Links Jennifer Rexford COS 461: Computer Networks

Slides:



Advertisements
Similar presentations
Ethernet “dominant” LAN technology: cheap $20 for 100Mbs!
Advertisements

Review r Error Detection: CRC r Multiple access protocols m Slotted ALOHA m CSMA/CD r Homework 3 out r Project 3 out, link state only. Some slides are.
5: DataLink Layer5-1 Mac Addressing, Ethernet, and Interconnections.
1 Links Reading: Chapter 2 COS 461: Computer Networks Spring 2006 (MW 1:30-2:50 in Friend 109) Jennifer Rexford Teaching Assistant: Mike Wawrzoniak
The ALOHA Protocol “Free for all”: whenever station has a frame to send, it does so. –Station listens for maximum RTT for an ACK. –If no ACK after a specified.
1 Ethernet EECS 489 Computer Networks Z. Morley Mao Wednesday Feb 21, 2007 Acknowledgement: Some slides taken.
5: DataLink Layer5-1 MAC Addresses and ARP r 32-bit IP address: m network-layer address m used to get datagram to destination IP subnet r MAC (or LAN or.
1 Announcement r Homework #3 was due last night r Homework #4 is out.
CSCI 4550/8556 Computer Networks Comer, Chapter 11: Extending LANs: Fiber Modems, Repeaters, Bridges and Switches.
Local Area Networks: Ethernet, Switching COS 461: Computer Networks Spring 2011 Mike Freedman
EE 122: Ethernet and Ion Stoica September 18, 2002 (* this talk is based in part on the on-line slides of J. Kurose & K. Rose)
MAC Addresses and ARP 32-bit IP address: –network-layer address –used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address:
EEC-484/584 Computer Networks Lecture 13 Wenbing Zhao
5-1 Data Link Layer r Today, we will study the data link layer… r This is the last layer in the network protocol stack we will study in this class…
5: DataLink Layer5-1 Chapter 5 Link Layer and LANs Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross.
16 – CSMA/CD - ARP Network Layer4-1. 5: DataLink Layer5-2 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit.
IP Address 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class to to
Review r Error Detection: CRC r Multiple access protocols m Slotted ALOHA m CSMA/CD r LAN addresses and ARP r Ethernet Some slides are in courtesy of J.
COMPUTER NETWORKS.
Introduction 1 Lecture 25 Link Layer (Ethernet, Switch) slides are modified from J. Kurose & K. Ross University of Nevada – Reno Computer Science & Engineering.
Connecting LANs, Backbone Networks, and Virtual LANs
Link Layer Zeinab Movahedi Some slides are from Computer networks course thought by Jennifer Rexford at Princeton University.
Introduction1-1 Data Communications and Computer Networks Chapter 5 CS 3830 Lecture 26 Omar Meqdadi Department of Computer Science and Software Engineering.
5: DataLink Layer5-1 LAN technologies Data link layer so far: m services, error detection/correction, multiple access Next: LAN technologies m addressing.
Links Mike Freedman COS 461: Computer Networks Lectures: MW 10-10:50am in CS 104
Lecture 17 Ethernet r Widely deployed because: m First LAN technology m Simpler and less expensive than token LANs and ATM m Kept up with the speed race:
Introduction1-1 Data Communications and Computer Networks Chapter 5 CS 3830 Lecture 27 Omar Meqdadi Department of Computer Science and Software Engineering.
LAN Technologies MAC protocols used in LANs, to control access to the channel Token Rings: IEEE (IBM token ring), for computer room, or department.
