CS422 ClassNotes

Oct.10 - Oct. 17 by Qian Gong

Delay and Throughput

n Delay causes: (seconds)

v Propagation Delay (Network Media)

v Switching/Routing Delay (Router Hardware)

v Queuing delay – Time that the packet stays in buffer waiting for transmission

v Length of the network link.

n Throughput

v Given by network bandwidth.

n Relationship between delay and network utilization

v Network utilization --percentage of time network is used.

n When network is idle:

v Network Utilization = 0 %

v Queuing Delay = 0

n When the network is running at 100% capacity (full capacity)

v Network Utilization = 100 %

v Queuing Delay = infinitive

v Example: A router connects to two networks. One is 10Mbps (max), the other one is 20 Mbps.

If the traffic that R receives N2 into N1 is 11Mbps(> 10Mbps), The queue of R will grow until it overflows and packets are discarded. This situation is called “ Network Congestion”.

In between:

0 < network usage < 100

0 < queuing delay < infinitive

Do à Minimum Delay (i.e. Propagation & Routing Delay): (Queuing Delay = 0)

U à Utilization of Network(between 0 and 1 , i.e. between 0% and 100%)

D à Total Delay

D = Do/ (100 – U)

Consequence: Any network that operates with utilization to 100% of capacity is doomed.

Solution: Increase bandwidth so the network usage is always close to 0.

Delay throughput Product

Delay – Time it takes for a bit to cross the network (sec)

Throughput – Number of bits per second, crossing the network. (bit/sec)

Delay × Throughput – Number of bits in transit in the network. (bit)

Networking Protocols

A protocol is:

-- An agreement to allow communication between different computers.

-- A protocol specifies

ü Formats of message

ü Rules of exchange

ü Handling problems

n It is common to divide a network communication problem into different protocols.

n Each protocol addresses a part of the problem.

n This group of protocols is known as protocol suite or sanity of protocols.

n Protocols address a different layer of the problem.

ISO-7 Network Layer Reference Model

Invented for protocol designers (7 layers)

.

Application	--Individual Application Program
Presentation	--How data is presented
Session	--Login and Passwords
Transport	--Reliability
Network	--Packet Forwarding
Data Link	--Hardwork Frame/packet Format
Physical	--Underlying hardware

n This design is outdated. It was created when networking was used to connect terminals to computers.

n It does not include the “Internet “ layer that allows networking using different network technologies.

Layering with TCP/IP Protocols ( 5 layers)

Application	--Application Program	e.g. HTTP/Telnet/ FTTP/ MTP/DNS
Transport	--Application to Application	e.g. TCP/UDP
Internet	--Forwarding across Internet (Hide network hardware)	e.g. IP
Network Interface	--Network Interface hardware format	e.g. Ethernet , serial line, token ring, ATM
Physical	--Basic Network Hardware

Notes Regarding Lab4:

Use realpath(path) to get the path without “..” or symbolic links and make sure that file requested is inside http-root-dir.

Another option is to make sure that <document> does not contain “..” in the request.

For cgi-bin programs you need to fork another process.

There are 2 semantics for fork ( ):

1) If you use Solaris threads (thr_create), fork creates a new process with a copy of all the threads of parent process. What we would like is to create a new process with only one thread, the thread calling fork( ), to get this semantics use fork1( ).

à create a new process with only the caller thread.

2) If you use pthread (pthread_create), then fork will always creak a child process with only one thread (the caller thread) like using fork1( ).