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Transcript
Chapter 5 Link Layer and LANs A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Thanks and enjoy! JFK/KWR All material copyright 1996-2005 J.F Kurose and K.W. Ross, All Rights Reserved 5: DataLink Layer 5-1 Chapter 5: The Data Link Layer Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies 5: DataLink Layer 5-2 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS 5: DataLink Layer 5-3 Link Layer: Introduction Some terminology: “link” hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 5: DataLink Layer 5-4 Adaptors Communicating datagram sending node frame frame adapter link layer implemented in “adaptor” (aka NIC) RAM, DSP chips, host bus interface, and link interface Ethernet card, PCMCIA card, 802.11 card sending side: rcving node link layer protocol encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. adapter receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semi-autonomous link & physical layers 5: DataLink Layer 5-5 Link layer Protocol stack picture M Ht M Hn Ht M Hl Hn Ht M application transport network link physical data link protocol phys. link network link physical Hl Hn Ht M frame adapter card 5: DataLink Layer 5-6 Link layer: context Datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link Each link protocol provides different services e.g., may or may not provide rdt over link transportation analogy trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm 5: DataLink Layer 5-7 Link Layer Functions Framing encapsulate datagram into frame, adding header, trailer Medium access and quality of service channel access if shared medium Addressing “MAC” addresses used in frame headers to identify source, dest • different from IP address! Reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit error link (i.e. fiber, some twisted pair) wireless links: high error rates • Q: why both link-level and end-end reliability? 5: DataLink Layer 5-8 Link Layer Functions (more) Flow Control pacing between adjacent sending and receiving nodes Error Detection errors caused by signal attenuation, noise. receiver detects presence of errors: • signals sender for retransmission or drops frame Error Correction receiver identifies and corrects bit error(s) without resorting to retransmission Security Demux to upper protocol 5: DataLink Layer 5-9 Framing Data encapsulation for transmission over physical link Data embedded within a link-layer frame before transmission Data-link header and/or trailer added Physical addresses used in frame headers to identify source and destination (not IP) 5: DataLink Layer 5-10 Fixed length framing Length delimited Beginning of frame has length Single corrupt length can cause problems • Must have start of frame character to resynchronize • Resynchronization can fail if start of frame character is inside packets as well 5: DataLink Layer 5-11 Variable length framing Byte stuffing Special start of frame byte (e.g. 0xFF) Special escape byte value (e.g. 0xFE) Values actually in text are replaced (e.g. 0xFF by 0xFEFF and 0xFE by 0xFEFE) Worst case – can double the size of frame Bit stuffing Special bit sequence (0x01111110) 0 bit stuffed after any 11111 sequence 5: DataLink Layer 5-12 Clock-Based Framing Used by SONET Fixed size frames (810 bytes) Look for start of frame marker that appears every 810 bytes Will eventually sync up 5: DataLink Layer 5-13 Demux to upper protocol Protocol type specification interfaces to network layer Data-link layer can support any number of network layers Type field in data-link header specifies network layer of packet Each data-link layer defines its own protocol type numbering for network layer IP is one of many network layers 5: DataLink Layer 5-14 Demux to upper protocol http://www.cavebear.com/CaveBear/Ethernet/type.html Some Ethernet protocol types – – – – – 0800 DOD Internet Protocol (IP) 0806 Address Resolution Protocol (ARP) 8037 IPX (Novell Netware) 80D5 IBM SNA Services 809B EtherTalk (AppleTalk over Ethernet) 5: DataLink Layer 5-15 Security Mainly for broadcast data-link layers Encrypt payload of higher layers Hide IP source/destination from eavesdroppers Important for wireless LANs especially • Parking lot attacks • 802.11b and WEP If time permits, security will be covered at the end of the course…. 5: DataLink Layer 5-16 Flow control Pacing between sender and receiver Sender prevented from overrunning receiver Ready-To-Send, Clear-To-Send 5: DataLink Layer 5-17 Reliable delivery Reliability at the link layer Handled in a similar manner to transport protocols ARQ, Stop-and-wait, Go-back-N, Selective Repeat When and why should this be used? ● ● Rarely done over twisted-pair or fiber optic links Usually done over lossy links for performance improvement (versus correctness) 5: DataLink Layer 5-18 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-19 Error detection/correction Errors caused by signal attenuation, noise. Receiver detects presence of errors Possible actions Signal sender for retransmission Drops frame Correct bit errors if possible and continue 5: DataLink Layer 5-20 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction 5: DataLink Layer 5-21 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0 5: DataLink Layer 5-22 Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only) Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. 5: DataLink Layer 5-23 Cyclic Redundancy Check (CRC) Polynomial code Treat packet bits a coefficients of n-bit polynomial Choose r+1 bit generator polynomial G • G well known – chosen in advance Add r bits to packet so that message is divisible by G • At receiver, divide payload by generator polynomial • If result not zero, error detected Better loss detection properties than checksums All single bit errors, all double bit errors, all oddnumbered errors, burst errors less than r widely used in practice (ATM, HDCL, now in SCTP) 5: DataLink Layer 5-24 Cyclic Redundancy Check (CRC) Calculate code using modulo 2 division of data by generator polynomial Subtraction equivalent to XOR Weak definition of magnitude • X >= Y iff position of highest 1 bit of X is the same or greater than the highest 1 bit of Y Record remainder after division and send after data Result divisible by generator polynomial 5: DataLink Layer 5-25 Cyclic Redundancy Check (CRC) 5: DataLink Layer 5-26 CRC example Data: 101110 Generator Polynomial: x3 + 1 (1001) Send: 101110011 5: DataLink Layer 5-27 CRC example Data: 10000 Generator Polynomial: x2 + 1 (101) Send: 1000001 G 101 101 1000000 101 010 000 100 101 010 000 100 101 01 D R 5: DataLink Layer 5-28 Cyclic Redundancy Check (CRC) CRC-16 implementation Shift register and XOR gates 5: DataLink Layer 5-29 CRC polynomials