5: DataLink Layer5-1 Ethernet “dominant” wired LAN technology: r cheap $20 for 100Mbs! r first widely used LAN technology r Simpler, cheaper than token.
5: DataLink Layer5-1 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing.
1 Computer Communication & Networks Lecture 13 Datalink Layer: Local Area Network Waleed Ejaz
Link Layer: MAC Ilam University Dr. Mozafar Bag-Mohammadi.
CCNA 3 Week 4 Switching Concepts. Copyright © 2005 University of Bolton Introduction Lan design has moved away from using shared media, hubs and repeaters.
Review: –Ethernet What is the MAC protocol in Ethernet? –CSMA/CD –Binary exponential backoff Is there any relationship between the minimum frame size and.
Data Link Layer Moving Frames. Link Layer Protocols: ethernet, wireless, Token Ring and PPP Has node-to-node job of moving network layer.
5: DataLink Layer5-1 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing.
Sem1 - Module 8 Ethernet Switching. Shared media environments Shared media environment: –Occurs when multiple hosts have access to the same medium. –For.
5: DataLink Layer5-1 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame r If channel sensed.
McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Connecting Devices CORPORATE INSTITUTE OF SCIENCE & TECHNOLOGY, BHOPAL Department of Electronics and.
Chapter 11 Extending LANs 1. Distance limitations of LANs 2. Connecting multiple LANs together 3. Repeaters 4. Bridges 5. Filtering frame 6. Bridged network.
5: DataLink Layer5-1 Chapter 5 Link Layer and LANs Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross.
Ch 5. The Link Layer and Local Area Networks Myungchul Kim
EE 122: Lecture 6 Ion Stoica September 13, 2001 (* this talk is based in part on the on-line slides of J. Kurose & K. Rose)
1 Multiple Access: Ethernet Section Point-to-Point vs. Broadcast Media Point-to-point –PPP for dial-up access –Point-to-point link between Ethernet.
Computer Communication and Networking Lecture # 4 by Zainab Malik 1.
Links Mike Freedman COS 461: Computer Networks Lectures: MW 10-10:50am in Architecture N101
CS 457 – Lecture 3 Link Layer Protocols Fall 2011.
5-1 Last time □ Multiple access protocols ♦ Channel partitioning MAC protocols TDMA, FDMA ♦ Random access MAC protocols Slotted Aloha, Pure Aloha, CSMA,
Link Layer 5.1 Introduction and services
Link Layer 5.1 Introduction and services
Random Access Protocols
MAC Addresses and ARP 32-bit IP address:
Computer Communication Networks
Chapter 4 Data Link Layer Switching
Hubs Hubs are essentially physical-layer repeaters:
University of Pittsburgh
CS 457 – Lecture 8 Switching and Forwarding
CS 457 – Lecture 6 Ethernet Spring 2012.
Data Link Issues Relates to Lab 2.
Services of DLL Framing Link access Reliable delivery
Mac Addressing, Ethernet, and Interconnections
Hubs Hubs are essentially physical-layer repeaters:
Data Link Issues Relates to Lab 2.
CS4470 Computer Networking Protocols
Protocol layering and data
Link Layer and LANs Not everyone is meant to make a difference. But for me, the choice to lead an ordinary life is no longer an option 5: DataLink Layer.
18: Ethernet, Hubs, Bridges, Switches
Protocol layering and data
LAN Addresses and ARP IP address: drives the packet to destination network LAN (or MAC or Physical) address: drives the packet to the destination node’s.
Link Layer 5.1 Introduction and services
Presentation transcript:

