref A

A

C

How it works (Mobility). 

Updates also start at local leaf, – geographic locality -> root is not traversed, – worst-case is to root

Only need to update until we find an existing reference, then update.  E.g. A moves to A’

root Ref A, Ref B, ...

Ref A



Ref A’ A

Ref A

A’ A

Problems  Root

must know all references (Scale!).

– Uses heuristics to partition the root onto n physical distributed machines.  Searching

is fast, but:

– only for logically close objects (A&C).  Updating

location due to mobility,

– slower the further the object moves.  So,

resolution times are inconsistent.

Nomad  The

Nomad Location Service is designed

– specifically to handle mobility, in – large scale distributed systems.  Application

Object Cluster Model:

– n application objects per cluster, – cluster is the unit of identity, and location.  Developed

in New Zealand

applications

Review the Transport Layer

technology

transport protocol

Layer Architecture Application

Transport

mail, file transfer, web

tcp, udp

Application

Transport

ip Internet

Internet

Internet

Media access

Media access

Media access

Roles and Responsibilities  Logical

communication between processes  Message oriented – Best effort, single message delivery  Connection

oriented

– Sequenced, error free, stream – Flow and congestion control

The Transport Layers application ? UDP  User

Datagram Protocol  Independent segments  Best effort delivery

TCP  Transmission

Control

Protocol  Reliable stream  Manages segments (parts of stream)

Multiplexing

application

UDP

TCP IP

  

IP (network layer) receives all segments for host IP delivers to appropriate transport layer Transport protocol delivers to socket (application)

The Role of Headers transport header network header

source destination other port port header

source destination protocol host host

 Each

protocol has a header  Network header addresses machines – Denotes protocol (TCP or UDP)  Transport

header addresses ports (sockets)

data

Port and Sockets  Previously

seen from the application layer.  Port numbers 16 bits, 0->65535.  Port numbers 0->1023 are well known port numbers, and are restricted.  Why does a transport header have both source and destination port numbers?  Surely the destination port number alone is sufficient to identify the recipient?

Simple Example  Client

is assigned some random unused port X by its host.  The SMTP server runs on well known port 25.  Reply

is to destination port X, with src port 25

host A

source port: x dest. port: 25

server B

source port:25 dest. port: x

port use: simple smtp

What About Two Clients? host A

source port: x dest. port: 25

server B

source port:25 dest. port: x

Source IP: C Dest IP: B source port: y dest. port: 80

port use: simple smtp

Web client host A

Web client host C

Source IP: A Dest IP: B source port: x dest. port: 80

Source IP: C Dest IP: B source port: x dest. port: 80

Web server B port use: Web server

Ports and Sockets - Detail  Server

listens on a socket for a connection  Connect request causes new socket on same port  Server uses new thread to service connection

listener thread

worker worker

TCP server socket socket port

UDP  “best

effort” service, segments may be: – lost – delivered out of order

 connectionless:

– no handshaking between sender, receiver – each segment handled independently

 Why 

  

have UDP?

no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired

More on UDP includes  UDP used

for

header

– DNS – SNMP – Multimedia streaming Loss tolerant  Rate sensitive

src port no dest port no length

checksum



 reliable

data

transfer

– add reliability at application layer

32 bits

UDP Checksum Goal: detect “errors” in transmitted segment Sender  



treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field

Receiver  

compute checksum of received segment check if computed checksum equals checksum field value: – NO - error detected – YES - no error detected. But maybe errors nonetheless?

TCP Socket Structure Client process

Server process 3-way handshake

client socket stream data

“public” socket “connection” socket

TCP Steps Client Server (on hostId) Create public socket with port = x

Create socket Connect to hostId, port x Send request on client socket

Wait for connection create connection socket Read request from connection socket Write reply to connection socket

Read reply from client socket Close client socket

Java server (TCP) import java.io.*; import java.net.*; class TCPServer {

Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket

public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));

Java server (TCP), cont Create output stream, attached to socket Read in line from socket

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n';

Write out line to socket

outToClient.writeBytes(capitalizedSentence); } }

}

End of while loop, loop back and wait for another client connection

Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; Create input stream Create client socket, connect to server Create output stream attached to socket

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Example: Java client (TCP), cont. Create input stream attached to socket

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine();

Send line to server

outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine();

Read line from server

System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); }

}

The End.  We

learned about:

– the Socket API connecting them to the transport layer. 

reliable (TCP) vs. unreliable (UDP) msg transfer

 Thank

you for your attention !

Sources  For

the preparation of this lectures a lot of sources where used – my special thanks go to : – – – –

Univ. Auckland Univ. California – San Diego (UCSD) Univ. Hongkong … many others …