CRC-16 = x16 + x15 + x2+ 1 (used in HDLC) CRC-CCITT = x16 + x12 + x5 + 1 CRC-32 = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 (used in Ethernet) 5: DataLink Layer 5-30 Forward error correction FEC Use error correcting codes to repair losses Add redundant information which allows receiver to correct bit errors Suggest looking at information and coding theory work. 5: DataLink Layer 5-31 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-32 Multiple Access Protocols and QoS How and when can a node transmit? Directly controls per-hop quality-of-service Two types of “links”: point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) traditional Ethernet 802.11 wireless LAN 5: DataLink Layer 5-33 Media access problem Point-to-point link and switched media no problem Broadcast links? Network arbitration Give everyone a fixed time/freq slot? • Ok for fixed bandwidth (e.g., voice) • What if traffic is bursty? Centralized arbiter • Ex: cell phone base station • Single point of failure Distributed arbitration • Aloha/Ethernet Humans use multiple access protocols all the time 5: DataLink Layer 5-34 Multiple access protocols single shared communication channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time multiple access protocol: distributed algorithm that determines how stations share channel, i.e., determine when station can transmit communication about channel sharing uses channel itself! what to look for in multiple access protocols: • synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance 5: DataLink Layer 5-35 Ideal Mulitple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple 5: DataLink Layer 5-36 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions “recover” from collisions “Taking turns” tightly coordinate shared access to avoid collisions Nodes take turns, but nodes with more to send can take longer turns Goal: efficient, fair, simple, decentralized 5: DataLink Layer 5-37 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access channel divided into N time slots, one per user access to channel in "rounds" inefficient with low duty cycle users and at light load each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 5: DataLink Layer 5-38 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency frequency bands bands 2,5,6 idle 5: DataLink Layer 5-39 Channel Partitioning MAC protocols CDMA (Code Division Multiple Access) unique “code” assigned to each user; ie, code set partitioning used mostly in wireless broadcast channels (cellular, satellite,etc) each user has own “chipping” sequence (ie, code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 5: DataLink Layer 5-40 Channel Partitioning MAC protocols CDMA Encode/Decode 5: DataLink Layer 5-41 Channel Partitioning MAC protocols CDMA: two sender interference 5: DataLink Layer 5-42 Random Access Protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes ➜ “collision”, To avoid deterministic collisions: randomize random access MAC protocol specifies: • how to detect collisions • how to recover from collisions (e.g., via delayed retransmissions) • “Asynchronous” TDMA Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA 5: DataLink Layer 5-43 Slotted ALOHA Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success 5: DataLink Layer 5-44 Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization 5: DataLink Layer 5-45 Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p)N-1 prob that any node has a success = Np(1-p)N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1 For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37 At best: channel used for useful transmissions 37% of time! 5: DataLink Layer 5-46 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame arrives Send without awaiting for beginning of slot collision probability increases: frame sent at t0 collides with other frames sent in [t0-1,t0+1] 5: DataLink Layer 5-47 Pure Aloha efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0] . P(no other node transmits in [p0,p0+1] = p . (1-p)(N-1) . (1-p) (N-1) P(success by any of N nodes) = N p . (1-p) (N-1). (1-p) (N-1) S = throughput = “goodput” (success rate) … choosing optimum p as n -> infty ... = 1/(2e) = .18 0.4 0.3 Slotted Aloha 0.2 0.1 Pure Aloha 0.5 1.0 1.5 2.0 G = offered load = Np protocol constrains effective channel throughput! 5: DataLink Layer 5-48 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle Non-persistent CSMA: retry after random interval Human analogy: don’t interrupt others! 5: DataLink Layer 5-49 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability 5: DataLink Layer 5-50 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals • Collisions detected within short time • Abort transmission as soon as collision detected to reduce channel waste human analogy: the polite conversationalist 5: DataLink Layer 5-51 CSMA/CD collision detection 5: DataLink Layer 5-52 CSMA/CD problems Can CSMA/CD work over wireless LANs Collision detection difficult in wireless LANs: receiver shut off while transmitting Hidden terminal problem 5: DataLink Layer 5-53 Hidden Terminal effect A, C cannot hear each other obstacles, signal attenuation Neither A or C can tell if they collide at B 5: DataLink Layer 5-54 CSMA/CA Use base CSMA Add acknowledgements Receiver acknowledges receipt of data Avoids hidden terminal problem Avoid collisions explicitly via channel reservation Sender sends “request-to-send” (RTS) messages • Transmitted without reservation using CSMA with ACKs Receiver sends “clear-to-send” (CTS) messages • Transmitted without reservation using CSMA with ACKs Sender sends data packet using reservation • Explicitly indicates length of so others know how long to back off Used in 802.11 wireless LAN networks 5: DataLink Layer 5-55 “Taking Turns” MAC protocols Recall, channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead “taking turns” protocols look for best of both worlds! 