# Chapter 5: The Data Link Layer

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4 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) 5-7 Error Detection EDC= Error Detection and Correction bits (redundant check bits) D = Data protected by error checking, may include header fields Error detection not 100% reliable protocol may miss some errors more check bits yield better detection and correction Data Link Layer (SSL) 5-8 4

5 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors error Even parity: total number of 1s is odd means there is a bit error or, more precisely, an odd number of bit errors 0 0 Data Link Layer (SSL) 5-9 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 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) 5-11 CRC Theory and Example Want: (D*2 r ) XOR R = ng add R to both sides: D*2 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)

8 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) 5-15 Ideal Multiple 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, time slots 4. simple Data Link Layer (SSL)

9 MA Protocols: a taxonomy Three broad classes: Channel Partitioning divide id channel into smaller pieces (time slots, frequency bands, codes) allocate a piece to each node for exclusive use Random Access shared channel, collisions allowed recover from collisions Taking turns nodes take turns a node with more to send can take a longer turn Data Link Layer (SSL) 5-17 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 * there are multiple transmitters Data Link Layer (SSL)

10 Channel Partitioning protocols FDMA: frequency division multiple access each station assigned a fixed frequency band unused transmission time in frequency bands go idle FDM cable frequency bands Data Link Layer (SSL) 5-19 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: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA Data Link Layer (SSL)

11 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) 5-21 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)

12 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 Data Link Layer (SSL) 5-23 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)

13 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) 5-25 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)

14 CSMA: Carrier Sense Multiple Access CSMA: listen before transmit: 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) 5-27 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)

15 Vulnerable period of a transmission Let be the maximum one-way propagation p delay between two nodes in a subnet If sender A detects no collision after 2 seconds, then it knows that its transmission will be successful 2 Vulnerable period is 2 <- node D will not transmit after sensing A s transmission Data Link Layer (SSL) 5-29 CSMA/CD collision detection (& abort) Data Link Layer (SSL)

16 CSMA/CD (Collision Detection) 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) 5-31 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 2e t prop. (Why?) CSMA/CD channel efficiency = t trans / (2e t prop + t trans ) Data Link Layer (SSL)

17 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) 5-33 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 activering interface token loss (nothing to send) T T data Data Link Layer (SSL)

18 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) 5-35 Summary of MA protocols channel partitioning, by time, frequency (or code) Time Division, Frequency Division (also CDMA) random access ALOHA, slotted ALOHA, CSMA, CSMA/CD collision detection: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA A used in (Chap. 6) taking turns polling by a central site, e.g., Bluetooth token passing - IBM Token Ring, FDDI Data Link Layer (SSL)

22 Addressing: routing to another LAN walkthrough: A sends datagram to B via R. focus on addressing - at both IP (datagram) and MAC layer (frame) A knows B s IP address A knows IP address of first-hop router, R A knows MAC address of first hop router s interface (how?) A C-E8-FF-55 R A-23-F9-CD-06-9B B BD-D2-C7-56-2A E6-E BB-4B 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Data Link Layer (SSL) 5-43 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)

23 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) 5-45 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 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)

24 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) 5-47 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)

25 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) 5-49 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)

26 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) 5-51 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern followed by one byte with pattern used to synchronize receiver, sender clocks long preamble used due to burst nature of transmissions, unlike a synchronous point to point link Data Link Layer (SSL)

28 802.3 Ethernet Standards: Link & Physical Layers many different Ethernet standards same MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10Gbps different physical layer media and technologies: cable, twisted pair, fiber 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) 5-55 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)

29 Hubs Hubs are physical-layer repeaters: bits coming from one link go out all other links at the same rate no frame buffering -> collisions like a coax cable adapters detect collisions as usual twisted pair hub Data Link Layer (SSL) 5-57 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 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)

30 Switch: allows multiple simultaneous transmissions hosts have dedicated, direct connection ( (full duplex) to switch switch buffers packets switching: A-to-A and B- to-b simultaneously, without collisions not possible with dumb hub Ethernet protocol used on each link not possible with dumb hub B C 6 5 A A switch with six interfaces (1,2,3,4,5,6) B C Data Link Layer (SSL) 5-59 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)

31 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: incoming LAN segment records sender/location pair in switch table What is required to make this work? C B MAC addr interface TTL 6 A A A A A B C Switch table (initially empty) Data Link Layer (SSL) 5-61 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)

32 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) 5-63 Interconnecting 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)

33 Institutional network to external network router mail server web server IP subnet Data Link Layer (SSL) 5-65 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 switches (layer 2) maintain switch tables, implement filtering, learning algorithms Layer 2 switch Data Link Layer (SSL)

34 VLANs: motivation Computer Science Electrical Engineering Computer Engineering Suppose a single broadcast domain is configured all layer-2 broadcast frames (ARP, DHCP, switch-table cache miss, etc.) cross entire LAN => security/privacy, i efficiency issues Data Link Layer (SSL) 5-67 VLANs Port-based VLAN: switch ports grouped (by switch management software) for a single physical switch which Virtual Local Area Network A switch that supports VLAN capabilities can be configured to have multiple virtual LANs 1 2 Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) operates as multiple virtual switches Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-16) Data Link Layer (SSL)

35 Port-based VLAN 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 1 2 Electrical Engineering (VLAN ports 1-8) forwarding between VLANS: done via a router (just as with separate switches) in practice the router is built into the switch 7 8 router Computer Science (VLAN ports 9-15) Data Link Layer (SSL) 5-69 VLANs spanning multiple switches Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) Ports 2,3,5 belong to EE 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 adds/removes an additional header field for each frame forwarded between trunk ports another example of tunnelling! Data Link Layer (SSL)

36 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) 5-71 Link Virtualization: A Network as a Link Virtual circuits provided by ATM, frame relay, which are packet-switching networks in their own right with service models, addressing, routing different from Internet A subnet of MPLS capable routers Each is invisible to IP and viewed as a link connecting two IP nodes Data Link Layer (SSL)

37 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) 5-73 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)

38 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) 5-75 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)

39 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) 5-77 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 numbers of clients managing/balancing load, avoiding bottlenecks in processing and networking Inside a 40-ft Microsoft container, Chicago data center Data Link Layer (SSL)

40 Data center networks Load balancer: receives external client requests for service directs workload within data center Internet returns results to external client NAT functionality - hiding data center internals from client Load balancer B Border router Access router Load balancer Tier 1 switches A C Tier 2 switches TOR switches Server racks Data Link Layer (SSL) 5-79 Subnet below an access router rich interconnection among switches as well as duplication of routers/switches: increased reliability via redundancy increased throughput between racks (multiple routing paths possible) partitioned into smaller VLANs to localize ARP broadcast Tier 1 switches Tier 2 switches TOR switches Server racks focus of much recent research: routing, congestion control, etc. Data Link Layer (SSL)

41 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, 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 Chapter5_A_Day_animation.ppt file on your own and see the animation) Data Link Layer (SSL) 5-81 The end Data Link Layer (SSL)

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