# Chapter 6 The Data Link layer

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4 Cyclic Redundancy Check (CRC) - sender View data bits, D, as a binary number Goal: choose r CRC bits, R, such that <D,R> is exactly divisible by G using modulo 2 arithmetic Choose r+1 bit pattern (generator), G Modulo 2 arithmetic there is no carry in addition, and no borrow in subtraction addition and subtraction same as bitwise exclusive OR (XOR) Data Link Layer (SSL) 6-7 Cyclic Redundancy Check (CRC) - receiver Bit string <D,R> sent is exactly divisible by G Receiver knows G, performs division. If non-zero remainder, error detected! can detect all burst errors less than r+1 bits; longer burst errors are detectable with probability 1-(0.5) r Data Link Layer (SSL) 6-8 4

5 CRC Theory and Example Want: (D*2 r ) XOR R = ng add R to both sides: D*2 r XOR R XOR R = (ng) XOR R Equivalently: the remainder from dividing D*2 r by G is equal to R; the desired CRC bit string is D*2 r R = remainder[ ] G Data Link Layer (SSL) 6-9 Chapter 6 The Data Link layer 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs addressing, ARP Ethernet layer-2 switches VLANS 6.5 link virtualization: MPLS 6.6 data center networks 6.7 a day in the life of a web request (play animation in.ppt slide on your own) Data Link Layer (SSL)

6 Links and Multiple Access Protocols Two types of links : point-to-point fiber optic link link between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet shared coax cable in HFC (hybrid fiber cable), e.g., Spectrum wireless ( LAN and others), etc. shared cable (e.g., old Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a party (shared air, acoustics) Data Link Layer (SSL) 6-11 Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes may interfere with each other collision if a node receives two or more signals at the same time Need a protocol to determine when nodes can transmit no out-of-band channel for coordination Data Link Layer (SSL)

7 MA Protocols: a taxonomy Three broad classes: Channel Partitioning (e.g., cell phones) divide id channel into smaller pieces (frequency bands, time slots, codes) allocate a piece to each node for exclusive use Random Access (e.g., early Ethernet, wifi) shared channel, collisions allowed recover from collisions does not provide QoS Taking turns (e.g., token-ring LAN, FDDI) nodes take turns a node with more to send can take a longer turn Data Link Layer (SSL) 6-13 Channel Partitioning protocols FDMA: frequency division multiple access* each station assigned a fixed frequency band (note: MIMO antenna can use multiple frequencies) unused transmission time in frequency bands go idle FDM cable frequency bands * multiple transmitters Data Link Layer (SSL)

8 Channel Partitioning protocols TDMA: time division multiple access* each station gets fixed length slot (length = pkt trans time) in each frame requires time synchronization unused slots go idle 6-slot frame * multiple transmitters Data Link Layer (SSL) 6-15 Random Access Protocols When node has packet to send transmit at full channel data rate no a priori coordination among nodes two or more transmitting nodes collision random access MA protocol specifies: how to detect collision how to recover from collision (e.g., via delayed retransmissions) examples (chronological): ALOHA slotted ALOHA CSMA, CSMA/CD, CSMA/CA Data Link Layer (SSL)

9 Slotted Aloha time is divided into equal size slots (pkt trans. times) requires time synchronization node with new arriving pkt: transmit at beginning of next slot if collision: retransmit pkt in a future slot with probability p (or one of K slots at random), until successful. Success (S), Collision (C), Empty (E) slots Data Link Layer (SSL) 6-17 Slotted Aloha efficiency Long-term fraction of time slots that are successful? Suppose N nodes have packets to send each transmits in slot with probability p prob. successful transmission S is by a particular node: S = p (1-p) (N-1) by any of N nodes: S = Prob [one of N nodes transmits] = N p (1-p) (N-1) choosing optimum p, let N -> infinity = 1/e =.37 as N -> infinity Channel occupied by useful transmissions < 37% of time Data Link Layer (SSL)

