1. Text has no relevant information
Computer Networks
Text has no relevant information

2. Learning Outcomes
Be able to explain the differences between different types of computer networks
Be able to identify differences in connection based and connectionless communication protocols
Be able to explain the way data are exchanged between two machines in term of the standard protocols and their data headers
Unit 19

3. What do we know about Networking?
So far we have
looked at the socket API
discussed the basic actions that applications have to perform to communicate
been aware of threads and how to use them
seen how to use semaphores to deal with problems that require the application to process more than one thing at a time
The question we want to raise at this point is what happens when we send our packet is there anything else we need to know about that might be of interest to a programmer?
Unit 19

4. Is this enough?
What happens when we send our data from one machine to another?
With respect to network programming:
Is it certain that the data will always arrive?
Should we care how long it might take for data to arrive?
Should we care about the data arrival rate?
etc.
Would knowledge about any of these things change the way we design a program?
Are there other issues that might be of interest to us and help us write more robust programs?
What we really need to talk about is would knowledge about any of these things change the way we design a program
Unit 19

5. How can computers be connected?
Ask the students for ideas on how networks are connected in particular go looking for technologies
- should get back various comments about wired and hopefully wireless networks the
Unit 19

6. Communication paths
When data is exchanged between computers it may traverse many different network types and travel at different speeds
100 Mb/s - local Ethernet (copper wire)
1 Gb/s - local backbone (fiber)
155 Mbs/s - OC3 leased line
10 Gb/s – OC192 backbone
11 Mb/s g to the laptop
Unit 19

7. What’s a Network?
A network is a Collection of interconnected computers.
Depending on the number of computers and the distance between them we can form:
Local Area Networks (LAN)
cover small local area, like a home, office, or campus
are fast
can be wireless
Metropolitan Area Networks (MAN)
usually spanning a campus or a city
typically use wireless structure or optical fiber connections to link their sites.
Wide Area Networks (WAN)
cover a wide area, involve a large number of computers
usually connect LAN’s and MAN’s
Internet is a WAN
Unit 19

8. Local Area Networks or
Bus: Computers are connected to a central cable (bus). Also called Ethernet.
Ring: Computers are connected into a loop.
Star: Computers are connected to a hub or a switch
most popular nowadays
Network Hub:
has ports where computers are attached
repeats the signal received at one port out each of the other ports
Network Switch:
Like a hub, but it only sends the signal to one other port
Unit 19

9. Metropolitan Area Networks[Tan02]
Unit 19

10. Wide Area Networks [Tan02]
Backbone
Notes in the subnet are usually routers.
A router acts as a junction between two or more networks to transfer data among them.
Unit 19

11. Satellite Links
Unit 19

12. Architecture of the Internet [Tan02]
Network Access Point: huge interconnection points
Points Of Presence: connects clients to an ISP
To connect a computer to Internet, we connect it to an ISP computer via a modem.
Unit 19

13. Distinguishing Network Characteristics
Latency - how long it takes data to get from one location to the next
Bandwidth – the amount of data that can be moved in a given unit of time
Jitter – the variability in latency
Error rate – how frequently data becomes corrupted during transmission
Public transit vs cars Latecy is the same but the BW is difference
Unit 19

14. Data transmission
Signal transmission over cables, fiber optics, or other media generates errors; Sender may have to re-send a message many times
For efficiency, messages are divided into fix-size packets before they sent
Two ways to send the packets:
Using Connectionless Service (or Datagram)
Using Connection-Oriented Service (or virtual circuit )
Unit 19

15. Connection Based Service(TCP/IP)
Sending and receiving hosts establish a connection
Sender
Splits sequence of outgoing bytes into packets
Remembers sent packets in case they are lost and need to be resent
Receiver
Receives packets
Reassembles packets into a sequence of bytes
If packets don’t arrive in a given time requests the sender to retransmit them
Produces a reliable streamed delivery of data
The most common protocol supported by most OS’s is TCP/IP
TCP – Transmission Control Protocol
IP – Internet Protocol
Remind about the Socket interface so we know how to do the connection so this is the other type of work that needs to be done
Explain what is mean by reliable and streamed
Unit 19

16. Datagram (UDP)
Not streamed – data arrives in the “chunks” that the sender sent, called datagrams
Datagrams have a maximum size
Datagrams are individually addressed
Datagrams may be lost
Datagrams may arrive out of order
No connection setup
No connection state
The most common protocol supported by most OS’s is
UDP – User Datagram Protocol
Talk about postcards as being like packets and what happens if you drop them into the mailbox – they don’t come out in that order at the destination
Unit 19

17. Protocol
A formally agreed upon convention for performing or engaging in some sort of activity
A data communication protocol is an example of a protocol to exchange data between machines
A connection protocol describes how two machines establish a connection. (Usually a subset of a data communication protocol)
Not sure if this is an important slide or not, it is a bit of an aside and the important point to get across is that everyone has to agree on what the rules are for things to work
Unit 19

