Performance and Internet Architecture

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

2. Announcements • Project 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. 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

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 • 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

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

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: • 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

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

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

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

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.

33. ISO/OSI Reference Model (cont’d) • Physical layer • Transmitting raw bits (0/1) over wire • Examples: 802.11 (2.4GHz wireless), Copper, Fiber

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