5: DataLink Layer 5-56 “Taking Turns” MAC protocols Token passing: Polling: control token passed from master node one node to next “invites” slave nodes sequentially. to transmit in turn token message RTS, CTS messages concerns: concerns: polling overhead latency single point of failure (master) token overhead latency single point of failure (token) 5: DataLink Layer 5-57 Taking-turns protocols Distributed Polling: time divided into slots begins with N short reservation slots reservation slot time equal to channel end-end propagation delay station with message to send posts reservation reservation seen by all stations after reservation slots, message transmissions ordered by known priority 5: DataLink Layer 5-58 Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code • Time Division, Frequency Division Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technologies (wire), hard in others (wireless) • CSMA/CD used in Ethernet • CSMA/CA used in 802.11 Taking Turns • polling from a central site, token passing 5: DataLink Layer 5-59 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-60 MAC Addresses 32-bit IP address: network-layer address used to route between networks to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address: used to get frame from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM • ifconfig –a • arp -a 5: DataLink Layer 5-61 Physical addressing Why have separate IP and hardware addresses? Assign adapters an IP address • Hardware only works for IP (no IPX, DECNET) • Must be reconfigured when moved Use hardware address as network address • Need standardized fixed length hardware address • No route aggregation 5: DataLink Layer 5-62 LAN Addresses and ARP Each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD 71-65-F7-2B-08-53 LAN (wired or wireless) Broadcast address = FF-FF-FF-FF-FF-FF = adapter 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 5: DataLink Layer 5-63 LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address ➜ portability can move LAN card from one LAN to another IP hierarchical address NOT portable depends on IP subnet to which node is attached 5: DataLink Layer 5-64 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 237.196.7.78 1A-2F-BB-76-09-AD 237.196.7.23 Each IP node (Host, Router) on LAN has ARP table ARP Table: IP/MAC address mappings for some LAN nodes 237.196.7.14 LAN 71-65-F7-2B-08-53 237.196.7.88 < IP address; MAC address; TTL> 58-23-D7-FA-20-B0 TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 0C-C4-11-6F-E3-98 5: DataLink Layer 5-65 ARP protocol: Same LAN (network) A knows B’s IP address and wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address Dest MAC address = FF-FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to- MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator 5: DataLink Layer 5-66 Routing to another LAN walkthrough: send datagram from A to B via R assume A know’s B IP address A R B Two ARP tables in router R, one for each IP network (LAN) 5: DataLink Layer 5-67 A creates datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s adapter sends frame R’s adapter receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B A R B 5: DataLink Layer 5-68 RARP, BOOTP, DHCP ARP: Given an IP address, return a hardware address RARP: Given a hardware address, give me the IP address DHCP, BOOTP: Similar to RARP Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play” host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg 5: DataLink Layer 5-69 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-70 Specific data-link layers Specific data-link layers Ethernet (802.3) Token Ring (802.5) WiFi (802.11) X.25 Frame relay Special link layers covered later (PPP, ATM) 5: DataLink Layer 5-71 Ethernet's implementation of data-link layer Framing (special pre-amble within frame) Physical addressing (6 byte hardware addresses) Demux to upper protocol (type field in header) Flow control (none) Error detection and correction (CRC-32) Reliable delivery (none) Security (none) Media access and quality of service (CSMA/CD with adaptive, randomized wait) Digital to analog conversion (Manchester encoding) 5: DataLink Layer 5-72 Ethernet “dominant” wired LAN technology: First practical local area network, built at Xerox PARC in 70’s cheap: $3 for 100Mbs NIC first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch 5: DataLink Layer 5-73 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates Ethernet II frame standard, some others…. Original circa 1970 Xerox PARC IEEE 802.3 LLC frame IEEE 802.3 SNAP frame Novell Proprietary Not all compatible with each other 5: DataLink Layer 5-74 Ethernet Frame Structure (more) Addresses: 6 bytes Globally unique, allocated to manufacturers if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol otherwise, adapter discards frame Type: indicates the higher layer protocol mostly IP but others include Novell IPX and AppleTalk Data – 46 to 1500 bytes CRC: 4 bytes checked at receiver, if error is detected, frame is dropped CRC-32 • (x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1) 5: DataLink Layer 5-75 Unreliable, connectionless service Connectionless: No handshaking between sending and receiving adapter. Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter stream of datagrams passed to network layer can have gaps gaps will be filled if app is using TCP otherwise, app will see the gaps 5: DataLink Layer 5-76 Ethernet uses CSMA/CD No slots adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random access 5: DataLink Layer 5-77 General Ethernet CSMA/CD Packet? No Sense Carrier Send Detect Collision Yes Discard Packet attempts < 16 b=CalcBackoff(); wait(b); attempts++; attempts == 16 5: DataLink Layer 5-78 Ethernet CSMA/CD algorithm 1. Adaptor receives 4. If adapter detects datagram from net layer & another transmission while creates frame transmitting, aborts and sends jam signal 2. If adapter senses channel idle, it starts to transmit 5. After aborting, adapter frame. If it senses enters exponential channel busy, waits until backoff before returning channel idle and then to Step 2 transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 5: DataLink Layer 5-79 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits (more later) Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential backoff calculation: If deterministic delay after collision, collision will occur again in lockstep If random delay with fixed mean • Few senders needless waiting • Too many senders too many collisions Exponentially increasing random delay • Infer senders from # of collisions • More senders increase wait time 5: DataLink Layer 5-80 Ethernet’s CSMA/CD (more) Exponential backoff calculation: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K·512 bit times first collision: choose K from {0,1}; delay is K· 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten collisions, choose K from {0,1,2,3,4,…,1023} See/interact with Java applet on AWL Web site: highly recommended ! 