1. Performance and Internet Architecture
Networking
CS 3470, Section 1
Sarah Diesburg

2. Announcements Project 1 Assignment 1 Due in two weeks
Start now, if you haven’t already
Will go over missing pieces and answer questions on Friday
Assignment 1
Due in one week
Hand in during class (instead of eLearning)

3. Performance

4. The University of Adelaide, School of Computer Science
14 December 2017
Bandwidth
Bandwidth
Width of the frequency band
Number of bits per second that can be transmitted over a communication link
1 Mbps: 1 x 106 bits/second
1 x 10-6 seconds to transmit each bit
Can imagine this on a timeline
Chapter 2 — Instructions: Language of the Computer

5. Bandwidth
Bits transmitted at a particular bandwidth can be regarded as having some width:
(a) bits transmitted at 1Mbps (each bit 1 μs wide);
(b) bits transmitted at 2Mbps (each bit 0.5 μs wide).

6. The length of bits How wide/long is a bit in the network?
Propagation speed?
Electrons in copper: 2.3x108m/s
Light pulses in fiber: 2.0x108m/s
Transmission rates
10Mbps
100Mbps
1Gbps
10Gbps

7. The length of bits How wide is a bit in the network?
If your transmission rate is 10Mbps, how long does it take to put one bit on the line?
10Mbps = 10x106 bits in one second
= 1 bit in 1/ 10x106 seconds
= 1 bit in 1x10-7 seconds
= 1 bit in 0.1 μs

8. The length of bits How long is a bit in the network?
10Mbps = 1 bit every 1x10-7 seconds
In copper, electrons travel at 2.3x108 m/s
2.3x108 m/s x 1x10-7 seconds ≈ 23 meters

9. The length of bits How long is a bit in the network?
What about wireless?
2.4GHz spectrum (802.11b)
Transmission rate 11Mbps
Transmission medium is taken to be the speed of light
Unless there are physical considerations:
Wood,Glass, Plastic (low)
Water, Bricks, Living Animals (medium)
Ceramic, Paper, Bullet-proof glass, Concrete (high)
Metal (very high)

10. Latency Latency Latency = Propagation + Transmit + Queue
How long it takes for a message to travel from the source to the destination
Always measured in time
Lots of factors can affect this – any ideas?
Latency = Propagation + Transmit + Queue

11. Latency Three main factors affect latency
Propagation delay deals with the speed of light over the medium
Electrons in copper: 2.3x108m/s
Light pulses in fiber: 2.0x108m/s
Propagation = Distance/SpeedOfLight

12. Latency Three main factors affect latency Transmit time
Amount of time it takes to transmit a unit of data
Transmit = Size/Bandwidth

13. Latency Three main factors affect latency
Queue delay deals with delays in the network
E.g., switches that store and forward

14. All Together… Latency = Propagation + Transmit + Queue
Propagation = Distance/SpeedOfLight
Transmit = Size/Bandwidth
Sometimes, we are concerned with round-trip time (RTT)
Time it takes to send a message from source to destination and back to source
One-way latency time X 2

15. The Delay x Bandwidth “Pipe”
Okay, so it takes “latency” seconds for a bit to go from one end to another (plus a fraction for the transmission of the bit!).
While that one bit is “on its way,” you can still send more bits.
How many bits can you stuff in the pipe?
That is, how many bits can be “in transit” that the sender knows have been sent, but the receiver has not yet been made aware of?

16. The Delay x Bandwidth “Pipe”
Think of the link as a pipe.
The “length” of the pipe as the latency
The cross-sectional area as the transmission rate
Then, the Delay x Bandwidth product is the volume (in bits) of the pipe.

17. And yet another time
The transfer time refers to the amount of time sending the data plus the overhead in setup/teardown of the transfer.
We'll see a lot of these when we talk about TCP, but for now, look at it like this:
Transfer request
Transmission
Time
Data Transmission
Acknowledgment
RTT

18. Jitter
Packets that go through several congested routers must contend for transmission slots.
The result is that an application sending packets at a constant interval would be perceived by the receiver to have variations in the interpacket gap, or the time between successive packets.
This is observable by variations in latency, referred to as “jitter.”

19. Internet Architecture

20. Layered Architecture
Layering simplifies the architecture of complex system
Layer N relies on services from layer N-1 to provide a service to layer N+1
Interfaces define the services offered
Service required from a lower layer is independent of it’s implementation
Layer N change doesn’t affect other layers
Network software is normally designed and implemented using a layered approach to make the complex system manageable.
Consider the figure at the right hand side which illustrates the basic layered concept. Layer N implements certain network functionalities, or services. Layer N implements these services using the functionalities (services) provided by the lower layer N-1. In turn, Layer N also provides services to the upper layer N+1. Each layer access the services provided by a lower layer via the service interface at the lower layer. Service interface precisely defines the services offered by the layer.
Layer N uses the services provided by the lower layers to offer a service to upper layers. Interfaces define the services offered by a layer.
Such layer architecture has great advantages to make a complex system manageable. For example, we can partition a complex system into several layers, and each layer only implements some limited (simple) functions. In addition, by keeping the interface same, the implementation of a layer can be changed without affecting the rest of the system. This type of information and complexity hiding is similar to a combination of divide-and-conquer and object oriented approach.

