10/23

Two types of Networks

- Connectionless (IP, UDP)

- Connection Oriented (TCP)

Connectionless Network

- No setup is required before transmitting data.

- Each packet is handled independently

- No cleanup is necessary after sending data

- Think of “postcards”

Connection Oriented

- Sender requests connection to server

- Sender waits until connection is formed

- It leaves the connection open while sending

- Terminates connection when there is no more data to transmit

- Think of “telephone calls”

Connection Oriented vs Connectionless

Connection Oriented

- Has a set up overhead

- Is reliable

- Can reserve bandwidth in some networks (plane line, ATM)

- Examples: TCP Protocol, telephone system, ATM

Connectionless

- Has less overhead

- Application can have more control in message retransmission, error handling, etc

- Allows broadcasts and multicasts

- Examples: UPC, IP

When to use TCP or UPD ?

TCP:

- Used in applications that need high (complete) reliability.

- Email, file transfer

UDP:

- Used in applications that do no need a high degree of reliability but require high response.

- Examples : voice over internet

- Also used in applications that want to have more control in the delivery of messages.

- These applications have to implement reliability on top of UDP.

- Danger: Application may “reinvent” TCP

- Also used in applications that need broadcast/multicast

- TCP is a point to point protocol (host to host)

Network Performance Measurement

- Delay

o Time it takes for a packet to arrive at its destination

o Cases :

§ Propagation delay (intrinsic network media)

§ Switching delay (hardware)

§ Queuing delay (time a packet stays in a buffer waiting to be delivered)

o Think of “Length of Pipe”

- Throughput

o Capacity in bits/sec

o Width of pipe

Relationship between delay and throughput:

- when network is idle

o Queue is empty

o Queuing delay is zero

o There is no need to store data in buffer queue

- When load increases

o Queuing delay increases

Load is defined as the ratio between throughputs to capacity

Capacity -> Max throughput

D -> total Delay

D₀ = delay when network is idle

U = Utilization (load) 0 <= U <= 1

U=0 (Network is idle)

U=1 ( Network is used)

D = D₀ / (1 –U)

Network is idle U =0 -> D = D₀

Network is highly used (congested), U =1 -> D= infinite

Consequence:

Any network that operates with utilization close to 100% of capacity is doomed

Delay Throughput Product

· Delay - Time to cross network in seconds

· Throughput – Capacity in bits per second

· Delay(sec) x Throughput(bit) = D x T bits

Delay = 0.5 seconds

Throughput = 128 Kbps

#bits in transit = 0.5 seconds x 128 Kbps = 64Kbits

Protocol:

• Agreement about communication
• Necessary for interoperation among different hardware and software
• Specifies
• Format of messages
• Rules of exchange
• Handling of problems

Family of protocols

• Designed to work together
• Each protocol in the family solves a different communication problem
• It is also called “protocol suite” or “protocol family”
• Designed by layers

ISO-7 layer reference model

• Intended for protocol designers

Layer 7	Application	- Individual Application
	Presentation	- Data representation
	Session	- Login and Passwords
	Transport	- Reliability
	Network	- Packet Forwarding
	Data Link	- Hardware and frame definitions
Layer 1	Physical	- Underlying hardware

• Outdated
• It was designed when networking was mostly used to connect terminals & mainframe
• It does not include “internet” layer

Internetworking

• Heterogeneity is inevitable
• No single network technology is best for all needs
• Internet goal

o Connects physical networks and creates a software that makes the resulting system homogeneous

Router

· A device used in the internet to connect heterogeneous networks.

· It is also called - Internet router

- Internet gateway

• A router can connect networks that have different
• Technologies
• Media
• Addressing scheme
• Frame formats

The Internet is a collection of

• Networks
• Routers that interconnect networks
• Hosts connected to networks

Goal of Internet

• To hide heterogeneity to user

• The internet is a virtual network that runs on top of other networks and that has its own addressing and name schemes

Friday (27 Oct 2000)

ss = accept (…)

# if defined (DEGUB)

Process HTTP Request(ss);

Close (ss)

# else

// standard procedure

intpd = fork()

if (pd ==0)

{ processHTTPRequest(ss);

exit;

}

close(ss);

#endif

- Debugging a “core” file + process

Crashes write a “core” file

gdb myserver core

gdb > whre (prints stack trace)

strcpy

processHTTPRequest line 100 server.cc

main line 1801 server.cc

gdb > up } go to correct any in stack trace

gdb > down }

gdb > print <variable>

Debugging a server

- Disable fork(), so the master server is going to crash if there is a problem.

- Running gdb on a program

gdb myserver

gdb run

……. Running …..

to stop program, ctrl-C

> where

> break file:line -> stop process when it reaches this line

> break

>cont

> help

> next -> execute function, execute program name by line

> step -> Step into function

CGI-BIN:

If a request has the form GET /cig-bin/<prog>?a=b&c=d

Then:

processHTTPrequest()

{ if cgi-bin… in request

- get <prog>

- get <query>

// set env variable

Build string

S1 = query_string = <query>

S2 = Requestmehtod = To_get;

// use putenv to set

putenv(s1)

putenv(s2)

- Redirect stdout to slave socket

- execvp for

http-root-dir/cgi-bin/<prog>