Links Jennifer Rexford COS 461: Computer Networks Lectures: MW 10-10:50am in Architecture N101 http://www.cs.princeton.edu/courses/archive/spr12/cos461/

Course Announcements Get a Piazza account http://piazza.com/class#spring2012/cos461 See assignment #0 on socket programming Posted on the course Web site Due 11:59pm Thu Feb 16 Counts in course participation Friday precepts P01: 10-10:50am in Friend 109 (Rob Kiefer) P02: 11-11:50am in Friend 109 (Peng Sun) P02A: 11-11:50am in Friend 108 (Xiaozhou Li)

Protocol Layers host host HTTP message TCP segment router router IP IP packet IP packet IP packet IP IP IP Ethernet interface Ethernet interface SONET interface Ethernet interface SONET interface Ethernet interface Ethernet frame SONET frame Ethernet frame

Link = Medium + Adapters

What is a Link? Communication Medium Network Adapter

Broadcast Links: Shared Media

Adaptors Communicating packet packet link layer protocol frame frame sending node adapter adapter receiving node Sending side Encapsulates packet in a frame Adds error checking bits, flow control, etc. Receiving side Looks for errors, flow control, etc. Extracts datagram and passes to receiving node

Link-Layer Services Encoding Framing Error detection Error correction Represent the 0s and 1s Framing Encapsulate packet into frame, adding header/trailer Error detection Receiver detecting errors with checksums Error correction Receiver optionally correcting errors Flow control Pacing between sending and receiving nodes

Addresses

Medium Access Control Address Identify the sending and receiving adapter Unique identifier for each network adapter Identifies the intended receiver(s) of the frame … and the sender who sent the frame

Medium Access Control Address MAC address (e.g., 00-15-C5-49-04-A9) Numerical address used within a link Unique, hard-coded in the adapter when it is built Flat name space of 48 bits Hierarchical allocation Blocks: assigned to vendors (e.g., Dell) by the IEEE Adapters: assigned by the vendor from its block Broadcast address (i.e., FF-FF-FF-FF-FF-FF) Send the frame to all adapters

As an Aside: Promiscuous Mode Normal adapter: receives frames sent to The local MAC address Broadcast address FF-FF-FF-FF-FF-FF Promiscuous mode Receive everything, independent of destination MAC Useful for packet sniffing Network monitoring E.g., wireshark, tcpdump

Why Not Just Use IP Addresses? Links can support any network protocol Not just for IP (e.g., IPX, Appletalk, X.25, …) Different addresses on different kinds of links An adapter may move to a new location So, cannot simply assign a static IP address Instead, must reconfigure the adapter’s IP address Must identify the adapter during bootstrap Need to talk to the adapter to assign it an IP address

Who Am I: Acquiring an IP Address 71-65-F7-2B-08-53 1A-2F-BB-76-09-AD ???? 1.2.3.5 DHCP server 0C-C4-11-6F-E3-98 1.2.3.6 Dynamic Host Configuration Protocol (DHCP) Broadcast “I need an IP address, please!” Response “You can have IP address 1.2.3.4.”

Who Are You: Discovering the Receiver 71-65-F7-2B-08-53 1A-2F-BB-76-09-AD 1.2.3.4 1.2.3.5 0C-C4-11-6F-E3-98 1.2.3.6 Address Resolution Protocol (ARP) Broadcast “who has IP address 1.2.3.6?” Response “0C-C4-11-6F-E3-98 has 1.2.3.6!”

Sharing the Medium

Collisions Single shared broadcast channel 71-65-F7-2B-08-53 1A-2F-BB-76-09-AD 0C-C4-11-6F-E3-98 Single shared broadcast channel Avoid having multiple nodes speaking at once Otherwise, collisions lead to garbled data

Multi-Access Protocol Divide the channel into pieces In time In frequency Take turns Pass a token for the right to transmit Punt Let collisions happen … and detect and recover from them

Like Human Conversation… Carrier sense Listen before speaking …and don’t interrupt! Collision detection Detect simultaneous talking … and shut up! Random access Wait for a random period of time … before trying to talk again!

Carrier Sense Multiple Access Listen for other senders Then transmit your data Collisions can still occur Propagation delay Wasted transmission

CSMA/CD Collision Detection Detect collision Abort transmission Jam the link Wait random time Transmit again Hard in wireless Must receive data while transmitting

Comparing the Three Approaches Channel partitioning Efficient and fair at high load Inefficient at low load “Taking turns” Eliminates empty slots without collisions Vulnerable to failures (e.g. lost token) Random access Efficient at low load Collision overhead at high load

Ethernet

Ethernet Dominant wired LAN technology First widely used LAN technology Kept up with speed race: 10 Mbps – 40 Gbps Metcalfe’s Ethernet sketch

Ethernet Uses CSMA/CD Carrier Sense: wait for link to be idle Channel idle: start transmitting Channel busy: wait until idle Collision Detection: listen while transmitting No collision: transmission is complete Collision: abort transmission, and send jam signal Random Access: exponential back-off After collision, wait random time before trying again After mth collision, choose K randomly from {0, …, 2m-1} … and wait for K*512 bit times before trying again