10 S 0 P S P N 1 = [NP (1 P ) ] P N 1 N 1 = NP (1 P ) + (1 P ) N P N 2 N 1 = NP (N 1) (1 P ) + N(1 P ) N 2 = + N(1 P) { P(N 1) 1 P} N 2 = + + N(1 P) { NP P 1 P} S 1 = 0 when P = to maximize i S P N My terminology : Probability Division Multiplex Division of probability does not have to be fair, i.e., P 1 +P 2 + +P N = 1 is condition for maximum Data Link Layer (SSL) 6-19 S = NP(1 P) 1 e max N 1 1 P = N N = N 1 N N N 1 1 N = 1 e N which is maximum throughput (efficiency) of the slotted ALOHA protocol Data Link Layer (SSL)

11 Pure (unslotted) ALOHA unslotted Aloha: no time synchronization when frame arrives send immediately (without waiting for beginning g of slot) collision probability increases: frame sent at t 0 can collide with another frame sent within [t 0-1, t 0 +1] Vulnerable period is twice that of slotted ALOHA Data Link Layer (SSL) 6-21 Pure Aloha (cont.) P(success by any of N nodes) choosing optimum P, let N -> infinity = 1/(2e) = Slotted Aloha Pure Aloha G = offered load = NP Data Link Layer (SSL)

12 CSMA: Carrier Sense Multiple Access CSMA: listen before transmit (for a channel with short propagation delay) If channel sensed idle: transmit entire packet If channel sensed busy, defer transmission; retry after some random interval human analogy: don t interrupt when someone else is speaking Data Link Layer (SSL) 6-23 CSMA collisions spatial layout of nodes along cable collisions can occur: it takes time for two nodes to hear each other s transmission due to propagation delay collision: entire packet transmission time wasted Data Link Layer (SSL)

13 Vulnerable period of a transmission Let τ be the maximum one-way propagation p delay between two nodes in a subnet If sender A detects no collision after 2τ seconds, then it knows that its transmission will be successful 2τ Vulnerable period is 2τ <- node D will not transmit after sensing A s transmission Data Link Layer (SSL) 6-25 CSMA/CD collision detection (& abort) Data Link Layer (SSL)

14 CSMA/CD carrier sensing, deferral as in CSMA CD useful for channels where collisions are detectable within a short time colliding transmissions aborted, reducing channel wastage collision detection is easy in wired LANs: measure signal strength, compare transmitted and received signals difficult in wireless LANs: received signal overwhelmed by local transmission signal high channel utilization possible by sending very long packets (relative to propagation delay) Data Link Layer (SSL) 6-27 CSMA/CD channel efficiency Channel efficiency = t trans /(contention period + t trans ) where t trans is average transmission time of a frame Let t prop denote the maximum propagation delay between any two nodes. Then a good estimate of the average contention period is 2t prop e. (Why?) CSMA/CD channel efficiency = t trans / (2t prop e + t trans ) Data Link Layer (SSL)

15 Taking Turns MA protocols Polling: master node invites slave nodes to transmit in turn concerns: polling overhead latency (for large N) single point of failure (master) slaves data data poll master Data Link Layer (SSL) 6-29 Taking Turns MA protocols Token passing: control token (short msg) passed from one node to next sequentially. Data removed from ring by its sender => broadcast concerns: latency (for large N) single point of failure ring interface is an active repeater token loss (nothing to send) T T data Data Link Layer (SSL)

16 Solution: Star-shaped Ring Topology Example: Token ring (IEEE 802.5) with wiring closet Today s Eh Ethernet uses a star topology Data Link Layer (SSL) 6-31 Chapter 6 The Data Link layer 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs addressing, ARP Ethernet layer-2 switches VLANS 6.5 link virtualization: MPLS 6.6 data center networks 6.7 a day in the life of a web request (play animation in.ppt slide on your own) Data Link Layer (SSL)