5: DataLink Layer 5-81 Ethernet: uses CSMA/CD if packet then { A: sense channel if idle then { transmit and monitor the channel; } } if detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} else {wait until ongoing transmission is over and goto A} 5: DataLink Layer 5-82 Ethernet CSMA/CD and Packet Size What if two people sent really small packets How do you find collision? Must have a minimum packet size 5: DataLink Layer 5-83 Ethernet Collision Detect & Packet Size Min packet length > 2x max prop delay If A, B are at opposite sides of link, and B starts one link prop delay after A Jam signal Jam network for after collision, then stop sending Ensures that everyone notices collision 5: DataLink Layer 5-84 Propagation delay & packet size Propagation delay determines min. packet size to prevent undetected collisions Modern 10Mb Ethernet Minimum packet size calculation • 500m maximum segment length • Can add repeaters up to a maximum 5 segments (2500m) • c in cable = 60% * c in vacuum = 1.8 x 10^8 m/s • ~ 12.5us one-way delay • Add repeater and tranceiver delay • To be safe IEEE specifies a 512 “bit-time” slot for Ethernet = 51.2us • 512 bits = 64 bytes (data payload = 46 bytes) 5: DataLink Layer 5-85 Minimum packet size What about scaling? 100Mbit, 1Gbit... Make network smaller? Solution for 100BaseT Make min pkt size larger? 512bits @ 1Gbps = 512ns 512ns * 1.8 * 10^8 = 92meters Gigabit ethernet uses collision extension for small pkts 5: DataLink Layer 5-86 10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub Bus topology popular through mid 90s (10Base2, co-ax) Half-duplex Nodes at both ends of link can not transmit at same time twisted pair hub 5: DataLink Layer 5-87 10Base2 Ethernet Sifting through the jargon (10Base2) 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to DataLink Layer its other interfaces: physical layer device5:only! 5-88 Gbit Ethernet uses standard Ethernet frame format allows for point-to-point links and shared broadcast channels in shared mode, CSMA/CD is used; short distances between nodes required for efficiency Full-Duplex at 1 Gbps for point-to-point links Nodes can transmit and receive at 1Gbps simultaneously 10 Gbps now ! 5: DataLink Layer 5-89 Ethernet Problems Ethernet unstable at high loads Peak utilization = 1/e = 37% Peak throughput worse with More hosts – more collisions needed to identify single sender Smaller packet sizes – more frequent arbitration Longer links – collisions take longer to observe, more wasted bandwidth 5: DataLink Layer 5-90 Token Rings Packets broadcast around ring Token “right to send” rotates around ring Fair, real-time bandwidth allocation • Every host holds token for limited time • Higher latency when only one sender Higher bandwidth • Point to point links electrically simpler than bus 5: DataLink Layer 5-91 Token Passing: IEEE802.5 standard 4 Mbps max token holding time: 10 ms (limits frame length) • SD, ED mark start, end of packet • AC: access control byte: ● ● ● token bit: value 0 means token can be seized, value 1 means data follows FC priority bits: priority of packet reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free 5: DataLink Layer 5-92 Why Did Ethernet Win? Better failure modes Token rings – network unusable Ethernet – node detached Good performance in common case Volume lower cost higher volume …. Adaptable To higher bandwidths (vs. FDDI) To switching (vs. ATM) Completely distributed, easy to maintain/administer Easy incremental deployment Cheap cabling, etc 5: DataLink Layer 5-93 IEEE 802.11 Wireless LAN Wireless LANs: untethered (often mobile) networking IEEE 802.11 standard: Defines specific implementations of data-link functions Framing, error detection, MAC, etc. Unlicensed frequency spectrum: 900Mhz, 2.4Ghz • Organized into cells called Basic Service Sets ● ● ● Wireless hosts Access Point (base station) Combined to form distribution system 5: DataLink Layer 5-94 Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can dynamically form network without AP Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group 5: DataLink Layer 5-95 IEEE 802.11 MAC Allows for several modes CSMA (with explicit ACK to indicate collision) CSMA/CA: reservations Polling from AP 5: DataLink Layer 5-96 IEEE 802.11 MAC Protocol: CSMA 802.11 CSMA sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -if sense channel busy then backoff (random, exponential) 802.11 CSMA receiver if received OK return ACK after SIFS 802.11 CSMA others NAV: Network Allocation Vector 802.11 frame has transmission time field others (hearing data) defer access for NAV time units SIFS and DIFS Used to separate and prioritize frames SIFS < DIFS allows acks to grab channel with higher priority 5: DataLink Layer 5-97 IEEE 802.11 MAC Protocol CSMA/CA Same as previous mode but with explicit channel reservation Send short reservation messages via CSMA to reserve channel • Sender RTS (request to send), Receiver CTS (clear to send) • CTS notifies all hidden stations of sender's reservation • Short messages so that collision less likely and of short duration Send data unobstructed on reserved channel • End result similar to CSMA/CD 5: DataLink Layer 5-98 X.25 and Frame Relay Like ATM: wide area network technologies virtual circuit oriented origins in telephony world Not really a link layer but.... Viewed as link layers by IP protocol Used mostly to carry IP datagrams between IP routers Going the way of the dinosaurs.... 5: DataLink Layer 5-99 X.25 X.25 builds VC between source and destination for each user connection Per-hop control along path error control (with retransmissions) on each hop using LAP-B • variant of the HDLC protocol • developed when bit error rates over long-haul copper links were orders of magnitude higher per-hop flow control using credits • congestion arising at intermediate node propagates to previous node on path • back to source via back pressure 5: DataLink Layer 5-100 IP versus X.25 X.25: reliable in-sequence end-end delivery from end-to-end “intelligence in the network” built for dumb terminals accessing mainframes IP: unreliable, out-of-sequence end-end delivery “intelligence in the endpoints” 2000 gigabit routers: limited processing possible CPU capacity at end-hosts IP wins 5: DataLink Layer 5-101 Frame Relay Designed in late ‘80s, widely deployed in the ‘90s Second-generation X.25 Frame relay service: no error control no flow control End-to-end congestion control Some QoS mechanisms 5: DataLink Layer 5-102 Frame Relay (more) Designed to interconnect corporate customer LANs typically permanent VC’s: “pipe” carrying aggregate traffic between two routers switched VC’s: as in ATM corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT) 5: DataLink Layer 5-103 Frame Relay (more) flags address data CRC flags Flag bits, 01111110, delimit frame address: 10 bit VC ID field 3 congestion control bits • FECN: forward explicit congestion notification (frame experienced congestion on path) • BECN: congestion on reverse path • DE: discard eligibility Precursor to IP DiffServ and ECN 5: DataLink Layer 5-104 Frame Relay -VC Rate Control Committed Information Rate (CIR) defined, “guaranteed” for each VC negotiated at VC set up time customer pays based on CIR DE bit: Discard Eligibility bit Edge FR switch measures traffic rate for each VC; marks DE