CS 3640: Introduction to Networks and Their Applications

Transcription

1 CS 3640: Introduction to Networks and Their Applications Fall 2018, Lecture 5: The Link Layer I Errors and medium access Instructor: Rishab Nithyanand Teaching Assistant: Md. Kowsar Hossain 1

2 You should Be checking Piazza regularly for announcements. Have started working on assignment 1. Know and understand: The three Internet design principles. Encapsulation. The components of the Internet. Circuit- vs. packet- switched networks. Components of end-to-end delay. How do you estimate each of them? How do you identify bottlenecks and address each delay? 2

3 This week in class 1. Recap: Performance metrics Link layer principles: Errors and medium access Thursday: Assignment in class 3

4 Recap: How do we assess the performance of a packet-switched network? Delay How long does it take a packet to get to its destination? Transmission delay: How fast can your link interface convert bits into link signals? Propagation delay: How fast can your link send bits from one end to the other? Processing delay: How fast can your processor handle an incoming packet? Queueing delay: How full is the buffer? Loss What fraction of packets that are sent end up getting dropped? Throughput At what rate is data being received by the destination?

5 Recap: What is the end-to-end transmission delay? Switch Gbps Switch 2.5 Gbps Transmission delay: How much time for the switch/router to put all bits on the link? Source transmission delay: 8x10 9 b/1x10 9 bps = 8s Switch 1 transmission delay: 8x10 9 b/100x10 9 bps =.08s Switch 2 transmission delay: 8x10 9 b/.5x10 9 bps = 16s End-to-end transmission delay: 24.08s

6 Recap: What is the end-to-end propagation delay? 2000m Switch Gbps 2000m Switch 2.5 Gbps 4000m Propagation delay: How much time for the link to send data from one end to the other? Source -> switch 1 propagation delay: 2x10 3 m/2x10 8 mps = 10-5 s Switch 1 -> switch 2 propagation delay: 2x10 3 m/2x10 8 mps = 10-5 s Switch 2 -> destination propagation delay: 4x10 3 m/2x10 8 mps = 2x10-5 s End-to-end transmission delay: x10-5 s = 4x10-5 s

7 Recap: What is the end-to-end processing delay? 2000m Switch Gbps 2 GHz 2000m Switch 2.5 Gbps 1 GHz 4000m Processing delay: How much time to check packet for errors and decide next hop? Switch 1 processing delay per packet: 1000 cycles per packet/2x10 9 cycles per second =.5x10-6 seconds per packet Switch 2 processing delay per packet: 1000 cycles per packet/.5x109 cycles per second = 2x10-6 seconds per packet End-to-end processing delay per packet: 2.5x10-6 seconds per packet

8 Recap: What is the end-to-end (average) queueing delay? Switch Gbps Switch 2.5 Gbps Queueing delay: How much time, on average, does data spend in buffers? Switch 1. Arrival rate: 1 Gbps Departure rate: 100 Gbps A packet comes in. Queue is empty. It gets sent out. Average queueing delay at switch 1 = 0 s

9 Maximum queueing delay: 8s. Total bits: 8x10 9 Average queueing delay for a bit at switch 2: (0 + a + 2a + + (8x10 9-1)a)/8x10 9 = a(.5x(8x10 9 )x(8x10 9-1))/8x10 9 =.5x(8x10 9-1)a =.5x8 = 4 s. Recap: What is the end-to-end (average) queueing delay? Switch Gbps Switch 2.5 Gbps Queueing delay: How much time, on average, does data spend in buffers? Switch 2. Arrival rate: 1 Gbps (bottlenecked by source sending rate). Departure rate:.5 Gbps For every 2 bits coming in, 1 has to wait in the queue. First bit arrives at t 0 sec. Last bit arrives at t 0 + 8s (time for a 1 GB file to arrive at 1Gbps). Last bit leaves at t s (time for a 1 GB file to leave at.5gbps).

10 This week in class 1. Recap: Performance metrics Link layer principles: Errors and medium access Thursday: Assignment in class 10

11 Recap: The link layer What are the functions of the link layer? Write bits to and read bits from the network interface. How does the link layer perform error detection and correction? How do multiple devices share a link? Figure out which wire to put bits on. What does the link layer need to know to put things on the right wire? The link layer protocol can be different things on different devices. Can be Ethernet on one hop, WiFi on the next. They are usually implemented on an adaptor (NIC for Ethernet). We will focus on the principles of the link layer, not any specific protocol.

12 Recap: The link layer How do adaptors communicate? At the senders side: Link layer gets a datagram from Network layer. Figures out which wire to use for the network layer destination. Encapsulates datagram with link layer header. (Frame) Adds error checking (or, correction) bits to header. Writes frame to link.

13 Recap: The link layer How do adaptors communicate? At the receivers side: Link layer gets a frame from the link. Figures out which wire to use for the network layer destination. Looks for errors and corrects them if required/possible. Strips link layer headers and passes the datagram up to the network layer.