20 Addressing: routing to another LAN IP Eth Phy A A creates IP datagram with IP source A, destination B A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram C-E8-FF-55 MAC src: C-E8-FF-55 9C MAC dest: E6-E BB-4B IP src: IP dest: R A-23-F9-CD-06-9B B BD-D2-C7-56-2A CC-49-DE-D0-AB-7D E6-E BB-4B B2-2F-54-1A-0F Data Link Layer (SSL) 6-39 Addressing: routing to another LAN IP Eth Phy A frame sent from A to R frame received at R, datagram passed up to IP MAC src: C-E8-FF-55 MAC dest: E6-E BB-4B IP src: IP dest: C-E8-FF-55 IP Eth Phy R A-23-F9-CD-06-9B B BD-D2-C7-56-2A CC-49-DE-D0-AB-7D E6-E BB-4B B2-2F-54-1A-0F Data Link Layer (SSL)

21 Addressing: routing to another LAN A R forwards datagram with IP source A, destination B R looks up B s MAC address R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram C-E8-FF-55 IP Eth Phy R A-23-F9-CD-06-9B MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: IP dest: IP Eth Phy B BD-D2-C7-56-2A CC-49-DE-D0-AB-7D E6-E BB-4B B2-2F-54-1A-0F Data Link Layer (SSL) 6-41 Addressing: routing to another LAN R sends frame to B A C-E8-FF-55 IP Eth Phy R A-23-F9-CD-06-9B MAC src: 1A-23-F9-CD-06-9B 9B MAC dest: 49-BD-D2-C7-56-2A IP src: IP dest: IP Eth Phy B BD-D2-C7-56-2A CC-49-DE-D0-AB-7D E6-E BB-4B B2-2F-54-1A-0F Data Link Layer (SSL)

22 Addressing: routing to another LAN R sends frame to B B s IP layer receives datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: IP dest: IP Eth Phy A C-E8-FF CC-49-DE-D0-AB-7D R E6-E BB-4B A-23-F9-CD-06-9B B BD-D2-C7-56-2A B2-2F-54-1A-0F Data Link Layer (SSL) 6-43 Link layer, LANs 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 link virtualization: MPLS 5.6 data center networks 5.7 a day in the life of a web request (play animation in.ppt slides on your own) Data Link Layer (SSL)

23 Ethernet dominant wired LAN technology: cheap, \$20 for NIC first widely used LAN technology simpler, cheaper than competitors token-ring (16 Mbps), FDDI (100 Mbps), and ATM (155 Mbps) kept up with speed race: 10 Mbps 10 Gbps Data Link Layer (SSL) 6-45 Star topology bus topology popular through mid 90s, and later star topology with hub at center all nodes in same collision domain (their transmissions can collide with each other) today: star topology with active switch (layer 2) at center no collision switch bus: coaxial cable star Data Link Layer (SSL)

25 Unreliable, connectionless service Connectionless: No handshaking between sending and receiving adapters Unreliable: receiving adapter doesn t sendacksor nacks to sending adapter stream of datagrams passed to network layer can have gaps gaps will be filled only if app is using TCP Ethernet s MAC protocol: CSMA/CD with binary backoff Interval for random retransmission doubles after every additional collision Data Link Layer (SSL) Ethernet Standards: Link & Physical Layers many different Ethernet standards different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10Gbps different physical layer media and technologies: coax cable, twisted pair, fiber same frame format and MAC protocol application transport network link physical 100BASE-TX MAC protocol and frame format 100BASE-T2 100BASE-FX 100BASE-T4 100BASE-SX 100BASE-BX copper (twisted pair) physical layer fiber physical layer Data Link Layer (SSL)

26 Chapter 6 The Data Link layer 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs addressing, ARP Ethernet layer-2 switches VLANS 6.5 link virtualization: MPLS 6.6 data center networks 6.7 a day in the life of a web request (play animation in.ppt slide on your own) Data Link Layer (SSL) 6-51 Layer-2 Switches vs. Routers both store-and-forward devices routers: network layer devices (examine network layer headers) layer-2 switches are link layer devices routers maintain forwarding tables, implement routing protocols layer-2 switches maintain switch tables, perform filtering and learning Layer 2 switch Data Link Layer (SSL)