bit DE = 0: high priority, rate compliant frame; deliver at “all costs” DE = 1: low priority, eligible for discard when congestion Precursor to IP DiffServ Can be used to support higher layer QoS mechanisms 5: DataLink Layer 5-105 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Interconnections: Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-106 Link-layer devices Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: 802.3 specifies maximum cable length large “collision domain” (can collide with many stations) limited number of stations: 802.5 have token passing delays at each station 5: DataLink Layer 5-107 Hubs Hubs are essentially physical-layer, multi-port repeaters: bits coming from one link go out all other links at the same rate no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality twisted pair hub 5: DataLink Layer 5-108 Hubs (more) Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: simple, inexpensive device extends maximum distance between node pairs Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions 5: DataLink Layer 5-109 Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain single collision domain results in no increase in max throughput multi-tier throughput same as single segment throughput Can’t interconnect 10BaseT & 100BaseT hub hub hub hub 5: DataLink Layer 5-110 Switches Link Layer device Stores and forwards Ethernet frames Examines frame header and selectively forwards frame based on destination MAC address Two-port switch known as a “bridge” Switches known as “multi-port” bridges A switch isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit Transparent to hosts Plug-and-play, self-learning (do not need to be configured) 5: DataLink Layer 5-111 Switches (more) Switch advantages: Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters 5: DataLink Layer 5-112 Switch operation switch 1 2 hub 3 hub hub How do determine onto which LAN segment to forward frame? Looks like a routing problem... 5: DataLink Layer 5-113 Self learning Approach Monitor traffic to build a cache of which nodes are downstream of which ports Selectively forward frames based on cache entries Flood network for frames with unknown (MAC) destinations Algorithm Each switch has a switch table entry in switch table: • (MAC Address, Interface, Time Stamp) • stale entries in table dropped (TTL can be 60 min) switch learns which hosts can be reached through which interfaces • when frame received, switch “learns” location of sender: incoming LAN segment 5: DataLink Layer • records sender/location pair in switch table 5-114 Filtering/Forwarding When switch receives a frame: index switch table using MAC dest address if entry found for destination then{ if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 5: DataLink Layer 5-115 Switch example Suppose C sends frame to D 1 B C A B E G 3 2 hub hub hub A address interface switch 1 1 2 3 I D E F G H Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table, switch forwards frame into interfaces 2 and 3 frame received by D 5: DataLink Layer 5-116 Switch example Suppose D replies back with frame to C. address interface switch B C hub hub hub A I D E F G A B E G C 1 1 2 3 1 H Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table, switch forwards frame only to interface 1 frame received by C 5: DataLink Layer 5-117 Switch: traffic isolation switch installation breaks subnet into LAN segments switch filters packets: same-LAN-segment frames not usually forwarded onto other LAN segments segments become separate collision domains switch collision domain hub collision domain hub collision domain hub 5: DataLink Layer 5-118 Switches: dedicated access Switch with many interfaces A Hosts have direct connection to switch No collisions; full duplex Much greater aggregate bandwidth Data backplane of switches typically large to support simultaneous transfers amongst ports Switching: A-to-A’ and B-to-B’ simultaneously, no collisions C’ B switch C B’ A’ 5: DataLink Layer 5-119 Switches and Spanning Trees for increased reliability, desirable to have redundant, alternate paths from source to destination with multiple simultaneous paths, cycles result bridges may multiply and forward frame forever solution: organize switches in a spanning tree by disabling subset of interfaces Disabled switch switch 5: DataLink Layer 5-120 Switches vs. Routers both store-and-forward devices routers: network layer devices (examine network layer headers) switches/bridges are link Layer devices routers maintain routing tables, implement routing algorithms swtiches maintain filtering tables, implement filtering, learning and spanning tree algorithms why can't the Internet be one great big switch? 5: DataLink Layer 5-121 Routers vs. Switches Switches + and + Switch operation is simpler requiring less processing bandwidth - Topologies are restricted with switches: a spanning tree must be built to avoid cycles - Switches do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a switch) 5: DataLink Layer 5-122 Routers vs. Switches Routers + and + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher processing bandwidth switches do well in small (few hundred hosts) while routers used in large networks (thousands of hosts) 5: DataLink Layer 5-123 Summary comparison hubs routers switches traffic isolation no yes yes plug & play yes no yes optimal routing cut through no yes no yes no yes 5: DataLink Layer 5-124 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM 5: DataLink Layer 5-125 Point to Point Data Link Control Point-to-point links One sender, one receiver, one link Easier than shared broadcast links • No media access control • No need for explicit MAC addressing (ie ARP) Goal of Point-to-Point protocols Layer generic “higher-level” data-link layer functions on top of a variety of point-to-point links • Dial-up phone line, DSL, ISDN etc. • Each different link does its own digital-analog conversion (ie provides bits) • Implement pseudo-link layer on top that implements common functions – Framing, Demux to upper layer, etc. Examples PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!) 