14 The link layer: Error detection and correction The link layer may perform error detection and correction on frames when possible and if asked to do so. Why? The physical world is inherently noisy. Interference from radio transmissions and microwaves. Discuss: Is this a violation of the end-to-end principle? When is it OK?

15 The link layer: Error detection and correction Discuss: How would you detect errors in frames? Hint: Redundancy might help.

16 The link layer: Error detection and correction A simple solution: Send multiple copies of each frame. An error has occurred if copy 1!= copy 2!=!= copy n. Problems Overhead! n transmissions for each frame.

17 The link layer: Error detection and correction A better idea: Parity bits Add extra bits to make sure number of 1s is even. Example: 7-bit ASCII chars. Make 8 th bit the parity bit. If number of 1s in any 8bit sequence is odd, error occurred. Overhead: 1 parity bit for every 7 frame bits (14%). Problem? Discuss: What s the minimum number of bits that need to change for this to fail?

18 The link layer: Error detection and correction A better idea: Two-dimensional parity bits Imagine your frame as a two-dimensional array of bits. Add one parity bit for each row and for each column. Can detect most errors with only 14% overhead! Discuss: What's the minimum number of bits that need to change for this to fail?

19 The link layer: Error detection and correction An even better idea: Checksums What if you just added all the bytes in your frame. Included the sum in the frame. Requires only 16 bits overhead for the whole frame! (~1% overhead) This is used in the transport layer and IP layer. Problem: Still can have errors. Example (data): = Error: =

20 The link layer: Error detection and correction What we actually use in the link layer: Cyclic Redundancy Check (CRC) A more complicated version of checksum. Idea: Perform a series of mathematical operations on the frames data bits to get a semi-unique value. Typically, 32 bit overhead per frame (3%). Chance of error: 1/2 32! Very cheap to implement in hardware. Very quick.

21 The link layer: Error detection and correction How does a sender know if a frame was: Received error free? Received at all? Discuss: How do you do this IRL?

22 Time Time The link layer: Error detection and correction The Stop and Wait protocol I ll wait for an ACK before I send the next frame. If I don t get an ACK in x ms, I ll resend the frame. Used by Bluetooth. Problem: Poor efficiency. At best, only one frame every RTT seconds. Discuss: How would you improve the efficiency of this protocol.

23 Time Time The link layer: Error detection and correction The Sliding Window Protocol We can allow multiple frames to be unacknowledged. This is the window. Better efficiency: Can allow n frames per RTT

24 The link layer: Medium access protocols What is a medium access control (MAC) protocol? A transmission medium can be shared by many devices. If everyone talks at the same time, we have collisions and unintelligible data. MAC: Rules for sharing a common transmission medium.

25 The link layer: Medium access protocols General strategies for MAC protocols Idea 1: Partition the transmission channel so each host has its share. We briefly saw this Time and Frequency division. Each host has a fixed share (time or frequency band) in the medium. What if a host has nothing to send? Idea 2: Pass a transmit now token to hosts. Like me asking a question in class and selecting one person (from many ) to answer. What if multiple people really want to give an answer? Problem: Transmission channel utilization isn t great.

26 The link layer: Medium access protocols General strategies for MAC protocols Idea 3: Allow collisions, we ll figure out how to recover data. Allows much higher utilization. Now we have new problems: How to identify when a collision has occurred. How to recover from a collision. This strategy is called Random access MAC or Contention-based MAC. Used by Ethernet, mobile transmission protocols, and others.

27 The link layer: The ALOHA MAC protocol (1970s) Idea: Send a frame as soon as you need to. If you receive a frame while transmitting, a collision has occurred. Wait for some time and transmit again. Eventually, a frame will get transmitted without collisions. Assumptions: All frames are equally sized. Errors (collisions) are detectable. Problem: For successful transmissions, no other frame from any other host should start within T time before or after you. Sensitive transmission period: 2T for each frame. Scales terribly. (Theoretical maximum throughput for large number of hosts: 18%).

28 The link layer: The Slotted ALOHA MAC protocol (1970s) Idea: If we allow transmissions only at certain time points, the sensitive period for transmissions reduces. Transmissions on the channel can occur only every T seconds. The channel is divided into slots, but anyone can send in any slot. Sensitive transmission period: T for each frame. If a collision occurs, wait some number of time slots and retransmit. Assumptions: All frames are equally sized. All host clocks are synchronized (!!!) Errors (collisions) are detectable. Sensitive period reduced from 2T to T. Theoretical maximum throughput is doubled to 36%. Still not great.

29 Announcement No lecture on Thursday. I am traveling. In-class working hours. Kowsar (TA) will be in class if you need help with your assignment or any of the material we ve covered so far. 29

30 What you should remember from this lecture What is the role of the link layer? Where is it implemented? How is error detection/correction done at the link layer? What are the different approaches? General ideas and what goes into selection of an approach. When is it not violating the end-to-end principle? Why is stop-and-wait less efficient than the sliding window? What function to MAC protocols provide? Why is slotted ALOHA better than ALOHA? 30