27 Switch (layer 2) Link layer device stores and forwards Ethernet frames examines frame header and may selectively l forward frame to just one outgoing interface (instead of broadcast) it still uses CSMA/CD (just in case an outgoing interface is connected to a hub) transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured Data Link Layer (SSL) 6-53 Switch: allows multiple simultaneous transmissions hosts have dedicated, direct connection (full duplex) to switch switch buffers packets switching: A-to-A and B- 5 4 to-b simultaneously, C without collisions not possible with dumb hub B A Ethernet protocol used on each link switch with six interfaces (1,2,3,4,5,6) C 6 A B Data Link Layer (SSL)

28 Switch Table Q: how does switch know that A reachable via interface 4, B reachable via interface 5? A: each switch has a switch table, each entry: (MAC address of host, interface to reach host, time stamp) looks like a forwarding table for routing Q: how are entries created, maintained in switch table? no routing protocol is used C B 6 5 A A B switch with six interfaces (1,2,3,4,5,6) C Data Link Layer (SSL) 6-55 Switch: self-learning Source: A Dest: A switch learns which hosts can be reached through which interfaces when frame received, switch learns location of sender in incoming LAN segment records sender/location pair in switch table What is required to make this work for a network of switches? C B MAC addr interface TTL 6 A A A A A B C Switch table (initially empty) Data Link Layer (SSL)

29 Switch: frame filtering/forwarding When frame received: 1. record interface associated with sending host 2. index switch table using MAC destination address 3. if entry in table found for destination then { if dest is on interface 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 Data Link Layer (SSL) 6-57 Self-learning, forwarding: example C A A A Source: A Dest: A B destination A unknown: flood destination A location known: selective send B 1 2 A 6A A A A C MAC addr interface TTL A 1 60 A 4 60 Switch table (initially empty) Data Link Layer (SSL)

30 Interconnecting layer-2 switches switches can be connected together S 4 A B C S 1 S 2 S 3 D E (note: some links are idled if physical topology has loops) Q: sending from A to G -how does S 1 know to forward frame destined to G via S 4 (and S 3 )? A: self learning (works exactly the same as in single-switch case) F G H I Data Link Layer (SSL) 6-59 Institutional network to external network router mail server web server IP subnet Data Link Layer (SSL)

31 Scope of broadcast domain Computer Science Electrical Engineering Computer Engineering a single broadcast domain all layer-2 broadcast frames (ARP, DHCP, switch-table cache miss, etc.) cross entire LAN => security/privacy, efficiency issues multiple broadcast domains Data Link Layer (SSL) 6-62 Port-based VLANs ports grouped by switch management software for a single physical switch to operate CSRES (VLAN ports 1-8) Computer Science (VLAN ports 9-15) as multiple virtual switches CSRES (VLAN ports 1-8) Computer Science (VLAN ports 9-16) Data Link Layer (SSL)

32 Port-based VLANs (cont.) traffic isolation: frames to/from ports of a VLAN can only reach its ports can also define a VLAN based on MAC addresses of endpoints, rather than switch ports dynamic membership: ports can be dynamically assigned among VLANs f d b VL N forwarding between VLANS: router CSRES Computer Science (VLAN ports 1-8) (VLAN ports 9-15) done via a router (just as with separate switches) in practice the router is built into the switch Data Link Layer (SSL) 6-63 VLANs spanning multiple switches CSRES (VLAN ports 1-8) Computer Science (VLAN ports 9-15) Ports 2,3,5 belong to CSRES VLAN Ports 4,6,7,8 belong to CS VLAN trunk ports: carry frames between VLANs defined over multiple physical switches frames forwarded within a VLAN between physical switches must carry VLAN ID info 802.1q protocol inserts/removes an additional header field (4 byte VLAN tag) for each frame forwarded between trunk ports Data Link Layer (SSL)