5: DataLink Layer 5-126 PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address 5: DataLink Layer 5-127 PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers! 5: DataLink Layer 5-128 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) 5: DataLink Layer 5-129 PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection 5: DataLink Layer 5-130 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> data byte Receiver: two 01111101 byte followed by 01111110 byte: discard first byte, continue data reception single 01111110: flag byte 5: DataLink Layer 5-131 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 5: DataLink Layer 5-132 PPP Data Control Protocol Before exchanging networklayer data, data link peers must configure PPP link (max. frame length, authentication) learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 5: DataLink Layer 5-133 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet and other data link layers 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS 5: DataLink Layer 5-134 Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: computing examples: virtual memory, virtual devices Virtual machines: e.g., java IBM VM os from 1960’s/70’s layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly 5: DataLink Layer 5-135 The Internet: virtualizing networks 1974: multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha) packet radio network ARPAnet "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648. … differing in: addressing conventions packet formats error recovery routing satellite net 5: DataLink Layer 5-136 The Internet: virtualizing networks Internetwork layer (IP): addressing: internetwork appears as a single, uniform entity, despite underlying local network heterogeneity network of networks Gateway: “embed internetwork packets in local packet format or extract them” route (at internetwork level) to next gateway gateway ARPAnet satellite net 5: DataLink Layer 5-137 Cerf & Kahn’s Internetwork Architecture What is virtualized? two layers of addressing: internetwork and local network new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology cable satellite 56K telephone modem today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! 5: DataLink Layer 5-138 Virtual links and tunneling Many options of encapsulating or tunneling packets through a “virtual link” (VPN) Generic Routing Encapsulation (GRE) • PPTP (Point-to-point Tunneling Protocol) • L2F (Layer 2 Forwarding) • L2TP (Layer 2 Tunneling Protocol) Example • Encrypt data at a layer below network layer • IPsec only works for IP packets • Allows encryption at data-link layer Can also be done at network layer via IPsec 5: DataLink Layer 5-139 ATM and MPLS ATM, MPLS separate networks in their own right different service models, addressing, routing from Internet viewed by Internet as logical link connecting IP routers just like dialup link is really part of separate network (telephone network) 5: DataLink Layer 5-140 Multiprotocol label switching (MPLS) initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address! PPP or Ethernet header MPLS header label 20 IP header remainder of link-layer frame Exp S TTL 3 1 5 5: DataLink Layer 5-141 MPLS capable routers a.k.a. label-switched router forwards packets to outgoing interface based only on label value (don’t inspect IP address) MPLS forwarding table distinct from IP forwarding tables signaling protocol needed to set up forwarding RSVP-TE forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! use MPLS for traffic engineering must co-exist with IP-only routers 5: DataLink Layer 5-142 MPLS forwarding tables in label out label dest 10 12 8 out interface A D A 0 0 1 in label out label dest out interface 10 6 A 1 12 9 D 0 R6 0 0 D 1 1 R3 R4 R5 0 0 R2 in label 8 out label dest 6 A out interface in label 6 outR1 label dest - A A out interface 0 0 5: DataLink Layer 5-143 Chapter 5: Summary principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Ethernet switched LANS PPP virtualized networks as a link layer: ATM, MPLS 5: DataLink Layer 5-144 Physical Layer Plethora of physical media Fiber, copper, air Specifies the characteristics of transmission media Too many to cover in detail, not the focus of the course Many data-link layer protocols (i.e. Ethernet, Token-Ring, FDDI. ATM run across multiple physical layers) Physical characteristics dictate suitability of data-link layer protocol and bandwidth limits 5: DataLink Layer 5-145 Digital to analog conversion Bits sent as analog signals Photonic pulses of a given wavelength over optical fiber Electronic signals of a given voltage 5: DataLink Layer 5-146 Digital to analog conversion Will cover electronic transmission (optical transmission left for you to research) Biggest issue When to sample voltage? Detecting sequences involves clocking with the same clock • How to synchronize sender and receiver clocks? Need easily detectible event at both ends • Signal transitions help resync sender and receiver • Need frequent transitions to prevent clock skew http://www.mouse.demon.nl/ckp/telco/encode.h tm 5: DataLink Layer 5-147 RZ Return to Zero (RZ) 1=pulse to high, dropping back to low 0=no transition 5: DataLink Layer 5-148 NRZ-L Non-Return to Zero Level (NRZ-L) 1=high signal, 0=lower signal Long sequence of same bit causes difficulty • DC bias hard to detect – low and high detected by difference from average voltage • Clock recovery difficult Used by Synchronous Optical Network (SONET) • SONET XOR’s bit sequence to ensure frequent transitions Used in early magnetic tape storage 5: DataLink Layer 5-149 NRZ-L 5: DataLink Layer 5-150 NRZ-M Non-Return to Zero Mark Less power to transmit versus NRZ 1=signal transition at start of bit, 0=no change No problem with string of 1’s NRZ-like problem with string of 0’s Used in SDLC (Synchronous Data Link Control) Used in modern magnetic tape storage 5: DataLink Layer 5-151 NRZ-S Non-Return to Zero Space 1=no change, 0=signal transition at start of bit No problem with string of 0’s NRZ-like problem with string of 1’s 5: DataLink Layer 5-152 Manchester (Bi-Phase-Level) coding Used by Ethernet 0=low to high transition, 1=high to low transition Transition for every bit simplifies clock recovery Not very efficient Doubles the number of transitions Circuitry must run twice as fast 5: DataLink Layer 5-153 Manchester coding Encoding for 110100 Bit stream 1 1 0 1 0 0 Manchester encoding 5: DataLink Layer 5-154 Other coding schemes Bi-Phase-Mark, Bi-Phase-Space Level change at every bit period boundary Mid-period transition determines bit • Bi-Phase-M: 0=no change, 1=signal transition • Bi-Phase-S: 0=signal transition, 1=no change 5: DataLink Layer 5-155 Other coding schemes Differential Bi-Phase-Space, Differential Bi-Phase-Mark Level change at every mid-bit period boundary Bit period boundary transition determines bit • Diff-Bi-Phase-M: 0=signal transition, 1=no change • Diff-Bi-Phase-S: 0=no change, 1=signal transition 5: DataLink Layer 5-156 Common Cabling Copper Twisted Pair • Unshielded (UTP) – CAT-1, CAT-2, CAT-3, CAT-4, CAT-5, CAT-5e • Shielded (STP) Coaxial Cable Fiber Single-mode Multi-mode 5: DataLink Layer 5-157 Twisted Pair Most common LAN interconnection Multiple pairs of twisted wires Twisting to eliminate interference More twisting = Higher data rates, higher cost Standards specify twisting, resistance, and