Limitations on Ethernet Length B A latency d Latency depends on physical length of link Time to propagate a packet from one end to other Suppose A sends a packet at time t And B sees an idle line at a time just before t+d … so B happily starts transmitting a packet B detects a collision, and sends jamming signal But A doesn’t see collision till t+2d

Limitations on Ethernet Length B A latency d A needs to wait for time 2d to detect collision So, A should keep transmitting during this period … and keep an eye out for a possible collision Imposes restrictions on Ethernet Maximum length of the wire: 2500 meters Minimum length of the packet: 512 bits (64 bytes)

Ethernet Frame Structure Sending adapter encapsulates packet in frame Preamble: synchronization Seven bytes with pattern 10101010, followed by one byte with pattern 10101011 Used to synchronize receiver, sender clock rates

Ethernet Frame Structure Addresses: source and destination MAC addresses Adaptor passes frame to network-level protocol If destination is local MAC address or broadcast address Otherwise, adapter discards frame Type: indicates the higher layer protocol Usually IP But also Novell IPX, AppleTalk, … CRC: cyclic redundancy check Checked at receiver If error is detected, the frame is simply dropped

Unreliable, Connectionless Service No handshaking between send and receive adapter Unreliable Receiving adapter doesn’t send ACKs or NACKs Packets passed to network layer can have gaps Gaps can be filled by transport protocol (e.g., TCP) Otherwise, the application will see the gaps

Hubs and Switches

Physical Layer: Repeaters Distance limitation in local-area networks Electrical signal becomes weaker as it travels Imposes a limit on the length of a LAN Repeaters join LANs together Analog electronic device Continuously monitors electrical signals Transmits an amplified copy

Physical Layer: Hubs Joins multiple input lines electrically Designed to hold multiple line cards Do not necessarily amplify the signal Very similar to repeaters Also operates at the physical layer hub hub hub hub

Limitations of Repeaters and Hubs One large shared link Each bit is sent everywhere So, aggregate throughput is limited Cannot support multiple LAN technologies Does not buffer or interpret frames Can’t interconnect between different rates/formats Limitations on maximum nodes and distances Shared medium imposes length limits E.g., cannot go beyond 2500 meters on Ethernet

Link Layer: Bridges Connects two or more LANs at the link layer Extracts destination address from the frame Looks up the destination in a table Forwards the frame to the appropriate segment Each segment can carry its own traffic host host host host host host Bridge host host host host host host

Link Layer: Switches Typically connects individual computers A switch is essentially the same as a bridge … though typically used to connect hosts Supports concurrent communication Host A can talk to C, while B talks to D B A C switch D

Bridges/Switches: Traffic Isolation Switch filters packets Frame only forwarded to the necessary segments Segments can support separate transmissions switch/bridge segment hub hub hub segment segment

Advantages Over Hubs/Repeaters Only forwards frames as needed Avoid unnecessary load on segments Wider geographic span Separate segments allow longer distances Improves privacy Hosts can “snoop” traffic traversing their segment … but not all the rest of the traffic Can join segments using different technologies

Self Learning: Building the Table When a frame arrives Inspect the source MAC address Associate the address with the incoming interface Store the mapping in the switch table Use a timer to eventually forget the mapping B A C Switch learns how to reach A. D

Self Learning: Handling Misses When frame arrives with unfamiliar destination Forward the frame out all of the interfaces … except for the one where the frame arrived Hopefully, this case won’t happen very often! B When in doubt, shout! A C D

Summary: Multiple Layers Different devices switch different things Network layer: packets (routers) Link layer: frames (bridges and switches) Physical layer: electrical signals (repeaters and hubs) Application Transport Frame header Packet header TCP header User data Router Bridge, switch Repeater, hub

Conclusion Links Coming next Get started Connect two or more network adapters … each with a unique address … over a shared communication medium Coming next Friday: “links” between application processes Monday: network layer (IP) Get started On assignment #0 on socket programming