33 Chapter 6 The Data Link layer 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs addressing, ARP Ethernet layer-2 switches VLANS 6.5 link virtualization: MPLS 6.6 data center networks 6.7 a day in the life of a web request (play animation in.ppt slide on your own) Data Link Layer (SSL) 6-65 Link Virtualization: A Network as a Link Virtual circuits provided by ATM, frame relay, which are packet-switching networks in their own right (obsolete) with service models, addressing, routing different from Internet A subnet of MPLS capable routers Each is viewed as a link connecting two IP nodes Data Link Layer (SSL)

34 Multiprotocol label switching (MPLS) initial goal: speed up IP forwarding by using fixedlength label (instead of variable-length IP prefix) to do forwarding borrowing ideas from Virtual Circuit (VC) approach MPLS routers insert and remove MPLS header but IP datagram still keeps IP address PPP or Ethernet header MPLS header IP header remainder of link-layer frame label Exp S TTL Data Link Layer (SSL) 6-67 MPLS capable routers a.k.a. label-switched router forward packets to outgoing interface based only on label value (does not inspect IP address) Much faster than longest prefix match MPLS forwarding table distinct from IP forwarding tables flexibility: MPLS forwarding decisions can differ from those of IP Data Link Layer (SSL)

35 MPLS forwarding tables IP-only in out out label label dest interface 10 A 0 12 D 0 8 A 1 in out out label label dest interface 10 6 A D 0 MPLS capable R6 R5 R4 0 1 R2 in out out label label dest interface 8 7 A 0 R3 0 0 D 1 0 A in out R1 out label label dest interface 6 - A A 0 There are two predetermined routes from R4 to A Data Link Layer (SSL) 6-69 MPLS applications Fast failure recovery - rerouting flows quickly to pre-computed backup paths (useful for VoIP) Traffic engineering network operator can override IP routing and allocate traffic toward the same destination to multiple paths Resource provision i for virtual private networks Data Link Layer (SSL)

36 Chapter 6 The Data Link layer 6.1 introduction, services 6.2 error detection, correction 6.3 multiple access protocols 6.4 LANs addressing, ARP Ethernet layer-2 switches VLANS 6.5 link virtualization: MPLS 6.6 data center networks 6.7 a day in the life of a web request (play animation in.ppt slide on your own) Data Link Layer (SSL) 6-71 Data center networks 10 s to 100 s of thousands of hosts in close proximity supporting cloud applications e-business (e.g. Amazon) content-servers (e.g., YouTube, Akamai, Apple, Microsoft) search engines, data mining (e.g., Google) challenges: multiple applications, each serving massive number of clients managing/balancing load, avoiding bottlenecks in processing and networking Inside a 40-ft Microsoft container, Chicago data center Data Link Layer (SSL)

37 Data center networks Load balancer: NAT functionality - hiding data center internals from outside receives external client requests for Internet service directs workload within data center returns results to external client Load balancer Border router Access router B Load balancer Tier-1 switches A C Tier-2 switches TOR switches Server racks Data Link Layer (SSL) 6-73 Link layer below an access router rich interconnection among switches as well as duplication of switches: increased reliability via redundancy partitioned into smaller VLANs to localize ARP broadcast increased throughput between server racks (how to enable multiple routing paths) Tier-1 switches Tier-2 switches TOR switches Server racks focus of recent research: revisit routing for layer 2, congestion control, etc. Data Link Layer (SSL)

38 Chapter 6: 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, VLANs virtualized networks as a link layer: MPLS data center networks synthesis: a day in the life of a web request (be sure to open Chapter6_A_Day_animation.ppt file in cs356/slides folder on your own and see the animation) Data Link Layer (SSL) 6-75 The end Data Link Layer (SSL)

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