maximum cable length for use with particular data-link layer 5: DataLink Layer 5-158 Twisted pair 5 categories Category 1 • Voice only (telephone wire) Category 2 • Data to 4Mbs (LocalTalk) Category 3 • Data to 10Mbs (Ethernet) Category 4 • Data to 20Mbs (16Mbs Token Ring) Category 5 (100 MHz) • Data to 100Mbs (Fast Ethernet) Category 5e (350 MHz) • Data to 1000Mbs (Gigabit Ethernet) 5: DataLink Layer 5-159 Twisted Pair Common connectors for Twisted Pair RJ11 (3 pairs) RJ45 (4 pairs) • Allows both data and phone connections • (1,2) and (3,6) for data, (4,5) for voice • Crossover cables for NIC-NIC, Hub-Hub connection (Data pairs swapped) 5: DataLink Layer 5-160 UTP Unshielded Twisted Pair Limited amount of protection from interference Commonly used for voice and ethernet • Voice: multipair 100-ohm UTP 5: DataLink Layer 5-161 STP Shielded Twisted Pair Not as common at UTP UTP susceptible to radio and electrical interference Extra shielding material added Cables heavier, bulkier, and more costly Often used in token ring topologies • 150 ohm STP two pair (IEEE 802.5 Token Ring) 5: DataLink Layer 5-162 Coaxial cable Single copper conductor at center Plastic insulation layer Highly resistant to interference Braided metal shield Support longer connectivity distances over UTP 5: DataLink Layer 5-163 Coaxial cable Thick (10Base5) Large diameter 50-ohm cable N connectors Thin (10Base2) cables Small diameter 50-ohm cable BNC, RJ-58 connector Video cable 75-ohm cable BNC, RJ-59 connector Not compatible with RJ-58 5: DataLink Layer 5-164 Fiber Center core made of glass or plastic fiber Transmit light versus electronic signals Protects from electronic interference, moisture Plastic coating to cushion core Kevlar fiber for strength Teflon or PVC outer insulating jacket 5: DataLink Layer 5-165 Fiber Single-mode fiber Smaller diameter (12.5 microns) One mode only Preserves signal better over longer distances Typically used for SONET or SDH Lasers used to signal More expensive Multi-mode fiber Larger diameter (62.5 microns) Multiple modes LEDs used to signal WDM and DWDM Photodiodes at receivers 5: DataLink Layer 5-166 Fiber connectors ESCON Duplex SC ST MT-RJ (multimode) Duplex LC 5: DataLink Layer 5-167 Physical-link lingo Specifies capacities over physical media Electronic T1/DS1=1.54 Mbps T3/DS3=45Mbps Optical (OC=optical carrier) OC1=52 Mbps OC3/STM1=156 Mbps OC12=622 Mbps OC48=2488 Mbps OC192=10 Gbps OC768=40 Gbps 5: DataLink Layer 5-168 Wireless Entire spectrum of transmission frequency ranges Radio Infrared Lasers Cellular telephone Microwave Satellite Acoustic (see ESE sensors) Ultra-wide band http://www.ntia.doc.gov/osmhome/allochrt.html 5: DataLink Layer 5-169 5: DataLink Layer 5-170 What runs on them? Protocol Summary Protocol Cable Speed Topology Ethernet Twisted Pair, Coaxial, Fiber 10 Mbps Linear Bus, Star, Tree Fast Ethernet Twisted Pair, Fiber 100 Mbps Star LocalTalk Twisted Pair .23 Mbps Linear Bus or Star Token Ring Twisted Pair 4 Mbps - 16 Mbps Star-Wired Ring FDDI Fiber 100 Mbps Dual ring ATM Twisted Pair, Fiber 155-2488 Mbps Linear Bus, Star, Tree 5: DataLink Layer 5-171 Extra slides 5: DataLink Layer 5-172 DL: ARQ Automatic Repeat Request (ARQ) Receiver sends acknowledgement (ACK) when it receives packet Sender waits for ACK and timeouts if it does not arrive within some time period 5: DataLink Layer 5-173 DL: Stop and Wait Sender Time Receiver Timeout • Simplest ARQ protocol • Send a packet, stop and wait until acknowledgement arrives 5: DataLink Layer 5-174 ACK lost Timeout Timeout Timeout Timeout Timeout Time Timeout DL: Recovering from Error Packet lost Early timeout 5: DataLink Layer 5-175 DL: Stop and Wait Problems How to recognize a duplicate? Performance Can only send one packet per round trip 5: DataLink Layer 5-176 DL: How to Recognize Resends? • Use sequence numbers – both packets and acks • Sequence # in packet is finite -- how big should it be? – For stop and wait? • One bit – won’t send seq #1 until received ACK for seq #0 5: DataLink Layer 5-177 DL: How to Keep the Pipe Full? Send multiple packets without waiting for first to be acked Number of pkts in flight = window How large a window is needed Round trip delay * bandwidth = capacity of pipe Reliable, unordered delivery Several parallel stop & waits Send new packet after each ack Sender keeps list of unack’ed packets; resends after timeout Receiver same as stop&wait 5: DataLink Layer 5-178 DL: Sliding Window Reliable, ordered delivery Receiver has to hold onto a packet until all prior packets have arrived Sender must prevent buffer overflow at receiver Circular buffer at sender and receiver Packets in transit <= buffer size Advance when sender and receiver agree packets at beginning have been received 5: DataLink Layer 5-179 DL: Sender/Receiver State Sender Max ACK received Receiver Next expected Next seqnum … … … … Sender window Sent & Acked Sent Not Acked OK to Send Not Usable Max acceptable Receiver window Received & Acked Acceptable Packet Not Usable 5: DataLink Layer 5-180 DL: Window Sliding – Common Case On reception of new ACK (i.e. ACK for something that was not acked earlier Increase sequence of max ACK received Send next packet On reception of new in-order data packet (next expected) Hand packet to application Send cumulative ACK – acknowledges reception of all packets up to sequence number Increase sequence of max acceptable packet 5: DataLink Layer 5-181 DL: Loss Recovery On reception of out-of-order packet Send nothing (wait for source to timeout) Cumulative ACK (helps source identify loss) Timeout (Go Back N recovery) Set timer upon transmission of packet Retransmit max ACK received sequence + 1 Restart from max ACK received sequence + 1 Performance during loss recovery No longer have an entire window in transit Can have much more clever loss recovery • Covered in TCP lectures 5: DataLink Layer 5-182 DL: Sequence Numbers How large do sequence numbers need to be? Must be able to detect wrap-around Depends on sender/receiver window size E.g. Max seq = 7, send win=recv win=7 If pkts 0..6 are sent succesfully and all acks lost • Receiver expects 7,0..5, sender retransmits old 0..6 Max sequence must be >= send window + recv window 5: DataLink Layer 5-183 Checksumming: Cyclic Redundancy Check view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits widely used in practice (ATM, HDCL) 5: DataLink Layer 5-184 Manchester encoding Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized, global clock among nodes! Hey, this is physical-layer stuff! More later 5: DataLink Layer 5-185 Backbone Bridge 5: DataLink Layer 5-186 Interconnection Without Backbone Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment 5: DataLink Layer 5-187 