21. Protocols Protocols are rules by which network elements communicate
The format and the meaning of messages exchanged
Protocols in everyday life
Examples: traffic control, open round-table discussion etc
Protocol is a common term you hear while talking about networks. Protocol is simply the rules by which two peer entities communicate with each other. Protocol defines the syntax of a message, semantics of messages, and actions to be taken upon receipt of a message. In other words, protocols define the format and meaning of the messages exchanged.
Protocols are common in everyday life. For example, traffic control. We know what the meaning of a signal and what action to take when you see a signal. Similarly round table discussion, question and answer session in the class etc.
Traffic control also helps explain why there are so many protocols. You have signal lights, two-way stop, four-way stop, yield. Each of them are ideal for some traffic conditions. When there is lot of traffic, its better to have signals each way getting a time slice. Under light traffic, its better to have four-way stop type coordination.

22. Protocol Stacks and Layering
Layering leads to separation of tasks, which makes it easier for programmers and hardware vendors to implement the interface to the neighboring layers.
Protocols lead to standardization and well-defined behaviors and expectations.

23. Encapsulation
Encapsulation refers to the embedding of a data representation at one protocol layer into the data representation of another layer.

24. Fragmentation Packets at one layer might be too large.
In this case, the packet might be fragmented into smaller pieces, encapsulated into the data representation of the underlying protocol, and then defragmented (reassembled) at the destination, or at a node later on in the link.

25. Common Standards ISO: IETF International Standards Organization
Defined reference model known as OSI (Open Systems Interconnection)
IETF
Internet Engineering Task Force
Defined the Internet Model

26. The OSI Model Also known as the seven-layer salad. Application
Presentation
Session
Transport
Network
Data Link
Physical (All pizzas sent through Nick digest promptly)

27. The Internet Model
Commonly four layers—with the physical layer implied.
Application
Transport
Network
Link
(Physical)

28. ISO/OSI and Internet Reference Models
The layered approach is fine but it is not trivial to decide which layer should provide what functionality. International Standards Organization (ISO) defined a reference model specifying the tasks of each layer. This reference model is called ISO/OSI reference model (where OSI stands for open systems interconnection) It is a seven layer model. Though its protocols are not very popular (in terms of real implementation support), it is still considered a good reference model.
The most popular internet protocol stack has only 5 layers. It doesn’t have presentation and session layers. A bit of presentation layer job is done by the application layer. Other layers also don’t exactly correspond to OSI layers but roughly similar. We first talk about ISO/OSI reference model and then discuss internet protocols.

29. ISO/OSI Reference Model
Application layer
Examples: http, ftp, smtp etc
Process-to-process communication
All layers exist to support this layer
Presentation layer (OSI only)
Conversion of data to common format
Example: Little endian vs big endian byte orders
All the layers exist to support the application layer because this is where the end user applications reside (or put in another way, this is what end users really care for). Examples of application layer protocols are smtp (simple mail transfer protocol) for , http (hypertext transfer protocol) for web, and ftp (file transfer protocol) for file transfer.
Presentation layer deals with representation of data. The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system.
Different brands of computers use different internal representations for integers, characters etc. So there is a need for conversion of data to common format. For example, some machines like Intel PCs use little endian representation. That means low order byte is stored at lowest address. Other machines like Mac, SUN use big endian. So when you communicate between these two machines you need to communicate using a common format.

30. ISO/OSI Reference Model (cont’d)
Session layer (OSI only)
Session setup (authentication)
Recovery from failure (broken session)
Transport layer
Examples: TCP, UDP
End-to-end delivery
(Some typical) functions include reliable in-order delivery and flow/error control
Session layer protocols deal with session setup and authentication. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. They also deal with recovery from a failure during a session. This layer is present only in ISO/OSI reference model and such functionality is not provided in Internet protocols.
Transport layer is responsible for end to end delivery of packets. It is an end to end layer and thus a transport protocol is between peer entities in the end-systems. Some of the functions of transport layer are reliable in-order delivery and flow control. This is one of the most complex layers. TCP and UDP are the transports protocols in the Internet. TCP is a connection-oriented reliable transport protocol, while UDP is connectionless datagram service.

31. ISO/OSI Reference Model (cont’d)
Network layer
Examples: IP
Used to determine how packets are routed from source to destination
Congestion control
Accounting
Network layer protocols specify how addresses are assigned to nodes in the network and how packets are routed within the network. Also responsible for avoiding congested or failed links. Example network protocols are IP in the Internet protocol stack.

32. ISO/OSI Reference Model (cont’d)
Data link layer
Examples: Ethernet, PPP
Responsible for taking a raw transmission facility and transforming it into a line that appears free of undetected transmission errors.
Accomplished by sending data in frames, and transmitting frames in sequence.
Acknowledgment frames.
Special delineation bit patters used to distinguish frames.
Data link layer’s main task is to organize the data into frames and transmit them without errors between neighboring elements. Data link layer protocols are responsible for ensuring that speed matching sender and receiver. When the underlying medium is shared, a sublayer known as media access control (MAC) regulates the access to the medium (who should access the shared media now). Ethernet and PPP are some of the data link layer protocols.

33. ISO/OSI Reference Model (cont’d)
Physical layer
Transmitting raw bits (0/1) over wire
Examples: (2.4GHz wireless), Copper, Fiber
Physical layer is concerned with transmitting raw bits over the wire. How 1 and 0 are coded etc.

34. More on Layers
The lower three layers are implemented on all network nodes
The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers

35. Protocol Stacks and Layering
The OSI 7-layer Model
OSI – Open Systems Interconnection