18. Introducing Protocol Layers
Communication protocols are usually designed as a stack of layers
Similar idea to virtual machine
Add layers to perform various functions
Lowest layer deals with hardware
Next layer deals with moving data to the destination machine
Next layer handles correcting errors and lost data (if appropriate) and getting data to application
Last layer is the application
Unit 19

19. Layering Yet Again! Application Application Transport Unix I/O Network
Application programs
Operating system
Hardware
General Layering
Structure
File System
Link
Disk Drive
Draw lines from the operating system part of things to indicate that the Unix/IO model and transport and network part of things are all part of the OS
On the network side of things talk about all the things that we learned at each part
Network
Layering
File System
Layering
Unit 19

20. Ethernet, token ring, wireless
Protocol Layers
Application
Application
Transport
Network
Link
telnet, FTP, SMTP, HTTP
TCP, UDP IP
Ethernet, token ring, wireless
Transport
Network
Link
Draw a physical layer on the bottom to indicate that there is some form of actual signaling going on
Unit 19

21. Layers of Functionality
Each layer hides the details from the layer above and uses the services of the layer below
A layer in a node A interacts with the same layer in another node B (using the layers below it )
Specifically:
Link – moves the data between directly connected nodes on the path to the destination
Network – used to route the data between the source/destination end points
Transport – application to application delivery
Application – application level interchanges
Unit 19

22. Encapsulation
To achieve the desired functionality each layer adds “headers” to the data it receives
Headers are meant for the corresponding layer at the destination
Headers are removed at the destination layer
Unit 19

23. Protocol headers Application Application data Transport TCP Header
A. Data
Application data
A. Data
Transport
TCP Header
A. Data
TCP Header
A. Data
Network
IP Header
TCP Header
A. Data
Link
Ethernet Header
IP Header
TCP Header
A. Data
Unit 19

24. Protocol headers: Going Down
Application
A. Data
Transport
TCP Header
A. Data
Network
IP Header
TCP Header
A. Data
Link
It should be possible to animate this slide so that we can actually see it move through the layers. Idea would be to put the moving stuff on the top and have a transparent version below so that we can see what was done at the previous layers
Ethernet Header
IP Header
TCP Header
A. Data
Unit 19

25. Protocol headers: Coming up
Application
Transport
Network
Link
It should be possible to animate this slide so that we can actually see it move through the layers. Idea would be to put the moving stuff on the top and have a transparent version below so that we can see what was done at the previous layers
Ethernet Header
IP Header
TCP Header
A. Data
Unit 19

26. Header Contents TCP IP Ethernet Source Port Destination Port
Sequence Number
ACK Number
Additional Fields
Control Fields
(Includes transport protocol type)
Source IP Address
Destination IP Address
Options
Dest Ethernet Addr
Source. Ethernet Addr
Type/Size
TCP Port can be the application port or the running program’s port.
Unit 19

27. Routing example Source Destination Application data TCP Header
Transport
Network
Link
Application
Transport
Network
Link
Network
Link
Draw out each part of the movement through the stack
Application data
TCP Header
IP Header
802.11G Header
Ethernet Header
Unit 19

28. Routing
Unit 19

29. Protocol Processing AP1 Application Protocol 1 AP2 Application
TCP
UDP IP
Relate this back to the socket API and how the calls tell these layers what to do. In particular point out how bind is needed in this situation so that the application can inform the system of where it should be accepting packets.
Ethernet
Wireless
Unit 19

30. TCP Acknowledgement Normal Operation, no loss
Send pkt0
pkt0
recv pkt0
Send ACK0
ACK0
recv ACK0
Send pkt1
pkt1
recv pkt1
Send ACK1
ACK1
recv ACK1
Send pkt2
pkt2
recv pkt2
Send ACK2
recv ACK2
ACK2
Unit 19

31. TCP Acknowledgement Lost Packet
Send pkt0
pkt0
recv pkt0
Send ACK0
ACK0
recv ACK0
Send pkt1
pkt1 X
loss
Timeout
Send pkt1
pkt1
recv pkt1
Send ACK1
ACK1
recv ACK0
pkt2
Unit 19

32. Extending Device I/O Layering makes it “easy” to Example:
incorporate networking in the OS
extend I/O to devices that are connected to computer through a network
Example:
Network File Systems (NFS)
Just insert the new service at the appropriate layer
Unit 19

33. Inserting New Functionality
UNIX I/O
Application
UNIX FFS
File System
Logging FS
NFS Client
Other Devices
Virtual Disk Interface
Virtual Disk Interface
Network Protocol Stack
SCSI
IDE
Unit 19

34. From the beginning to the end
STDIO
GCC
Mozilla
emacs
STDIO
GCC
Firefox
emacs
Application programs
UNIX I/O
FFS
LFS
Virtual Disk interface
IDE
SCSI
Other devices
Protocol
Stack
Link IP
Transport
Operating system
UNIX I/O
FFS
LFS
Virtual Disk interface
IDE
SCSI
Other devices
Protocol
Stack
Virtual Machine Monitor
CPU
Motherboard
Disks
Display
Network card
etc
Hardware
Virtual Machine Monitor
CPU
Motherboard
Disks
Display
Network card
etc
Unit 19