CSMA/CD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame efficiency 1 1 5t prop / ttrans Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA, but still decentralized, simple, and cheap 5: DataLink Layer 5-188 More on Switches cut-through switching: frame forwarded from input to output port without first collecting entire frame slight reduction in latency combinations of shared/dedicated, 10/100/1000 Mbps interfaces 5: DataLink Layer 5-189 Institutional network to external network mail server web server router switch IP subnet hub hub hub 5: DataLink Layer 5-190 ATM ATM Replace existing Internet protocols with a more “robust” architecture Network architecture to support • Multiple service classes and per-flow guarantees • Virtual circuits to support real-time applications • Explicit rate signaling and resource allocation Covered as a data-link layer… 5: DataLink Layer 5-191 Internet vs. ATM Internet “elastic” datagram service, no strict timing req. Computer communication only “smart” end systems (computers) • can adapt, perform control, error recovery • simple inside network, complexity at “edge” many link types • different characteristics • uniform service difficult ATM evolved from telephony, strict timing and reliability requirements Computer and human communication • need for guaranteed service “dumb” end systems • telephones • complexity inside network 5: DataLink Layer 5-192 Asynchronous Transfer Mode (ATM) 1980s/1990’s standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture Take strengths of IP, learn from its shortcomings Packet switching good Packet switching without explicit network-level connections and reservations bad Packet switching using large headers for small packets bad (voice) Design new network to address emerging applications while allowing for efficient support for non-real-time data applications Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world packet-switching (fixed length packets, called “cells”) using virtual circuits Covered now since it is used mostly as a data-link layer 5: DataLink Layer 5-193 ATM architecture adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer 5: DataLink Layer 5-194 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” ATM is a network technology Reality: used to connect IP backbone routers “IP over ATM” ATM as switched link layer, connecting IP routers IP network ATM network 5: DataLink Layer 5-195 ATM Adaptation Layer (AAL) ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells analogy: TCP segment in many IP packets 5: DataLink Layer 5-196 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell 5: DataLink Layer 5-197 ATM Layer Service: transport cells across ATM network analogous to IP network layer very different services than IP network layer Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no 5: DataLink Layer 5-198 ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. Permanent VCs (PVCs) long lasting connections typically: “permanent” route between to IP routers Switched VCs (SVC): dynamically set up on per-call basis 5: DataLink Layer 5-199 ATM VCs Advantages of ATM VC approach: QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic one PVC between each source/dest pair) does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived connections 5: DataLink Layer 5-200 ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload Why?: small payload -> short cell-creation delay for digitized voice halfway between 32 and 64 (compromise!) Cell header Cell format 5: DataLink Layer 5-201 ATM cell header VCI: virtual channel ID will change from link to link thru net PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum cyclic redundancy check 5: DataLink Layer 5-202 ATM Physical Layer (more) Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Physical Medium Dependent: depends on physical medium being used TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send 5: DataLink Layer 5-203 ATM Physical Layer Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle) 5: DataLink Layer 5-204 IP-Over-ATM Classic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses IP over ATM replace “network” (e.g., LAN segment) with ATM network ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs 5: DataLink Layer 5-205 IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy phy ATM phy ATM phy app transport IP AAL ATM phy 5: DataLink Layer 5-206 Datagram Journey in IP-over-ATM Network at Source Host: IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL5 AAL5 encapsulates data, segments cells, passes to ATM layer ATM network: moves cell along VC to destination at Destination Host: AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP 5: DataLink Layer 5-207 IP-Over-ATM Issues: IP datagrams into ATM AAL5 PDUs from IP addresses to ATM addresses just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs 5: DataLink Layer 5-208 ATM and “IP switching” ATM advantages Lookup of VCID = O(1), Lookup of IP routes O(log n) One-time route lookup and circuit establishment, all subsequent traffic switched ATM disadvantages Complex signaling and routing for establishing communication Difficulty in mapping IP traffic dynamically onto ATM circuits Goal Maintain IP infrastructure Accelerate it with labels to support O(1) lookups a la ATM Solution Ipsilon and “IP switching” http://pnewman.org/papers/infocom96.pdf 5: DataLink Layer 5-209 IP over ATM versus IP switching IP network control IP routing IP network control IP routing ATM network control ATM label switching IP network control IP routing IP network control ATM label switching IP network control IP routing 5: DataLink Layer 5-210 ATM and “IP switching” In a nutshell Start with ATM switch Rip out ATM signaling and routing Add IP routing software Add Flow classifier to map unknown flows to underlying ATM virtual circuit ID Attach VCID and allow downstream nodes to do the same Operation Upon arrival of first packet in flow • Record unknown incoming VCID • Lookup IP flow and map it to an outgoing virtual circuit ID (label) using IP routing software • Create incomingVCID to outgoingVCID table entry for subsequent packets Subsequent packets • Switched in hardware using VCID after flow classified at edge • IP packet forwarding done as label index lookup O(1) versus IP route lookup O(log n) 5: DataLink Layer 5-211 ATM and “IP switching” Later generalized as MPLS (multi-protocol label switching) “Layer 2 ½” Not tied to ATM Extensible to IPv6 Half-way in between data-link addresses and IP addresses • Labeling done within a cloud versus link-local (data-link addresses) and global (IP addresses) http://www.rfc-editor.org/rfc/rfc3031.txt Used as a tool for traffic engineering http://www.rfc-editor.org/rfc/rfc2702.txt 5: DataLink Layer 5-212